OXYGEN ISOTOPE FRACTIONATION BETWEEN CHLORITE AND WATER FROM 170 TO 350°C: A PRELIMINARY ASSESSMENT BASED ON PARTIAL EXCHANGE AND FLUID/ROCK EXPERIMENTS

Abstract

Oxygen isotope fractionations in laboratory systems have been determined between chlorite and water at 170-350°C. In one series of experiments, the Northrop-Clayton partial exchange method was used where three (sometimes four) isotopically different waters were reacted with chlorite [(#Fe)/#Fe+Mg = 0.483] for four durations (132-3282 h) at 350°C and 250 b. The percents of exchange determined for the four times from shortest to longest are 4.4, 6.5, 8.0, and 11.9. The fractionations calculated from the Northrop and Clayton (1966) method are in modest agreement for the four run durations: 0.13, 0.26, -0.46, and -0.55 per mil. Errors associated with each of these fractionations are quite large (e.g., +/-1.2 per mil for the longest run). The value determined for the longest run of ~20 weeks is the most reliable of the group and compares very closely with a value of ~ -0.7 per mil estimated by Wenner and Taylor (1971) based on natural chlorites. Good agreement is also observed with the estimates, -1.2 and -1.3%% calculated at 350°C for chlorite compositions with [(#Fe)/#Fe+Mg] = 0.313 and 0.444, respectively, from equations given by Savin and Lee (1988) based on their empirical bond-type method.Additional fractionation data have been estimated from hydrothermal granite-fluid experiments where chlorite formed from biotite. Detailed thin section, scanning electron microscope (SEM), x-ray diffraction (XRD), and electron microprobe analyses demonstrate that biotite is altered exclusively to chlorite in 13 granite-fluid experiments conducted at the following conditions: T = 170-300°C, P = vapor saturation - 200 b, salinity = H2O, 0.1 and 1 m NaCl, fluid/biotite mass ratios = 3-44, run durations = 122-772 h. The amount of chlorite, quantified through point counting and XRD, increased with increasing temperature, salinity, and time. The isotope compositions of chlorite were calculated from mass balance and compared to the final measured δ18O of the fluids. The 103ln α values averaged 0.14, 0.8 and 2.9 per mil for 300°, 250°, and 200°C, respectively. A least-squares regression model of the combined data set (all T's) gives the following expression for fractionation: 1000 ln αchl-w=2.693 (109/T3)-6.342 (106/T2)+2.969 (103/T) The curve described by this equation is in very good agreement with empirical curves given by Wenner and Taylor (1971), Savin and Lee (1988), and Zheng (1993).