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Внутренние океанические комплексы и гидротермальный процесс

Журнальные статьи

Abell R.E. et al. Variability of particulate flux over the Mid-Atlantic Ridge // Deep-Sea Res. Part II-Top. Stud. Oceanogr. 2013. Vol. 98. P. 257–268.

The magnitude and composition of the sinking-particle flux were studied over the northern Mid-Atlantic Ridge (MAR) from June 2007 to July 2010. Four moorings equipped with dual sediment traps, 100 m and 1000 m above the sea floor, sampled regions north and south of the Charlie Gibbs Fracture Zone (between 49 degrees N and 54 degrees N) and east and west of the MAR. Biogenic data were coupled with satellite estimates of primary production and modelled particle source to assess the variability in export flux. Large variations were found in the seasonality, chemical composition, magnitude and source of sinking particulate material between mooring sites. The northern moorings recorded both greater mean primary production and greater particle mass flux than the southern moorings, although, the large inter-annual variability within the sites exceeded inter-site differences. While estimates of primary production and organic carbon fluxes are comparable to other investigations of this type, they are notably lower than previous estimates for the abyssal plain of the North Atlantic. The deeper traps consistently recorded a higher mass flux compared to the shallower traps. However, we suggest that the overall flux recorded by the shallower traps was reduced by trapping inefficiency, which in the light of the low current velocities, may largely be due to the physical nature of the sinking material. Although deep-trap flux estimates may be more susceptible to errors due to re-suspended and advected material from nearby topography, mass flux and current velocity are not linked. In addition, the relatively low aluminium concentration of the deep-trap material indicates that this contribution is relatively small. The organic carbon flux to the NE, NW, SE and SW station was 0.8, 1.2, 1.1 and 1.1 g m(-2) y(-1) respectively, corresponding to an export flux of 0.6% over this region of the MAR.

Andreani M. et al. Tectonic structure, lithology, and hydrothermal signature of the Rainbow massif (Mid-Atlantic Ridge 36° 14'N) // Geochem. Geophys. Geosyst. 2014. Vol. 15, № 9. P. 3543–3571.

Rainbow is a dome-shaped massif at the 36 degrees 14N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (approximate to 350 degrees C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220 degrees C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at approximate to 160-260 degrees C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.

Arnulf A.F. et al. Anatomy of an active submarine volcano // Geology. 2014. Vol. 42, № 8. P. 655–658.

Most of the magma erupted at mid-ocean ridges is stored in a mid-crustal melt lens that lies at the boundary between sheeted dikes and gabbros. Nevertheless, images of the magma pathways linking this melt lens to the overlying eruption site have remained elusive. Here, we have used seismic methods to image the thickest magma reservoir observed beneath any spreading center to date, which is principally attributed to the juxtaposition of the Juan de Fuca Ridge with the Cobb hotspot (northwestern USA). Our results reveal a complex melt body, which is similar to 14 km long, 3 km wide, and up to 1 km thick, beneath the summit caldera. The estimated volume of the reservoir is 18-30 km(3), more than two orders of magnitude greater than the erupted magma volumes of either the A. D. 1998 or 2011 eruption. Our images show a network of sub-horizontal to shallow-dipping (<30 degrees) features that we interpret as pathways facilitating melt transport from the magma reservoir to the eruption sites.

Boettger J. et al. Energy yields from chemolithotrophic metabolisms in igneous basement of the Juan de Fuca ridge flank system // Chemical Geology. 2013. Vol. 337–338. P. 11–19.

The permeable rocks of the upper oceanic basement contain seawater-sourced fluids estimated to be ~ 2% of the global ocean volume. This represents a very large potential subsurface biosphere supported by chemosynthesis. Recent collection of high integrity samples of basement fluid from the sedimented young basaltic basement on the Juan de Fuca Ridge flanks, off the coasts of Vancouver Island (Canada) and Washington (USA), and subsequent chemical analyses permit numerical modeling of metabolic redox reaction energetics. Here, values of Gibbs free energy for potential chemolithotrophic net reactions were calculated in basement fluid and in zones where basement fluid and entrained seawater may mix; the energy yields are reported both on a per mole electrons transferred and on a per kg of basement fluid basis. In pure basement fluid, energy yields from the anaerobic respiration processes investigated are anemic, releasing < 0.3 J/kg basement fluid for all reactions except methane oxidation by ferric iron, which releases ~ 0.6 J/kg basement fluid. In mixed solutions, aerobic oxidation of hydrogen, methane, and sulfide is the most exergonic on a per mole electron basis. Per kg of basement fluid, the aerobic oxidation of ammonia is by far the most exergonic at low temperature and high seawater:basement fluid ratio, decreasing by more than two orders of magnitude at the highest temperature (63 °C) and lowest seawater:basement fluid ratio investigated. Compared with mixing zones in deep-sea hydrothermal systems, oceanic basement aquifers appear to be very low energy systems, but because of their expanse, may support what has been labeled the ‘starving majority’.

Boschi C. et al. Serpentinization of mantle peridotites along an uplifted lithospheric section, Mid Atlantic Ridge at 11 degrees N // Lithos. 2013. Vol. 178. P. 3–23.

Mantle peridotites from an exposed lithospheric section (Vema Lithospheric Section, VLS), generated during similar to 26 Ma at a similar to 80 km long Mid Atlantic Ridge segment (11 degrees N), have been sampled and studied to understand the evolution of the serpentinization process. The VLS was uplifted due to a 10 Ma transtensional event along the Vema transform. Before the uplift residual mantle rocks were lying beneath a 0.8-1.3 km thick basaltic crustal layer. The major and trace element compositions of the serpentinites, as well as their H, O, Sr, Cl and B isotopic compositions were interpreted based on thermal models of lithospheric spreading from ridge axis. The results suggest that serpentinization occurred mostly near the ridge axis. Serpentinization temperatures, estimated from stable isotopes, are consistent with resetting of the closure temperatures during the tectonic uplift of the lithospheric sliver, reflected by decreasing delta O-18 and increasing delta B-11 values. Modeling shows that the thermal influence of the transtensional event affected mainly the region close to the RTI (ridge-transform intersection). Petrological, elemental and isotopic data suggest that, when the ultramafic basal unit of the VLS was uplifted and exposed on the ocean floor, serpentinization became superseded by low temperature water-rock reactions, with Fe-Mn crust formation, which is still progressing, as recorded by delta D. Ultramafic mylonites, prevalent in a short stretch of the VLS, show only a partial serpentinization process, together with pervasive contamination by low-temperature Fe-Mn crust.

Bowles J.A. et al. Eruptive timing and 200 year episodicity at 92 degrees W on the hot spot-influenced Galapagos Spreading Center derived from geomagnetic paleointensity // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 6. P. 2211–2224.

Eruptive timing in mid-ocean ridge systems is relatively poorly constrained, despite being an important variable in our understanding of many mid-ocean ridge processes, including volcanic construction; magma recharge, flux, and storage; and the stability of hydrothermal systems and biological communities. Only a handful of absolute eruption chronologies exist, yet they are essential in understanding how eruptive timing varies with important controlling variables. To construct an eruptive history at one location on the Galapagos Spreading Center, we present age determinations derived from geomagnetic paleointensity. To aid interpretation of the paleointensity data, we also present results from on-bottom magnetic anomaly measurements and forward modeling of topographic-induced magnetic anomalies. Anomalies may lead to a 1-2 mu T bias in flow-mean paleointensities, which does not significantly affect the overall interpretation. Paleointensity results for the three youngest sampled units are indistinguishable, consistent with the flows being emplaced in relatively rapid succession. Comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100-200 years. The fourth sampled unit has a significantly higher paleointensity, consistent with an age of roughly 400 years. The possible bias in paleointensity data allows for ages as young as similar to 50 years for the youngest three flows and 200-400 years for the oldest flow. This age distribution demonstrates an episodicity in the emplacement of the largest flows at this location, with a 200-300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units.

Caetano M. et al. The Condor seamount at Mid-Atlantic Ridge as a supplementary source of trace and rare earth elements to the sediments // Deep-Sea Res. Part II-Top. Stud. Oceanogr. 2013. Vol. 98. P. 24–37.

The Condor Seamount rises from seabed to 180 m water depth, being located 10 nautical miles southwest of the island of Faial, Azores Archipelago at the Mid-Atlantic Ridge (MAR). The vertical distribution of major, minor, trace and rare earth elements (REE) and Pb isotopes was studied in four sediment cores: one from the top of the Condor Seamount (200 m, MC9), two from the seamount base (1400 m, MC2 and MC4), and one from a deep area (1900 m, MC8). Sediments from the top of the Condor were composed by coarser particles being the fine fraction lower than 1%. Conversely the other sediments were constituted by 51-92% of fine particles (<63 mu m). Individual fragments of volcanic material (>2 mm) were found at several depths of the cores sampled at the base of the seamount. The core collected in the top of the Condor showed higher carbonate content (76-86%) compared with the other cores (41-64%). The chemical compositions of MC2 and MC4 point to an enhancement of V, Cr, Co, Ni and Fe concentrations. Lower concentrations in MC8 hypothesis that Condor seamount constitutes a supplementary source of trace elements. The most plausible explanation for the enhancement found in sediments of the seamount base is the weathering of slopes with volcanic activities, which supply particles with higher element concentrations than pelagic sediments. This hypothesis is corroborated by REE data, showing increased chondrite normalized ratios in MC2 and MC4. Moreover, the REE pattern found in those cores was comparable to that existing in volcanic material with Light REE enrichment in comparison to Heavy REE. These results indicate a substantial contribution of particles derived from volcanic activities to sediments settled in the vicinity of the Condor Seamount. It is argued the potential use of REE in sediments from this region as tracers of volcanic activities. Depth profiles of Pb-206/Pb-207 and Pb-206/Pb-208 showed lower ratios in the first 8 cm sediment layers, reflecting atmospheric input of anthropogenic Pb in the last century. On the basis of Pb profiles it is proposed a baseline Pb concentration of 3.6 +/- 0.2 mu g g(-1) for pelagic sediments of the region with an isotopic signature of Pb-206/Pb-207 = 1.227 +/- 0.003 and Pb-206/Pb-208 = 0.492 +/- 0.001 signature. The isotope plots of Pb-206/Pb-207 versus Pb-208/Pb-206 showed a linear trend indicating the mixing between more radiogenic pre-industrial end-members and less radiogenic anthropogenic lead. The Pb isotope composition of sediments from the Condor area falls closer to North Atlantic Sediment Line. Sediments showed a Pb-206/Pb-204 signature closer to the basalts of the Capelo volcanic complex than from Mid-Ocean Ridge Basalts (MORB), which suggests the contribution of similar geological formations to sedimentary material.

Cannat M. et al. High-resolution bathymetry reveals contrasting landslide activity shaping the walls of the Mid-Atlantic Ridge axial valley // Geochem. Geophys. Geosyst. 2013. Vol. 14, № 4. P. 996–1011.

Axial valleys are found along most slow-spreading mid-ocean ridges and are one of the most prominent topographic features on Earth. In this paper, we present the first deep-tow swath bathymetry for the axial valley walls of the Mid-Atlantic Ridge. These data allow us to analyze axial valley wall morphology with a very high resolution (0.5 to 1 m compared to 50 m for shipboard multibeam bathymetry), revealing the role played by landslides. Slow-spreading ridge axial valleys also commonly expose mantle-derived serpentinized peridotites in the footwalls of large offset normal faults (detachments). In our map of the Ashadze area (lat. 13 degrees N), ultramafic outcrops have an average slope of 18 degrees and behave as sliding deformable rock masses, with little fragmentation. By contrast, the basaltic seafloor in the Krasnov area (lat. 16 degrees 38N) has an average slope of 32 degrees and the erosion of the steep basaltic rock faces leads to extensive fragmentation, forming debris with morphologies consistent with noncohesive granular flow. Comparison with laboratory experiments suggests that the repose angle for this basaltic debris is > 25 degrees. We discuss the interplay between the normal faults that bound the axial valley and the observed mass wasting processes. We propose that, along axial valley walls where serpentinized peridotites are exposed by detachment faults, mass wasting results in average slopes 20 degrees, even in places where the emergence angle of the detachment is larger.

9. U10290
Carazzo G., Jellinek A.M., Turchyn A.V. The remarkable longevity of submarine plumes: Implications for the hydrothermal input of iron to the deep-ocean // Earth and Planetary Science Letters. 2013. Vol. 382. P. 66–76.

The longevity of submarine plumes generated at sea-floor hydrothermal systems constrains the hydrothermal input of chemical species into the deep-ocean. Decades of observations of episodic “event plumes” suggest that a key process governing the dynamics of an hydrothermal cloud spreading out laterally from a buoyant rising plume is the production of internal layering. Here, we use analog experiments on turbulent, hot particle-laden plumes and clouds to show that this layering occurs where particle diffusive convection driven by the differential diffusion of heat and small mineral precipitates gives rise to a large scale double diffusive instability. Where hydrothermal clouds are enriched in fine minerals, this “particle diffusive convection” can extend the longevity of an event plume to 2 yr after its emplacement. The very long residence time imposed by diffusive convective effects enables complete dissolution of fine sulfide and sulfate minerals. We develop a new theoretical model that includes both sedimentation and dissolution processes to quantify the potential amount of iron produced by the dissolution of iron-sulfide minerals settling through the cloud by diffusive convection. A key prediction is that the concentration of dissolved iron in hydrothermal clouds can reach up to 19 ± 3 nM , which represents about 5% of the global hydrothermal discharge. If these results are representative of all hydrothermal vent fields, hydrothermal systems could provide 75% of the global budget of dissolved iron in the deep-ocean. Regionally, this flux is expected to scale in magnitude with mid-ocean ridge heat flow, consistent with observations and global ocean models.

Castelain T., McCaig A.M., Cliff R.A. Fluid evolution in an Oceanic Core Complex: A fluid inclusion study from IODP hole U1309 D-Atlantis Massif, 30 degrees N, Mid-Atlantic Ridge // Geochem. Geophys. Geosyst. 2014. Vol. 15, № 4. P. 1193–1214.

In the detachment mode of slow seafloor spreading, convex-upward detachment faults take up a high proportion of the plate separation velocity exposing gabbro and serpentinized peridotite on the seafloor. Large, long-lived hydrothermal systems such as TAG are situated off axis and may be controlled by fluid flow up a detachment fault, with the source of magmatic heat being as deep as 7 kmbsf. The consequences of such deep circulation for the evolution of fluid temperature and salinity have not previously been investigated. Microthermometry on fluid inclusions trapped in diabase, gabbro, and trondjhemite, recovered at the Atlantis Massif Oceanic Core Complex (30 degrees N, Mid-Atlantic Ridge), reveals evidence for magmatic exsolution, phase separation, and mixing between hydrothermal fluids and previously phase-separated fluids. Four types of fluid inclusions were identified, ranging in salinity from 1.4 to 35 wt % NaCl, although the most common inclusions have salinities close to seawater (3.4 wt % NaCl). Homogenization temperatures range from 160 to >400 degrees C, with the highest temperatures in hypersaline inclusions trapped in trondjhemite and the lowest temperatures in low-salinity inclusions trapped in quartz veins. The fluid history of the Atlantis Massif is interpreted in the context of published thermochronometric data from the Massif, and a comparison with the inferred circulation pattern beneath the TAG hydrothermal field, to better constrain the pressure temperature conditions of trapping and when in the history of exhumation of the rocks sampled by IODP Hole U1309D fluids have been trapped.

Chen Z. et al. Influence of igneous processes and serpentinization on geochemistry of the Logatchev Massif harzburgites (14° 45'N, Mid-Atlantic Ridge), and comparison with global abyssal peridotites // Int. Geol. Rev. 2013. Vol. 55, № 1. P. 115–130.

We acquired bulk-rock analyses of Mid-Atlantic Ridge (MAR) harzburgites in order to understand the influence of submarine igneous and metamorphic processes on the distribution of incompatible elements (especially rare Earth elements or REEs) in abyssal peridotites. The geochemical characteristics of these Logatchev Massif serpentinized and talc-altered harzburgites, and spatially associated metagabbros were then compared with a compilation of global abyssal peridotites. The Logatchev harzburgites show light rare earth element (LREE) enrichments (average La-N/Yb-N = 2.81), positive correlations between LREEs (e. g. La, Ce, Pr, and Nd) and high field strength elements (HFSEs; e. g. Nb and Zr), and positive correlations between HFSEs and Th. Most global abyssal peridotites show similar trends. We suggest that the systematic enrichment of incompatible elements probably reflects a post-partial fusion magmatic refertilization. The compositional scatter exhibited by some serpentinized peridotites in Nb-LREE diagrams is probably due to the elimination of diopside during partial melting and significant impregnation by a melt produced in the Opx-Ol-Sp melting field rather than to later hydrothermal alteration. The correlation between Pb and Nd observed for most global abyssal peridotites, including the Logatchev harzburgites, indicates magmatic generation. The scatter of Pb in some rocks suggests that lead is likely mobile during serpentinization or weathering. Low to moderate water/rock (W/R) ratios in the harzburgites calculated from Sr isotopic compositions (5.98-26.20 for a close system and 1.66-2.72 for an open system), and the low abundance of REEs in Logatchev hydrothermal fluids indicate that the REE contents of abyssal peridotites probably were little influenced by hydrothermal alteration. Compared to this later alteration, the presence of small proportions of gabbroic melt (from 1:30 to 1:3 in our sample) that crystallized in the residual harzburgites modified their REE patterns significantly by elevating the LREEs.

Chiodini G. et al. Evidence of thermal-driven processes triggering the 2005–2014 unrest at Campi Flegrei caldera // Earth and Planetary Science Letters. 2015. Vol. 414. P. 58–67.

An accelerating process of ground deformation that began 10 years ago is currently affecting the Campi Flegrei caldera. The deformation pattern is here explained with the overlapping of two processes: short time pulses that are caused by injection of magmatic fluids into the hydrothermal system; and a long time process of heating of the rock. The short pulses are highlighted by comparison of the residuals of ground deformation (fitted with an accelerating polynomial function) with the fumarolic CO2/CH4 and He/CH4 ratios (which are good geochemical indicators of the arrival of magmatic gases). The two independent datasets show the same sequence of five peaks, with a delay of ?200 days of the geochemical signal with respect to the geodetic signal. The heating of the hydrothermal system, which parallels the long-period accelerating curve, is inferred by temperature–pressure gas geoindicators. Referring to a recent interpretation that relates variations in the fumarolic inert gas species to open system magma degassing, we infer that the heating is caused by enrichment in water of the magmatic fluids and by an increment in their flux. Heating of the rock caused by magmatic fluids can be a central factor in triggering unrest at calderas.

Coogan L.A., Dosso S.E. Alteration of ocean crust provides a strong temperature dependent feedback on the geological carbon cycle and is a primary driver of the Sr-isotopic composition of seawater // Earth and Planetary Science Letters. 2015. Vol. 415. P. 38–46.

On geological timescales there is a temperature dependent feedback that means that increased degassing of CO2 into the atmosphere leads to increased CO2 drawdown into rocks stabilizing Earth's climate. It is widely considered that this thermostat largely comes from continental chemical weathering. An alternative, or additional, feedback comes from dissolution of seafloor basalt in low-temperature (tens of degrees C), off-axis, hydrothermal systems. Carbonate minerals precipitated in these systems provide strong evidence that increased bottom water temperature (traced by their O-isotopic compositions) leads to increased basalt dissolution (traced by their Sr-isotopic compositions). Inversion of a simple probabilistic model of fluid-rock interaction allows us to determine the apparent activation energy of rock dissolution in these systems. The high value we find (92 +/- 7 kJ mol(-1)) indicates a strong temperature dependence of rock dissolution. Because deep-ocean temperature is sensitive to global climate, and the fluid temperature in the upper oceanic crust is strongly influenced by bottom water temperature, increased global temperature must lead to increased basalt dissolution. In turn, through the generation of alkalinity by rock dissolution, this leads to a negative feedback on planetary warming; i.e. off-axis, hydrothermal systems play an important role in the planetary thermostat. Changes in the extent of rock dissolution, due to changes in bottom water temperature, also lead to changes in the flux of unradiogenic Sr into the ocean. The decreased flux of unradiogenic Sr into the ocean due to the cooling of ocean bottom water over the last 35 Myr is sufficient to explain most of the increase in seawater Sr-87/Sr-86 over this time.

Dean S.L., Sawyer D.S., Morgan J.K. Galicia Bank ocean-continent transition zone: New seismic reflection constraints // Earth and Planetary Science Letters. 2015. Vol. 413. P. 197–207.

The West Iberia continental margin is a type locale for magma-poor rifting, and studies there have been instrumental in changing the classical view of the ocean-continent transition (OCT) from a discrete boundary juxtaposing continental and oceanic crust, into a more complicated zone of varying width that can include exhumed mantle. This study examines two new seismic lines in the Galicia Bank area extending west of the Peridotite Ridge, showing high resolution images of five new ridges. These ridges could be hyperextended continental crust, exhumed continental mantle, or rough ultra-slow spreading oceanic crust. There are no tilted fault blocks with pre-syn rift stratigraphy that would indicate continental crust. There are also no faults indicating mid-ocean spreading with seismic layer stratigraphy indicating normal oceanic crust. The ridges have no coherent internal seismic structure, and some resemble the topographic profile of the Peridotite Ridge. Therefore, it is likely the western ridges are also mainly composed of serpentinized mantle. These western ridges are also similar to small oceanic core complexes observed along the active part of the Mid-Atlantic Ridge, which also contain exhumed serpentinized mantle. This implies that there is a gradual transition within our study area from continental extension to seafloor spreading. Exhumation of continental mantle results in the formation of peridotite ridges, then transitions to episodic volcanism, which produces local thin basaltic crust, and exhumation of oceanic core complexes. Asymmetric processes during initial rifting and spreading result in contrasting structures on the two resulting margins.

Eickmann B. et al. Barite in hydrothermal environments as a recorder of subseafloor processes: a multiple-isotope study from the Loki’s Castle vent field // Geobiology. 2014. Vol. 12, № 4. P. 308–321.

Barite chimneys are known to form in hydrothermal systems where barium-enriched fluids generated by leaching of the oceanic basement are discharged and react with seawater sulfate. They also form at cold seeps along continental margins, where marine (or pelagic) barite in the sediments is remobilized because of subseafloor microbial sulfate reduction. We test the possibility of using multiple sulfur isotopes (delta S-34, Delta S-33, Delta S-36) of barite to identify microbial sulfate reduction in a hydrothermal system. In addition to multiple sulfur isotopes, we present oxygen (delta O-18) and strontium (Sr-87/Sr-86) isotopes for one of numerous barite chimneys in a low-temperature (similar to 20 degrees C) venting area of the Loki's Castle black smoker field at the ultraslow-spreading Arctic Mid-Ocean Ridge (AMOR). The chemistry of the venting fluids in the barite field identifies a contribution of at least 10% of high-temperature black smoker fluid, which is corroborated by Sr-87/Sr-86 ratios in the barite chimney that are less radiogenic than in seawater. In contrast, oxygen and multiple sulfur isotopes indicate that the fluid from which the barite precipitated contained residual sulfate that was affected by microbial sulfate reduction. A sulfate reduction zone at this site is further supported by the multiple sulfur isotopic composition of framboidal pyrite in the flow channel of the barite chimney and in the hydrothermal sediments in the barite field, as well as by low SO4 and elevated H2S concentrations in the venting fluids compared with conservative mixing values. We suggest that the mixing of ascending H-2-and CH4-rich high-temperature fluids with percolating seawater fuels microbial sulfate reduction, which is subsequently recorded by barite formed at the seafloor in areas where the flow rate is sufficient. Thus, low-temperature precipitates in hydrothermal systems are promising sites to explore the interactions between the geosphere and biosphere in order to evaluate the microbial impact on these systems.

Erdman M.E., Lee C.T.A. Oceanic- and continental-type metamorphic terranes: Occurrence and exhumation mechanisms // Earth-Science Reviews. 2014. Vol. 139. P. 33–46.

Understanding the fate of subducted materials has important implications for the chemical and physical differentiation of the Earth, particularly the compositional evolution of the continental crust. Of interest here is how deeply-subducted materials return to the Earth's surface. We present a comprehensive global compilation of high-pressure, low-temperature metamorphic terranes for which peak metamorphic conditions have been constrained. These metamorphic terranes are classified based on tectonic setting: terranes in oceanic plate subduction zones were classified as oceanic-type and those in continent-continent collision zones were classified as continental-type. We show that oceanic-type terranes form under pressures less than ~. 2.7. GPa whereas continental-type terranes develop under greater pressure and slightly higher prograde geothermal gradients. Whereas these two terrane types probably share common descent paths (i.e. subduction), their separation in pressure-temperature space suggests that the mechanism and pathways of their exhumation likely differ. Here we present a simple buoyancy-driven model to explain the bifurcation of subducted material at depth and how exhumation regimes may change in different tectonic settings during the evolution of convergent margins. We explore two exhumation modes. In one, the hydrous nature of subducted sediments leads to a low-density, low-viscosity channel bounded by relatively rigid walls, thereby driving channel-like flow along the dipping slab surface. In the other mode, channel viscosity approaches that of the overlying mantle wedge, preventing channel flow but permitting vertical exhumation via diapirism. We show that the exhumation mode depends on slab dip and the viscosity ratio between the buoyant material and the overlying mantle (described by a dimensionless parameter, M). Due to a significant change in channel viscosity with the breakdown of hydrous minerals, we suggest that the transition in exhumation mode coincides with slab dehydration; at what depth this transition occurs depends on plate velocity and the initial thermal state of the slab. Such a model predicts channel flow to be limited to shallow depths and diapiric exhumation to greater depths, providing an internally consistent explanation for the apparent differences in peak metamorphic conditions of oceanic- and continental-type terranes if the former exhume via channel flow and the latter via diapirism. Because young, hot slabs dehydrate at shallower depths than old, cold slabs, the maximum depth to which channel flow can operate is greater in the latter. Finally, our model also predicts how the exhumation mode changes as the nature of subduction zones evolves during the closure of an ocean basin, beginning with oceanic plate subduction and culminating in continent-continent collision. In such a scenario, channel flow is favored during the subduction of dense, steeply dipping oceanic lithosphere, but a growing continental (low density) character to the subducted materials as the ocean basin closes should gradually shift the mode of exhumation to diapirism.

Escartin J. et al. Lucky Strike seamount: Implications for the emplacement and rifting of segment-centered volcanoes at slow spreading mid-ocean ridges // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 11. P. 4157–4179.

The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow spreading Mid-Atlantic Ridge at the Lucky Strike Segment is deduced using near-bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (<1 Myr), associated with focused eruptions, and with effusion rates feeding lava flows that bury tectonic features developed prior to and during volcano construction. This volcanic phase likely requires efficient melt pooling and a long-lived crustal magma chamber as a melt source. A reduction in melt supply triggers formation of an axial graben rifting the central volcano, and the onset of seafloor spreading may eventually split it. At Lucky Strike, this results in two modes of crustal construction. Eruptions and tectonic activity focus at a narrow graben that bisects the central volcano and contains the youngest lava flows, accumulating a thick layer of extrusives. Away from the volcano summit, deformation and volcanic emplacement is distributed throughout the rift valley floor, lacking a clear locus of accretion and deformation. Volcanic emplacement on the rift floor is characterized by axial volcanic ridges fed by dikes that propagate from the central axial magma chamber. The mode of rapid volcano construction and subsequent rifting observed at the Lucky Strike seamount is common at other central volcanoes along the global mid-ocean ridge system.

Faccenna C. et al. Mantle dynamics in the Mediterranean // Reviews of Geophysics. 2014. Vol. 52, № 3. P. 283–332.

The Mediterranean offers a unique opportunity to study the driving forces of tectonic deformation within a complex mobile belt. Lithospheric dynamics are affected by slab rollback and collision of two large, slowly moving plates, forcing fragments of continental and oceanic lithosphere to interact. This paper reviews the rich and growing set of constraints from geological reconstructions, geodetic data, and crustal and upper mantle heterogeneity imaged by structural seismology. We proceed to discuss a conceptual and quantitative framework for the causes of surface deformation. Exploring existing and newly developed tectonic and numerical geodynamic models, we illustrate the role of mantle convection on surface geology. A coherent picture emerges which can be outlined by two, almost symmetric, upper mantle convection cells. The downwellings are found in the center of the Mediterranean and are associated with the descent of the Tyrrhenian and the Hellenic slabs. During plate convergence, these slabs migrated backward with respect to the Eurasian upper plate, inducing a return flow of the asthenosphere from the back-arc regions toward the subduction zones. This flow can be found at large distance from the subduction zones and is at present expressed in two upwellings beneath Anatolia and eastern Iberia. This convection system provides an explanation for the general pattern of seismic anisotropy in the Mediterranean, first-order Anatolia, and Adria microplate kinematics and may contribute to the high elevation of scarcely deformed areas such as Anatolia and eastern Iberia. More generally, the Mediterranean is an illustration of how upper mantle, small-scale convection leads to intraplate deformation and complex plate boundary reconfiguration at the westernmost terminus of the Tethyan collision.

Fontaine F.J. et al. Along-axis hydrothermal flow at the axis of slow spreading Mid-Ocean Ridges: Insights from numerical models of the Lucky Strike vent field (MAR) // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 7. P. 2918–2931.

The processes and efficiency of hydrothermal heat extraction along the axis of mid-ocean ridges are controlled by lithospheric thermal and permeability structures. Hydrothermal circulation models based on the structure of fast and intermediate spreading ridges predict that hydrothermal cell organization and vent site distribution are primarily controlled by the thermodynamics of high-temperature mid-ocean ridge hydrothermal fluids. Using recent constraints on shallow structure at the slow spreading Lucky Strike segment along the Mid-Atlantic Ridge, we present a physical model of hydrothermal cooling that incorporates the specificities of a magma-rich slow spreading environment. Using three-dimensional numerical models, we show that, in contrast to the aforementioned models, the subsurface flow at Lucky Strike is primarily controlled by across-axis permeability variations. Models with across-axis permeability gradients produce along-axis oriented hydrothermal cells and an alternating pattern of heat extraction highs and lows that match the distribution of microseismic clusters recorded at the Lucky Strike axial volcano. The flow is also influenced by temperature gradients at the base of the permeable hydrothermal domain. Although our models are based on the structure and seismicity of the Lucky Strike segment, across-axis permeability gradients are also likely to occur at faster spreading ridges and these results may also have important implications for the cooling of young crust at fast and intermediate spreading centers.

Gale A. et al. Constraints on melting processes and plume-ridge interaction from comprehensive study of the FAMOUS and North Famous segments, Mid-Atlantic Ridge // Earth Planet. Sci. Lett. 2013. Vol. 365. P. 209–220.

Detailed major element, trace element and isotopic study of the FAMOUS and North Famous segments within the geochemical gradient south of the Azores platform provides new constraints on controls on chemical variations at the segment scale and the origin of plume geochemical gradients. A comprehensive investigation of 110 samples along the entire length of the FAMOUS segment, coupled with a recent extensive melt inclusion study by Laubier et al. (2012), shows large trace element diversity within a single segment and substantial isotopic variability that largely correlates with trace element variations. Substantial variations are also present along the short (18 km) North Famous segment despite the presence of an axial volcanic ridge. These results confirm multiple supply of magmas along the length of these segments, the lack of a centrally supplied magma chamber, and the ability of melting processes to deliver highly diverse melts over short distances and times. With the exception of one group of high Al2O3, low SiO2 magmas (HiAl-LoSi) largely recovered in the original small FAMOUS area, the data can be simply explained by a two-component mixing model coupled with melting variations. The HiAl-LoSi magmas reflect assimilation and mixing in the crust, an interpretation supported by the diverse melt inclusions in these lavas. Since the mantle heterogeneity reflects two-component mixing, the end members can be constrained. Surprisingly, source mixing between the Azores plume and depleted mantle cannot produce the observations. This is evident regionally from the fact that nearly all basalts have highly incompatible trace element ratios (e.g., Th/La, Nb/La) as high or higher than the most plume-influenced MORE near the Azores hotspot, despite being over 300 km farther south and much less enriched isotopically. To account for the elevated highly incompatible trace element ratios, a metasomatic component formed by adding deep, low-degree melts of Azores plume material to a depleted mantle is required. The regional gradient south of the Azores then requires different processes along its length. Close to the Azores, plume material mixes with depleted mantle. The pure plume influence is spatially restricted, and enrichment farther to the south is caused by shallow mantle metasomatized by low-degree melts from deep plume flow. North Famous lavas are spatially closer to the Azores and yet are more depleted in trace elements and isotopes than FAMOUS lavas, suggesting delivery of the enriched component to individual segments is influenced by additional factors such as segment size and offset. The extent to which these processes operate in other regions of plume-ridge interaction remains to be investigated.

Gale A., Langmuir C.H., Dalton C.A. The Global Systematics of Ocean Ridge Basalts and their Origin // Journal of Petrology. 2014. Vol. 55, № 6. P. 1051–1082.

Tests of models of melt generation and mantle source variations beneath mid-ocean ridges require a definitive set of mid-ocean ridge basalt (MORB) compositions corrected for shallow-level processes. Here we provide such a dataset, with both single sample and segment means for 241 segments from every ocean basin, which span the entire range of spreading rate, axial depth, and MORB chemical composition. Particular attention is paid to methods of fractionation correction. Values corrected to 8 wt % MgO are robust as they are within the range of the data. Extrapolation to equilibrium with mantle olivine is a non-unique procedure that is critically dependent on the MgO content where plagioclase first appears. MORB data, trace element ratios and calculated liquid lines of descent provide consistent evidence that plagioclase fractionation primarily occurs between 8 and 9 wt % MgO, with the exception of hydrous magmas mainly from back-arc segments. Varying the MgO content of plagioclase appearance over large ranges does not produce the observed systematics at 8 wt % MgO, but may contribute to the spread of the data. Data were evaluated individually for each segment to ensure reliable fractionation correction, and segment means are reported normalized both to MgO of 8 wt % and also to a constant Mg/(Mg + Fe) in equilibrium with Fo(90) olivine. Both sets of corrected compositions show large variations in Na2O and FeO, good correlations with segment depth, and systematic relationships among the major elements. A particularly good correlation exists between Al-90 and Fe-90. These new data are not in agreement with the presentation of Niu & O'Hara (Journal of Petrology 49, 633-664, 2008), whose results relied on an inaccurate fractionation correction procedure, which led them to large errors for high- and low-FeO magmas. The entire dataset is provided in both raw and normalized form so as to have a uniform basis for future evaluations. The new data compilation permits tests of competing models for the primary causes of variations in MORB parental magmas: variations in mantle composition, mantle temperature, reactive crystallization or lithospheric thickness. The principal component of chemical variation among segment mean compositions is remarkably consistent with variations in mantle temperature of some 200A degrees C beneath global ocean ridges. Comparisons with experimental data, pMELTS and other calculations show that variations in mantle fertility at constant mantle potential temperature produce trends that are largely orthogonal to the observations. At the same time, there is clear evidence for mantle major element heterogeneity beneath and around some hotspots and beneath back-arc basins. Super slow-spreading ridges display a characteristic chemical signature of elevated Na-90 and Al-90 and lowered Si-90 relative to faster-spreading ridges. If this signature were produced by reactive crystallization, Si-90 should be higher rather than lower in these environments owing to the thicker lithosphere and lower temperatures of mantle-melt reaction. Instead, the data are consistent with lower extents of mantle melting beneath a thicker lithosphere. Hence, variations in extent of melting appear to be the dominant control on the major element compositions of MORB parental magmas. Trace elements, in contrast, require a large component of mantle heterogeneity, apparent in the factor of 50 variation in K-90. Such variations do not correlate with the other major elements, showing that major element and trace element (and isotope) heterogeneity reflect different processes. This supports the model of movement of low-degree melts for the creation of trace element and isotope mantle heterogeneity, and is inconsistent with large variations in the amount of recycled crust in most ocean ridge mantle sources.

Gardner J.V., Calder B.R., Malik M. Geomorphometry and processes that built Necker Ridge, central North Pacific Ocean // Mar. Geol. 2013. Vol. 346. P. 310–325.

Necker Ridge is an enigmatic 650-km long, narrow, linear aseismic bathymetric feature that rises 2500 to 3000m above the abyssal seafloor south of the Hawaiian Ridge. The ridge is the largest of a series of aseismic ridges that emanate from the eastern side of the Mid-Pacific Mountains outward towards the northeast. The trend of Necker Ridge is at an angle to fracture zones and spreading centers in the region, so its origin is controversial, yet it is a major feature on this part of the Cretaceous Pacific Plate. The entire feature, from Necker Island on the Hawaiian Ridge to the eastern Mid-Pacific Mountains, including the adjacent abyssal seafloor, was mapped in 2009 and 2011 with the latest generation of multibeam echosounders. The detailed bathymetry shows the ridge to be constructed of a series of stacked, thick (200-400 m) volcanic flows that can be traced along the trend of Necker Ridge for 100s of km. This continuity suggests that the volcanism erupted simultaneously along almost the entire length of the feature and not as spatially episodic areas of extrusion. Three relatively flat platforms occur on the summit region, presumably constructed of shallow-water carbonates when these portions of the ridge were at sea level. A conspicuous lack of thick pelagic sediment on the non-platform ridge summit and flanks is seen throughout the ridge. The lack of landslides along the length of the ridge is equally puzzling. The southern end of the ridge is connected by a saddle to the Mid-Pacific Mountains whereas the northern end of the ridge is buried by an archipelagic apron of the southern flank of the Hawaiian Ridge.

Grad M. et al. Geophysical investigations of the area between the Mid-Atlantic Ridge and the Barents Sea: From water to the lithosphere-asthenosphere system // Polar Sci. 2015. Vol. 9, № 1. P. 168–183.

As a part of the large international panel "IPY Plate Tectonics and Polar Gateways" within the "4th International Polar Year" framework, extensive geophysical studies were performed in the area of southern Svalbard, between the Mid-Atlantic Ridge and the Barents Sea. Seismic investigations were performed along three refraction and wide-angle reflection seismic lines. Integrated with gravity data the seismic data were used to determine the structure of the oceanic crust, the transition between continent and ocean (COT), and the continental structures down to the lithosphere-asthenosphere system (LAB). We demonstrate how modeling of multiple water waves can be used to determine the sound velocity in oceanic water along a seismic refraction profile. Our 2D seismic and density models documents 4-9 km thick oceanic crust formed at the Knipovich Ridge, a distinct and narrow continent-ocean transition (COT), the Caledonian suture zone between Laurentia and Barentsia, and 30-35 km thick continental crust beneath the Barents Sea. The COT west of southern Spitsbergen expresses significant excess density (more than 0.1 g/cm(3) in average), which is characteristic for mafic/ultramafic and high-grade metamorphic rocks. The results of the gravity modeling show relatively weak correlation of the density with seismic velocity in the upper mantle, which suggests that the horizontal differences between oceanic and continental mantle are dominated by mineralogical changes, although a thermal effect is also present. The seismic velocity change with depth suggests lherzolite composition of the uppermost oceanic mantle, and dunite composition beneath the continental crust.

Grevemeyer I., Reston T.J., Moeller S. Microseismicity of the Mid-Atlantic Ridge at 7°S - 8°15'S and at the Logatchev Massif oceanic core complex at 14°40’N-14°50’N // Geochem. Geophys. Geosyst. 2013. Vol. 14, № 9. P. 3532–3554.

Lithospheric formation at slow spreading rates is heterogeneous with multiple modalities, favoring symmetric spreading where magmatism dominates or core complex and inside corner high formation where tectonics dominate. We report microseismicity from three deployments of seismic networks at the Mid-Atlantic Ridge (MAR). Two networks surveyed the MAR near 7 degrees S in the vicinity of the Ascension transform fault. Three inside corner high settings were investigated. However, they remained seismically largely inactive and major seismic activity occurred along the center of the median valley. In contrast, at the Logatchev Massif core complex at 14 degrees 45N seismicity was sparse within the center of the median valley but concentrated along the eastern rift mountains just west of the serpentine hosted Logatchev hydrothermal vent field. To the north and south of the massif, however, seismic activity occurred along the ridge axis, emphasizing the asymmetry of seismicity at the Logatchev segment. Focal mechanisms indicated a large number of reverse faulting events occurring in the vicinity of the vent field at 3-5 km depth, which we interpret to reflect volume expansion accompanying serpentinization. At shallower depth of 2-4 km, some earthquakes in the vicinity of the vent field showed normal faulting behavior, suggesting that normal faults facilitates hydrothermal circulation feeding the vent field. Further, a second set of cross-cutting faults occurred, indicating that the surface location of the field is controlled by local fault systems.

Howell S.M. et al. The origin of the asymmetry in the Iceland hotspot along the Mid-Atlantic Ridge from continental breakup to present-day // Earth Planet. Sci. Lett. 2014. Vol. 392. P. 143–153.

The Iceland hotspot has profoundly influenced the creation of oceanic crust throughout the North Atlantic basin. Enigmatically, the geographic extent of the hotspot influence along the Mid-Atlantic Ridge has been asymmetric for most of the spreading history. This asymmetry is evident in crustal thickness along the present-day ridge system and anomalously shallow seafloor of ages similar to 49-25 Ma created at the Reykjanes Ridge (RR), SSW of the hotspot center, compared to deeper seafloor created by the now-extinct Aegir Ridge (AR) the same distance NE of the hotspot center. The cause of this asymmetry is explored with 3-D numerical models that simulate a mantle plume interacting with the ridge system using realistic ridge geometries and spreading rates that evolve from continental breakup to present-day. The models predict plume-influence to be symmetric at continental breakup, then to rapidly contract along the ridges, resulting in widely influenced margins next to uninfluenced oceanic crust. After this initial stage, varying degrees of asymmetry along the mature ridge segments are predicted. Models in which the lithosphere is created by the stiffening of the mantle due to the extraction of water near the base of the melting zone predict a moderate amount of asymmetry; the plume expands NE along the AR similar to 70-80% as far as it expands SSW along the RR. Without dehydration stiffening, the lithosphere corresponds to the near-surface, cool, thermal boundary layer; in these cases, the plume is predicted to be even more asymmetric, expanding only 40-50% as far along the AR as it does along the RR. Estimates of asymmetry and seismically measured crustal thicknesses are best explained by model predictions of an Iceland plume volume flux of similar to 100-200 m(3)/s, and a lithosphere controlled by a rheology in which dehydration stiffens the mantle, but to a lesser degree than simulated here. The asymmetry of influence along the present-day ridge system is predicted to be a transient configuration in which plume influence along the Reykjanes Ridge is steady, but is still widening along the Kolbeinsey Ridge, as it has been since this ridge formed at similar to 25 Ma.

Kato S. et al. Elemental dissolution of basalts with ultra-pure water at 340°C and 40 Mpa in a newly developed flow-type hydrothermal apparatus // Geochemical Journal. 2013. Vol. 47, № 1. P. 89–92.

To simulate the discharge zone of hydrothermal systems in the laboratory, we developed a flow-type hydrothermal apparatus that can reproduce water-rock reactions at elevated temperatures and pressures under flow-through conditions and also the mixing of hydrothermal fluids with seawater. Using this apparatus, basaltic rocks were reacted with ultra-pure water at 340°C and 40 MPa for 1271 h under flow-through conditions. An increase in Al, Fe, K, Na, P and Si in the reacted water was observed during the experiment, reproducing the process of dissolution from the basaltic rocks. An increase in pH to 9 during the experiment can be attributed to the dissolution of silicate minerals in the basaltic rocks. That the pH in the experiment is higher than the pH observed in natural hydrothermal fluids at mid-ocean ridges is a result of higher Si concentrations observed in the experiment than those of natural hydrothermal fluids.

Kelley D.S., Delaney J.R., Juniper S.K. Establishing a new era of submarine volcanic observatories: Cabling Axial Seamount and the Endeavour Segment of the Juan de Fuca Ridge // Marine Geology. 2014. Vol. 352. P. 426–450.

At least 70% of the volcanism on Earth occurs along the 65,000 km network of mid-ocean ridge (MOR) spreading centers. Within these dynamic environments, the highest fluxes of heat chemicals, and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions. However, because underwater volcanoes are difficult and expensive to access, researchers are rarely, if ever, in the right place at the right time to characterize these events. Therefore, our knowledge is limited about the linkages among hydrothermal, chemical and biological processes during seafloor formation and crustal evolution. To make significant advancements in understanding the evolution of MOR environments, the United States and Canada have invested in the first plate-scale submarine cabled observatory linked through the global Internet. Spanning the Juan de Fuca tectonic plate, these two networks include >1700 km of cable and 14 subsea terminals that provide 8-10 kW power and 10 Gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column - resulting in a 24/7/365 presence in the oceans. Data and imagery are available in real- to near-real time. The initial experimental sites for monitoring volcanic processes include the MOR volcanoes called Axial Seamount and the Endeavour Segment that are located on the Juan de Fuca Ridge. Axial, a hot-spot influenced seamount, is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years. In contrast the Endeavour Segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known. A non-eruptive spreading event lasting 6 years was documented at Endeavour between 1999 and 2005. Hydrothermal venting intensity, chemistry, and temperature, as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events. This paper presents the similarities and differences between the Axial and Endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies. The U.S. has made a 25-year commitment for sustained observations using the cabled infrastructure. The highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences, robotics, genomics and sensor miniaturization. Comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins.

Kelley K.A., Kingsley R., Schilling J.-G. Composition of plume-influenced mid-ocean ridge lavas and glasses from the Mid-Atlantic Ridge, East Pacific Rise, Galapagos Spreading Center, and Gulf of Aden // Geochem. Geophys. Geosyst. 2013. Vol. 14, № 1. P. 223–242.

The global mid-ocean ridge system is peppered with localities where mantle plumes impinge on oceanic spreading centers. Here, we present new, high resolution and high precision data for 40 trace elements in 573 samples of variably plume-influenced mid-ocean ridge basalts from the Mid-Atlantic ridge, the Easter Microplate and Salas y Gomez seamounts, the Galapagos spreading center, and the Gulf of Aden, in addition to previously unpublished major element and isotopic data for these regions. Included in the data set are the unconventional trace elements Mo, Cd, Sn, Sb, W, and Tl, which are not commonly reported by most geochemical studies. We show variations in the ratios Mo/Ce, Cd/Dy, Sn/Sm, Sb/Ce, W/U, and Rb/Tl, which are expected not to fractionate significantly during melting or crystallization, as a function of proximity to plume-related features on these ridges. The Cd/Dy and Sn/Sm ratios show little variation with plume proximity, although higher Cd/Dy may signal increases in the role of garnet in the mantle source beneath some plumes. Globally, the Rb/Tl ratio closely approximates the La/Sm-N ratio, and thus provides a sensitive tracer of enriched mantle domains. The W/U ratio is not elevated at plume centers, but we find significant enrichments in W/U, and to a lesser extent the Mo/Ce and Sb/Ce ratios, at mid-ocean ridges proximal to plumes. Such enrichments may provide evidence of far-field entrainment of lower mantle material that has interacted with the core by deeply-rooted, upwelling mantle plumes.

Kimura J.-I., Kawabata H. Trace element mass balance in hydrous adiabatic mantle melting: The Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 6. P. 2467–2493.

A numerical mass balance calculation model for the adiabatic melting of a dry to hydrous peridotite has been programmed in order to simulate the trace element compositions of basalts from mid-ocean ridges, back-arc basins, ocean islands, and large igneous provinces. The Excel spreadsheet-based calculator, Hydrous Adiabatic Mantle Melting Simulator version 1 (HAMMS1) uses (1) a thermodynamic model of fractional adiabatic melting of mantle peridotite, with (2) the parameterized experimental melting relationships of primitive to depleted mantle sources in terms of pressure, temperature, water content, and degree of partial melting. The trace element composition of the model basalt is calculated from the accumulated incremental melts within the adiabatic melting regime, with consideration for source depletion. The mineralogic mode in the primitive to depleted source mantle in adiabat is calculated using parameterized experimental results. Partition coefficients of the trace elements of mantle minerals are parameterized to melt temperature mostly from a lattice strain model and are tested using the latest compilations of experimental results. The parameters that control the composition of trace elements in the model are as follows: (1) mantle potential temperature, (2) water content in the source mantle, (3) depth of termination of adiabatic melting, and (4) source mantle depletion. HAMMS1 enables us to obtain the above controlling parameters using Monte Carlo fitting calculations and by comparing the calculated basalt compositions to primary basalt compositions. Additionally, HAMMS1 compares melting parameters with a major element model, which uses petrogenetic grids formulated from experimental results, thus providing better constraints on the source conditions.

30. U10290
Kinsey J.C., German C.R. Sustained volcanically-hosted venting at ultraslow ridges: Piccard Hydrothermal Field, Mid-Cayman Rise // Earth and Planetary Science Letters. 2013. Vol. 380. P. 162–168.

At slow spreading mid-ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically-controlled hydrothermal circulation unrelated to young volcanic activity. Here, by contrast, we show that the Piccard Hydrothermal Field (PHF), on the ultraslow spreading Mid-Cayman Rise, represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting. The lateral extent and apparent longevity associated with the PHF are comparable to some of the largest tectonically-hosted vent sites known along the slow-spreading Mid-Atlantic Ridge. If such systems recur along all ultraslow ridges, which comprise ?20% of the ?55,000 km global ridge crest, potential implications would include (i) a higher probability of locating large, economically valuable, mineral deposits along ultraslow ridges together with (ii) larger fluxes than previously anticipated of chemicals released from high-temperature venting entering the oceans along the Atlantic–Indian Ocean sectors of the deep-ocean thermohaline conveyor.

Kusky T.M. et al. Recognition of ocean plate stratigraphy in accretionary orogens through Earth history: A record of 3.8billion years of sea floor spreading, subduction, and accretion // Gondwana Research. 2013. Vol. 24, № 2. P. 501–547.

Ocean plate stratigraphy (OPS) is a term used to describe the sequence of sedimentary and volcanic rocks deposited on oceanic crust substratum from the time it forms at a spreading center, to the time it is incorporated into an accretionary prism at a convergent margin. In this study, we review the major geological characteristics of relict Cenozoic to early Archean oceanic crust and OPS persevered in Alaska, Japan, California (Franciscan Complex), Central Asia, British Isles, Canada (Slave Province), Australia (Pilbara craton), and Greenland (Isua and Ivisaartoq belts). An assessment of OPS in accretionary orogens spanning the duration of Earth's rock record shows remarkable similarities between OPS of all ages in terms of structural style, major rock components, sequence of accretion, and trace element geochemical signatures. Volcanic rocks preserved in the orogenic belts are characterized predominantly by oceanic island arc basalts, island arc picrites, mid-ocean ridge basalts, back-arc basalts, oceanic plateau basalts, ocean island basalts, and boninites, with extremely rare komatiites. This demonstrates that sea-floor spreading, lateral movement of oceanic plates with accompanying sedimentation over the oceanic substratum, and accretion at convergent margins have been major Earth processes since at least 3.8. Ga ago. There have been some secular changes in the rock types in OPS, such as changes in carbonates and radiolarian cherts whose sources were in the biota in existence in Phanerozoic times but absent in the Precambrian, but overall, there have been few changes in the style of OPS accretion with time. Komatiites and banded iron formations occur predominantly in Archean orogenic belts, reflecting higher mantle temperatures and less oxic seawater composition, respectively, before 2.5. Ga. This is clear documentation that plate tectonics, including the lateral movement of oceanic lithosphere, has been a major heat loss mechanism on Earth since the early Precambrian.

Lagabrielle Y., Brovarone A.V., Ildefons B. Fossil oceanic core complexes recognized in the blueschist metaophiolites of Western Alps and Corsica // Earth-Science Reviews. 2015. Vol. 141. P. 1–26.

Tethyan ophiolites show an apparent poorly organized association of ultramafic and mafic rocks. By contrast to the complete mantle-crustal sections of Semail-type ophiolite sheets, Tethyan ophiolites are characterized by a smaller amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by the frequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associated gabbroic intrusions. Therefore, they are considered as typical remnants of oceanic lithosphere formed in slowspreading environment or in ocean-continent transition at distal passive margins. In the very first models of formation of the Tethyan ophiolites, in the years 1980, the geodynamical processes leading to mantle unroofing were poorly understood due to the paucity of data and concepts available at that time from the present-day oceans. In particular, at that time, little work had focused on the distribution, origin and significance of mafic rocks with respect to the dominant surrounding ultramafics. Here, we reconsider the geology of some typical metaophiolites from the Western Alps and Corsica, and we show how results from the past decade obtained in the current oceans ask for reassessing the significance of the Tethyan ophiolites in general. Revisited examples include a set of representative metaophiolites from the blueschists units of the Western Alps (Queyras region) and from Alpine Corsica (Gob o Valley). Field relationships between the ophiolitic basement and the metasedimentary/metavolcanic oceanic cover are described, outlining a typical character of the Tethyan ophiolite lithological associations. Jurassic marbles and polymictic ophiolite metabreccias are unconformably overlying the mantle-gabbo basement, in a way strictly similar to what is observed in the non-metamorphic Appennine ophiolites or Chenaillet massif. This confirms that very early tectonic juxtaposition of ultramafic and mafic rocks occurred in the oceanic domain before subduction. This juxtaposition resulted from tectonic activity that is now assigned to the development of detachment faults and to the formation of Oceanic Core Complexes (OCCs) at the axis of slow spreading ridges. This fundamental Plate Tectonics process is responsible for the exhumation and for the axial denudation of mantle rocks and gabbros at diverging plate boundaries. In addition, field relationships between the discontinuous basaltic formations and the ultramafic-mafic basement indicate that this tectonic stage is followed or not by a volcanic stage. We discuss this issue in the light of available field constraints.

Laigle M. et al. Seismic structure and activity of the north-central Lesser Antilles subduction zone from an integrated approach: Similarities with the Tohoku forearc // Tectonophysics. 2013. Vol. 603. P. 1–20.

The 300-km-long north-central segment of the Lesser Antilles subduction zone, including Martinique and Guadeloupe islands has been the target of a specific approach to the seismic structure and activity by a cluster of active and passive offshore-onshore seismic experiments. The top of the subducting plate can be followed under the wide accretionary wedge by multichannel reflection seismics. This reveals the hidden updip limit of the contact of the upper plate crustal backstop onto the slab. Two OBS refraction seismic profiles from the volcanic arc throughout the forearc domain constrain a 26-km-large crustal thickness all along. In the common assumption that the upper plate Moho contact on the slab is a proxy of its downdip limit these new observations imply a three times larger width of the potential interplate seismogenic zone under the marine domain of the Caribbean plate with respect to a regular intra-oceanic subduction zone. Towards larger depth under the mantle corner, the top of the slab imaged from the conversions of teleseismic body-waves and the locations of earthquakes appears with kinks which increase the dip to 10-20° under the forearc domain, and then to 60° from 70. km depth.At 145. km depth under the volcanic arc just north of Martinique, the 2007 M 7.4 earthquake, largest for half a century in the region, allows to document a deep slab deformation consistent with segmentation into slab panels. In relation with this occurrence, an increased seismic activity over the whole depth range provides a new focussed image thanks to the OBS and land deployments. A double-planed dipping slab seismicity is thus now resolved, as originally discovered in Tohoku (NE Japan) and since in other subduction zones. Two other types of seismic activity uniquely observed in Tohoku, are now resolved here: "supraslab" earthquakes with normal-faulting focal mechanisms reliably located in the mantle corner and "deep flat-thrust" earthquakes at 45. km depth on the interplate fault under the Caribbean plate forearc mantle.None such types of seismicity should occur under the paradigm of a regular peridotitic mantle of the upper plate which is expected to be serpentinized by the fluids provided from the dehydrating slab beneath. This process is commonly considered as limiting the downward extent of the interplate coupling. Interpretations are not readily available either for the large crustal thickness of this shallow water marine upper plate, except when remarking its likeness to oceanic plateaus formed above hotspots.The Caribbean Oceanic Plateau of the upper plate has been formed earlier by the material advection from a mantle plume. It could then be underlain by a correspondingly modified, heterogeneous mantle, which may include pyroxenitic material among peridotites. Such heterogeneity in the mantle corner of the present subduction zone may account for the notable peculiarities in seismic structure and activity and impose regions of stick-slip behavior on the interplate among stable-gliding areas.

Le Voyer M. et al. The effect of primary versus secondary processes on the volatile content of MORB glasses: An example from the equatorial Mid-Atlantic Ridge (5 degrees N-3 degrees S) // J. Geophys. Res.-Solid Earth. 2015. Vol. 120, № 1. P. 125–144.

We report microanalysis of volatile and trace element compositions, as well as Fe3+/sigma Fe ratios, from 45 basaltic glasses from cruise RC2806 along the equatorial Mid-Atlantic Ridge. The along-strike variations in volatiles result from the complex geodynamical setting of the area, including numerous transform faults, variations in ridge depth, melting degree, and source composition. The strongest gradient is centered on 1.7 degrees N and encompasses an increase of H2O, Cl, and F contents as well as high F/Zr ratio spatially coincident with radiogenic isotope anomalies. We interpret these variations as source enrichment due to the influence of the nearby high--type Sierra Leone plume. South of the St. Paul fracture zone, H2O and F contents, as well as H2O/Ce and F/Zr ratios, decrease progressively. This gradient in volatiles is consistent with progressive dilution of an enriched component in a heterogeneous mantle due to the progressive increase in the degree of melting. These two large-scale gradients are interrupted by small-scale anomalies in volatile contents attributed to (1) low-degree melts preferentially sampling enriched heterogeneities near transform faults and (2) local assimilation of hydrothermal fluids in four samples from dredge 16D. Finally, 20 RC2806 samples described as popping rocks during collection do not show any difference in volatile content dissolved in the glass or in vesicularity when compared to the RC2806 nonpopping samples. Our observations lead us to question the interpretation of the CO2 content in the highly vesicular 2D43 popping rock as being representative of the CO2 content of undegassed mid-ocean ridge basalt.

Lemaitre N. et al. Trace element behaviour at cold seeps and the potential export of dissolved iron to the ocean // Earth and Planetary Science Letters. 2014. Vol. 404. P. 376–388.

Seawater samples were collected by submersible above methane seeps in the Gulf of Guinea (Regab and Baboon pockmarks) in order to investigate the behaviour of iron (Fe), manganese (Mn) and rare earth elements (REE) during fluid seepage. Our aim was to determine whether cold seeps may represent potential sources of dissolved chemical species to the ocean. Dissolved (<0.45 mu m filtered samples) and total dissolvable (unfiltered samples) concentrations were determined over similar to 50 m long vertical transects above the seafloor and at various discrete locations within the pockmarks. We show that substantial amounts of Fe and Mn are released into seawater during seepage of methane-rich fluids. Mn is exported almost quantitatively in the dissolved form (more than 90% of total Mn; mean Mn-DISS similar to 12 +/- 11 nmol/kg). Although a significant fraction of Fe is bound to particulate phases, the dissolved iron pool still accounts on average for approximately 20 percent of total iron flux at vent sites (mean Fe-DISS similar to 22 +/- 11 nmol/kg). This dissolved Fe fraction also appears to remain stable in the water column. In contrast, there was no evidence for any significant benthic fluxes of pore water REE associated with fluid seepage at the studied sites. Overall, our results point towards distinct trace element behaviour during fluid seepage, with potential implications for the marine geochemical budget. The absence of any dissolved REE enrichments in bottom waters clearly indicates effective removal in sub-surface sediments. Most likely, precipitation of authigenic mineral phases at cold seeps (i.e. carbonates) represents a net sink for these elements. While Mn appears to behave near-conservatively during fluid seepage, the observed relative stability of dissolved Fe in the water column above seepage sites could be explained by complexation with strong organic ligands and/or the presence of Fe-bearing sulfide nanoparticles, as reported previously for submarine hydrothermal systems. Considering the ubiquitous occurrence of methane vents at ocean margins, cold seeps could represent a previously unsuspected source of dissolved Fe to the deep ocean.

Lissner M. et al. Selenium and tellurium systematics in MORBs from the southern Mid-Atlantic Ridge (47-50 degrees S) // Geochim. Cosmochim. Acta. 2014. Vol. 144. P. 379–402.

Selenium and tellurium concentrations along with sulfur and Highly Siderophile Element (HSE) contents as well as Os-187 signatures were determined in 20 Mid-Ocean-Ridge Basalts (MORBs) from the southern Mid-Atlantic Ridge (SMAR; 47-50 degrees S), ranging in composition from depleted N-MORBs to mantle plume-related E-MORBs. A comparison between glassy rims and crystalline pillow interiors reveal that secondary processes are only reflected in seawater-overprinted Os-187/Os-188 signatures and degassing-related low S contents of the crystalline pillow cores but did not affect the Se and Te abundances. In contrast, the segregation of sulfide liquids during MORB differentiation lowers the Se and Te concentrations (similar to 35% and 60%, respectively) and leads to higher Se/Te ratios. Recomputed primitive melt Se contents broadly overlap for both, N- and E-MORBs, while primitive E-MORB melts have systematically higher Te contents and lower Se/Te ratios compared to those of the N-MORBs (13-14 ppb Te and Se/Te approximate to 18 vs. 9-11 ppb Te and Se/Te approximate to 25). As suggested by lithophile trace element and Sr-Nd-Pb isotopic constraints, the Se-Te systematics of the E-MORB mantle source traces the involvement of a recycled component likely derived from the nearby Discovery mantle plume. Bulk mixing models suggests an addition of either 20% pyroxenitic melts, or only 10 ppm of metasomatic sulfides to account for the Te-richer E-MORB compositions. A conservative correction of our MORB data for sulfide segregation combined with a near fractional melting model predicts a Te-depleted MORB mantle reservoir with a non-chondritic Se/Te of 18-25, significantly higher than the primitive mantle Se/Te estimates (6.3-9.9). The existence of these different Se-Te signatures between the E-MORB mantle source, the N-MORB mantle source and the primitive mantle support an incompatible behavior of both Se and Te during partial melting, with Te being slightly more incompatible. More importantly, this stresses the necessity of considering the full spectrum of the terrestrial silicate reservoirs to realistically constrain the budget of these volatile and highly siderophile elements in the bulk silicate Earth in order to discuss large scale planetary processes.

Marchesi C. et al. Platinum-group elements, S, Se and Cu in highly depleted abyssal peridotites from the Mid-Atlantic Ocean Ridge (ODP Hole 1274A): Influence of hydrothermal and magmatic processes // Contrib. Mineral. Petrol. 2013. Vol. 166, № 5. P. 1521–1538.

Highly depleted harzburgites and dunites were recovered from ODP Hole 1274A, near the intersection between the Mid-Atlantic Ocean Ridge and the 15A degrees 20'N Fracture Zone. In addition to high degrees of partial melting, these peridotites underwent multiple episodes of melt-rock reaction and intense serpentinization and seawater alteration close to the seafloor. Low concentrations of Se, Cu and platinum-group elements (PGE) in harzburgites drilled at around 35-85 m below seafloor are consistent with the consumption of mantle sulfides after high degrees (> 15-20 %) of partial melting and redistribution of chalcophile and siderophile elements into PGE-rich residual microphases. Higher concentrations of Cu, Se, Ru, Rh and Pd in harzburgites from the uppermost and lowest cores testify to late reaction with a sulfide melt. Dunites were formed by percolation of silica- and sulfur-undersaturated melts into low-Se harzburgites. Platinum-group and chalcophile elements were not mobilized during dunite formation and mostly preserve the signature of precursor harzburgites, except for higher Ru and lower Pt contents caused by precipitation and removal of platinum-group minerals. During serpentinization at low temperature (< 250 A degrees C) and reducing conditions, mantle sulfides experienced desulfurization to S-poor sulfides (mainly heazlewoodite) and awaruite. Contrary to Se and Cu, sulfur does not record the magmatic evolution of peridotites but was mostly added in hydrothermal sulfides and sulfate from seawater. Platinum-group elements were unaffected by post-magmatic low-temperature processes, except Pt and Pd that may have been slightly remobilized during oxidative seawater alteration.

Melekestseva I.Y. et al. Barite-rich massive sulfides from the Semenov-1 hydrothermal field (Mid-Atlantic Ridge, 13° 30.87'N): Evidence for phase separation and magmatic input // Mar. Geol. 2014. Vol. 349. P. 37–54.

The ultramafic-hosted Semenov-1 hydrothermal field is a part of the Semenov group of sulfide deposits located at 13 degrees 30 ' N Mid-Atlantic Ridge. The hydrothermal deposits comprise fine-crystalline, colloform, nodular, and banded barite-marcasite-pyrite assemblages, which are a result of moderately- to low-temperature venting of (Fe, Ba)-rich hydrothermal fluids. The precipitates contain low concentrations of Cu (0.02-0.55 wt.%) and Zn (0.01-0.08 wt.%), and variable Au content (0.35-4.76 ppm). Fe-disulfides are relatively enriched in Au and typical low-temperature trace elements (As, Ag, Pb, Mn, TI). The concentrations of Au and of most trace elements are higher in the earliest fine-grained Fe-disulfides and decrease in the late coarse-crystalline pyrite. Based on fluid inclusion data, barite precipitated from moderately- to low-temperature (244-83 degrees C), low-salinity (0.6-3.8 wt.% NaCl eq.), carbonate-sulfate-aqueous fluids, which incorporated a low-density component produced by fluid phase separation. The presence of CO2 and SO2 in fluid inclusions in barite, as revealed by Raman spectroscopy, may indicate a magmatic volatile contribution to the hydrothermal fluid. The main amount of marcasite and pyrite was formed at temperatures below 240 C, after the precipitation of barite. The occurrence of isocubanite, chalcopyrite and pyrrhotite in precipitates from station 292 indicates a later high-temperature (similar to 300 degrees C) overprint. The sulfur isotopic composition of barite ( + 21.0 and + 21.3%.) closely matches that of seawater. The isotopic composition of sulfur in sulfides from station 186 ( -3.26 to -0.08%.) suggests a possible contribution of light reduced sulfur derived from disproportionation of magmatic SO2 or from leaching of basalt that has degassed SO2. The higher delta S-34 values in sulfides from station 292 ( -0.08 to + 1.53%.) may reflect a heavy sulfur contribution from reduced seawater sulfate. Although the Semenov-1 field is considered to be associated with serpentinized ultramafic rocks (Beltenev et al., 2007), the mineralogical composition of the hydrothermal precipitates, particularly, their high barite and pyrite contents, is more typical of EMORB-hosted seafloor hydrothermal deposits (e.g., Lucky Strike or Menez Gwen fields). Thermodynamic modeling of fluid-rock hydrothermal systems using a flow reactor model demonstrates that basalt-seawater interaction generates fluids that can produce only minor amounts of barite, irrespective of the Ba content of the basalt. Addition of a magmatic gas to the system produces highly acidic hydrothermal fluids capable to extract larger amounts of Ba and Fe, and form mineral assemblages similar to the studied ones. In contrast, interaction between ultramafic rock (peridotite) and seawater, with or without magmatic gas, results in precipitation of barite-free mineral assemblages. The above results do not support an ultramafic signature for the Semenov-1 hydrothermal system and suggest a mafic control on the hydrothermal fluid composition.

Murphy J.B. et al. Highly depleted isotopic compositions evident in Iapetus and Rheic Ocean basalts: implications for crustal generation and preservation // International Journal of Earth Sciences. 2014. Vol. 103, № 5. P. 1219–1232.

Subduction of both the Iapetus and Rheic oceans began relatively soon after their opening. Vestiges of both the Iapetan and Rheic oceanic lithospheres are preserved as supra-subduction ophiolites and related mafic complexes in the Appalachian-Caledonian and Variscan orogens. However, available Sm-Nd isotopic data indicate that the mantle source of these complexes was highly depleted as a result of an earlier history of magmatism that occurred prior to initiation of the Iapetus and Rheic oceans. We propose two alternative models for this feature: either the highly depleted mantle was preserved in a long-lived oceanic plateau within the Paleopacific realm or the source for the basalt crust was been recycled from a previously depleted mantle and was brought to an ocean spreading centre during return flow, without significant re-enrichment en-route. Data from present-day oceans suggest that such return flow was more likely to have occurred in the Paleopacific than in new mid-ocean ridges produced in the opening of the Iapetus and Rheic oceans. Variation in crustal density produced by Fe partitioning rendered the lithosphere derived from previously depleted mantle more buoyant than the surrounding asthenosphere, facilitating its preservation. The buoyant oceanic lithosphere was captured from the adjacent Paleopacific, in a manner analogous to the Mesozoic-Cenozoic "capture" in the Atlantic realm of the Caribbean plate. This mechanism of "plate capture" may explain the premature closing of the oceans, and the distribution of collisional events and peri-Gondwanan terranes in the Appalachian-Caledonian and Variscan orogens.

Njome M.S., de Wit M.J. The Cameroon Line: Analysis of an intraplate magmatic province transecting both oceanic and continental lithospheres: Constraints, controversies and models // Earth-Science Reviews. 2014. Vol. 139. P. 168–194.

The near 1700. km long Cameroon Line (CL) is an African intraplate 'fan-shaped' alkaline volcano-plutonic rift zone of variable width (<. 200. km), with a ca. 66. Ma history of magmatic activity without any systematic internal pattern of age variation. Although the CL features prominently in discussions about active rift tectonics and magmatism during the growth of the African Plate and the opening of the Central Atlantic Ocean, there is a significant diversity of opinions about its local geometry, origin and evolution. Here, we review various geodynamic models for the CL based on large data-sets of field geology, petrology, geochemistry and geochronology. Results indicate that the Line traverses both oceanic and continental domains, extending NE-SW for about 700. km on oceanic lithosphere of the Gulf of Guinea (ca. 80-120. Ma), cutting obliquely across oceanic transform faults and across the ocean-continent boundary (OCB), and then continuous for ~. 1000. km (to Kapsiki), with a significant bifurcation at about 320. km, to the Adamawa and the Biu Plateaus along complex Neoproterozoic continental lithosphere of Central and West Africa (500-800. Ma, with embedded fragments of Paleoproterozoic-Archean crust). The continental lithosphere is transected by a dense network of major late Neoproterozoic shear zones (500-650. Ma) that were linked to conterminous extensions in South America before the opening of the Atlantic Ocean. The shear zones define a "Cameroon Block" to which the continental sector of the Cameroon Line is confined, but neither this continental tectonic anisotropy, nor its oceanic transform faults, appear to have influenced the geometry of the CL. Geochemical data confirm that the mostly alkaline CL magmas originate from deep mantle upwelling with variable input from at least four different mantle end-members (DMM, HIMU, FOZO and EM-I), and with different degrees of partial melts derived from above the 410-discontinuity. These melts then interacted diversely with lithospheric mantle, but without extensive crustal contamination. The oldest igneous activity is documented on the continent at the far end of the CL by the Late Cretaceous-Early Paleocene (on the Kapsiki Plateau at Golda Zuelda), and by the Miocene, contemporaneous volcanic activity spread along the oceanic and continental sectors. The intraplate magmatism along the CL is coincident with continent-wide extensional tectonics across central and northern Africa during the Cretaceous and throughout the Cenozoic, a time during which the African Plate experienced rotation of ~. 7° and slow but variable rates of convergence with Europe, involving several episodic changes in the pole of rotation between the African and South American plates that affected the opening of the South-Central Atlantic. The CL reflects a unique long-lived geodynamic setting and there appears to be no simple explanation for its origin.

Parnell-Turner R.E. et al. Crustal manifestations of a hot transient pulse at 60 degrees N beneath the Mid-Atlantic Ridge // Earth Planet. Sci. Lett. 2013. Vol. 363. P. 109–120.

Since its inception at 62 Ma, mantle convective upwelling beneath Iceland has had a significant influence on Cenozoic vertical motions, magmatism and paleoceanography in the North Atlantic Ocean. Crucially, intersection of the Reykjanes Ridge with the Icelandic Plume provides us with a useful window into the transient activity of this plume. Here, the spreading ridge acts as a linear sampler of plume activity, which is recorded as a series of time-transgressive V-shaped ridges and troughs. We present the results of a detailed study of the spreading ridge close to 60 degrees N, where the youngest V-shaped ridge of thickened oceanic crust is forming today. A combination of multibeam bathymetry and seismic reflection profiles, acquired along and across the ridge axis, is used to map the detailed pattern of volcanism and normal faulting. Along the ridge axis, the density of volcanic seamounts varies markedly, increasing by a factor of two between 59 degrees N and 62 degrees N. Within this zone, seismic imaging shows that there is enhanced acoustic scattering at the seabed. These observations are accompanied by a decrease in mean fault length from similar to 12 km to similar to 6 km. A 1960-2009 catalog of relocated teleseismic earthquake hypocenters indicates that there is a pronounced gap in seismicity between 59 degrees N and 62 degrees N where the cumulative moment release is two orders of magnitude smaller than that along adjacent ridge segments. A steady-state thermal model is used to show that a combination of increased melt generation and decreased hydrothermal circulation accounts for this suite of observations. The predicted decrease in the thickness of the brittle seismogenic layer is consistent with geochemical modeling of dredged basaltic samples, which require hotter asthenospheric material beneath the spreading axis. Thus, along-axis variation in melt supply caused by passage of a pulse of hot material modulates crustal accretion processes and rheological properties.

Perez F. et al. Coupled modeling of thermics and hydrogeology with the Cast3M code: application to the Rainbow hydrothermal field (Mid-Atlantic Ridge, 36A degrees 14’N) // Comput. Geosci. 2013. Vol. 17, № 2. P. 217–237.

We present a new numerical tool developed in the Cast3M software framework to model hydrothermal circulation. Thermodynamic properties of one-phase pure water are calculated from the International Association for the Properties of Water and Steam formulation. This new numerical tool is validated on several test cases of convection in closed-top and open-top boxes. Simulations of hydrothermal circulation in a homogeneous-permeability porous medium also give results in good agreement with already published simulations. This new numerical tool is used to construct a geometric and physical conceptual model of the Rainbow vent site at 36A degrees 14(')N on the Mid-Atlantic Ridge. Several configurations are discussed, showing that high temperatures and high mass fluxes measured at the Rainbow site cannot be modeled with hydrothermal circulation in a homogeneous-permeability porous medium. These high values require the presence of a fault or a preferential pathway right below the venting site. We propose and discuss a two-dimensional single-pass model that allows us to simulate both high temperatures and high mass fluxes. This modeling of the hydrothermal circulation at the Rainbow site constitutes the first but necessary step to understand the origin of high concentrations of hydrogen issued from this ultramafic-hosted vent field.

Rausch S. et al. Calcium carbonate veins in ocean crust record a threefold increase of seawater Mg/Ca in the past 30 million years // Earth and Planetary Science Letters. 2013. Vol. 362. P. 215–224.

Chemical (Sr, Mg) and isotopic (?18O, 87Sr/86Sr) compositions of calcium carbonate veins (CCV) in the oceanic basement were determined to reconstruct changes in Sr/Ca and Mg/Ca of seawater in the Cenozoic. We examined CCV from 10 basement drill sites in the Atlantic and Pacific, ranging in age between 165 and 2.3 Ma. Six of these sites are from cold ridge flanks in basement <46 Ma, which provide direct information about seawater composition. CCV of these young sites were dated, using the Sr isotopic evolution of seawater. For the other sites, temperature-corrections were applied to correct for seawater–basement exchange processes. The combined data show that a period of constant/low Sr/Ca (4.46–6.22 mmol/mol) and Mg/Ca (1.12–2.03 mol/mol) between 165 and 30 Ma was followed by a steady increase in Mg/Ca ratios by a factor of three to modern ocean composition. Mg/Ca–Sr/Ca relations suggest that variations in hydrothermal fluxes and riverine input are likely causes driving the seawater compositional changes. However, additional forcing may be involved in explaining the timing and magnitude of changes. A plausible scenario is intensified carbonate production due to increased alkalinity input to the oceans from silicate weathering, which in turn is a result of subduction-zone recycling of CO2 from pelagic carbonate formed after the Cretaceous slow-down in ocean crust production rate.

Roumejon S., Cannat M. Serpentinization of mantle-derived peridotites at mid-ocean ridges: Mesh texture development in the context of tectonic exhumation // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 6. P. 2354–2379.

At slow spreading ridges, axial detachment faults exhume mantle-derived peridotites and hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid-Atlantic and Southwest Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic stresses and subsequently served as channels for serpentinizing fluids. Additional reaction-induced cracks developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of similar to 60 mu m but most serpentinization occurs next to a subset of these microfractures that define mesh cells 100-400 mu m in size. Apparent mesh rim thickness is on average 33 +/- 19 mu m corresponding to serpentinization extents of 70-80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the serpentinization domain in the detachment fault's footwall are probably variable in time and space and as a result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a spatially heterogeneous distribution.

Saeid S. et al. A prototype design model for deep low-enthalpy hydrothermal systems // Renewable Energy. 2015. Vol. 77. P. 408–422.

This paper introduces a prototype design model for deep low-enthalpy hydrothermal systems. The model predicts, empirically, the lifetime of a hydrothermal system as a function of reservoir porosity discharge rate, well spacing, average initial temperature of the reservoir, and injection temperature. The finite element method is utilized for this purpose. An extensive parametric analysis on a wide range of physical parameters and operational scenarios, for a typical geometry, has been conducted to derive the model. The proposed model can provide geothermal engineers and decision makers with a preliminary conjecture about the lifetime of a deep low-enthalpy hydrothermal system. The proposed modelling technique can be utilized as a base to derive elaborate models that include more parameters and operational scenarios.

Samuel H., King S.D. Mixing at mid-ocean ridges controlled by small-scale convection and plate motion // Nature Geoscience. 2014. Vol. 7, № 8. P. 603–606.

Oceanic lavas are thought to be derived from different sources within the Earth's mantle, each with a distinct composition(1-4). Large-scale plate motions provide the primary mechanism for mixing these sources, yet the geochemical signature of lavas erupted at different mid-ocean ridges can still vary significantly(5,6). Geochemical variability is low where plate spreading rates are high, consistent with plate-scale mixing(5,6). However, slow-spreading centres, such as the Southwest Indian Ridge in the Indian Ocean, are also geochemically homogeneous, which is inconsistent with plate-scale mixing(6,7). Here we use numerical simulations of mantle flow to study mantle mixing at mid-ocean ridges, under conditions with variable plate length and spreading rate. Our simulations reveal that small-scale convection in the mantle contributes significantly to mantle mixing at slow spreading rates; faster plate velocities and smaller plates inhibit small-scale convection. We conclude that whereas fast-spreading ridge lavas are well mixed by plate-scale flow, slow-spreading ridge lavas are mixed by small-scale convection.

Schmeling H., Marquart G. A scaling law for approximating porous hydrothermal convection by an equivalent thermal conductivity: theory and application to the cooling oceanic lithosphere // Geophysical Journal International. 2014. Vol. 197, № 2. P. 645–664.

In geodynamic models of mid-ocean ridges hydrothermal cooling processes are important to control the temperature and thus the rheological behaviour of the crust. However, the characteristic time scale of hydrothermal convection is considerably shorter than that of viscous flow of mantle material or cooling of the oceanic lithosphere and can hardly be addressed in a conjoined model. To overcome this problem we present an approach to mimic hydrothermal cooling by an equivalent, increased thermal conductivity. First the temperature and pressure dependence of crack related porosity and permeability are derived based on composite theory. A characteristic pore closure depth as a function of pressure, temperature and pore aspect ratio is defined. 2-D porous convection models are used to derive scaling laws for parameterized convection including a Rayleigh-Nusselt number relation for a permeability exponentially decreasing with depth. These relations are used to derive an equivalent thermal conductivity to account for consistently evolving hydrothermal heat transport in thermally evolving systems. We test our approach using a 1-D model for cooling of the oceanic lithosphere. Within the context of our modelling parameters we found a pronounced effect for young lithosphere (younger than 10 Ma) down to about 20 km. Significant deviations of the heat flux versus age from the 1/root t law may occur due to hydrothermal convection. For the bathymetry versus age curves slopes steeper than 1/root t slopes already occur for very young lithosphere. Hydrothermal convection leads to an increase of the total heat flux and heat loss with respect to the classical purely conductive cooling model. Comparison of the total heat flow and its conductive contribution with observations confirm previous suggestions that for young lithosphere heat flow measurements represent only the conductive part, while at older ages the total heat flow is observed. Within their scatter and uncertainties heat flow and bathymetry data are in general agreement with our hydrothermally enforced cooling model suggesting that hydrothermal convection may be important even up to high ages.

Smith D.K. et al. Development and evolution of detachment faulting along 50 km of the Mid-Atlantic Ridge near 16.5 degrees N // Geochemistry Geophysics Geosystems. 2014. Vol. 15, № 12. P. 4692–4711.

A multifaceted study of the slow spreading Mid-Atlantic Ridge (MAR) at 16.5°N provides new insights into detachment faulting and its evolution through time. The survey included regional multibeam bathymetry mapping, high-resolution mapping using AUV Sentry, seafloor imaging using the TowCam system, and an extensive rock-dredging program. At different times, detachment faulting was active along ?50 km of the western flank of the study area, and may have dominated spreading on that flank for the last 5 Ma. Detachment morphologies vary and include a classic corrugated massif, noncorrugated massifs, and back-tilted ridges marking detachment breakaways. High-resolution Sentry data reveal a new detachment morphology; a low-angle, irregular surface in the regional bathymetry is shown to be a finely corrugated detachment surface (corrugation wavelength of only tens of meters and relief of just a few meters). Multiscale corrugations are observed 2–3 km from the detachment breakaway suggesting that they formed in the brittle layer, perhaps by anastomosing faults. The thin wedge of hanging wall lavas that covers a low-angle (6°) detachment footwall near its termination are intensely faulted and fissured; this deformation may be enhanced by the low angle of the emerging footwall. Active detachment faulting currently is limited to the western side of the rift valley. Nonetheless, detachment fault morphologies also are present over a large portion of the eastern flank on crust >2 Ma, indicating that within the last 5 Ma parts of the ridge axis have experienced periods of two-sided detachment faulting.

Syverson D.D. et al. Fe isotope fractionation during phase separation in the NaCl-H2O system: An experimental study with implications for seafloor hydrothermal vents // Earth and Planetary Science Letters. 2014. Vol. 406. P. 223–232.

Phase separation has been proposed as a possible mechanism contributing to the Fe isotope composition of hydrothermal fluids at mid-ocean ridges. The uncertainty results largely from the emphasis on field data that can involve competing processes that obscure cause and effect of any one process. To better understand the potential significance of phase separation in the NaCl-Fe-H2O system on Fe isotope fractionation, temperature and pressure of a Fe-bearing NaCl fluid in a titanium flow reactor were carefully adjusted to produce vapor +/- liquid +/- halite, while the Fe isotope composition between coexisting phases was monitored. Two different P-T regions were emphasized: (1) 424-420 degrees C, 35.2-31.5 MPa; and (2) 464-466 degrees C, 29.8-24.7 MPa. Both regions were chosen to simulate the range of physical conditions that are experienced by hydrothermal fluids at mid-ocean ridges (MORs). Decompression induced phase separation in both P-T regions results in the vapor phase becoming enriched in the heavier isotopes of Fe, as the Fe/Cl ratio decreases. The coexisting NaCl-rich liquid phase remains essentially constant with respect to Fe/CI ratio and Fe isotope composition. Coinciding with the lowest vapor chlorinity in the vapor-liquid stability field, the Fe/Cl ratio of the vapor abruptly increases, while the Fe isotope fractionation between the vapor and liquid (10(3) ln alpha(56/54)(V/L)) reached a maximum value of +0.145 +/- 0.048 parts per thousand). Subsequently, Fe isotope fractionation decreased upon transition into the vapor-halite stability field (P-T region 2). We infer that the observed Fe isotope fractionation between vapor +/- liquid +/- halite is caused by differences in Fe speciation among coexisting chloride-bearing phases. The experimental study confirms for the first time that measurable Fe isotope variability can result from phase separation in high temperature hydrothermal systems. The species-dependent Fe isotope fractionation reported here is small relative to predicted mineral-mineral and mineral-fluid fractionations, especially if redox effects are involved as might occur during vent fluid-seawater mixing reactions and/or magmatic activity associated with seafloor eruptive episodes.

Taneja R., O’Neill C. Constraining the age and origin of the seamount province in the Northeast Indian Ocean using geophysical techniques // Mar. Geophys. Res. 2014. Vol. 35, № 4. P. 395–417.

The breakup of western margin of Australia from Greater India started around 155 Ma and progressed southwards. After the separation, the interceding intraplate region experienced large volumes of submarine volcanism, extending over 100 Myrs. The Christmas Island Seamount Province (CHRISP, as it has been dubbed) lies south of the Java-Sunda Trench, and contains numerous submerged volcanic seamounts, and two sub-aerially exposed island groups-Cocos (Keeling) Islands, and Christmas Island. While recent geochronological investigations have shed light on the diverse eruption ages of the volcanics of this region, some islands/seamounts have demonstrated protracted volcanic histories, and it is not clear how the volcanic loading, tectonic subsidence, and subsequent emergence history of the islands relates to these discrete volcanic episodes. This study utilises a number of geophysical techniques to determine the crustal structure, loading and subsidence history, and last sub-aerial exposure age for the CHRISP. The study shows that flexural and subsidence modelling are reliable techniques in constraining the age of the seamounts when geochronological techniques are not possible. Utilising regional gravity signatures, we model the crustal structure underneath the Cocos (Keeling) Island, and constrain the thickness of the limestone cover between 900 and 2,100 m. Using age-depth subsidence curves for oceanic lithosphere the time since these seamounts were exposed above sea-level was determined, and a trend in exposure ages that youngs towards the west is observed. Two episodes of volcanism have been recorded at Christmas Island and they are of different origin. The younger phase in the Pliocene is a manifestation of flexure induced cracks produced in the lithosphere as it rides the subduction fore-bulge, whereas a low velocity seismic zone rising from the lower mantle, and tectonic reorganization, may be associated with the older Eocene volcanic phase, as well as much of the rest of the province. Our modelling also supports the existence of an older, undated volcanic core to Christmas Island, based on the loading ages from flexural modelling.

Tominaga M. “Imaging” the cross section of oceanic lithosphere: The development and future of electrical microresistivity logging through scientific ocean drilling // Tectonophysics. 2013. Vol. 608. P. 84–96.

A detailed understanding of the architecture of volcanic and magmatic lithologies present within the oceanic lithosphere is essential to advance our knowledge of the geodynamics of spreading ridges and subduction zones. Undertaking sub-meter scale observations of oceanic lithosphere is challenging, primarily because of the difficulty in direct continuous sampling (e.g., by scientific ocean drilling) and the limited resolution of the majority of geophysical remote sensing methods. Downhole logging data from drillholes through basement formations, when integrated with recovered core and geophysical remote sensing data, can provide new insights into crustal accretion processes, lithosphere hydrogeology and associated alteration processes, and variations in the physical properties of the oceanic lithosphere over time. Here, we introduce an alternative approach to determine the formation architecture and lithofacies of the oceanic sub-basement by using logging data, particularly utilizing downhole microresistivity imagery (e.g. Formation MicroScanner (FMS) imagery), which has the potential to become a key tool in deciphering the high-resolution internal architecture of the intact upper ocean crust. A novel ocean crust lithostratigraphy model based on meticulously deciphered lava morphology determined by in situ FMS electrofacies analysis of holes drilled during Ocean Drilling Program legs (1) advances our understanding of ocean crust formation and accretionary processes over both time and space; and (2) allows the linking of local igneous histories deciphered from the drillholes to the regional magmatic and tectonic histories. Furthermore, microresistivity imagery can potentially allow the investigation of (i) magmatic lithology and architecture in the lower ocean crust and upper mantle; and, (ii) void space abundances in crustal material and the determination of complex lithology-dependent void geometries.

Verney-Carron A. et al. Lithium isotopes in hydrothermally altered basalts from Hengill (SW Iceland) // Earth and Planetary Science Letters. 2015. Vol. 411. P. 62–71.

The Li isotope signatures of hydrothermal fluids are remarkably constant (delta Li-7 = 8.0 +/- 1.97 parts per thousand) irrespective of the water/rock ratio (W/R), permeability, temperature or fluid involved (seawater or meteoric). High temperature hydrothermal fluids represent the second most significant source of Li to the ocean, yet the homogeneity of the Li isotopic signatures of this source remains to be explained and in this context, the lack of data for the corresponding altered phases is problematic. We measured Li contents and Li isotope signatures (as well as mineralogy, composition and local fluid temperature) in hyaloclastites collected from a borehole in the Hellisheidi geothermal system (Iceland) which have been altered by high temperature aqueous fluids (from 170 to 300 degrees C). Li is more enriched in the solid phases than the other alkali metals, highlighting its greater ability to be incorporated into secondary phases, especially at high temperatures (>250 degrees C). Mass balance calculations show that the low Li concentrations in hydrothermal fluids are best explained by a high water/rock ratio and a high permeability of this system. The Li isotopic signature of the altered hyaloclastites (delta Li-7 between +1.9 and +4.0 parts per thousand) remains close to the fresh basalt at deep levels and high temperatures (290-300 degrees C) (as measured delta Li-7 range between +3.7 and +4.0 parts per thousand), and decreases at shallower depths and lower temperatures (150-270 degrees C) (delta Li-7 between +1.9 and +3.1 parts per thousand). A mass balance model involving basalt dissolution, secondary phase formation, and successive isotope equilibrium during the migration and the cooling of the percolating fluid was developed. The corresponding apparent mineral-fluid Li isotope fractionation factors resulting from precipitation of secondary phases (Delta Li-7(minerals-fluid)) range between 0 parts per thousand at 300 degrees C and -8.5 parts per thousand at 170 degrees C and highlight a key role of chlorite. Applying the same approach to mid-ridge oceanic hydrothermal systems allows the relatively homogeneous isotope signatures of high temperature fluids of various locations to be explained.

Weis P. The dynamic interplay between saline fluid flow and rock permeability in magmatic-hydrothermal systems // Geofluids. 2015. Vol. 15, № 1-2. P. 350–371.

Magmatic-hydrothermal ore deposits document the interplay between saline fluid flow and rock permeability. Numerical simulations of multiphase flow of variably miscible, compressible H2O-NaCl fluids in concert with a dynamic permeability model can reproduce characteristics of porphyry copper and epithermal gold systems. This dynamic permeability model uses values between 10(-22) and 10(-13)m(2), incorporating depth-dependent permeability profiles characteristic for tectonically active crust as well as pressure- and temperature-dependent relationships describing hydraulic fracturing and the transition from brittle to ductile rock behavior. In response to focused expulsion of magmatic fluids from a crystallizing upper crustal magma chamber, the hydrothermal system self-organizes into a hydrological divide, separating an inner part dominated by ascending magmatic fluids under near-lithostatic pressures from a surrounding outer part dominated by convection of colder meteoric fluids under near-hydrostatic pressures. This hydrological divide also provides a mechanism to transport magmatic salt through the crust. With a volcano at the surface above the hydrothermal system, topography-driven flow reverses the direction of the meteoric convection as compared to a flat surface, leading to discharge at distances of up to 7km from the volcanic center. The same physical processes at similar permeability ranges, crustal depths, and flow rates are relevant for a number of active systems, including geothermal resources and excess degassing at volcanos. The simulations further suggest that the described mechanism can separate the base of free convection in high-enthalpy geothermal systems from the magma chamber as a driving heat source by several kilometers in the vertical direction in tectonic settings with hydrous magmatism. These root zones of high-enthalpy systems may serve as so-called super-critical geothermal resources. This hydrology would be in contrast to settings with anhydrous magmatism, where the base of the geothermal systems may be closer to the magma chamber.

Wheeler A.J. et al. Moytirra: Discovery of the first known deep-sea hydrothermal vent field on the slow-spreading Mid-Atlantic Ridge north of the Azores // Geochem. Geophys. Geosyst. 2013. Vol. 14, № 10. P. 4170–4184.

Geological, biological, morphological, and hydrochemical data are presented for the newly discovered Moytirra vent field at 45 degrees N. This is the only high temperature hydrothermal vent known between the Azores and Iceland, in the North Atlantic and is located on a slow to ultraslowspreading mid-ocean ridge uniquely situated on the 300 m high fault scarp of the eastern axial wall, 3.5 km from the axial volcanic ridge crest. Furthermore, the Moytirra vent field is, unusually for tectonically controlled hydrothermal vents systems, basalt hosted and perched midway up on the median valley wall and presumably heated by an off-axis magma chamber. The Moytirra vent field consists of an alignment of four sites of venting, three actively emitting "black smoke," producing a complex of chimneys and beehive diffusers. The largest chimney is 18 m tall and vigorously venting. The vent fauna described here are the only ones documented for the North Atlantic (Azores to Reykjanes Ridge) and significantly expands our knowledge of North Atlantic biodiversity. The surfaces of the vent chimneys are occupied by aggregations of gastropods (Peltospira sp.) and populations of alvinocaridid shrimp (Mirocaris sp. with Rimicaris sp. also present). Other fauna present include bythograeid crabs (Segonzacia sp.) and zoarcid fish (Pachycara sp.), but bathymodiolin mussels and actinostolid anemones were not observed in the vent field. The discovery of the Moytirra vent field therefore expands the known latitudinal distributions of several ventendemic genera in the north Atlantic, and reveals faunal affinities with vents south of the Azores rather than north of Iceland.

Whitney D.L. et al. Continental and oceanic core complexes // Geol. Soc. Am. Bull. 2013. Vol. 125, № 3-4. P. 273–298.

Core-complex formation driven by lithospheric extension is a first-order process of heat and mass transfer in the Earth. Core-complex structures have been recognized in the continents, at slow-and ultraslow-spreading mid-ocean ridges, and at continental rifted margins; in each of these settings, extension has driven the exhumation of deep crust and/or upper mantle. The style of extension and the magnitude of core-complex exhumation are determined fundamentally by rheology: (1) Coupling between brittle and ductile layers regulates fault patterns in the brittle layer; and (2) viscosity of the flowing layer is controlled dominantly by the synextension geotherm and the presence or absence of melt. The pressure-temperature-time-fluid-deformation history of core complexes, investigated via field- and modeling-based studies, reveals the magnitude, rate, and mechanisms of advection of heat and material from deep to shallow levels, as well as the consequences for the chemical and physical evolution of the lithosphere, including the role of core-complex development in crustal differentiation, global element cycles, and ore formation. In this review, we provide a survey of similar to 40 yr of core-complex literature, discuss processes and questions relevant to the formation and evolution of core complexes in continental and oceanic settings, highlight the significance of core complexes for lithosphere dynamics, and propose a few possible directions for future research.

XianBin W. et al. Serpentinization, abiogenic organic compounds, and deep life // Science China-Earth Sciences. 2014. Vol. 57, № 5. P. 878–887.

The hydrocarbons and other organic compounds generated through abiogenic or inorganic processes are closely related to two science subjects, i.e., energy resources and life's origin and evolution. "The earth's primordial abiogenic hydrocarbon theory" and "the serpentinization of abiogenic hydrocarbon theory" are the two mainstream theories in the field of related studies. Serpentinization generally occurs in slow expanding mid-ocean ridges and continental ophiolites tectonic environment, etc. The abiogenic hydrocarbons and other organic compounds formed through the serpentinization of ultramafic rocks provide energy and raw materials to support chemosynthetic microbial communities, which probably was the most important hydration reaction for the origin and early evolution of life. The superposition of biological and abiological processes creates big challenge to the identification of the abiogenic organic materials in serpentinite-hosted ecosystem. Whether abiotic (inorganic) process can form oil and gas resource is a difficult question that has been explored continuously by scientific community for more than a century but has not yet been solved. However, some important progress has been made. The prospecting practice of abiogenic hydrocarbons in commercial gases from the Songliao Basin, China, provides an important example for exploring abiogenic natural gas resources.

Yeo I.A., Searle R.C. High-resolution Remotely Operated Vehicle (ROV) mapping of a slow-spreading ridge: Mid-Atlantic Ridge 45 degrees N // Geochem. Geophys. Geosyst. 2013. Vol. 14, № 6. P. 1693–1702.

Axial volcanic ridges (AVRs) are found on most slow-spreading mid-ocean ridges and are thought to be the main locus of volcanism there. In this study we present high-resolution mapping of a typical, well-defined AVR on the Mid-Atlantic Ridge at 45 degrees N. The AVR is characterized by hummocky terrain, composed typically of hummocks with pillowed or elongate pillowed flanks with pillowed or lobate lava flow summits, often with small haystacks sitting on their highest points. The AVR is surrounded by several areas of flat seafloor, composed of lobate and sheet lava flows. The spatial and morphological differences between these areas indicate different eruption processes operating on and off the AVR. Volcanic fissures are found all around and on the AVR, although those with the greatest horizontal displacement are found on the ridge crest and flat seafloor. Clusters of fissures may represent volcanic vents. Extremely detailed comparisons of sediment coverage and examination of contact relations around the AVR suggest that many of the areas of flat seafloor are of a similar age or younger than the hummocky terrain of the AVR. Additionally, all the lavas surveyed have similar degrees of sediment cover, suggesting that the AVR was either built or resurfaced in the same 50ka time frame as the flat seafloor.

Yi S.-B. et al. Geochemistry and petrogenesis of mafic-ultramafic rocks from the Central Indian Ridge, latitude 8 degrees-17 degrees S: denudation of mantle harzburgites and gabbroic rocks and compositional variation of basalts // International Geology Review. 2014. Vol. 56, № 14. P. 1691–1719.

This study investigates the formation of lower oceanic crust and geochemical variations of basalts along the Central Indian Ridge (CIR, lat. 7 degrees 45 '-17 degrees 10 ' S). Harzburgites, various gabbroic cumulates, medium- to fine-grained oxide gabbros, diabases, and pillow basalts were recovered by dredging from segment ends such as ridge-transform intersections (RTIs), non-transform discontinuities (NTDs), and transform offset areas. The occurrence of both harzburgites and gabbroic rocks with minor basalts at all segments ends, and leucogabbro intrusive into harzburgite at the 12 degrees 45 ' S NTD indicates that oceanic crust at segment ends exposes mantle-derived harzburgites and gabbroic intrusions with a thin basaltic cover due to sparse magmatic activity. Basalts collected along the entire ridge show wide compositional variations between N (normal)- and E (enriched)-mid-ocean ridge basalt (MORB). T (transitional)-MORBs with enriched affinities are more prominent than N-MORBs. There is no tendency of enrichment towards specific directions. (La/Sm)(N) variations in MORB along the CIR (8 degrees-21 degrees S) fluctuates at a regional scale with local high positive anomalies reflecting compositional heterogeneity of the sub-CIR mantle domain.

Аранович Л.Я., Зингер Т.Ф., Бортников Н.С., Шарков Е.В., Антонов А.В. Циркон из габброидов осевой зоны Срединно-Атлантического хребта (впадина Маркова, 6° С.Ш.): корреляция геохимических особенностей с петрогенетическими процессами // Петрология. 2013. Т. 21. № 1. С. 4-19.

Проведено детальное петролого-геохимическое исследование цирконов и вмещающих их пород медленно-спредингового Срединно-Атлантического хребта, драгированных в районе впадины Маркова. Породы, в разной степени катаклазированные, представлены габбро-норитом, содержащим прожилки океанического плагиогранита (ОПГ), лейкократовым габбро (примитивным габбро) и роговообманковым Fe-Ti оксидным габбро-норитом (феррогаббро), не содержащими ОПГ. Изученные цирконы различаются по морфологии, внутренней структуре, набору минеральных включений (сростков) и по содержанию редких элементов. Отдельные зерна также неоднородны по составу. Спектры редкоземельных элементов (РЗЭ) в цирконах характеризуются плавным ростом от легких РЗЭ к тяжелым, с отчетливо выраженной положительной аномалией Се и отрицательной Eu, и в целом попадают в диапазон спектров цирконов магматических пород. Океанические цирконы отчетливо отличаются от континентальной популяции на дискриминационных диаграммах в координатах U/Yb?Y и U/YbHf, в первую очередь за счет более низкого значения U/Yb отношения при широких вариациях концентрации Y и Hf. Цирконы, содержащие включения кислого стекла и, следовательно, кристаллизовавшиеся из расплава ОПГ, относительно обеднены РЗЭ (в особенности тяжелыми). Это указывает на образование ОПГ путем частичного плавления габбро в присутствии концентрированного водно-солевого флюида, который экстрагировал РЗЭ из плагиогранитного расплава. Цирконы из габброидов, не содержащих обособлений ОПГ, отличаются от цирконов из ОПГ более высокой суммарной концентрацией РЗЭ. Поздние гидротермальные изменения циркона, отчетливо устанавливаемые по формированию в нем новообразованной колломорфной (пористой) структуры и/или по составу минеральных включений, сопровождаются заметным обогащением циркона La. Неоднородность содержания Ti в цирконе может быть связана не с изменением температуры его кристаллизации, а с вариациями отношения активности кремнезема и оксида титана в породах, обусловленными их взаимодействием с гидротермальным раствором переменной кислотности. Совместное изучение структурно-морфологических и геохимических особенностей океанических цирконов и фазового состава вмещающих пород и включений позволяет более надежно охарактеризовать процессы, приводившие к кристаллизации и последующей эволюции этого минерала в породах океанической литосферы.

Глебовский В.Ю., Астафурова Е.Г., Черных А.А., Корнева М.С., Каминский В.Д., Поселов В.А. Мощность земной коры в глубоководной части Северного Ледовитого океана: результаты 3-d гравитационного моделирования // Геология и геофизика. 2013. Т. 54. № 3. С. 327-344.

Использованный метод 3-D гравитационного моделирования основан на вычислении гравитационных эффектов от основных плотностных границ литосферы, вычитании этих эффектов из наблюденного поля силы тяжести и последующем пересчете остаточных гравитационных аномалий вначале в глубины залегания раздела Мохоровичича (Мохо), а затем в значения суммарной мощности земной коры и мощности ее консолидированной части. При моделировании были учтены также гравитационные эффекты, связанные с возрастанием плотности осадков при увеличении глубины их залегания и с поднятием кровли астеносферы под океаническим хр. Гаккеля. Полученные результирующие 3-D модели рельефа Мохо и мощности земной коры хорошо согласованы с данными глубинной сейсмометрии. Они подтверждают существенные различия в строении земной коры Евразийского и Амеразийского бассейнов и дают представление о региональных вариациях мощности земной коры под основными глубоководными поднятиями и котловинами Северного Ледовитого океана.

Лаверов Н.П., Лобковский Л.И., Кононов М.В., Добрецов Н.Л., Верниковский В.А., Соколов С.Д., Шипилов Э.В. Геодинамическая модель развития Арктического бассейна и примыкающих территорий для Мезозоя и Кайнозоя и внешняя граница континентального шельфа России // Геотектоника. 2013. № 1. С. 3-35.

Рассматривается тектоническое развитие Арктического региона в мезозое и кайнозое с учетом более раннего палеозойского этапа эволюции древнего континента Арктида. Предложена новая геодинамическая модель эволюции Арктики, основанная на представлении о развитии верхнемантийной конвекции под континентом, обусловленной процессом субдукции Тихоокеанской литосферы под Евразийскую и Северо-Aмериканскую литосферные плиты. Показано последовательное образование структур Амеразийского и Евразийского бассейнов Арктики в контексте разрушения древнего континента Арктида, сохранившийся фрагмент которой представлен структурами центрального сегмента Северного Ледовитого океана, включая хребет Ломоносова, поднятие Альфа-Менделеева, котловины Подводников и Макарова. Предлагаемая модель рассматривается как научное обоснование обновленной заявки России в комиссию ООН на установление внешней границы континентального шельфа в Арктике.

Ласточкин А.Н., Егоров И.В., Кузнецов Т.В. Содержание, легенда и интерпретация морфогеодинамической карты при поисково-разведочных работах на гидротермальный рудогенез (на примере приосевой зоны срединно-атлантического хребта между параллелями 12°40? и 15°10? С. Ш.) // Вестник Санкт-Петербургского университета. Серия 7: Геология. География. 2014. № 1. С. 125-135.

Настоящая статья является очередной работой по изучению рельефа и рельефообразующих процессов Северо-Атлантического хребта между параллелями 12°40? и 15°10? c. ш., в данной статье рассматриваются содержание, условные обозначения и интерпретация новой морфогеодинамической карты, отражающей статические и динамические связи между рельефом срединно-океанических хребтов (СОХ) и рельефообразующими породами и создавшими и моделирующими их тектоно-магматическими рельефо-, структуро- и рудообразующими процессами. В статье представлена характеристика различных типов морфоструктур и дана их динамическая характеристика, а также дан анализ выявленных дизъюнктивных дислокаций. Новизна объектов поисковой геоморфологии потребовала ввести в понятийно-терминологический аппарат термин «морфогеодинамика», под которым понимаются исследования выраженных в рельефе подводной поверхности не только тектонических движений (предмет морфотектоники), но и имеющих не меньшее рельефо- и структурообразующее значение в приосевых зонах СОХ вулканических процессов.

Ласточкин А.Н., Егоров И.В., Кузнецов Т.В. Теоретическое обоснование морфогеодинамических исследований при поисково-разведочных работах (на примере приосевой зоны срединно-атлантического хребта между параллелями 12°40? и 15°10? С. Ш.) // Вестник Санкт-Петербургского университета. Серия 7: Геология. География. 2013. № 1. С. 49-56.

Продолжая комплекс системных геоморфологических исследований осевых зон срединно-океанических хребтов на примере Северо-Атлантического хребта между параллелями 12°40’ и 15°10’, актуальность которых обусловлена тем, что к настоящему времени уже определены границы Заявочного района Российской Федерации в Северной Атлантике, данная статья является теоретическим обоснованием морфогеодинамичсеких исследований. Большое значение имеют статическое и динамическое определение понятия «морфоструктура». Новые названия карт и картировочных единиц оправдываются новизной их содержания — объектами (рельефом СОХ и рельефообразующими породами) и создавшими и моделирующими их тектоно-магматическими рельефо-, структуро- и рудо-образующими процессами.

Маракушев А.А., Панеях Н.А., Маракушев С.А. Образование сульфидных руд и углеводородов в срединно-океанических хребтах // Глубинная нефть. 2014. Т. 2. № 5. С. 689-698.

Грандиозные подводные поднятия в океанах (срединно-океанические хребты) формировались в связи со спредингом океанического дна, скорость которого достигала 16 см/год. С замедлением и прекращением спрединга в них развивались продольные и поперечные депрессии, которыми контролировалось развитие в них сульфидного Zn-Cu рудообразования. Генетически оно связывалось с развитием на глубине депрессионного магматизма со свойственной ему генерацией богатых железом дифференциатов (Fe2SiO4). Они подвергались флюидной сульфуризации с генерацией сульфидных расплавов. Вовлечение в этот процесс оксидов углерода (СО2, СО) порождает углеводороды: 3,5Fe2SiO4+14H2S+2CO2=7FeS2+3,5SiO2+11H2O+C2H6. Подобными процессами определяется наблю- даемая пространственная связь углеводородов и сульфидных руд. Ими вносится существенный вклад в углеводородную специализацию депрессионных структур срединно-океанических хребтов.

Моргунова И.П., Иванов В.Н., Литвиненко И.В., Петрова В.И., Степанова Т.В., Черкашёв Г.А. Геохимия органического вещества донных отложений гидротермального поля Ашадзе (13° С.Ш., Срединно-Атлантический хребет) // Океанология. 2012. Т. 52. № 3. С. 372-380.

Проведено сравнительное изучение состава и распределения рассеянного органического вещества (РОВ) донных осадков активных участков гидротермального поля Ашадзе (Срединно-Атлантический хребет, 13° с.ш.) и фоновых отложений в районе исследований, отобранных в ходе рейсов НИС “Профессор Логачёв” в 2003 и 2007 гг. Были зафиксированы существенные различия в групповом и молекулярном составе РОВ сопоставляемых областей. Содержание углеводородов (УВ) в гидротермальных образцах превышает фоновое почти в 20 раз, причем в их составе обнаружены как слабо преобразованные (изопреноиды, гопены, ??-гопаны, ???27R-стераны), так и геологически зрелые (н-алканы С16 - С35, геогопаны, моретаны, полиароматические углеводороды) хемофоссилии. Полиароматические углеводороды (ПАУ) представлены в основном фенантреном и его алкил-гомологами возможными продуктами диагенетической трансформации биогенных предшественников. Полученные результаты указывают на смешанный генезис гидротермального РОВ, обусловленный, прежде всего, разнообразием состава биоты рассматриваемого региона, а также специфичностью процессов созревания РОВ под действием экстремальных условий среды.

Русаков В.Ю., Шилов В.В., Рыженко Б.Н., Габлина И.Ф., Рощина И.А., Кузьмина Т.Г., Кононкова Н.Н., Добрецова И.Г. Минералого-геохимическая зональность осадков гидротермального узла “Семенов” (13°3113°30 С.Ш. Срединно-Атлантический хребет) // Геохимия. 2013. Т. 51. № 8. С.717-742.

На оригинальном материале восьми колонок, полученных в ходе 32-й экспедиции на НИС “Профессор Логачев” в 2009 г., выполнены литолого-фациальные, биостратиграфические, минералогические и геохимические исследования осадков, расположенных в пределах Северо-Западного (активного), и Восточного (не активного) гидротермальных полей рудного узла “Семенов”. Выделяются минеральные типы осадков, и предложена принципиальная схема вертикального строения гидротермально-осадочного разреза, перекрывающего массивные сульфидные руды. Установлено, что рудоносные осадки характеризуются вертикальной зональностью из последовательно сменяющих друг друга минеральных ассоциаций, которые в свою очередь контролируются активностью кислорода. Рассмотрены механизмы формирования атакамита CuCl2 · 3Cu(OH)2, широко развитого в оксидно-железистых красноцветных апосульфидных отложениях (sulfide gossans).

Силантьев С.А., Бортников Н.С., Шатагин К.Н., Бычкова Я.В., Краснова Е.А., Бельтенев В.Е. Перидотит-базальтовая ассоциация САХ на 19°42–19°59 С.Ш.: оценка условий петрогенезиса и баланса вещества при гидротермальном преобразовании океанической коры // Петрология. 2015. Т. 23. № 1. С. 3-25.

Реконструированы условия петрогенезиса в слабо изученном районе осевой зоны САХ. Установлено, что мантийные перидотиты САХ на 19°42 19°59 с.ш. характеризуются узким интервалом низких и умеренных степеней плавления. По геохимическим параметрам исследованные перидотиты близки к типичным абиссальным перидотитам, вмещающим активные гидротермальные поля Атлантики (например, гидротермальный кластер Ашадзе-Семенов-Логачев), Однако в изученных перидотитах присутствуют плагиоклазовые лерцолиты. Среди отобранных образцов встречены породы, представленные магнезиальным метабазитами, которые, возможно, были образованы за счет метасоматического преобразования (родингитизации) гарцбургитов на их контакте с габброидами. Установлено, что по уровню содержания хрома и никеля в серпентине можно судить о том, по какому первичному минералу он развивается. Судя по характеру распределения точек изотопных составов абиссальных перидотитов САХ на диаграмме в координатах 87Sr/86Sr 143Nd/144Nd, можно прийти к заключению, что обнажения мантийных перидотитов и ассоциирующих с ними плутонических пород в гребневой зоне хребта сложены блоками глубинных пород, выведенных на поверхность океанического дна в разное время. Акваторию, включающую осевую зону САХ между 19°42 и 19°59 с.ш., можно рассматривать как перспективную для поисков гидротермальных рудопроявлений. Параметры состава базальта из изученной коллекции соответствуют наиболее деплетированным разновидностям MORB Атлантики.

Силантьев С.А., Кепке Ю., Арискин А.А., Аносова М.О., Краснова Е.А., Дубинина Е.О., Зур Г. Геохимическая природа и возраст плагиогранит/габбро-норитовой ассоциации внутреннего океанического комплекса Срединно-Атлантического хребта на 5°10 Ю.Ш // Петрология. 2014. Т. 22. № 2. С. 126-146.

Представлены первые данные о геохимии и возрасте плагиогранит/габбро-норитовой ассоциации внутреннего океанического комплекса Срединно-Атлантического хребта (САХ) на 5°10 ю.ш. Геохимические особенности океанического плагиогранита (ОПГ) и габбро-норита из изученного сегмента хребта позволяют предполагать тесную генетическую связь этих пород. U/Pb оценка возраста циркона из исследованного образца плагиогранита составляет 1.059 ± 0.055 млн лет и хорошо согласуется с возрастом циркона, выделенного из плутонических пород внутренних океанических комплексов северного САХ. Характерным геохимическим признаком рассмотренных пород является необычно деплетированный тип изотопных отношений 87Sr/86Sr и 143Nd/144Nd, которые позволяют рассматривать плутонические породы из габбро-норитплагиогранитной ассоциации САХ на 5°10 ю.ш. как производные продуктов плавления наиболее деплетированного мантийного резервуара из всех известных под осевой зоной САХ. Предпринятое с помощью программы КОМАГМАТ-5.2 термодинамическое моделирование возможных кристаллизационных связей между плагиогранитами и габбро-норитами из сегмента САХ на 5°10 ю.ш. позволило прийти к заключению, что главную роль в формировании плагиогранитов играл двухстадийный процесс, включающий частичное плавление габбро-норита и последующее фракционирование новообразованного расплава. Существующие региональные различия в изотопно-геохимических характеристиках плагиогранитов САХ, возможно, отражают локальные особенности “гидротермального анатексиса”, заключающиеся как в геохимической специфике магматических пород, в нем участвующих, так и в параметрах гидротермального процесса, например, различия в величине отношения W/R.

Силантьев С.А., Портнягин М.В., Краснова Е.А., Хауфф Ф., Вернер Р., Кузьмин Д.В. Петрология и геохимия плутонических пород северо-западной части Тихого океана и их геодинамическая интерпретация // Геохимия. 2014. № 3. С. 195-213.

Рассмотрены петрологические и геохимические особенности плутонических пород, драгированных в районе разломной зоны Стелмейт (СЗ Пацифика) при проведении SO201-1b рейса НИС “Зонне”. Предложена также реконструкция условий образования и интерпретация их тектонической эволюции. Показано, что среди плутонических пород, слагающих мел-палеогеновый фундамент северо-западной акватории Тихого океана, участвуют габброиды, происхождение которых связано с магматизмом в древнем центре спрединга и отражает эволюцию магматических расплавов, родительских для N-MORB. Эти породы вместе с ассоциирующими с ними перидотитами, базальтами и долеритами могут быть отнесены к дезинтегрированному разрезу тихоокеанской океанической литосферы мел-палеогенового возраста. Малоглубинная мантия под древней океанической корой рассматриваемого региона сложена деплетированными шпинелевыми лерцолитами и гарцбургитами а также дунитами магматического происхождения. Присутствие на юго-восточном окончании разломной зоны Стелмейт габбро-диоритов и диоритов, генетически не связанных с породами мел-палеогенового океанического фундамента северо-запада Тихого океана, возможно, отражает сложное строение тектонического коллажа пород, образованных в разное время в различных геодинамических обстановках и тектонически совмещенных вблизи фронтальной части Алеутской островной дуги. Эти породы, судя по их изотопно-геохимическим особенностям, не могут рассматриваться как представители семейства океанических плагиогранитов. Вдоль всего простирания разломной зоны Стелмейт широко проявлены процессы деформации океанического фундамента, в результате которых слагающие его породы испытывали брекчирование и крупноамплитудные вертикальные движения внутри океанической литосферы.

Сколотнев С.Г. Природа многообразия вулканитов экваториальной части Срединно-Атлантического хребта // Электронное научное издание Альманах Пространство и Время. 2013. Т. 4. № 1. С. 6.

На основании изучения состава, геохимии и изотопии вулканитов из осевой части Срединно-Атлантического хребта в Приэкваториальной Атлантике и анализа характера структурно-пространственных вариаций их различных типов показана природа многообразия состава вулканитов региона. Высказывается мнение о том, что блоки континентальной литосферы появились в результате тектонической эрозии Экваториального сегмента Гондваны, расколовшегося и раскрывшегося позднее соседних областей Атлантики.

Сколотнев С.Г., Ескин А.Е. Состав и строение 3-го слоя океанической коры в приэкваториальном сегменте Срединно-Атлантического хребта (5° - 7° С.Ш.) // Геохимия. 2013. № 9. С. 773-808.

Изучен валовой и минеральный состав, концентрации элементов-примесей и составы минералов плутонических пород полигона Сьерра Леоне, расположенного в гребневой зоне Срединно-Атлантического хребта между трансформными разломами Страхова и Богданова. По отношению к структуре дна выделяются породные ассоциации рифтовых долин и зон нетрансформных смещений. Троктолиты и оливиновые габбро, образующие рифтовую ассоциацию, формировались на ранней стадии фракционирования расплавов океанических толеитов в нестационарных магматических камерах небольших размеров. Под зонами нетрансформных смещений основная часть пород кристаллизовалась в ходе длительно протекавших процессов фракционирования расплава в крупных камерах, окруженных серпентинизированными перидотитами. Эта часть представлена рядом кумулатов от троктолитов до габбродиоритов. В зонах проникновения в частично консолидированные участки этих камер глубинных тектонических срывов происходили процессы взаимодействия между расплавом и субстратом. Флюид, возникавший при дегидратации серпентина и концентрировавший гигрофильные элементы, локально изменял состав расплава, приводя к образованию амфиболсодержащих пород. При стрессах в тектонически ослабленные зоны выжимались интеркумулусные расплавы, где они смешивались между собой и также взаимодействовали с субстратом. Из смешанных расплавов сформировались рудные FeTi габброиды, в том числе и путем фракционной кристаллизации. Из остаточных порций этих расплавов образовалась основная часть диоритов и плагиогранитов. Однако высоконатровые диориты, по-видимому, кристаллизовались из кислых расплавов, генерированных при частичном плавлении уже сформировавшихся габброидов на участках циркуляции водных флюидов, возникавших при десерпентинизации субстрата, в зонах тектонизации камер.

Углов Б.Д., Егоров И.В. Геолого-геофизическое моделирование районов развития глубоководных полиметаллических сульфидов на примере рудного поля ТАГ Срединно-Атлантического хребта // Отечественная геология. 2015. № 1. С. 54-63.

Показаны возможности применения комплекса геофизических и геоморфологических методов при глубинном ге- ологическом моделировании районов развития глубоководных полиметаллических сульфидов (ГПС) на примере гидротермального рудного поля ТАГ Срединно-Атлантического хребта. В пределах 3-го слоя океанической коры выявлена магматическая камера, инициирующая функционирование рециклинговой гидротермальной системы, под влиянием которой образуются гидротермальные рудные объекты в глубоководных частях Мирового океана. Этот факт подтверждает основную роль рециклинговых процессов при образовании глубоководных полиметал- лических сульфидов.

Хуторской М.Д., Поляк Б.Г. Геотермические модели геодинамических обстановок разного типа // Геотектоника. 2014. № 1. С. 77-96.

Рассматриваются особенности распределения теплового потока, глубинных температур и отношения изотопов гелия в зонах осевого спрединга (срединно-океанических хребтах), в зонах “рассеянного спрединга” (задуговых бассейнах), в активных частях трансформных разломов, во внутриконтинентальных и периконтинентальных рифтовых зонах, в палеозойских линейных и мозаичных складчатых поясах, в осадочных бассейнах нагрузки и растяжения. Отмечаются резкие различия в этих структурах величин теплового потока (от 15 до 1500 мВт/мРассматриваются особенности распределения теплового потока, глубинных температур и отношения изотопов гелия в зонах осевого спрединга (срединно-океанических хребтах), в зонах “рассеянного спрединга” (задуговых бассейнах), в активных частях трансформных разломов, во внутриконтинентальных и периконтинентальных рифтовых зонах, в палеозойских линейных и мозаичных складчатых поясах, в осадочных бассейнах нагрузки и растяжения. Отмечаются резкие различия в этих структурах величин теплового потока (от 15 до 1500 мВт/м2) и, соответственно, мощности термической литосферы. В палеозойских складчатых поясах количественно оценена радиогенная теплогенерация, обеспечивающая 40-50% фонового теплового потока. Показано, что нестационарность теплового потока проявляется не только в современных, но и в позднепалеозойских тектонических поясах. Объясняется природа положительных и отрицательных геотермических аномалий. Подчеркнута связь таких аномалий в осадочных бассейнах с локализацией месторождений углеводородов.) и, соответственно, мощности термической литосферы. В палеозойских складчатых поясах количественно оценена радиогенная теплогенерация, обеспечивающая 40-50% фонового теплового потока. Показано, что нестационарность теплового потока проявляется не только в современных, но и в позднепалеозойских тектонических поясах. Объясняется природа положительных и отрицательных геотермических аномалий. Подчеркнута связь таких аномалий в осадочных бассейнах с локализацией месторождений углеводородов.

Черкашёв Г.А., Иванов В.Н., Бельтенёв В.И., Лазарева Л.И., Рождественская И.И., Самоваров М.Л., Порошина И.М., Сергеев М.Б., Степанова Т.В., Добрецова И.Г., Кузнецов В.Ю. Сульфидные руды северной приэкваториальной части Срединно-Атлантического хребта // Океанология. 2013. Т. 53. № 5. С. 680-693.

Приведены результаты многолетних экспедиционных исследований, направленных на поиск и изучение сульфидного оруденения в северной приэкваториальной зоне Срединно-Атлантического хребта. Определен оптимальный набор поисковых методов, включающий геофизические (электроразведочные), геологические (минералого-геохимические) и гидрологические наблюдения. С использованием данного набора методов открыто шесть и значительно расширены еще два рудных объекта. Проанализировано их геологическое положение, в т.ч. связь с габбро-перидотитовыми породами и пологозалегающими глубинными разломами (детачментами). Руды, связанные с ультраосновными породами, характеризуются повышенным содержанием меди, золота и кобальта. Отмечена длительная многоэтапная история формирования руд, приводящая к накоплению крупных залежей (более 10 млн тонн).

Шилов В.В., Бельтенев В.Е., Иванов В.Н., Черкашёв Г.А., Рождественская И.И., Габлина И.Ф., Добрецова И.Г., Наркевский Е.В., Густайтис А.Н., Кузнецов В.Ю. Новые гидротермальные рудные поля на Срединно-Атлантическом хребте: Зенит-Виктория (20°08 С.Ш.) и “Петербургское” (19°52 С.Ш.) // Доклады Академии наук. 2012. Т. 442. № 3. С. 383-389.

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