Determination of "Sub-Solidus" Equilibration of Fertile Peridotite in Piston-Cylinder Experiments

A 1.0 GPa piston-cylinder experiment was performed to investigate sub-solidus re-equilibration of PbO-fluxed fertile peridotite composition FER-A (Pickering-Witter &Johnston, 2000), equivalent to MM3 (Baker & Stolper, 1994). The starting material was constructed from olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), and spinel (sp) mineral separates of a spinel lherzolite xenolith from Kilbourne Hole, New Mexico. The mineral separates were combined by weight to create initial ol:opx:cpx:sp ratio of 0.50:0.30:0.17:0.03 (weight %) and ground to ≤12μm. A portion of the FER-A peridotite was mixed with 1 wt% PbO, ground under ethanol, then stored in 110°C furnace. The PbO served as a flux, lowering the solidus temperature of the peridotite. While the experiments are not truly sub-solidus, the resulting PbO-fluxed melt increases the rate of chemical equilibration of the minerals by providing a pathway for diffusion that would be much slower if restricted to solid state diffusion. The starting material was sealed within a graphite-lined Pt capsule and placed in the hot spot of a graphite/CaF2 and crushable MgO cell furnace assembly in a 1.27 cm diameter piston-cylinder apparatus and run at 1220°C for 48 hours. Run temperature was selected to be ~20°C below the 1.0 GPa solidus of MM3 and FER-A. Based on back scattered electron (BSE) imaging and electron microprobe analysis of the dissected run product, ol, opx, cpx, sp, and PbO-fluxed melt are present. Quantitative microprobe analysis of &~&20μm olivine grains show slight zoning in Cr# and Mg# from core to rim. Olivine cores have an average Mg# of 90.7 with average Mg# of 90.9 toward the rims. Olivine cores also have an average Cr# of 61.2 with average Cr# of 64.7 toward the rim. Compared to starting xenolith mineral separates, average olivine (rim) Mg# (90.9) are higher than the 90.5 of the starting olivine grains. Average FeO and NiO concentrations of olivine in the run product, 8.77 and 0.04 wt%, respectively, are lower than those of the starting olivine (9.31 wt% FeO and 0.41 wt% NiO). Cpx in the run products also display some compositional zoning with average cpx Mg#'s varying from 90.8 to 91.2 from core to rim. This rim value is higher than the Mg# (90.3) of the starting cpx. This is also reflected in the higher MgO concentration in the run product cpx (19.37 wt% vs. 14.91 wt% in the starting cpx). Concentrations of Na2O, Al2O3, and CaO in the run product cpx are all lower than starting cpx values (0.27, 4.0, and 18.27 vs. 1.52, 6.38, and 19.49, respectively). Analyses of opx and sp, as well as the PbO-fluxed melt are necessary to completely characterize the experiment and will permit determination of mode by mass balance. These analyses, as well as a 2.0 GPa PbO-fluxed experiment on the same starting material, are underway.