¹Damanveer S.Grewal,¹Rajdeep Dasgupta,^1,2Alexandra Farnell
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.04.023]
¹Department of Earth, Environmental, and Planetary Sciences, Rice University, 6100 Main Street, MS 126, Houston, TX 77005, USA
²St. John’s School, 2401 Claremont Ln, Houston, TX 77019, USA
Copyright Elsevier

The composition of the atmospheres and the resulting potential for planetary habitability in the rocky bodies of our Solar System and beyond is strongly controlled by the volatile exchange between their silicate reservoirs and exosphere. The initial budget and speciation of major volatiles, like carbon (C), nitrogen (N) and water (H₂O), in the silicate reservoirs and atmospheres was set during the formation stages of rocky bodies. However, the speciation of these major volatiles in reduced silicate melts prevalent during the differentiation stages of rocky bodies and its effect on the partitioning of volatiles between major rocky body reservoirs is poorly known. Here we present SIMS and vibrational spectroscopy (FTIR and Raman) data, determining C solubility, H content, and speciation of mixed C-O-N-H volatiles in graphite saturated silicate glasses from high P (1-7 GPa)-T (1500-2200 °C) experiments reported in Grewal et al. (2019b, 2019a). The experiments recorded oxygen fugacity (log fO₂) between IW–4.3 and IW–0.8. C-O-N-H speciation varied systematically as function of fO₂ at any given P–T. We find out that C≡N⁻, , N₂, and OH⁻ are the dominant species in the oxidized range (> IW–1.5), along with some contributions from C-H, N-H, and C≡O bearing species. Between IW–3.0 and IW–1.5, C is bonded as C≡O either in the form of isolated C≡O molecules or Fe-carbonyl complexes, or as C-H in hydrocarbons, or as combination of both in esters, while almost all of the H is bonded with the dominant N species, i.e., NH²⁻ or . At the most reduced conditions (<IW–3.0), C is present mostly in the form of C-H bearing species, while anhydrous N³⁻ followed by N-H bearing molecules are the dominant N bearing species. Magma oceans (MOs) in highly reduced bodies like Mercury would contain most of their C as graphite if MO is carbon saturated and the dissolved C and N would be chemically bonded with the silicate network either in the form of anhydrous C⁴⁻ and N³⁻, or hydrogenated C-H and N-H bearing species depending on H content of the silicate melts. MOs relevant for Mars, the Moon, Vesta, and angrite parent body would contain C and N mostly in the form of C≡O and N-H bearing species, respectively. If the composition of Earth’s accreting material evolved from reduced to oxidized, then initially a significant amount of the C and N budget would be locked in the silicate reservoirs, which would subsequently be released to the proto-atmosphere(s) at later stages. The retention of proto-atmosphere(s) formed by MO degassing on Earth could have provided important precursors for prebiotic chemistry which possibly led to the eventual habitability of our planet. Additionally, based on the dominant speciation of N versus C in silicate melt as a function of fO₂, we also predict that is unaffected by fH₂ under highly reduced conditions (< IW–3), while is. Therefore, caution must be taken during the application of experimentally determined and to nominally anhydrous MOs.

¹Zhen Tian,¹Bradley L.Jolliff,¹Randy L.Korotev,¹Bruce FegleyJr,¹Katharina Lodders,²James M.D.Day,^1,3Heng Chen,¹KunWang(王昆)
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.04.021]
¹Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University in St. Louis, One Brookings Drive, St. Louis, MO 63130, USA
²Scripps Institution of Oceanography, University of California San Diego, La Jolla, CA, 92093 USA
³Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964, USA
Copyright Elsevier

The Moon is depleted in water and other volatiles compared to Earth and the bulk solar composition. Such depletion of volatile elements and the stable isotope fractionations of these elements can be used to better understand the origin and early differentiation history of the Moon. In this study, we focus on the moderately volatile element, potassium, and we report the K elemental abundances and isotopic compositions (δ⁴¹K relative to NIST SRM 3141a) for nineteen Apollo lunar rocks and lunar meteorites (twenty-two subsamples), spanning all major geochemical and petrologic types of lunar materials. The K isotopic compositions of low-Ti and high-Ti basalts are indistinguishable, providing a lunar basalt average δ⁴¹K of –0.07 ± 0.09‰ (2SD), which we also consider to be the best estimate of the lunar mantle and the bulk silicate Moon. The significant enrichment of K in its heavier isotopes in the bulk silicate Moon, compared with the bulk silicate Earth (δ⁴¹K = –0.48 ± 0.03‰), is consistent with previous analyses of K isotopes and other moderately volatile elements (e.g., Cl, Cu, Zn, Ga, and Rb). We also report the first analyses of K isotopes for lunar nonmare samples, which show large variations of K isotopic ratios compared to lunar basalts. We interpret this large K isotopic fractionation as the result of late-stage magma ocean degassing during urKREEP formation, which is also coupled with Cl isotope fractionation. UrKREEP degassing likely triggered redistribution of K isotopes in the Moon, enriching the urKREEP reservoir in heavy K isotopes while implanting the light K isotopic signatures onto the lunar surface. This scenario suggests a heterogeneous distribution of K isotopes in the Moon as a consequence of its magmatic evolution.