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Experimental Constraints on the Role of Garnet Pyroxenite in the Genesis of High-Fe Mantle Plume Derived Melts 总被引：10，自引：1，他引：9
The anhydrous phase relations of an uncontaminated (primitive),ferropicrite lava from the base of the Early Cretaceous Paraná–Etendekacontinental flood basalt province have been determined between1 atm and 7 GPa. The sample has high contents of MgO (14·9wt %), FeO* (14·9 wt %) and Ni (660 ppm). Olivine phenocrystshave maximum Fo contents of 85 and are in equilibrium with thebulk rock, assuming a of 0·32. A comparison of our results with previous experimental studiesof high-Mg rocks shows that the high FeO content of the ferropicritecauses an expansion of the liquidus crystallization field ofgarnet and clinopyroxene relative to olivine; orthopyroxenewas not observed in any of our experiments. The high FeO contentalso decreases solidus temperatures. Phase relations indicatethat the ferropicrite melt last equilibrated either at 2·2GPa with an olivine–clinopyroxene residue, or at 5 GPawith a garnet–clinopyroxene residue. The low bulk-rockAl2O3 content (9 wt %) and high [Gd/Yb]n ratio (3·1)are consistent with the presence of residual garnet in the ferropicritemelt source and favour high-pressure melting of a garnet pyroxenitesource. The garnet pyroxenite may represent subducted oceaniclithosphere entrained by the upwelling Tristan starting mantleplume head. During adiabatic decompression, intersection ofthe garnet pyroxenite solidus at 5 GPa would occur at a mantlepotential temperature of 1550°C and yield a ferropicriteprimary magma. Subsequent melting of the surrounding peridotiteat 4·5 GPa may be restricted by the thickness of theoverlying sub-continental lithosphere, such that dilution ofthe garnet pyroxenite melt component would be significantlyless than in intra-oceanic plate settings (where the lithosphereis thinner). This model may explain the limited occurrence offerropicrites at the base of continental flood basalt sequencesand their apparent absence in ocean-island basalt successions. KEY WORDS: continental flood basalt; ferropicrite; mantle heterogeneity; mantle melting; phase relations; pyroxenite 相似文献
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介绍一个产生玄武岩的模型 总被引：1，自引：0，他引：1
地幔柱存在的一个主要证据是大规模高熔玄武岩省的出现,而且多认为玄武岩的来源依赖于地幔柱从下地幔输送。Michele Lustrino研究了造山时下地壳和岩石圈地幔的拆沉和拆离作用,提出了产生玄武岩的一个新模型。该模型认为即使地幔柱不存在,拆沉到地幔的下地壳物质再循环同样可以解释小规模的板内(大洋岛弧和大陆内部)火山岩和大洋、大陆溢流玄武岩及洋中脊玄武岩的生成及其常见的几种地球化学特征。在陆-陆碰撞过程中,下地壳中的变质反应生成石榴石,导致岩石的密度增大,致使过厚岩石圈底部(下地壳和岩石圈地幔)和上地壳分离并沉入上地幔。下地壳发生部分熔融形成富SiO2的熔体,和上涌的软流圈地幔(充填在下沉的岩石圈地幔和下地壳的空间)发生变质交代反应,导致具有强烈的地壳特点的富含斜方辉石层的形成。这个变质交代地幔体可以在拆沉后保持不变长达几个百万年。这种源的部分熔体可以保有下地壳的明显特征,产生类似富集地幔1型玄武岩浆作用。因此,该模型是提供了玄武岩浆来源的一个新选择。 相似文献
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The Qilin geotherm established by Xu et al. [Xu, X.S., O'Reilly, S.Y., Zhou, X.M. and Griffin, W.L., 1996. A xenolith-derived geotherm and the crust-mantle boundary at Qilin, southeastern China. Lithos, 38: 41–62.] using the Ellis and Green [Ellis D.J. and Green D.H., 1979. An experimental study of the effect of Ca upon garnet-clinopyroxene Fe-Mg exchange equilibria. Contrib. Mineral. Petrol., 71: 13–22]/Wood [Wood B.J., 1974. Solubility of alumina in orthopyroxene coexisting with garnet. Contrib. Mineral. Petrol. 46: 1-15] combination is in need of revision on the basis of re-evaluation of geothermobarometers, comparison of calculated pressures and temperatures with experimentally determined phase relationships and geological/geophysical data. The invalid assumption that all iron is present as Fe2+ in minerals, and the thermal destruction of equilibrium Fe–Mg exchange between clinopyroxene and garnet that may have resulted from heating of the Qilin xenoliths by the host magma resulted in unrealistically high temperatures estimated by the Ellis and Green's thermometer. An important implication arising from this study is that care must be taken when applying thermobarometers to Fe-rich pyroxenites for the purpose of geotherm construction and a comprehensive analysis is often required. 相似文献