In an article recently published in the journal Geochimica et Cosmochimica Acta, researchers discussed the calcite and fluid exchange rates for carbon and oxygen isotopes during chemical equilibrium.
Background
A window into Earth's past is created through the analysis of the isotopic composition of carbonate minerals, which keep a record of historical geochemical and climatic circumstances. Calcium carbonate minerals are excellent archives of the planet's previous climatic conditions. Effective use of any of these proxies requires knowledge of the mechanisms of isotope fractionation as well as preservation of isotopic and trace element compositions across timescales up to millions of years.
Despite the fact that numerous studies have documented the isotopic alteration of calcium carbonate and a number of other minerals while they were in chemical equilibrium and without morphological alteration visible at the level of electron microscope analysis, the mechanism underlying this alteration of isotopic composition is still not fully understood.
The theory of simultaneous forward and backward reactions has been applied in recent experimental studies to estimate the near-equilibrium reaction rates of a variety of minerals, including calcite, to account for the equilibration of isotopic compositions between fluid and solid after initial precipitation, and to explain isotope exchange at or near chemical equilibrium.
It is unclear what motivates dynamic equilibrium as an isotope exchange process; however, it may be related to isotopic disequilibrium. It is important to figure out the method by which the isotopic composition of calcite changes. For weakly crystalline materials, Ostwald ripening is probably a significant isotopic exchange control in the early stages of mineral precipitation and development. However, well-crystallized and old materials probably have less of a need for this mechanism.
About the Study
In this study, the authors carried out batch reactor experiments in chemical equilibrium between calcite and a fluid enriched in 13C and 18O relative to the solid at 25°C to assess the rates and processes by which C and O isotopes were exchanged between calcite and fluid. To assess the effect of mineral surface area and size on C and O isotope exchange rates, different grain sizes of natural and synthesized calcite were examined.
The team indicated the change of the O and C isotope compositions of calcite at ambient temperatures, which occurred easily over short time scales, while it was unclear how to determine the amount to which this process continued over geologic time scales.
The researchers assessed the rates of change in the C and O isotope composition of calcite at 25 °C and chemical equilibrium, as well as to produce new insights into a potential process. The ion activity product of Ca and CO32- and the solubility product of pure calcite were equivalent in this definition of chemical equilibrium. At chemical calcite-fluid equilibrium, a fluid enriched in 18O and 13C relative to calcite was exposed to both natural and synthetic calcite of various grain sizes in a series of batch reactor studies.
Observations
The specific process of isotope exchange was unclear between 72 hours and 2112 hours during the large calcite experiments and between 72 hours and the conclusion of the tiny and synthetic calcite studies at 3000 hours. The projected dissolving rate of calcite at a pH of 8 was four orders of magnitude lower than the C and O isotope exchange rate. In the synthetic, small, and first and second big calcite tests, the values of the 13C diffusion coefficients for the long-term exchange were 1.3 x 10-24, 1.1 x 10-23, 4.2 x 10-26, and 1.2 x 10-25 m2/s, respectively. In the synthetic, small, and second and first big calcite studies, the equivalent values for 18O diffusion coefficients were 1.3 x 10-24, 1.0 x 10-23, 2.0 x 10-26, and 1.1 x 10-25 m2/s, respectively.
According to the experimental findings, both C and O isotope exchange occurred quickly within 72 hours for all of the calcite grain sizes examined. The C and O isotope exchange rates slowed down after 72 hours but remained very stable over thousands of hours. For all calcite grain sizes examined, surface-area normalized O and C isotope exchange rates were comparable, and C and O were exchanged in a ratio of about 3:1, consistent with the exchange of CO32-.
The rates of O and C exchange were approximately four orders of magnitude lower than the rates of far-from-equilibrium calcite dissolution, which indicated that exchange was either governed solely by the dissolution-precipitation of the pre-existing reactive sites or by a combination of solid-state/aqueous mediated diffusion and dissolution-precipitation.
The findings of this study could serve as a further reminder of the significance of developing a mechanistic understanding of the isotope exchange mechanism in chemical equilibrium and as a first step in that direction.
Conclusions
In conclusion, this study indicated that the alteration of calcite's O and C isotope compositions at Earth's surface temperatures could occur easily over short time periods, while it was currently difficult to determine how far this process would continue over geologic time scales. When there was isotopic disequilibrium, there was a rapid initial exchange between fluid and solid that was mostly due to species exchange on the surface.
Uncertainty persists regarding the mechanism underlying the observed slower rate of change in the C and O isotope composition of calcite across the longer time scales in the performed tests. Longer time periods of greater than 72 hours could see a combination of solid-state/aqueous mediated diffusion and dissolution-reprecipitation led to the change of carbon and oxygen isotopes.
However, the significant utility of carbonate isotopic and trace element compositions underscores the need for a mechanistic understanding of the solid-fluid exchange to enable reliable interpretation of sample selection and archives.
The authors mentioned that this is the first study to examine O and C exchange simultaneously and at bulk calcite-fluid equilibrium with high purity calcite, opening up fresh perspectives on the processes and rates of isotope exchange.
International Conference on Organic Chemistry
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