Mathematics – Logic
Scientific paper
Jun 1987
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987gecoa..51.1523c&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 51, Issue 6, pp.1523-1538
Mathematics
Logic
Scientific paper
Oxygen and hydrogen isotopic exchange reactions between basalt and seawater at T = 300° to 500°C were investigated, using oceanic tholeiitic basalt ( 18 O = ~5.7%.; D = ~-70%.), natural seawater ( 18 O = 1.2%.; D = + 5%.) and artificial seawater ( 18 O = -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 18 O 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 18 O 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.
Cole David R.
Mottl Michael J.
Ohmoto Hiroshi
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