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The diffusion behavior of hydrogen in plagioclase feldspar at 800–1000 °C: Implications for re-equilibration of hydroxyl in volcanic phenocrysts

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Published/Copyright: March 7, 2015
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American Mineralogist
From the journal American Mineralogist Volume 98 Issue 10

Abstract

To use structural hydroxyl (OH) concentrations preserved in volcanic phenocrysts to constrain magmatic water contents prior to eruption, it is first necessary to understand the diffusive behavior of hydrogen in plagioclase. In this study, diffusion coefficients for a natural OH-bearing plagioclase feldspar (Ab66An31Or3) are determined from a series of integrated loss heating experiments performed at 800-1000 °C and 1 atm under air, nitrogen gas, and a CO2-H2 mixture at the FMQ oxygen buffer. Hydrogen diffusion is found to be isotropic within analytical error. Using a one-dimensional diffusive loss model for an infinite slab, the diffusion behavior for hydrogen in plagioclase is described by the diffusion parameters log D0 = -1.62 ± 0.31 (m2/s) and Ea = 266 ± 77 kJ/mol, and log D0 = -0.97 ± 0.35 (m2/s) and Ea = 278 ± 90 kJ/mol for experiments only conducted under nitrogen gas. Nearly complete (83-97%) loss of OH from the andesine was achieved in 900 and 1000 °C heating series, except for the 900 °C FMQ buffer experiment in which only 64% of the total OH was lost after 21.6 days of cumulative heating. The diffusion rates of hydrogen in the plagioclase after 800-1000 °C are similar to interpolated diffusion rates for sodium diffusion in An30 feldspar, implying that Na+ and H+ both diffuse via Frenkel defects involving the large cation sites and interstitial ions. The diffusion coefficient (D) values for hydrogen in plagioclase are lower than most reported diffusion data for hydrogen in nominally anhydrous minerals, and are most similar to D reported for pure forsterite, unaffected by iron redox reactions. Based on the hydrogen diffusion parameters in this study, a 1 mm spherical plagioclase phenocryst experiencing dehydration under lowered water activity during ascent and eruption at 800 °C retains 50% of its initial OH concentration after 34 days. At 900 and 1000 °C, a 1 mm phenocryst retains 50% of its initial OH concentration after only 1.3 days and 0.25 day, respectively. OH concentrations in plagioclase are therefore most indicative of magmatic water contents during the latest stages of ascent and eruption.

Keywords : IR spectroscopy; OH in plagioclase; diffusion; H in plagioclase; high-temperature studies; igneous petrology
Received: 2013-3-1
Accepted: 2013-6-15
Published Online: 2015-3-7
Published in Print: 2013-10-1

© 2015 by Walter de Gruyter Berlin/Boston

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https://doi.org/10.2138/am.2013.4521
Keywords for this article
IR spectroscopy; OH in plagioclase; diffusion; H in plagioclase; high-temperature studies; igneous petrology

Articles in the same Issue

  1. Review article. The crystal structure and vibrational spectroscopy of jarosite and alunite minerals
  2. Versatile monazite: Resolving geological records and solving challenges in materials science. Petrogenesis of the Kulyk Lake monazite-apatite-Fe(Ti)-oxide occurrence revealed using in-situ LA-(MC)-ICP-MS trace element mapping, U-Pb dating, and Sm-Nd isotope systematics on monazite
  3. Amorphous Materials: Properties, structure, and durability. Analysis of H2O in silicate glass using attenuated total reflectance (ATR) micro-FTIR spectroscopy
  4. Unlocking the secrets of Al-tobermorite in Roman seawater concrete
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  7. Petrology and geochemistry of lunar granite 12032,366-19 and implications for lunar granite petrogenesis
  8. Extreme fractionation from zircon to hafnon in the Koktokay No. 1 granitic pegmatite, Altai, northwestern China
  9. Controls of P-T path and element mobility on the formation of corundum pseudomorphs in Paleoproterozoic high-pressure anorthosite from Sittampundi, Tamil Nadu, India
  10. Aluminum speeds up the hydrothermal alteration of olivine
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  12. Effects of fluid and melt density and structure on high-pressure and high-temperature experimental studies of hydrogen isotope partitioning between coexisting melt and aqueous fluid
  13. DFT simulation of the occurrences and correlation of gold and arsenic in pyrite
  14. Crystal structure and hydration/dehydration behavior of Na2Mg(SO4)2·16H2O: A new hydrate phase observed under Mars-relevant conditions
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  24. Olivine from spinel peridotite xenoliths: Hydroxyl incorporation and mineral composition
  25. Determination of the melting temperature of kaolinite by means of the Z-method
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