VOLATILE (H2O, CO2, S, CL) CONCENTRATIONS AND THE LOW EXPLOSIVITY OF CALC-ALKALINE ANDESITES FROM MOUNT HOOD VOLCANO, OREGON

Mount Hood, Oregon, is an unusual silicic arc volcano in that it is characterized by a lack of large explosive eruptions, and homogenous magma compositions over the lifetime of the current edifice (~0.5 My). Two possible explanations for the lack of explosive activity at Mount Hood are 1) intrinsically lower volatile element concentrations than in more explosive Cascade volcanoes, and/or 2) a plumbing system that allows volatile loss prior to eruption. To distinguish between these two scenarios, we have measured volatile concentrations in plag-, amphibole-, and opx-hosted melt inclusions from pumice produced during the last three eruptive periods at Mount Hood.

Volatile data from SIMS and FTIR analyses indicate that these melt inclusions contain up to 5.5-6 wt% H₂O and 2300 ppm CO₂, equivalent to saturation pressures up to 5 kbar. H₂O decreases with decreasing CaO and Al₂O₃ and increasing SiO₂ and Na₂O, suggesting concurrent degassing and crystallization during magma ascent. Concentrations of S range from 50-400 ppm, and Cl generally varies between 1500-2500 ppm (but reaches values as high as 4000 ppm). Measured F contents are consistently ~500 ppm. Within a given eruption, H₂O and S appear highest in opx-hosted inclusions. No clear correlation exists between host mineral phase and CO₂, F, or Cl concentrations.

These new measurements of volatiles in melt inclusions from Mount Hood indicate significant overlap with the range of H₂O contents observed in melt inclusions from the 1980 Plinian eruption of Mount St. Helens (up to 6.7 wt% H₂O) and the Holocene eruptions of Mount Mazama (up to 5.3 wt% H₂O). Variable H₂O and CO₂ concentrations suggest that Mount Hood’s magmas experienced various extents of degassing prior to entrapment. Decreasing S/Cl ratios as H₂O decreases are consistent with the progressive loss of volatiles in the order CO₂-H₂O-S-Cl, as observed by others during degassing of hydrous basalts.

Our data indicate that Hood’s volatile concentrations are typical of silicic arc volcanoes. The overall low explosivity of Mount Hood seems to be the result of a plumbing system that allows for extensive pre-eruptive degassing, slower ascent rates, and/or eruption of only small magma volumes rather than representing a source with intrinsically lower volatile concentrations.