Calcic amphibole equilibria and a new amphibole-plagioclase geothermometer

Abstract

There is currently a dearth of reliable thermobarometers for many hornblende and plagioclase-bearing rocks such as granitoids and amphibolites. A semi-empirical thermodynamic evaluation of the available experimental data on amphibole+plagioclase assemblages leads to a new thermometer based on the Aliv content of amphibole coexisting with plagioclase in silica saturated rocks. The principal exchange vector in amphiboles as a function of temperature in both the natural and experimental studies is \(\left( {Na\square _{ - 1} } \right)^A \left( {AlSi_{ - 1} } \right)^{T1}\). We have analysed the data using 3 different amphibole activity models to calibrate the thermometer reactions $$1. Edenite + 4 Quartz = Tremolite + Albite$$ (1.) $$2. Pargasite + 4 Quartz = Hornblende + Albite.$$ (2.) The equilibrium relation for both (1) and (2) leads to the proposed new thermometer $$T = \frac{{0.677P - 48.98 + Y}}{{ - 0.0429 - 0.008314 ln K}} and K = \left( {\frac{{Si - 4}}{{8 - Si}}} \right)X_{Ab}^{Plag} ,$$ where Si is the number of atoms per formula unit in amphiboles, with P in kbar and T in K; the term Y represents plagioclase non-ideality, RTlnγab, from Darken's Quadratic formalism (DQF) with Y=0 for Xab>0.5 and Y=-8.06+25.5(1-Xab)2 for Xab<0.5. The best fits to the data were obtained by assuming complete coupling between Al on the T1 site and Na in the A site of amphibole, and the standard deviation of residuals in the fit is ±38°C. The thermometer is robust to ferric iron recalculation procedures from electron probe data and should yield temperatures of equilibration for hornblende-plagioclase assemblages with uncertainties of around ±75° C for rocks equilibrated at temperatures in the range 500°–1100° C. The thermometer should only be used in this temperature range and for assemblages with plagioclase less calcic than An92 and with amphiboles containing less than 7.8 Si atoms pfu. Good results have been attained on natural examples from greenschist to granulite facies metamorphic rocks as well as from a variety of mafic to acid intrusive and extrusive igneous rocks. Our analysis shows that the pressure dependence is poorly constrained and the equilibria are not suitable for barometry.