Experimental trace element partition coefficients for upper mantle pyroxenites coexisting with H2O with CO2, NaCl and NH3 fluids

Abstract

Experimental determinations of trace element distribution between rocks/minerals and fluids at mantle pressures and temperatures have concentrated on eclogite and on fluids containing H₂O with or without the addition of NaCl. Here we present data for websterite (pyroxenite) coexisting with five different fluids: H₂O, H₂O+CO₂, H₂O+NaCl, H₂O+CO₂+NaCl and H₂O+NH₃. All experiments were conducted at 1.5 GPa and 800˚C using glassy carbon fluid traps that were kept in a frozen state following experiments until trace elements in the fluid traps were analysed by laser cryoablation-ICP-MS. Results show that Rb, Ba, Sr, Pb and Li are fluid-mobile, with lower rock/fluid partition coefficients (D_Rock/Fluid) for all mixed fluids relative to pure H₂O. Lowest values (0.0004–0.123) are found for H₂O+NH₃ fluids. Almost all for the REE are < 1, contrasting with results at higher pressures for eclogite, and show very little fractionation of LREE from HREE. The REE are least mobile in H₂O+CO₂+NaCl and H₂O+NH₃ fluids, as are the HFSE, which also show lower D_Rock/Fluid than for eclogites, with or without NaCl. Re and Pt are fluid-mobile, with D_Re < D_Pt and lowest values for both in H₂O+CO₂ fluids. The first row transition elements show D-values ranging from 0.0004 to 2.88 with only 4 of 35 data points > 1. D_Rock/Fluid increase in the order Cu < Ni ≈ Zn < Co < Cr, V, Sc. Rock/fluid partitioning of trace elements for pyroxenite resembles peridotite more than eclogite due to the abundance of minerals with low D_Rock/Fluid,particularly low-Al clinpyroxenes.