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The phase relations of mafic rocks within the Earths mantle and geobarometers for eclogite and pyroxenite rocks to conditions of the Earths transition zone.

地幔内镁铁质岩石的相关系以及榴辉岩和辉石岩岩石的地质气压计与地球过渡带条件的关系。

基本信息

批准号：
264531645
负责人：
Professor Dr. Daniel J. Frost
金额：
--
依托单位：
Bayerisches Forschungsinstitut für Experimentelle Geochemie und Geophysik (BGI)
依托单位国家：
德国
项目类别：
Research Grants
财政年份：
2014
资助国家：
德国
起止时间：
2013-12-31 至 2015-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/264531645?language=en
关键词：
phase relations mafic rocks within

项目摘要

While the bulk of the Earths upper mantle is considered to be comprised of olivine dominated ultramafic rocks, there is evidence from mantle xenoliths and inclusions in diamonds for the presence of mafic rock compositions such as eclogites and pyroxenites. Eclogites are likely present in the mantle as a result of subduciton of oceanic crust, however, many pyroxenite rocks may also be present as a result of melting or metasomatic processes occurring in the mantle. As they provide evidence for mantle processes, studying these rocks and interpreting their depth of origin and also their influence on mantle seismic properties is of great importance. Some studies on seismic properties have even proposed that the convecting mantle may comprise a mechanical mixture of ultramafic and mafic rocks. In this study the phase relations of such eclogitic/pyroxenitic rocks will be studied up to conditions of the Earths transition zone, and mainly in the range where these rocks are composed principally of clinopyroxene and garnet. Using these relations, two geobarometers will be calibrated; one that employs the proportion of Al hosted in the clinopyroxene Si-site coexisting with garnet and the other that employs the majorite-garnet transition within such mafic assemblages. Due to the high chemical variance and small number of phases, the mineral compositions within such assemblages are strongly dependent on bulk compositions. In this study phase relations over a wide range of plausible eclogite and pyroxenite compositions will be studied and to maintain internal consistency multiple sample chambers will be employed so many bulk compositions will be equilibrated at the same pressure and temperature simultaneously. In addition further experiments will be performed to asses the effects of oxygen fugacity and H2O on these barometer equilibria. These barometers are essential for determining the depth of formation of mineral inclusions in diamonds. It is increasingly recognised that many such inclusions are formed at sublithospheric conditions in the Earths convecting mantle. This growing field of diamond study presents a remarkable opportunity to investigate processes in the deep mantle. Using the results the conditions of origin of a range of diamond inclusions and xenoliths will be investigated. In addition by combining a thermodynamic model for mafic rock phase relations with elasticity data, the seismic velocity of subducted oceanic crust compositions will be calculated and compared with seismic observations.

虽然地球上地幔的大部分被认为是由橄榄石为主的超镁铁质岩石组成的，但从地幔包体和钻石包裹体中有证据表明存在镁铁质岩石成分，如榴辉岩和辉石岩。榴辉岩可能是由于海洋地壳的俯冲作用而存在于地幔中，然而，许多辉石岩也可能是由于地幔中发生的熔融或交代作用而存在的。由于它们为地幔过程提供了证据，研究这些岩石并解释它们的起源深度及其对地幔地震性质的影响是非常重要的。一些关于地震性质的研究甚至提出，对流地幔可能由超基性岩和基性岩的机械混合物组成。在本研究中，这类辉生岩/辉生岩的相关系将研究到地球过渡带的条件，主要是在这些岩石主要由斜辉石和石榴石组成的范围内。利用这些关系，将校准两个地球气压计；一种是利用斜辉石硅位中与石榴石共存的铝的比例，另一种是在这种基性组合中利用多数-石榴石过渡。由于化学变化大，相数少，这种组合中的矿物组成强烈依赖于体积组成。在本研究中，将研究广泛的似是而非的榴辉岩和辉石岩组成的相关系，为了保持内部一致性，将使用多个样品室，因此将在相同的压力和温度下同时平衡许多大块组成。此外，还将进行进一步的实验来评估氧逸度和H2O对这些气压计平衡的影响。这些气压计对于确定钻石中矿物包裹体的形成深度是必不可少的。人们越来越认识到，许多这样的包裹体是在地球对流地幔的岩石圈下形成的。钻石研究的这一不断发展的领域为研究深部地幔的过程提供了一个绝佳的机会。利用这些结果，将研究一系列金刚石包裹体和捕虏体的起源条件。此外，将基性岩相关系热力学模型与弹性资料相结合，计算俯冲洋壳成分的地震速度，并与地震观测结果进行比较。