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 for MORB generation involves low-pressure fractionation the near-solidus partial melting systematics of modally variable of magmas generated by polybaric melting of adiaperidotites. Starting materials consisted of compositionally interbatically upwelling mantle beneath mid-ocean spreading mediate (i.e. containing moderate incompatible element abundances) centers. Because the slope of the mantle adiabat is steeper minerals, separated from a spinel lherzolite xenolith from Mt. than that of the mantle solidus (McKenzie, 1984; McNoorat, SE Australia, and recombined to create five starting mixtures Kenzie & Bickle, 1988), a rising parcel of hot peridotite varying in their proportions of olivine, orthopyroxene, clinopyroxene will cross its solidus and begin to melt. The resulting (Cpx), and spinel. These modes match those of starting materials melt is buoyant relative to residual solid phases and can made with fertile (FER) minerals from a different xenolith, inrise faster than the upwelling solids once an intervestigated in a companion study. A layer of vitreous carbon spheres connected pathway develops. Isotopic and trace element provided a melt sink in the experiments. Solidus temperatures for evidence in MORBs and abyssal peridotites suggests that the five peridotites are similar and estimated to be >1260 ± this can occur at melt fractions (F ) as low as 0·001 (i.e. 10°C based on the zero-F (melt fraction) intercepts of F vs 0·1%) (Salters & Hart, 1989; Johnson et al., 1990). These temperature curves. Melt productivity (∂F/∂T) varies significantly melts aggregate throughout the melting column and are between the starting materials, but unlike the FER runs, the overall eventually emplaced as oceanic crust, with the solid melt productivity does not correlate with the initial Cpx abundance. residuum forming abyssal peridotite. Although the proThis suggests that the compositional fertility of Cpx plays a greater cesses occurring between the onset of melting and the role in determining F at a given temperature than simply its overall ultimate eruption of MORBs are too complex to simulate source abundance. Low-F glasses have elevated SiO2 and Na2O experimentally, data generated by equilibrium batch contents relative to average basalts, as high as >53·9 and partial melting experiments, like those presented here, >2·5 wt %, respectively, but are less enriched than in equivalent are integral to any attempt to model the process mathFER melts. ematically (e.g. Klein & Langmuir, 1987; McKenzie & Bickle, 1988; McKenzie, 1989; Niu & Batiza, 1991; Kinzler & Grove, 1992a, 1992b; Langmuir et al., 1992;