Geochemistry and petrography of martian meteorite Northwest Africa
11115: A rare earth element-enriched olivine-phyric shergottite closely
linked to Northwest Africa 1068
Abstract
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).