Computer Science
Scientific paper
Nov 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994gecoa..58.4767r&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 58, Issue 21, pp.4767-4777
Computer Science
5
Scientific paper
A three-component, diffusion-limited, open-system exchange model for pyroxene, olivine and fluid is presented that reproduces the range of oxygen isotope compositions of coexisting silicates from mantle-derived samples. Closed-system exchange cannot reproduce observed disequilibrium fractionations. Measured oxygen self-diffusion data were used to constrain fluid-silicate reaction rates. The magnitudes of fluid-mineral fractionations for typical mantle fluids are presented and show that equilibrated fluid will always be more 18 O-rich than coexisting silicates. Three conditions are necessary to generate the disequilibrium pyroxene-olivine oxygen isotope fractionations observed in the mantle dataset. There must be at least an order of magnitude difference between the fluid-pyroxene and fluid-olivine reaction rates, fluid and solid phases must be out of equilibrium prior to reaction, and fluid must be moving several orders of magnitude faster than it is reacting with the silicates. Assuming initial isotopic equilibrium between pyroxene and olivine and reasonable porosities, the following constraints apply. If pyroxene is reacting faster than olivine, to be expected when pyroxene and olivine grain sizes are subequal, a fluid flow rate of at least 5 × 10 3 times the pyroxene/fluid reaction rate is needed to generate the most extreme negative pyroxene-olivine fractionations seen in natural samples. If olivine reacts faster, the case when its effective grain size is <1% that of pyroxene, flow rates of 1 × 10 4 to 5 × 10 4 times the fluid/pyroxene reaction rate are required to have the same effect. The composition of the fluid phase and starting mantle necessary to reproduce the mantle dataset are dependent on which silicate is the faster reactant. Using self-diffusion data to constrain reaction rates, flow rates >0.2 cm/y at 1200°C and >3 cm/y at 1400°C are needed when pyroxene is the faster reactant. Flow rates >1 cm/y at 1200°C and >15 cm/y at 1400°C are necessary in the olivine dominated system. Regardless of which silicate reacts faster, large amounts of fluid are required, with fluid/rock ratios approaching unity, implying a fluid-focusing mechanism operating in the mantle source region of the xenoliths. Measurable, negative, disequilibrium pyroxene-olivine 18 O fractionations are very short-lived at mantle conditions, surviving <10 5 y at temperatures above 1200°C.
Kurtis Kyser T.
Rosenbaum Jeffrey M.
Walker David
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