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