На главную К списку выставокАрхив выставок

Базальт. Месторождения. Добыча. Технологии

Журнальные статьи

1. U01624
Baker, Leslie L.; Rember, William C.; Sprenke, Kenneth F.; et al.Celadonite in continental flood basalts of the Columbia River Basalt Group// AMERICAN MINERALOGIST Volume: 97 Issue: 8-9 Pages: 1284-1290 Published: AUG-SEP 2012

Celadonite is a common alteration product of basalts in marine environments. It has been argued that marine fluids are necessary for celadonite formation, possibly by providing a source of K and other dissolved cations. Laterally extensive deposits of celadonite occur in basalts of the Grande Ronde Basalt of the Columbia River Basalt Group. The celadonite is found in scoriaceous flow tops of layered basalt flows, where it fills vesicles and replaces the surrounding groundmass. Evolved interstitial glasses are present in the basalts and dissolution of these glasses may provide sufficient K for celadonite formation, whereas dissolution of groundmass augite provides a source of Mg and Fe. These observations show that alteration by seawater or any other external source of dissolved ions is not necessarily required for celadonite formation.

2. U10290
Charlier, Bernard; Grove, Timothy L.; Zuber, Maria T. Phase equilibria of ultramafic compositions on Mercury and the origin of the compositional dichotomy //EARTH AND PLANETARY SCIENCE LETTERS Volume: 363 Pages: 50-60 Published: FEB 1 2013

Measurements of major element ratios obtained by the MESSENGER spacecraft using x-ray fluorescence spectra are used to calculate absolute element abundances of lavas at the surface of Mercury. We discuss calculation methods and assumptions that take into account the distribution of major elements between silicate, metal, and sulfide components and the potential occurrence of sulfide minerals under reduced conditions. These first compositional data, which represent large areas of mixed high-reflectance volcanic plains and low-reflectance materials and do not include the northern volcanic plains, share common silica- and magnesium-rich characteristics. They are most similar to terrestrial volcanic rocks known as basaltic komatiites. Two compositional groups are distinguished by the presence or absence of a clinopyroxene component. Melting experiments at one atmosphere on the average compositions of each of the two groups constrain the potential mineralogy at Mercury's surface, which should be dominated by orthopyroxene (protoenstatite and orthoenstatite), plagioclase, minor olivine if any, clinopyroxene (augite), and tridymite. The two compositional groups cannot be related to each other by any fractional crystallization process, suggesting differentiated source compositions for the two components and implying multi-stage differentiation and remelting processes for Mercury. Comparison with high-pressure phase equilibria supports partial melting at pressure < 10 kbar, in agreement with last equilibration of the melts close to the crust mantle boundary with two different mantle lithologies (harzburgite and lherzolite). Magma ocean crystallization followed by adiabatic decompression of mantle layers during cumulate overturn and/or convection would have produced adequate conditions to explain surface compositions. The surface of Mercury is not an unmodified quenched crust of primordial bulk planetary composition. Ultramafic lavas from Mercury have high liquidus temperatures (1450-1350 degrees C) and very low viscosities, in accordance with the eruption style characterized by flooding of pre-existing impact craters by lava and absence of central volcanoes. (c) 2012 Elsevier B.V. All rights reserved.

3. U08182
Filiberto, Justin; Wood, Justin; Dasgupta, Rajdeep; et al. Effect of fluorine on near-liquidus phase equilibria of an Fe-Mg rich basalt //CHEMICAL GEOLOGY Volume: 312 Pages: 118-126 Published: JUN 18 2012

Volatile species (H2O, CO2, F, Cl, etc.) have important effects on the formation and crystallization history of basaltic magmas. Here, we have experimentally investigated the effects of F on phase equilibria of Fe-Mg-rich basalt. Our results show that fluorine has large effects on the liquidus temperature and the chemistry of crystallizing minerals. Compared to the F-free system, addition of similar to 2 wt.% F moves the olivine-pigeonite liquidus point down similar to 2 kbar and 95 degrees C (from 12 kbar, 1375 degrees C to 10 kbar, 1280 degrees C). With increasing fluorine concentrations, Kd(Mineral-Melt)(Fe-Mg) dramatically increases for both pyroxene and olivine, suggesting that fluorine in basaltic magmas complexes primarily with MgO. Complexing with MgO in the melt decreases its MgO activity, and forces the crystallizing minerals to greater Fe/Mg, and so increases Kd(Mineral-Melt)(Fe-Mg). Models of basalt generation, where the magma is fluorine-rich, need to include the effect of not only water but fluorine on liquidus depression and minerals crystallizing/melting. Our results suggest that fluorine may significantly aid in the petrogenesis of silica-poor, alkali-rich magmas in the Earth and Mars. (C) 2012 Elsevier B.V. All rights reserved.

4. U09363
Giehl, Christopher; Marks, Michael; Nowak, Marcus. Phase relations and liquid lines of descent of an iron-rich peralkaline phonolitic melt: an experimental study //CONTRIBUTIONS TO MINERALOGY AND PETROLOGY Volume: 165 Issue: 2 Pages: 283-304 Published: FEB 2013

We experimentally investigated the phase relations of a peralkaline phonolitic dyke rock associated with the Ilimaussaq plutonic complex (South Greenland). The extremely evolved and iron-rich composition (magnesium number = 2, alkalinity index = 1.44, FeO* = 12 wt%) may represent the parental magma of the Ilimaussaq complex. This dyke rock is therefore perfectly suited for performing phase-equilibrium experiments, since in contrast to the plutonic rocks of the complex, no major cumulate formation processes complicate defining a reasonable starting composition. Experiments were carried out in hydrothermal rapid-quench cold-seal pressure vessels at P = 100 MPa and T = 950-750 A degrees C. H2O contents ranging from anhydrous to H2O saturated (similar to 5 wt% H2O) and varying fO(2) (similar to Delta logFMQ -3 to +1; where FMQ represents the fayalite-magnetite-quartz oxygen buffer) were applied. Reduced and dry conditions lead to substantial crystallization of alkali feldspar, nepheline, hedenbergite-rich clinopyroxene, fayalite-rich olivine and minor amounts of ulvospinel-rich magnetite, which represent the phenocryst assemblage of the natural dyke rock. Oxidized and H2O-rich conditions, however, suppress the crystallization of olivine in favor of magnetite and clinopyroxene with less or no alkali feldspar and nepheline formation. Accordingly, combined low fO(2) and aH(2)O force the evolution of the residual melt toward decreasing SiO2, increasing FeO* and alkalinity index (up to 3.55). On the contrary, high fO(2) and aH(2)O produce residual melts with relatively low FeO*, high SiO2 and a relatively constant alkalinity index. We show that variations of aH(2)O and fO(2) lead to contrasting trends regarding the liquid lines of descent of iron-rich silica-undersaturated peralkaline compositions. Moreover, the increase in FeO* and alkalinity index (reduced and dry conditions) in the residual melt is an important prerequisite to stabilize late-magmatic minerals of the dyke rock, for example, aenigmatite (Na2Fe5TiSi6O20), coexisting with the most evolved melts at 750 A degrees C. Contrary to what might be expected, experiments with high aH(2)O and interlinked high fO(2) exhibit higher liquidus T's compared with experiments performed at low aH(2)O and fO(2) for experiments where magnetite is liquidus phase. This is because ulvospinel-poor magnetite crystallizes at higher fO(2) and has a higher melting point than ulvospinel-rich magnetite, which is favored at lower fO(2

5. U12332
Hastie, Alan R.; Mitchell, Simon F.; Kerr, Andrew C.; et al. Geochemistry of rare high-Nb basalt lavas: Are they derived from a mantle wedge metasomatised by slab melts? // GEOCHIMICA ET COSMOCHIMICA ACTA Volume: 75 Issue: 17 Pages: 5049-5072 Published: SEP 1 2011

Compositionally, high-Nb basalts are similar to HIMU (high U/Pb) ocean island basalts, continental alkaline basalts and alkaline lavas formed above slab windows. Tertiary alkaline basaltic lavas from eastern Jamaica, West Indies, known as the Halberstadt Volcanic Formation have compositions similar to high-Nb basalts (Nb > 20 ppm). The Halberstadt high-Nb basalts are divided into two compositional sub-groups where Group 1 lavas have more enriched incompatible element concentrations relative to Group 2. Both groups are derived from isotopically different spinel peridotite mantle source regions, which both require garnet and amphibole as metasomatic residual phases. The Halberstadt geochemistry demonstrates that the lavas cannot be derived by partial melting of lower crustal ultramafic complexes, metasomatised mantle lithosphere, subducting slabs, continental crust, mantle plume source regions or an upper mantle source region composed of enriched and depleted components. Instead, their composition, particularly the negative Ce anomalies, the high Th/Nb ratios and the similar isotopic ratios to nearby adakite lavas, suggests that the Halberstadt magmas are derived from a compositionally variable spinel peridotite source region(s) metasomatised by slab melts that precipitated garnet, amphibole, apatite and zircon. It is suggested that high-Nb basalts may be classified as a distinct rock type with Nb > 20 ppm, intraplate alkaline basalt compositions, but that are generated in subduction zones by magmatic processes distinct from those that generate other intraplate lavas. (C) 2011 Elsevier Ltd. All rights reserved.

6. U18998
Lambart, Sarah; Laporte, Didier; Schiano, Pierre Markers of the pyroxenite contribution in the major-element compositions of oceanic basalts: Review of the experimental constraints // LITHOS Volume: 160 Pages: 14-36 Published: FEB 2013

Based on previous and new results on partial melting experiments of pyroxenites at high pressure, we attempt to identify the major element signature of pyroxenite partial melts and to evaluate to what extent this signature can be transmitted to the basalts erupted at oceanic islands and mid-ocean ridges. Although peridotite is the dominant source lithology in the Earth's upper mantle, the ubiquity of pyroxenites in mantle xenoliths and in ultramafic massifs, and the isotopic and trace elements variability of oceanic basalts suggest that these lithologies could significantly contribute to the generation of basaltic magmas. The question is how and to what degree the melting of pyroxenites can impact the major-element composition of oceanic basalts. The review of experimental phase equilibria of pyroxenites shows that the thermal divide, defined by the aluminous pyroxene plane, separates silica-excess pyroxenites (SE pyroxenites) on the right side and silica-deficient pyroxenites (SD pyroxenites) on the left side. It therefore controls the melting phase relations of pyroxenites at high pressure but, the pressure at which the thermal divide becomes effective, depends on the bulk composition; partial melt compositions of pyroxenites are strongly influenced by non-CMAS elements (especially FeO, TiO2, Na2O and K2O) and show a progressive transition from the liquids derived from the most silica-deficient compositions to those derived from the most silica-excess compositions. Another important aspect for the identification of source lithology is that, at identical pressure and temperature conditions, many pyroxenites produce melts that are quite similar to peridotite-derived melts, making the determination of the presence of pyroxenite in the source regions of oceanic basalts difficult; only pyroxenites able to produce melts with low SiO2 and high FeO contents can be identified on the basis of the major-element compositions of basalts. In the case of oceanic island basalts, high CaO/Al2O3 ratios can also reveal the presence of pyroxenite in the source-regions. Experimental and thermodynamical observations also suggest that the interactions between pyroxenite-derived melts and host peridotites play a crucial role in the genesis of oceanic basalts by generating a wide range of pyroxenites in the upper mantle: partial melting of such secondary pyroxenites is able to reproduce the features of primitive basalts, especially their high MgO contents, and to impart, at least in some cases, the major-element signature of the original pyroxenite melt to the oceanic basalts. Finally, we highlight that the fact the very silica depleted compositions (SiO2<42 wt.) and high TiO2 contents of some ocean island basalts seem to require the contribution of fluids (CO2 or H2O) through melting of either carbonated lithologies (peridotite or pyroxenite) or amphibole-rich veins. (C) 2012 Elsevier B.V. All rights reserved.

7. U10290
Martin, A. M.; Righter, K.; Treiman, A. H. Experimental constraints on the destabilization of basalt plus calcite plus anhydrite at high pressure-high temperature and implications for meteoroid impact modeling //EARTH AND PLANETARY SCIENCE LETTERS Volume: 331 Pages: 291-304 Published: MAY 15 2012

Calcite CaCO3 and anhydrite CaSO4 are two sedimentary components or alteration products of basalts on the Earth, Venus, and Mars. The fate of anhydrite-, calcite-bearing crust during a meteoroid impact must be addressed in order to evaluate: (1) the potential S- and C-gas release to the atmosphere, (2) the formation of S- and C-rich melts, and (3) the crystallization of S- and C-rich minerals which may be recognized by spectral analyses of planetary surfaces. We performed piston-cylinder experiments at 1 GPa, between 1200 and 1750 degrees C, on a mixture of 70 wt.% tholeiitic basalt + 15 wt.% anhydrite + 15 wt.% calcite. Up to similar to 1440 degrees C, an ultracalcic (CaO > 19.8 wt.%; CaO/Al2O3 > 1 wt.%) picrobasaltic (SiO2 similar to 39-43 wt.%; Na2O + K2O < 2 wt.%) melt containing up to 5.7 wt.% SO3 and up to 5.1 wt.% CO2 + H2O (calculated by difference) is present in equilibrium with fassaitic clinopyroxene, anhydrite, scapolite, chromian spinel and a gas composed mainly of CO and, occasionally, aliphatic thiols like CH3(CH2)(3)SH. Hydrogen was incorporated either by contact between the starting material and air or by diffusion through the capsule during the experiments. Above similar to 1440 degrees C, a CaO-rich (similar to 35 wt.%) sulfate-carbonate (SC) melt which contains 41-47 wt.% SO3, 7-12 wt.% CO2 + H2O and a few percent of Na2O, forms in equilibrium with the picrobasaltic melt. This study shows that a meteoroid impact onto an anhydrite- and calcite-bearing basaltic crust is likely to release CO gas to the atmosphere, while S is trapped in solid or liquid phases. Under hydrous conditions, however, the S/C in the gas may increase. The importance of the temperature parameter on the impact phase relations is also demonstrated. In particular, SC melt may form by meteoroid impact, and flow rapidly on a planetary surface. Physical modeling must therefore be combined with high P-high T phase diagrams of complex assemblages similar to planetary lithologies in order to evaluate the effects of a meteoroid impact. Published by Elsevier B.V.

8. U56487
McCollom, Thomas M.; Hynek, Brian M.; Rogers, Karyn; et al.Chemical and mineralogical trends during acid-sulfate alteration of pyroclastic basalt at Cerro Negro volcano and implications for early Mars // // JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS Volume: 118 Issue: 9 Pages: 1719-1751 Published: SEP 2013

Acid-sulfate alteration of pyroclastic basalts in active fumaroles at Cerro Negro volcano, Nicaragua, was studied as a means to infer the mineralogical and chemical consequences of basalt alteration in analogous environments on early Mars. At this site, recently erupted basaltic cinders are undergoing alteration by SO2-bearing steam. During alteration, silicate phenocrysts, including plagioclase, olivine, and augite, react much more rapidly than basaltic glass. Secondary mineralogy is dominated by a very limited number of phases that include amorphous silica, gypsum, Fe-bearing natroalunite, and Fe-oxides/oxyhydroxides, including hematite and magnetite. The major element chemistry of the deposits is controlled by two processes: gradual depletion of the major cations other than Si as the basalt components decompose and elements are mobilized out of the deposits, and enrichment in Ca and S from precipitation of gypsum, with Ca apparently supplied from sources below the surface. Reaction path models constrained by these observations but extrapolated to Martian conditions predict that alteration of pyroclastic deposits in similar environments on Mars should produce a secondary mineral assemblage that includes amorphous silica, Fe-bearing natroalunite, anhydrite, kieserite, and hematite. Iron-bearing natroalunite was found to produce a Mossbauer signal similar to that of jarosite, suggesting that this phase should be considered as an alternative to the jarosite component identified at Meridiani Planum. Spheroidal hematite formed in close association with natroalunite suggests a pathway for formation of hematite deposits on Mars.

9. U56487
McCollom, Thomas M.; Robbins, Mark; Moskowitz, Bruce; et al.Experimental study of acid-sulfate alteration of basalt and implications for sulfate deposits on Mars // JOURNAL OF GEOPHYSICAL RESEARCH-PLANETS Volume: 118 Issue: 4 Pages: 577-614 Published: APR 2013

Acid-sulfate alteration of basalt by SO2-bearing volcanic vapors has been proposed as one possible origin for sulfate-rich deposits on Mars. To better define mineralogical signatures of acid-sulfate alteration, laboratory experiments were performed to investigate alteration pathways and geochemical processes during reaction of basalt with sulfuric acid. Pyroclastic cinders composed of phenocrysts including plagioclase, olivine, and augite embedded in glass were reacted with sulfuric acid at 145 degrees C for up to 137 days at a range of fluid:rock ratios. During the experiments, the phenocrysts reacted rapidly to form secondary products, while the glass was unreactive. Major products included amorphous silica, anhydrite, and Fe-rich natroalunite, along with minor iron oxides/oxyhydroxides (probably hematite) and trace levels of other sulfates. At the lowest fluid:rock ratio, hexahydrite and an unidentified Fe-silicate phase also occurred as major products. Reaction-path models indicated that formation of the products required both slow dissolution of glass and kinetic inhibitions to precipitation of a number of minerals including phyllosilicates and other aluminosilicates as well as Al- and Fe-oxides/oxyhydroxides. Similar models performed for Martian basalt compositions predict that the initial stages of acid-sulfate alteration of pyroclastic deposits on Mars should result in formation of amorphous silica, anhydrite, Fe-bearing natroalunite, and kieserite, along with relict basaltic glass. In addition, analysis of the experimental products indicates that Fe-bearing natroalunite produces a Mossbauer spectrum closely resembling that of jarosite, suggesting that it should be considered an alternative to the component in sulfate-rich bedrocks at Meridiani Planum that has previously been identified as jarosite.

10. U60463
Nkouandou, Oumarou F.; Temdjim, Robert Petrology of spinel lherzolite xenoliths and host basaltic lava from Ngao Voglar volcano, Adamawa Massif (Cameroon Volcanic Line, West Africa): equilibrium conditions and mantle characteristics // JOURNAL OF GEOSCIENCES Volume: 56 Issue: 4 Pages: 375-387 Published: 2011

Spinel-bearing lherzolite xenoliths have been recovered from Mio-Pliocene alkaline basalt flows of the Ngao Voglar volcano, 35 km northwest of Ngaoundere in the Adamawa volcanic Massif (Cameroon). They have been examined to characterize the petrography, mineralogical composition, and equilibrium conditions of the upper mantle beneath the Ngaoundere region. The xenoliths exhibit protogranular textures and consist of four main minerals: olivine (Fo(89-90)), Mg-enstatite (En(89-91) Wo(1)Fs(8-10)), Cr-diopside (En(49-52)Wo(44-49)Fs(1.5-5)) and spinel (Mg#similar to 79.2, Cr#similar to 10.7). Thermobarometric calculations show equilibrium temperatures ranging between 850 and 950 degrees C and pressures of 8 to 17 kbar consistent with the spinel lherzolite stability field. These data suggest that the xenoliths come from a depth of 28-31km in the uppermost mantle situated just below a thinned crust; they are in agreement with the geophysical data previously determined in the Adamawa Massif. On the basis of these features, and considering the evidence for textural, mineralogical, and chemical equilibrium in the studied xenoliths prior to their entrainment in the host magma, we conclude that the source of these xenoliths was a chemically and petrographically homogeneous, spinel lherzolite lithospheric mantle. But the occurrence of mantle-derived xenoliths of various types (dunite, lherzolite, wehrlite, harzburgite, websterite, clinopyroxenite, and orthopyroxenite) in alkali basalts from many other localities of Cameroon (Oku Massif, Lake Nyos area and Mount Cameroon) is consistent with upper-mantle heterogeneities on a regional scale and implies that the nature of the upper mantle beneath the continental sector of the Cameroon Volcanic Line varies under its different volcanic centres.

11. U10290
Portnyagin, Maxim; Hoernle, Kaj; Storm, Sonja; et al. H2O-rich melt inclusions in fayalitic olivine from Hekla volcano: driving forces of explosive volcanism on Iceland// EARTH AND PLANETARY SCIENCE LETTERS Volume: 357 Pages: 337-346 Published: DEC 1 2012

Silicic Icelandic magmas are widely believed to contain low to moderate H2O content prior to degassing, and that their high explosivity mostly results from the interaction of the magmas with ice or meteoric water. Here we report the compositions of glass inclusions (SiO2=57-72 wt%, K2O=1.3-2.6 wt%) in Fe-rich olivines (Fo(2-42)) from the largest Holocene eruptions of Hekla volcano (H3 and H4) on Iceland, which preserved quenched melts with very high primary H2O contents (3.3-6.2 wt%). The silicic Hekla melts originate primarily by extensive (similar to 90%) crystal fractionation of H2O-poor (similar to 0.6 wt%) basalts and represent an end member in the systematics of terrestrial magmas because they originate at low fO(2) (Delta QFM similar to-0.1 to -0.4) and have as high H2O contents as significantly more oxidized island-arc magmas (Delta QFM >= 1). This demonstrates that H2O and Delta QFM do not correlate in silicic magmas from different tectonic settings, and that fO(2), not H2O content, shows a major difference between silicic ocean-island (e.g.. Icelandic) and island-arc magmas. Analysis of available experimental data suggests that high H2O activity and low fO(2) expand the field of olivine stability in silicic melts. Low fO(2) and low MgO content could also suppress crystallization of amphibole. On the basis of these results we propose that an anhydrous mineral assemblage bearing Fe-rich olivine in evolved volcanic and Skaergaard-type intrusive rocks does not imply low H2O in magmas prior to degassing but, in contrast to the commonly held view, is an indicator of H2O-rich silicic parental magmas crystallized at low fO(2). Finally, the high H2O content in magma was a major driving force of the largest explosive eruptions of Hekla volcano and must be at least as important for driving silicic explosive volcanism on Iceland as magma-ice interaction. (C) 2012 Elsevier B.V. All rights reserved.

12. U17586
Putirka, Keith; Ryerson, F. J.; Perfit, Michael; et al. Mineralogy and Composition of the Oceanic Mantle //JOURNAL OF PETROLOGY Volume: 52 Issue: 2 Pages: 279-313 Published: FEB 2011

The mineralogy of the oceanic basalt source region is examined by testing whether a peridotite mineralogy can yield observed whole-rock and olivine compositions from (1) the Hawaiian Islands, our type example of a mantle plume, and (2) the Siqueiros Transform, which provides primitive samples of normal mid-ocean ridge basalt. New olivine compositional data from phase 2 of the Hawaii Scientific Drilling Project (HSDP2) show that higher Ni-in-olivine at the Hawaiian Islands is due to higher temperatures (T) of melt generation and processing (by c. 300 degrees C) related to the Hawaiian mantle plume. Pm is low at high T, so parental Hawaiian basalts are enriched in NiO. When Hawaiian (picritic) parental magmas are transported to shallow depths, olivine precipitation occurs at lower temperatures, where D-Ni is high, leading to high Ni-in-olivine. Similarly, variations in Mn and Fe/Mn ratios in olivines are explained by contrasts in the temperatures of magma processing. Using the most mafic rocks to delimit Siqueiros and Hawaiian Co and Ni contents in parental magmas and mantle source compositions also shows that both suites can be derived from natural peridotites, but are inconsistent with partial melting of natural pyroxenites. Whole-rock compositions at Hawaii and Siqueiros are also matched by partial melting experiments conducted on peridotite bulk compositions. Hawaiian whole-rocks have elevated FeO contents compared with Siqueiros, which can be explained if Hawaiian parental magmas are generated from peridotite at 4-5 GPa, in contrast to pressures of slightly greater than 1 GPa for melt generation at Siqueiros; these pressures are consistent with olivine thermometry, as described in an earlier paper. SiO2-enriched Koolau compositions are reproduced if high-Fe Hawaiian parental magmas re-equilibrate at 1-1.5 GPa. Peridotite partial melts from experimental studies also reproduce the CaO and Al2O3 contents of Hawaiian (and Siqueiros) whole-rocks. Hawaiian magmas have TiO2 contents, however, that are enriched compared with melts from natural peridotites and magmas derived from the Siqueiros depleted mantle, and consequently may require an enriched source. TiO2 is not the only element that is enriched relative to melts of natural peridotites. Moderately incompatible elements, such as Ti, Zr, Hf, Gamma, and Eu, and compatible elements, such as Gamma b and Lu, are all enriched at the Hawaiian Islands. Such enrichments can be explained by adding 5-10% mid-ocean ridge basalt (crust) to depleted mantle; when the major element composition of such a mixture is recast into mineral components, the result is a fertile peridotite mineralogy.

13. U10290
Rollinson, Hugh Geochemical constraints on the composition of Archaean lower continental crust: Partial melting in the Lewisian granulites //EARTH AND PLANETARY SCIENCE LETTERS Volume: 351 Pages: 1-12 Published: OCT 15 2012

New geochemical data for the Lewisian granulites of NW Scotland show that the protolith to the neoArchaean Scourie granulites was a tonalite containing a mafic hydrous phase, most probably hornblende. This observation when combined with recent thermobarometric calculations and new partial melting studies on the Lewisian makes it very probable that the Lewisian granulites experienced fluid-absent melting. Partial melting calculations based upon a proxy for the unmelted granulite protolith demonstrate that Scourie granulite protolith was depleted in both LILE (Rb, Th, U) and HFSE (Ta) relative to unmelted amphibolite facies Lewisian gneisses of the same age. Both granulite facies and amphibolite facies gneisses are members of the tonalite-trondhjemite-granidiorite (TTG) magmatic suite. Nb/Ta ratios in the TTG granulite protolith are suprachondritic and can be attributed to the partial melting of a basaltic source with a rutile eclogite residue, whereas the amphibolite facies TTG gneisses possess Nb/Ta ratios which are subchondritic and can be attributed to the partial melting of a basaltic source with a garnet amphibolite residue. However, these differences do not account for the differences in LILE between the two suites implying that in addition the two basaltic precursors were different. It is proposed that the depleted character of the tonalitic protolith to the Scourie granulites was inherited from ultra-depleted basalt with very low concentrations of Rb, Th, U and Ta. Felsic melts formed during partial melting are no longer present in the granulite terrane and were probably removed to the upper crust, now removed by erosion. Thus partial melting of the Lewisian granulites contributed to the process of crustal fractionation. The process of fluid-absent melting in the Lewisian took place ca 200 Ma after crust formation. This long time interval indicates that the stabilisation and differentiation of the crust was probably a separate event from that of crust formation. (c) 2012 Elsevier B.V. All rights reserved.

14. U09363
Tsuno, Kyusei; Dasgupta, Rajdeep Melting phase relation of nominally anhydrous, carbonated pelitic-eclogite at 2.5-3.0 GPa and deep cycling of sedimentary carbon //CONTRIBUTIONS TO MINERALOGY AND PETROLOGY Volume: 161 Issue: 5 Pages: 743-763 Published: MAY 2011

We have experimentally investigated melting phase relation of a nominally anhydrous, carbonated pelitic eclogite (HPLC1) at 2.5 and 3.0 GPa at 900-1,350A degrees C in order to constrain the cycling of sedimentary carbon in subduction zones. The starting composition HPLC1 (with 5 wt% bulk CO2) is a model composition, on a water-free basis, and is aimed to represent a mixture of 10 wt% pelagic carbonate unit and 90 wt% hemipelagic mud unit that enter the Central American trench. Sub-solidus assemblage comprises clinopyroxene + garnet + K-feldspar + quartz/coesite + rutile + calcio-ankerite/ankerite(ss). Solidus temperature is at 900-950A degrees C at 2.5 GPa and at 900-1,000A degrees C at 3.0 GPa, and the near-solidus melt is K-rich granitic. Crystalline carbonates persist only 50-100A degrees C above the solidus and at temperatures above carbonate breakdown, carbon exists in the form of dissolved CO2 in silica-rich melts and as a vapor phase. The rhyodacitic to dacitic partial melt evolves from a K-rich composition at near-solidus condition to K-poor, and Na- and Ca-rich composition with increasing temperature. The low breakdown temperatures of crystalline carbonate in our study compared to those of recent studies on carbonated basaltic eclogite and peridotite owes to Fe-enrichment of carbonates in pelitic lithologies. However, the conditions of carbonate release in our study still remain higher than the modern depth-temperature trajectories of slab-mantle interface at sub-arc depths, suggesting that the release of sedimentary carbonates is unlikely in modern subduction zones. One possible scenario of carbonate release in modern subduction zones is the detachment and advection of sedimentary piles to hotter mantle wedge and consequent dissolution of carbonate in rhyodacitic partial melt. In the Paleo-NeoProterozoic Earth, on the other hand, the hotter slab-surface temperatures at subduction zones likely caused efficient liberation of carbon from subducting sedimentary carbonates. Deeply subducted carbonated sediments, similar to HPLC1, upon encountering a hotter mantle geotherm in the oceanic province can release carbon-bearing melts with high K2O, K2O/TiO2, and high silica, and can contribute to EM2-type ocean island basalts. Generation of EM2-type mantle end-member may also occur through metasomatism of mantle wedge by carbonated metapelite plume-derived partial melts.

15. U60656
Wang FuDong; Zhu XiaoQing; Wang ZhongGang.ZhongGang Madouzi-type (nodular) sedimentary copper deposit associated with the Emeishan basalt// SCIENCE CHINA-EARTH SCIENCES Volume: 54 Issue: 12 Pages: 1880-1891 Published: DEC 2011

Ore minerals in the sedimentary-type Cu deposits in the Xuanwei Formation overlying the Emeishan basalt are dominated by copper sulfides and native copper. As the ores mostly exhibit concretionary structure, previous researchers named them the "Madouzi-type Copper Deposit". Here the authors carried out mineralogical and isotopic studies on copper nodules in this ore deposit. The mineralogical study shows that copper nodules are composed of copper sulfides that have been cemented by ferruginous amorphous minerals, clay, and carbonaceous fragments in the modes of metasomatism and sedimentation. The nodules are preliminarily present as aggregates of gelatinous material. The isotopic analysis shows that the delta (13)C(PDB) values of anthraxolite are within the range of -24.8aEuro degrees aEuro"a'23.9aEuro degrees, indicating that the anthraxolite is the product of sedimentary metamorphism of in-situ plants. The delta (34)S(V-CDT) values of chalcocite are within the range of 7.6aEuro degrees aEuro"13.1aEuro degrees, close to those (about 11aEuro degrees) of Permian seawater. The delta (34)S(V-CDT) values of bornite and chalcopyrite are 21.6aEuro degrees aEuro"22.2aEuro degrees, similar to the sulfur isotopic composition (20aEuro degrees) of marine sulfate, indicative of different sources of sulfur. The above characteristics indicate that the copper nodules were formed in such a process that Cu-bearing basalt underwent weathering-leaching and copper-bearing material was transported into waters (e.g., rivers, lakes, and swamps) and then adsorbed on clay and ferruginous amorphous mineral fragments. Then, the copper-bearing material was suspended and transported in the form of gelinite. In lake or swamp environment, it was co-deposited with sediments to form copper nodules. At later stages there occurred metasomatism and hydrothermal superimposition, followed by the replacement of chalcocite by bornite and the superimposition of chalcopyrite over bornite, finally resulting in the formation of the "Madouzi-type" nodular copper deposit.

На главную К списку выставокАрхив выставок