Modal abundances of pyroxene, olivine, and mesostasis in nakhlites: Heterogeneity, variation, and implications for nakhlite emplacement

Abstract

Nakhlites, clinopyroxenite meteorites from Mars, share common crystallization and ejection ages, suggesting that they might have been ejected from the same place on Mars by the same ejection event (impact) and are different samples of the same thick volcanic flow unit or shallow sill. Mean modal abundances and abundance ranges of pyroxene, olivine, and mesostasis vary widely among different thin-sections of an individual nakhlite. Lithologic heterogeneity is the main factor contributing to the observed modalabundance variations measured in thin-sections prepared from different fragments of the same stone. Two groups of nakhlites are distinguished from one another by which major constituent varies the least and the abundance of that constituent. The group consisting of Nakhla, Lafayette, Governador Valadares, and the Yamato nakhlite pairing group is characterized by low modal mesostasis and pyroxene-olivine covariance, whereas the group consisting of the Miller Range nakhlite pairing group and Northwest Africa 5790 is characterized by low modal olivine and pyroxene-mesostasis covariance. These two groups sample the slowest-cooled interior portion and the chilled margin, respectively, of the nakhlite emplacement body as presently understood, and appear to be also related to recently proposed nakhlite groups independently established using compositional rather than petrographic observations. Phenocryst modal abundances vary with inferred depth in the nakhlite igneous body in a manner consistent with solidification of the nakhlite stack from dynamically sorted phenocryst-rich magmatic crystal-liquid mush.

Noble gases in 18 Martian meteorites and angrite Northwest Africa 7812—Exposure ages, trapped gases, and a re‐evaluation of the evidence for solar cosmic ray‐produced neon in shergottites and other achondrites

“…Perhaps, the petrographic features which distinguish this specimen from the other nakhlites (Corrigan et al. ; Tomkinson et al. ) might be consistent with a second launch site.…”

“…), this specimen differs from other nakhlites in certain petrologic features such as low modal olivine concentration, high mesostasis content, and very limited aqueous alteration (Corrigan et al. ; Tomkinson et al. ).…”

Northwest Africa 5790: A previously unsampled portion of the upper part of the nakhlite pile

“…In this study, we determined a mesostasis abundance of 44%; other authors have reported values of 35–44% (Corrigan et al. ; Tomkinson et al. ).…”

“…() and Corrigan et al. (). Augite is found as large, sub‐mm to mm‐sized, subhedral to euhedral phenocrysts (Figs.…”

“…Initial reports of NWA 5790 have also shown variation in the abundance of crystals and mesostasis in different portions of the meteorite (Corrigan et al. ; Tomkinson et al. ).…”