Multi-generational carbonate assemblages in martian meteorite Allan Hills 84001: Implications for nucleation, growth, and alteration

The carbonate mineralogy of several complex carbonate-rich regions in Allan Hills (ALH) 84001 has been examined. These regions contain familiar forms of carbonate, as well as textural forms previously unreported including carbonate rosettes, planiform stab carbonates, distinct post-slab magnesites, and carbonates interstitial to feldspathic glass and orthopyroxene. Slab carbonates reveal portions of the carbonate growth sequence not seen in the rosettes and suggest that initial nucleating compositions were calcite-rich. The kinetically controlled growth of rosettes and slab carbonates was followed by an alteration event that formed the magnesite-siderite layers on the exterior surfaces of the carbonate. Post-slab magnesite, intimately associated with silica glass, is compositionally similar to the magnesite in these exterior layers but represents a later generation of carbonate growth. Feldspathic glasses had little or no thermal effect on carbonates, as indicated by the lack of thermal decomposition or any compositional changes associated with glass/carbonate contacts.