2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 9
Presentation Time: 8:00 AM-12:00 PM


SOCKI, Richard A., Astromaterials Research and Exploration Science, NASA/LM, Mail Code C-23, 2400 NASA Road, Houston, TX 77058, GIBSON, Everett K., NASA/Johnson Space Ctr, 2101 NASA Rd. 1, Houston, TX 77058 and PERRY, Eugene C., Department of Geology and Environmental Geosciences, Northern Illinois Univ, 312 Davis Hall, De Kalb, IL 60115-2854, rsocki@ems.jsc.nasa.gov

We report the C and O isotope composition of microcrystalline carbonates (caliche) from two low-temperature terrestrial environments: 1) the tsekel zone of N. Yucatan, Mexico; and 2) Two volcanic fields in Arizona, USA. These data are used as terrestrial analogs to gauge whether carbonates found within Martian meteorites could have formed under similar conditions on the planet Mars.

Yucatan carbonate δ18O(SMOW) values range from +23.78 to +31.07‰, while δ13C(PDB) values range from -13.64 to +1.68‰. The most 18O and 13C enriched carbonates consist of mud formed near the edge of a saltpan; the result of intense evaporation of HCO3- -rich fluids. 18O and 13C isotope composition of tsekel zone caliches, consisting of single hand specimens and layered samples, are more depleted. The layered samples contain caliche “crusts” which form layers within vugs and fractures in the host rock. In all cases the caliche crusts are isotopically lighter than their hosts. 13C depletion within the crusts implies alteration in the vadose zone. In the Yucatan, light soil-gas CO2, derived from oxidation of organic matter, dissolves in vadose water and mixes with heavier marine limestone-derived carbon. The result is HCO3- fluid with an intermediate δ13C composition. Eventually the HCO3- precipitates as layers of caliche or microcrystalline calcite in vugs and veins on or within the host limestone and is δ13C-depleted.

Arizona samples consist of caliche collected from two volcanic fields. δ18O(SMOW) values range from +22.88 to +33.56‰, while δ13C(PDB) values range from -5.02 to +11.21‰. Pedogenic caliches, presumed to have been in contact with isotopically light soil gas, are δ13C-depleted, similar to the Yucatan caliches. Carbonates with heavier 18O and 13C values show the effects of forming from 18O-rich evapo-concentrated HCO3- water. These data suggest 13C enrichment due either to removal of 12C by photosynthesizers in the evaporating drops or from evaporation effects and/or during freezing.

If indeed Mars once had a hydrologic cycle where HCO3- fluids were subjected to evaporation and/or freezing, precipitated carbonate minerals would in turn be 18O-and 13C-enriched. Subsequent alteration could produce Martian carbonates whose δ13C and δ18O composition is highly fractionated with respect to the evaporation-produced carbonates.