DECIDING WHAT REACTION RATE TO USE IN YOUR MODEL

Modern research in metamorphism frequently employs complex physical models. Many of these models require some quantification (or assumption) of the rate at which metamorphic reactions, or chemical exchange rates, proceed in natural systems. It has long been accepted that most laboratory-based kinetic experiments – particularly those employing powders bathed in fluids – do not accurately reproduce natural metamorphic conditions and tend to overestimate reaction rates. But, in the absence of other quantitative information, the modeler has a dilemma. If available lab-based kinetic parameters overestimate natural reaction rates, how great is the discrepancy? A factor of two? A hundred? A million? By compiling published quantitative field-based constraints on high temperature (>400 °C) reaction rates from a variety of sources we can begin to address this question. 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±1, where Rnet is the net reaction rate in g/cm2/yr and T is temperature (°C) (Baxter 2003). At the same time, 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. Thus, laboratory-based kinetic data may be judiciously applied to some contact metamorphic systems. Regional metamorphic reaction rates may be limited by comparatively lesser (or transient) availability of aqueous fluid in the intergranular medium. The general field-based rate law may be applied to regional metamorphic, and other environments (i.e. UHP or UHT metamorphism), if similar system characteristics can be inferred.

Reference: Baxter (2003) Natural Constraints on Metamorphic Reaction Rates. In Geochronology: linking the isotopic record with petrology and textures, eds. Vance, Muller, & Villa. Geological Society of London, Special Publication (in press November 2003).