Garnet-Biotite-Tourmaline Thermometry at High Mn Content

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Intellectual Merit. This proposal requests funding for new experimental programs that will expand the utility of existing Fe-Mg exchange geothermometers, and that may introduce new thermometers based on the composition of coexisting tourmaline. If successful, the research will extend the application of garnet-biotite thermometry to some common Mn-rich metamorphic rocks, and to a variety of silicic igneous rocks in which garnet and tourmaline coexist, with or without accessory biotite and muscovite. Although Mn readily substitutes for Fe in common ferromagnesian silicates, previous calibrations of equilibrium between these minerals were conducted in Mn-free systems thereby raising concern that there may be systematic errors in P-T estimates based on these calibrations. Essentially, the accuracy of existing thermobarometers wanes as mineral compositions diverge from those of the experimental design. Thus, many rock types and conditions are outside of the range of confidence for which most thermometers can be applied. New experiments using Mn-bearing starting materials will help to resolve current discrepancies in the estimation of the thermodynamic mixing parameters for garnet and lead to improved solid solution models. Experiments are also proposed to calibrate the fractionation of major and minor elements (Fe, Mg, Mn, Ca, Ti, and V) between the bipolar ends of tourmaline crystals as a function of temperature, in hopes of developing a single-phase geothermometer. Finally, experiments will be conducted to quantify Fe-Mg exchange between tourmaline and biotite, cordierite, and garnet to develop a new tourmaline- ferromagnesian silicate thermometers. If successful multiple tourmaline-based thermometers could be used simultaneously in the same rock to confirm internal equilibrium or document lack thereof. For igneous rocks, tourmaline-garnet thermometry will be inherently more accurate than conventional thermometry with micas (biotite or muscovite), owing to the refractory nature of garnet and tourmaline as compared to biotite and muscovite, for which low closure temperatures and low modal abundances in igneous rocks hinder their value in thermobarometry.Broader Impacts. This project will support one graduate student. Funding also will support the experimental lab of the PI, and the contributions to mineral thermobarometry could have profound influence on petrogenetic interpretations.