Geochemical implications of subaqueous molten sulfur at Yugama crater lake, Kusatsu-Shirane volcano, Japan.

Abstract

Crater lakes with active subaqueous fumaroles often contain molten sulfur pools on the lake floor. Volcanic gases passing through the sulfur pools carry hollow spherules of solidified molten sulfur to the surface of crater lakes. This sulfur dissolves SO2 and H2S gases and releases these gases into the water. The sulfur also contains homocyclic sulfur (cycl. S, x = 6-16) and probably sulfane monosulfonates. The concentration of cyclic S7 increases with increasing temperature between 120 and 175°C, which is useful to estimate the temperatures of subaqueous molten sulfur pools. The gases drastically lower viscosity of the molten sulfur. This may be due to blockage of growing long-chain sulfur molecules by the dissolved gases. Thus a jump in viscosity at 159°C observed for pure sulfur is not likely to be present in subaqueous molten sulfur at crater lakes. Based on the chemistry and morphology of sulfur slicks, activity of subaqueous fumaroles can be divided into four stages (I-IV), each of which may serve for qualitative in situ monitoring of crater lakes. At Stage I, no molten sulfur pools exist on the lake floor and fumaroles discharge low temperature gases (<119°C) containing only traces of SO2; at Stage II, subaqueous molten sulfur pools (119°C < T < 150°C) are formed, releasing yellow hollow spherules of sulfur with no tails; at Stage III, the fumarolic temperature increases to >150°C, resulting in an increase in molten sulfur viscosity; and at Stage IV, frequent phreatic or geyser-like eruptions are observed. The molten sulfur pools are dispersed into pieces on the lake floor at this stage.

Idiosyncrasies of Volcanic Sulfur Viscosity and the Triggering of Unheralded Volcanic Eruptions

“…A decrease in polythionate (S 4 O 2− 6 , S 6 O 6 ) concentrations was observed also at Ruapehu's Crater Lake during very low temperatures for 3 months in 1988 prior to the Dec. 8, 1988 unheralded eruption (Takano et al, 1994). This apparently indicated that the liquid S e pool under the lake remained open to fumarolic degassing (H 2 S and SO 2 ) even during periods of low lake temperatures, and therefore it could not be a barrier for magmatic heat transfer.…”

“…Pure sulfur viscosity is highly temperature dependent, but also extremely sensitive to the presence of impurities (Bacon and Fanelli, 1943;Meyer, 1976). This has been noted in volcanic settings (Takano, 1987;Rowe et al, 1992;Takano et al, 1994), but not followed up. Experiments carried out in the early 1940's (Bacon and Fanelli, 1943) showed that organic material, trivalent elements, ammonia, halogens, or the exposure to certain gases (e.g., H 2 S), almost always lead to lower viscosities on heating than for the pure S e case.…”

“…In a similar case, Kusatsu-Shirane volcano (Japan) ejected molten sulfur through the Yugama acidic crater-lake during several phreatic eruptions since 1805 (Takano et al, 1994). Takano (1987) noted that 2 months before an unheralded eruption on 26 October 1982, the concentration of polythionates ions (S n O 2− 6 ) in the lake waters decreased, and that of sulfates increased.…”

“…Previous studies (i.e., Takano, 1987;Rowe et al, 1992;Takano et al, 1994), noted the role of some types of impurities (H 2 S, Se, As, Te) as S-viscosity modifiers, but did not examine these further. Multiple distillations of sulfur containing organic material showed how hydrogen persulfides are responsible for maintaining low S viscosity for prolonged periods of heating, releasing H 2 S when hydrogen persulfides break up (Fanelli, 1942).…”

“…This apparently indicated that the liquid S e pool under the lake remained open to fumarolic degassing (H 2 S and SO 2 ) even during periods of low lake temperatures, and therefore it could not be a barrier for magmatic heat transfer. Alternatively, Takano et al (1994), suggest cyclic variation of the Ruapehu Crater Lake temperatures reflected variations in the deep P, T, and gas composition beneath the S-pool. This, however, is unable to explain the occurrence of unheralded eruptions.…”