Kazuhiro Amita

Color change of lake water at the active crater lake of Aso volcano, Yudamari, Japan: is it in response to change in water quality induced by volcanic activity?

One feature of volcanic lakes influenced by subaqueous fumaroles existing at lake bottoms (called active crater lakes) is the remarkable color of their waters: turquoise or emerald green. The active crater lake named Yudamari at Mt. Nakadake of Aso volcano, Japan, takes on a milky pale blue-green. The particular blue component of the lake water color results from Rayleigh scattering of sunlight by very fine aqueous colloidal sulfur particles; the green component is attributable to absorption of sunlight by dissolved ferrous ions. An objective color observation conducted during 2000–2007 revealed that the lake water color changed from blue-green to solid green. The disappearance of the blue ingredient of the water color will result in diminution of aqueous colloidal sulfur from chemical analyses of lake waters sampled simultaneously. The aqueous sulfur is produced by the reaction of sulfur dioxide and hydrogen sulfide supplied from subaqueous fumaroles. However, its production efficiency decreases by domination of sulfur dioxide in the subaqueous fumarolic sulfur gas species with increasing subaqueous fumarolic temperature. The disappearance of blue ingredients from the blue-green color of the lake water may be attributed to activation of subaqueous fumarole activity.

Entrainment of atmospheric air into the volcanic system during the 1995 phreatic eruption of Kuju Volcano, Japan

Abstract On 11 October 1995, Kuju Volcano (Central Kyushu, Japan) erupted steam and ash from new vents opened south of the active Kuju–Iwoyama fumarole area. The He/Ar atomic ratio of fumarolic gas, which is a possible indicator of the ratio of magmatic gas to air, suddenly decreased from 0.12–0.22 to 0.04–0.06 at the beginning of the 1995 eruption. The flux of magmatic He soon after the eruption is estimated to be almost 12 times larger than that before the eruption: values before and after the eruption are 57×10 −3 and 694×10 −3 kg/day, respectively. In contrast, the flux of air-derived He increased after the eruption by nearly 40 times than before the event: values before and soon after the eruption are 0.2×10 −3 and 7.6×10 −3 kg/day, respectively. The sudden decrease in the He/Ar ratio suggests that air has been suctioned into the flow path of uprising volcanic fluid through the surrounding formation of altered rocks piled loosely. The volume of air entrained is estimated to be nearly 1×10 4 m 3 /day.

Origin of saline waters distributed along the Median Tectonic Line in southwest Japan: Hydrogeochemical investigation on possibility of derivation of metamorphic dehydrated fluid from subducting oceanic plate

To identify of metamorphic dehydrated fluid as source fluid of hot spring water, we conducted chemical and isotopic analyses of water and accompanied gas samples collected from hot-spring wells along the Median Tectonic Line (MTL) in the forearc region of the southwestern part of Japan. As a result, we found the hot spring waters having anomalous δD and δ18O compositions as compared with modern seawater and shallow groundwater in Wakayama and Shikoku regions. Judging from data in relative B–Li–Cl composition and He isotopic systematics, the source fluid of the hot springs in Shikoku could be identified to be one of diagenetic fluids. On the other hand, the source fluid of the hot springs of Wakayama had different B–Li–Cl composition and higher 3He/4He ratio in comparison with diagenetic dehydrated fluids and then the fluid was thought to be originated from metamorphic dehydrated fluid as well as Oita plain. There was another striking contrast between the source fluid of Wakayama and Oita and that of Shikoku and Miyazaki; accompanied gases by the former were rich in CO2, whereas those with the latter were rich in CH4, and CO2 in the accompanied gases of Wakayama and Oita is mostly derived from marine carbonate like volcanic gases in subduction zones. Moreover, the Li–B–Cl compositions of them showed transitive values between the relative composition of diagenetic fluids and those of volcanic thermal waters. Consequently, the source fluid of hot springs in Wakayama and Oita was likely to be dehydrated metamorphic fluids released from the subducting Philippine-Sea plate.