¹Jennifer T.Mitchell,¹Andrew G.Tomkins,²Christopher Newton,³Tim E.Johnson
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2021.117105]
¹School of Earth, Atmosphere & Environment, Monash University, Melbourne, Australia
²School of Physics & Astronomy, Monash University, Melbourne, Australia
³School of Earth & Planetary Sciences, The Institute for Geoscience Research, Curtin University, Perth, Australia
Copyright Elsevier

Combined phase equilibrium and thermal modelling has been used to investigate the evolution of asteroid 4 Vesta. Orthopyroxene compositions of 200 natural diogenite meteorites are used as a basis for constructing a staged mantle melting model for Vesta, which is then used to develop a staged thermal evolution model. Our pMELTS models find that removal of 15–20% of a mean eucrite component from an initial Vestan mantle composition allows a second stage of melting that crystallises low-calcium orthopyroxenes that match the observed compositions of those in natural diogenites, whereas single stage melting produces orthopyroxenes that are too calcic. Using the compositions generated by the pMELTS modelling, THERMOCALC models were created for an initial Vestan mantle composition and an evolved composition generated by a melt extraction stage. These models suggest that melt production for second-stage diogenite generation required considerably hotter temperatures (>1340 °C) than for eucrites (<1240 °C). Staged and layered thermal evolution models developed using these composition and temperature constraints, based on the decay of 26Al and 60Fe, suggest that Vesta accreted 1.50 to 1.75 Myr after calcium-aluminium inclusion (CAI) formation. Earlier accretion results in conditions that are inconsistent with the petrology of the HED meteorites, whereas later accretion predicts temperatures that are insufficient to produce diogenites. We suggest that upward migration of 26Al-rich melt initially created a convecting shallow magma ocean of <20 km depth that rapidly crystallised to form a 26Al-rich eucritic crust that acted as a hot insulating lid. The second stage of crust formation began once the depleted mantle residue reached high enough temperatures to produce diogenite-forming magmas. These results further support the view that diogenites likely formed as crustal intrusions rather than as magma ocean cumulates.

^1,2Brant M.Jones,¹Aleksandr Aleksandrov,³Charles A.Hibbitts,^1,2,4Thomas M.Orlando
Earth and Planetary Science Letters (in Press) Link to Article [https://doi.org/10.1016/j.epsl.2021.117107]
¹School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA, United States of America
²Center for Space Technology and Research, Georgia Institute of Technology, Atlanta, GA, United States of America
³John Hopkins Applied Physics Laboratory, Laurel, MD, United States of America
⁴School of Physics, Georgia Institute of Technology, Atlanta, GA, United States of America
Copyright Elsevier

The evolution of water and molecular hydrogen from Apollo lunar sample 15221, a mature mare soil, was examined by temperature program desorption (TPD) experiments conducted under ultra-high vacuum conditions. Desorption at the grain/vacuum interface with re-adsorption as water transports though the void space of the grains and activated sub-surface diffusion were found to reproduce the experimental TPD signal. Signal from the grain/vacuum interface yielded the second order desorption activation energies and site probability distributions. Water from sample 15221 exhibited a broad distribution of activation energies peaking at 130 kJ mol−1 extending up to 350 kJ mol−1 at zero coverage limit with an onset of 110 kJ mol−1 at full coverage. Our results suggest that water and hydrogen originating from lunar regolith contributes a minor amount to the observed mass in the LCROSS impact event. The abnormal amount of molecular hydrogen observed in the ejecta plume of the LCROSS impact may indicate that the radiolytic production of H2 from electron and galatic cosmic rays of physisorbed water is a contributor to the vast quantity of molecular hydrogen detected.

¹Sergey N. Britvin,¹Maria G. Krzhizhanovskaya, ¹Andrey A. Zolotarev, ¹Liudmila A. Gorelova,³Edita V. Obolonskaya,⁴Natalia S. Vlasenko,^4,5Vladimir V. Shilovskikh,¹Mikhail N. Murashko
The American Mineralogist 106, 1520–1529 Link to Article [http://www.minsocam.org/msa/ammin/toc/2021/Abstracts/AM106P1520.pdf]
¹Institute of Earth Sciences, St. Petersburg State University, Universitetskaya Nab. 7/9, 199034 St. Petersburg, Russia
²Kola Science Center, Russian Academy of Sciences, Fersman Str. 14, 184209 Apatity, Russia
³The Mining Museum, Saint Petersburg Mining University, 2, 21st Line 199106 St. Petersburg, Russia
⁴Centre for Geo-Environmental Research and Modelling, St. Petersburg State University, Ulyanovskaya ul. 1, 198504 St. Petersburg, Russia
⁵Institute of Mineralogy, Urals Branch of Russian Academy of Science, Miass 456317, Russia
Copyright: The Mineralogical Society of America

Schreibersite, (Fe,Ni)3P, the most abundant cosmic phosphide, is a principal carrier of phosphorus in the natural Fe-Ni-P system and a likely precursor for prebiotic organophosphorus compounds at the early stages of Earth’s evolution. The crystal structure of the mineral contains three metal sites allowing for unrestricted substitution of Fe for Ni. The distribution of these elements across the structure could serve as a tracer of crystallization conditions of schreibersite and its parent celestial bodies. However, discrimination between Fe (Z = 26) and Ni (Z = 28) based on the conventional X-ray structural analysis was for a long time hampered due to the proximity of their atomic scattering factors. We herein show that this problem has been overcome with the implementation of area detectors in the practice of X-ray diffraction. We report on previously unknown site-specific substitution trends in schreibersite structure. The composition of the studied mineral encompasses a Ni content ranging between 0.03 and 1.54 Ni atoms per formula unit (apfu): the entire Fe-dominant side of the join Fe3P-Ni3P. Of 23 schreibersite crystals studied, 22 comprise magmatic and non-magmatic iron meteorites and main group pallasites. The near end-member mineral (0.03 Ni apfu) comes from the pyrometamorphic rocks of the Hatrurim Basin, Negev desert, Israel. It was found that Fe/Ni substitution in schreibersite follows the same trends in all studied meteorites. The dependencies are nonlinear and can be described by second-order polynomials. However, the substitution over the M2 and M3 sites within the most common range of compositions (0.6 < Ni <1.5 apfu) is well approximated by a linear regression: Ni(M2) = 0.84 × Ni(M3) – 0.30 apfu (standard error 0.04 Ni apfu). The analysis of the obtained results shows a strong divergence between the variation of unit-cell parameters of natural schreibersite and those of synthetic (Fe,Ni)3P. This indicates that Fe/Ni substitution trends in the mineral and its synthetic surrogates are different. A plausible explanation might be related to the differences in the system equilibration time of meteoritic schreibersite (millions of years) and synthetic (Fe,Ni)3P (~100 days). However, regardless of the reason for the observed difference, synthetic (Fe,Ni)3P cannot be considered a structural analog of natural schreibersite, and this has to be taken into account when using synthetic (Fe,Ni)3P as an imitator of schreibersite in reconstructions of natural processes

^1,2Naoya Imae,¹Makoto Kimura
American Mineralogist 106, 1470–1479 Link to Article [http://www.minsocam.org/msa/ammin/toc/2021/Abstracts/AM106P1470.pdf]
¹National Institute of Polar Research, 10-3 Midori-cho, Tachikawa-shi, Tokyo 190-8518, Japan 2
²SOKENDAI, 10-3 Midori-cho, Tachikawa-shi, Tokyo 190-8518, Japan
Copyright: The Mineralogical Society of America

We currently do not have a copyright agreement with this publisher and cannot display the abstract here