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

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.

of helium isotopes were reported also in fumarolic gases during the 2002-2003 eruption of Stromboli volcano, Italy 13 . All above helium isotopic anomalies were related to magmatic eruptions, none has yet been reported for hydro-volcanic eruptions. Here we show a ten-year helium anomaly related to the 2014 Mt Ontake eruption. A hydrodynamic dispersion model applied to the data provides an explanation for temporal variation of helium-3 flux at the conduit. The helium-3 flux can be converted into magmatic volatile flux, which may have led to the accumulation of steam in the volcanic edifice and the hydro-volcanic eruption.

Results
Helium isotopes and helium/neon ratios of gas samples. We measured helium isotopes and helium/neon ratios of 92 gas samples in seven bubbling hot and minerals springs around Mt Ontake (Fig. 1). Samples were collected once every few years since November 1981 14 (STable 1) and 12 samples were collected after the 2014 eruption. The 3 He/ 4 He and 4 He/ 20 Ne ratios vary significantly from 1.25 Ra to 7.38 Ra (where Ra is the atmospheric ratio 15 of 1.382 × 10 −6 ) and from 0.34 to 285 respectively. All helium isotopic ratios are higher than the air value, suggesting the influence of a mantle signature typical for arc volcanoes (7.4 ± 1.3 Ra 9 ). Observed 3 He/ 4 He ratios are corrected for atmospheric contamination using helium/neon ratios 11 . Hereafter we use only corrected values, while we identified five samples collected between 1993 and 2007 with significant air contamination. During the whole observation period, the 3 He/ 4 He ratio generally decreases with increasing distance from the central cone to the sampling site (SFig. 1) suggesting that the most primitive magmatic 3 He is carried with fluid flowing through the volcanic conduit 14 . As helium moves from the volcanic conduit through fissures and permeable channels to surrounding hot and mineral springs, the magmatic helium is diluted by radiogenic helium (0.02 Ra 16 ) produced in aquifer rocks. This process results in lower 3 He/ 4 He ratios at more distant sites. However, monitoring of distant mineral springs still provides data that are, to a large extent, the direct result of variations of 3 He/ 4 He ratios in the volcanic conduit 10,11,12 . Secular variations of helium isotopes. Figure 2 shows secular variations of helium isotopes in seven natural springs where Fig. 2a indicates those in the northwest section of Mt Ontake and Fig. 2b those in the southeast. These data cover 3 He/ 4 He ratios of bubbling gas samples collected for 34 years since November 1981, comprising the longest record of hydrothermal helium isotope data in the noble gas literature 8,9 . In the northwest sites, 3 He/ 4 He ratios were generally constant within 2σ error from November 1981 to June 2000. Then the ratios increased significantly from June 2003 to November 2014 at Nigorigo hot spring, the closest site to the central cone. In contrast distant from the cone, the ratios stayed constant during the same period at Akigami and Yuya mineral springs (Fig. 2a). In the southeast sites, 3 He/ 4 He ratios were mostly variable ( Fig. 2b). At the Kanose site close to the cone, the ratio increased gradually and with a constant rate from November 1981 to November 2014. On the other hand, there are two step changes of helium isotope values at Shirakawa, Kakehashi and Shojima, sites located relatively distant from the cone. The 3 He/ 4 He ratios increased significantly from November 1981 to June 2003 and then suddenly decreased and remained at a constant value until after the 2014 eruption. In summary spatial and secular variations of helium isotopes are complex and there is not a simple relationship except for recent increases of helium isotopes at Nigorogo site closest to the cone.

Discussion
In order to study how the activation of Ontake volcano led to the fatal hydro-volcanic eruption, precise data analysis and hydro-geochemical modeling is necessary. In addition, the recent history of geotectonic events reported in the region is important for the interpretation of helium isotopes. These events are summarized as follows: The last magmatic activity was estimated to have occurred about 23,000 years ago 17 and the volcano had been believed to be dormant, even though weak fumarolic activity was observed at the southwestern flank of the central cone. The first historical hydro-volcanic eruption occurred on 28 th October 1979, forming several new craters and ejecting large amounts of volcanic ash, rock and steam 18 . Five years later, a large earthquake (M6.8; the 1984 Western Nagano Earthquake) at shallow depth (2 km 19 ) occurred about 10 km southeast of Mt Ontake on 14 th September 1984. Immediately after the earthquake, a large-scale landslide took place near the top of the volcano, killing 29 people on the southern slope. On 12 th November 1992, seismic activity occurred beneath the summit, followed by a white plume rising to 100 m above the crater 20 25 . Subsequent emission of mantle helium has ceased by the time of ground uplift, probably due to exhaustion of mantle volatiles in the small magma volume. Constant increase of 3 He/ 4 He ratio at Kanose site ( Fig. 2b) may be due to a switch of the source of mantle helium from the diapiric magma in southeast flank to the central cone plumbing system during the time from 2002 to 2004. Different patterns at the sites Kanose and Shirakawa may be due to the distance from the fault (SFig. 2). Kanose is located further than Shirakawa and influence of diapiric magma may be smaller. Figure 3 indicates the relationship between the distance from the central cone of Mt Ontake and the TROC of helium isotopes after June 2003. There is a negative relationship between the distance and TROC, suggesting that the source of excess mantle helium is attributable to reactivated magma beneath the central crater. Decrease of crustal helium contribution into natural springs by aquifer rock dilatancy 26 is not likely because there is not significant seismic activity in the northwest section. Therefore the recent ten years of increases in 3 He/ 4 He ratios at the Nigorigo and Kanose sites (Fig. 2) are mostly related to the central magma source of Mt Ontake, which may be related to the hydro-volcanic eruption.
There are two types of hydro-volcanic eruptions 5 ; explosions of confined geothermal systems with or without the direct influence of magmatic fluids and those caused by the vaporization of surface fluids percolating into the temporarily plugged hot conduit of the volcano. The most likely cause of the 2014 eruption could be the former type of explosion because there is not a plugged hot conduit. Heating of shallow groundwater may have occurred during the magma rise, which may have increased the volatile pressure in the volcanic edifice. Ten years increase of helium isotopes at Nigorigo and Kanose sites suggests that the eruption process is slow (Fig. 4), caused by the gradual, rather than fast, accumulation of mantle volatiles during the rapid increase of volatile pressure produced by groundwater contact with the magma. Prior to the small hydro-volcanic eruption in March 2007, a very-long-period (VLP) volcanic event was detected by seismic observation 27 . The VLP event was explained as the response of a hydrothermal system to magma intrusion about 3 km beneath the summit of Mt Ontake. Therefore accumulation of volatile pressure was ongoing at least since 2007, which corresponds to the increase of helium isotope ratios at Nigorigo (Fig. 2). To evaluate the risk of a possible hydro-volcanic eruption, it is important to study the rate of volatile input into the volcanic edifice. Monitoring of volcanic SO 2 flux measurement may be useful to estimate this rate, but it has not been conducted at the central cone of Mt Ontake before the 2014 eruption. Using our data it is possible to estimate helium-3 flux at the conduit by a hydrodynamic dispersion model applied to the spatial variation of the helium isotopes in a given year 28 (see Methods). Assuming that the fringe of the conduit is 1 km away from the center, which is the same size of the dike model 23  Assuming that the depth of the aquifer is 30 m with an uncertainty of a factor of three and using the volcanic conduit diameter of 2 km, the hypothetical area of helium emission is 1.9 × 10 5 m 2 and the total helium-3 flux from the conduit of Mt Ontake before June 2003 is 78 nmol/day. The magmatic CO 2 / 3 He and H 2 O/CO 2 ratios of high temperature subduction zone volcanic gases are well documented and summarized as 1 × 10 10 and 100, respectively 29 . Using these values, the magmatic water flux is calculated as 1.4 tons/day based on the helium-3 flux and H 2 O/ 3 He ratio. The magmatic water flux increased to 1.7 ton/day in June 2005 as the helium-3 flux was enhanced. This excess water supply of 0.3 ton/day, which likely continued over the last 10 years, led to an accumulated water amount of 1000 tons. This amount of water was introduced into the surrounding hydrothermal system and excess water vapor may have been trapped in the conduit just beneath the central cone (Fig. 4). This excess water vapor could have provided the driving force for the 2014 eruption.
In summary, we have observed a clear helium isotope increase at the hot spring close to Mt Ontake since June 2003, ten years before the 2014 fatal eruption. There were no consistent change at the distant sites. The helium anomaly is likely related to the recent activation of magma and is valuable for the mitigation of volcanic hazard in future. Methods Sampling, analysis and data reduction. Hot and mineral spring gases were collected by water displacement method using an inverted funnel, a manual pump and a lead glass container 9 . All sampling sites are natural springs and we did not use any lifting pump system. A portion of gas sample was introduced into a metallic high vacuum line in the laboratory, where helium and neon were purified by hot Ti getters and charcoal traps at liquid nitrogen temperature. Then the 4 He/ 20 Ne ratios were measured by a quadrupole mass spectrometer and helium was separated from neon by a cryogenic charcoal trap. Samples before 1990 and 2003 were measured by a Nuclide noble gas mass spectrometer without separating helium from neon 30 , while those after 1990 except for 2003 were analyzed by a VG5400 mass spectrometer 31 . There is an experimental bias of about 9% between the two systems. However the difference was well corrected by a careful treatement 32,33 . Samples collected after the 2014 eruption were measured by the same system as for the 1993-2009 samples. Therefore there is no bias expected among them. Correction of the 3 He/ 4 He ratio for air contamination was made based on the 4 He/ 20 Ne ratio. If the 4 He/ 20 Ne ratio is close to the air value, the correction could be significantly erroneous 11 . Therefore we masked five samples with low 4 He/ 20 Ne ratios (STable 1).
Hydrodynamic dispersion model. In order to explain the observed helium isotope trend around the volcano (SFig. 1), a hydrodynamic dispersion model was developed 28 Assuming that thermal fluids are supplied from a magma reservoir to the conduit at a constant rate, and that the boundary conditions are such that the height of the piezometric head has the same distribution in any vertical section through the axis of the conduit, it is possible to estimate the fluid flow and thus helium isotopes based on the dispersion model. The equation governing helium isotopes at distance (r) under steady-state, homogeneous and isotropic conditions is as follows: where 3 P, 4 P, 3 C and 4 C denote nucleogenic and radiogenic production rate of 3 He and 4 He, hypothetical concentration of 3 He and 4 He at conduit, respectively. Assuming typical sedimentary material composing the aquifer, 3 P and 4 P is 1.5 × 10 6 atoms/m 3 sec and 3 × 10 −2 atoms/m 3 sec, respectively. It is possible to calculate 3 C and 4 C values by fitting the observed helium isotope distribution to the above equation by the least-squares method. Despite the model being simplistic, it reproduced well the spatial distribution of helium isotopes at several volcanoes (Mt Nevado del Ruiz, Mt Hakone, Mt Kusatsu and Mt Unzen) 9 . The method is applied to the spatial data set of year 1981, 1984, 1985, 1991, 1993, 1996, 1998, 2003, 2005, 2007, 2009, and 2014. It is difficult to calculate the data of 2000 because the number of data is too small. Helium-3 flux at the conduit is estimated by the term of " 3 C/r" in above equation for each year. Secular variation of the helium-3 flux is plotted in SFig. 3 where the error is 2 sigma obtained by the least-squares method.