Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations

Mid-ocean-ridge basalts (MORBs) are probably a mixture of liquids formed by near-fractional melting of upwelling mantle over a range of pressures. Thus, an understanding of MORB 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 that at 10 kbar, near-solidus melts (melt fraction F = 0.02-0.05) of fertile peridotite are enriched in SiO_2, A1_(2)O_3 and Na_(2)O and depleted in FeO* (all iron as FeO), MgO and CaO relative to higher-degree melts. At F≈0.02, the partial melt has ~57 wt% SiO_2 and is qualitatively similar to silica-rich melt inclusions found in spinel peridotites worldwide. At low melt fractions (F≤0.08), measured and calculated clinopv roxene/ liquid (cpx/liq) partition coefficients for TiO_2 are larger than those calculated from cpx-liq pairs in higher-melt-fraction experiments. This change in titanium partitioning just above the fertile peridotite solidus implies that other highly charged elements (such as Hf, Zr, Th and U) exhibit similarly complex behaviour during the initial stages of mantle melting.