Solubility of water in peridotite liquids and the prevalence of steam atmospheres on rocky planets

1Paolo A.Sossi,1Peter M.E.Tollan,2James Badro,3Dan J.Bower
Earth and Planetary Science Letters 601, 117894 Link to Article []
1Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
2Institut de Physique du Globe de Paris, Université de Paris, 75005 Paris, France
3Center for Space and Habitability, Universität Bern, 3012 Bern, Switzerland
Copyright Elsevier

Atmospheres are products of time-integrated mass exchange between the surface of a planet and its interior. On Earth and other planetary bodies, magma oceans likely marked significant atmosphere-forming events, during which both steam- and carbon-rich atmospheres may have been generated. However, the nature of Earth’s early atmosphere, and those around other rocky planets, remains unclear for lack of constraints on the solubility of water in liquids of appropriate composition. Here we determine the solubility of water in 14 peridotite liquids, representative of Earth’s mantle, synthesised in a laser-heated aerodynamic levitation furnace. We explore oxygen fugacities (fO2) between −1.9 and +6.0 log units relative to the iron-wüstite buffer at constant temperature (2173 ± 50 K) and total pressure (1 bar). The resulting fH2O ranged from nominally 0 to 0.027 bar and fH2 from 0 to 0.064 bar. Total H2O contents were determined by transmission FTIR spectroscopy of doubly-polished thick sections from the intensity of the absorption band at 3550 cm−1 and applying the Beer-Lambert law. The mole fraction of water in the liquid is found to be proportional to (fH2O)0.5, attesting to its dissolution as OH. The data are fitted by a solubility coefficient of 524 ± 16 ppmw/bar0.5, given a molar absorption coefficient, , of 6.3 ± 0.3 m2/mol for basaltic glasses or 647 ± 25 ppmw/bar0.5, for a preliminary m2/mol for peridotitic glasses. These solubility constants are roughly 10 – 25% lower than those for basaltic liquids at 1623 K and 1 bar. Higher temperatures result in lower water solubility in silicate melts, wholly offsetting the greater depolymerisation of peridotite melts that would otherwise increase H2O solubility relative to basaltic liquids at constant temperature. Because the solubility of water remains high relative to that of CO2, steam atmospheres are rare, although they may form under moderately oxidising conditions on telluric bodies, provided sufficiently high H/C ratios prevail.


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