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Published June 1, 2005 | public
Journal Article

Microscale carbon isotope variability in ALH84001 carbonates and a discussion of possible formation environments

Abstract

The carbonates in martian meteorite ALH84001 preserve a record of aqueous processes on Mars at 3.9 Ga, and have been suggested to contain signatures of ancient martian life. The conditions of the carbonate formation environment are critical for understanding possible evidence for life on Mars, the history of water on Mars, and the evolution of the martian atmosphere. Despite numerous studies of petrographic relationships, microscale oxygen isotope compositions, microscale chemical compositions, and other minerals associated with the carbonates, formation models remain relatively unconstrained. Microscale carbon isotope analyses of ALH84001 carbonates reveal variable δ¹³C values ranging from +27 to +64‰. The isotopic compositions are correlated with chemical composition and extent of crystallization such that the Mg-poor, early-formed carbonates are relatively ¹³C depleted and the Mg-rich, later forming carbonates, are ¹³C enriched. These data are inconsistent with many of the previously proposed environments for carbonate formation, and a new set of hypotheses are proposed. Specifically, two new models that account for the data involve low temperature (<100°C) aqueous processes: (1) the carbonates formed during mixing of two fluids derived from separate chemical and isotopic reservoirs; or (2) the carbonates formed from high pH fluids that are exposed to a CO₂-rich atmosphere and precipitate carbonate, similar to high pH springs on Earth.

Additional Information

Reviews and comments by John Eiler, James Farquhar, Allan Treiman, and editorial handling by Thomas Chacko significantly improved the manuscript and are gratefully acknowledged. Helpful discussions with Rick Hervig, Paul Knauth, and Mariana Cosarinsky are also valued and appreciated. NASA grants NAG5-10523 and NCC2-1051, as well as NASA GSRP grant NGT-962 are gratefully acknowledged for support of this work and the ASU GeoSIMS laboratory.

Additional details

Created:
August 22, 2023
Modified:
October 25, 2023