COMPOSITIONS OF NEAR-SOLIDUS PERIDOTITE MELTS FROM EXPERIMENTS AND THERMODYNAMIC CALCULATIONS

MID-ocean-ridge basalts (MORBs) are probably a mixture of liquids form ed by near-fractional melting of upwelling mantle over a range of pres sures(1-3). Thus, an understanding of MORE genesis requires knowledge of the compositions of near-solidus melts of mantle peridotite, which have not been measured experimentally. Here we present the results of melting experiments on peridotite using a two-stage diamond-aggregate extraction technique, and the results of thermodynamic calculations of peridotite melting. Both the experiments and the calculations show th at at 10 kbar, near-solidus melts (melt fraction F=0.02-0.05) of ferti le peridotite are enriched in SiO2, Al2O3 and Na2O and depleted in FeO (all iron as FeO), MgO and CaO relative to higher-degree melts. At F approximate to 0.02, the partial melt has similar to 57 wt% SiO2 and is qualitatively similar to silica-rich melt inclusions found in spine l peridotites worldwide(4,5). At low melt fractions (F less than or eq ual to 0.08), measured and calculated clinopyroxene/liquid (cpx/liq) p artition coefficients for TiO2 are larger than those calculated from c px-liq pairs in higher-melt-fraction experiments. This change in titan ium partitioning just above the fertile peridotite solidus implies tha t other highly charged elements (such as Hf, Zr, Th and U) exhibit sim ilarly complex behaviour during the initial stages of mantle melting.