Olivine in Martian meteorite ALH 84001: Evidence for a high-temperature origin and implications for signs of life.
Olivine from martian meteorite ALH 84001 occurs as clusters within orthopyroxene adjacent to fractures containing disrupted carbonate globules and feldspathic shock-glass. The inclusions are irregular in shape and range in size from approximately 40 microns to sub-micron. Some of the inclusions are elongate and boudinage-like. The olivine grains are in sharp contact with the enclosing orthopyroxene, and often contain small inclusions of chromite. The olivine exhibits a very limited range of composition from Fo65 to Fo66 (n= 25). The (18O values of the olivine and orthopyroxene analyzed by ion microprobe range from +4.3 to +5.3deg and are indistinguishable from each other within analytical uncertainty. The mineral chemistries, oxygen isotopic data, and textural relationships indicate that the olivine inclusions were produced at a temperature greater than 800 deg C. It is unlikely that the olivines formed during the same event that gave rise to the carbonates in ALH 84001, which have more elevated and variable (18O values, and were probably formed from fluids that were not in isotopic equilibrium with the orthopyroxene or olivine. The reactions most likely instrumental in the formation of olivine could be either the dehydration of hydrous silicates that formed during carbonate precipitation or the reduction of orthopyroxene and spinel. If the olivine was formed by either reaction during a post-carbonate heating event, the implications are profound with regards to the interpretations of McKay et al. (1996). Due to the low diffusion rates in carbonates, this rapid, high-temperature event would have resulted in the preservation of the fine-scale carbonate zoning, while partially devolatilizing select carbonate compositions on a sub-micron scale (Brearley, 1998). This may have resulted in the formation of the minute magnetite grains that McKay et al (1996) attributed to biogenic activity.