Vaporization and Thermodynamics of Forsterite-Rich Olivine and some Implications for Silicate Atmospheres of Hot Rocky Exoplanets

1Gustavo C.C. Costa, 1Nathan S. Jacobson, 2Bruce Fegley Jr.
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2017.02.006]
1NASA Glenn Research Center, 21000 Brookpark Road, MS 106-1, Cleveland, OH, 44135, USA
2Planetary Chemistry Laboratory, McDonnell Center for the Space Sciences, Department of Earth and Planetary Sciences, Washington University in St. Louis, Campus Box 1169, 1 Brookings Dr, St. Louis, MO, 63130-4899, USA
Copyright Elsevier

We describe an experimental and theoretical study of olivine [Mg2SiO4 (Fo) – Fe2SiO4 (Fa)] vaporization. The vaporization behavior and thermodynamic properties of a fosterite-rich olivine (Fo95Fa5) have been explored by high-temperature Knudsen effusion mass spectrometry (KEMS) from 1750 – 2250 K. The gases observed (in order of decreasing partial pressure) are Fe, SiO, Mg, O2 and O. We measured the solidus temperature (∼ 2050 K), partial pressures of individual gases, the total vapor pressure, and thermodynamic activities and partial molal enthalpies of MgO, ‘FeO’, and SiO2 for the Fo95Fa5 olivine. The results are compared to other measurements and models of the olivine system. Our experimental data show olivine vaporizes incongruently. We discuss this system both as a psuedo-binary of Fo-Fa and a psuedo-ternary of MgO-‘FeO’-SiO2. Iron/magnesium molar ratios in the sample before (∼ 0.05) and after (∼ 0.04) vaporization are consistent with the small positive deviations from ideality of fayalite (γ ∼ 1.17) in olivine of the composition studied (e.g., Nafziger & Muan 1967). Our data for olivine + melt confirm prior theoretical models predicting fractional vaporization of Fe relative to Mg from molten silicates (Cameron & Fegley 1987, Schaefer & Fegley 2009, Ito et al. 2015). If loss of silicate atmospheres occurs from hot rocky exoplanets with magma oceans the residual planet may be enriched in magnesium relative to iron.

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