Summit outgassing as indicated by radon, mercury and pH mapping, Kilauea volcano, Hawaii

Surface geochemical mapping of Rn, Hg and pH on the summit of Kilauea volcano, Hawaii, has shown some of the characteristics of outgassing. Rn concentrations measured in shallow ground gas are highest (10 to 155 units, i.e. up to 0.016 nCi/l²²²Rn) over deep structures associated with the summit caldera, low (1 to 5 units) over the upper rift zones (which are underlain by shallow intrusions) and low to negative (1 to -3 units) over most of the volcanically inactive peripheral areas. High Rn concentrations were measured over peripheral well-developed structures such as the Kaoiki and Koae fault systems (45 and 12 units, respectively). The pattern of Rn values broadly suggests the existence of a summit-wide convection system interrupted locally by specific permeable structures. The low pH (3.5 to 5.6) of soils over the caldera is suggested to be largely due to the sulfurous component of the ground gas. The low concentrations of Hg (5 to 80 ppb) in these soils are postulated as being partly a function of subsurface complexing of Hg with the abundant sulfur. The shallow intrusions below the upper rift zones appear to have outgassed much of their sulfur content, so soils in these areas are only slightly acid (5.6 to 6.2), allowing the accumulation of Hg to concentrations of several thousands of ppb. These conditions change during extrusive (and intrusive) events with the increases in shallow subsurface temperatures and volatiles in ground gas producing an increase in Rn concentration, and a decrease in soil Hg due to volatilization. After such events, Rn concentration decreases, and soil Hg increases significantly.