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 H
2O 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. I = 120,470 ± 11,360
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 in the literature are too high by up to a factor
of two, requiring revision of previously estimated plagioclase-melt H
partitioning coefficients.