• journal_title：American Mineralogist
• Contributor：Subhadip Bhadra ; Abhijit Bhattacharya
• Publisher：Mineralogical Society of America
• Date：2007-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2007.2067
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：92
• issue：4
• firstpage：491
• section：Articles

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 γ_Ab are the mole fraction and activity coefficient of albite in plagioclase, respectively, and ΔV 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 ΔH° and ΔS°, 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, ΔĤ(−9.326 kJ) = ΔH° + a_9, where a₉ is a mixing parameter, and the second term ΔS° = 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 (1σ) 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.