FLUID-FLOW AND STABLE ISOTOPIC ALTERATION IN ROCKS AT ELEVATED-TEMPERATURES WITH APPLICATIONS TO METAMORPHISM

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 H2O/cm2 flowing through a carbonate or quartzo-feldspathic rock at 600-degrees-C along a temperature gradient of +25-degrees-C/km will cause deltaO-18 to decrease by almost-equal-to 5 parts per thousand. 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 O-18-depletions of 5-8 parts per thousand in contact and regional metamorphic terranes can be explained by flow of 2-50 . 10(4) mol/cm2 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.