Generation of felsic melts within fast-spreading oceanic crust: Experimental partial melting of hydrothermally altered sheeted dike

In recent oceanic crust and in ophiolites, felsic lithologies are observed. Different processes, like fractional crystallization of MORB and partial melting of mafic rocks are discussed to form these lithologies. Partial melting is expected as a major process in forming felsic lithologies at the base of the sheeted dike complex of fast-spreading ridges, where the axial melt lens is assumed to be located directly beneath the sheeted dikes.It is widely accepted that this melt lens has the potential to trigger partial melting of mafic lithologies at the gabbro/dike transition zone. In this experimental study, the influence of partial melting on the generation of felsic lithologies is examined. Therefore, partial melting experiments at a pressure of 100 MPa were performed. As starting material, a natural basalt from the IODP (Integrated Ocean Drilling Program) drilling at Site 1256 (equatorial East Pacific Rise) was chosen, which is representative for the lower sheeted dike complex. It is characterized as a moderately altered dolerite containing plagioclase (An_50-57), clinopyroxene (Mg# 0.55-0.60) and quartz, with chlorite as secondary phase; sulfides and Fe-Ti-oxides are present as accessory minerals. The partial melting experiments were conducted in an H₂-controlled IHPV at the Institute of Mineralogy in Hanover, Germany. To investigate the evolution of the partial melts, different experiments were performed at temperatures between 1030°C and 910°C and a constant pressure of 100 MPa. All experiments were water saturated leading to a fO₂ corresponding to QFM +1 (QFM = quartz-fayalite-magnetite oxygen buffer). This is slightly more oxidized than MORB crystallization due to the influence of a hydrous fluid which generally increases the oxygen activity. The experimental products were analyzed using electron microprobe for major elements, and a SIMS (CRPG Nancy, France) for trace elements. We present here our first results on phase relations and mineral compositions. The analyzed melts vary in their SiO₂ contents between ~50wt% (1030°C) and ~71wt% (910°C) and can be classified as basaltic respectively dacitic. The residual mineral assemblage in the experiments performed at 1030°C contains olivine + plagioclase + clinopyroxene. At lower temperatures, orthopyroxene crystallizes at the expense of olivine, and Fe-Ti-oxides are stabilized. A comparison with corresponding naturally formed lithologies shows similarities both in terms of melt composition and residual phase compositions in those experimental runs performed at lower temperature. First trace element analyses of the experimental melts reveal similar trace element patterns known from naturally formed felsic lithologies recorded in the gabbro/dike transition zone of fast-spread oceanic crust.