Passive degassing of magmatic volatiles from Iwate volcano, NE Japan, based on three‐dimensional measurement of helium isotopes in groundwater

Abstract

Magmatic volatile supply to the groundwater around Iwate volcano, which is an active volcano with weak fumarolic activity, is investigated using chemical and isotopic compositions of 180 groundwater samples from various depths. Magmatic 3He flux is estimated by an applied method using helium isotopes. The three‐dimensional distribution of 3He flux can be obtained as well as that of the concentrations. High 3He flux with high concentration is found in shallow groundwaters at N and NE flanks, and in shallow and deep groundwater around the fault zone. Low 3He flux with high concentration in deep groundwater suggests the slow and long‐term accumulation of magmatic volatiles. High 3He fluxes and concentrations around the fault zone indicate that the fault acts as an ascending path of magmatic volatiles. Distribution of flux indicates the three‐dimensional anisotropy of magmatic volatile supply, which is due to the groundwater flow variation restricted by the structure of the volcanic body. Supply rates of other magmatic volatiles can be also obtained using 3He flux, and the total supply rates are estimated to be 2.3 × 10−3 (3He), 5.1 × 108 (magmatic C), 1.3 × 108 (Cl), and 1.3 × 108 (S) mol/y. Comparison of supply rate to groundwater with volcanic gas emission rate reveals that the magmatic volatiles are largely supplied to the groundwater. However, the total emission combining the groundwater and volcanic gas is not prominently large compared with other active volcanoes. We infer that the method combining the magmatic volatile concentration and flux to groundwater is available for the evaluation of passive activity of volcano.

Magmatic Carbon and Helium in Springs Reveals the Vitality of a Dormant Volcano, Taranaki, New Zealand

“…It is not known if diffuse CO 2 degassing occurs from Taranaki's upper flanks above the staturated zone. The inset shows the expected range of CO 2 emission from Taranaki's springs (in gray) compared to emissions from cold springs on Mammoth Mountain (Evans et al., 2002), individual cold springs in the Cascades (James et al., 1999), Lassen volcano (Barry et al., 2021; Rose & Davisson, 1996), and Iwate volcano in Japan (Ohwada et al., 2012).…”

Helium in Stream Water as a Volcanic Monitoring Tool

“…Radiogenic 4 He is added to groundwater in volcanic systems due to alpha decay in crustal minerals and subsequent release to adjacent groundwater due to chemical weathering, mechanical deformation, and mineral diffusion (Torgersen, ). In volcanic systems, magmatic 3 He can be added both from active and passive gas release (Lowenstern et al, ; Ohwada et al, ; Padrón et al, ; Sano et al, ). The helium budget for a given isotope can be written where [ i He ] atm is the concentration of atmospherically derived helium, including excess air added due to air‐water exchange processes, [ i He ] rad is the concentration radiogenically derived helium, and [ i He ] m is the concentration of magmatic helium for helium isotope i .…”

“…Radiogenic 4 He is added to groundwater in volcanic systems due to alpha decay in crustal minerals and subsequent release to adjacent groundwater due to chemical weathering, mechanical deformation, and mineral diffusion (Torgersen, 1980). In volcanic systems, magmatic 3 He can be added both from active and passive gas release (Lowenstern et al, 2014;Ohwada et al, 2012;Padrón et al, 2013;Sano et al, 2015). The helium budget for a given isotope can be written…”

“…The spatial distribution of volcanic volatiles in groundwater has been shown to provide information on a variety of volcanic processes including: passive degassing (Ohwada et al, 2012), volcanic fluid movement (Aizawa et al, 2016), and local to regional strain and seismicity (Sano et al, 2015;Padrón et al, 2013;Kennedy and van Soest, 2007). In particular, helium isotopic ratios of groundwater have been shown to respond to both volcanic disturbance (Padrón et al, 2013) and seismic deformation (Brauer et al, 2003).…”

“…Groundwater flow can distribute volcanic volatiles, including helium, tens to hundreds of kilometers away from a volcanic center (Saar et al, ). Helium isotopes in groundwater systems can be used to monitor passive degassing in volcanic systems with little fumarolic activity (Ohwada et al, ). Recent studies have shown magmatically derived helium in groundwater responds in precursory manner to volcanic activity (Padrón et al, ; Sano et al, ) and could potentially be a monitoring signal of volcanic unrest if the frequency and latency of groundwater analyses can be improved.…”