Carbon-oxygen isotopic covariation in hydrothermal calcite during degassing of CO₂

Abstract

The effect of the outgassing of CO₂ from a hydrothermal fluid on the C- and O-isotopic compositions of calcite, which is precipitated from this fluid, is quantitatively modelled in terms of batch and Rayleigh distillation equations. Both CO₂ degassing and calcite precipitation are considered to be the removal mechanisms for dissolved carbon species from the fluid. Combined degassing-precipitation models are then developed by taking H₂CO₃ and HCO ⁻₃ , respectively, as the dominant dissolved carbon species. A positive correlation array between δ ¹³C and δ ¹³O values of calcite can be yielded by the precipitation of calcite from a H₂CO ⁻₃ -dominant fluid, accompanied by a progressive decrease in temperature during CO₂ degassing, whereas calcite precipitated from a HCO ⁻₃ -dominant fluid under the same conditions tends to display much smaller variation in δ ¹³C values than in δ ¹⁸O values. The combined processes of CO₂ degassing and calcite precipitation result in lowering the δ ¹³C value of calcites with respect to those precipitated in a closed system simply due to temperature effect. Carbon and oxygen isotopic data for calcite from the Kushikino gold-mining area in Japan illustrate the application of quantitative modelling, and degassing of CO₂ is suggested as a more likely cause for the precipitation of the calcite and quartz in this mining area.