Kohei Kazahaya

Excessive degassing of Izu-Oshima volcano : magma convection in a conduit

Excess degassing of magmatic H2O and SO2 was observed at Izu-Oshima volcano during its latest degassing activity from January 1988 to March 1990. The minimum production rate for degassed magma was calculated to be about 1×104 kg/s using emission rates of magmatic H2O and SO2, and H2O and S contents of the magma. The minimum total volume of magma degassed during the 27-month period is estimated to be 2.6×108 m3. This volume is 20 times larger than that of the magma ejected during the 1986 summit eruption. Convective transport of magma through a conduit is proposed as the mechanism that causes degassing from a magma reservoir at several kilometers depth. The magma transport rate is quantitatively evaluated based on two fluid-dynamic models: Poiseuille flow in a concentric double-walled pipe, and ascent of non-degassed magma spheres through a conduit filled with degassed magma. This process is further tested for an andesitic volcano and is concluded to be a common process for volcanoes that discharge excess volatiles.

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.

Volatile transport in a convecting magma column: Implications for porphyry Mo mineralization

We propose that convection in a column of silicic magma allows transport of volatile components from large (>50 km3) magma chambers to the sites of shallow (≈3 km) porphyry-type Mo deposits. Using constraints from the Henderson and Pine Grove systems, we show that even at temperatures as low as 700 °C, a granitic magma with 30 vol% phenocrysts can flow through a magma column with a 150 m radius at a rate >10 km3/yr—enough magma to contribute the Mo necessary to form an ore deposit in a geologically reasonable time frame. This process requires an efficient mechanism for bubble separation at the top of the magma column to produce degassed magma that can descend down through the column. It also requires slow rates of crystallization in the silicic magma, consistent with experimental studies. Hydrothermal fluids released from the convecting magma column can explain many geologic features of the deposits.

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.

Degassing process of Satsuma-Iwojima volcano, Japan: Supply of volatile components from a deep magma chamber

Satsuma-Iwojima volcano continuously releases magmatic volatiles from the summit of Iwodake, a rhyolitic lava dome. The temperature of fumaroles is high, between 800° and 900°C, and the water-rich composition of volcanic gases has not changed essentially over the past 10 years. Sulfur dioxide flux measured by COSPEC is almost constant with an average of 550 t/d since 1975. The present volcanic gas is likely degassed from a rhyolitic magma whose composition is similar to that erupted in 1934, 2 km east of Satsuma-Iwojima. Comparison of silicate melt inclusions and volcanic gas compositions indicates that the magma degassing pressure is very low, implying magma-gas separation at a very shallow level. The mass rate of magma degassing is estimated at 10 m3/s using the volatile content of the magma and the fluxes of magmatic volatiles. The rhyolitic parental magma is volatile-undersaturated in the deep magma chamber, as suggested by melt inclusion studies. Magma convection in a conduit, driven by the density difference between higher density degassed and lower density non-degassed magmas, explains the high emission rate of magmatic volatiles released at shallow depth from such a magma chamber, that is gas-undersaturated at depth. Model calculations require the conduit diameter to be greater than 50 m as a necessary condition for convection of the rhyolitic magma. Long-term convective degassing has resulted in the rhyolitic magma in the deep chamber to become depleted in volatile components. The melt-inclusion studies indicate that the rhyolitic magma responsible for discharging the present volcanic gas has been degassed during the long degassing history of the volcano and is now supplied with CO2-rich volatile components from an underlying basaltic magma. The total volcanic gas flux over 800 years requires degassing of 80–120 km3 of basaltic magma.

Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojima volcano, Japan: Formation of a new degassing vent, 1990–1999

Large changes in the surface manifestation of degassing activity were observed from 1990 to 1999 at the summit crater of Iwodake cone of Satsuma-Iwojima volcano. During this period, a new high-temperature fumarolic area formed in the center of the crater floor and became a degassing vent with a diameter of 40 m. Altered volcanic rocks were ejected during the course of vent formation. Although glass fragments were observed in the ejected ash, the glass comes from altered Iwodake rhyolite that covers the crater floor. The highest fumarolic temperature and equilibrium temperatures of volcanic gases had a maximum of about 900°C at the beginning of the vent formation. The flux of SO2, measured by COSPEC, varied from 300 to 700 ton/day and correlated directly with maximum fumarole temperature. During this period, open fractures formed along the southern rim of the crater almost contemporaneously with the vent formation and changes in the nature of fumarolic discharges. The continuous and intense degassing at Satsuma-Iwojima is likely caused by volatile transport from a deep magma chamber through a convecting magma column. An increase in the magma convection rate might have caused these large changes in surface manifestations, including increase in the SO2 flux and fumarolic temperatures, ground deformation, and the vent formation.

Mass and heat flux of volcanic gas discharging from the summit crater of Iwodake volcano, Satsuma-Iwojima, Japan, during 1996–1999

Abstract Volcanic gas of Iwodake has been discharged continuously from high-temperature (900°C maximum) fumaroles and a degassing vent which formed on the central floor of the summit crater after 1994. Although most of the high-temperature fumaroles located on the crater wall before 1991, many fumaroles appeared on the crater floor associated with the vent opening, suggesting a shift of thermal activity from the peripheral to the central crater. A large amount of volcanic gas has been discharged from the fumaroles and vent. Since heat has been transferred from ascending gas to the surrounding soil, a region showing a surface temperature anomaly has developed around the fumaroles and vent. To quantify Iwodake thermal activity, heat and volcanic gas mass fluxes were estimated during 1996–1999 using infrared thermal images, and plume velocity and temperature data which were observed by a pitot tube and thermocouple. The estimated gas mass flux was compared with the COSPEC data to investigate the accuracy of our estimation. The gas mass flux had been decreased from 230 kg/s in October 1996 to 30 kg/s in November 1999. Although the vent diameter had increased from 20 to 70 m during the same period, this mass flux variation had indicated the decline of degassing rate. The degassing depth was estimated from the volcanic gas mass flux and temperature. The depth showed a tendency of magma head migration to shallower depth during 1996–1999, which was consistent with the drastic change of the surface manifestation.

Degassing process of miyakejima volcano: implications of gas emission rate and melt inclusion data

Abstract Various techniques of volcanic plume studies were applied to monitor degassing activity of Miyakejima volcano, Japan, including SO 2 emission rate measurement with COSPEC, heliborne-CO 2 /SO 2 ratio measurements, Cl/S ratio measurements with alkaline-trap methods. The degassing activity of Miyakejima, that started mid-2000, was characterized by its larger emission rate (up to 40 kt/d SO 2 ), gradual decrease of the emission rate and constant composition even during the emission decrease. Composition of the volcanic gas was estimated by combination of these plume studies. Volatile contents of the magma were estimated by the analyses of melt inclusions in the Miyakejima 2000 basalt, and close similarity was found between the estimated melt composition and composition of the volcanic gas, indicating a shallow degassing of the magma. The results of volcanic plume and melt inclusion studies were integrated to evaluate the degassing process of the volcano, and the intense degassing was modeled based on the volatile-transport through a convecting magma conduit

Soil gas emission of volcanic CO2 at Satsuma-Iwojima volcano, Japan

Soil gas surveys were carried out in November 1999 and October 2000 at Satsuma-Iwojma volcano, southwest Japan. The chemical composition of the soil gas was a mixture of CO2 and air components, with CO2 concentrations ranging from 0.03 to 59 vol%. The origin of the soil CO2 was evaluated on the basis of the variation of δ13CCO2 and CO2 concentration. Although most of the CO2 is of biogenic origin, large volcanic contributions of greater than 50% were found close to the caldera rim, where soil temperature anomalies were observed. Emission rate of volcanic CO2 was estimated by assuming simple mixing among volcanic, atmospheric, and biogenic CO2. High rates of diffuse emission of volcanic CO2 were also observed at the caldera rim and close to some hot springs, indicating that part of the volcanic-gas discharge ascends through fissures along the caldera rim. The flux of the volcanic CO2 through the soil is estimated to be 20 t day−1; this is 25% of the total CO2 flux through soil and 20% of the CO2 flux from volcanic fumaroles in the main crater of Iwodake.

Escape of volcanic gas into shallow groundwater systems at Unzen Volcano (Japan) : evidence from chemical and stable carbon isotope compositions of dissolved inorganic carbon

Abstract Chemical and stable carbon isotopic analyses of dissolved inorganic carbon (DIC) were carried out for groundwater samples collected from cold springs and shallow wells in the Unzen volcanic region in 1999 and 2000. All of the data sets plotted on the carbon isotope ratio (δ13C) vs 1/DIC diagram can be explained by mixing of volcanic CO2 with DIC equilibrated with soil CO2. Groundwater DIC showing a high mixing ratio of volcanic CO2 appears to have a tendency to distribute along two major faults near the activity center of the 1990–1995 eruption. This suggests that these faults are escape routes of volcanic CO2 diffused into the volcanic edifice. The total flux of the volcanic DIC discharged from the cold springs is shown to be one to two orders of magnitude lower than the roughly estimated flux of volcanic CO2 discharged from the summit during the eruptive period.