^1,2Lauren A. Jennings, ¹Stephan Klemme, ³Max Collinet, ²Julia Maia, ^1,2Carianna Herrera, ²Ana-Catalina Plesa
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2026.116986]
¹Institut für Mineralogie, Universität Münster, Corrensstraße 24, Münster 48149, Germany
²Institute of Space Research, German Aerospace Center (DLR), Berlin, Germany
³Institute of Life, Earth and Environment, Geology Department, University of Namur, Namur, Belgium
Copyright Elsevier

The mantle composition of Venus is often assumed to be similar to Earth, albeit with a lower iron content to account for the density differences between the two planets. However, it has yet to be tested whether partial melting of proposed Venusian mantle compositions can produce melts that are similar to the measured basaltic rock compositions analysed in-situ during the Venera 14 and Vega 2 missions. In this study, we used Perple_X to calculate melt compositions from several bulk mantle compositions of Venus and found they were unable to reliably produce primary melt compositions that are similar to the Venera 14 or Vega 2 basalts, regardless of the oxidation state or degree of fractional crystallisation. As such, we used an iterative approach to identify new mantle compositions for Venus that are able to produce Vega 2- and/or Venera 14-like melts over a large pressure and temperature range. We found 23 mantle compositions that are similar to the terrestrial composition of KLB-1, but have a high Al2O3 and low CaO abundance, resulting in a sub-chondritic CaO/Al2O3 and SiO2/Al2O3. We recommend two of these as new mantle compositions for Venus as they were the most successful at producing Venus-like melts. Lastly, we propose that the sub-chondritic ratios of these new mantle compositions are the result of igneous processes, such as magma ocean differentiation and Ca-rich carbonatite melt extraction, that altered the mantle composition prior to the melting that produced the basalts sampled by the Venera 14 and Vega 2 missions.

¹M.C. Benner, ^1,2T.J. Zega, ¹B.S. Prince, ³Z.E. Wilbur,^1,4,5H.C. Connolly Jr., ¹D.S. Lauretta
Geochimica et Cosmochimica Acta (in Press) Open Access Link to Article [https://doi.org/10.1016/j.gca.2026.01.056]
¹Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA
²Department of Materials Science & Engineering, University of Arizona, Tucson, AZ, USA
³Department of Mineral Sciences, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
⁴Department of Geology, Rowan University, Glassboro, NJ, USA
⁵Department of Earth and Planetary Science, American Museum of Natural History, New York, NY, USA
Copyright Elsevier

We report the discovery of fibrous mackinawite in samples of asteroid Bennu returned by the OSIRIS-REx mission. Mackinawite occurs primarily in particles belonging to Bennu’s hummocky lithology, with fibers that range from 75 to 250 nm in length and 10 to 30 nm in width. In Bennu particles, mackinawite displays both fibrous and tabular habits and forms flower-like clusters that resemble the texture of coarse-grained phyllosilicates previously described. Energy-dispersive X-ray spectroscopy indicates an Fe/S ratio of 1, and four-dimensional scanning transmission electron microscopy reveals a tetragonal structure consistent with mackinawite. Similar to terrestrial occurrences, the activities of Fe2+ and S2– in aqueous solution are likely the main drivers of mackinawite precipitation within Bennu’s parent body. We suggest that mackinawite formed via precipitation from solution following the dissolution of accreted metal and sulfides when Fe and S activities were high enough to support mackinawite stability. Based on comparison to terrestrial Pourbaix diagrams, we hypothesize that mackinawite precipitation within Bennu’s parent body was possible at 7 < pH < 10, –0.5 < Eh < –0.15, 10–9 ≤ aFe ≤ 10–6, and temperatures up to 70°C.