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.