Hiroshi Shinohara

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.

Partition of chlorine compounds between silicate melt and hydrothermal solutions: I. Partition of NaCl-KCl

Abstract The partition experiments of NaCl and KCl between silicate melts and aqueous chloride solutions were carried at a temperature of 810°C in the pressure range from 0.6 to 6.0 kb. The chloride concentration in the melt (CClm) was constant in certain ranges of chloride concentration in the aqueous phase (CClaq) at 0.6 and 1.2 kb, which reveals the presence of vapor-liquid immiscibility of the aqueous solution. The variation diagram of CClm and CClaq can be applied to the study of aqueous phases as a new method. The partition ratio of chloride ( D Cl m aq = C Cl m C Cl aq ) exhibits a strong negative pressure dependence, which is attributed to the large negative partial molar volume of chlorides in the aqueous phase. The distribution coefficient of Na and K ( D Na K M Aq = ( C Na m C K m / C Na aq C K aq )) is about 0.75 and has little pressure dependence at pressures higher than 2.2 kb. The distribution coefficient, however, has a positive pressure dependence at pressures lower than 1.2 kb.

Exsolution of immiscible vapor and liquid phases from a crystallizing silicate melt: Implications for chlorine and metal transport

Abstract The compositional variation of a Cl-bearing aqueous fluid exsolving from a crystallizing silicate melt is evaluated for the system haplogranitic melt-(Na, K)C1-H 2 O, with attention to the effect of vaporliquid immiscibility on the aqueous fluid. Above critical pressures for the aqueous fluid ( P > 1.5 kb at 800°C), where the fluid is single phase, the Cl concentration in the aqueous fluid ( C cl aq ) gradually decreases with crystallization of a silicate melt under Rayleigh fractionation, as demonstrated by previous studies. However, the variation of Ca is quite different below critical pressures, where vapor-liquid immiscibility occurs, in particular at low pressures ( P Cl H 2 O ratio in a melt is small and Ca will increase with crystallization. A high Cl H 2 O ratio at low pressure results in liquid exsolution and decreasing Ca with crystallization. Eventually both vapor and liquid phases will exsolve simultaneously regardless of the initial exsolving phase. During this simultaneous exsolution, the composition of the exsolving vapor and liquid phases, the mass ratio of these phases and the Cl concentration of the silicate melt will all remain constant. Comparison of the model and published experimental data indicates that the simultaneous exsolution of vapor and liquid may occur from a normal rhyolite crystallized at pressures below the critical pressure for the aqueous fluid. Therefore, simultaneous exsolution of the immiscible aqueous phases is likely to be a common phenomenon in magmatic hydrothermal systems at conditions for the vapor-liquid immiscibility. Model calculations at 0.6 and 1.2 kb indicate that major amounts of Cl in the system will be distributed to the liquid phase particularly at the lower pressure. This suggests that the majority of chloridecomplexed metals exsolving from a magma may be transported in the liquid phase under shallow crustal conditions.

Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of Satsuma Iwojima volcano

The Satsuma Iwojima volcano, southwest Japan, degasses 5 x 10 6 t/yr H 2 O, 9 x 10 4 t/yr S, and 6 x 10 4 t/yr Cl from high-temperature (≤880 °C) fumaroles atop a now-altered rhyolite dome that erupted 1200 yr ago. Acidic hot springs (pH 1.1-1.8) discharge ∼20 x 10 6 t/yr H 2 O to the sea; this water is composed of ∼1 part magmatic vapor absorbed by 6 parts meteoric ground water. The Cl and SO 4 in solution originate from the vapor, whereas cation components are derived largely by dissolution of the rhyolite. The flux of Pb, Zn, Cu, and Mo in the vapor and acidic springs ranges from 0.1 to 10 t/yr each, whereas the Au flux is 10 -5 and 10 -3 t/yr, respectively. The low concentrations of NaCl and metals in the vapor are due to the condensation of a hypersaline liquid from the vapor during ascent and depressurization, meaning that the atmospheric-pressure vapor does not reflect the composition of the fluid exsolving from the magma. Neither this low-pressure vapor nor the acidic waters can account for high-sulfidation Cu-Au ore deposits deduced to have formed in this environment; such mineralization requires the subsequent ascent of a metal-rich fluid.

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.

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.

Experimental study in the system albite-andalusite-quartz-NaCl-HCl-H2O at 600°C and 400 to 2000 bars

Abstract The composition of aqueous fluid equilibrated with a mineral assemblage of quartz, albite, and andalusite was experimentally obtained at 600°C, with pressure ranging from 400 to 2000 bars and NaCl concentrations ranging from 0.05 to almost 100 m. This allowed us to determine 1. (1) the pressure dependence of the apparent equilibrium constant (Ka) of the following reaction under critical and subcritical conditions, 1 2 Al 2 SiO 5 + 5 2 SiO 2 + 1 2 H 2 O + NaCl = NaAlSi 3 O 8 + HCl 2. (2) the composition of coexisting vapor and liquid phases in the NaCl-HCl-H2O system. The Ka was obtained as the HCl NaCl concentration ratio in dilute fluid (〈 2 m). The equilibrium constant (K) of the reaction, which is equivalent to the Ka with an assumption of a unit activity coefficient ratio of HCl and NaCl, varies from 0.013 at 2000 bars to 1.7 at 400 bars with (∂lnK/∂P) being greater at lower pressure. The large pressure dependence of the equilibrium constant is attributed to the large difference in partial molar volumes of HCl and NaCl in the aqueous phase at high temperature and low pressure. The compositions of the coexisting vapor and liquid phases are graphically estimated on a diagram showing concentrations of NaCl and HCl by regarding the above reaction as a buffer reaction. The concentration ratio of HCl in vapor to liquid phase ( m HCl vapor m HCl liquid ) ranges from 1.5 at 1000 bars to 7 at 400 bars. The addition of HCl into the NaCl-H2O system increases the critical pressure and also increases the concentration of NaCl in the vapor phase at low pressure. These results indicate that the effects of pressure are comparable but inverse to temperature effects and must be considered in modeling the geochemical structure of hydrothermal systems.

Pathways for escape of magmatic carbon dioxide to soil air at Unzen Volcano, SW Japan.

Estimation of the magmatic contribution to soil air at Unzen Volcano, SW Japan, was carried out using carbon isotopes, both (super 14) C and (super 13) C, and a mixing model of isotopic mass balance in order to assess the spatial variation of magmatic influence from the volcano. The advantage of using soil air samples is that a wide range of gas sampling sites can be selected. Magmatic CO (sub 2) contributed mostly in the eastern region from Unzen Volcano. The high magmatic contribution to soil air appeared along the Akamatsudani fault zone located southeast of the volcano. Our observations across the fault also showed remarkable peaks of CO (sub 2) concentration and delta (super 13) C values, suggesting that magmatic fluid comes up along the fracture zone as for the normal fault system of the graben.