An experimental study of phase equilibria and Fe oxy-component in kaersutitic amphibole; implications for the f_H2and alpha_H2Oin the upper mantle

Abstract

Other| June 01, 1995 An experimental study of phase equilibria and Fe oxy-component in kaersutitic amphibole: Implications for the fH2 and alpha aH2O in the upper mantle Robert K. Popp; Robert K. Popp Texas A&M University, Department of Geology and Geophysics, College Station, TX, United States Search for other works by this author on: GSW Google Scholar David Virgo; David Virgo Search for other works by this author on: GSW Google Scholar Hatten S. Yoder; Hatten S. Yoder Search for other works by this author on: GSW Google Scholar Thomas C. Hoering; Thomas C. Hoering Search for other works by this author on: GSW Google Scholar Michael W. Phillips Michael W. Phillips Search for other works by this author on: GSW Google Scholar American Mineralogist (1995) 80 (5-6): 534–548. https://doi.org/10.2138/am-1995-5-613 Article history first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Robert K. Popp, David Virgo, Hatten S. Yoder, Thomas C. Hoering, Michael W. Phillips; An experimental study of phase equilibria and Fe oxy-component in kaersutitic amphibole: Implications for the fH2 and alpha aH2O in the upper mantle. American Mineralogist 1995;; 80 (5-6): 534–548. doi: https://doi.org/10.2138/am-1995-5-613 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu nav search search input Search input auto suggest search filter All ContentBy SocietyAmerican Mineralogist Search Advanced Search Abstract Experiments have been carried out from 500 to 1200 °C, 1 atm to 10 kbar, and fH2 from that of the IQF buffer to air, to quantify the variation of Fe oxy-component content in a titanian pargasite megacryst amphibole from Vulcan’s Throne, Arizona. The results document the operation of the following substitution mechanism in the amphibole crystal structure: Fe2+ + OH− = Fe3+ + O2− + ½H2 whereby the Fe3+/Fetot of the amphibole is controlled by T, P, and fH2. For the amphibole composition that was investigated, there is a linear variation of log fH2 as a function of log (Fe3+/Fe2+) at fixed T and P of the form log fH2 = a + b log(Fe3+/Fe2+). Values of a and b are: T (°C) . P (kbar) . a . b . 700 1 −2.66 −5.14 800 1 −1.84 −4.31 900 1 −1.01 −3.76 1000 1 −0.30 −3.95 900 5 −0.70 −4.69 900 10 −0.50 −4.60 T (°C) . P (kbar) . a . b . 700 1 −2.66 −5.14 800 1 −1.84 −4.31 900 1 −1.01 −3.76 1000 1 −0.30 −3.95 900 5 −0.70 −4.69 900 10 −0.50 −4.60 View LargeTwo different expressions were defined for the equilibrium constant for the amphibole Fe end-member reaction Ca2Fe52+Si8O22(OH)2=Ca2Fe32+Fe23+Si8O24+H2⁠. In the random mixing model, in which it is assumed that Fe3+ and Fe2+ mix randomly on the five M1, M2, and M3 crystallographic sites,K=fH2(28.94)(XFe3+)2(X[ ])2(XFe2+)2(XOH)2for whichlog⁡K=4.25−4363/T(K)+0.11(P−1)(kbar).From the appropriate values of K, fH2 of the experiments can be predicted to within ~0.5 log units from knowledge of the absolute amounts of Fe3+, Fe2+, and OH in the amphibole. For the nonrandom mixing model, in which observed Fe3+ and Fe2+ site populations are used to define the mole fraction terms,K=fH2(28.94)(XFe3+M1)0.8(XFe3+M2)0.8(XFe3+M3)0.4(X[ ])2(XFe2+M1)0.8(XFe2+M2)0.8(XFe2+M3)0.4(XOH)2for whichlog⁡K=5.29−5903/T(K)+0.13(P−1)(kbar).With this model, fH2 of the experiments can be predicted to within ~0.3 log units from knowledge of the relevant ionic contents. The random mixing model yields slightly poorer estimates of fH2 but can be used for literature data in applications because Fe3+ and Fe2+ site populations for natural kaersutite are seldom reported. In order to use the K expressions, the OH content of the amphibole must be known. In kaersutite for which H content has not been measured, OH apfu can be estimated as (2.0 − Fe3+ − Ti).Because both the reactant and product amphiboles in the end-member reaction refer to components in a single homogenous amphibole phase, the K expressions should apply to any calcic amphibole in which Fe3+ and Fe2+ mix on the five M1, M2, and M3 crystal-lographic sites, regardless of the amphibole bulk composition.The study confirms that the relatively high Fe3+/Fetot of most natural kaersutitic amphiboles can result from P-T-fH2 conditions characteristic of the upper mantle, rather than from oxidation during ascent or eruption. Closed-system cooling favors the reduction, not oxidation, of amphibole. With K values from the equations above, it is possible to predict fH2 of amphibole crystallization, presumably from a melt, if P and T, as well as the relevant amphibole composition terms, are known. Calculated values of fH2 for the majority of kaersutitic amphiboles reported in the literature range from approximately 0.01 to 100 bars. Such fH2 values are generally consistent with estimated redox states and H2O activities of mantle processes. If fH2 estimates are combined with fO2 estimates made on the same xenolith assemblages, H2O activity in the environment of formation can be predicted. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not currently have access to this article.