STABLE ISOTOPE DISEQUILIBRIA IN TRAVERTINE FROM HIGH PH WATERS - LABORATORY INVESTIGATIONS AND FIELD OBSERVATIONS FROM OMAN

Ultramafic rocks in northern Oman host unusual, hyperalkaline groundwaters (pH > 11.5) which precipitate inorganic travertine by diffusive uptake of atmospheric CO2 after discharge. Stable isotope fractionation during CO2 uptake results in strong depletions of the carbonate phase (delta-C-13 values as low as -27.5% and delta-O-18 values of - 16.9% PDB). Experiments under closed and open system conditions show that a kinetic depletion in the order of 15.5% for C-13 occurs during hydroxylation of aqueous CO2 (CO2(aq) + OH- reaction), attributed to a lower activation energy for C-12-O vs C-13-O. The observed O-18 depletion can be accounted for by CO2 reaction with OH- (epsilon-O-18OH--H2O is-approximately-equal-to -40%) without subsequent exchange and with no evidence of significant additional kinetic effects. Calculated reaction rates show hydroxylation to be the rate limiting step and that CO2(g)-CO2(aq) exchange at the gas/solution interface achieves isotopic equilibrium.