Two populations of carbonate in ALH84001: Geochemical evidence for discrimination and genesis

We present major and trace-element, oxygen isotope, textural, and structural data for carbonates and related phases in the SNC meteorite ALH84001. These data document the existence of at least two distinct carbonate populations: one composed of finely zoned, chemically and isotopically heterogeneous concretions of magnesio-siderite with distinct white magnesite rims, and a second composed of relatively homogeneous, isotopically and compositionally simple domains of ankeritic carbonate and intimately intergrown glass and fine-grained pyroxene. We suggest on the basis of textural evidence and geochemical systematics that the first population consists of low-temperature aqueous precipitates, and the second is produced by shock melting of the first. Values of delta(18)O and Sr/Ca ratios are correlated with one another in magnesio- side rite concretions; the trend formed by these data is consistent with the predicted relationship for inorganic precipitation of carbonate from a solution of constant composition between temperatures of similar to 190degreesC (for concretion cores) to 20degreesC (for magnesite-rich concretion rims). Given the assumptions inherent in this temperature estimate, the aqueous fluid parental to carbonate concretions is constrained to have a delta(18)O of -5parts per thousand, VSMOW (significantly mass fractionated compared with expected juvenile martian volatiles) and minor-element abundances broadly similar to terrestrial seawater. Copyright (C) 2002 Elsevier Science Ltd.