Fluid flow and stable isotopic alteration in rocks at elevated temperatures with applications to metamorphism

Abstract

A simple model quantitatively predicts the shifts in stable isotope composition that result from fluid flow through rocks along a temperature gradient in flow systems where local fluid-rock isotope exchange equilibrium is attained. Equilibrium fluid flow along a temperature gradient is a potent mechanism for stable isotopic alteration in rocks: for example, a time-integrated fluid flux of 1 · 10 5 mol H 2O/ cm 2 flowing through a carbonate or quartzo-feldspathic rock at 600° C along a temperature gradient of +25°C/km will causeδ 18O to decrease by ≈5‰. Using the model, measured isotopic shifts in rocks can be quantitatively interpreted as records of time-integrated fluid flux and of flow direction relative to fossil temperature gradients in metamorphic terranes and other high-temperature, deep-crustal fluid flow systems. Inferred whole-rock 18O-depletions of 5–8‰ in contact and regional metamorphic terranes can be explained by flow of 2–50 · 10 4 mol/cm 2 aqueous fluids through rocks in the direction of increasing temperature if local mineral-fluid equilibrium is maintained. Stable isotope alteration in metamorphic rocks does not require infiltration of chemically exotic, non-equilibrium fluids, and therefore does not necessarily provide information about the source of infiltrating fluids.