Melting of Mantle Peridotite at Pressures Approaching the Spinel to Garnet Transition

Melting of the Earth's upper mantle beneath midocean ridge spreading centers is likely to be a dynamic, near-fractional process during which pressure (P), temperature (T), and source composition change as melting proceeds (Cooper and Kohlstedt, 1986; Johnson et al., 1990; Kinzler and Grove, 1992a,b; Langmuir et al., 1992). Well constrained melting models are required in order to use the compositions of mid-ocean ridge basalts to infer characteristics of the melting processes that lead to ocean ridge volcanism. These include the pressure range over which melting occurs, the extent to which the melting process approaches fractional melting; the geometry of the melting region, the role played by garnet in the source region, and the extent to which melts react with their surroundings as they migrate to the surface, etc. Current models for the generation of portions of the oceanic crust generally predict initial pressures of melting of 25-30 kbar (Klein and Langmuir, 1987, 1989; Kinzler and Grove, 1992b, Langmuir et al., 1992), however, the experimental data base available upon which to build full major element peridotite melting models has been limited by a lack of data at pressures greater than ~ 15 kbar. This abstract presents new experimental data relevant for melting of spinel lherzolite obtained over the pressure range of 15-25 kbar. A modified version of the method of Kinzler and Grove (1992a) is used to provide a preliminary parameterization of these data, combined with data from the literature.