Exsolution of H2O, CO2, and S during eruptive episodes at Kilauea Volcano, Hawaii

Abstract

A model for the exsolution of H2O, CO2, and S from Kilauea magma as it rises from a shallow crustal reservoir predicts that vigorous exsolution occurs only after magma has ascended to shallow depths (≲ 150 m lithostatic) where pressures are <3 MPa (30 bars). Exsolved vapor at saturation (30–100 MPa) is CO2‐rich (70 vol %, maximum), but H2O increases and ultimately predominates in the vapor as pressure drops. The model indicates that most H2O remains dissolved, however, down to pressures as low as 2–3 MPa (20–30 bars) rather than being exsolved abundantly below 8 MPa (80 bars) as is commonly assumed. Most CO2 is exsolved above 10 MPa, whereas most S, like H2O, is exsolved below 2–3 MPa (<100–150 m lithostatic). The critical condition for major outgassing and disruption of magma into spray is reached at 1.0–0.6 MPa (∼40–25 m lithostatic). Gas exsolution apparently continues after disruption. The total quantity of exsolved volatiles as predicted by the exsolution model agrees with that predicted by models of fire fountain dynamics. At 0.3–0.4 MPa (≲20 m lithostatic), the exsolved gases resemble volcanic gases of east rift zone eruptions and coexist with melts containing volatiles at concentrations similar to those observed in fountain spatter. Exsolution from coalesced spatter of previously disrupted melt produces residual gases that are H2O‐rich and sharply depleted in CO2. The low pressures and shallow depths required for extensive exsolution imply that magma must be transported nearly to the surface before disruption, outgassing, and fountaining are possible. Therefore eruption forecasting techniques based on detection of volatiles outgassed from ascending magma may not provide useful advance warning at Kilauea and other tholeiite basalt systems.