| Popis: |
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: \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&\mathit{P}\_{1}(kbar)\ =\ [{-}9.326\ +\ 0.01462T\ (K)\ +\ R\mathit{T}ln\mathit{K}\_{ideal}\ {-}\ 98.698\mathit{X}^{A}\ \_{Na}\ {-}\ 33.213\mathit{X}^{A}\ \_{K}\ {-}\ 20.338\mathit{X}^{M4}\_{Na}\ {-}\ 39.101\mathit{X}^{M13}\_{Fe^{2+}}\\&&+\ 100.392\mathit{X}^{M2}\ \_{Al}\ +\ 131.03\mathit{X}^{M2}\ \_{Fe^{2+}}\ +\ 82.479\mathit{X}^{M2}\ \_{Fe^{3+}}\ {-}\ 118.653\mathit{X}^{T1}\ \_{Al}\ {-}\ 2R\mathit{T}ln{\gamma}_{Ab}]/({-}{\Delta}\mathit{V})\end{eqnarray*} \end{document} and \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \begin{eqnarray*}&&\mathit{P}\_{2}(kbar)\ =\ [{-}1.869\ +\ 0.0076T\ (K)\ +\ R\mathit{T}ln\mathit{K}\_{ideal}\ {-}\ 102.692\mathit{X}^{A}\_{Na}\ {-}35.251\mathit{X}^{A}\_{K}\ {-}\ 15.969\mathit{X}\_{Na}^{M4}\ {-}\ 40.499\mathit{X}^{M13}\ \_{Fe^{2+}}\\&&+\ 93.069\mathit{X}^{M2}\ \_{Al}\ +\ 130.750\mathit{X}^{M2}\ \_{Fe^{2+}}\ +\ 74.226\mathit{X}^{M2}\ \_{Fe3+}\ {-}\ 104.402\mathit{X}^{T1}\ \_{Al}\ {-}\ 2R\mathit{T}ln{\gamma}_{Ab}]/({-}{\Delta}\mathit{V})\end{eqnarray*} \end{document} where \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[\mathit{K}\_{ideal}\ =\ \left[\frac{16(\mathit{X}\_{Na}\mathit{^{A}})(\mathit{X}^{T1}\_{Al})}{(\mathit{X}\_{{\square}}\mathit{^{A}}(\mathit{X}^{T1}\_{Si})(\mathit{X}\_{Ab})}\right]^{2}.\] \end{document} X i j denotes the mole fraction of cation i in site j of amphibole, X Ab and γAb are the mole fraction and activity coefficient of albite in plagioclase, respectively, and Δ V is the volume change for the reaction. In the expression for P 2, the first two terms in the numerator on the right-hand side corresponding to Δ H ° and Δ S °, respectively, were adopted from an internally consistent thermodynamic database. In the expression for P 1, the first term on the right-hand side of the numerator, Δ Ĥ (−9.326 kJ) = Δ H ° + a 9, where a 9 is a mixing parameter, and the second term Δ 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 (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. |
