FAST GRAIN-BOUNDARY - A FORTRAN-77 PROGRAM FOR CALCULATING THE EFFECTS OF RETROGRADE INTERDIFFUSION OF STABLE ISOTOPES

Exchange of stable isotopes between coexisting minerals is recognized widely as an important factor in the interpretation of stable isotope geochemistry of plutonic and high-grade metamorphic rocks. Where retro gression has occurred without major recrystallization events, the rate limiting step for stable isotope exchange will be diffusion. The math ematics of diffusion are well known for many problems, but no analytic al solution, including that for closure temperature, adequately descri bes the complex and highly variable controls of rate and mass balance that will dominate many diffusion processes in rocks. We have implemen ted a model describing diffusional exchange for rocks in which grain b oundary diffusion is sufficiently rapid that a representative volume o f rock (typically millimeter to centimeter) is able to have mutual equ ilibration of all grain boundaries for the time scale of cooling. This Fast Grain Boundary model explicitly links intracrystalline diffusion rates and abundances of all minerals in a rock, and allows study of t he impact of rock type on stable isotope thermometry, retrogression, a nd zonation. The FORTRAN-77 program for the Fast Grain Boundary model presented here can be used with a personal computer to solve typical p roblems in minutes. Input includes the grain size(s), model abundance( s), diffusion coefficient, and fractionation factor for each constitue nt mineral, and a cooling rate for the rock. Output includes the diffu sion profile and integrated (bulk) composition of every mineral in a r ock, as well as the apparent temperatures that would be observed by ap plying bulk-mineral stable isotope thermometry to such a rock.