Petrology and geochemistry of olivine-phyric shergottites LAR 12095 and LAR 12240: Implications for their petrogenetic history on Mars.

Emilie T Dunham,Meenakshi Wadhwa,Harry Y Mcsween,J Brian Balta,Thomas G Sharp

doi:10.1111/maps.13262

Abstract

Larkman Nunatak (LAR) 12095 and LAR 12240 are recent olivine-phyric shergottite lnds. We report the results of petrographic and chemical analyses of these two samples to understand their petrogenesis on Mars. Based on our analyses, we suggest that these samples are likely paired and are most similar to other depleted olivine-phyric shergottites, particularly Dar al Gani (DaG) 476 and Sayh al Uhaymir (SaU) 005 (and samples paired with those). The olivine megacryst cores in LAR 12095 and LAR 12240 are not in equilibrium with the groundmass olivines. We infer that these megacrysts are phenocrysts and their major element compositions have been homogenized by diffusion (the cores of the olivine megacrysts have Mg# ~70, whereas megacryst rims and groundmass olivines typically have Mg# ~58–60). The rare earth element (REE) microdistributions in the various phases (olivine, low- and high-Ca pyroxene, maskelynite, and merrillite) in both samples are similar and support the likelihood that these two shergottites are indeed paired. The calculated parent melt (i.e., in equilibrium with the low-Ca pyroxene, which is one of the earliest formed REE-bearing minerals) has an REE pattern parallel to that of melt in equilibrium with merrillite (i.e., one of the last-formed minerals). This suggests that the LAR 12095/12240 paired shergottites represent the product of closed-system fractional crystallization following magma emplacement and crystal accumulation. Utilizing the europium oxybarometer, we estimate that the magmatic oxygen fugacity early in the crystallization sequence was ~IW. Finally, petrographic evidence indicates that LAR 12095/12240 experienced extensive shock prior to being ejected from Mars.

Highlights

At present, there are 123 known distinct Martian meteorites totaling a mass of ~140 kg (NASA 2018)
As is typical for Martian samples (e.g., Balta et al 2015b), we found that pyroxene grain boundaries were too poorly defined in Backscattered electron (BSE) to analyze by Crystal size distribution (CSD) and would require techniques designed to target that mineral (e.g., Lentz and McSween 2005), which was beyond the scope of this study
Some small chromium- and aluminum-rich oxide inclusions were found in large olivine grains in Larkman Nunatak (LAR) 12095 and LAR 12240, but in contrast to Tissint, these inclusions were clustered within grains and there were large areas free of these inclusions

Summary

Introduction

There are 123 known distinct Martian meteorites totaling a mass of ~140 kg (NASA 2018). Shergottites (presently totaling 108 distinct samples) are the largest group of Martian meteorites and, based on their modal mineralogy and petrography, they are categorized into basaltic, olivine-phyric, and poikilitic classes (e.g., Goodrich 2002). Based on their minor and trace element abundances, initial Sr, Nd, Hf, and Pb isotope compositions and crystallization ages, the shergottites are categorized into enriched, intermediate, and depleted groups (e.g., Symes et al 2008). We report the petrology and geochemical characteristics of these shergottites to evaluate their pairing, relationship to other shergottites, and their petrogenetic history on Mars

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Results

Discussion

Conclusion