Secular variations in helium isotope ratios in an active volcano: Eruption and plug hypothesis

Earth and Planetary Science Letters, 107 (1991) 95 100

95

Elsevier Science Publishers B.V., Amsterdam

[CH]

Secular variations in helium isotope ratios in an active volcano: Eruption and plug hypothesis Yuji Sano 1, Kenji N o t s u , J u n - i c h i r o Ishibashi, G e o r g e Igarashi a n d H i r o s h i W a k i t a Laboratory for Earthquake Chemistry, Faculty of Science, University of Tokyo, Bunkyo-Ku, Tokyo 113, Japan Received February 25, 1991; revision accepted August 1, 1991

ABSTRACT Secular variations in 3 H e / 4 H e and 4 H e / 2 ° N e ratios are reported for October 1986 to April 1991 in gas samples from a 90°C steam well located about 3 km north of Mt. Mihara, an active volcano in Izu-Oshima Island, Japan. The 3 H e / 4 H e and 4 H e / 2 ° N e ratios increased anomalously from 4.18 Rat m to 4.59 R~t m and from 0.93 to 1.08, respectively, about 3 months prior to the explosive eruption of the volcano on 4 October 1990. The 3 H e / 4 H e enhancement may be related to the concurrent decrease in height of volcanic smoke at the central cone. The gas pressure in the conduit may have significantly increased before the eruption owing to solidified cap lava or ash in the cone, which may have strengthened the transfer of magmatic helium from the source to the observation well through fissures or permeable channels in the volcanic edifice.

I. Introduction

3He is the most important isotope among volatile species in geochemical studies because of its primordial signature [1,2]. It is well established that volcanic and geothermal systems above subduction zones trap mantle-derived helium with higher 3He/4He ratios than atmospheric helium [3]. This may be attributed to the ascending magma which may bring primordial 3He from the upper mantle [4]. Geographical distribution of 3 H e / 4 H e ratios around an active volcano revealed that the ratio decreases with distance from the central cone [5]. This trend suggests that more primitive 3He is carried with fluid flowing through a conduit during volcanic activity. A data set on the secular variation of the 3He/4He ratio, if available, would be very important in understanding the behaviour of the magmatic fluid flow in volcanic systems. Data on temporal variations in 3 H e / 4 H e ratios are scarce, however [6,7], so we therefore present i Present address: Institute of Geology and Mineralogy, Faculty of Science, Hiroshima University, Naka-Ku, Hiroshima 730, Japan 0012-821X/91/$03.50

here secular variations in 3He/4He and 4He/2°Ne ratios in the hydrothermal system of Oshima volcano, Japan. Mount Mihara, a stratovolcano (elevation 758 m), is located on lzu-Oshima Island (34°44'N, 139°23'E), about 110 km south-southwest of Tokyo, in Sagami Bay [8]. Large-scale eruptive activity has been occurring repeatedly every 100150 years [9]. The most recent magmatic eruption lasted from 15 November 1986 until 18 December 1986, with 5 × 107 tons of lava erupted [10]. After a year of quiescence, Mt Mihara exploded on 16 November 1987, ejecting large amounts of volcanic ash and steam [11]. The eruption was interpreted as the explosion of solidified lava on the surface of the central crater, which was caused by the gas pressure accumulated in a cavity beneath the lava [12]. Small eruptions (2 × 103 tons) occurred subsequently on 25 January 1988 [13]. The next explosive event occurred on 4 October 1990 [14]. No lava was erupted, although ash fell extensively over the northern half of the island. The column reportedly reached 1200 m [14]. Considering the presence of a new collapse pit in the crater floor, there may have been a cave beneath the solidified

© 1991 - Elsevier Science Publishers B.V. All rights reserved

96

Y. S A N O E T AL.

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Fig. 1, Sampling site for steam well gas on Izu-Oshima.

and the other to a manual pump. The container was then lowered into the well about 4 m below the ground surface. After air in the container was completely replaced by the steam gas using the pump, the container was recovered and both valves were closed. Since the permeability of helium in lead glass is quite low, significant helium loss or gain is assumed to be negligible during the storage time. In the laboratory, the helium and neon in the sample were purified using hot Ti Zr collectors and activated charcoal traps held at 77 K. After 4 H e / 2 ° N e measurement with a quadrupole mass spectrometer, the helium was completely separated from neon using a cryogenic charcoal trap held at 40 K. The 3 H e / 4 H e ratio was measured with a high-precision mass spectrometer. Experimental details have been given elsewhere [15]. Preliminary chemical analysis of the gas sample with a quadrupole mass spectrometer system indicates that the major compositions are CO 2 and N 2 with a little H20. Oxygen concentrations were below 4% in almost all samples, suggesting that they were not significantly compromised by air contamination.

3. Results lava before the eruption. The cavity may have been under high pressure from magmatic volatiles, which exploded the surface envelope of the conduit in the central crater. For the purposes of monitoring volcanic activity, in October 1986 we began, approximately once monthly, periodic gas collection of steam well gas on Izu-Oshima and helium isotope measurements [6]. The anomalous enhancement of the 3 H e / a H e ratio prior to the October 1990 event is discussed.

2. Methods and analysis The sampling site is No. 3 steam well at the Oshima Onsen Hotel, located about 3 km north of the central cone at the bottom of the inner slope of the caldera (Fig. 1). The well is a drilled hole reaching a depth of 369 m. Fumarolic gas with a temperature of about 90°C escapes through a crack in the casing pipe at a depth of 150 m and flows up through the pipe to the ground surface. The gas sample was collected in a lead-glass container with vacuum valves at both ends. One end of the container was connected to a thick-walled tygon tube

The measured 3 H e / 4 H e and 4 H e / 2 ° N e ratios are listed in Table 1 together with the sampling dates. Errors in the 3 H e / 4 H e and 4 H e / 2 ° N e determinations are about 1% and 10%, respectively (10). The 3 H e / 4 H e and 4 H e / 2 ° N e ratios vary significantly, from 1.70 R a t m t o 5.49 R atm, and from 0.37 to 2.00, respectively ( R a t m is the atmospheric 3 H e / a H e ratio at Ueno Park, central Tokyo, Japan [16], 1.343 × 1 0 - 6 ) . Figure 2 shows the temporal variations in the 3 H e / 4 H e and 4 H e / 2 ° N e ratios. Arrows indicate the dates of the volcanic eruptions. Both 3 H e / 4 H e and 4He/Z°Ne ratios increased significantly after the magmatic eruptions of November 1986. The enhancement of mantle-derived helium in the well was attributed to the influence of volcanic eruptions on hydrothermal systems. The time lag between the eruptions and the helium enhancement could reflect the travel time of the magmatic component through fissures or channels occurring in the volcanic edifice and the basement [6]. After the eruption of 16 November 1987 the 3 H e / 4 H e ratio in the steam well increased gradu-

SECULAR VARIATIONSIN HELIUM ISOTOPE RATIOS IN AN ACTIVE VOLCANO a l l y f r o m 5.02 R a t m t o a m a x i m u m

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J u l y 1988. T h i s v a l u e is c l o s e t o t h e h e l i u m i s o t o p e

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ratio

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elsewhere

in

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July

1988

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[3]. O v e r

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the end

of March

TABLE 1 Temporal variation in 3He/4He and 4He/Z°Ne ratios in the steam well at Oshima Onsen Hotel, Izu-Oshima No 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43

Date of sampling

3He/4He ( R atm )

4He/2°Ne

Helium Air

Component magma

3-Oct-1986 3-Dec-1986 3-Dec-1986 16-Jan-1987 9-Feb-1987 21-Feb-1987 12-Mar-1987 12-Mar-1987 2-Apr-1987 16-Apr-1987 17-Apr-1987 29-May-1987 26-Aug-1987 1-Sep-1987 21-Oct-1987 20-Nov-1987 4-Dec- 1987 8-Dec-1987 24-Dec-1987 24-Feb-1988 2-Mar-1988 31-Mar-1988 28-Apr-1988 23-Jun-1988 5-Ju1-1988 15-Jul-1988 6-Oet-1988 14-Nov-1988 1-Feb-1989 1-Apr-1989 20-May-1989 6-Ju1-1989 6-Jul-1989 12-Aug-1989 27-Sep- 1989 16-Nov-1989 16-Nov-1989 30-Jan-1990 26-Mar-1990 2-Apr-1990 20-Apr-1990 11-May-1990 20-Jul-1990 6-Oct-1990 11-Dec-1990 18-Jan-1991 26-Apr-1991

1.71 1.89 1.70 3.07 3.49 4.00 3.61 3.54 4.04 3.98 4.15 4.74 4.74 4.73 5.02 5.02 5.04 5.01 5.08 5.19 5.28 5.22 5.40 5.39 5.49 5.38 5.21 5.05 5.05 5.12 5.02 4.98 5.00 4.89 4.75 4.58 4.54 4.45 4.27 4.18 4.27 4.42 4.59 4.46 4.25 4.05 3.88

0.37 * 0.38 *

86% 84%

14% 16%

0.49 0.58 0.81 0.62 0.62 0.79 0.82 0.85 0.97 1.10 1.10 1.50 1.80 1.60 1.50 1.80 1.50 1.60 1.60 1.80 1.80 1.80 2.00 1.70 1.40 1.30 1.39 1.30 1.21 1.43 1.23 1.11 1.06 0.98 0.98 0.80 0.93 0.82 0.99 1.08 0.98 0.76 0.74 0.76

65% 55% 39% 51% 51% 40% 39% 37% 33% 29% 29% 21% 18% 20% 21% 18% 21% 20% 20% 18% 18% 18% 16% 19% 23% 24% 23% 24% 26% 22% 26% 29% 30% 32% 32% 40% 34% 39% 32% 29% 32% 42% 43% 42%

35% 45% 61% 49% 49% 60% 61% 63% 67% 71% 71% 79% 82% 80% 79% 82% 79% 80% 80% 82% 82% 82% 84% 81% 77% 76% 77% 76% 74% 78% 74% 71% 70% 68% 68% 60% 66% 61% 68 % 71% 68% 58% 57% 58%

* Data from Sano et al. [6]

* * * * * * * * * * * * * * * *

* * * * * * * * * * * * *

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1986

1987

1986

1987

1 9 8 8 1989

1990

1 9 8 8 1989

1990

1991

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I 1991

Year Fig. 2. Secular variations in the 3 H e / 4 H e (top) a n d 4He/Z~lNe ratios (bottom) observed in a steam well located a b o u t 3 km north of the central cone of Mr. M i h a r a on Izu-Oshima. A r r o w s indicate the dates of volcanic eruptions. Error bars in the 4 H e / Z ° N e ratios are 1 SD.

both 3 H e / 4 H e and 4He/2°Ne ratios decreased significantly, to minima of 4.2 Rat m and 0.8 respectively. From March 1990 to June 1990 both ratios increased slightly, but significantly. An apparent peak of the 3 H e / 4 H e ratio was found in July 1990 (Fig. 2). With the exception of the volcanic eruption no significant geophysical events, such as large earthquakes, heavy precipitation or tropical cyclones, were reported in the vicinity of the island. The 3 H e / 4 H e enhancement observed in M a r c h - D e c e m b e r 1990 may be a precursor of the 4 October 1990 eruption of Mt. Mihara. 4. Discussion

After the eruptions of 20 November 1987 a n d / o r 25 January 1988, there may have been a conduit between the magma reservoir and the central cone of Mt. Mihara. Appreciable amounts of magmatic gas and fluids, including mantle-derived helium, were released from the central cone

through the conduit. This is confirmed by the correlation spectrometer (COSPEC) measurements of the SO 2 flux in the central crater [17]. The SO 2 flux was 350 t o n s / d a y in May June 1988, and it did not vary significantly until July 1989. In addition to emission from the central cone, gases are also released through fissures and aquifers in and around the volcanic edifice [5]. Magmatic helium observed in the steam well was transferred from the m a g m a reservoir through the channels by gas and fluid flow. We present a hypothesis to explain the 3 H e / 4 H e anomaly observed in March December 1990. Sometime in March or April 1990, the volcanic conduit apparently became covered and plugged by solidified cap lava or ash in the cone. Then, magmatic volatiles that had been emitted from the reservoir could not escape into the atmosphere and started to accumulate in the conduit. The gas pressure in the cave may have increased significantly, which strengthened the release of magmatic gas and fluid through fissures or permeable channels in the volcanic edifice. The elevated ~ H e / 4 H e ratio in the steam well is consistent with the intensified transportation of the helium from the magmatic gas source. The height of volcanic smoke measured by eye presumably reflects the intensity of gas emission in the central cone of Mt. Mihara. Thus, our "plug" hypothesis can be verified by the temporal variation in the height of the smoke. Figure 3 shows the variation in the monthly averaged height data reported by the Japan Meteorological Agency [18] together with the temporal variation in the 3 H e / a H e ratio in the steam well with an enlarged scale. Generally, the height of the smoke increased from January 1988 to February 1990, while the 3 H e / n i l e ratio decreased with time except for the small 3 H e / 4 H e peak in July 1988. The height began to decrease in March 1990 and was almost zero in June (i.e., no volcanic smoke present in the central cone of Mt. Mihara). At the same time, the SO 2 flux was about one order of magnitude smaller than that of November 1988 [17]. Concurrently the 3 H e / 4 H e ratio increased and reached its maxim u m in July 1990. Thus there is a negative correlation between the smoke height and the 3 H e / 4 H e ratio in the steam well, suggesting that the model described above is valid. When the emission of magmatic gas and fluid was blocked in the central

SECULAR VARIATIONS IN HELIUM ISOTOPE RATIOS IN AN ACTIVE VOLCANO

cone, the gas and fluid may have travelled to the observation site through channels; this would make the 3He/4He ratio high. There appears to have been a time lag of about one or two months between the zero smoke height in June 1990 and the 3He/4He maximum in July 1990, although no gas samples were collected in June. This lag could be caused by the travel time of the magmatic gas and fluid from the source to the steam well, as was suggested previously [6]. On 6 October 1990, two days after the eruption, the 3He/4He ratio was slightly lower than that of July 1990. The smoke height, however, began to increase in September 1990. This suggests that leakage of magmatic volatiles may have started in the central cone one month before the eruption. A small decrease in the 3 H e / 4 H e ratio in October 1990 may be attributable to leakage in the cone and weakened emission to the surrounding channels. The 3He/4He ratio subsequently decreases gradually until April 1991, although the

E 600 500

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1991

Year Fig. 3. Secular variation in the measured height (by eye) of volcanic smoke at Mt. Mihara (top) and in 3He/4He ratios with an enlarged scale (bottom). The height data are from the Japan Meteorological Agency [18]. Arrows indicate the dates of volcanic eruptions. Error bars in the 3He/4He ratios are 1

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4 He/20 Ne Fig. 4. A correlation diagram between the 3He/4He and 4 H e / 2°Ne ratios in the steam gas samples. The solid square is the atmospheric ratio. Arrows show temporal variations from May 1987 to November 1987 and from March 1990 to July 1990. Note that in the period May 1987 to November 1987 the 4He/2°Ne ratio increased significantly, whereas the increase in the 3He/4He ratio was not so great in this period.

smoke height does not vary significantly from November 1990 to April 1991. Considering the long-term decrease in the 3 H e / 4 H e ratio since July 1988, there is a significant 3He/4He peak in July 1990 (Fig. 3). Similar explosive eruptions occurred on 16 November 1987 [8] and 25 January 1988 [11], and the "plug" hypothesis can be applied to them as well. Although significant increase in the 4 H e / 2°Ne ratio was observed prior to the November 1987 event [6], the change in the helium isotope ratio was not so large. This may be attributed to the difference in the mixing rate of the magmatic and air helium. Figure 4 shows the relationship between the observed 3 H e / 4 H e and 4He/2°Ne ratios. The positive correlation between the ratios strongly suggests that there is a mixture of two components in the steam well samples. One end member is obviously atmospheric rare gas with the 3 H e / 4 H e ratio of 1 Rat m. The other is probably magmatic rare gas with the 3 H e / n i l e ratio of about 6.2 R~t m. Because the magmatic gas shows a 4He/2°Ne ratio that is significantly larger than that of air it is possible to calculate fractions of helium in each based on a simple mixing model and the measured 4 H e / 2 ° N e ratios (Table 1). As is shown by the arrows in Fig. 4, one can trace the mixing line with the elapsed time. The 4He/2°Ne

100

ratio increased significantly from May 1987 to November 1987, whereas the increase in the 3 H e / 4He ratio was not vary so great. Since the 3He/4He ratio was almost saturated with the magmatic value, no meaningful change was found even though an appreciable amount of magmatic gas was added to the sample. Therefore the 3He/4He ratio was not so sensitive at that time because it was already magmatic, while the 4He/2°Ne ratio had the potential to vary. Both 3He/4He and a He/20 Ne ratios were relatively low in March-July 1990 compared with M a y - N o v e m b e r 1987 however. In addition, there has been a long-term trend of decreasing 3He/4He ratios since July 1988. The 3He/4He ratio could have varied with the increase in magmatic helium in March-July 1990 but may not have in M a y - N o v e m b e r 1987. This may be the reason why there is no apparent 3He/4He change prior to the volcanic eruptions on 16 November 1987 and 25 January 1988. This idea, however, should be tested in future projects. In conclusion, the magmatic component of helium in the steam well significantly increased prior to the eruption of Mt. Mihara on 4 October 1990, apparently as a result of the elevated magmatic gas pressure in the conduit owing to a solidified lava cap. Helium isotope research provides useful information on the velocity of magmatic gas flow and may be of practical use in forecasting volcanic eruptions.

Acknowledgements We are grateful to Drs. B. Marty and W.C. Evans, and an anonymous reviewer, for their comments and suggestions. Unpublished data on the height of volcanic smoke at Mt. Mihara from November 1990 to April 1991 were provided by the Japan Meteorological Agency.

Y. S A N O ET AL.

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