4th Swiss Geoscience Meeting, Bern 2006 Experimental constrain on the origin of OIB: evidence for a metasomatized lithospheric source.

The source of alkali basalts erupted in oceanic island and continental alkali massif settings is commonly assumed to contain a component of recycling oceanic crust. High-pressure melting experiments on silica-deficient garnet clinopyroxenite compositions (formed by silica extraction from the oceanic crust during subduction or hybridization of MORB-eclogite with peridotite) produce ne-normative liquids that are close to alkali basalt compositions (Hirschmann et al., 2003), but are unable to reproduce low silica basanite compositions and the transitional compositions between basanite and alkali basalt that are observed in alkali massif worldwide (Keshav et al., 2004). Moreover, melting of oceanic crust seems unlikely to reproduce the high trace element content observe in OIBs. Here, we present the results of high-pressure melting experiments on natural amphibole-clinopyroxenite and hornblendite lithologies from the French Pyrenees (1.5 GPa, 1150–1350°C, Pt–graphite capsules) and explore the alternative hypothesis for the origin of OIBs — that these basalts result from the partial melting of recycled metasomatized continental or oceanic lithosphere. The compositions o