Ten-year helium anomaly prior to the 2014 Mt Ontake eruption

Abstract

Mt Ontake in central Japan suddenly erupted on 27th September 2014, killing 57 people with 6 still missing. It was a hydro-volcanic eruption and new magmatic material was not detected. There were no precursor signals such as seismicity and edifice inflation. It is difficult to predict hydro-volcanic eruptions because they are local phenomena that only affect a limited area surrounding the explosive vent. Here we report a long-term helium anomaly measured in hot springs close to the central cone. Helium-3 is the most sensitive tracer of magmatic volatiles. We have conducted spatial surveys around the volcano at once per few years since November 1981. The 3He/4He ratios of the closest site to the cone stayed constant until June 2000 and increased significantly from June 2003 to November 2014, while those of distant sites showed no valuable change. These observations suggest a recent re-activation of Mt Ontake and that helium-3 enhancement may have been a precursor of the 2014 eruption. We show that the eruption was ultimately caused by the increased input of magmatic volatiles over a ten-year period which resulted in the slow pressurization of the volcanic conduit leading to the hydro-volcanic event in September 2014.

Volcanic plume measurements using a UAV for the 2014 Mt. Ontake eruption

“…Input of magmatic fluid into Mt. Ontake's volcanic system had also been suggested by He isotopic ratios of summit fumaroles and flank hot springs (Sano et al 1984(Sano et al , 1986 He ratio at hot springs at about 4 km from the summit had been reported and is concluded as precursory increased input of magmatic fluid from the depth (Sano et al 2015). Supporting evidences for the input of magmatic fluids are also found in seismological studies.…”

“…Ontake, the volume of the system needed may be drastically decreased. In fact, sustained magmatic fluid supply to the hydrothermal system has been indicated by helium-3 studies (Sano et al 2015). Sulfur brought by the long-term magmatic fluid supply might be a candidate for such accumulated sulfur in the hydrothermal system.…”

“…In their review of phreatic eruptions, Barberi et al (1992) grouped the eruption into two categories: "explosions of confined geothermal systems with or without the direct action of magmatic fluids" and "explosions caused by the vaporization of surface fluids percolating into the temporarily plugged hot conduit of an active volcano. " Sano et al (2015) concluded that the 2014 eruption belongs to the former group. The significant SO 2 flux observed implies that the eruption belongs to the former group with direct magmatic fluid.…”

Opal-A in Glassy Pumice, Acid Alteration, and the 1817 Phreatomagmatic Eruption at Kawah Ijen (Java), Indonesia

“…Though the event was purely hydrothermal in that no juvenile material was erupted, there is considerable geochemical evidence that magmatic input was involved in the triggering of hydrothermal fluid migration. Gas samples over 10 years indicated a progression of He isotopes toward increasingly magmatic values prior to the 2014 event (Sano et al, 2015). Immediately after the eruption, SO 2 emission rates of >2,000 t/d were measured, and rapidly decreased thereafter, though hydrothermal H 2 S emissions continued (Mori et al, 2016).…”

“…Immediately after the eruption, SO 2 emission rates of >2,000 t/d were measured, and rapidly decreased thereafter, though hydrothermal H 2 S emissions continued (Mori et al, 2016). It appears likely that the eruption was at least partially triggered by input of magmatic gas at the base of the acidaltered hydrothermal system (Sano et al, 2015;Mori et al, 2016;Yamaoka et al, 2016).…”