Simple models of coupled fluid infiltration and redox reactions in the crust |
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Authors: | Jay J Ague |
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Institution: | (1) Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520-8109, USA; E-mail: jay.ague@yale.edu; Fax: (203) 432-3134, US |
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Abstract: | Simple one-dimensional numerical models are presented for coupled advection-hydrodynamic dispersion and kinetically controlled
oxidatioin-reduction reactions in graphite-free porous media containing magnetite coexisting with silicate assemblages. Fluid-solid
interactions involving either OH (O2-H2O-H2) or COH (O2-H2O-H2-CO2-CO-CH4) fluids are considered at ∼500 ∘C and 5 kbar. The major implications of the modeling are as follows: (1) Regional (km scale) reduction of typical magnetite-bearing
rocks originally at f
O2 near NNO may be possible during long-term metamorphic fluid flow if the infiltrating fluids have sufficiently low f
O2 and sufficiently large concentrations of CH4 and/or H2. Regional oxidation of such rocks by highly oxidized OH or COH fluids appears to be difficult to achieve. (2) Nearly identical
mineral assemblages and modes may be produced by very different kinetic reaction pathways. The model implies that “equilibrium”
assemblages preserved in rocks may not always reflect the true kinetic reaction path that evolved during fluid flow, and highlights
the need for quantitative measurements of metamorphic reaction rates. (3) Preservation of sharp lithologic contacts between
rocks of very different redox states containing accessory amounts of oxides may be unlikely if fluid-rock interaction times
exceed 103104 years. Substantial contact disruption over these times scales is predicted even for oxide-rich rocks if redox contrasts between
layers are large. Flow across lithologic contacts may produce asymmetric patterns of metasomatic mineral zonation that may
prove useful for mapping flow directions in metamorphic sequences. (4) For fluid flow in typical T gradients through originally homogeneous rock, significant major element metasomatism (e.g., K, Na, Ca) may be possible without
producing large changes in oxide abundances.
Received:12 November 1997 / Accepted: 9 March 1998 |
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