TY - JOUR
T1 - δ44Ca evolution in a carbonate aquifer and its bearing on the equilibrium isotope fractionation factor for calcite
AU - Jacobson, Andrew D.
AU - Holmden, Chris
N1 - Funding Information:
This work was supported by NSF grant EAR-0617585 awarded to A. Jacobson. T. D. Bullen and A. Galy provided comments on an earlier version of this manuscript. Two anonymous reviewers helped improve the version in print. The authors thank editor R. W. Carlson for handling the manuscript.
PY - 2008/6/30
Y1 - 2008/6/30
N2 - To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ44Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ44Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ44Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δcal-w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ44Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.
AB - To improve understanding of Ca isotope transport during water-rock interaction on the continents, we measured dissolved δ44Ca values along a 236 km flow path in the Madison aquifer, South Dakota, where fluids have chemically evolved according to dolomite and anhydrite dissolution, calcite precipitation, and Ca-for-Na ion-exchange over a timescale spanning ~ 15 kyr. We used a reactive transport model employing rate data constrained from major ion mass-balances to evaluate the extent to which calcite precipitation and ion-exchange fractionate Ca isotopes. Elevated δ44Ca values during the initial and final stages of water transport possibly result from calcite precipitation under supersaturated conditions and Ca-for-Na ion-exchange, respectively. However, for the bulk of the flow path, δ44Ca values evolve by mixing between anhydrite and dolomite dissolution, with no fractionation during calcite precipitation under saturated conditions. We attribute the absence of Ca isotope fractionation to the long timescale of water-rock interaction and slow rate of calcite precipitation, which have enabled fluids to chemically and isotopically equilibrate with calcite. We therefore conclude that the equilibrium Ca isotope fractionation factor between calcite and water (Δcal-w) is very close to zero. To the extent that the Madison aquifer typifies other groundwater systems where calcite slowly precipitates from solutions at or near chemical equilibrium, this study suggests that groundwater contributions to δ44Ca variability on the continents can be modeled according to simple mixing theory without invoking isotope discrimination.
KW - calcite precipitation
KW - calcium isotopes
KW - fractionation
KW - groundwater
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U2 - 10.1016/j.epsl.2008.03.039
DO - 10.1016/j.epsl.2008.03.039
M3 - Article
AN - SCOPUS:44649177588
VL - 270
SP - 349
EP - 353
JO - Earth and Planetary Sciences Letters
JF - Earth and Planetary Sciences Letters
SN - 0012-821X
IS - 3-4
ER -