^1,2Bruce Fegley Jr., ³Nathan S. Jacobson, ²K. B. Williams, ⁴J. M. C. Plane, ⁵L. Schaefer, ^1,2Katharina Lodders
The Astrophysical Journal 824, 103 Link to Article [http://dx.doi.org/10.3847/0004-637X/824/2/103]
¹Planetary Chemistry Laboratory, McDonnell Center for the Space Sciences, Washington University, St. Louis, MO 63130, USA
²Department of Earth & Planetary Sciences, Washington University, St. Louis, MO 63130, USA
³Materials Division, NASA Glenn Research Center, MS106-1, 21000 Brookpark Road, Cleveland, OH 44135, USA
⁴School of Chemistry, University of Leeds, Leeds LS2 9JT, UK
⁵Harvard—Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA

Extensive experimental studies show that all major rock-forming elements (e.g., Si, Mg, Fe, Ca, Al, Na, K) dissolve in steam to a greater or lesser extent. We use these results to compute chemical equilibrium abundances of rocky-element-bearing gases in steam atmospheres equilibrated with silicate magma oceans. Rocky elements partition into steam atmospheres as volatile hydroxide gases (e.g., Si(OH)4, Mg(OH)2, Fe(OH)2, Ni(OH)2, Al(OH)3, Ca(OH)2, NaOH, KOH) and via reaction with HF and HCl as volatile halide gases (e.g., NaCl, KCl, CaFOH, CaClOH, FAl(OH)2) in much larger amounts than expected from their vapor pressures over volatile-free solid or molten rock at high temperatures expected for steam atmospheres on the early Earth and hot rocky exoplanets. We quantitatively compute the extent of fractional vaporization by defining gas/magma distribution coefficients and show that Earth’s subsolar Si/Mg ratio may be due to loss of a primordial steam atmosphere. We conclude that hot rocky exoplanets that are undergoing or have undergone escape of steam-bearing atmospheres may experience fractional vaporization and loss of Si, Mg, Fe, Ni, Al, Ca, Na, and K. This loss can modify their bulk composition, density, heat balance, and interior structure.

¹V. Clesi, ¹M.A. Bouhifd, ¹N. Bolfan-Casanova, ¹G. Manthilake, ¹A. Fabbrizio, ¹D. Andrault
Geochimica et Cosmochmica Acta (in Press) Link to Article [doi:10.1016/j.gca.2016.07.029]
¹Laboratoire Magmas et Volcans, Université Blaise Pascal, CNRS UMR 6524, OPGC-IRD, Campus Universitaire des Cézeaux, 6 Avenue Blaise Pascal, 63178 Aubie‘re Cedex, France
Copyright Elsevier

This study investigates the metal-silicate partitioning of Ni, Co, V, Cr, Mn and Fe during core mantle differentiation of terrestrial planets under hydrous conditions. For this, we equilibrated a molten hydrous CI chondrite model composition with various Fe-rich alloys in the system Fe-C-Ni-Co-Si-S in a multi-anvil over a range of P, T, fO2fO2 and water content (5 – 20 GPa, 2073 – 2500 K, from 1 to 5 log units below the iron-wüstite (IW) buffer and for XH2OXH2O varying from 500 ppm to 1.5 wt%). By comparing the present experiments with the available data sets on dry systems, we observes that the effect of water on the partition coefficients of moderately siderophile elements is only moderate. For example, for iron we observed a decrease in the partition coefficient of Fe (View the MathML sourceDmet/silFe) from 9.5 to 4.3, with increasing water content of the silicate melt, from 0 to 1.44 wt%, respectively. The evolution of metal-silicate partition coefficients of Ni, Co, V, Cr, Mn and Fe are modelled based on sets of empirical parameters. These empirical models are then used to refine the process of core segregation during accretion of Mars and the Earth. It appears that the likely presence of 3.5 wt% water on Mars during the core-mantle segregation could account for ∼∼ 74% of the FeO content of the Martian mantle. In contrast, water does not play such an important role for the Earth; only 4 to 6% of the FeO content of its mantle could be due to the water-induced Fe-oxidation, for a likely initial water concentration of 1.8 wt%. Thus, in order to reproduce the present-day FeO content of 8 wt% in the mantle, the Earth could initially have been accreted from a large fraction (between 85 to 90%) of reducing bodies (similar to EH chondrites), with 10 to 15% of the Earth’s mass likely made of more oxidized components that introduced the major part of water and FeO to the Earth. This high proportion of enstatite chondrites in the original constitution of the Earth is consistent with the 17O17O, 48Ca48Ca, 50Ti50Ti, 62Ni62Ni and 90Mo90Mo isotopic study by Dauphas2014. If we assume that the CI-chondrite was oxidized during accretion, its intrinsically high water content suggests a maximum initial water concentration in the range of 1.2 to 1.8 wt% for the Earth, and 2.5 to 3.5 wt% on Mars.

¹Tasha L. Dunn,^2,3Juliane Gross,⁴Marina A. Ivanova,⁵Simone E. Runyon,⁶Andrea M. BruckMeteoritics & Planetary Sciences (in Press) Link to Article [DOI: 10.1111/maps.12691]
¹Department of Geology, Colby College, Waterville, Maine, USA
²Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey, USA
³Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York, USA
⁴Vernadsky Institute of Geochemistry, Moscow, Russia
⁵Department of Geosciences, University of Arizona, Tucson, Arizona, USA
⁶Department of Chemistry, SUNY Stony Brook, Stony Brook, New York, USA
Published by arrangement with John Wiley & Sons

Bulk isotopic and elemental compositions of CV and CK chondrites have led to the suggestion that both originate from the same asteroid. It has been argued that magnetite compositions also support this model; however, magnetite has been studied almost exclusively in the equilibrated (type 4-6) CKs. Magnetite in seven unequilibrated CKs analyzed here is enriched in MgO, TiO2, and Al2O3 relative to the equilibrated CKs, suggesting that magnetite compositions are affected by metamorphism. Magnetite in CKs is compositionally distinct from CVs, particularly in abundances of Cr2O3, NiO, and TiO2. Although there are minor similarities between CV and equilibrated CK chondrite magnetite, this is contrary to what we would expect if the CVs and CKs represent a single metamorphic sequence. CV magnetite should resemble CK3 magnetite, as both were metamorphosed to type 3 conditions. Oxygen fugacities and temperatures of CVox and CK chondrites are also difficult to reconcile using existing CV-CK parent body models. Mineral chemistries, which eliminate issues of bulk sample heterogeneity, provide a reliable alternative to techniques that involve a small amount of sample material. CV and CK chondrite magnetite has distinct compositional differences that cannot be explained by metamorphism.