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

高级变质岩中的多矿物包裹体是从熔体成分、流体机制和地球动力学环境等方面研究部分熔体形成的一种有前景的工具。迄今为止，已报道的多矿物包裹体研究要么发生在深熔壳岩和麻粒岩中，要么发生在俯冲陆壳的岩石中。相比之下，坦桑尼亚Usagaran带的榴辉岩相变质岩代表了俯冲洋壳的一部分，为研究洋岩的部分熔融过程提供了难得的机会。这些变质岩中的多矿物包裹体是独特的，这是因为含硅云母矿物组合赋存于碳质矿物组合旁边，表明存在以CO2为主的COH流体，熔体和/或流体的解混可能是形成过程。此外，该研究区域提供了研究已知最古老的俯冲带之一(2Ga)的难得机会，其结果可能有助于揭示地球早期历史上的地温梯度。拟议的研究将集中在：(1)最古老的俯冲带之一的熔体形成时间和过程，(2)产生的熔体的组成和二氧化碳的存在的影响。样品将通过光学显微镜、电子探针、拉曼光谱和LA-ICPMS进行分析。此外，多矿物包裹体将通过活塞筒重熔实验进行重均一化。

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.