Bull. Bull. Volcano 1. Soc. Letter Letter Vo 1. 47 (2002) No. 6， pp. 751-755

The Discovery of Y oung Dacite Lava in Island ， ，Japan

Katsuhiko FURUYAMA* ，Masahiro DAISHI** ，Keisuke NAGAO***' **** and Mugihiko EGUCHI*****

(Received (Received March 15 ， 2002; Accepted September 17 ， 2002)

hitherto A hitherto unreported dacite lava was found on Akusekijima Island ， Tokara Islands ，Japan. The lava ，named Omune Lava ，is 100 m thick. K-Ar dating of the lava by the sensitivity method considering mass fractionation of initially initially trapped atmospheric argon ， shows it to be less than 0.08 Ma. The surface of the lava is much less dissected than than surrounding areas ， and is not covered by any tephra laye r. It suggests an eruption even as recent as the Holocene. Holocene. The range of Si0 2 w t. % of andesites (two pyroxene andesites) is between 56 and 63%. On the other hand ，Omune Lava (hornblende dacite) contains 68-69% Si02. Two trends are recognized with only a small difference difference among Akusekijima rocks. Our results show that dacite magma ，instead of andesitic magma ，concerned the the latest volcanic activity in Akusekijima Island.

1. 1. Introduction ty of further volcanological study of the eruptive Akusek 討ima Island is one of the Tokara Islands in history in Akusekijima Island. Southwest Southwest Japan ， located 200 km SSW from the southem end of Kyushu (Fig. la). Our recent 2. Geography and Geology geological geological survey resulted in the discovery of a dacite The circumference of Akusekijima Island is about lava lava flow (c1 assification based on the total alkali- 11. 5 km ， with N-S and E-W diameters of 3.2 km si1i ca diagram after Le Bas et al. ， 1986) which con- and 2.5 km respectively (Fig. lb). The island is tains tains high Si0 2 (68-69% wt. %)， derived from the mos t1 y surrounded by steep cliffs ，and beaches are most recent eruption on the island. Previously report 圃 found only on the southwestem and southeastem ed volcanic rocks on the island were andesites only parts of the island. The island represents the upper (Daishi (Daishi and Matsumoto ， 1979). This newly dis- part of a cone-like mountain ，which rises 800 m from covered covered dacite not only has the highest Si0 2 content the sea floor ，and about 12 km in diameter at the on Ak usekijima Island ， but also has the highest value base. The highest peak is M t. Mitake ，584 m above among all the Tokara Islands ex c1 uding rhyolite in sea leve 1. There is a gen t1 y eastward sloping plateau Kikai Kikai caldera volcano (Ono et al. ， 1982) based on (N-S 500 m X E-S 1500 m) on the top of the cliff at data data by Daishi (1992). We describe the geology ， the northern end of the island ，which is named as petrography ，a K-Ar dating result and chemical “Omune" (Fig. 1 b ). During several recent surveys ， compositions compositions of this newly discovered dacite lava. we were able to collect dacite lava samples at new This This discovery of younger eruption implies a necessi- road cuttings on “Omune" and below the northeast- em cliff ， although “Omune" had been covered by a * Department of Bio- and Geosciences ， Graduate dense growth of bamboo at the first survey by Daishi School School of Science ， Osaka City University ，Sumiy- and Matsumoto (1979). Here it is named as oshi-ku ， Osaka 558-8585 ，Japan. “Omune Lava ぺwhich forms the gentle sloping pla- ** ** Geochronology Japan Inc. ，Chuo-ku ， Osaka 542 一 teau ，“ Omune ぺwith an average thickness estimated 0012 ，Japan. at 100 m based on the height of the cliff surrounding *** *** Institute for Study of the Earth's Interior ，Oka- the northem part of “Omune" . The distribution of yama University ，Misasa-cho ，Tottori 682 -0 193 ， Omune lava is shown as the shaded area in the Japan. Japan. northem part of the island in Fig. 1b. The surface of **** **** Present: Laboratory for Earthquake Chemistry ， Graduate Graduate School of Science ，The University of the lava lacks severe dissections ， in contrast to the other other variously dissected regions of the island. The Tokyo ，Bunkyo ・ku ，Tokyo 113 -00 33 ，Japan. ***** ***** Division of Earth & Planetary Sciences ， Graduate vent of the lava could not be determined. School School of Science ， Kyoto University ，Sakyo 主u， Daishi and Matsumoto (1979) divided the main Kyoto Kyoto 606-8502 ，Japan. volcanic products in Akusekijima Island into N onze- 752 752 Katsuhiko FUR UYA MA ， Masahiro DAISHI ， Keisuke NAGAO and Mugihiko EGUCHI

128' 128' vent vent around Mt. Nakadake. We could not 血ld any tephra tephra layers including the Ueson Pumice Flow de- 129 ・36 ' 12 9' 37 ・ posit ， overlying the Omune Lava. This infers that b Omune Lava was formed by the latest eruption in Ak usekijima Island .

3. 3. Petrography Surface Surface exposures of the upper part of Omune Lava show white to gray colour with fluidal texture ， and sometimes very vesicular (pumiceous). We were able able to collect fresh samples from new outcrops . Phenocrysts Phenocrysts of Omune Lava comprise of plagioclase (く3 mm) ， hornblende (0.5 - 1 mm) ， hypersthene (< 0.5 mm) ，and augite (< 0.5 mm) in decreasing order order of abundance. Groundmass is holohyaline to cryptocrystalline cryptocrystalline in texture. On the other hand ， andesites andesites from Ak usekijima Island are classified as two pyroxene andesites with groundmass minerals of plagioclase ，augite ， hypersthene and iron oxides.

q 1，oq Om 4. 4. K-Ar Dating 129 ・36 ・ 129 ・37 ・ Sampled Omune Lava (sample OMD) was

Fig. Fig. 1. Location and geomorphology of Akuseki ・ crushed and sieved. The crushed fragments between jima jima Island . 60 and 100 mesh (150μm to 250μm) were divided a: a: location of Akusekijima Island in the Tokara into groundmass parts and phenocrysts by an isody- Islands. Islands. 1 -7: Volcanic islands of the Tokara namic separator. The groundmass parts were used Islands Islands (1; Satsuma-Iojima ， 2; Kuchinoerabu- for K-Ar dating. K content was measured at Osaka jima ， 3; ， 4; ， 5; City University by flame photometry based on the ，6 ; Ak usekijima ，7; Yokoat 吋ima ， method by N agao et al. (1984). The experimental 8; 8; 10 ・Torishima). b: topographic map of Ak u- error of the K content was assumed to be 3% based sekijima sekijima Island. Upper shaded area is the dis- on the reproducibility in replicate analyses. Ar anal- tribution tribution of Omune Lava . X ; sampling locali- ysis was performed on a noble gas mass spectrometer ties ties of Omune Lava (OMD ，OM1 and OM2). (VG5400) at the Institute for Study of the Earth's Lower shaded area is the distribution distribution of the Interior ，Okayama University ，by the sensitivity Ueson Ueson Pumice F10w deposi t. method. method. In the method ， the sensitivity is determined with with calibrated atmospheric argon and is used for misaki misaki Andesites ，Biroyama An desites ， Kazanshita argon analysis of the unknown sample (N agao and An desites (dyke) ，Nakadake Andesites ，Mitake An- It aya ， 1988; Takaoka et al. ， 1989; Matsumoto et al. ， desites ，and Y ounger Pyroclastic Rocks in ascending 1989). Experimental procedures basically followed order . The Y ounger Pyroclastic Rocks were further those of Nagao et al. (1991). This method is practi- divided divided into 7 layers (l ayer 1; coarse ash fall deposit ， cal for very young volcanic rocks ，which have a large layer layer 2; pumice fall deposit ， layer 3; pumice flow atmospheric fraction ， because it enables the correc- deposit ， layer 4; ash fall deposit ， layer 5; pumice fall tion of mass fractionation of initially trapped atmos- deposit ， layer 6; bedded ash fall deposit ， layer 7; ash pheric argon (It aya and Nagao ， 1988; Takaoka et al. ， fall fall deposit ， in ascending order). They considered 1989; Matsumoto et al. ， 1989). The K-Ar dating that that the Biroyama and the Nakadake Andesites result is shown in Table 1. When initial 4oAr/36Ar formed the first and second somma ，respectively ，and ratio is assumed to be the same value to the atmos- the the Mitake Andesites formed a central dome. We pheric one (296; Nier ，1950) ， the estimated age is recognized recognized that the layer 3 of the Y ounger Pyroclas- negative (a in Table 1). However ，precisely meas ・ tic tic Rocks (maximum thickness 7 m) overlies the ured 38 Ar/ 36 Ar ratio in the sample (0. 18629 ::t Nakadake and Biroyama An desites. It forms a slight- 0.00042 ， Table 1) is below the atmospheric value ly ly dissected pumice pl 出n around Ueson Village (0.188; Nier 1950) and it shows that the fractionated (shaded (shaded area of the southern part of the island in atmospheric argon had been trapped. The correction Fig. Fig. lb) ， here it is named as Ueson Pumice Flow of mass fractionation (Takaoka et al. ， 1989 ; Matsu- deposit. deposit. It might have originated from an unknown moto et al. ， 1989) of the trapped argon yields the The Discovery of Young Dacite Lava in Akusekijima Island ， Tokara Islands ， Japan 753

estimation estimation of a small amount of radiogenic 40 Ar and calc-alkaline (Miyashiro ， 1974) and the medium-K a geologically meaningful K-Ar age as a result (b in series (G i11， 1981; Fig. 1. 2). The range of Si0 2 wt. % Table 1). Although the measurement of a small of andesite lava and pumice from Ueson Pumice amount of radiogenic 40 Ar accompanied by a large Flow deposit is between 56 and 63%. On the other atmospheric fraction in the sample is very di 血cult ， hand ，Omune Lava (sample OMl and OM2) con- the the possible upper limit of the K-Ar age could be tains 68-69% Si0 2. It is recognized that there are determined as 0.08 Ma (result of mass fractionation two trends showing slight difference between older correction ，b in Table 1). (Nonze to Biroyama Andesites) and younger (Nak- adake Andesites to Ueson Pumice Flow deposit) 5. 5. Major and Trace Element Chemistry andesites in Figs. 2 and 3. Daishi and Matsumoto Major and some trace elements of Omune Lava (1992) rejected the simple fractional crysta 11i zation and other volcanic rocks from Akusekijima Island for the chemical variation among Akusekijima an- were analyzed by XRF using a glass bead ，自 ux and desites based on the common existence of com- sample ratio of 2:1 ，and following the method de- positional reverse zoning in hypersthene and aug 江e veloped by Yamada et al. (1995) at Osaka City phenocrysts in the Biroyama ，Nakadake and Mitake University. University. Igneous rocks of the GSJ geochemical Andesites. reference reference samples ，which were supplied by the Geo- logical logical Survey of Japan ，AIST ，were used as standard 6. Discussion and Summary materials. materials. The calibration curves corrected by theor- The K-Ar result of Omune Lava implies a young etical etical matrix coefficients were calculated using the eruption age less than 0.08 島1a. The undissected software software installed in the machine (RIX 2100 ，Rigaku young geomorphology and thin cover lacking any Ltd.). Ltd.). Analytical results are shown in Table 2，and tephra layer also suggests that the lava is very young ， Figs. Figs. 2 and 3. Akusekijima rocks belong to the even as recent as the Holocene. Further detailed

Table Table 1. K-Ar dating result of Omune Lava. 36 40 Sample Sample K Weight 38 Ar ;J 6 Ar 40 Ar /36 Ar Ar C.M. 。Ar rad K-Ar age Atm. Ar (wt.%) (wt.%) (g) (10- 10 cm 3STP!g) (10- scm 3STP!g) (Ma) (%) OMD 1. 915 0.45404 0.18629 291.01 35.4 a 296 -1. 77 -0.24 10 1.7 ::!::0.058 ::!::0.058 ::!::0.00042 ::!::0.22 ::!::1.8 ::!::0.12 ::!::0.02 b 290.6 0.14 0.02 99.9 土 1.3 土 0.47 土 0.06 10 10 K-Ar K-Ar age was calculated using decay constants λ.=0.581 x1 0- /y， λs =4.962x1 0- /y and 4O K/K=0.0001167 (Steiger and Jager ，1977). 40 40 Ar ;J 6Ar ，廿叩 :initially trapped 40 Ar /36 Ar ratio ，40 Ar rairadiogenic 40 Ar ， Atm. 40 Ar:initially trapped atmosoheric 40 Ar (%) in all Ar in sample. C.M. C.M. :calculated method. (a) 40Ari6Artrap is assumed to be atmospheric ratio. (b) 40Ar/36Artrap is corrected for mass f悶 ctionation e作ect based on the measured 38 Ar /36 Ar ratio. Error of K is 3 wt.% ，other errors are 1σ.

Table Table 2. M 吋or and trace element compositions of Omune lava and andesites from Ak usekijima Volcano. sample sample No. OM1 OM2 2 3 4 16 5 6 8 9 10 24 13 14 Omune Omune Omune Nonze Biro Biro Biro Biro Kazan Nakadake Nakadake Nakadake Nakadake pumice Mitake Mitake Si0 2 68.14 68.92 60.82 58.07 57.06 60.36 62.58 56.05 59 .4 7 57.81 59.19 60.29 60.65 60.82 61.20 Ti0 2 0.4 2 0.4 1 0.56 0.58 0.58 0.58 0.55 0.59 0.67 0.72 0.72 0.68 0.60 0.68 0.67 AI 203 16.04 16.30 16.56 16.83 17.00 16.96 16.34 17.03 16.74 17.23 17.08 16.55 16.73 16.99 16.65 T- Fe 203 2.81 2.75 6.69 7.22 7.65 6.4 8 5.77 7.94 7.36 8.01 7.75 7.23 6.27 7.07 7.00 MnO 0.08 0.08 0.11 0.13 0.14 0.12 0.12 0.14 0.14 0.15 0.15 0.13 0.12 0.13 0.13 MgO 0.81 0.80 2.60 3.84 4.12 2.87 2.4 3 4.50 3.4 5 3.53 3.27 3.21 2.62 2.74 2.67 CaO CaO 3.97 4.04 5.31 7.87 8.27 6.04 5.74 8.71 6.57 7.33 6.96 6.90 5.82 6.24 6.06 Na20 Na20 4.14 4.12 3.01 2.91 2.67 3.13 3.41 2.70 3.03 3.02 3.13 3.15 3.23 3.47 3.4 6 K20 2.24 2.26 1. 45 1.37 1.22 1.62 1.74 1.03 1.52 1.28 1. 45 1. 57 1. 64 1. 52 1.56 P20 5 0.09 0.09 0.07 0.08 0.07 0.09 0.10 0.07 0.10 0.11 0.11 0.10 0.10 0.11 0.11 Total Total 98.74 99.77 97.17 98.89 98.77 98.24 98.77 98.76 99.04 99.20 99.79 99.79 97.77 99.78 99.50 Sr Sr 177 180 185 200 212 189 192 209 200 228 220 208 195 217 199 Rb Rb 63 64 36 37 32 46 46 28 42 33 39 43 46 41 42 Ba Ba 269 278 192 170 149 194 223 140 189 174 184 191 227 190 197 Nb Nb 5 4 3 3 3 3 3 tr 5 3 3 4 5 4 4 Zr Zr 131 128 78 75 67 84 104 60 95 86 90 95 107 93 95 Y 29 29 23 19 18 20 24 17 24 25 29 24 26 25 24 Major Major elements in wt.% ，trace elements in ppm. Omune; Omune Lava ，Nonze; Nonzemisaki Andesites ，Biro; Biroyama Andesites ，Kazan; Kazanshita Andesites ，Nakadake; Nakadake Andesites ，pumice; pumice from Ueson Pumice Flow deposit ，Mitake; Mitake Andesites. Sample localities of Omune Lava Lava are shown in Fig.1 b. Andesite Lavas were collected based on the stratigraphy by Oaishi and Matsumoto (1979). 754 754 Katsuhiko FURUYAMA ， Masahiro DAISHI ， Keisuke NAGAO and Mugihiko EGUCHI study is required to understand the cause of the concerned the latest volcanic activity in Akusekijima chemical differences among the Akusekijima rocks Island. which exhibit two sligh t1 y differing trends (Figs. 2 and 3) ，and chemical reverse zoning of pyroxene Acknowledgements phenocrysts in andesites (Daishi and Matsumoto ， We wish to thank Dr. Andrew J. Martin (Tono 1992). 1992). Our results show that dacite magma ，which Geoscience Center ，Japan Nu c1 ear Cy c1 e Develop- had not been found before in Ak usekijima Islands ， ment Institute) for critical reading of the manuscript and helpful comments. Comments from the revie- (wt.%) (wt.%) wers were useful for improving the manuscript. We Omune L

• MltakeAd also thank residents in Ak usekijima Island for vari 同 Ueson Ueson Pm 企 d‘ ous help during field work. 0.6 0.6 ‘~.. NakadakeAd ロ+。ロ Kazanshita Ad ロ Biroyama Ad References 0.2 0.2 o Nonzemisaki Ad (wt.%) (wt.%) Daishi ，M. (1992) Cenozoic volcanic activities and rocks T。ロ旬 rreo nMU''ト ， -a 19 19 A ロ& -- 'a from the . In Exploration of volcanoes ロ AA •・ロ o + and rocks in Japan ，China ，and Ant αrctic α(Commemora-

企 ー l 企 ロ 日・ 『，，企 凶 十 tive Work Group for Professor Yukio Matsumoto ，eds.) ， 「 ロ A 。 A ロ 49-60. Commemorative Work Group for Professor Yukio Matsumoto ，Yamaguchi (in Japanese with English 15 15 abstract). 0.2 0.2 Daishi ，M. and Matsumoto ， Y. (1979) Volcanic geology of ロ the Akuseki-jima ， Prefecture ， Japan. Geolog- 企 a- 企 & 0.1 0.1 ロ・ ic α1 Studies ofthe Ryukyu Islands. 4， 63-78 (in Japanese 圃 +O ロ with English abstract). Daishi ，M. and Matsumoto ， Y. (1992) Volcanic rocks from Akuseki-jima ， the Tokara Islands ，Kagoshima Prefec- ロ CaO ture ， Japan. In Exploration of volc α noes and rocks in ~ ~ -.~ ・・+。ロ Japan ，China ，and Antarctica (Commemorative Work Group for Professor Yukio Matsumoto ，eds.) ， 81-94. 4 ト Commemorative Work Group for Professor Yukio Ma- -圃ロ ロ + tsumoto ，Yamaguchi (in Japanese with English ab- A A O 」ー --企 D stract). Na 0 G i11， J. B. (1981) Orogenic Andesites and Plate Tectonics. 2 •E nJ-nJa- Springler-Verlag ，New York ，39 Op. ロ nHUW n+ - It aya ， T. and Nagao ，K. (1988) K-Ar age determination of M 園田 ロa-。 企 PO 口 2 5 volcanic rocks younger than 1 Ma. Mem. Geol. Soc. 0.1 0.1 Japan ，29 ， 143-161 (in with Japanese English abstract). KO 2 Le Bas ，M. J. ，Le Maitre ， R. W. ，Streckeisen ，A.， Zanettin ， B. B. (1986) A chemical c1 assification of volcanic rocks 55 55 60 55 7065 60 7065 based on the total alkali-s i1i ca diagram. J. Petro l.， 27 ， 745-750. 745-750. 5i0 2 (Wt.%) 5i0 2 (wt.%) Matsumoto ，A. ，Uto ，K. and Shibata ，K. (1989) Argon Fig. Fig. 2. Si02 (wt. %) vs. oxides (w t. %) diagrams of isotopic ratios in historic lavas 一寸 mportance of correc- the the Akusekijima rocks. Data are recalculated to tion for the initial argon in K-Ar dating of young volcar ト total total oxides= 100%. (L; Lava ，Ad; Andesites ， ic rocks-. Mass めec. ，37 ， 353-363 (in Japanese with Ueson Pm; pumce from Ueson Pumice Flow English abstract). deposit) deposit) Miyashiro ， A. (1974) Volcanic rock series in island arcs and active continental margins. Amer. J. Sci. 274 ，321- 0.5 0.5 355. Rb/Ba Rb/Ba Nagao ，K. and It aya ， T. (1988) K-Ar age determination. Mem. Geol. Soc. Japan ，29 ， 5-21 (in Japanese with Eng- 0.25 1-円 0.5 1- .門口 口角 企ロ"， 04 ....圃袖+。ロ 。 】~ ~ゐ企圃圃 +U ロ lish abstract) Nagao ，K. ，Nishido ，H. ，It aya ， T. and Ogata ，K. (1984) An age age deterniination by K-Ar method. Bulletin ~ ~ ~ ro ~ ~ ~ ro of the 510 (wt. %) 5i0 (wt.%) Hiruzen Research 2 2 Institute ，Okayama University of Sci- ence ，9 ， 19-38 (in with Japanese English abstract). Fig. Fig. 3. Si0 2 (w t. %) vs. Rb/Ba and Rb/Zr ratios Nagao ，K. ，Ogata ，A. ，Miura ，Y. ，Matsuda ，J. and Aki ・ diagrams. diagrams. Si0 2 wt. % and symbols are as same moto ，S. (199 1) Highly reproducible 13 and 17 ka K-Ar as as those in Fig. 3. ages of two volcanic rocks. Geochem. よ， 25 ，447-45 1. The Discovery of Young Dacite Lava in Akusekijima Island ，Tokara Islands ， Japan 755

Nier ，A. O. (1950) A redetermination of the relative abun- Takaoka ，N. ，Konno ， K.， Oba ， Y. and Konda ， T. (1989) dances dances of the isotopes of carbon ，nitrogen ，oxygen ， K-Ar dating of lavas from Zao Volcano ，north-eastern argon ，and potassium. Phys. Rev. ，77 ， 789-793. Japan. J. Geol. Soc. Japan ，95 ， 157-170 (in Japanese Ono ，K. ，Soya ， T. and Hosono ， T. (1982) Geology of the with English abstract).

Satsuma-io ・jima distric t. Quadrangle series ， scale 1: Yamada ，Y. ，Kohno ，H. and Murata ，M. (1995) A low 50 ，000 ， Geological Survey of Japan. 80 p (in Japanese dilution fusion method for major and trace element anal- with with English abstract). ysis of geological samples. Advances in X-ray Chemical Steiger ，R. H and Jager ，E (1977) Subcommission on geo ・ Analysis Jap αn. 26 ， 33-44 (in Japanese with English chronology: chronology: Convention on the use of decay constants in abstract). geo ・and cosmochronology. Earth Planet. Sci. Lett. ，36 ， (Editorial handling Jun-ichi It oh) 359-362. 359-362.

トカラ列島，悪石島における若いデイサイト溶岩の発見

古山勝彦・大四雅弘・長尾敬介・江口麦彦

おおむね 西南日本， トカラ列島の一つ悪石島に厚さ約 100m の未報告のデイサイト溶岩をみつけた.本溶岩を大峰溶岩と称 する.本溶岩は初成同位体比の質量分別を考慮した K-Ar 年代値から 0.08 Ma より若いといえる.本溶岩はほとんど 開析をうけていない若い地形を示し，テフラ層にもおおわれていないことから，噴出は完新世にまでさかのぼるかも

しれない.悪石島の他の火山岩が Si0 2 56-63 wt. %の両輝石安山岩なのに対し，大峰溶岩は約 68% の角閃石デイサイ トである.安山岩の化学組成には 2 つのトレンドがみられるがその差は小さい.悪石島の最新の活動にはそれまでの 安山岩に代わって新たにデイサイトマグマが関与した.