Oxygen isotope fractionation in the portlandite-water and brucite-water systems from 125 to 450 °C, 50 MPa

Equilibrium oxygen isotope fractionation factors were determined for the portlandite-water and brucite-water systems from 125 to 425 degrees C, 50 MPa using the partial exchange technique. Reagent grade cryptocrystalline Ca(OH)(2) and amorphous Mg(OH)(2) were reacted with three waters having different initial delta O-18 compositions. Isotope exchange occurred via recrystallization with exchange varying from 40% to 95% at 200 to 425 degrees C, respectively. Equilibrium O-18 brucite-water fractionation factors (10(3)ln alpha) increase from -4.7 +/- 3.5 parts per thousand at 200 degrees C to -3.5 +/- 2.5 parts per thousand at 425 degrees C. These data connect smoothly with previous experimental calibrations at lower and higher temperatures to define a single function valid from 15 to 450 degrees C, as follows: 10(3)ln alpha(brucite-water) = 4.39 x10(6)/T-2-16.95 x 10(3)/T+11.19 where T is temperature in Kelvin. These results confirm the existence of a broad minimum in the fractionation factor for brucite at similar to 250 degrees C. The equilibrium O-18 fractionation factor for portlandite-water varies from -11.1 +/- 2.7 parts per thousand at 125 degrees C to -6.6 +/- 0.1 parts per thousand at 425 degrees C, and can be described by the following function: 10(3)ln alpha(brucite-water) = 5. 61 x 10(6)/T-2-26.29 x 10(3)/T+19.72 where T is temperature in Kelvin. These experimental results indicate that brucite favors O-18 relative to portlandite with brucite- portlandite fractionation decreasing from 8 parts per thousand to 3 parts per thousand from 125 to 425 degrees C. A significant temperature dependent cation mass effect is therefore indicated for cation-OH bonds in hydroxide minerals. The observed fractionation is consistent with quantum theory which predicts that bonds with less massive cations have higher vibrational frequencies and will display a relative affinity for O-18 to stabilize the structure. Brucite-portlandite 18 O fractionation predicted using the increment method is extremely small, opposite in sign (-0.1 parts per thousand to -0.2 parts per thousand), and shows very little dependence on temperature, in poor agreement with the experimental calibration. This indicates that the method does not adequately account for the effect of cation mass on O-18 fractionation within hydroxide minerals. It is suggested that cation-specific parameters within the increment method could be fit to the experimental calibrations reported here to improve prediction of fractionation factors for hydroxides and hydroxyl-bearing aluminosilicates, particularly at low temperate where the cation-mass effect is more significant. (C) 2015 Elsevier Ltd. All rights reserved.