Kinetic Fractionation of Stable Isotopes in Carbonates on Mars: Terrestrial Analogs

An ancient Martian hydrosphere consisting of an alkali-rich ocean would likely produce solid carbonate minerals through the processes of evaporation and/or freezing. We postulate that both (or either) of these kinetically-driven processes would produce carbonate minerals whose stable isotopic compositions are highly fractionated (enriched) with respect to the source carbon. Various scenarios have been proposed for carbonate formation on Mars, including high temperature formation, hydrothermal alteration, precipitation from evaporating brines, and cryogenic formation. 13 C and 18 O-fractionated carbonates have previously been shown to form kinetically under some of these conditions, ie.: 1) alteration by hydrothermal processes, 2) low temperature precipitation (sedi-mentary) from evaporating bicarbonate (brine) solutions, and 3) precipitation during the process of cryogenic freezing of bicarbonate-rich fluids. An important feature of carbonate mineral behavior in sediments and during diagenesis is the result of their unique kinetics of dissolution and precipitation. The degree of disequilibrium is one of the primary factors controlling the rate of reaction of carbonate minerals in aqueous solutions and, in general, the rate of reaction tends to increase with increasing disequilibrium. Additionally , different chemical and physical processes are typically involved in given chemical reactions. For a given set of conditions any one of these processes could be slower than all the others, thus this process is the rate controlling (kinetic) step [1]. If indeed a Martian hydrologic cycle included the presence of alkali-rich oceans, which were subject to either evaporation or freezing (both kinetically-controlled processes), the result would undoubtedly be an increase in salinity and alkalinity of those oceans. Under these conditions an enrichment in the carbon isotope composition of the dissolved inorganic carbon (DIC) in the brine would be expected. Subsequent carbonates precipitating from that brine would also be isotopically enriched. This process could explain the formation of 18 O-and 13 C-enriched carbonates similar those found in ALH84001 [2]. Here we examine several terrestrial field settings within the context of kinetically-controlled carbonate precipitation where stable isotope enrichments have been observed. Low temperature carbonate precipitation from evaporating brines-Wide variations in O and C isotopes have been shown within the weathering products (caliche) on < 1 Ma basalts in three volcanic fields in Arizona [3]. In all cases, the most 18 O and 13 C enriched samples were of calcite developed on pinnacles, knobs, and flow lobes that protruded above tephra, rubble , and soil. Here calcite had δ 18 O values of +19‰ to …