C AND O ISOTOPE COMPOSITION OF CALICHE FORMED IN TWO LOW-TEMPERATURE TERRESTRIAL ENVIRONMENTS AS MARTIAN ANALOGS

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.