Melting systematics of modally variable, compositionally intermediate peridotites and the effects of mineral fertility

Abstract

Piston-cylinder experiments were performed at 10 kbar to investigate the near-solidus partial melting systematics of modally variable peridotites. Starting materials consisted of compositionally intermediate (i.e. containing moderate incompatible element abundances) minerals, separated from a spinel lherzolite xenolith from Mt. Noorat, SE Australia, and recombined to create five starting mixtures varying in their proportions of olivine, orthopyroxene, clinopyroxene (Cpx), and spinel. These modes match those of starting materials made with fertile (FER) minerals from a different xenolith, investigated in a companion study. A layer of vitreous carbon spheres provided a melt sink in the experiments. Solidus temperatures for the five peridotites are similar and estimated to be 1260 ± 10°C based on the zero-F (melt fraction) intercepts of F vs temperature curves. Melt productivity (F/T) varies significantly between the starting materials, but unlike the FER runs, the overall melt productivity does not correlate with the initial Cpx abundance. This suggests that the compositional fertility of Cpx plays a greater role in determining F at a given temperature than simply its overall source abundance. Low-F glasses have elevated SiO2 and Na2O contents relative to average basalts, as high as 53.9 and 2.5 wt %, respectively, but are less enriched than in equivalent FER melts. © 2001 The American Physical Society.