Composition of volcanic gases emitted during repeating Vulcanian eruption stage of Shinmoedake, Kirishima volcano, Japan
© 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; TERRAPUB. 2013
Received: 15 August 2012
Accepted: 14 November 2012
Published: 8 July 2013
Volcanic gas compositions of Shinmoedake, Kirishima volcano, Japan were measured by Multi-GAS during the persistent degassing period with repeating Vulcanian eruptions from March 2011 to March 2012. In order to avoid risks due to eruptions, the measurements were performed with the Unmanned Aerial Vehicles (UAV) that fly through the plume with the Multi-GAS and by an automatic Multi-GAS monitoring station installed 5 km away from the summit. Based on the UAV measurements on May 18, 2011, most of the major volcanic gas components were quantified as CO2/SO2 = 8, SO2/H2S = 0.8, H2O/CO2 = 70 and H2/SO2 = 0.03 (mol ratio), and the SO2/H2S ratio of the plume was quantified as 8 on March 15, 2011. The Multi-GAS monitoring station occasionally detected a dilute plume with an SO2/H2S ratio ranging from 0.8 to 3.3 from April 2011 to March 2012. The decrease of the SO2/H2S ratio from March 15, 2011, to May 18, 2011, is interpreted as the result of a ten times increase of the degassing pressure. Based on the SO2 fluxes and the gas compositions, the conduit magma convection is considered to be the gas supply mechanism at the Shinmoedake, and the degassing pressure changes are attributed to the change of depth of the convecting magma column top.
Volcanic gas composition is controlled by volatile contents in magmas and their degassing condition, and is an important indicator of magmatic plumbing and degassing conditions that help to monitor volcanic activities. Classical studies of volcanic gases by direct sampling and analysis can be applied only for accessible fumaroles, and studies of large-scale degassing activities were quite limited. The development of instrumental techniques has enabled the measurement of volcanic gas composition of large-scale degassing by measuring the volcanic plume. Conventional manned aircraft can transport fairly large instruments and have been applied to measure the volcanic plume composition (Gerlach et al., 1997; Shinohara et al., 2003a). The application of manned aircraft, however, is limited to quiescent states of volcanoes to avoid risks to the operators. FT-IR remote-sensing has enabled composition measurement of eruptive degassing and has revealed variations of the gas composition during lava fountain, lava effusion and Strom-bolian eruptions (Allard et al., 2005; Oppenheimer et al., 2006). Continuous monitoring of the plume gas composition by the Multi-GAS has also been applied to detect gas composition variation prior to an eruption (Aiuppa et al., 2009). The application of FT-IR and the Multi-GAS is also limited by accessibility to the degassing vent.
Unmanned aerial vehicles (UAV) commonly are small aircrafts with payloads ranging from a few to tens of kilograms and are widely used in atmospheric science studies because of their low cost operation and flight planning flexibility (Lin, 2006; Watai et al., 2006). In volcanology, the UAV has another important advantage in that they can work even in high-risk areas, such as near a vent with frequent explosions. This advantage is important for a volcanic plume study, because measurements near a vent are necessary to reduce the error due to dilution of the plume with distance. The first report of a UAV application to a volcanic plume study was given by McGonigle et al. (2008), who conducted SO2 flux measurements with a compact UV spectrometer and measured the CO2/SO2 ratio with a simple Multi-GAS system mounted on a small unmanned helicopter at La Fossa crater, Vulcano, Italy. Although the surveyed area was not under risk during the operation, they demonstrated the capability of the UAV for volcanic plume measurements under conditions of risk.
In this report, we present the results of composition measurements of volcanic plumes by the application of a UAV during a persistent degassing stage at Shinmoedake, Kirishima volcano, Japan. Since Vulcanian eruptions were repeated during this study, an area of 4 km from the summit was closed, even for manned aircrafts, and a UAV was necessary to perform near vent plume measurement. We also report the results of volcanic plume composition monitoring with a Multi-GAS automatic station installed 5 km from the summit, where dilute plumes occasionally reached. Based on the results of these measurements, we estimate composition variation with time and discuss its origin.
2. Shinmoedake Volcano
Before the eruptive activity started in January 2011, degassing activity was not significant at the Kirishima volcano. The significant degassing plume, which could be traced further than 10 km from the summit, was recognized on January 22, 2011, four days before the sub-Plinian eruption (Kinoshita et al., 2011), but SO2 flux measurement was not carried out before the eruption. The early SO2 flux measurements were carried out during pause periods of the sub-Plinian eruptions and resulted in quite large fluxes of >10 kt/d (Mori and Kato, 2013). The SO2 flux rapidly decreased after the lava effusion and reached less than 1,000 t/d in March. From March 2011 to the present, the SO2 flux varies between 100 and 400 t/d.
A Multi-GAS (Aiuppa et al., 2005; Shinohara, 2005) was modified to fit the limited volume and weight capacity of the UAV payload space. The Multi-GAS consists of a non-dispersive infrared CO2-H2O analyzer (LI-840, LI-COR, Inc., Lincoln, USA), SO2 and H2S electrochemical sensors (KTS-512 and KHS-5TA, respectively, Komyo Rikagaku K.K., Kawasaki, Japan), and an H2 semi-conductor sensor (Model GM12s, Sensor Tech K.K., Rittou, Japan). Filter-type scrubbers for H2S and SO2 were placed in front of the KTS-512 and KHS-5TA sensors, respectively, to reduce cross-sensitivity of these sensors (Shinohara et al., 2011). The power was supplied by a small 12 V 1.5 Ah Lithium Ion Polymer battery, which can run the system for a few hours and the results are recorded every second by a data logger (NR-1000, Keyence Co. Japan). The gases are pumped at a rate of 1 l/min from an air inlet at the front of the UAV into the Multi-GAS through Teflon tubing (Fig. 2).
A Multi-GAS was installed about 5 km southeast of the Shinmoedake volcano to monitor the variation of the volcanic gas compositions (Fig. 1(b)). The Multi-GAS consists of the same set of gas sensors listed above. The H2S sensor was not equipped with an SO2 scrubber, because the capacity of an SO2 scrubber is small and it can be saturated after a long monitoring period. The concentration of H2S is calculated by subtracting the SO2 contribution to the H2S sensor, based on the SO2 concentration measured by the SO2 sensor and the cross sensitivity to SO2 calibrated in the laboratory. In order to save the power consumption and extend the lifetime of the sensors, the Multi-GAS ran only for 1.5 h everyday at about noon. The data were transmitted to a data server by a mobile phone and then collected through the Internet. The system was powered by a lead battery charged by solar panels.
4.1 UAV plume measurements
The plume measurements by the UAV were performed on March 15 and May 18, 2011. The UAV was launched from a campground 7 km southwest from the Shinmoedake (Fig. 1(b)). Three flights of 60 min duration were conducted each day. Flight courses were variable taking into consideration wind direction and plume height estimated by visual observation from the base station and web cameras. The wind directions were also determined by SO2 tracking with COMPUSS (Mori et al., 2007) prior to the UAV flights (T. Mori, pers. comm.). The degassing activity was not intense with 500 and 200 ton/day of SO2 on March 15 and May 18, 2011, respectively (Mori and Kato, 2013). Figure 1(b) shows an example of the track of the second flight on May 18, 2011. With an SSE wind, the UAV scanned between the summit and the downwind of south to southeast at 1600-and 1800-m altitudes.
4.2 Monitoring station results
Results of the Multi-GAS remote station.
5.1 Chemical composition
The long-term stable degassing with a large emission rate is characteristic of persistent degassing, which is caused by conduit magma convection (Shinohara, 2008). Considering the large magma degassing rate, the conduit magma convection is the likely mechanism of the degassing at Shin-moedake after the lava effusion. The Shinmoedake volcanic gas composition, however, is quite S-poor compared with other persistently degassing volcanoes at subduction zones (Fig. 8). Melt inclusion analyses suggests that the H2O and S contents of the bulk magma are about 2 wt.% and 500 ppm (Saito et al., 2011), implying that the H2O/S mol ratio of the bulk magma is 70, which is similar to other persistently degassing volcanoes, such as the Satsuma-Iwojima and Miyakejima volcanoes (Saito et al., 2001, 2010). The S-poor volcanic gas composition of the Shinmoedake can be due to high-pressure degassing. Based on empirically estimated gas solubility, Giggenbach (1996) modeled the evolution of the gas composition with degassing. The model suggests that a shallow degassing results in a S-rich gas and the composition becomes S-poor with increasing pressure, then becomes also H2O-poor with further pressure increase (Fig. 8). This degassing trend qualitatively agrees with the results of recent experimental work by Lense et al. (2011) for basaltic magmas. This degassing trend suggests that the composition difference between the volcanic gases of the Shinmoedake and other persistently degassing volcanoes can be caused by higher pressure degassing at the Shinmoedake than at other volcanoes. The quantitative modeling of the gas composition evolution, however, is still not possible because of the lack of the experimental sulfur partitioning coefficient; in particular, with silicic magmas at low pressure ranges (Webster and Botcharnikov, 2011). The H2O-rich composition of the Shimoedake gases can also be due to the incorporation of meteoric water heated by the lavas in the crater.
5.2 Composition variation
Variation of the SO2/H2S ratios was monitored for about one year (Fig. 7). The ratio decreased from 8 on March 15, 2011, to 0.8 on May 18, 2011, then stayed between 0.8 and 3.3 with gradual fluctuation. In a gas phase, the SO2/H2S ratio changes by reaction (1), whose equilibrium constant is given by the Eq. (2). If the H2/H2O ratio, therefore the oxygen fugacity, is constant in the gas phase, a decrease of the SO2/H2S ratio can be caused by a temperature decrease. Hydrogen, however, is a minor species compared with SO2 and H2S, and the H2/H2O ratio commonly decreases with temperature as a result of reaction (1) under a constant SO2/H2S ratio (gas buffer; Giggenbach, 1987). The H2/H2O ratio of the Shimoedake gas is 5 × 10−5, that is three orders of magnitude lower than that controlled by a QFM buffer, a common rock buffer, and it is unlikely that the SO2/H2S decrease is caused by a temperature decrease under a constant H2/H2O ratio. A pressure increase at a constant temperature and the H2/H2O ratio results in the SO2/H2S ratio increase (Eq. (2)). The temperature and the oxygen fugacity is likely to be constant at the magma degassing condition, whereas the pressure can be variable. The ten times decrease of the ratio can be caused by a ten times increase of the degassing pressure. The gradual variation of the SO2/H2S ratios between 0.8 and 3.3 can be similarly caused by a variation of the degassing pressure by four times. The degassing pressure increase coincides with the high CO2/SO2 ratio measured on May 18, 2011, which suggests high pressure degassing.
5.3 Origin of volcanic gas
Large amounts of volcanic gases were discharged even after the sub-Plinian eruptions in January and the subsequent lava effusion in early February 2011 (Mori and Kato, 2013). The continuous volcanic gas emission, with a rate of 100–400 t/d SO2, after March 2011 amounts to a total SO2 emission of 7 × 104 t in one year, which requires the degassing of 7 × 107 t (3 × 107 m3) of magma. The continuous volcanic gas emission requires a mechanism of degassing and gas transport from a large volume of magmas. Such persistent degassing is modeled by either the permeable gas flow model (Edmonds et al., 2003, 2010), or the conduit magma convection model (Kazahaya et al., 1994; Shinohara, 2008). The permeable gas flow model considers that the intensive and continuous gas emission is caused by the transport of volcanic gas exsolved at a deep and large magma chamber through a permeable volcanic conduit connecting the chamber to the surface (Edmonds et al., 2003, 2010). In such a case, volcanic gas compositions are likely be CO2-rich due to gas exsolution at high pressure. In contrast, the Shinmoedake volcanic gas is H2O-rich (Fig. 8), which cannot be produced by high-pressure degassing because the Shinmoedake magma is relatively H2O-poor (Saito et al., 2011). The water content of the magmas is about 2 wt.% whose saturation pressure in an andesitic melt is <50 MPa at about 1000° C (Baker and Alletti, 2012). The magma chamber depth is estimated as about 10 km based on the crustal deformation before and after the eruptions measured by GPS (Nakao et al., 2011). As efficient H2O discharge can occur only at a low pressure, the intensive and continuous degassing at the Shinmoedake volcano cannot be caused by permeable gas flow from the magma chamber.
In the conduit magma convection model, degassing occurs under low pressure conditions at the top of the convecting magma column and the dense degassed magma descends through the magma column where a volatile-rich less dense magma ascends from a magma chamber to compensate the descending magma (Kazahaya et al., 1994; Shinohara, 2008). The low pressure degassing is consistent with the efficient H2O discharge from the H2O-poor magma at Shinmoedake. In contrast, the S-poor composition of the volcanic gas and the variation of the SO2/H2S ratio suggests that the degassing pressure of the Shinmoedake volcanic gases is higher than that at other persistently degassing volcanoes and changed with time (Figs. 7 and 8). However, quantitative evaluation of the degassing pressure for the S-poor composition is not possible due to the lack of the sulfur partition coefficient data.
We can evaluate the possible range of the degassing pressure variation based on the observed H2O/CO2 ratio in the gas and the H2O content in melt inclusions, and H2O-CO2 solubility in a silicate melt. The carbon dioxide content of the magma should less than 700 ppm, because the H2O/CO2 mol ratio of the volcanic gas on May 18, 2011, is 70 and H2O content in the magma is 2 wt.% (Saito et al., 2011). If the CO2 content in the magma is 500 ppm, the degassing pressure to exsolve the gases with H2O/CO2 = 70 is 3.0 MPa (Newman and Lowenstern, 2002). This calculation was conducted with solubility data for a rhyolitic melt at 900°C, because a systematic H2O-CO2 solubility dataset is not available for an andesitic melt, but the difference in the solubility is not large between rhyolitic and andesitic melts (Baker and Alletti, 2012). The ten times larger SO2/H2S ratio on March 15 compared with that of May 18, 2011, suggests a ten times smaller degassing pressure. The degassing pressure change from 0.3 MPa to 3.0 MPa corresponds to a depth change from 12 to 120 m under a lithostatic pressure gradient. The degassing pressure change will also cause a H2O/CO2 ratio change in the exsolved gas phase from 100 to 70. If the original CO2 content is 300 ppm, the degassing pressure on March 15 is 1.2 MPa and that on May 18 is 12 MPa, corresponding to a depth change from 48 to 480 m. Therefore the variation of the volcanic gas composition can be caused by changes of the degassing depth in the shallow part of the conduit system at <500-m depth.
The continuous degassing with the small composition variation at the Shinmoedake volcano during March 2011 is likely caused by the conduit magma convection with a variation of the degassing depth at the relatively shallow region. The intensive degassing started a few days prior to the sub-Plinian eruption which started on 26 January, 2011 (Kinoshita et al., 2011) and the SO2 flux quickly decreased from more than 10 kt/d at the end of January to less than 1,000 t/d in mid March 2011 (Mori and Kato, 2013). Since the intensive degassing continued even after lava effusion, the conduit magma convection process could have been established just after the end of the lava effusion activity. Small-scale Vulcanian eruptions were frequent in early February and the frequency and scale decreased with time. The last eruption occurred in September 2011, and since then the degassing continues without eruptive activity. Vulcanian eruptions with various frequencies are often observed at persistently degassing volcanoes of dacitic-andesitic magmas and the presence, or absence, of Vulca-nian eruptions is attributed to slight changes in the conditions at the top of the convecting magma (Shinohara, 2008). Based on the conduit magma convection model, the relatively stable SO2 flux and gas composition indicate a stability of the magma plumbing and degassing conditions after March 2011, and the absence of eruptive activity since September 2011 may not readily indicate a decrease of the eruptive potential of the volcano. The lava filling the summit crater is likely to be still molten in the interior and it is likely that the instability of the magma convection causes a drain back of the large volume of the accumulated lavas to the depth. In order to evaluate the stability of the magma convection, it is important to monitor the variation of the volcanic gas compositions.
Compositions of volcanic gases emitted from Shin-moedake during persistent degassing with repeating Vulca-nian eruption were measured by the Multi-GAS mounted on the UAV and the Multi-GAS monitoring station. The UAV measurements were performed on March 15 and May 18, 2011; on March 15, only the SO2/H2S ratio of the plume was quantified as 8, and on May 18, the UAV flew through plumes with a maximum SO2 concentration of 2.5 ppm, and most of the major volcanic gas components were quantified as CO2/SO2 = 8, SO2/H2S = 0.8, H2O/CO2 = 70 and H2/SO2 = 0.03 (mol ratio). The Multi-GAS monitoring station was installed on April 12, 2011, about 5 km southeast of the crater and could occasionally detect diluted plumes with a maximum SO2 concentration of 0.22 ppm. Because of the low concentration of the plume, SO2/H2S and CO2/SO2 ratios were obtained only 15 and 3 times during the one-year observation, respectively. The SO2/H2S ratio gradually varied in a range from 0.8 to 3.3. The CO2/SO2 ratios were obtained in January and February 2012 as 12, which agrees with the ratio obtained on May 18 by the UAV measurement within the error. Combining the SO2/H2S ratios obtained both by the UAV measurements and the monitoring station, the rapid decrease of the SO2/H2S ratio from 8 on March 15 to 0.8 on May 18 was quantified.
The volcanic gas composition obtained on May 18, 2011, is S-poor compared with other intensively degassing volcanoes, such as Satsuma-Iwojima or Miyakejima. Comparison with the melt inclusion volatile contents and their solubility, the S-poor composition is attributed to high-pressure degassing and contamination of meteoric water. The decrease of the SO2/H2S ratio from March 15 to May 18 is likely due to a ten times increase of the degassing pressure, which agrees with the high-pressure degassing suggested by the S-poor composition on May 18, 2011. The H2O-poor composition of the Shinmoedake magmas, however, requires a maximum degassing pressure less than 1.2 MPa (<500-m depth), implying that the degassing pressure variation should occur at the shallow region of the magma plumbing system. Based on the SO2 fluxes and the gas compositions, the conduit magma convection is considered as the gas supply mechanism at Shinmoedake, and the degassing pressure change is attributed to the change of depth of the convecting magma column top. The stable gas composition after May 18 suggests a stable condition of the conduit magma convection, although the eruption frequency decreased with time.
I would like to thank Drs. I. Miyagi and A. Tanaka for their assistance during the UAV measurements, Dr. T. Mori for his assistance to estimate the plume direction during the UAV measurements by COMPUSS measurements, Dr. G. Saito for discussion on the volatile contents in magmas, the journal editor, Dr. M. Ichihara for the careful editing and two anonymous reviewers for constructive reviews. This study is supported by the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government under the program of Special Coordination Funds for Promoting Science and Technology, “Urgent Study on the Eruption of Mt. Shinmoe-dake, Kirishima, 2011”.
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