TY - JOUR
T1 - Ca and Mg isotope fractionation during the stoichiometric dissolution of dolomite at temperatures from 51 to 126 °C and 5 bars CO2 pressure
AU - Perez-Fernandez, A.
AU - Berninger, U. N.
AU - Mavromatis, V.
AU - Pogge von Strandmann, P. A.E.
AU - Oelkers, E. H.
PY - 2017/9/20
Y1 - 2017/9/20
N2 - Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1 mol/kg NaCl solutions at 51, 75, 121, and 126 °C and a pressure of 5 bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47 days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9 ± 0.3. All aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75 °C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126 °C was significantly greater such that Δ44/42Casolid-fluid = − 0.6 ± 0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126 °C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126 °C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.
AB - Natural polycrystalline hydrothermal Sainte Colombe dolomite was dissolved in stirred titanium closed system reactors in aqueous 0.1 mol/kg NaCl solutions at 51, 75, 121, and 126 °C and a pressure of 5 bars CO2. In total, 52, 27, 16, and 12%, respectively, of the dolomite placed in the reactors dissolved into the fluid phase during these experiments. Each experiment lasted from 12 to 47 days and the fluid phase in each evolved towards, but did not exceed, ordered dolomite equilibrium at a pH of 5.9 ± 0.3. All aqueous reactive fluids were undersaturated with respect to all potential secondary phases including calcite and magnesite. The reactive fluid compositions at the end of the experiments had a molar Ca/Mg ratio equal to that of the dissolving dolomite, and the dolomite recovered after the experiments contained only pure dolomite as verified by scanning electron microscopy. The Ca and Mg isotopic ratios of the reactive fluids remained within uncertainty equal to that of the dissolving dolomite in the experiments performed at 51 and 75 °C. In contrast, the Ca isotopic composition of the reactive fluid in the experiment performed at 121 and 126 °C was significantly greater such that Δ44/42Casolid-fluid = − 0.6 ± 0.1‰, whereas that of Mg was within uncertainty equal to that of the dissolving mineral. The equilibrium fractionation factors for both divalent cations favor the incorporation of isotopically light metals into the dolomite structure. Our results at 121 and 126 °C, therefore, are consistent with the one-way transfer of Mg from dolomite to the fluid but the two-way transfer of Ca from and to dolomite as equilibrium is approached during its stoichiometric dissolution. The lack of Mg returning to the dolomite structure at these conditions is attributed to the slow dehydration kinetics of aqueous Mg. As more than 12% of the dolomite dissolved during the 121 and 126 °C closed system experiments, our observations indicate a significant change in the Ca isotopic signature of the dolomite during its stoichiometric dissolution. Moreover, as there is no visual evidence for dolomite recrystallization during this experiment, it seems likely that the resetting of Ca isotopic signatures of carbonate minerals can be readily overlooked in the interpretation of natural systems.
KW - Ca isotopes
KW - Dissolution
KW - Dolomite precipitation
KW - Isotopes
KW - Mg
KW - Mineral growth
UR - http://www.scopus.com/inward/record.url?scp=85027217206&partnerID=8YFLogxK
U2 - 10.1016/j.chemgeo.2017.07.026
DO - 10.1016/j.chemgeo.2017.07.026
M3 - Article
AN - SCOPUS:85027217206
SN - 0009-2541
VL - 467
SP - 76
EP - 88
JO - Chemical Geology
JF - Chemical Geology
ER -