VAPOR SATURATION AND ACCUMULATION IN MAGMAS OF THE 1989-1990 ERUPTIONOF REDOUBT VOLCANO, ALASKA

The 1989-1990 eruption of Redoubt Volcano, Alaska, provided an opportu nity to compare petrologic estimates of SO2 and Cl emissions with esti mates of SO2 emissions based on remote sensing data and estimates of C l emissions based on plume sampling. In this study, we measure the sul fur and chlorine contents of melt inclusions and matrix glasses in the eruption products to determine petrologic estimates of SO2 and Cl emi ssions. We compare the results with emission estimates based on COSPEC and TOMS data for SO2 and data for Cl/SO2 in plume samples. For the e xplosive vent clearing period (December 14-22, 1989), the petrologic e stimate for SO2 emission is 21,000 tons, or similar to 12% of a TOMS e stimate of 175,000 tons. For the dome growth period (December 22, 1989 to mid-June 1990), the petrologic estimate for SO2 emission is 18,000 tons, or similar to 3% of COSPEC-based estimates of 572,000-680,000 t ons. The petrologic estimates give a total SO2 emission of only 39,000 tons compared to an integrated TOMS/COSPEC emission estimate of simil ar to 1,000,000 tons for the whole eruption, including quiescent degas sing after mid-June 1990. Petrologic estimates also appear to underest imate Cl emissions, but apparent HCl scavenging in the plume complicat es Cl emission comparisons. Several potential sources of 'excess sulfu r' often invoked to explain petrologic SO2 deficits are concluded to b e unlikely for the 1989-1990 Redoubt eruption - e.g., breakdown of sul fides, breakdown of anhydrite, release of SO2 from a hydrothermal syst em, degassing of commingled infusions of basalt in the magma chamber, and syn-eruptive degassing of sulfur from melt present in non-erupted magma. Leakage and/or diffusion of sulfur from melt inclusions do not provide convincing explanations for the petrologic SO2 deficits either . The main cause of low petrologic estimates for SO2 is that melt incl usions do not represent the total sulfur content of the Redoubt magmas , which were vapor-saturated magmas carrying most of their sulfur in a n accumulated vapor phase. Almost all the sulfur of the SO2 emissions was present prior to emission as accumulated magmatic vapor at 6-10 km depth in the magma that supplied the eruption; whole-rock normalized concentrations of gaseous excess S in these magmas remained at similar to 0.2 wt.% throughout the eruption, equivalent to similar to 0.7 vol .% at depth. Data for CO2 emissions during the eruption indicate that CO2 at whole-rock concentrations of similar to 0.6 wt.% in the erupted magma was a key factor in creating the vapor saturation and accumulat ion condition making a vapor phase source of excess sulfur possible at depth. When explosive volcanism involves magma with accumulated vapor , melt inclusions do not provide a sufficient basis for predicting SO2 emissions. Thus, petrologic estimates made for SO2 emissions during e xplosive eruptions of the past may be too low and may significantly un derestimate impacts on climate and the chemistry of the atmosphere.