TERRESTRIAL ANALOGS PROVIDE INSIGHTS TO CARBONATE OCCURRENCES IN MARTIAN METEORITE ALLAN HILLS 84001

Martian meteorite Allan Hills 84001 (ALH 84001) is an orthopyroxenite with rare but intriguing carbonate globules (also called rosettes) and disseminated carbonate. These carbonates could tell us about water on Mars, and near-surface conditions, but interpretations of their deposition vary widely. Insight may be gained by examining terrestrial analog sites. For example, Treiman et al. (2002: EPSL 204, 323 – 332) and Koziol and Treiman (2018: GSA Abstracts w/ Program, Abstract 284-6) reported on carbonate globules observed in basalts and peridotite xenoliths from Spitsbergen, Svalbard, Norway. Similarly, MacArtney (2018: Ph.D. thesis, University of Glasgow) examined carbonates from the Semail ophiolite, Oman. Here we use both analogs to provide insight into ALH 84001 carbonates.

Semail, a dry and warm Mars analog, has carbonates that occur as chemically zoned rosettes and slabs (following the nomenclature of Corrigan and Harvey, (2004) MAPS 39, 17-30). They are dolomitic and vary in Ca content. They appear similar to ALH 84001 rosettes but chemically are Mg-rich as the ophiolite is Mg-rich. It is proposed that slow growth and limited nucleation sites led to rosettes, while rapid growth and more nucleation sites led to slab carbonates. Mg-rich shallow subsurface aquifers draining and mixing is thought to lead to carbonate deposition. In Spitsbergen, carbonate zoning trends from Fe-rich Ca-bearing magnesite-siderite compositions to more Mg-rich magnesite compositions, very similar to zoning see in Martian carbonates. Inhomogeneity between zoned rosettes suggests inhomogeneous hydrothermal fluids.

Similar occurrences of zoned carbonates on Earth and in Martian meteorites imply similar origins. Could the ALH 84001 carbonates be formed in a shallow aquifer connected to the atmosphere, as suggested by the Semail occurrences? Or were they formed by hydrothermal waters, as suggested by the Spitsbergen occurrences? Halevy et al. (2011: PNAS, 108, 16895-16899) provided low temperature constraints on the ALH 84001 carbonates. Low temperature hydrothermal or shallow aquifer origins both seem possible.