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.

Project Report

This research project had two primary goals: (1) to assess the effects of Mn substitution, mostly in garnet, on the Fe-Mg exchange thermometer as calibrated by Ferry and Spear (F-S) (1978), and (2) to develop a new geothermometer based on elemental exchange between garnet and tourmaline. The project formed the M.S. thesis research of James L. Maner (M.S. spring 2013), who conducted 106 experiments in pursuit of these goals. For project (1), we attempted to synthesize garnet - biotite assemblages from granitic melt rather than from aqueous-carbonic vapor (as Ferry and Spear did), to avoid the problem of contamination by nickel (Ni) in the compositions of the synthetic biotite. Ferry and Spear synthesize biotite by conventional hydrothermal methods at the oxygen buffer maintained between iron - wustite (IW). In their garnet-biotite exchange experiments, however, they used the graphite-water buffer (CCO), which produces graphite plus CO-H2O fluid that buffers the oxidation state higher than but close to IW. We replicated their hydrothermal method by taking a large mass fraction of garnet (almandine-spessartine solid solution) and equilibrating a small quantity of biotite (annite-phlogopite solid solution) with that. At 750°C, 200 MPa, 14 days, and oxidation state buffered by CCO within the capsule, the synthetic biotite contains 5.9 wt% NiO. The temperature calculated from F-S is 399°C. Progress toward Goal 1: All of our attempts to synthesize biotite from melt produced crystals whose width was < 1 mm (Fig. 1), too small for electron probe microanalysis. For this reason, and because biotite synthesized by hydrothermal means contains > 5 wt% NiO, even at CCO, we abandoned this project. Progress toward Goal 2: We have attempted to calibrate the exchange of Fe, Mg, Mn, and Ca between garnet and tourmaline synthesized from granitic melt. We utilized two synthesis methods: (a) referred to as the "sandstone" method in which a large fraction of sand-sized mineral grains of mafic silicates (Mn-fayalite, rhodonite, forsterite, etc.) was reacted with a small volume (~ 20 vol%) of granitic melt; and (b) bulk compositions that are preconditioned to 850°C, 200 MPa, to produce a hydrous melt, then quenched, and run forward to the temperature of interest for the crystal syntheses. We have conducted these experiments at three different oxidation states: IW, CCO, and NNO (nickel - nickel oxide), with the oxidation state increasing to the right. (2a) The sandstone method (a) produced garnet-tourmaline pairs. However, those phases crystallized before the melt had time to homogenize by the dissolution of the seed crystals and the diffusion of their components through the melt. As a result, the compositions of garnet and tourmaline are extremely heterogeneous (Fig. 2), and averages of their compositions possess standard deviations large enough to invalidate any utilization of that data. Moreover, tourmaline saturates in Mn at low concentrations, and garnet fractionates Mg out of the melt so effectively that the resultant slopes of element variations, e.g. Fe/Mn or Fe/Mg of garnet/tourmaline versus temperature (T), possess slopes of ~ zero with respect to T. In short, there is no usefully measurable temperature dependence of fractionation among pairs of Fe, Mg, Mn, or Ca between garnet and tourmaline, and thus a geothermometer based on the exchange of major and minor elements in this system is not likely to be viable. Van Hinsberg and Schumacher (2009) also tried and failed to develop an experimental calibration for a biotite-tourmaline geothermometer. We have, therefore, two significant negative outcomes to report: (1) the biotite synthesized by Ferry and Spear (1978) likely contained wt% levels of Ni, and their biotite was probably well off the composition of their intended calibration, and (2) the petrologic value of tourmaline, which has been heavily promoted in recent publications (e.g., Dutrow and Henry 2011), has been greatly oversold. (2b) The preconditioned experiments (b) that crystallize from a melt derived from glass have so far failed to yield garnet-tourmaline pairs. We have, however, been able to track the fractionation Mg, Fe, and Mn within granitic liquids (melts) and coexisting tourmaline, or cordierite + garnet + oxide ± amphibole. The liquid line of descent, which tracks the change of melt composition upon crystallization with decreasing T, migrates away from Mg and toward Mn, as do the compositions of cordierite, garnet, and amphibole (Fig. 3). Publications stemming from this work: Maner, J.L. IV, London, D., Morgan, G.B. VI (2013) Toward an experimentally calibrated garnet-tourmaline geothermometer. (abstr) Geological Society of America Abstracts with Programs, 45 (3), 17. Other citations: Dutrow, B.L. and Henry, D.J. (2011) Tourmaline. Elements, 7, 291-338. Ferry, J.M. and Spear, F.S. (1978) Experimental calibration of the partitioning of Fe and Mg between biotite and garnet. Contributions to Mineralogy and Petrology, 66, 113-117. Van Hinsberg, V.J. and Schumacher, J.C. (2009) The geothermobarometric potential of tourmaline, based on experimental and natural data. American Mineralogist, 94, 761-770.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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University of Oklahoma
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