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

Paolo A. Sossi , Peter M. E. Tollan , James Badro (1) , Dan J. Bower

1 IPGP (UMR_7154) - Institut de Physique du Globe de Paris

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/bar^0.5, given a molar absorption coefficient, ε₃₅₅₀, of 6.3 ± 0.3 m²/mol for basaltic glasses or 647 ± 25 ppmw/bar^0.5, for a preliminary ε₃₅₅₀ = 5.1 ± 0.3 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.

Mots clés: water; peridotite; solubility; magma ocean; steam atmosphere; Astrophysics - Earth and Planetary Astrophysics; Physics - Geophysics

Domaines: Planète et Univers [physics]

Paolo A. Sossi, Peter M. E. Tollan, James Badro, Dan J. Bower. Solubility of water in peridotite liquids and the prevalence of steam atmospheres on rocky planets. Earth and Planetary Science Letters, 2023, 601, ⟨10.1016/j.epsl.2022.117894⟩. ⟨insu-03947011⟩