Other| April 01, 1998 Reversed determination of the reaction; phlogopite+quartz = enstatite+potassium feldspar+H 2 O in the ranges 750-875 degrees C and 2-12 kbar at low H 2 O activity with concentrated KCl solutions L. Ya. Aranovich; L. Ya. Aranovich University of Chicago, Department of the Geophysical Sciences, Chicago, IL, United States Search for other works by this author on: GSW Google Scholar R. C. Newton R. C. Newton Search for other works by this author on: GSW Google Scholar Author and Article Information L. Ya. Aranovich University of Chicago, Department of the Geophysical Sciences, Chicago, IL, United States R. C. Newton Publisher: Mineralogical Society of America First Online: 02 Mar 2017 Online Issn: 1945-3027 Print Issn: 0003-004X GeoRef, Copyright 2004, American Geological Institute. American Mineralogist (1998) 83 (3-4): 193–204. https://doi.org/10.2138/am-1998-3-401 Article history First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation L. Ya. Aranovich, R. C. Newton; Reversed determination of the reaction; phlogopite+quartz = enstatite+potassium feldspar+H 2 O in the ranges 750-875 degrees C and 2-12 kbar at low H 2 O activity with concentrated KCl solutions. American Mineralogist 1998;; 83 (3-4): 193–204. doi: https://doi.org/10.2138/am-1998-3-401 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract The reaction of phlogopite plus quartz to enstatite, potassium feldspar, and aqueous fluid in the system KMASH-KCl was reversed at 2-12 kbar and 750-875 degrees C and at low H 2 O activities by reversal of the H 2 O content of concentrated KCl solutions equilibrated with product and reactant assemblages. Synthetic 1M phlogopite [KMg 3 AlSi 3 O 10 (OH) 2 ] and enstatite (MgSiO 3 ) maintained end-member stoichiometry throughout, and the potassium feldspar (KAlSi 3 O 8 ) was a high sanidine, based on unit-cell refinements. The broad P-T-XH 2 O and narrow reversal ranges of this investigation were possible because of the low and well-defined H 2 O activity, yet powerful fluxing action, of concentrated KCl solutions. Solubility experiments on quartz and potassium feldspar in our P-T-XH 2 O range showed that fluid-phase solution of silicate constituents was too small to have affected the H 2 O activity in the experiments. The new determinations are more definitive than previous work done at very low pressures with pure H 2 O or in CO 2 -H 2 O mixtures. They establish the standard free energy of the reaction in the experimental range with an uncertainty of about 1 kJ and indicate that the synthetic phlogopite has maximal (Al-Si) disorder under our experimental conditions. The standard enthalpy of reaction at 298 K is 106.54+ or -2.0 kJ (2sigma ) based on our reversals, a value 6 kJ less positive than that currently used by many workers in calculations of biotite stability and H 2 O activity in the petrogenesis of high-grade metamorphic rocks. The lower thermal stability that we find for phlogopite requires revision in estimates of H 2 O activity of granulite facies metamorphism: typical values for the natural assemblage orthopyroxene-biotite-garnet-potassium feldspar-plagioclase-quartz at deep-crustal metamorphic conditions (750-850 degrees C, 5-10 kbar) are alpha H2O = 0.4-0.6 compared with values of 0.15-0.30 which would have been estimated with previously available data on phlogopite stability. An important consequence of the expanded H 2 O activity range of granulites is that alkali chloride solutions of only moderate concentration [XH 2 O = 0.5-0.7], which are the values observed in actual fluid inclusions in many kinds of igneous and metamorphic rocks, are a feasible alternative to the vapor-absent conditions considered necessary by many workers based on previous low estimates of H 2 O. Participation of concentrated brines in deep-crust/upper mantle metamorphic processes enables alkali metasomatism and other kinds of chemical transport in an aqueous fluid without large-scale melting of the crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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Aranovich et al. (1998) studied this question.