Abstract
Commonly used thermometer and barometer calibrations are sensitive to mineral assemblage and, thus, bulk-rock composition. Calculated mineral stabilities for an average pelitic rock over a pressure–temperature (PT) range appropriate for normal, thickened, heated and shallowly subducted continental crust (400–900°C at 0.1–3.0 GPa) reveal more than one hundred possible assemblages. Individual phase compositions are dependent on the assemblage in which they belong and combining isopleth sets to represent \({\left({X^{\rm Mg} /X^{\rm Fe}} \right)}_{\rm garnet} /{\left({X^{{\rm Mg}} /X^{{\rm Fe}}} \right)}_{{{{\rm biotite}}}}\) and \(X^{{\rm Ca}}_{{\rm garnet}} /X^{{\rm Ca}}_{{\rm plagioclase}}\) reveals several PT-ranges where commonly used mineral thermobarometers are less effective. For example, the garnet-biotite thermometer becomes increasingly P dependent in the absence of muscovite in high T melt-bearing assemblages, and biotite and plagioclase are not stable at pressures appropriate for lower thickened continental crust. Compositional thermobarometers involving equilibration between alternative phases (namely garnet, phengite and omphacite) are presented. Although the equilibrium compositions of phases at any P and T may change significantly as a function of bulk-rock composition, compositional-ratio thermobarometers are typically insensitive to this, unless a pseudo-univariant reaction is crossed and the buffering assemblage is altered. Quantification of the limits of efficacy of various thermobarometers allows the mineralogy of metapelites to be used to precisely determine segments of PT paths and infer their likely tectonic controls.
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Acknowledgments
We would like to thank James Connolly for advice on and continued development of Perple_X. This work benefited particularly from discussion with James and Tim Holland. We thank two anonymous reviewers and our editor for helpful and insightful comments. This work is supported by ETH Research Funds.
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Appendices
Appendix 1: Activity-composition models and mineral abbreviations
The principle site occupancies permitted by the solution models used, and their source references are outlined here. Further details are available from the Perplex website of JAD Connolly (Table 2).Footnote 2
Additional mineral abbreviations: Lawsonite (Law), rutile (Rut), zoisite (Zoi), sphene (Sph), sillimanite (Sil), andalusite (And), kyanite (Kya), quartz (Qtz), coesite (Coe), end-member albite (alb), anorthite (ano), almandine (alm), grossular (gro), pyrope (pyr), spessartine (spe), diopside (dio), phlogopite (phl), annite (ann).
Appendix 2: Comparing thermobarometer calibrations
The mineral thermobarometers calibrated in the text have been examined to show both how well the simple regressions model the composition data and how they compare with published thermobarometer calibrations. Firstly the minimum-free-energy phase compositions calculated as a function of P and T by Perplex were input to the thermobarometer calibrations and the predicted PTs plotted. For example, Gar–Bio calibration (2) yields T within 50°C of the ‘expected’ value over the entire examined PT range and within 15°C over most of that range (Fig. 8). The same composition data were also input to published thermobarometer calibrations, and the differences in predicted P or T between these calibrations and ours were calculated. For example, the difference between calibration (2) and the Gar–Bio calibration of Ferry and Spear (1978) is less than 15°C at pressures of ca. 5 kbars, increasing to over 200°C in extreme cases of high P, low T (Fig. 8). Finally, our calibrations have been compared to others from the literature using experimentally derived phase compositions or those from well-studied natural samples. For example, the seven Mt. Moosilauke compositions of Hodges and Spear (1982) yield temperatures of 477–538°C with the Hodges and Spear thermometer (assumed P = 0.376 GPa and W MgMn = 0) and 507–559°C with our calibration (2) (Fig. 8). Similar plots and descriptions are available for each of the calibrated thermobarometers as electronic supplemental material.
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Caddick, M.J., Thompson, A.B. Quantifying the tectono-metamorphic evolution of pelitic rocks from a wide range of tectonic settings: mineral compositions in equilibrium. Contrib Mineral Petrol 156, 177–195 (2008). https://doi.org/10.1007/s00410-008-0280-6
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DOI: https://doi.org/10.1007/s00410-008-0280-6