Complete chemical analyses of metamorphic hornblendes: implications for normalizations,calculated H2O activities,and thermobarometry |
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| Authors: | Michael A. Cosca Eric J. Essene Johns R. Bowman |
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| Affiliation: | (1) Department of Geological Sciences, University of Michigan, 48109-1063 Ann Arbor, MI, USA;(2) Department of Geology and Geophysics, University of Utah, 84112 Salt Lake City, UT, USA;(3) Present address: Institut de Minéralogie et Petrographie BFSH2, Université de Lausannc, CH-1015 Lausanne, Switzerland |
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| Abstract: | ![]()
Twenty-two hornblendes separated from amphibolites and granulites of the Grenville Orogen of Ontario have been quantitatively analyzed for major and minor elements by electron microprobe, for FeO/Fe2O3 by wet chemistry, and for H2O by manometric measurement as H2. Hornblende formulae were calculated on the basis of 24O+OH+Cl+F. Most samples are magnesio-hornblendes, ferroan pargasitic hornblendes and ferroan hastingsitic hornblendes, with weight fractions of Fe3+/(Fe2++Fe3+) ranging from 0.15 to 0.50. An oxy-amphibole component of 0–25 mol%, with an average value of 17 mol%, is obtained for these complete analyses. When compared with structural formulae determined solely from microprobe data, normalization based on 13=Si+Ti+Al+Fe+Mn+Mg cations provides the best approximation to hornblende formulae calculated from the complete analyses. Less satisfactory agreement is obtained from a normalization scheme based on 15=Si+Ti+Al+Fe+Mn+Mg+Ca, while worst agreement is obtained from normalization to 23 oxygens assuming all Fe is Fe2+. No normalization scheme based on microprobe data alone consistently replicates the measured FeO, Fe2O3, and H2O; accurate determination of these values requires complete chemical analysies. Ionic solution models previously have been proposed to evaluate the activity of Ca2Mg5Si8 O22(OH)2(aTrem) in hornblende for use in equilibria that constrain the activity of H2O (aH2O) in igneous and metamorphic rocks. Application of ionic models to typical hornblendes produces low aTrem (usually<0.01), consequetly yielding extremely low aH2O. If an oxy-amphibole component is present, the calculated aTrem and H2O is further reduced. An oxy-amphibole component of 25% reduces the calculated H2O activity and that of any hydroxyl-amphibole component by 50% below that calculated with simplified assumptions regarding XOH in the hydroxyl site (i.e., XOH=1, or XOH=1–XCl–Xf). Thus, methods of amphibole normalizations appear to have a substantial effect on calculated amphibole and H2O activites. Before quantitative hornblende thermobarometry can be calibrated and applied, the amounts of FeO, Fe2O3 and H2O must be measured in order to fully characterize hornblende solid solutions.Contribution No. 478 from the Mineralogical Laboratory, University of Michigan |
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