HR: 0800h
AN: V21A-0590    [Abstracts]
TI: Field Based Constraints on Reaction Rates in the Crust
AU: * Baxter, E F
EM: efb@bu.edu
AF: Boston University, Dept. of Earth Sciences, 685 Commonwealth Ave. , Boston, MA 02215 United States
AB: Modern research in plate boundary processes involving metamorphism frequently employs complex physical models. Such models require some quantification (or assumption) of the rate at which metamorphic reactions, or chemical exchange, proceed in natural systems. Here, a compilation of available quantitative field-based constraints on high temperature reaction rates will be presented. These include quantifications based on isotopic exchange, porphyroblast and reaction corona growth models, geochronology, and textural analysis. Additionally, natural strain rates provide an important upper bound on simultaneous reaction rates by virtue of a direct mechanistic link between reaction and strain that applies in most situations within the deforming crust. These data show that reaction rates attending regional metamorphism are 4-7 orders of magnitude slower than most laboratory-based predictions. A general rate law for regional metamorphic reactions has been derived which best describes these field-based data: log10(Rnet) = .0029T-9.6$\pm$1, where Rnet is the net reaction rate in g/cm2/yr and T is temperature (C) (Baxter 2003, JGSL). Reaction rates attending contact metamorphism differ from laboratory-based predictions by less than 2 orders of magnitude, and are in closest agreement at higher temperatures. Regional metamorphic reaction rates may be limited by comparatively lesser (or transient) availability of aqueous fluid in the intergranular medium, slower heat input, and smaller deviations from equilibrium. Implications of slow natural metamorphic reaction rates may include a delay in the completion of metamorphic reactions which release (or take in) volatiles, and transform the mineralogy of the crust in dynamic plate boundary settings such as subduction zones.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 3660 Metamorphic petrology
DE: 3902 Creep and deformation
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting