Stable isotope disequilibria in travertine from high pH waters: Laboratory investigations and field observations from Oman

Abstract

Ultramafic rocks in northern Oman host unusual, hyperalkaline groundwaters ( pH > 11.5) which precipitate inorganic travertine by diffusive uptake of atmospheric CO 2 after discharge. Stable isotope fractionation during CO 2 uptake results in strong depletions of the carbonate phase (δ 13C values as low as −27.5‰ and δ 18O values of −16.9‰ PDB). Experiments under closed and open system conditions show that a kinetic depletion in the order of 15.5‰ for 13C occurs during hydroxylation of aqueous CO 2 (CO 2(aq) + OH − reaction), attributed to a lower activation energy for 12C-O vs 13C-O. The observed 18O depletion can be accounted for by CO 2 reaction with OH − ( ϵ 18O OH −-H 2O ≅ −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 CO 2(g)-CO 2(aq) exchange at the gas/solution interface achieves isotopic equilibrium.