Carbonate minerals in martian meteorite Allan Hills 84001: Implications for environmental conditions on ancient Mars

The carbonate mineralogy of unique carbonate-rich regions in ALH 84001 has been examined. These regions contain carbonate "rosettes" and "slabs", "post-slab" magnesites and "interstitial" carbonates. Slab carbonates reveal early portions of the carbonate growth sequence that suggest that initial nucleation compositions were calcite-normative. The kinetically-controlled growth of rosettes and slabs was followed (and possibly terminated) by an alteration event that formed the MSM layers on the exterior carbonate surfaces. Post-slab magnesite is compositionally similar to the magnesite in the MSM rims but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of textural or compositional changes at glass/carbonate contacts seen on the millimeter, micron and nanometer scale.; Isotopic analyses of carbonates extend the previous range of chemical compositions by ∼28 mol% Ca and the range of isotopic compositions by ∼+9‰. The new carbonate textures were sampled, including post-slab magnesites and slab carbonates. Isotopic analyses also suggest that post-slab magnesites are of a later generation. Impact-induced infiltration of the rock by high-temperature CO2-rich fluids (possibly initially water-rich, later mixing with atmospheric CO2) likely caused significant isotopic fractionation as carbonates precipitated quickly under kinetically controlled conditions, while temperatures decreased.; All ALH 84001 carbonates exhibit similar trace element patterns, and are LREE-depleted relative to HREEs, indicating that they formed from a consistent initial fluid (possibly in trace elemental equilibrium with orthopyroxene), precipitating quickly from high-temperatures. Variations in individual trace elements confirm the presence of multiple generations of carbonate and suggest localized mineral interaction. ToF SIMS analyses also confirm the presence of multiple generations of carbonate.; Martian meteorites exhibit little aqueous alteration, even taking into account the multiple generations of carbonate seen here. This supports suggestions that the martian hydrosphere and lithosphere interacted with less frequency than is sometimes suggested. Future missions may provide information on the unresolved inequality between martian volatiles and alteration minerals.