Interdiffusion between coexisting minerals affects all rocks and causes
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
fractionations and intracrystalline zonation which result from closed
system interdiffusion (EILER et al., 1991, 1992). This model assumes
that grain boundary diffusion is much faster than volume diffusion, and
it accounts for exchange among all minerals in a rock. Previous models
of closure temperature violate mass balance restrictions and will be
inaccurate in most rocks.
Modeling results are described for amphibolites and hornblende granites
and gneisses; biotite granites, schists, and gneisses, pelitic and
semi-pelitic rocks; garnet peridotites; anorthosites, gabbros,
pyroxenites, and related rocks; and calc-silicate rocks. Examples of
mineral pairs and specific rock types that allow accurate stable isotope
thermometry include plagioclase-pyroxene in pyroxene bearing
anorthosites and garnet-quartz in garnetiferous quartzites. In contrast,
the same mineral pairs in related rocks such as pyroxenites and pelitic
schists will exhibit reset apparent temperatures. Closed-system
processes are capable of producing a variety of patterns of stable
isotope resetting, discordance, mineral zonation, and fractionation
reversals. Examples include large reversals of quartz-feldspar
fractionations in micaceous rocks, and oscillatory zonation in feldspar
from some quartz-rich rocks. These results permit reinterpretation of
many studies of stable isotope thermometry, speedometry, and retrograde
alteration history. FGB modeling of mineral zonation provides an
important new guide to applying recently developed microanalytical tools
to slowly cooled rocks. Application of the FGB model to
quartzo-feldspathic gneisses from the Adirondack Mountains, New York,
demonstrates the usefulness of diffusion modeling in discriminating
closed-system, diffusion controlled retrogression from open-system
retrogression, and illustrates the possible importance of incorporating
the effect of water activity on mineral diffusivity.