ELECTRON MICROPROBE ANALYSIS OF CARBONATES FROM MARTIAN METEORITE ALLAN HILLS 84001 AND SPITSBERGEN NORWAY: IMPLICATIONS OF CHEMICAL HETEROGENEITY

Martian meteorite Allan Hills 84001 (ALH 84001), an orthopyroxenite, may contain a history of early conditions on Mars, and is notable for its rare chemically zoned carbonates. A terrestrial analog site is Spitsbergen, Norway. Basaltic volcanic rocks host mantle xenoliths with a somewhat similar mineralogy to that in ALH 84001. They have been affected by hydrothermal fluids, leading to deposition of carbonate disks or globules very similar to those seen in ALH 84001. The carbonates occur within the mantle xenoliths and in the fine-grained basalts, particularly in vesicles.

This study reports data collected on the Cameca SX-100 electron microprobe at Johnson Space Center, Houston. Spitsbergen globule carbonates were described by Treiman et al. (2002: EPSL 204, 323 – 332) as cored by ankerite-siderite-magnesite solid solutions, with distinct dolomite (dolomite, magnesite, and/or calcite) rims. In this study, chemical zoning seen in backscatter electron images corresponds with zoning in color seen optically, reported by Koziol (2018: 49th LPSC, Abstract #1446). However, there is not a straightforward match between the chemistry of the clear/light zones in the carbonate and orange-brown/dark zones. Optically dark orange-brown rims could be a magnesite-siderite composition or a dolomite composition. Dark rims were magnesite-siderite compositions with variable Fe content. Clear cores or clear rims were nearly pure magnesite. ALH84001 disks (globules) were cored by Ca, Fe, Mg compositions in between dolomite-ankerite and magnesite-siderite. Rims were Mg-rich magnesite-siderite solid solutions. Interstitial carbonates proved to be too small and too disseminated to analyze.

For both Spitsbergen and ALH 84001 carbonates, trends to more magnesian-rich rim compositions were seen. However, Spitsbergen carbonate compositions and Fe contents were inhomogeneous within and between thin sections. For example, in vesicles of sample 01-SVF-10, some globules were cored by a dolomite composition, and nearby vesicles contained carbonate globules with magnesite-siderite solid solutions. If Spitsbergen is a good analog for Mars, this implies similar inhomogeneity in carbonate chemistry for the source region on Mars of ALH 84001.