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Preferential formation of ^(13)C–^(18)O bonds in carbonate minerals, estimated using first-principles lattice dynamics

Schauble, Edwin A. and Ghosh, Prosenjit and Eiler, John M. (2006) Preferential formation of ^(13)C–^(18)O bonds in carbonate minerals, estimated using first-principles lattice dynamics. Geochimica et Cosmochimica Acta, 70 (10). pp. 2510-2529. ISSN 0016-7037.

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Equilibrium constants for internal isotopic exchange reactions of the type: Ca^(12)C^(18)O^(16)O_2+Ca^(13)C^(16)O_3 ↔ Ca^(13)C^(18)O^(16)O_2+Ca^(12)C^(16)O_3 for individual CO_3^(2−) groups in the carbonate minerals calcite (CaCO_3), aragonite (CaCO_3), dolomite (CaMg(CO_3)_2), magnesite (MgCO_3), witherite (BaCO_3), and nahcolite (NaHCO_3) are calculated using first-principles lattice dynamics. Calculations rely on density functional perturbation theory (DFPT) with norm-conserving planewave pseudopotentials to determine the vibrational frequencies of isotopically substituted crystals. Our results predict an ∼0.4‰ excess of ^(13)C^(18)O^(16)O_2^(2-) groups in all studied carbonate minerals at room-temperature equilibrium, relative to what would be expected in a stochastic mixture of carbonate isotopologues with the same bulk ^(13)C/^(12)C, ^(18)O/^(16)O, and ^(17)O/^(16)O ratios. The amount of excess ^(13)C^(18)O^(16)O^(2-)_2 decreases with increasing temperature of equilibration, from 0.5‰ at 0 °C to <0.1‰ at 300 °C, suggesting that measurements of multiply substituted isotopologues of carbonate could be used to infer temperatures of ancient carbonate mineral precipitation and alteration events, even where the δ^(18)O of coexisting fluids is uncertain. The predicted temperature sensitivity of the equilibrium constant is ∼0.003‰/°C at 25 °C. Estimated equilibrium constants for the formation of ^(13)C^(18)O^(16)O^(2-)_2 are remarkably uniform for the variety of minerals studied, suggesting that temperature calibrations will also be applicable to carbonate minerals not studied here without greatly compromising accuracy. A related equilibrium constant for the reaction: Ca^(12)C^(18)O^(16)O_2+Ca^(12)C^(17)O^(16)O_2 ↔ Ca^(12)C^(18)O^(17)O^(16)O+Ca^(12)C^(16)O_3 in calcite indicates formation of 0.1‰ excess ^(12)C^(18)O^(17)O^(16^O^(2−) at 25 °C. In a conventional phosphoric acid reaction of carbonate to form CO_2 for mass-spectrometric analysis, molecules derived from ^(13)C^(18)O^(16)O_2^(2-) dominate (∼96%) the mass 47 signal, and ^(12)C^(18)O^(17)O^(16)O^(2−) contributes most of the remainder (3%). This suggests that carbonate internal equilibration temperatures can be recovered from acid-generated CO_2 if abundances of isotopologues with mass 44–47 can be measured to sufficient precision. We have also calculated ^(18)O/^(16)O and ^(13)C/^(12)C reduced partition function ratios for carbonate minerals, and find them to be in good agreement with experiments and empirical calibrations. Carbon and oxygen isotope fractionation factors in hypothetical ^(40)Mg—magnesite and ^(40)Ba—witherite indicate that M^(2+)-cation mass does not contribute significantly to equilibrium isotopic fractionations between carbonate minerals.

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Additional Information:© 2006 Elsevier Inc. Received 2 September 2005; accepted in revised form 3 February 2006; Available online 18 April 2006. Associate editor: Juske Horita. This work is supported by the National Science Foundation, including grant EAR-0345433 to E.A.S. and a period two technician support grant to P.G. and J.M.E., and by a Packard Fellowship to J.M.E. We thank Juske Horita and an anonymous reviewer for thoughtful and constructive reviews of this work. Discussions with Weifu Guo about stable-isotope fractionation and ^(13)C–^(18)O clumping effects were particularly helpful during the initial preparation of this manuscript.
Funding AgencyGrant Number
Packard FellowshipUNSPECIFIED
NSF Period Two Technician Support GrantUNSPECIFIED
Issue or Number:10
Record Number:CaltechAUTHORS:20121031-092521573
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Official Citation:Edwin A. Schauble, Prosenjit Ghosh, John M. Eiler, Preferential formation of 13C–18O bonds in carbonate minerals, estimated using first-principles lattice dynamics, Geochimica et Cosmochimica Acta, Volume 70, Issue 10, 15 May 2006, Pages 2510-2529, ISSN 0016-7037, 10.1016/j.gca.2006.02.011. (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:35200
Deposited By: Ruth Sustaita
Deposited On:31 Oct 2012 18:47
Last Modified:03 Oct 2019 04:26

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