Near Equilibrium ^(13)C-^(18)O Bonding During Inorganic Calcite Precipitation under Chemo-Stat Conditions

Abstract

We report results of ^(13)C/^(12)C, ^(18)O/^(16)O, and ^(13)C-^(18)O "clumped" isotope analyses from a series of calcite precipitation experiments from aqueous solutions under laboratory conditions. Chemo‐stat precipitation experiments were performed to synthetically form calcite from aqueous solution onto ^(43)Ca‐labeled calcite seed crystals. Formation rate was controlled during the experiments to investigate the effect of precipitation rate and temperature on ^(13)C-^(18)O bonding in calcite, where rates ranged from 10^(−6.88) to 10^(−8.20)mol m^(−2) s^(−1) at three temperatures (10, 20, and 30°C). No relation was observed between precipitation rate and ^(13)C-^(18)O bonding proportion over the range of precipitation rates used. The relation between Δ_(47) and temperature produced was comparable to calibration studies which report a relatively high sensitivity of ^(13)C-^(18)O bonding to temperature over the range investigated. Comparing solution conditions across multiple experimental data sets indicates an inverse relation between saturation state and ^(13)C-^(18)O bonding, where high super‐saturation conditions are likely to be furthest from equilibrium ^(13)C-^(18)O partitioning. Carbon fractionation between calcite and HCO-_(3(aq)) was found to be a temperature independent value of +1.6‰. The temperature‐dependent calcite‐water ^(18)O/^(16)O fractionation relation determined in this study is slightly different (larger α_(calcite-H_2O) value) than those measured in several previous investigations. Significantly, we observe a dependence of the 18O/16O isotope fractionation factor on growth rate. Taken together, these findings suggest carbonate growth in our experiments approached equilibrium more closely than previous experiments of this type, yet did not achieve full O isotope equilibrium.

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