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A thermodynamic model for Ca–Na clinoamphiboles in Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–O for petrological calculations
Author(s) -
DALE J.,
POWELL R.,
WHITE R. W.,
ELMER F. L.,
HOLLAND T. J. B.
Publication year - 2005
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2005.00609.x
Subject(s) - amphibole , geothermobarometry , tremolite , plagioclase , thermodynamics , mineralogy , hornblende , phase diagram , context (archaeology) , geology , quartz , chemistry , materials science , phase (matter) , metamorphic rock , physics , geochemistry , paleontology , organic chemistry , asbestos , biotite , metallurgy
A calibration is presented for an activity–composition model for amphiboles in the system Na 2 O–CaO–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–O (NCFMASHO), formulated in terms of an independent set of six end‐members: tremolite, tschermakite, pargasite, glaucophane, ferroactinolite and ferritschermakite. The model uses mixing‐on‐sites for the ideal‐mixing activities, and for the activity coefficients, a macroscopic multicomponent van Laar model. This formulation involves 15 pairwise interaction energies and six asymmetry parameters. Calibration of the model is based on the geometrical constraints imposed by the size and shape of amphibole solvi inherent in a data set of 71 coexisting amphibole pairs from rocks, formed over 400–600 °C and 2–18 kbar. The model parameters are calibrated by combining these geometric constraints with qualitative consideration of parameter relationships, given that the data are insufficient to allow all the model parameters to be determined from a regression of the data. Use of coexisting amphiboles means that amphibole activity–composition relationships are calibrated independently of the thermodynamic properties of the end‐members. For practical applications, in geothermobarometry and the calculation of phase diagrams, the amphibole activity–composition relationships are placed in the context of the stability of other minerals by evaluating the properties of the end‐members in the independent set that are in internally consistent data sets. This has been performed using an extended natural data set for hornblende–garnet–plagioclase–quartz, giving the small adjustments necessary to the enthalpies of formation of tschermakite, pargasite and glaucophane for working with the Holland and Powell data set.