Determination of activities and condensation temperatures of GaO_1.5 and InO_1.5 in anorthite-diopside eutectic melts by Knudsen Effusion Mass Spectrometry

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 (T-c(50)). 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 anorthitediopside (An-Di) eutectic glasses with similar to 1000 and similar to 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(fO(2)) range Delta IW+1.5 to Delta IW+2.5 (IW = iron-wustite buffer). We detect Ga-0 and In-0 as the dominant vapour species and determine activity coefficients of gamma(GaO1.5) = 0.036(6) at 1700 K and of gamma(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 similar to In > Zn, different from those predicted from their T-c(50) 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.