PRELIMINARY RESULTS FROM 1.5 GPa PARTIAL MELTING EXPERIMENTS USING FER-A AND FER-E PERIDOTITE COMPOSITIONS AND VITREOUS CARBON SPHERES

An initial series of twelve 1.5 GPa piston-cylinder experiments were performed at 1340-1400ºC to investigate the near-solidus partial melting behavior of two compositionally fertile (FER) peridotites. Starting materials were constructed by recombining olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), and spinel (sp) mineral separates from a spinel lherzolite xenolith from Kilbourne Hole, New Mexico. Starting mix FER-A is a typical sp-lherzolite assemblage (ol:opx:cpx:sp ratio of 0.50:0.30:0.17:0.03) and mix -E represents an equal-pyroxene mode (0.50:0.235:0.235:0.03). These same starting compositions were studied previously at 1.0 GPa (Baker & Stolper, 1994; Pickering-Witter & Johnston, 2000). Starting powders were placed in graphite-lined Pt capsules with vitreous carbon spheres as a melt sink and run in straight-walled graphite/CaF₂ furnace assemblies with crushable MgO inner parts in a 1.27 cm piston-cylinder apparatus for 46-74 hours. Post-run capsules were mounted in epoxy, cut, and polished for backscattered electron (BSE) imaging and analysis by electron microprobe at the University of Oregon. Mass balance calculations performed using starting material compositions with glass (quenched melt) and residual crystalline phase compositions permit determination of residual mineral modes and melt fraction. Results from six FER-A experiments (1350-1400ºC) generally show an increasing proportion of glass (melt fraction) with increasing temperature (~6 to ~18%), while cpx and sp abundance decrease. The slopes of linear regression curves, fit to plots of residual ol, opx, cpx, and sp abundances plotted against calculated melt fraction, yield stoichiometric coefficients of a melting reaction: 0.88 Cpx + 0.13 Opx + 0.09 Sp = 1 Glass + 0.10 Ol. The reaction shows that cpx, opx, and sp all contribute to the melt while olivine is a product of melting. The form of this reaction is the same as the published 1.0 GPa reaction of equivalent starting material, MM3 (Baker & Stolper, 1994). The FER-A 1.5 GPa solidus temperature is estimated to be ~1305ºC, as determined by projecting the linear fit of calculated glass percent vs. temperature to 0% glass. Additional work is underway to further refine these results, compare the 1.0 GPa and 1.5 GPa datasets, and to analyze the results of the FER-E experiments.