The Effect of Fe on OH Content of Olivine: Implications for Extraction of H2O from the Martian Mantle. A

Introduction: The concentration of H2O in the Martian mantle is of importance to understanding the dynamics of convection, the spatial and temporal occurrence of partial melting, and the history of volatile fluxes from the interior to the surface, but is subject to considerable uncertainty and vigorous debate [1-6] Simulation of the cosmochemical and accretionary origin of Mars suggests an initial H2O inventory similar to 0.06-0.27 terrestrial oceans [4]. If all of this H2O were incorporated into the mantle, the Martian mantle would initially have 200-1000 ppm H2O, which is consistent with the bulk H2O mantle concentration (288 ppm) estimated by Lodders and Fegley [8], but considerably more than the 36 ppm value favored by Dreibus and Wanke [1]. Estimates of the fraction of this original H2O remaining in the mantle at the present time range from 50 to 95% [8,4]. Determination of solubilities and mineral/melt partition coefficients of H2O for Martian magmas and minerals may constrain H2O concentrations in the Martian mantle by allowing analogies to non-volatile species such as K and REE, as has been done for Earth's mantle [9,10] Experimental and Analytical Methods: Starting materials consisted of mixtures of MgO, MgOH, FeO and SiO2 such that the bulk composition in each experiment was equivalent to 90% olivine + 10% pyroxene, with Mg# (100*Mg/[Mg+Fe]) ranging from 50 to 100, and H/Si =1. Experiments were run at 3 GPa and 1200-1500 °C in a piston cylinder and at 6 GPa and 1200 °C in a multi-anvil apparatus. Starting materials were sealed in 2 mm AuPd capsules that, in the case of the 3 GPa/1200 °C experiments, were embedded in FeO within an outer capsule of Fe. Run duration was 6 h at 1200 °C and 3 h at 1500 °C. Run products were analysed for H2O using SIMS calibrated against FTIR measurements of mineral standards [11,12]. Mineral H2O concentrations were determined from OH/Si mass ratios by means of mineral specific calibration curves that were determined at the time of analysis. Raman analysis of areas probed by SIMS verified homogeneity of the phases analysed, and mineral compositions were measured using electron microprobe. Results: Run products consist of olivine crystals (≤300 μm in diameter), orthopyroxene (≤ 40 μm) and quenched liquid (fine grained, elongate exsolved crystals interspersed with H2O). Experiments with bulk Mg# ≤ 60 contain Fe intergrown with opx and olivine at the lower end of the capsule. Olivine Mg# varies from 55 to 100, and measurements of Mg Fe D K between olivine and opx are consistent with equilibrium values [13].