^1,2Amanda C. Stadermann,¹Jessica J. Barnes,³Timmons M. Erickson,²Tabb C. Prissel,⁴Zachary D. Michels
Journal of Geophysical Research (Planets) (in Press) Open Access Link to Article [https://doi.org/10.1029/2022JE007728]
¹Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd, Tucson, AZ, 85721 USA
²Astromaterials Research and Exploration Science at NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX, 77058 USA
³Jacobs JETS at NASA Johnson Space Center, 2101 NASA Parkway, Houston, TX, 77058 USA
⁴Department of Geosciences, University of Arizona, 1040 E 4th St, Tucson, AZ, 85721 USA
Published by arrangement with John Wiley & Sons

The magnesian suite (Mg-suite) of rocks record some of the earliest intrusive magmatism on the Moon. Studies of these Mg-suite rocks find they are plutonic or hypabyssal, formed typically kilometers under the lunar surface. Several models exist to explain the formation and evolution of the Mg-suite but distinguishing between hypotheses can be difficult given limited sample availability. The global extent of Mg-suite magmatism remains in debate and is key to constraining models of early secondary crust building. In this study, we present magnesian clasts within Apollo impact melt rock 68815. These clasts contain olivine, plagioclase, with minor amounts of Mg-Al-spinel and pyroxene similar to spinel troctolites of the Mg-suite, but they lack plutonic textures. We provide evidence that some of the clasts may be of extrusive volcanic origin akin to terrestrial komatiites while others might represent crystalline impact melts. There exists a large breadth of evidence for Mg-suite intrusives, whereas here we present possible evidence of Mg-rich volcanic counterparts. If valid, this would broaden the known diversity of lunar volcanism during the initial stages of secondary crust building. We anticipate this finding to provide a greater constraint onto models of Mg-suite ascent and emplacement, which only currently consider intrusive magmatism, as well as a renewed motivation to examine impact melt breccias for rare and understudied lithologies. Future trace element studies or radiometric dating could be used to further interrogate the connections of these clasts to the Mg-suite.

¹Robert M. Hazen et al. (>10)
Journal of Geophysical Research (Planets) (in Press) Link to Article [https://doi.org/10.1029/2023JE007865]
¹Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, 20015 USA
Published by arrangement with John Wiley & Sons

A systematic survey of 161 known and postulated minerals originating on Mars points to 20 different mineral-forming processes (paragenetic modes), which are a subset of formation modes observed on Earth. The earliest martian minerals, as on Earth, were primary phases from mafic igneous rocks and their ultramafic cumulates. Subsequent primary igneous minerals were associated with products of limited fractional crystallization, including alkaline and quartz-normative lithologies. Significant mineral diversification occurred via precipitation of primary phases from aqueous and atmospheric fluids, including authigenesis, hydrothermal and cryogenic precipitation, and evaporites, including freeze drying during eras of low atmospheric pressures. In particular, hydrothermal mineral formation associated with both volcanic fluids and sustained hydrothermal activity in impact fracture zones may have triggered significant mineral diversification, though as yet undocumented. At least 65 such primary minerals have been identified by flown missions to Mars and from martian meteorites. A host of secondary martian minerals were produced by near-surface processes related to water/rock interactions, including hydration/dehydration, oxidation/reduction, serpentinization, metasomatism, and a variety of diagenetic alterations. Additional mineral diversity resulted from metamorphic events, including thermal and shock metamorphism, lightning, and bolide impacts. However, several dominant sources of mineral diversity on Earth, including: (1) extensive fluid/rock interactions and element concentration associated with plate tectonics; (2) high-pressure regional metamorphism associated with plate tectonics; and (3) biologically mediated mineralization—are not known to be in play on Mars. Consequently, we estimate the total mineral diversity of Mars to be an order of magnitude smaller than on Earth.