Isotopic exchange in mineral-fluid systems; I, Theoretical evaluation of oxygen isotopic exchange accompanying surface reactions and diffusion

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doi: 10.1016/0016-7037(83)90018-2
Authors:Cole, David R.; Ohmoto, Hiroshi; Lasaga, Antonio C.
Author Affiliations:Primary:
Pa. State Univ., Dep. Geosci., University Park, PA, United States
Volume Title:Geochimica et Cosmochimica Acta
Source:Geochimica et Cosmochimica Acta, 47(10), p.1681-1693. Publisher: Pergamon, Oxford, International. ISSN: 0016-7037
Publication Date:1983
Note:In English
Summary:Oxygen isotopic exchange between minerals and fluids proceeds through two mechanisms: 1) surface reactions (dissolution, precipitation), and 2) diffusion of O-bearing species along lattice planes or crystal imperfections. The rate constants for the first type of mechanism are related as follows: r = -ln(l - F)(WS)/(W + S)tA, where W and S are the number of moles of oxygen in the fluid and solid, respectively, A is the surface area, t is time and F is the fraction of O-isotopic exchange. Examination of published experimental data on O-isotopic exchange reactions between various phases (e.g. silicates, carbonates, sulphates) and aqueous fluids, using the above equation, yields rate constants of between 10-4 and 10-8 moles/m2/sec in the T range 800-250°C; activation energies range 8-22 kcal/mol and are similar to those of chemical reactions involving fluids and mineral surfaces. Rates of O-isotopic exchange accompanying diffusion of O-bearing compounds through mineral lattices are also calculated from published data using equations that account for fluid/mineral mass ratios of the systems. These rates are found to be several orders of magnitude less than those accompanying surface reactions when the T, fluid/mineral mass ratio and the grain radius of the mineral are similar. These data suggest that the O-isotopic exchange reactions between fluids and rocks in natural systems may proceed in two steps: the first through a surface-controlled mechanism when the fluids and minerals are out of chemical equilibrium, and then through a diffusional mechanism once the systems attain chemical equilibrium. [T.R.]
Subjects:Crystal chemistry; Diffusion; Fluid phase; Geochemistry; Ion exchange; Isotopes; Mechanism; O-18/O-16; Oxygen; Phase equilibria; Precipitation; Processes; Solid phase; Solution; Stable isotopes; Theoretical studies; Transformations; Reactions
Abstract Numbers:84M/2659
Record ID:1984006012
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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