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Geochemistry and petrography of martian meteorite Northwest Africa 11115: A rare earth element-enriched olivine-phyric shergottite closely linked to Northwest Africa 1068
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  • Mohit Melwani Daswani,
  • Nicolas Greber,
  • Jinping Hu,
  • Richard Greenwood,
  • Philipp Heck
Mohit Melwani Daswani
Jet Propulsion Laboratory, California Institute of Technology

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Nicolas Greber
Institute of Geological Sciences, University of Bern
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Jinping Hu
Division of Geological and Planetary Sciences, California Institute of Technology
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Richard Greenwood
The Open University
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Philipp Heck
Field Museum of Natural History
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The shergottite family of meteorites shows a remarkable petrographic and geochemical variety, revealing information about mantle processes and basalt formation on Mars. Northwest Africa (NWA) 11115, found in Morocco in 2016, is one of the newest meteorites in this family. We report bulk-rock major and trace element abundances of NWA 11115, bulk oxygen isotope systematics, and the petrography and mineralogy of a thick section, and compare the geochemistry of this recent find to other martian rocks. NWA 11115 is an olivine-phyric shergottite with an enriched rare earth element pattern, and shares similarities with NWA 1068, another enriched olivine-phyric shergottite. The large (< 2.5 mm) olivine phenocrysts are likely to be cumulates, similar to NWA 1068. However, the abundant maskelynite (~30 vol. %) in NWA 11115 places the bulk chemistry somewhat closer to the basaltic shergottites. We suggest that NWA 11115 is genetically linked to NWA 1068, perhaps crystallizing slightly above in the same cumulate pile. NWA 11115 contains one of the lowest K/Th ratios among the martian meteorites (K/Th = 2987 ± 810), and far lower than the surface of Mars (K/Th = 5300). Finally, while NWA 11115 contains abundant (~0.4 vol. %) fracture-filling calcite (presumably from hot desert alteration during its terrestrial residence), diagnostic bulk element mass ratios were not indicative of the presence of terrestrial alteration (Th/U ≈ 4.09, Sr/Nd ≈ 12.26, K/La ≈ 526.94, Ce/Ce* ≈ 1.01).