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American Mineralogist

Journal of Earth and Planetary Materials

Ed. by Baker, Don / Xu, Hongwu / Swainson, Ian


IMPACT FACTOR 2018: 2.631

CiteScore 2018: 2.55

SCImago Journal Rank (SJR) 2018: 1.355
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1945-3027
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Volume 100, Issue 5-6

Issues

Hydrous species in feldspars: A reassessment based on FTIR and SIMS

Jed L. Mosenfelder
  • Corresponding author
  • Division of Geological and Planetary Sciences, California Institute of Technology, M/C 170-25, Pasadena, California 91125-2500, U.S.A.
  • Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Minneapolis, Minnesota 55455, U.S.A.
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/ George R. Rossman
  • Division of Geological and Planetary Sciences, California Institute of Technology, M/C 170-25, Pasadena, California 91125-2500, U.S.A.
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/ Elizabeth A. Johnson
  • Department of Geology and Environmental Science, James Madison University, 395 S. High Street, MSC 6903, Harrisonburg, Virginia 22807, U.S.A.
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Published Online: 2015-05-12 | DOI: https://doi.org/10.2138/am-2015-5034

Abstract

Recent interest in hydrogen incorporation in feldspars has been driven by the potential of this common mineral species to record magmatic water contents. Accurate measurement of H concentrations in feldspars by Fourier transform infrared (FTIR) spectroscopy is hampered by the need to collect polarized spectra in three mutually perpendicular directions, which can be impractical for crystals characterized by small dimensions, polysynthetic twinning, and/or chemical zoning. SIMS is an attractive alternative to FTIR, offering high spatial resolution, high precision, and the feasibility of attaining low detection limits. In this study we compare FTIR and SIMS data for 19 feldspars, including plagioclase, anorthoclase, sanidine, microcline, and orthoclase. We present adjustments to previously published FTIR data on some of these samples. Our new SIMS and FTIR data are well correlated and we demonstrate the feasibility of quantitatively measuring H concentrations as low as 1-2 ppmw H2O using SIMS. Combination of the new data together with re-evaluation of the NMR calibration of Johnson and Rossman (2003) indicates that the IR absorption coefficients for hydrous species in feldspar increase with decreasing frequency of their O-H absorptions, in accord with theory. We derive new molar integral IR absorption coefficients (I) for feldspars with the following hydrous species as defined by Johnson and Rossman (2003):

Type I and II H2O (microcline and orthoclase): I = 120 470 ± 11 360 L·mol-1 H2O cm-2

Type IIb OH (sanidine): I = 150 000 ± 15 000 L·mol-1 H2O cm-2

Type IIa OH (plagioclase and anorthoclase): I = 202 600 ± 20 260 L·mol-1 H2O cm-2

These absorption coefficients depend on critical assumptions with regards to SIMS matrix effects. If accurate, one important implication is that the H concentrations of plagioclase crystals estimated in the literature are too high by up to a factor of two, requiring revision of previously estimated plagioclasemelt H partitioning coefficients.

Keywords: FTIR; SIMS; feldspar; calibration; nominally anhydrous minerals; hygrometry; water; crust

About the article

Received: 2014-05-05

Accepted: 2014-11-30

Published Online: 2015-05-12

Published in Print: 2015-05-01


Citation Information: American Mineralogist, Volume 100, Issue 5-6, Pages 1209–1221, ISSN (Online) 1945-3027, ISSN (Print) 0003-004X, DOI: https://doi.org/10.2138/am-2015-5034.

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© 2015 by Walter de Gruyter Berlin/Boston.

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