ACCURATE ICE THICKNESS ESTIMATES FROM VOLATILE CONTENTS OF SUBGLACIAL GLASSES FROM NORTHERN BRITISH COLUMBIA, CANADA

Volatile concentrations (H₂O and CO₂) of glaciovolcanic pillow glasses can be used to estimate overlying ice thickness based on the pressure dependence of volatile solubility in magma. The few existing studies that have estimated ice thickness using volatile concentrations have suffered from errors associated with accurately measuring wafer thickness for FTIR measurements of H₂O and the lack of CO₂ data for the pillow glasses. In this study, we have attempted to overcome these two deficiencies to accurately estimate ice thicknesses in northern British Columbia. Basaltic pillow lavas erupted from Tennena Cone at the Mount Edziza Volcanic Complex (MEVC) in northern British Columbia occur as sinuous ridges with near vertical margins. Basaltic pillow lavas with steep margins suggest confinement by glacier ice. We have collected multiple samples of a pillow unit from Tennena Cone to attempt accurate ice thickness estimates using both H₂O and CO₂ concentrations of the glassy rims.

Initial H₂O contents by manometry and hydrogen isotopic compositions for the 2006 samples from the Tennena Cone pillow rinds range from 0.66 to 0.83 wt% and -94.5 to -127.1 per mil, respectively. H₂O content ranges from 0.719 to 0.988 wt% based on Fourier Transform Infrared (FTIR) measurements for 2006 and 2007 samples. Wafer thickness was accurately determined in the FTIR studies using interference fringes in reflected light. CO₂ concentrations of 25 ppm in the matrix glass were determined by manometry during the step heating analysis. To our knowledge, this is the first time that ice thickness estimates are based on the combination of measured FTIR H₂O and manometric CO₂ concentrations. Saturation pressures can be determined using the measured volatile data and a program called VolatileCalc. Assuming a basaltic composition with 49 wt% SiO₂, an eruption temperature of 1000°C, and CO₂ concentrations of 0 ppm, the measured H₂O contents by FTIR yield eruption pressures that range from 48 to 94 bars. Assuming an ice density of 0.931 g cm^-3, these pressures correspond to ice thicknesses between 540 and 1057 m. For the samples that have measured CO₂ concentrations by manometry, ice thickness varies from 1336 to 1566 m.