A NEW LOOK AT STABLE-ISOTOPE THERMOMETRY

Interdiffusion between coexisting minerals affects all rocks and cause s resetting and discordance of stable isotope geothermometers that is commonly observed in slowly cooled igneous and metamorphic rocks. The Fast Grain Boundary (FGB) model describes the stable isotope fractiona tions and intracrystalline zonation which result from closed system in terdiffusion (EILER et al., 1991, 1992). This model assumes that grain boundary diffusion is much faster than volume diffusion, and it accou nts for exchange among all minerals in a rock. Previous models of clos ure temperature violate mass balance restrictions and will be inaccura te in most rocks. Modeling results are described for amphibolites and hornblende granites and gneisses; biotite granites, schists, and gneis ses, pelitic and semi-pelitic rocks; garnet peridotites; anorthosites, gabbros, pyroxenites, and related rocks; and calc-silicate rocks. Exa mples of mineral pairs and specific rock types that allow accurate sta ble isotope thermometry include plagioclase-pyroxene in pyroxene beari ng anorthosites and garnet-quartz in garnetiferous quartzites. In cont rast, the same mineral pairs in related rocks such as pyroxenites and pelitic schists will exhibit reset apparent temperatures. Closed-syste m processes are capable of producing a variety of patterns of stable i sotope resetting, discordance, mineral zonation, and fractionation rev ersals. Examples include large reversals of quartz-feldspar fractionat ions in micaceous rocks, and oscillatory zonation in feldspar from som e quartz-rich rocks. These results permit reinterpretation of many stu dies of stable isotope thermometry, speedometry, and retrograde altera tion history. FGB modeling of mineral zonation provides an important n ew guide to applying recently developed microanalytical tools to slowl y cooled rocks. Application of the FGB model to quartzo-feldspathic gn eisses from the Adirondack Mountains, New York, demonstrates the usefu lness of diffusion modeling in discriminating closed-system, diffusion controlled retrogression from open-system retrogression, and illustra tes the possible importance of incorporating the effect of water activ ity on mineral diffusivity.