Polymineralic inclusions in eclogite-facies metapelites from Tanzania – insights on partial melting and geodynamic processes in a 2 Ga old subduction zone

高变质岩中的多矿物包裹体在熔体组成、流体状态和地球动力学背景等方面对部分熔体的形成有新的认识。迄今为止所报道的关于多矿物包裹体的研究要么发生在无水性地壳岩石和麻粒岩中，要么发生在俯冲大陆地壳的岩石中。相比之下，坦桑尼亚Usagaran带榴辉岩相变长岩代表了俯冲洋壳的一部分，为研究大洋岩石的部分熔融过程提供了难得的机会。这些变长岩中的多矿物包裹体是独特的，因为硅质云母矿物组合出现在碳质包裹体旁边，表明存在以二氧化碳为主的COH流体，可能是熔体和/或流体的分离形成过程。此外，研究区域提供了一个特殊的机会来研究已知最古老的俯冲带之一（2 Ga），结果可能会揭示早期地球历史上的地热梯度。建议的研究重点应集中在：(1)最古老的俯冲带之一的熔体形成的时间和过程；(2)产生的熔体的组成以及CO2存在的影响。样品将通过光学显微镜、电子探针、拉曼光谱和LA-ICP-MS进行分析。此外，通过活塞缸重熔实验对多矿物包裹体进行了再均匀化。

Polymineralic inclusions in high-grade metamorphic rocks are a promising tool to obtain new insights on the formation of partial melts with respect to melt composition, fluid regime and geodynamic setting. All studies on polymineralic inclusions reported so far occur either in anatectic crustal rocks and granulites or in rocks from subducted continental crust.By contrast, the eclogite-facies metapelites of the Usagaran belt in Tanzania represent a part of the subducted oceanic crust, providing the rare opportunity to investigate the process of partial melting in oceanic rocks. Polymineralic inclusions in these metapelites are distinct, because siliceous mica-bearing mineral assemblages occur beside carbonaceous ones indicating the presence of a CO2-dominated COH fluid, with unmixing of melt and/or fluid as a possible formation process. Furthermore, the study area provides the exceptional opportunity to investigate one of the oldest known subduction zones (2 Ga) and the results might shed light on the geothermal gradients in the early Earth’s history.The proposed research shall focus on: (1) Timing and process of melt formation in one of the oldest ancient subduction zones, (2) composition of the melts produced and the influence of the presence of CO2. The samples will be analyzed by means of optical microscopy, electron microprobe, Raman spectroscopy and LA-ICP-MS. Furthermore, the polymineralic inclusions will be rehomogenized by remelting experiments with a piston cylinder.