REACTIVE BULK COMPOSITION AND MODELING GARNET ZONING

Extracting P-T paths from chemical zoning in garnet requires knowledge of the assemblages and reactions responsible for producing the zoning, and assurance that diffusion has not significantly modified the growth zonation. Methods involving only intensive thermodynamic variables (i.e. the Gibbs method) are preferred because no assumptions about the reactive bulk composition (RBC) are necessary, although it can only be applied in low-variance assemblages. Methods that incorporate mass balance require only two independently measurable monitors (e.g. garnet compositional zoning), but are limited because of the uncertainty of the RBC at any point in time. Approaches that use a presumed equilibrium reference point and examine changes in zoning from this point are least susceptible to errors of extrapolation. Use of pseudosections is least precise because of the strict reliance on the thermodynamic data and solution models, the amount of extrapolation from presumed equilibrium points required, and the uncertainty in the RBC.

Garnet is chemically zoned, so the RBC must change with time. Pseudosections created through forward modeling of fractionating garnet are not strictly equilibrium diagrams (they are path-dependent) but can be useful for examining the effects of changing bulk composition. In metapelites, models created this way reveal that the P-T interval over which garnet grows is smaller than that calculated in equilibrium models, which suggests that a sizeable gap in the garnet record exists in most amphibolite facies garnet samples. Recent studies of quartz reveals that even this ubiquitous phase is chemically zoned in Ti (and probably other elements). This suggests that the RBC may not involve the interior of any phases, but is, instead, restricted to grain boundaries. Calculations of RBCs assuming that only the grain margins react reveal significant shifts in the RBC relative to the whole-rock BC, depending on the phases and grain sizes. Forward models that incorporate evolving RBCs inferred from grain margins areas reveal significant differences in parageneses and will be discussed with implications for the interpretation of garnet zoning.