Carbon Isotope Fractionation between CO2 and Carbonate Melt, an Experimental Approach
Abstract
The fractionation of carbon isotopes during the degassing of carbonatite and carbonated-silicate melts is unconstrained. Theoretical and experimental studies investigated the fractionation in the solid calcite/aragonite - CO2 system [1,2], but their applicability to molten carbonate systems is poor. Here, we present our experimental results on carbon isotope fractionation in the carbonate melt - CO2 system, obtained between 750 and 1000 °C under atmospheric pressure. Our experimental approach used an isotopically labelled mixture of Ca-K-Li carbonates that was equilibrated within a continuous stream of labelled CO2 in a vertical gas mixing furnace. The eutectic mix of CaCO3, K2CO3, and Li2CO3 was chosen to supress the high melting point of CaCO3 and assure batch melting. Initial (disequilibrated) isotopic contrasts between carbonate melt and CO2 gas is +5.65‰, -17.76‰ and -31.32‰, forcing the system to approach a common equilibrium fractionation value from two sides. Our approach chose a continuous stream of CO2 to provide a quasi-infinite carbon reservoir, whose carbon isotope composition ( d 13 CCO2) governs the adjustment of the carbonates 13C/12C ratio towards a common equilibrium fractionation. In a set of time-series experiments, the rapidly quenched carbonate melt was characterized for d 13 Ccarbonate melt and subtracted from d 13 CCO2, yielding the fractionation factor Δ13CCO2-carbonate melt for a given experimental temperature.
Our results suggest rapid isotope exchange between CO2 and carbonate melt where isotopic equilibration proceeds within hours, contrasting the very slow equilibration in the solid-state system where progressed partial equilibration requires days to months. Equilibrium Δ13CCO2-carbonate melt amounts to 3.85 - 2.49‰ (±0.1‰) at 750 - 1000 °C, respectively. The results appear in good agreement with previous experimental and theoretical studies on calcite/aragonite, suggesting that the bonding environment in the CO3-groups in calcite/aragonite and carbonate melt are similar enough to not cause any significant differences in isotope fractionation. [1] Bottinga (1969), GCA 33:49-64; [2] Chacko et al. (1991), GCA 55:2867-2882; [3] Scheele & Hoefs (1992), CMP 112:35-45.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMDI43A0036P
- Keywords:
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- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 1009 Geochemical modeling;
- GEOCHEMISTRY;
- 1030 Geochemical cycles;
- GEOCHEMISTRY;
- 3620 Mineral and crystal chemistry;
- MINERALOGY AND PETROLOGY