^1,2Amanda Ostwald,¹Arya Udry,^3,4Juliane Gross,⁵James M.D. Day,⁶Sammy Griffin
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2024.06.033]
¹Department of Geoscience, University of Nevada, Las Vegas, Lilly Fong Geoscience Building, 4505 S Maryland Pkwy, Las Vegas, NV 89154, USA
²Smithsonian National Museum of Natural History, 10th St. & Constitution Ave. NW, Washington, DC 20560, USA
³NASA Johnson Space Center, 2101 E NASA Pkwy, Houston, TX 77058, USA
⁴Department of Earth and Planetary Science, Rutgers University, Busch Campus, 610 Taylor Rd, Piscataway, NJ 08854, USA
⁵Scripps Institution of Oceanography, University of California San Diego, 9500 Gilman Dr, La Jolla, CA 92093-0244, USA
⁶University of Glasgow, Glasgow G12 8QQ, United Kingdom
Copyright Elsevier

Nakhlites (clinopyroxene-rich cumulates) and chassignites (dunites) are two types of meteorites that were emplaced onto — and subsequently ejected from— the surface of Mars together, but their petrogenetic history has been difficult to discern. We studied the primary magmatic history preserved in zoning patterns of cumulus phases from a suite of nakhlites and chassignites. Samples studied include nakhlites Northwest Africa (NWA) 11013, NWA 10645, Governador Valadares, Caleta el Cobre 022, Nakhla, Miller Range 090032, and NWA 817, as well as chassignites NWA 2737 and Chassigny. In nakhlite and chassignite olivine, phosphorous (P) preserves primary magmatic signatures, and P2O5 ranges from ∼<0.01 to 0.21 wt %; in nakhlite pyroxene, chromium (Cr) zoning corresponds to Cr2O3 abundances between ∼0.03 to 0.36 wt %. We find that nakhlite pyroxene cores uniformly formed rapidly for a time at high crustal pressures, and then slowly at near-equilibrium under lower crustal pressures. Pyroxene in the nakhlites were then stored through multiple injections of magma prior to remobilization, eruption, and final crystallization. Nakhlite olivine cores are morphologically heterogenous throughout the suite, but all record rapid initial crystallization prior to equilibrium formation, followed by resorption in changing magma compositions. Both olivine and pyroxene in the nakhlites are antecrysts, as they initially formed in a different magma than that in which they erupted. Chassignites underwent very rapid initial undercooling, and record later changes in magma conditions, resulting in thin elemental oscillatory zoning patterns in olivine grains. Together, the cumulus phases of the nakhlite and chassignite suite, combined with petrological evidence from martian shergottite meteorites, suggest that significant magmatic undercooling is the rule rather than the exception for martian magmatic systems. This may relate to the stalling of magmas within the thicker crust of Mars, fostering crystal storage with significant temperature differences between injected magmas and crystal mushes.