Oxygen isotope fractionation between chlorite and water from 170 to 350 deg C: A preliminary assessment based on partial exchange and fluid/rock experiments

摘要

Oxygen isotope fractionations in laboratory systems have been determined between chlorite and water at 170-350 deg 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 [(#SIGMA#Fe)/#SIGMA#Fe + Mg = 0.483] for four durations (132-3282 h) at 350 deg 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 approx 20 weeks is the most reliable of the group and compares very closely with a value of approx -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 per thousand calculated at 350 deg C for chlorite compositions with [(#SIGMA#Fe)/#SIGMA#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 deg C, P = vapor saturation -200 b, salinity = H_2O, 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 #delta#~(18)O of the fluids. The 10~3 In #alpha# values averaged 0.14, 0.8 and 2.9 per mil for 300 deg, 250 deg, and 200 deg C, respectively. A least-squares regression model of the combined data set (all T's) gives the following expression for fractionation: 1000 In #alpha#_(chl-w) = 2.693 (10~9/T~3) - 6.342 (10~6/T~2) + 2.969 (10~3/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).