Publication Date:
2013-10-02
Description:
We present a calibration for attenuated total reflectance (ATR) micro-FTIR for analysis of H 2 O in hydrous glass. A Ge ATR accessory was used to measure evanescent wave absorption by H 2 O within hydrous rhyolite and other standards. Absorbance at 3450 cm –1 (representing total H 2 O or H 2 Ot) and 1630 cm –1 (molecular H 2 O or H 2 O m ) showed high correlation with measured H 2 O in the glasses as determined by transmission FTIR spectroscopy and manometry. For rhyolite, \[ \hbox{ wt }\%{\hbox{ H }}_{2}\hbox{ O }=245(\pm 9)\cdot {A}_{3450}-0.22(\pm 0.03) \] and \[ \hbox{ wt }\%{\hbox{ H }}_{2}{\hbox{ O }}_{\hbox{ m }}=235(\pm 11)\cdot {A}_{1630}-0.20(\pm 0.03) \] where A 3450 and A 1630 represent the ATR absorption at the relevant infrared wavelengths. The calibration permits determination of volatiles in singly polished glass samples with spot size down to ~5 μm (for H 2 O-rich samples) and detection limits of ~0.1 wt% H 2 O. Basaltic, basaltic andesite and dacitic glasses of known H 2 O concentrations fall along a density-adjusted calibration, indicating that ATR is relatively insensitive to glass composition, at least for calc-alkaline glasses. The following equation allows quantification of H 2 O in silicate glasses that range in composition from basalt to rhyolite: \[ \hbox{ wt }\%\hspace{0.17em}{\hbox{ H }}_{2}\hbox{ O }=(\omega \cdot {A}_{3450}/\rho )+b \] where = 550 ± 21, b = –0.19 ± 0.03, = density, in g/cm 3 , and A 3450 is the ATR absorbance at 3450 cm –1 . The ATR micro-FTIR technique is less sensitive than transmission FTIR, but requires only a singly polished sample for quantitative results, thus minimizing time for sample preparation. Compared with specular reflectance, it is more sensitive and better suited for imaging of H 2 O variations in heterogeneous samples such as melt inclusions. One drawback is that the technique can damage fragile samples and we therefore recommend mounting of unknowns in epoxy prior to polishing. Our calibration should hold for any Ge ATR crystals with the same incident angle (31°). Use of a different crystal type or geometry would require measurement of several H 2 O-bearing standards to provide a crystal-specific calibration.
Print ISSN:
0003-004X
Electronic ISSN:
1945-3027
Topics:
Geosciences
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