A PLETHORA OF SNCS: CAPITALIZING ON THE LITHOLOGICAL DIVERSITY AMONG THE MARTIAN METEORITES

A major contribution to advancing the study of martian meteorites is the very direct and comprehensive review paper entitled “The SNC Meteorites are from Mars” (Treiman 2000, Planet. Space Sci.). This paper summarizes the prior two decades of work that established the origin of the shergottite-nakhlite-chassignite (“SNC”) meteorites, and remains required reading for students of Mars science today. Only 14 meteorites were known at the time; today that number is well over 170 (including pairings on the basis of petrography). The shergottites are the dominant group, comprising over 80% by number; the nakhlites and chassignites make up another 14% (Udry et al 2020, JGR-Planets). The remaining few are “one-offs”, including the infamous Allan Hills 84001, NWA 7034 (and pairs), and the so-called augite-rich shergottites (Herd et al. 2017, GCA; Lapen et al. 2017, Sci. Adv.). As noted by Treiman et al. (2000), the ages of the SNC meteorites (from 180 to over 4000 Ma) are consistent with the varied ages of the rocks at the surface of Mars based on cratering chronology; this remains so. The abundance of relatively young rocks in the martian meteorite suite (namely the shergottites, ~165 to ~700 Ma; Udry et al. 2020) is attributable to the bias in the impact-ejection process, which favors more competent (e.g., unaltered, igneous) rock types (e.g., Warren, 1994, Icarus; Walton et al. 2008, GCA).

With the increase number of martian meteorites, as well as the surface rock compositions provided by rover missions, we now have a plethora of igneous (and sedimentary) lithological diversity to explore. This provides an opportunity to apply a tested-and-true Treiman technique: the use of bulk element concentrations to compare different types of martian igneous rocks, and provide insights into their origins.

The augite-rich shergottites NWA 7635 and NWA 8159 present an interesting example in this regard. With ~0.36 wt% Ti and ~800 ppm K, these rocks are within the range of (K- and Ti-poor) shergottites (cf. Treiman and Filiberto, 2015, M&PS). However, they are distinct in crystallization ages of ~2400 Ma, augite compositions, bulk REE patterns, and oxygen fugacity (Herd et al. 2017; Lapen et al. 2017). A similar approach will be important for future studies of newly discovered martian meteorites, as well as returned samples from Mars.