Isotopic exchange in mineral-fluid systems. II. Oxygen and hydrogen isotopic investigation of the experimental basalt-seawater system

Abstract

Oxygen and hydrogen isotopic exchange reactions between basalt and seawater at T = 300° to 500°C were investigated, using oceanic tholeiitic basalt ( δ 18O = ∼5.7%.; δD = ∼−70%. ), natural seawater ( δ 18O = 1.2%.; δD = + 5%. ) and artificial seawater ( δ 18O = −5.3%. ) as starting materials. The starting basalts varied in the crystallinity (from holocrystalline to glass) but were ground to approximately the same grain size (−100 mesh). The water/rock mass ratios ranged from 1 to 3 and the duration of the experiments ranged from 167 to 576 days. In general, depletion of 18O and enrichment of D in basalts occur at all temperatures, with the magnitudes of change being greater as temperature, time and to a lesser degree, glass content increases. The trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the basalt ( e.g., smectite at 300°C, talc-actinoiite at 400°–500°C). The changes in the δ 18O values of basalts and seawater in the experimental systems were observed to follow closely with those expected from a first-order rate law. Rate constants for the oxygen isotopic exchange between rock and water range from 10 −9.5 to 10 −8.0 moles oxygen/m 2 of solid surface/sec for temperatures of 300° to 500°C. The activation energy for the isotopic exchange reaction was calculated to be 11.5 Kcal/ mol. An application of our experimental rate data to natural systems suggests that the oxygen isotope equilibrium between basalt and seawater in the mid-oceanic ridge may take place within approximately 1000 years at 350°C. Our experimental data also suggest the equilibrium oxygen isotopic fractionation factors between the altered basalt and seawater to be 3.5 ± 0.5%. at 300°C, 2.0 ± 0.4%. at 400°C and 0.5 ± 0.25%. at 500°C. The observed hydrogen isotopic fractionation factors between the altered basalts and seawater in our experimental systems were about −74%. at 300°C. about −62%. at 400°C and about −48%. at 500°C. These fractionation factors are probably attributable to the equilibrium fractionation between Fe-rich secondary phases and seawater.