INFLUENCE OF SOLUTION COMPOSITION AND PRESSURE ON THE RATES OF OXYGEN ISOTOPE EXCHANGE IN THE SYSTEM - CALCITE-H2O-NACL AT ELEVATED-TEMPERATURES

Variations in the extent of oxygen isotope exchange (F, where F = 0 at t = 0; and F = 1 at t = infinity) have been measured in the system calcite-H2O-NaCl as a function of temperature, pressure and concentration of NaCl. The conditions of these hydrothermal experiments are: T = 300-600-degrees-C, P = 1 kbar (0.25 kbar at 300-degrees-C; and 0.55, 1 and 2 kbar at 500-degrees-C only), m NaCl = 0, 0.1, 0.3, 1 and 4, W/S mass (moles) ratio approximately 4, and run time = 211-1956 hr. There is a pronounced increase in the F-values with increasing temperature, salinity, pressure (500-degrees-C example only), and degree of grain rounding and regrowth. The rate of change in F is initially steep but tends to flatten out with increasing NaCl at greater-than-or-similar-to 1 m. This greater degree of recrystallization is probably related to the enhanced solubility of calcite in the presence of NaCl(aq). Data at 500-degrees-C exhibit a trend of increasing F with increasing pressure. Rate constants (log r(f) in mol m-2 s-1 O) calculated from these data range from a maximum value of -6.75 at 600-degrees-C, 1 kbar, 4 m NaCl to a minimum rate of -9.46 at 300-degrees-C, 0.25 kbar, 0 m NaCl. Activation energies were calculated for each NaCl concentration at 1 kbar and vary as 15, 16, 19, 20 and 19 kcal mol-1 for 0, 0.1, 0.3, 1 and 4 m NaCl, respectively. Volumes of activation (DELTAV(double dagger)) estimated from the 500-degrees-C pressure data range from -40 to -9 cm3 mol-1 for 0 and 4 m NaCl, respectively. These data suggest that solvation of ions in solution during the activation step is the dominate process influenced by pressure. Model calculations indicate that isotopic equilibration times are very rapid, on the order of 10's to 100's of years at high temperature (> 500-degrees-C), high salinity (> 1 m) and high pressure ( > 1 kbar), depending on the W/S ratio and initial grain size.