Magmatic He distribution around Unzen volcano inferred from intensive investigation of helium isotopes in groundwater

Abstract

The ascending magmatic component around Unzen volcano was investigated by intensive sampling and measurements of helium concentration and isotopic ratios in groundwaters including thermal waters (and gases). In the eastern part of the Unzen graben, even shallow groundwaters contain large amount of dissolved magmatic helium with high 3He/ 4He ratio, about 7 times higher than the atmospheric value. This value is almost identical as those in thermal water of this region (Shimabara Spa). In contrast, no significant amount of magmatic He was incorporated into the shallow groundwaters in other regions. The 3He/ 4He distribution among thermal waters inside the graben does not follow the trend observed in many active volcanoes of decreasing the ratio with increasing the distance from a conduit. The distribution of 3He/ 4He ratios around Unzen volcano reflects the difference in residence time in thermal water reservoir and/or difference in the fluxes of magmatic gas components in each region. The results of model calculation, which considered the evolution of 4He concentration and 3He/ 4He ratio in the reservoir, indicated that the residence times of Obama Spa (western side of the graben) and Unzen Spa (central part) were about 28,000 and 11,000 years, respectively, which are significantly higher than those of Shimabara Spa (eastern part). These values are highly variable according to several parameters, such as crustal He flux, aquifer thickness and porosity etc, and should be taken as a first order approximation. Observed 3He/ 4He ratios, however, reflect the qualitative difference of thermal water residence time as well as the magnitude of magmatic He fluxes among three regions. The magmatic 3He flux out from the thermal water reservoir of Shimabara Spa is about four times higher than those of Obama and Unzen Spas. All of these results are not inconsistent with the concept that magmatic gas ascends through the densely existing EW faults in the eastern part of the Unzen graben as well as a conduit.