Determination of activities and condensation temperatures of GaO1.5 and InO1.5 in anorthite-diopside eutectic melts by Knudsen Effusion Mass Spectrometry

Abstract

The group 13 elements Ga and In are overabundant in bulk silicate Earth (BSE) when compared to lithophile elements of similar 50% nebular condensation temperature Tc50. To understand whether evaporation from silicate melts provides a more accurate description of volatility during the later stages of planetary accretion, namely, at higher temperatures and oxygen fugacities than in the solar nebula, knowledge of the activities of GaO1.5 and InO1.5 in silicate melts and their stable gaseous species are required. To this end, we doped anorthite-diopside (An-Di) eutectic glasses with ∼1000 and ∼10,000 ppm of Ga and In and determined their equilibrium partial pressures above the silicate liquid by Knudsen Effusion Mass Spectrometry (KEMS) using Ir cells at 1550–1740 K over the log(fO2) range ΔIW+1.5 to ΔIW+2.5 (IW = iron-wüstite buffer). We detect Ga0 and In0 as the dominant vapour species and determine activity coefficients of γ(GaO1.5) = 0.036(6) at 1700 K and of γ(InO1.5) = 0.017(12) at 1674 K. Using these activity coefficients, we calculate partial pressures of Ga and In, together with those of similarly volatile elements, K and Zn and show that their relative volatilities from An-Di eutectic melts are in the in order Ga > K ∼ In > Zn, different from those predicted from their Tc50 under nebular conditions but in line with their relative abundances in the BSE. This substantiates the view that the abundances of volatiles in BSE, such as Ga and In, may have been set by evaporation from silicate melts under oxidising conditions at later stages of planetary accretion. Moreover, chondrules likely never underwent significant evaporation during melting and their volatile-depleted nature is likely inherited from the earliest solid condensates.