Petrogenesis of TTG veins in oceanic gabbro: constraints from partial melting experiments in the presence of NaCl-rich H2O-CO2 fluids

在蛇绿岩杂岩和新近形成的洋壳中，广泛报道了长英质至长英质火成岩（文献中称为斜长花岗岩）。其成因机制包括后期分化的父母MORB熔体和部分熔融的辉长岩富水流体触发。最后一种机制已经得到了现实的实验证实（Koepke等人，2004年），尽管这些流体的性质仍在争论中（岩浆或热液海水衍生）。目前的实验项目的目的是有助于更好地了解富NaCl流体相的作用，在辉长岩部分熔融过程中产生的硅酸盐熔体的主要元素组成。此外，计划阐明NaCl对部分熔体、固相和低压下含Cl-H2O-CO2流体之间微量元素分配的作用。这些试验性部分熔融实验将在200 MPa、900-1000°C下进行，使用来自IODP站点U1309、远征304/305（MAR 30° 10.12 'N，42° 07.11' W，Atlantis地块中央圆顶）的典型辉长岩作为起始材料。对海洋地壳内由富含水和氯的流体驱动的深熔作用的研究也有可能揭示早期地球历史上第一个大陆地壳的形成，因为正如最近提出的那样（Rollinson，2008），在Haedean期间可能发生了类似的镁铁质原岩的含水熔融，产生了少量的长英质岩石。

Intermediate to felsic igneous rocks (referred to as plagiogranites in the literature) are widely reported in ophiolite complexes and from recently formed oceanic crust. The mechanisms proposed for their genesis include late-stage differentiation of a parental MORB melt and partial melting of gabbroic rocks triggered by water-rich fluids. This last mechanism has received a realistic experimental confirmation (Koepke et al., 2004) although the nature of these fluids is still under debate (magmatic or hydrothermal sea-water derived). The current experimental project is aimed to contribute to the better understanding of the role of NaCI-rich fluid phase on the major element composition of silicate melts generated during partial melting of gabbro. In addition, it is planned to elucidate the role of NaCI on trace element partitioning between partial melts, solid phases and CI-H2O-CO2-bearing fluids at low pressures. These pilot partial melting experiments will be performed at 200 MPa, 900-1000°C using a typical gabbro from the IODP site U1309, Expedition 304/305 (MAR 30°10.12'N, 42°07.11'W, central dome of Atlantis Massif) as starting material. The investigation of deep anatexis within the ocean crust driven by water- and Cl-rich fluids has also the potential to shed light on the formation of the first continental crust in the early Earth history, since as was suggested recently (Rollinson, 2008) similar hydrous melting of a mafic protolith may have operated during Haedean, to create small volumes of felsic rocks.