Field measurement of high temperature bulk reaction rates II: Interpretation of results from a field site near Simplon Pass, Switzerland

Abstract

Bulk metamorphic reaction rates have been measured using Sr isotopes as tracers of reactive transport processes at a field site near Simplon Pass, Switzerland, employing the technique described in Part I of this study (Baxter and DePaolo, 2002a). The reaction rate, R, inferred, similar to the value first reported in Baxter and DePaolo (2000), is 1.4_−0.4^+1.1 × 10⁻⁷ yr⁻¹ (or R̄ = 7.0_−2.0^+5.5 × 10⁻⁹ g/cm²/yr normalized to the geometric surface area of plagioclase) which is several orders of magnitude slower than extrapolations of laboratory data. This rate suggests that attainment of local equilibrium may require 10’s of millions of years, and consequently equilibrium may not be closely approached in dynamic metamorphic systems if significant changes in P-T-X occur over shorter timescales. These data suggest that there is a problem in current extrapolations or applicability of laboratory kinetic data to natural high-temperature systems. Possible reasons for this discrepancy include differences in reactive surface area, reactant supply limited by slow diffusion of major elements, or an armoring effect by which access to grain interiors is limited. On the basis of microchemical patterns in plagioclase, we identify grain boundary migration (GBM) as the rate limiting process for accessing grain interiors, and thus for bulk reaction. Calculation of reaction quotients for six balanced mineral reactions suggests local disequilibrium for garnet with respect to its Ca content. The characteristic equilibrium lengthscales (Le) for Sr and Ca are both about 1 meter. Equilibrium based geochemical tools such as isochron geochronology or geothermobarometry are compromised within a distance Le of the lithologic contact. Bulk Sr diffusivity measured at the field site changes from 10⁻⁹ to 10⁻⁷ m²/yr, suggesting that the characteristics of the intergranular transporting medium (ITM) changed significantly during the early retrograde history of the field site.