Why most "dry" rocks should cool "wet"

A new consideration of oxygen isotope resetting among metamorphic minerals is made accounting for (1) the possibility of f(H2O)-buffering by typical m ineral assemblages during cooling and (2) experimental data that show that high f(H2O) correlates with high diffusion rates. Isotope closure temperatu res in buffered rocks are intermediate between simpler predictions based on "wet" (1 kbar hydrothermal) and "dry" (P less than or equal to 1 atm, H2O- absent) diffusion experiments, but are typically within similar to 50 degre es C of closure temperature estimates that use "wet" diffusion rates, yet 2 00-300 degrees C different from "dry." Even though many rocks may be "dry" in that they lack a hydrous fluid that is physically present during cooling , buffering of f(H2O) results in quasi-"wet" diffusion rates. Re-evaluation of published data shows that most rocks indeed exhibit substantial isotope resetting that is best matched by predictions of f(H2O)-buffering models. Wet- and dry-diffusion models somewhat overestimate and greatly underestima te resetting respectively. Previous interpretations invoking "dry" diffusio n rates may derive from erroneous fractionation factors or faster cooling r ates than assumed, The rare preservation of isotope closure temperatures th at are higher than predicted may reflect faster than expected cooling rates or extraordinarily low f(H2O) in conjunction with anhydrous assemblages.