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The barometer tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz: Constraints from experimental data at unit silica activity, with application to garnet-free natural assemblages

¹ Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India
² Department of Earth Sciences, Pondicherry University, Kalapet, Pondicherry, 605014, India

Correspondence: ^* E-mail: abbhat55{at}yahoo.com

Garnet is notably absent in mafic to felsic lithologies, in low-P metamorphic terranes, and in Mg-rich bulk compositions in high-P regionally metamorphosed terranes. In the absence of garnet, existing mineral barometers cannot be applied for estimating metamorphic pressure. A new garnet-free, hornblende-plagioclase barometer has been formulated based on the reaction tremolite + tschermakite + 2 albite = 2 pargasite + 8 quartz. The intra- and cross-site mixing parameters for hornblende were retrieved exclusively from experimental data in the P-T window 1–15 kbar and 650–950 °C, after screening the available data for the presence of quartz. A linear least-squares fit to the coexisting hornblende and plagioclase composition in the experiments yields the following barometric expressions:

and

where

X_i^j denotes the mole fraction of cation i in site j of amphibole, X_Ab and _Ab are the mole fraction and activity coefficient of albite in plagioclase, respectively, and V is the volume change for the reaction. In the expression for P₂, the first two terms in the numerator on the right-hand side corresponding to H° and S°, respectively, were adopted from an internally consistent thermodynamic database. In the expression for P₁, the first term on the right-hand side of the numerator, (–9.326 kJ) = H° + a_9, where a₉ is a mixing parameter, and the second term S° = 0.01462 kJ/K, were retrieved by a linear least-squares fit to the experimental data. The formulations reproduced experimental P values within 2 kbar for 83% of the experimental runs. The cumulative precision (1) of pressure estimated using the formulations varied between 800 and 2000 bars for errors propagated due to uncertainties in P-T, volume, and thermometric estimate, and inaccuracies in estimated composition of phases. In natural assemblages spanning a pressure range of 2.5 to 12.5 kbar, the quartz–hornblende–plagioclase barometers yielded pressure within 2 kbar of the P-value recommended by the authors, and is consistent with the stable Al₂SiO₅ polymorph in associated metapelites.

Key Words: Thermobarometry • hornblende • plagioclase • quartz • thermodynamics • mixing parameters of hornblende • experimental petrology • mafic rocks • silica-saturated • metamorphic petrology