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

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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.