Earth Planets Space, 54, 327–335, 2002

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

Kohei Kazahaya, Hiroshi Shinohara, and Genji Saito

Geological Survey of Japan, AIST, 1-1-1 Higashi Tsukuba, Ibaraki 305-8567, Japan

(Received January 12, 2001; Revised January 17, 2002; Accepted January 17, 2002)

Satsuma-Iwojima volcano continuously releases magmatic volatiles from the summit of Iwodake, a rhyoliticlava dome. The temperature of fumaroles is high, between 800◦ and 900◦C, and the water-rich composition ofvolcanic gases has not changed essentially over the past 10 years. Sulfur dioxide flux measured by COSPEC isalmost constant with an average of 550 t/d since 1975. The present volcanic gas is likely degassed from a rhyoliticmagma whose composition is similar to that erupted in 1934, 2 km east of Satsuma-Iwojima. Comparison ofsilicate 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/susing the volatile content of the magma and the fluxes of magmatic volatiles. The rhyolitic parental magma isvolatile-undersaturated in the deep magma chamber, as suggested by melt inclusion studies. Magma convectionin a conduit, driven by the density difference between higher density degassed and lower density non-degassedmagmas, explains the high emission rate of magmatic volatiles released at shallow depth from such a magmachamber, that is gas-undersaturated at depth. Model calculations require the conduit diameter to be greater than50 m as a necessary condition for convection of the rhyolitic magma. Long-term convective degassing has resultedin the rhyolitic magma in the deep chamber to become depleted in volatile components. The melt-inclusion studiesindicate that the rhyolitic magma responsible for discharging the present volcanic gas has been degassed during thelong degassing history of the volcano and is now supplied with CO2-rich volatile components from an underlyingbasaltic magma. The total volcanic gas flux over 800 years requires degassing of 80–120 km3 of basaltic magma.

1. IntroductionA number of passively degassing volcanoes emit large

quantities of magmatic volatiles without eruption, i.e., Etna(Allard, 1997), Stromboli (Allard et al., 1994) and Izu-Oshima (Kazahaya et al., 1994). We can estimate the totalamount of the degassed magma if the degassing duration,degassing rate and gas content of the magma are known.These studies show that degassing of volatiles supplied fromlarge magma chambers is necessary to explain the long-termdegassing rates. Studies on volatile content of melt inclu-sions from Satsuma-Iwojima volcano further revealed thedegassing evolution of the deep magma chamber (Saito etal., 2001).

Magma convection in a conduit has been proposed asa mechanism responsible for intensive and continuous de-gassing of magmatic volatiles from magma chambers ofbasaltic to andesitic volcanoes (Kazahaya et al., 1994). Thedriving force for convection is the density difference be-tween a dense magma degassed at shallow level and lessdense undegassed magma supplied from a deeper level.Shinohara et al. (1995) first modeled magma convection inconduits of silicic systems related to porphyry Mo deposits.Stevenson and Blake (1998) modeled the dynamics of con-vective magma degassing in a revision of the approach by

Copy right c© The Society of Geomagnetism and Earth, Planetary and Space Sciences(SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan;The Geodetic Society of Japan; The Japanese Society for Planetary Sciences.

Kazahaya et al. (1994), in order to model silicic systems,and applied their model to the dacitic Mt. St. Helens magmasystem. Such modeling indicates that convective degassingis likely a common mechanism in basaltic to dacitic volca-noes (Stevenson and Blake, 1998).

Satsuma-Iwojima volcano is a rhyolitic example of in-tense and continuous degassing. This study summarizes thevolatile study of Satsuma-Iwojima volcano, including SO2flux measurements and melt inclusion data, and applies theabove-mentioned degassing mechanism to the more viscousrhyolitic magma systems of Satsuma-Iwojima volcano.

2. Outline of GeologyMt. Iwodake, the main rhyolitic lava dome on Satsuma-

Iwojima island, is located near the northwestern rim of thesubmarine Kikai caldera (ca. 18 km in diameter). This is theyoungest caldera in Japan and formed 6300 yr b.p. (Fig. 1).Early in the post-caldera stage, Mt. Iwodake started to erupt,mainly as lava effusions, but sometimes explosively, produc-ing pumice and pyroclastic flows. The latest explosive ac-tivity of Mt. Iwodake is dated at 500 yr b.p. (Kawanabe andSaito, 2002). Basaltic activities simultaneously occurred2 km distant from Iwodake central crater at about 3000yr b.p., creating the Inamuradake scoria cone and severalbasaltic lava flows (Ono et al., 1982). The latest submarineactivity occurred in 1934–35, 3 km east of Iwodake on 500-m-deep seafloor, with rhyolitic lava forming Showa-IwojimaIsland.

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328 K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO

100

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dake

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31°N

130°50’E

Kikai caldera50 km

Satsuma-Iwojima

Kyushu

Rim of Kikai caldera

N

Fig. 1. Topographic map of Satsuma-Iwojima, Kyushu, Japan, showing the location of Iwodake rhyolitic dome and Inamuradake scoria cone. Locationsof observation sites for COSPEC measurements by the tripod and the traverse method are also shown.

3. Magma SystemSaito et al. (2001) measured the chemical and volatile

composition of melt inclusions from rhyolites (pumice fromcaldera-forming eruption and lavas and ashes from the post-caldera stage) and from a basalt (scoria from the post-caldera stage). The post-caldera rhyolites were likely de-rived from a remnant of the caldera-forming magma, basedon the similarity of major-element compositions (Ono et al.,1982; Saito et al., 2001).

Each inclusion has a distinctive volatile composition(Fig. 2; Saito et al., 2001). The Takashima melt inclusionshave the highest water concentrations. The saturation pres-sures based on water and CO2 solubilities are calculated tobe 80–180 MPa, indicating that a rhyolitic magma cham-ber existed at 3–7 km depth just before the caldera-formingeruption. The saturation pressure of rhyolitic magma thatformed Iwodake, a post-caldera lava dome, was about 70MPa, corresponding to a depth of about 3 km. Inamuradakebasaltic activity simultaneously occurred with Iwodake rhy-olitic activity, and the magma-chamber depth is estimatedto be 3–5 km from the saturation pressure of water and CO2(70–130 MPa). This suggests that a layered magma chamberconsisting of upper rhyolitic magma (3-km depth) and lowerbasaltic magma (3–5 km depth) existed during Iwodake andInamuradake eruption.

Saito et al. (2002) estimated the temperatures of the mag-mas using pyroxene geothermometry. Adjacent orthopy-roxene and clinopyroxene phenocrysts from Iwodake andShowa-Iwojima rhyolites have compositions that indicatemagma temperatures of 960 ± 28◦C and 967 ± 29◦C, re-spectively, whereas those from Inamuradake basalt showeda temperature of 1125 ± 27◦C. The relatively high temper-ature of the rhyolites may have resulted from heating of therhyolitic magma by the underlying basalt.

Water contents of the rhyolitic melt inclusions show a de-creasing trend from older to younger lavas (Fig. 2). Becausethe major-element composition of the rhyolitic magma wasalmost constant during the post-caldera stage, the decreasein water content is likely due to degassing. The volatile-saturation pressure of the latest Showa-Iwojima magma,20–50 MPa (0.5–2 km depth), is too low to represent thereal pressure of the magma chamber. The present magmachamber is most likely located at 3-km depth, as estimatedfrom the Iwodake melt-inclusion constraints. As the re-sult of long-term and intense degassing, the Showa-Iwojimamagma should be undersaturated with volatiles at the depthof the present rhyolitic magma chamber. However, themelt inclusions from Showa-Iwojima, the latest product ofSatsuma-Iwojima volcano, have the highest CO2 concen-trations in rhyolites despite their water concentrations andsaturation pressures being the lowest. The origin of the highCO2 concentration in this rhyolite is discussed in Section 10.

4. Volcanic GasesMany fumaroles exist in and around the summit. The

chemical composition of volcanic gases from the summitcrater of Iwodake has been relatively constant since the1950s (Kamada, 1964) and there has been little change dur-ing the past 10 years (Shinohara et al., 2002). Isotopiccompositions of the summit volcanic gases indicate thatthe gases have a magmatic origin (Shinohara et al., 1993;Hedenquist et al., 1994). The highest fumarole temperaturemeasured was 900◦C in 1994 (Shinohara et al., 2002).

The summit area of the main Iwodake lava dome isstrongly silicified by volcanic gases (Hamasaki, 2002), in-dicating that degassing has continued for a long time. Anold manuscript “Heike Monogatari,” written about 800 yearsago, indicates that intense volcanic gas emission and depo-

K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO 329

200MPa

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Isobaric process under gas-saturated condition

IwodakeShowa-IwojimaInamuradakebasalt

0 2 4 6 8H O (wt%) in melt2

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) in

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elt

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Takeshima P.F.6300B.P.

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1934

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mixing between degassed magma and gas-charged magma

a)

b)

Fig. 2. Volatile composition of melt inclusions in pumices from Takeshima and Iwodake pyroclastic flows, scoria from Inamuradake and pumicious lavafrom Showa-Iwojima (revised from Saito et al., 2001). a) H2O and CO2, b) H2O and S. Isobaric solubility curves are drawn using solubilities of H2Oin rhyolite at 1000◦C (Miyagi et al., 1997), H2O in basalt at 1100◦C (Burnham, 1979), CO2 in rhyolite at 1200◦C (Fogel and Rutherford, 1990) andCO2 in basalt at 1200◦C (Stolper and Holloway, 1988). The dotted area indicates the range of CO2/H2O and S/H2O ratios of high temperature volcanicgases at the summit crater (Shinohara et al., 2002). This area is drawn assuming that magma does not degas all its H2O even at low-pressure conditions,as usually found from the lava compositions.

sition of sulfur already existed at that time (Kamada, 1964).Therefore, volcanic gases have discharged for greater than800 years from the summit crater of Iwodake.

5. Source of Volcanic GasesLong-term intense discharge of volcanic gases from

Iwodake implies that the underlying magma chamber is rel-atively degassed. Since the chemical composition of themelt inclusions in the rhyolites from Satsuma-Iwojima vol-cano is rhyolitic, these inclusions most likely represent theparental rhyolitic magma chamber at depth. The CO2/H2Oratio of the volcanic gas is found to be similar to that dis-solved in the melt inclusions from the Showa-Iwojima andthe Inamuradake melt inclusions (Fig. 2), and it is stronglysuggested that volcanic gas comes from either residual por-tion of the Showa-Iwojima rhyolitic magma or Inamuradakebasaltic magma.

Saito et al. (2001) measured the sulfur and chlorine con-tents of melt inclusions, and found that the Showa-Iwojimamelt inclusions have S/H2O ratios similar to the volcanicgases. By contrast, the Inamuradake melt inclusions contain

several times more sulfur than that expected on the basis ofthe volcanic gas composition (Fig. 2). As the composition ofvolcanic gas has been almost constant over the past 10 years(Shinohara et al., 2002), a stable degassing system is likelyto exist. This implies that the volcanic gases are emittedfrom magma with a constant volatile content and constantdegassing pressure.

For the following reasons, we assume that the present vol-canic gas is released from a rhyolitic magma with a volatilecontent similar to the Showa-Iwojima magma (erupted 67years ago), rather than from a basaltic magma:

1) Basaltic to andesitic magma has never erupted from theIwodake lava dome.

2) Less-dense rhyolitic magma may still be located in theupper part of a magma chamber above basaltic magma.Therefore, degassing of magmatic gases directly froman underlying basaltic magma is unlikely.

3) The high fumarolic temperature (900◦C) is close to thetemperature of the Showa-Iwojima rhyolite, estimated

330 K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO

from the two pyroxene geothermometer (967–990◦C;Saito et al., 2002). Thus, basaltic magma is not neces-sary to explain the high fumarole temperatures.

4) The Showa-Iwojima melt inclusions have volatile com-positions that are more similar to the volcanic gasesthan those of the Inamuradake melt inclusions. Thehigh fumarolic temperature also suggests a low pres-sure of degassing, and in this situation the volcanic gascomposition should be close to the volatile composi-tion of the magma. In particular, the Inamuradake meltinclusions have excessive sulfur concentrations to con-clude that the sulfur in volcanic gases is directly de-rived from the Inamuradake magma.

Therefore, the present volcanic gas is most likely re-leased from a rhyolitic magma similar in composition tothat of Showa-Iwojima magma, rather than from the basalticmagma underlying the rhyolitic magma.

6. Sulfur Dioxide Flux MeasurementsThe first sulfur dioxide flux measurement by COSPEC

III was performed in 1975 by the seaborne traverse method,measuring 570 t/d (Ohkita et al., 1977). We measured SO2flux since 1990 using COSPEC V with the ground-basedtripod method and the traverse method using a vehicle. Dur-ing COSPEC measurement, volcanic plume movement wasmonitored by a video camera to determine the wind velocity.The results for the COSPEC measurements are summarizedin Table 1.

For the tripod method, we used an automatic tilting mountand SO2 flux was monitored every minute. As the vol-canic plume was horizontal, during the monitoring, a ver-tical scan of the plume was performed during all the tri-pod measurements. The baseline changed during vertical

Table 1. Sulfur dioxide flux from the Iwodake summit measured byCOSPEC with the tripod and the traverse methods.

Date SO2 flux (t/d)min. max. ave.

tripod method1990.10.27 430 540 4701994.10.30 420 880 6401996.10.16 210 900 4601997.4.20 230 750 4701997.4.21 470 1100 7501998.11.8 120 600 320

traverse method1975.2.12∗ 510 620 5701995.10.21∗∗ (1370) (2070) (1760)1997.4.20∗∗ (920) (1430) (1210)1997.4.21∗∗ (1460) (1600) (1530)

average 550∗measured by Ohkita et al. (1977) using a boat.∗∗measured using a vehicle. values are not used (see text).

scanning, mainly due to the UV intensity variation in thescan range between low and high angles. Therefore, we al-ways corrected for this effect by using background SO2-freedata, determined in a direction without any volcanic plume.As the observation points were restricted on the island, wesometimes found that part of the volcanic plume was hiddenbehind the flank of Iwodake. Such data underestimated theflux and were omitted from Table 1.

The flux values obtained by the traverse method are al-ways higher than those obtained by the tripod method. OnSatsuma-Iwojima, the traverse route is restricted to a roadfrom southwest to northwest sites (Fig. 1). The route fol-lows the valley between Inamuradake cone and Yahazudakecaldera wall. Because of the topography, part of the plumeis likely trapped and stagnant in the valley when the windblows in the SW to NW directions. This causes an overesti-mation of the SO2 flux during the traverse method, becauseCOSPEC measures the SO2 signal not only from the flowingplume but also from the trapped plume. Thus, we use onlythe SO2 flux value obtained by the tripod method.

The SO2 flux from the Iwodake summit crater varied be-tween 320 and 750 t/d with the average flux of 550 t/d dur-ing the monitoring period from 1990 to 1998. Fluxes ofother magmatic volatiles can be estimated using the chem-ical composition of the volcanic gases (Shinohara et al.,2002) from the summit of Iwodake. These are 16000 t/d forwater and 150 t/d for CO2 (Table 2). The magmatic volatileflux condensed into groundwater and discharged from hotsprings has been neglected. Therefore, these are minimumflux values from the Satsuma-Iwojima volcano.

7. Volatile BudgetWe can calculate the mass rate of magma degassing us-

ing the magmatic volatile fluxes and volatile content of theShowa-Iwojima magma. A water content of 1 wt.% isused for the following calculation, because the highest watervalue of the Showa-Iwojima magma is 1.4 wt.% from meltinclusion analyses (Saito et al., 2001), and some water isretained in the magma. The mass rate of magma degassingcalculated on the basis of individual species, H2O, CO2 andS, varies in the range 17,000–30,000 kg/s or 7.5–13 m3/s,assuming complete degassing for S and CO2 (Table 2). Theaverage value of 22,000 kg/s (1.9 Mt/d) or 10 m3/s is usedin the following discussion for the mass rate of magma de-gassing.

Since degassing is believed to have continued for morethan 800 years (Yoshida and Ozawa, 1981), the estimateof the volume of degassed magma is 250 km3, if all thegases were initially contained in the rhyolitic magma. Thiscalculation assumes that the composition of volcanic gas andvolatile content of magma has been constant over 800 years;however, there is no evidence to check this assumption.The volatile content of the Showa-Iwojima magma used inthe above calculation is the highest value. Therefore, the250 km3 estimate for total volume of degassed magma isa minimum. The trend in volatile content of magmas overthe past 6300 years (Fig. 2) also indicates that degassing ofthe magma chamber occurred during the post-caldera stage(Saito et al., 2001). Consequently, long-term degassing of arhyolitic magma chamber might be required.

K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO 331

Table 2. High temperature volcanic gas composition, flux and melt inclusion concentration of H2O, CO2 and SO2, and mass rate of the magma degassingcalculated for each species.

H2O CO2 SO2

Mean chemical composition (mol%)∗ 97.4 0.037 0.093Flux (t/d)∗∗ 16000 150 550degassed from melt (wt%)∗∗∗ 1 0.010 0.021Mass rate of magma degassing average(m3/s) 8.2 7.5 13.1 10(kg/s) 19000 17000 30000 22000∗average of high temperature volcanic gas calculated from the data given by Shinohara et al. (2002).∗∗calculated using SO2 flux (550 t/d) and the mean chemical composition.∗∗∗reduced from the melt inclusion data given by Saito et al. (2001). See text.

8. Degassing MechanismDegassing of magma requires oversaturation of volatiles

in a melt. Therefore, degassing occurs only at relativelylow-pressure conditions. However, the magma chamber isconcluded to be located at a relatively high pressure, greaterthan 3 km for Satsuma-Iwojima volcano (Saito et al., 2001).Therefore, a huge amount of magmatic volatiles must betransported from such a deep magma chamber towards thesurface. Because the volcanic gases from the fumaroles col-lected at the summit crater are water-rich (>97%) and ofmagmatic origin (Shinohara et al., 1993; Hedenquist et al.,1994), the degassing pressure must be low enough to degaswater efficiently from the parental magma. As mentionedearlier, the highest fumarolic temperature of 900◦C also sup-ports the idea that volcanic gases were released from themagma at shallow depths. Therefore, it is necessary thatthe magma chamber system is connected to the surface byan open conduit and water-rich magmatic gases are sepa-rated at shallow depths. As effective water degassing frommagma only occurs at shallow depths, water must be trans-ported from a deep magma chamber either as bubbles orwater-bearing magma. Because the Showa-Iwojima magmais likely to be undersaturated in volatiles at the depth of themagma chamber, water transport by bubbles not possible.

Water transport by volatile-bearing magma is possible asthe result of magma convection in a conduit (Kazahaya etal., 1994). When a magma conduit extends to a shallowlevel and is allowed to degas, the upper-most part of themagma may then degas efficiently. Density of the degassedmagma increases by bubble separation and melt degassing,which makes the lower part of undegassed magma buoyant,and the degassed magma descend through the conduit.

9. Rhyolitic Magma Convection in a ConduitIn this section, we discuss whether convection of the

Showa-Iwojima rhyolitic magma could occur in an openconduit system with a sufficient rate to account for theobserved degassing rate. The degassing rate induced bymagma convection in a conduit is controlled by two pro-cesses: 1) the rate of counter flow in a magma conduit,and 2) bubble separation rate at the top of the conduit.The first process can be evaluated by fluid dynamic model.Stevenson and Blake (1998) provided fluid dynamic equa-

tions for magma convection in conduits based on experi-ments and theory, revising the treatment by Kazahaya et al.(1994):

Q = π(R∗)2Ps(g�ρR4/μd), (1)

where R∗(= Ra/Rc) is the dimensionless radius of the ris-ing flow and Ps is the Poiseuille number (Koyaguchi andBlake, 1989). Viscosity values for degassed magma witha water concentration of 0.4 wt.% (μd ), and undegassedmagma with a water concentration of 1.4 wt.% (μc), arecalculated with the method of Persicov (1990). The vis-cosity of rhyolitic magma using the chemical compositionof lava of Showa-Iwojima (Ono et al., 1982) is 106.1 Pa·sand 104.9 Pa·s at 1000◦C, respectively. The Ps is constant ata value of 0.064 when μd/μc > 12 (Stevenson and Blake,1998), and Ps of 0.064 is used for calculation. An R∗ valueof 0.6 is used according to the experiments by Stevensonand Blake (1998). Degassing of 1 wt.% water results in adensity difference of about 50 kg/m3, calculated from themethod of Lange and Carmichael (1987). This density dif-ference drives the magma convection in a conduit (Fig. 3).The diameter of the magma conduit obtained is 50 m usingEq. (1). This conduit diameter is realistic, based on estima-tions of conduit diameters at other volcanoes (80–120 m forMt. St. Helens in 1980; Stevenson and Blake, 1998).

Another rate control factor for magma convection is theseparation process of the bubbles from magma. This pro-cess might be very important for non-explosive eruptionsuch as dome-forming eruptions. Formation of permeablefoam in an ascending silicic magma would induce gas lossin a shallow conduit prior to eruption (Eichelberger et al.,1986; Westrich and Eichelberger, 1994). Melnik and Sparks(1999) showed theoretically that gas exsolution of a silicicmelt at shallow depth causes crystallization of magma dur-ing its ascent, and the effective gas release occurs through apermeable magma column. Although the above-mentionedprocess was modeled for dome-forming eruptions, it is ap-plicable to degassing of a convecting magma, because thedegassing process occurs in a shallow conduit prior to erup-tion and is controlled mainly by decompression rate and vis-cosity.

There are some volcanoes that have erupted effusivelywith an emission rate of volcanic gases similar to Satsuma-

K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO 333

700m

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900°C

Kikai Caldera

Magma convection in a conduit

magma chamber>80km3

Andesitic magma SiO =55%

Basaltic magma SiO =52%

0

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>50km3

Rhyolitic magmaVolatile undersaturatedat magma chamber

SiO =70%2

970°C

Degassed magmadescent

Undegassed magma ascent

degassing duration >800 yrs.

Gas and Heat transport

Magma mixing at interface

volcanic body including pre-caldera product and basement

submarine post-calderalava domes

Iwodake and Showa-Iwojima,post-caldera lava dome

Inamuradake,post-caldera scoria cone

gas-charged rhyolitic magma

degassed rhyolitic magma

andesitic layer made by mixingof upper rhyolite and basal basalt

basaltic magma

Fig. 5. Schematic illustration of magma system of Satsuma-Iwojima volcano (vertically exaggerated). The pressure of the basaltic magma and chemicalcompositions of the magma are from Saito et al. (2001). See text for details.

lite is not yet clear, but may result from bubble migration ordiffusion in the deep chamber. Bubble migration can trans-port volatiles oversaturated at the chamber pressure. There-fore, the transported volatles should be rich in CO2 and poorin H2O. As the solubility of sulfur compounds in a CO2-rich gas phase is not yet known, the S content in the bubblesmight be very low. This possibility is consistent with thepresent model but is not consistent with the commonly ac-cepted hypothesis of sulfur supply mechanism (e.g., Wallaceet al., 1995; Wallace, 2001). In contrast, transport of CO2and H2O may also be caused by diffusion, which is drivenby the activity gradient of a component. As sulfide solu-

bility in silicate melt is much higher in a basaltic melt thanin a rhyolitic melt (Carroll and Webster, 1994), the activitygradient of sulfur between a basalt and a rhyolite may be sosmall that little sulfur diffusion can take place.

Snyder and Tait (1998) theoretically modeled the chem-ical interaction caused by input of basalt to a more sili-cic magma chamber based on a flow-enhanced diffusivetransport model, and concluded that diffusive exchange be-tween silicic and basaltic magmas is easier to occur than thathas been thought previously. If the diffusive exchange oc-curs between the degassed rhyolite and gas-bearing basalt,volatile transport from the basalt to the silicic magma would

334 K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO

occur. This process can explain the volatile supply to themagma chamber system of Satsuma-Iwojima. Long-termconvective degassing results in the upper rhyolitic magma inthe chamber becoming degassed, and the volatiles containedin the underlying basaltic magma could be transported intothe upper degassed rhyolite.

The diffusion hypothesis can explain not only the CO2supply mechanism but also variation of volatile composi-tion in the Showa-Iwojima magma that seems to lie alonga straight line originating from 0.1–0.4 wt.% H2O and zeroCO2 (Fig. 2). This volatile-poor composition could resultfrom degassing at a low pressure of 0.5–3.0 MPa that mightcorrespond to the degassing pressure at the top of convect-ing magma. As neither degassing nor crystallization can cre-ate such a variation along a straight line that does not passthrough the origin, the variation is most likely created bymixing between volatile-poor rhyolite and volatile-rich rhy-olite. The volatile-rich rhyolite might be created by volatileand heat supply from basalt.

The present rhyolitic magma is thought to have fully de-gassed its original volatiles, such as noted from the Iwodakemagma or the Takeshima magma. It acts only as the carrierof volatile components from the underlying basaltic magmato the surface. The estimated mass of the degassed magmaof 250 km3 does not represent a real volume of the rhyolitein the chamber, but that circulated through the conduit. In-amuradake basalts contain H2O 2–3 wt.%, obtained by meltinclusion study (Saito et al., 2001). Assuming that all themagmatic water flux of 16000 t/d (Table 2) was suppliedfrom a basalt with this H2O content, we can estimate the to-tal mass of degassed basaltic magma to be 80–120 km3 overthe past 800 years.

11. SummaryA schematic model is shown in Fig. 5, which summa-

rizes the present magma system of Satsuma-Iwojima vol-cano deduced from the volatile studies (Shinohara et al.,1993, 2002; Kazahaya and Shinohara, 1996; Saito et al.,2001, 2002) and SO2 flux studies. Combination of repeatSO2 flux measurements using COSPEC and chemistry ofvolcanic gases (Shinohara et al., 2002) indicate an averageflux of volatiles of 550 t/d SO2, 16,000 t/d water and 150t/d CO2. Based on studies of melt inclusions and mafic in-clusions of the Satsuma-Iwojima magmas, a layered magmachamber is suggested to exist (Saito et al., 2001, 2002). Thedepth of the basaltic and rhyolitic magmas in the chamber isestimated to be 3–5 km and 3 km, respectively (Saito et al.,2001).

The Showa-Iwojima magma erupted in 1934–35, is likelyrepresentative of the present degassing magma at depth, be-cause of the similarity between the volcanic gas compositionand the volatile composition of its melt inclusions. Carbondioxide concentration of the Showa-Iwojima melt inclusionsis the highest in the rhyolitic melt inclusions, despite the wa-ter concentrations being the lowest. The CO2 in the Showa-Iwojima magma is thought to be supplied from the basalticmagma located beneath the rhyolitic magma. The volatiletransport from the underlying basaltic magma to the upperrhyolitic magma must occur at the rate of volatile emissionsfrom the summit. Thus, the rhyolitic magma acts as the

volatile transporter through the process of magma convec-tion in a conduit. This magma convection is driven by thedensity difference between degassed and undegassed mag-mas. The degassing has continued over at least the past800 years. An estimate for the total amount of the degassedmagma produced during this time is 80–120 km3, assum-ing that the chamber degasses magma with a water of 2–3 wt.%, corresponding to the water content of the Inamu-radake basaltic magma.

This process of magma convection in a conduit will workuntil the whole magma chamber is degassed, which includesboth the rhyolitic and basaltic magmas. The convective de-gassing model presented here is plausible even for a rhy-olitic magma system, and is consistent with the results byvolatile studies, such as compositions of volcanic gases andmelt inclusions, and the emission rate of magmatic gases.The specific mechanism of volatile supply from the basalticmagma to the upper rhyolitic magma is still unclear.

Acknowledgments. We thank Mishima-mura city office for sup-porting the COSPEC measurements by providing electricity andobservation sites. We also thank Prof. J. Hirabayashi who kindlypermitted us to use his COSPEC instrument for the 1990 measure-ments. This paper was greatly improved with critical and construc-tive comments from Drs. J. B. Lowenstern, P. Wallace, and J. W.Hedenquist.

ReferencesAllard, P., Endogenous magma degassing and storage at Mount Etna, Geo-

phys. Res. Lett., 24, 2219–2222, 1997.Allard, P., J. Carbobbelle, N. Metrich, H. Loyer, and P. Zettwoog, Sulfur

output and magma degassing budget of Stromboli volcano, Nature, 368,326–330, 1994.

Burnham, C. W., Magmas and hydrothermal fluids, in Geochemistry ofHydrothermal Ore Deposits, edited by H. Barnes, 2nd ed., pp. 71–136,John Wiley & Sons, New York, 1979.

Carroll, M. R. and J. D. Webster, Solubilities of sulfur, noble-gases, nitro-gen, chlorine, and fluorine in magmas, Volatiles in Magmas, Rev. Min-eral., 30, edited by M. R. Carroll and J. R. Holloway, pp. 231–279, Min-eral. Soc. Am., Washington, 1994.

Eichelberger, J. C., C. R. Carrigan, H. R. Westrich, and R. H. Price, Non-explosive silicic volcanism, Nature, 323, 598–602, 1986.

Fogel, R. A. and M. J. Rutherford, The solubility of carbon dioxide inrhyolitic melts: a quantitative FTIR study, Am. Mineral., 75, 1311–1326,1990.

Hamasaki, S., Volcanic-related alteration and geochemistry of Iwodakevolcano, Satsuma-Iwojima, Kyushu, SW Japan, Earth Planets Space,54, this issue, 217–229, 2002.

Hedenquist, J. W., M. Aoki, and H. Shinohara, Flux of volatiles and ore-forming metals from the magmatic-hydrothermal system of SatsumaIwojima volcano, Geology, 585–588, 1994.

Hirabayashi, J., T. Ohba, K. Nogami, and M. Yoshida, Discharge rate ofSO2 from Unzen volcano, Kyushu, Japan, Geophys. Res. Lett., 22, 1709–1712, 1995.

Kamada, M., Volcano and geothermy of Iwo-jima, Kagoshima prefecture,Jinetsu, 3, 1–23, 1964 (in Japanese).

Kawanabe, Y. and G. Saito, Volcanic activity of the Satsuma-Iwojima areaduring the past 6500 years, Earth Planets Space, 54, this issue, 295–301,2002.

Kazahaya, K. and H. Shinohara, Excess degassing of active volcanoes:processes and mechanisms, Mem. Geol. Soc. Japan, 46, 91–104, 1996(in Japanese with English abstract).

Kazahaya, K., H. Shinohara, and G. Saito, Excessive degassing of Izu-Oshima volcano: magma convection in a conduit, Bull. Volcanol., 56,207–216, 1994.

Koyaguchi, T. and S. Blake, The dynamics of magma mixing in a risingmagma batch, Bull. Volcanol., 52, 127–137, 1989.

Lange, R. and I. S. E. Carmichael, Densities of Na2O-K2O-CaO-MgO-FeO-Fe2O3-Al2O3-TiO2-SiO2 liquids: new measurements and derivedpartial molar properties, Geochim. Cosmochim. Acta, 51, 2931–2946

K. KAZAHAYA et al.: DEGASSING PROCESS OF SATSUMA-IWOJIMA VOLCANO 335

1987.Melnik, O. and R. S. J. Sparks, Nonlinear dynamics of lava dome extrusion,

Science, 402, 37–41, 1999.Miyagi, I., H. Yurimoto, and E. Takahashi, Water solubility in albite-

orthoclase join and JR-1 rhyolite melts at 1000◦C and 500 to 2000 bars,determined by microanalysis with SIMS, Geochem. J., 31, 57–61, 1997.

Nakada, S. and Y. Motomura, Petrology of the 1991–1995 eruption atUnzen: effusion pulsation and groundmass crystallization, J. Volcanol.Geotherm. Res., 89, 173–196, 1999.

Ohkita, T., M. Kawamura, and S. Takagi, Amount of SO2 emission fromSatsuma-Iwojima volcano, Kazan, 22, 107, 1977 (abstract in Japanese).

Ohminato, T. and D. Ereditato, Broadband seismic observation at Satsuma-Iwojima, Japan, Geophys. Res. Lett., 24, 2845–2848, 1997.

Ono, K., T. Soya, and T. Hosono, Geology of Satsuma-Io-jima district,Quadrangle series, Scale 1:50,000, 2, Geol. Surv. Japan, 80 p., 1982 (inJapanese with English abstract).

Persikov, E. S., The viscosity of magmatic liquids: experiment, generalizedpatterns, a model for calculation, applications, in Physical Chemistry ofMagmas, edited by L. L. Perchuk and I. Kushiro, pp. 1–40, Springer-Verlag, 1990.

Saito, G., K. Kazahaya, H. Shinohara, J. Stimac, and Y. Kawanabe, Vari-ation of volatile concentration in a magma system of Satsuma-Iwojimavolcano deduced from melt inclusion analyses, J. Volcanol. Geotherm.Res., 108, 11–31, 2001.

Saito, G., J. A. Stimac, Y. Kawanabe, and F. Goff, Mafic-felsic magmainteraction at Satsuma-Iwojima volcano, Japan: Evidence from maficinclusions in rhyolites, Earth Planets Space, 54, this issue, 303–325,2002.

Shinohara, H., W. F. Giggenbach, K. Kazahaya, and J. W. Hedenquist,Geochemistry of volcanic gases and hot springs of Satsuma-Iwojima,Japan: Following Matsuo, Geochem. J., 27, 271–285, 1993.

Shinohara, H., K. Kazahaya, and J. B. Lowenstern, Volatile transport in aconvecting magma column: Implications for porphyry Mo mineraliza-tion, Geology, 23, 1091–1094, 1995.

Shinohara, H., K. Kazahaya, G. Saito, N. Matsushima, and Y. Kawan-abe, Degassing activity from Iwodake rhyolitic cone, Satsuma-Iwojimavolcano, Japan: Formation of a new degassing vent, 1990–1999, EarthPlanets Space, 54, this issue, 175–185, 2002.

Snyder, D. and S. Tait, The imprint of basalt on the geochemistry of silicicmagmas, Earth Planet. Sci. Lett., 160, 433–445, 1998.

Stevenson, D. S. and S. Blake, Modelling the dynamics and thermodynam-ics of volcanic degassing, Bull. Volcanol., 60, 307–317, 1998.

Stolper, E. and J. R. Holloway, Experimental determination of the solubilityof carbon dioxide in molten basalt at low pressure, Earth Planet. Sci.Lett., 92, 107–123, 1988.

Wallace, P. J., Volcanic SO2 emissions and the abundance and distributionof exsolved gas in magma bodies, J. Volcanol. Geotherm. Res., 108, 85–106, 2001.

Wallace, P. J., A. T. Anderson, and A. M. Davis, Quantification of pre-eruptive exsolved gas contents in silicic magmas, Nature, 377, 612–616,1995.

Westrich, H. R. and J. C. Eichelberger, Gas-transport and bubble collapse inrhyolitic magma—an experimental approach, Bull. Volcanol., 56, 447–458, 1994.

Yoshida, M. and T. Ozawa, Abundance of some chemical elements pro-duced by Satsuma-Iwo-Jima Volcano in relation to their provenance,Bull. Volcanol. Soc. Japan, 26, 25–34, 1981.

K. Kazahaya (e-mail: kazahaya-k@aist.go.jp), H. Shinohara, and G.Saito