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 Na 2O and K 2O-rich and nepheline-normative in `fertile' mantle. The most extreme Na 2O-rich minimum melt composition is in equilibrium with an albite-bearing harzburgite residue at 1220°C. This melt composition is nepheline-normative with ∼64% SiO 2 and about ∼12% Na 2O. Our results disagree with recent reports [1, 2]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.