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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 · 10⁵ mol H₂O/ cm² 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 …