Garnet‐biotite geothermometry of Koettlitz Group metasediments, Wright Valley, South Victoria Land, Antarctica

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Abstract
Garnet‐biotite geothermometry is applied to three samples of Koettlitz Group metasediments from the Wright Valley, South Victoria Land, Antarctica: psammitic schist, pelitic schist, and melanosome. Spessartine‐rich garnets were homogenised by volume diffusion during metamorphism. Petrographic and chemical constraints are used to select garnet‐biotite pairs most likely to be in equilibrium. However, it is not conclusive as to which combination of either garnet core‐far biotite pairs, or garnet rim‐close biotite pairs, best represents peak metamorphic equilibria. Traditional geothermometry, involving the calculation of distribution coefficients and mean temperatures (± 2 standard deviations) for individual pairs using a 4.8 kbar pressure estimate, gives 864 ± 82°C, 716 ± 50°C, 680 ± 33°C, 719 ± 34°C, 722 ± 156°C, 668 ± 40°C, and 679 ± 103°C using seven calibrations of the garnet‐biotite geothermometer. There is no significant difference between mean temperatures calculated using garnet core‐far biotite pairs, and those using garnet rim‐close biotite pairs, although core‐far temperatures have smaller standard deviations. Comparison of different calibrations indicates some are not suitable for use on the samples of this study. The Kd is not consistently lowered or raised due to the presence of Mn or Ca in garnet, or due to Ti or AlVI in biotite, and compositional corrections are not applied in this study. The slope of an Mg/Fe garnet versus Mg/Fe biotite graph, for samples assumed to have undergone isofacial metamorphism, equals the Kd for garnet‐biotite Fe‐Mg exchange. The Kd and 95% confidence limits can be determined by regression, and used in some calibrations. This regression method merges analytical uncertainties with unknown disequilibrium and solution modelling uncertainties, but does not deal with calibration uncertainties. It results in a more constrained temperature range than that calculated using means and individual mineral pairs. A temperature of 700–730°C is estimated for metamorphism of Koettlitz Group metsediments in the Wright Valley.