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 m antle peridotite mineralogy at 1 GPa. We confirm earlier conclusions t hat anhydrous melts of Iherzolite at 1 GPa are basaltic with similar t o 15-20% normative diopside, > 10% normative olivine and at low degree s of melting are Na2O and K2O-rich and nepheline-normative in 'fertile ' mantle. The most extreme Na2O-rich minimum melt composition is in eq uilibrium with an albite-bearing harzburgite residue at 1220 degrees C . This melt composition is nepheline-normative with similar to 64% SiO 2 and about similar to 12% Na2O. Our results disagree with recent repo rts [1,2] that peridotitic minimum melt compositions have an 'andesiti c' character at 1 GPa, We present reversal experiments showing that th ese latter melts are not in equilibrium with a spinel or plagioclase I herzolite upper mantle assemblage. We use our new data and data from t he 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 mel t compositions are nepheline + olivine-normative for sodium-rich sourc es and hypersthene + olivine-normative for refractory or depleted comp ositions with very calcic plagioclase or high Ca/Ca + Na ratios in spi nel Iherzolite. It is not possible to derive quartz-normative basaltic or 'andesitic' melt compositions by partial melting of anhydrous Iher zolite at 1 GPa. (C) 1997 Elsevier Science B.V.