St. Catharines 2004
Technical Program


SS19:  Molecules to Planets: Infrared spectroscopy in geochemistry, exploration geochemistry and remote sensing 
Organizers / Organisateurs:  Penny King, Mike Ramsay, Gregg Swayze
Room / Salle:  AS 201

Date:  05/12/2004
Time:  8:20 AM
Presenter:  Julie Roberge


FTIR analysis of water and carbon dioxide in basaltic glass and rhyolitic glass inclusions in quartz phenocrysts

Roberge, J., and Wallace, P.J. Department of Geological Sciences, University of Oregon, Eugene, OR, USA, 97403-1272, jroberge@darkwing.uoregon.edu

Quenched glassy pillow rims of submarine-erupted lavas and melt (glass) inclusions trapped in crystals in subaerial volcanic rocks are important because they preserve information on pre-eruption volatile contents. Fourier transform infrared (FTIR) spectroscopy can be used to measure the H2O and CO2 contents of such glasses. To prepare the samples for transmission IR, fragments of unaltered basaltic glass from pillow margins and quartz crystals containing rhyolitic melt inclusions must be doubly ground and polished into wafers with two parallel sides. In the case of quartz crystals, the melt inclusions to be analyzed must be intersected on both sides of the wafer.

For basaltic glass, dissolved total H2O and carbonate (CO32–) can be calculated using Beer’s Law. Total dissolved H2O is measured using the intensity of the asymmetric band at 3550 cm-1. Dissolved molecular H2O can be measured using the intensity of the 1630 cm-1 absorption band. Because the high temperature speciation in basaltic melt is known (Dixon et al., 1995), the presence of excess molecular H2O can be used to recognize submarine glasses that have been affected by low temperature hydration.  Dissolved carbonate can be measured from the absorbance of the 1515 and 1430 cm-1 bands.  We measure absorbance intensities of these bands using a peak-fitting program that fits the sample spectrum with a straight line, a spectrum for carbonate-free glass, a pure 1630 cm-1 band for molecular H2O, and a pure carbonate doublet. The molar absorption coefficient for total H2O (3550 cm-1) is relatively independent of composition for basaltic glasses (Ihinger et al., 1994) but the molecular H2O (1630 cm-1) and carbonate (1515 and 1430 cm-1) absorption coefficients are compositionally dependent (Dixon et al., 1995; Dixon & Pan, 1995). For rhyolitic melt inclusions, total dissolved H2O can be measured by using the bands centered at 5200 cm-1 (molecular H2O) and 4500 cm-1 (OH- groups). Dissolved CO2 can be measured using the intensity of the band at 2350 cm-1 (molecular CO2). Molar absorption coefficients from Zhang et al. (1997) are used for molecular H2O and OH- in rhyolitic glasses and from Blank et al. (1989) for molecular CO2.

The H2O and CO2 data can be used to infer submarine eruption depths, pressures at which phenocrysts crystallize, degassing during magmatic processes, and volatile release to the atmosphere during volcanic eruptions.