The mantle composition of Venus: an investigation using thermodynamic modelling

The mantle composition of Venus is often assumed to be similar to Earth, albeit with a lower iron content to account for the density differences between the two planets. However, it has yet to be tested whether partial melting of proposed Venusian mantle compositions can produce melts that are similar to the measured basaltic rock compositions analysed in-situ during the Venera 14 and Vega 2 missions. In this study, we used Perple_X to calculate melt compositions from several bulk mantle compositions of Venus and found they were unable to reliably produce primary melt compositions that are similar to the Venera 14 or Vega 2 basalts, regardless of the oxidation state or degree of fractional crystallisation. As such, we used an iterative approach to identify new mantle compositions for Venus that are able to produce Vega 2- and/or Venera 14-like melts over a large pressure and temperature range. We found 23 mantle compositions that are similar to the terrestrial composition of KLB- 1, but have a high Al2O3 and low CaO abundance, resulting in a sub-chondritic CaO/Al2O3 and SiO2/Al2O3. We recommend two of these as new mantle compositions for Venus as they were the most successful at producing Venus-like melts. Lastly, we propose that the sub-chondritic ratios of these new mantle compositions are the result of igneous processes, such as magma ocean differentiation and Ca-rich carbonatite melt extraction, that altered the mantle composition prior to the melting that produced the basalts sampled by the Venera 14 and Vega 2 missions.