Experimental tests of low degree peridotite partial melt compositions: implications for the nature of anhydrous near-solidus peridotite melts at 1 GPa

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 Iherzolite at 1 GPa are basaltic with similar to 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 degrees C. This melt composition is nepheline-normative with similar to 64% SiO2 and about similar to 12% Na2O. 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 Iherzolite 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 Iherzolite 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 Iherzolite. It is not possible to derive quartz-normative basaltic or 'andesitic' melt compositions by partial melting of anhydrous Iherzolite at 1 GPa. (C) 1997 Elsevier Science B.V.