EXPERIMENTAL TESTS OF LOW DEGREE PERIDOTITE PARTIAL MELT COMPOSITIONS: IMPLICATIONS FOR THE NATURE OF ANHYDROUS NEAR-SOLIDUS PERIDOTITE MELTS AT 1 GPA

Abstract

We present results of an experimental study to determine the nature of minimum to near-minimum melt compositions in equilibrium with upper mantle peridotite mineralogy at 1 GPa. We confirm earlier conclusions that anhydrous melts of lherzolite at 1 GPa are basaltic with ~15-20% normative diopside, >10% normative olivine and at low degrees of melting are Na2O and K2O-rich and nepheline-normative in `fertile' mantle. The most extreme Na2O-rich minimum melt composition is in equilibrium with an albite-bearing harzburgite residue at 1220°C. This melt composition is nepheline-normative with ~64% SiO2 and about ~12% Na2O. Our results disagree with recent reports that peridotitic minimum melt compositions have an `andesitic' character at 1 GPa. We present reversal experiments showing that these latter melts are not in equilibrium with a spinel or plagioclase lherzolite upper mantle assemblage. We use our new data and data from the literature to define minimum melts (i.e. melts in equilibrium with olivine+orthopyroxene+clinopyroxene+plagioclase+/-spinel) for fertile or enriched to refractory lherzolite at 1 GPa. The minimum melt compositions are nepheline+olivine-normative for sodium-rich sources and hypersthene+olivine-normative for refractory or depleted compositions with very calcic plagioclase or high Ca/Ca+Na ratios in spinel lherzolite. It is not possible to derive quartz-normative basaltic or `andesitic' melt compositions by partial melting of anhydrous lherzolite at 1 GPa.