Theoretical constraints on the effects of added cations on clumped, oxygen, and carbon isotope signatures of dissolved inorganic carbon species and minerals
- California State Univ. (CalState), Long Beach, CA (United States)
- Univ. of California, Los Angeles, CA (United States)
Carbonate clumped isotope thermometry has emerged as a useful tool for the reconstruction of precipitation temperatures for minerals from surface and subsurface environments. Few studies have examined the impact of variable solution chemistry and mineral chemistry on clumping. Therefore, we used theoretical calculations to predict the impact of the presence of cations in a solution on the equilibrium clumped, oxygen (18O/16O and 17O/16O, θ and κ), and carbon isotopic (13C/12C) compositions of dissolved inorganic carbon species (DIC), and the impact of cation type and stoichiometry on carbonate mineral composition within the calcite-dolomite-magnesite solid solution. We examined the impact of the presence of Mg2+, K+, Na+, and Ca2+ on equilibrium Δ63 values of carbonate ion groups in DIC species and the composite DIC pool using the B3LYP/6-311++G(2d, 2p) model. We developed new cluster models for dolomite, magnesite, and high-magnesium calcite (Ca0.83Mg0.17)CO3 and (Ca0.67Mg0.33)CO3) and examined the effect of extraneous cations substituted into the crystal lattices of the carbonate minerals. We find the presence of added cations in the DIC pool increases equilibrium Δ63 values slightly, depending upon their solution concentrations. For the cations examined, the identity of the added cation does not seem to be significant. Cumulative effects may account for small-scale differences (<0.02‰) but are unlikely to be substantial unless cation/anion association and ion pairing are important in the solution that carbonates are growing from. We also find that at a given temperature, stoichiometric dolomite and magnesite Δ63 values are similar to calcite and less than aragonite. Surprisingly, substitution of an extraneous cation into a carbonate mineral (e.g., as observed in high-magnesium calcite) generally lowered Δ63 by a slight amount, which suggests that high and low-magnesian calcites may have resolvable differences in equilibrium clumped isotope signatures, potentially providing an explanation for observations of different Δ47 values in slow-growing minerals. These results imply the cationic composition of the solution may be a minor factor that should be considered for crystals that rapidly grow from solution, and that cationic substitution into carbonate mineral lattices will influence equilibrium compositions of carbonate minerals. Solution chemistry effects on mineral composition are likely to be observed at high growth rates. In contrast, equilibrium isotope compositions arising from cationic substitution in minerals, such as differences between high-Mg calcite and low-Mg calcite, or high-Mg calcite and dolomite, would likely be observed at slow growth rates.
- Research Organization:
- Univ. of California, Los Angeles, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- Grant/Contract Number:
- SC0010288
- OSTI ID:
- 1802072
- Alternate ID(s):
- OSTI ID: 1575806
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 269; ISSN 0016-7037
- Publisher:
- Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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Related Subjects
isotopologues
paleothermometry
carbonate speciation
solvation
carbonate minerals
dissolved inorganic carbon
clumped isotopes
clumped isotope thermometry
oxygen isotopes
carbon isotopes
triple oxygen isotopes
DIC with added cations
high and low magnesium calcite
isotopomeres
isotope fractionation
calcite
calcium carbonate