INVESTIGATION OF CARBONATE GLOBULES IN MARTIAN METEORITE ALLAN HILLS 84001 AND AN EARTH ANALOG FROM SVALBARD, NORWAY

Careful consideration must be made of terrestrial analogs when interpreting potential biosignatures on Mars, in Martian samples, or on other terrestrial planets or moons. Of interest here are the carbonate globules found in Allan Hills 84001, an orthopyroxene-rich Martian meteorite. The meteorite has had a multi-stage history involving shock events, carbonate deposition, and at least one later shock event (Treiman 1998, Met. & Planet. Sci. 33, 753 – 764). Putative biosignatures in this meteorite have been described by McKay et al. (1996, Science, 276, 924 – 930) but consensus is now centered on a low temperature hydrothermal/shock origin for these features.

An Earth analog to these carbonate globules is found in altered basaltic rocks from young volcanoes on Svalbard, Norway. Svalbard occurrences formed inorganically, from carbonate-rich hydrothermal alteration of host basaltic rocks and included mantle xenoliths (Treiman et al. 2002, Earth. Planet. Sci. Lett. 204, 323 – 332).

We examined thin sections from Svalbard and 2 thin sections of ALH 84001 (ALH 84001, 3 and ALH 84001, 308). The ALH 84001 thin sections contained chromite and very low amounts of carbonate, as well as abundant orthopyroxene. Crushed zones were pervasive. Only partial carbonate globules were observed, but a number of these showed the core to rim zoning described by other authors (McKay & Lofgren, 1997, LPSC XXVII abstract # 1799). Two Svalbard samples were glass-rich with plagioclase crystallites, and some clinopyroxene phenocrysts. Zoned carbonate globules occurred in vesicles in these samples. One sample was of an olivine and clinopyroxene-rich xenolith, which also contained carbonate globules. In this sample some globules were associated with interstitial glass, and some were in fracture zones between mineral grains. Using optical microscopy we observed the many similarities between the ALH 84001 and Svalbard occurrences, in particular the size, shape, and chemical zoning of the globules (as shown by changing optical characteristics from core to rim).

Further work will include chemical analysis by electron microprobe of the details of the chemical zoning of both the ALH 84001 samples and the Svalbard samples.