Response to the Comment by J. Horita and R.N. Clayton on “The studies of oxygen isotope fractionation between calcium carbonates and water at low temperatures”

Abstract

The apparent inconsistency in calcite–water fractionation does occur between the arithmetic combination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2003) An experimental study of oxygen isotope fractionation between inorganically precipitated aragonite and water at low temperatures. Geochim. Cosmochim. Acta 67, 387–399] and the experimental determination of Zhou and Zheng [Zhou G.-T., and Zheng Y.-F. (2005) Effect of polymorphic transition on oxygen isotope fractionation between aragonite, calcite and water: a low-temperature experimental study. Am. Mineral 90, 1121–1130]. To resolve this issue is to acknowledge whether or not the isotope salt effect of dissolved minerals would occur on oxygen isotope exchange between water and the minerals of interest. The question is whether or not a term of mineral–water interaction should be taken into account when calculating mineral–water 10 3ln α factors by an arithmetic combination between theoretical 10 3ln β factors for mineral and water, respectively. The hydrothermal experiments of Hu and Clayton [Hu G.-X., and Clayton R.N. (2003) Oxygen isotope salt effects at high pressure and high temperature, and the calibration of oxygen isotope geothermometers. Geochim. Cosmochim. Acta 67, 3227–3246] demonstrate the absence of isotope salt effect on the oxygen isotope fractionation between calcite and water, and this abnormal behavior reasonably explains the so-called inconsistency in the calcite–water fractionations of Zhou and Zheng (2003, 2005). We argue that the mineral–water correction is still necessary for calculation of fractionations in mineral–water systems. New experimental data for oxygen isotope fractionations involving dolomite and cerussite are consistent with the calculations of Zheng [Zheng Y.-F. (1999a) Oxygen isotope fractionation in carbonate and sulfate minerals. Geochem. J. 33, 109–126], but also shed light on the assumptions used in modifying the increment method. We argue that the modified increment method has developed into a theoretical mean of predictive power for calculation of oxygen isotope fractionation factors for crystalline minerals of geochemical interest.