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1.
Petrogenesis of Ultramafic Rocks from the Ultrahigh-pressure Metamorphic Kimi Complex in Eastern Rhodope (NE Greece) 总被引：1，自引：0，他引：1
Widespread bodies of garnet–spinel metaperidotites withpyroxenitic layers occur in the ultrahigh-pressure metamorphicKimi Complex. In this study we address the origin of such peridotite–pyroxeniteassociations in the context of polybaric melting regimes. Weconduct a detailed geochemical investigation of major and traceelement relations and compare them with a range of major elementmodelling scenarios. With increasing bulk-rock MgO content,the garnet–spinel metaperidotites exhibit decreasing CaO,Al2O3, TiO2, and Na2O along with increasing Ni and a graduallyincreasing Zr/Zr* anomaly, consistent with an origin as residuesafter variable degrees of melt extraction. The major elementmodelling further suggests a polybaric adiabatic decompressionmelting regime beginning at high to ultrahigh pressure, withan intermediate character between pure batch and fractionalmelting and a mean extent of melting of 9–11%. The pyroxenitesexhibit major element compositions that cannot be reproducedby experimental or calculated melts of peridotite. Moreover,the Kimi pyroxenites have highly variable Ni and Sc contentsand a wide range of Mg-number (0· 76–0·89), inconsistent with an origin as frozen melts or the productsof melt–peridotite interaction. However, both the majorelement systematics and the observed rare earth element patterns,with both convex and concave shapes, can be explained by anorigin as clinopyroxene-rich, high-pressure cumulates involvinggarnet and/or Cr-spinel. KEY WORDS: peridotite; pyroxenite; partial melting; UHP metamorphism; cumulate 相似文献
2.
Paul D Asimow 《Geochimica et cosmochimica acta》2004,68(9):1965-1966
3.
Chi Ma Jinping Hu Martin D. Suttle Yunbin Guan Thomas G. Sharp Paul D. Asimow Paul J. Steinhardt Luca Bindi 《Meteoritics & planetary science》2023,58(11):1642-1653
A recently described micrometeorite from the Nubian desert (Sudan) contains an exotic Al-Cu-Fe assemblage closely resembling that observed in the Khatyrka chondrite (Suttle et al., 2019; Science Reports 9:12426). We here extend previous investigations of the geochemical, mineralogical, and petrographic characteristics of the Sudan spherule by measuring oxygen isotope ratios in the silicate components and by nano-scale transmission electron microscopy study of a focused ion beam foil that samples the contact between Al-Cu alloys and silicates. O-isotope work indicates an affinity to either OC or CR chondrites, while ruling out a CO or CM precursor. When combined with petrographic evidence we conclude that a CR chondrite parentage is the most likely origin for this micrometeorite. SEM and TEM studies reveal that the Al-Cu alloys mainly consist of Al metal, stolperite (CuAl), and khatyrkite (CuAl2) together with inclusions in stolperite of a new nanometric, still unknown Al-Cu phase with a likely nominal Cu3Al2 stoichiometry. At the interface between the alloy assemblage and the surrounding silicate, there is a thin layer (200 nm) of almost pure MgAl2O4 spinel along with well-defined and almost perfectly spherical metallic droplets, predominantly iron in composition. The study yields additional evidence that Al-Cu alloys, the likely precursors to quasicrystals in Khatyrka, occur naturally. Moreover, it implies the existence of multiple pathways leading to the association in reduced form of these two elements, one highly lithophile and the other strongly chalcophile. 相似文献
4.
An experimental procedure has been developed that permits measurement of the partitioning of Ar and He between crystal interiors and the intergranular medium (ITM) that surrounds them in synthetic melt-free polycrystalline diopside aggregates. 37Ar and 4He are introduced into the samples via neutron irradiation. As samples are crystallized under sub-solidus conditions from a pure diopside glass in a piston cylinder apparatus, noble gases diffusively equilibrate between the evolving crystal and intergranular reservoirs. After equilibration, ITM Ar and He is distinguished from that incorporated within the crystals by means of step heating analysis. An apparent equilibrium state (i.e., constant partitioning) is reached after about 20 h in the 1450 °C experiments. Data for longer durations show a systematic trend of decreasing ITM Ar (and He) with decreasing grain boundary (GB) interfacial area as would be predicted for partitioning controlled by the network of planar grain boundaries (as opposed to ITM gases distributed in discrete micro-bubbles or melt). These data yield values of GB-area-normalized partitioning, , with units of (Ar/m3 of solid)/(Ar/m2 of GB) of 6.8 × 103-2.4 × 104 m−1. Combined petrographic microscope, SEM, and limited TEM observation showed no evidence that a residual glass phase or grain boundary micro-bubbles dominated the ITM, though they may represent minor components. If a nominal GB thickness (δ) is assumed, and if the density of crystals and the grain boundaries are assumed equal, then a true grain boundary partition coefficient , may be determined. For reasonable values of δ, is at least an order of magnitude lower than the Ar partition coefficient between diopside and melt. Helium partitioning data provide a less robust constraint with between 4 × 103 and 4 × 104 cm−1, similar to the Ar partitioning data. These data suggest that an ITM consisting of nominally melt free, bubble free, tight grain boundaries can constitute a significant but not infinite reservoir, and therefore bulk transport pathway, for noble gases in fine grained portions of the crust and mantle where aqueous or melt fluids are non-wetting and of very low abundance (i.e., <0.1% fluid). Heterogeneities in grain size within dry equilibrated systems will correspond to significant differences in bulk rock noble gas content. 相似文献
5.
We present new data for the Neoproterozoic mafic intrusion exposed in Wadi Nasb, south Sinai, Egypt (northernmost Arabian–Nubian Shield; ANS). The Nasb mafic intrusion (NMI) intrudes metasediments, Rutig volcanics, and diorite/granodiorite, and is intruded in turn by younger monzogranite and quartz-monzonite. Available geochronological data for the country rocks of the NMI provide a tight constraint on its age, between 619 and 610 Ma, during the hiatus between the lower and upper Rutig volcanics. The NMI is neither deformed nor metamorphosed, indicating post-collisional emplacement, and uralitization by late-magmatic and sub-solidus alteration is restricted to the margins of the intrusion. A quantitative fractionation model indicates a fractionating assemblage of 61% primary amphibole, 10% clinopyroxene, 28% plagioclase, 1% biotite, 0.4% apatite, and 0.15% Fe-Ti oxide. Contrary to the recent studies, we find that the nearby diorite of Gebel Sheikh El-Arab is not co-genetic with the appinitic gabbro of the NMI. Although there are volcanic xenoliths in the NMI, we find no chemical evidence requiring contamination by continental crust. A subduction-related signature in a post-orogenic intrusion requires the inheritance of geochemical tendencies from a previous subduction phase. Given that the fine-grained gabbro of the NMI is consistent with a near-primary mantle melt, we attribute this inheritance to persistence and later melting of the slab-modified mantle domains, as opposed to partial melting and assimilation of the juvenile continental crust. The fine-grained gabbro composition indicates derivation at temperature and pressure conditions similar to the sources of mid-ocean ridge basalts: mantle potential temperature near 1350°C and extent of melting about 7%. Such temperatures, neither so high as to require a plume nor so low as to be consistent with small degrees of melting of a volatile-rich source, are most consistent with a lithospheric delamination scenario, allowing the upwelling of fertile, subduction-modified asthenosphere to depths ≤50 km. 相似文献
6.
Emily A. Hamecher Paula M. Antoshechkina Mark S. Ghiorso Paul D. Asimow 《Contributions to Mineralogy and Petrology》2013,165(1):25-43
We define and calibrate a new model of molar volume as a function of pressure, temperature, ordering state, and composition for spinels in the supersystem (Mg, Fe2+)(Al, Cr, Fe3+)2O4 ? (Mg, Fe2+)2TiO4. We use 832 X-ray and neutron diffraction measurements performed on spinels at ambient and in situ high-P, T conditions to calibrate end-member equations of state and an excess volume model for this system. The effect on molar volume of cation ordering over the octahedral and tetrahedral sites is captured with linear dependence on Mg2+, Al3+, and Fe3+ site occupancy terms. We allow standard-state volumes and coefficients of thermal expansion of the end members to vary within their uncertainties during extraction of the mixing properties, in order to achieve the best fit. Published equations of state of the various spinel end members are analyzed to obtain optimal values of the bulk modulus and its pressure derivative, for each explicit end member. For any spinel composition in the supersystem, the model molar volume is obtained by adding excess volume and cation order-dependent terms to a linear combination of the five end-member volumes, estimated at pressure and temperature using the high-T Vinet equation of state. The preferred model has a total of 9 excess volume and order-dependent parameters and fits nearly all experiments to within 0.02 J/bar/mol, or better than 0.5 % in volume. The model is compared to the current MELTS spinel model with a demonstration of the impact of the model difference on the estimated spinel-garnet lherzolite transition pressure. 相似文献
7.
The GyPSM-S (Geodynamic and Petrological Synthesis Model for Subduction) scheme couples a petrological model with a 2-D thermal and variable viscosity flow model to describe and compare fundamental processes occurring within the subduction mantle wedge, including the development of a low-viscosity channel (LVC) (Hebert et al., 2009, Earth and Planetary Science Letters, v. 278, p. 243–256). Here we supplement the basic coupled model result with more sophisticated treatments of trace element partitioning in the fluid phase and melt transport regimes. We investigate the influences of slab fluid source lithology and fluid transport mechanisms on melt geochemistry, the implications of mantle source depletion related to fluid fluxing, and potential melt migration processes. This study describes two model cases that can be compared to geochemical datasets for the Izu–Bonin intra-oceanic subduction system and the Central Costa Rican part of the Central American arc. We find that there is a progression of geochemical characteristics described in studies of cross-arc and along-arc lavas that can be approximated assuming (i) limited fluid–rock interaction within the mantle wedge and (ii) that melt migration preserves the spatial distinction among melts initiated in different areas of the wedge. Specifically, volcanic front lavas have significant contributions from shallower slab fluid sources, and rear-arc lavas have significant contributions from deeper slab fluid sources. Evidence for limited fluid–rock interaction could imply either a rapid fluid transport mechanism or a fluid-dominated trace element budget within the LVC. Although we do not include a back-arc in these models, interpretations of the results lead to several potential mechanisms to explain hydrous inputs to back-arc source regions. 相似文献
8.
The whole-rock chemistry of eclogites, partially amphibolitized eclogites, and dyke amphibolites from the metamorphic Kechros complex in the eastern Rhodope Mountains preserves evidence of the geodynamic framework for the origin of their protoliths. Major and trace-element concentrations define two distinct protolith groups for the eclogites. The low-Fe–Ti (LFT) eclogites have low-TiO2 content (<0.67 wt%), negative high field strength element anomalies, and variable enrichments in large ion lithophile elements (LILE). The rare earth element (REE) patterns are characterized by strong light-REE (LREE) enrichment and heavy-REE (HREE) depletion. The high-Fe–Ti (HFT) eclogites have small to moderate LILE enrichment and lack Nb anomalies. The REE patterns of the HFT eclogites are characterized by LREE depletion and relatively flat MREE–HREE patterns. The rock compositions and petrographic features of the LFT eclogites resemble gabbros formed in a continental rift environment with minor to moderate contamination of a mantle-derived mafic magma by continental crust, whereas the HFT eclogites resemble mafic rocks formed in extensional oceanic environments. We interpret the HFT suite to represent a later stage in an evolution from continental rift to open ocean, following the origin of the LFT suite. Dyke amphibolite compositions, except for probable SiO2 loss associated with metamorphic dehydration reactions, appear to represent liquid compositions quenched in conduits through the lower crust. MELTS modeling shows that dyke amphibolite compositions can be related to each other by fractional crystallization under strongly oxidizing conditions at ~0.5 GPa pressure, and all can be derived from a low-degree melt of modified fertile peridotite from around 1.7 GPa. Cumulates crystallized from the parental liquids of the amphibolites under oxidizing conditions may have yielded the protoliths of the HFT suite. 相似文献
9.
I.?BaziotisEmail authorView author&#;s OrcID profile P.?D.?Asimow T.?Ntaflos J.?W.?Boyce F.?M.?McCubbin A.?Koroneos D.?Perugini S.?Flude M.?Storey Y.?S.?Liu S.?Klemme J.?Berndt 《Contributions to Mineralogy and Petrology》2017,172(7):58
Composite mantle xenoliths from the Cima Volcanic Field (CA, USA) contain glassy veins that cross-cut lithologic layering and preserve evidence of lithospheric melt infiltration events. Compositions and textures of minerals and glasses from these veins have the potential to place constraints on the rates and extents of reaction during infiltration. We studied glass-bearing regions of two previously undescribed composite xenoliths, including optical petrography and chemical analysis for major and trace elements by electron probe microanalysis and laser-ablation inductively coupled plasma mass spectrometry. The petrogenetic history of each vein involves melt intrusion, cooling accompanied by both wall-rock reaction and crystallization, quench of melt to a glass, and possibly later modifications. Exotic secondary olivine crystals in the veins display concentric phosphorus (P)-rich zoning, P-rich glass inclusions, and zoning of rapidly diffusing elements (e.g., Li) that we interpret as records of rapid disequilibrium events and cooling rates on the order of 10 °C/h. Nevertheless, thermodynamic modeling of the diversity of glass compositions recorded in one of the samples demonstrates extensive reaction with Mg-rich olivine from the matrix before final quench. Our results serve as a case study of methods for interpreting the rates and processes of lithospheric melt-rock reactions in many continental and oceanic environments. 相似文献
10.
Hisham Gahlan Mokhles Azer Paul Asimow Khaled Al-Kahtany 《Arabian Journal of Geosciences》2016,9(12):603
The Abu Rumeil syenitic rocks represent the inner ring dyke of the Katherina Ring complex, southern Sinai, Egypt. They are divided petrologically into two types, alkali feldspar syenite and quartz syenite. The mineralogy and geochemistry of the syenites indicate an alkaline nature with a shoshonitic affinity. Although rare mafic xenocrysts overgrown by primary K-feldspars and overlapping rare earth element (REE) patterns indicate some role for crustal contamination, the trace element chemistry shows a dominant mantle contribution. The geochronology and field relations imply that the Abu Rumeil syenites were emplaced in a post-collisional, within-plate tectonic setting, yet they express the enrichments in large-ion lithophile elements relative to high field strength elements generally characteristic of subduction influence. We suggest that this signature is inherited from partial melting of a lithospheric mantle source previously affected by subduction during assembly of the Arabian-Nubian Shield. Little evidence of the early evolution of the suite is preserved; there are no associated mafic rocks. We therefore restrict our attention to a petrogenetic model that can explain the relations among the observed felsic composition. The REE patterns of all samples are enriched in light REE and fractionated, but it is notable that there are small positive Eu anomalies in the alkali-feldspar syenites contrasting with small negative Eu anomalies in the quartz syenites. Positive Eu anomalies suggest a cumulate nature for the alkali-feldspar syenites; there are also breaks in the slopes of most variation trends between the alkali-feldspar syenites and the quartz syenites. The general trends in all major oxides and trace elements within the suite can be modeled by fractional crystallization of feldspars—with smaller roles for pyroxene, biotite, apatite, and Fe-Ti oxides—from an intermediate liquid to form the quartz syenites and by assimilation of the near-liquidus phases into the same starting liquid to form the alkali feldspar syenites. The geothermobarometry of pyroxenes and amphiboles suggests shallow emplacement (<10 km depth) and crystallization temperatures ranging from 1100 °C down to 800 °C. 相似文献