WATER CONCENTRATIONS IN BASALTIC GLASS: IMPLICATIONS FOR SUBGLACIAL ERUPTION PROCESSES

In certain cases, it is possible to calculate the thickness of a paleo-ice sheet by analyzing the volatile content in basaltic glasses that erupted beneath the ice sheet (Tuffen et al., 2010). This technique was evaluated by calculating the minimum ice thickness under which Hlöðufell, a tuya in south-central Iceland, erupted.

Hlöðufell is a Holocene tuya, an edifice that remains from a subglacial eruption once the ice sheet has retreated. It is located in the Western Rift Zone of Iceland, 9 km south of the modern edge of Langjokull ice cap. Stratigraphy of the tuya was described in detail by Skilling (2009). Concentrations of water dissolved in basaltic glass were measured using Fourier transform infrared (FTIR) spectroscopy. Quenching pressures were calculated using the VolatileCalc pressure-solubility model (Newman & Lowenstern, 2002). By relating quenching pressures, the density of ice, and the elevation of the sample, we were able to calculate an overlying ice thickness.

Water concentrations range from 0.068 to 0.54 wt % H₂O, representing pressures ranging from 0.66 to 31.45 bars. These pressures represent ice thicknesses between 0 and 340 m, which even at the maximum is 200 m less than expected, suggesting that the volatile concentrations in the basaltic glasses do not record the accurate quenching pressure. The overall low water concentrations may indicate that the samples degassed at or close to atmospheric pressures at higher elevations, and flowed downslope into areas of thicker ice or deeper melt-water before quenching. These results show that subglacial eruptions and degassing processes are complex and variable and the detailed analysis of flow microtextures is critical to interpreting volatile concentrations.