Isotopic exchange in mineral-fluid systems: III. Rates and mechanisms of oxygen isotope exchange in the system granite- H 2 O ± NaCl ± KCl at hydrothermal conditions

Details Isotopic exchange in mineral-fluid systems: III. Rates and mechanisms of oxygen isotope exchange in the system granite- H 2 O ± NaCl ± KCl at hydrothermal conditions Isotopic exchange in mineral-fluid systems: III. Rates and mechanisms of oxygen isotope exchange in the system granite- H 2 O ± NaCl ± KCl at hydrothermal conditions

Jan 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56..445c&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 56, Issue 1, pp.445-466

Mathematics

Logic

4

Scientific paper

Variations with time in the alteration mineralogy and the oxygen isotopic composition of solutions, minerals, and rocks have been experimentally investigated in the system granite- H 2 O ± NaCl ± KCl at T = 170° to 300° C , P = L / V to 0.3 kb , water / rock mass ratios (0.2 to 6), and grain sizes (~0.1 mm to 2.5 mm) for periods up to 1006 h. Alteration assemblages formed in the experiments are dominated by chlorite (after biotite), sericite-zeolite-albite (after K-feldspar and plagioclase), and hematite (after magnetite and pyrite). The abundance of these minerals continued to increase with increasing reaction time, temperature, NaCl (or KCl) concentration of solution, and surface area of solids. However, major element concentrations attained steady state in less than 330 h. Reaction of rock ( 18 O i ~- +8 ) and aqueous solution ( 18 O i ~- -10 ) resulted in depletion of 18 O in the solid and enrichment of 18 O in the fluid. The magnitudes of change increase as temperature, time, salt concentration, and surface area increase. The trends in isotopic shifts are directly related to changes in the style and intensity of mineralogic alterations in the granite. The degree of isotope exchange in the experimental systems between granite (minerals) and solutions was computed from the comparison with calculated equilibrium fractionation factors and yield values of less than 5 to 50% exchange. The 18 O changes of the rocks, minerals, and fluid were observed to follow closely with those expected from a firstorder rate law. At temperatures between 170 and 300°C, rate constants for the oxygen isotope exchange between granite and pure water range from 10 -9 to 10 -8.1 (moles 0 m -2 sec -1 ) whereas rates in the granite-0. 1m NaCl system range from 10 -8.5 to 10 -7.6 . Rate constants were also retrieved for biotite, K-feldspar, and plagioclase interacted with either pure water or 0.1-1 m NaCl. Based on a simple closed-system model with (W/S) mass ratios between 0.5 and 5, we estimate the minimum time required for granite-fluid isotopic equilibration to be roughly 200 years or less for grains 1 cm in radius or smaller (porous-media analog) reacted at 300°C, and between 10 4 and 10 5 years for grains ranging from 0.1 to 1 m in radius (fractured-media analog) reacted at 300°C.

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