STABLE ISOTOPE RECORD OF ARCHEAN AND CAMBRIAN SERPENTINIZING FLUID COMPOSITION FROM STICHTITE (MG-CR CARBONATE HYDROXIDE)

The rare mineral stichtite Mg₆Cr₂[(OH)₁₆|CO₃] · 4H₂O provides a record of the carbon and hydrogen isotope values of serpentizing fluids within ancient environments. This bears directly upon the study of early life on Earth and interpretation of methane signatures on Mars. Samples of stichtite from Cambrian serpentinites of Tasmania, Australia and Archean serpentinites of Barberton, South Africa were analyzed for carbon and hydrogen isotopic homogeneity and values using the ion microprobe at University of Hawaii. Carbon isotope values for stichtite are similar to marine kerogen of the same geological age, and distinct from marine carbonate and atmospheric carbon. Hydrogen isotope values are consistent with values measured for low temperature chrysotile serpentinization by fluids generated from metamorphic dewatering of subducted basalt. This isotopic evidence, plus observations of serpentinite veins cross-cutting stichtite mineralization, suggests that stichtite forms during serpentinization and is not a product of later atmospheric oxidation. It is therefore likely that stichtite H and C provides an isotopic record of serpentinizing fluids during the time of stichtite mineralization.