Jun-ichi Hirabayashi

Gigantic SO2 emission from Miyakejima volcano, Japan, caused by caldera collapse

An extremely large amount of volcanic gas has been released since mid-August 2000 from the volcanic island of Miyakejima, Japan, after formation of a summit caldera of 1.6 km diameter. The volcanic gas emission was continuous with very little extrusive magma activity. Variation of the SO 2 emission rate was monitored by repeated measurements with an airborne correlation spectrometer. In December 2000, the SO 2 emission rate averaged for the month peaked at 54 kt/d, which is twice the global SO 2 emission rate from nonerupting volcanoes evaluated before this activity. The SO 2 emission rate gradually decreased, almost linearly when plotted on a log scale, to 7 kt/d by the end of 2002, and then remained constant until at least December 2003. The total SO 2 emission amounts to 18 Mt, comparable to the emission of a large explosive eruption such as Pinatubo in 1991. A theoretical evaluation, based on the model of magma convection in a conduit, suggests that extremely large volcanic gas emissions can be caused by formation of a magma pathway with a slightly larger diameter than exists in common systems, because the magma-transport rate is proportional to the fourth power of the conduit radius. Because volcanic gas emissions were initiated by formation of a summit collapse caldera of 1.6 km diameter, the creation of a large magma-conduit system through fractures formed during caldera collapse is likely the underlying cause of the extremely large volcanic gas emissions from the volcano.

Evaluation of results from the fourth and fifth IAVCEI field workshops on volcanic gases, Vulcano island, Italy and Java, Indonesia

Abstract The major purpose of field workshops on volcanic gases, organized by the IAVCEI Commission on the Chemistry of Volcanic Gases, is the collection and analysis of volcanic gas discharges with the aim to develop and improve techniques for the geochemical surveillance of active volcanoes. The fourth and fifth workshops were held at Vulcano island, Italy, in 1991 and on Java island, Indonesia, in 1994, respectively. Gas samples were collected from four gas vents by nine groups at Vulcano and from eight gas vents by eight groups on Java. The quality (e.g. scatter of the data) of most of the results, reported from these two workshops, is sufficient to permit a broad chemical classification of the discharge and meaningful thermodynamic interpretation. In most cases, the majority of the data for individual gas vents cluster closely around the median values, suggesting that the median values are the best estimates of chemical composition. There is, however, also a considerable scatter of the analytical data, and this scatter warns us to not rely too heavily on a single analytical value, in particular on a value for CH4 and CO, because analytical data for these species often show a wide scatter. This warning is particularly relevant for chemical monitoring of volcanic activity. Further improvement of the sampling and analytical techniques as well as more detailed comparison of the techniques is required to reduce such uncertainty in order to interpret the volcanic activity and hydrothermal conditions.

Effect of UV scattering on SO2 emission rate measurements

We report the quantitative evaluation of the UV scattering effect on the SO 2 emission rate measurement by the compact UV spectrometer system. Plume spectra were obtained simultaneously at three measuring points with different distance to the volcanic plume. The apparent absorbance decreases with increasing distance to the plume and the attenuation becomes stronger at shorter wavelength bands. In addition, the attenuation intensity depends on the SO 2 column concentration. The underestimation of the measured absorbance caused by the UV scattering leads to the underestimation of the SO 2 emission rate. The attenuation was not significant with any wavelength band (<±10%) at 0.6 km but was 35-50% with shorter wavelength band at 2.6 km distance. The UV scattering effect on the SO 2 emission rate estimation can be evaluated by the comparison of the emission rates calculated with different wavelength bands.

Discharge rate of SO2 from Unzen volcano, Kyushu, Japan

The discharge rate of SO 2 from Unzen volcano, which started erupting in November 1990, has been measured with a correlation spectrometer (COSPEC) at a fixed ground-based station about 2.3 km SSW from the summit lava dome. The discharge rate of SO 2 increased greatly after the explosive eruptions on June 8th and 11th, 1991. High discharge rates of SO 2 (an average of 180 ton/day) were observed from July 1991 to January 1992, followed by a general decrease to an average of 110 ton/day during March 1992 and May 1994. The discharge rate of SO 2 correlates with the extrusion rate of magma. Judging from the sulfur inventory, i.e., 130-450 g of discharged SO 2 per ton of erupted magma. and the sulfur content of the extruded lava, the proportion of sulfur degassed from the magma is estimated to be ca. 80 %, suggesting an efficient volatile degassing. The discharge rate of the total volatiles is calculated to be 7,700 ton/day by combining the SO 2 discharge rate with the chemical composition of volcanic gases.

D/H and 18O/16O ratios of water in the crater lake at Kusatsu-Shirane volcano, Japan

Abstract The D/H and 18O/16O ratios of water in the active crater lake situated on the Kusatsu-Shirane volcano, Japan are about 20 and 6‰, respectively, higher than local meteoric water. The ratios show seasonal variations superimposed on a gradual change over nine years. The isotopic ratios started to increase in early 1990 and decrease in the spring of 1995. The seasonal variation which is high in winter and low in summer correlates with the temperature difference between lake water and ambient air. The large temperature difference in winter enhances the evaporation of lake water and produces the enriched isotopic ratios relative to the ratios in summer. The accumulation of snow and the decrease in the flux of meteoric water into the lake strengthens the winter-time isotopic enrichment. The enriched isotopic ratios of the lake water over a long time result from the addition of an end member with heavy isotopic ratios contained in a thermal fluid supplied to the lake. Considering the water balance in the lake, the isotopic ratios of the thermal fluid were found to be close to the lake water itself, suggesting the circulation of the lake water seeping through lake floor. Based on the correlation between Cl−concentration and the isotopic ratios, the contribution by the heavy end member was estimated to be 25–36% relative to the enrichment by evaporation. The heavy end member could be a liquid phase evolved from a parental fluid, which is a mixture of local meteoric water and a magmatic fluid as found in high-temperature volcanic gases.

Evolution of fumarolic gases — boundary conditions set by measured parameters: case study at Vulcano, Italy

Physical, chemical and isotopic parameters were measured in fumaroles at the Vulcano crater and in drowned fumaroles near the beach. The data were used to define boundary conditions for possible conceptual models of the system.Crater fumaroles: time variations of CO2 and SO2 concentrations indicate mixing of saline gas-rich water with local fresh water. Cl/Br ratios of 300– 400 favour sea-water as a major source for Cl, Brand part of the water in the fumaroles. Cl concentrations and δD values revealed, independently, amixing of 0.75 sea-water with 0.25 local freshwaterin furmarole F-5 during September 1982.Patterns of parameter correlation and mass balances reveal that CO2, S, NH3 and B originate from sources other than sea water. The CO2 value of δ13C = − 2%o favours, at least partial, origin from decomposition of sedimentary rocks rather than mantle-derived material. Radiogenic4He(1.3 × lO−3 ccSTP/g water) and radiogenic40Ar(10.6 × 10−4 ccSTP/g water) are observed, (4He/40Ar)radiogenic = 1.2, well in the range of values observed in geothermal systems.Drowned fumaroles: strongly bubbling gas at a pond and at the beachappears to have the same origin and initial compositionas the crater fumaroles (2 km away). The fumarolic gas is modified by depletion of the reactive gases, caused by dissolution in shallow-water. Atmospheric Ne, Ar, Kr and Xe are addeden route, some radiogenic He and Ar are maintained. The Vulcano system seems to be strongly influenced by the contribution of sea-water and decomposition of sedimentary rocks. Evidence of magmatic contributions is mainly derived from heat.

Helium and carbon fluxes in Lake Nyos, Cameroon: constraint on next gas burst

On 21 August, 1986, a lethal gas burst issued from Lake Nyos in Cameroon, western Africa, killing at least 1700 people. Although the consensus is that the victims died of asphyxiation by CO2 of magmatic origin, the frequency of such catastrophic degassing events is still unknown. The CO2 flux at the bottom of Lake Nyos is estimated based on the measurement of3He4He and4He20Ne ratios and the chemical composition of gases exsolved from the lake water. The calculated CO2 flux at the bottom of Lake Nyos is(4.4 ± 2.3) × 1013cm3STP/year. Combined with the estimated release in the August 1986 event of (8 ± 2) × 1014 cm3 STP, the CO2 flux suggests that the gas burst may happen about once in18 ± 10 years, although the significant uncertainty should be taken into account for the frequency resulting from assumptions such as steady-state fluxes in the lake and small fractionization of the C/He ratio during the degassing event. Several yearly measurements of hypolimnetic fluxes and seasonal measurements of epilimnetic fluxes are needed to constrain better the recurrence interval. In addition, the results of a regional3He4He survey of carbonated mineral springs in northwestern Cameroon are discussed in the context of the regional geotectonics.

Equilibrium temperature and redox state of volcanic gas at Unzen volcano, Japan

Abstract High-temperature volcanic gases were collected from a growing dacitic dome of Unzen volcano, Japan. Apparent equilibrium temperatures (AET) were calculated for the following two reactions, SO2+3H2=H2S+2H2O and CO2+H2=CO+H2O. Both the AETs are higher than the outlet temperature (To). Two hypotheses are examined as a cause of this disagreement; one is a partial advancement in the reaction SO2+3H2→H2S+2H2O, the other is an addition of external water after a final equilibrium attainment. Based on these hypotheses the equilibrium compositions, temperatures and redox potentials were estimated resulting in similar values. The estimated temperatures range from 850 to 911°C, which is consistent with a petrological estimation made with the lava; however, the estimated redox potential is one unit less in logfo2 scale than the petrological estimation.

Carbon and helium isotopic ratios at Kusatsu-Shirane Volcano, Japan

Abstract We have measured the chemical compositions, He/Ne ratios, He and C isotopes of 14 gas samples collected from the crater lake, fumaroles and hot springs associated with Kusatsu-Shirane Volcano, Japan. The 3 He/ 4 He ratio decreases with increasing distance from the central crater of the volcano to the sampling site whereas the δ 13 C value of CO 2 increases with the distance. This tendency suggests that high 3 He/ 4 He-low δ 13 C magmatic gas is mixed with and/or diluted by low 3 He/ 4 He-high δ 13 C crustal gas with increasing distance from the crater. Alternatively, the variation of δ 13 C values may be the result of isotope fractionation during migration since the isotope shift is relatively small. Based on the apparent mixing trend in a 3 He/ 4 He-δ 13 C diagram, the magmatic He and CO 2 in the volcano may have a 3 He/ 4 He ratio of 8 R atm and a δ 13 C value of −3.2%, respectively. The CO 2 / 3 He ratios show a positive correlation with temperature of fumaroles or hot springs, which may be caused by a difference of temperature dependencies of the solubilities between CO 2 and He.

Temporal variations in the constituents of volcanic ash and adherent water-soluble components in the Unzen Fugendake eruption during 1990-1991

A change in the chemical compositions of volcanic gases is one of the noticeable phenomena that frequently occurs prior to an eruption. Analysis of the water-soluble components adhering to volcanic ash is available for remote monitoring of volcanic gases from inaccessible volcanoes. It is a secure method for monitoring volcanic activity without using particular devices. Prolonged volcanic eruption at the Unzen Fugendake volcano from 1990 to 1995 started with a phreatic eruption after 198 years of dormancy. Volcanic activity changed from a phreatic and phreatomagmatic eruption to a magmatic eruption with pyroclastic flows in May 1991. The relationship between the chemical composition of volcanic ash and the contents of the water-soluble components adhering to it are discussed in relation to the early stage of the long-term eruption. Volcanic ash ejected by phreatic and phreatomagmatic eruption before dome formation was the product of the alteration in the volcanoclastic materials beneath the surface. The ash had a high content of water-soluble components, which was caused by the absorption of hydrogen chloride and sulfur dioxide gases from magma into wet debris before dome formation. Volcanic ashes which were generated by pyroclastic flows after dome formation were fresh lava fragments. While the contents of water-soluble sulfate adhering to the ash noticeably decreased, those of water-soluble chloride adhering to the ash hardly decreased. The considerable decrease in the contents of water-soluble sulfate was caused by the reaction of volcanic gases with dry lava fragments. Contrary to this, the concentration of hydrogen chloride gas in ash clouds was extremely high, which obstructed the decrease in the water-soluble chloride content in the ash. Volatility of chlorine and sulfur from volcanic rock suggests that the inner temperature of pyroclastic flows was higher than 600∼700°C at least.