Bull. Volcanol. Soc. Japan Letter Vol. /, (,**1)No..,pp.,.+ῌ,..

AMS Radiocarbon Dating of a Charcoal Fragment from the Irosin Ignimbrite, Sorsogon Province, Southern Luzon, Philippines

ῌ,ῌῌ ῌῌῌ ῌῌῌῌ Ma. Hannah T. MIRABUENO , Mitsuru OKUNO , Toshio NAKAMURA , ῌῌῌῌῌ ῌῌῌῌῌῌ Eduardo P. LAGUERTA , Christopher G. NEWHALL ῌῌῌῌῌῌῌ and Tetsuo KOBAYASHI

(Received, June ,2, ,**0; Accepted, May +., ,**1)

The eruption of dacitic to rhyolitic pyroclastic flows, Irosin ignimbrite, resulted in the formation of the Irosin caldera in Bicol Peninsula, southern Luzon, Philippines. This paper presents the AMS (Accelerator Mass Spectrometry) +.C date of charcoal fragment from the Irosin ignimbrite that is distributed in the province of Sorsogon. The obtained +.C age is -/,3-*ῌ,/* BP (NUTA,ῌ+*13/), and tentatively calibrated to the calendar year of .+,-,3ῌ+03 cal BP. This new age result contributes to the study of the Irosin caldera and to the database of widespread tephra deposits in the Philippines. Key words: Irosin caldera, Irosin ignimbrite, AMS +.C dates, Philippines

+. Introduction BP and ῍-0,*** BP (Newhall, unpublished data) for The Philippine Institute of Volcanology and Seismol- the ignimbrite plotted within the --ῌ-0 kyr BP range ogy (PHIVOLCS, ,**,) identifies ,, active and ,1 but near the limit of +.C detection. potentially active volcanoes in the Philippine archi- In order to refine the chronology of the caldera pelago. Among the active volcanoes, Bulusan volcano formation, we performed +.C dating with accelerator (+,῎.0.,῍N, +,.῎*-.*῍E), which is located in the prov- mass spectrometry (AMS) of charred wood fragments ince of Sorsogon in the southern end of the Bicol Arc, collected from the Irosin ignimbrite. This paper pre- southern Luzon (Fig. +) has erupted in recent historic sents the result of the +.C dating and discusses the times. Bulusan volcano is generally known for sudden eruption age of the ignimbrite. This new age is impor- occurrence of phreatic type of eruption. The most tant not only in understanding the eruptive history of recent volcanic activity involved a series of phreatic the Irosin caldera and associated volcanoes such as the eruptions which started from March ,**0 until ,. Jan- active Bulusan volcano, but also in providing a chrono- uary ,**1 (http://www.phivolcs.dost.gov.ph). logical framework of the volcanism of the Bicol Arc. This stratovolcano is one of the post-caldera cones of Irosin caldera. The caldera was formed by the erup- ,. Outline of the Irosin caldera tion of the Irosin ignimbrite, which is mostly massive, The Irosin caldera, together with the active Bulusan poorly to moderately sorted, dacitic to rhyolitic pyro- volcano and associated older volcanic centers in various clastic flows, and distributed widely around the caldera stages of erosion, comprise the Bulusan Volcanic Com- (Delfin et al., +33-; McDermott et al., ,**/). Previ- plex (BVC). The rim of the caldera forms a semicircle ous radiocarbon ( +.C) ages of --,/**ῌ+/* BP,῍-.,*** showing a strong topographic expression and steep gra-

῏Graduate School of Science and Engineering, Kago- ῏῏῏῏῏ Mayon Volcano-Lign˜on Hill Observatory, Philippine shima University, +ῌ,+ῌ-/ Korimoto, Kagoshima 23*ῌ Institute of Volcanology and Seismology (PHIVOLCS), **0/, Japan. Lign˜on Hill, Legazpi City, Philippines. ῏῏Philippine Institute of Volcanology and Seismology ῏῏῏῏῏῏ Formerly U.S. Geological Survey, United States of (PHIVOLCS), C.P. Garcia Avenue, University of the America. Philippines, Diliman, Quezon City, Philippines. ῏῏῏῏῏῏῏ Department of Earth and Environmental Science, ῏῏῏Department of Earth System Science, Faculty of Sci- Faculty of Science, Kagoshima University, +ῌ,+ῌ-/ ence, Fukuoka University, 2ῌ+3ῌ+ Nanakuma, Jonan- Korimoto, Kagoshima 23*ῌ**0/, Japan. ku, Fukuoka 2+.ῌ*+2*, Japan. ῏῏῏῏ Center for Chronological Research, Nagoya University, Corresponding author: M. H. T. Mirabueno Furo-cho, Chikusa-ku, Nagoya .0.ῌ20*,, Japan. e-mail: e*/[email protected] Ma. Hannah T. MIRABUENO, Mitsuru OKUNO, Toshio NAKAMURA, Eduardo P. LAGUERTA, 242 Christopher G. NEWHALL and Tetsuo KOBAYASHI

Fig. +. Map showing the volcanoes of the Bicol Arc Fig. ,. Map of the Bulusan Volcanic Complex. Shad- (after PHIVOLCS, ,**,). Box approximates the owed areas show distribution of the Irosin Ig- location of Bulusan Volcanic Complex (Fig. ,). nimbrite. The active volcano, Bulusan and other Inset shows a map of the Philippines; box in the volcanoes are represented by triangles. Star rep- inset map shows location of the Bicol Peninsula. resents the sampling location of charred wood fragment (Fig.-). Solid line traces the outline of Irosin caldera rim (after Delfin et al., +33-,Mc- dient in the south but the northern trace is absent. The Dermott et al., ,**/). northern rim was presumed to be covered by younger post-caldera deposits (Fig. ,: Delfin et al., +33-) how- ever, a gravity survey conducted at the BVC confirmed lite composition (Delfin et al., +33-; McDermott et al., the absence of the northern trace (Komazawa et al., ,**/). ,***). The charcoal fragment was embedded in the lower The largest single extrusion of dacite to rhyolite portion of the fine pyroclastic flow deposit exposed in magmas in the Bicol Arc led to a collapse and formation Juban, Sorsogon north-northwest of the caldera (Fig. of the ++ km-wide Irosin caldera (Delfin et al., +33-). ,). In this outcrop, the ignimbrite is approximately +/ The formation of the caldera represents the second stage meter-thick and consists of many thin flow units espe- of the --stage eruptive history of the BVC. Cone-build- cially at the upper horizon (Fig. -). ing episode constituted the first stage which commenced The charcoal sample was purified by acid-alkali-acid at +.+* Ma, and formed the pre-caldera andesitic vol- (AAA) treatments. The pretreated material was oxidized canoes (Delfin et al., +33-). Post-caldera volcanism by heating at 3**ῌ for , hours in a sealed Vycor῍ tube followed forming the stratovolcanoes, Sharp Peak and together with CuO. The produced CO, was reduced the currently active volcano, Bulusan (Fig. ,). catalytically to graphite on Fe-powder with hydrogen gas in a sealed Vycor῍ tube (Kitagawa et al., +33-). -. Experimental procedure We used a HVEE Tandetron AMS system at Nagoya At least two types of ignimbrite are recognized in the University to make +.C measurements of graphite targets field: lower fine and upper coarse ignimbrites. Both units with NIST oxalic acid (HoxII) as standards (Nakamura consist of beige, massive and pumice-rich pyroclastic et al., ,***). We corrected for carbon isotopic frac- +- +, +- flow deposits. The phenocrysts consist of plagioclase, tionation using the C/ C ratio (d CPDB). To esti- biotite, magnetite and trace amounts of amphibole and mate the +.C background level, the +.C age of commer- quartz. The bulk chemistry of the pumice sample cial graphite powder (dead carbon) was also measured shows 1/.2 wt.ῌ SiO,, which is consistent with a rhyo- in the same sequence of sample measurements. The AMS Radiocarbon Dating of a Charcoal Fragment from the Irosin Ignimbrite, Sorsogon Province, Southern Luzon, Philippines 243

Table +. Result of AMS +.C dating for the Irosin ignimbrite.

volcanism at BVC occurred around ,/ῌ-* ka based on the assumption that the typical timing of post-caldera volcanism in intermediate-sized calderas occurs within /ῌ +* kyr after caldera collapse (Mahood, +32*). Howev- er, it has been shown at Kikai and Aira calderas in Japan that post-volcanism has started +ῌ- kyr after the ignimbrite eruption (Okuno, ,**,; Okuno and Naka- mura, ,**-; Okuno et al., +331). These studies at Kikai and Aira calderas underscore the importance in deter- mining the age of the first activity in a post-caldera stage. The new +.C date provides additional data in constraining the age of post-caldera volcanoes in the BVC. Further, the study on widespread ash-fall depos- its associated with Irosin ignimbrite will be useful for establishing the chronological framework of the vol- canism in Luzon, and probably throughout the Philip- pines.

Acknowledgements This study was partly supported by a Grant-in-Aid for Scientific Research (nos.+/.*-**, and +0-,*+*2) from the Japan Society for the Promotion of Science (JSPS). We thank Hiroyuki Tsutsumi (Kyoto Univer- Fig. -. Photo of the Irosin ignimbrite in Juban, Sorso- sity) and PHIVOLCS for supporting field activities. gon (+,῍.3.-/῎N, +,-῍/1.-/῎E). Charcoal sample We also thank Kazutaka Mannen (Hot Springs Re- was taken from the lower part of the +/ meter-thick search Institute, Kanagawa Prefecture) and Teruki outcrop of the ignimbrite. Oikawa (Geological Survey of Japan, AIST) for review- ing manuscript, Lito Begonia and Joanna Ruvi Ayuson (PHIVOLCS) for their help. +.C age was calculated by subtracting the +.C concentra- tion of the background sample. References Catane, S.G., Taniguchi, H., Goto, A., Givero, A.P. and .. Result and discussion Mandanas A.A. (,**/) Explosive volcanism in the Philip- Table + shows the result of AMS dating and its pines. Center for Northeast Asian Studies, Tohoku Univ., calibration. The obtained +.Cageof-/,3-*ῌ,/* BP Sendai, CNEAS monograph series No. +2, +.0p. (NUTA,ῌ+*13/) is significantly younger than ..,01*ῌ Delfin, F.G., Panem, C.C. and Defant, M.J. (+33-) Erup- -+* BP (NUTA,ῌ+*123), which is used as the +.C back- tive history and petrochemistry of the Bulusan volcanic ground level. The obtained age is consistent with the complex: implications for the hydrothermal system and ages reported by Newhall (unpublished data), thus, volcanic hazards of Mt. Bulusan, Philippines. Geothermics, ,, .+1ῌ.-. further constrains the eruption age for the ignimbrite to , . +. Fairbanks, R.G., Mortlock, R.A., Chiu, T.-C., Cao, L., be -0 kyr BP. Currently, the C age calibration curve Kaplan, A., Guilderson, T.P., Fairbanks, T.W. and Bloom, IntCal*. (Reimer et al., ,**.) extends back to ,0 cal A.L. (,**/) Radiocarbon calibration curve spanning * to kyr BP but no precise calibration curve is available for ,-* ,-. ,-2 +. /*,*** years BP based on paired Th/ U/ Uand C the time range, ,0ῌ/* cal kyr BP (van der Plicht et al., dates on pristine corals. Quaternary Science Reviews, ,., ,**. ). Thus, we tentatively used the Fairbanks calibra- +12+ῌ+130. ,**/ tion program (Fairbanks et al., ), and obtained Kitagawa, H., Masuzawa, T., Nakamura, T. and Matsu- .+ -,3ῌ+03 calibrated year of , cal BP. moto, E. (+33-) A batch preparation method for graph- Catane et al. (,**/) suggested that the post-caldera Ma. Hannah T. MIRABUENO, Mitsuru OKUNO, Toshio NAKAMURA, Eduardo P. LAGUERTA, 244 Christopher G. NEWHALL and Tetsuo KOBAYASHI

ite targets with low background for AMS +.C measure- Okuno, M., Nakamura, T., Moriwaki, H. and Kobayashi, ments. Radiocarbon, -/, ,3/ῌ-**. T. (+331) AMS radiocarbon dating of the Sakurajima Komazawa, M., Pantig, J., Andico, B. and Listanco, E. tephra group, southern Kyushu, Japan. Nuclear Instru- (,***) Gravity anomalies of Bulusan volcano, South ments and Methods in Physics Research, B+,-, .1*ῌ.1.. Luzon.Report of international research and development Philippine Institute of Volcanology and Seismology (,**,) cooperation ITIT Projects. Research on Volcanic Hazard Volcanoes of the Philippines. Department of Science and Assessment in Asia, -/ῌ-2. Technology (DOST), .+p. Mahood, G. (+32*) Geological evolution of a Pleistocene Reimer, P.J., Baillie, M.G.L., Bard, E., Bayliss, A., Beck, rhyolitic center ῐ Sierra La Primavera, Jalisco, Mexico. J.W., Bertrand, C., Blackwell, P.G., Buck, C.E., Burr, Jour. Volcanol. Geotherm. Res., 2, +33ῌ,-*. G., Cutler, K.B., Damon, P.E., Edwards, R.L., Fairbanks, McDermott, F., Delfin, F.G., Defant, M.J., Turner, S. and R.G., Friedrich, M., Guilderson, T.P., Hogg, A.G., Maury, R. (,**/) The petrogenesis of volcanics from Hughen, K.A., Kromer, B., McCormac, F.G., Manning, Mt. Bulusan and Mt. Mayon in the Bicol arc, the Philip- S., Ramsey, C. Bronk, Reimer, R.W., Remmele, S., pines. Contribution to Mineralogy Petrology, +/*, 0/,ῌ Southon, J.R., Stuiver, M., Talamo, S., Taylor, F.W., 01*. van der Plicht, J. and Weyhenmeyer, C.E. (,**.) IntCal Nakamura, T., Niu, E., Oda, H., Ikeda, A., Minami, M., *. terrestrial radiocarbon age calibration, *ῌ,0 cal kyr Takahashi, H., Adachi, M., Pals, L., Gottdang, A. and BP. Radiocarbon, .0, +*,3ῌ+*/2. Suya, N. (,***) The HVEE Tandetron AMS system at van der Plicht, J., Beck, J.W., Bard, E., Baillie, M.G.L., Nagoya University. Nuclear Instruments and Methods in Blackwell, P.G., Buck, C.E., Friedrich, M., Guilderson, Physics Research, B+1,, /,ῌ/1. T.P., Hughen, K.A., Kromer, B., McCormac, F.G., Okuno, M. (,**,) Chronology of tephra layers in southern Ramsey, C Bronk, Reimer, P.J., Reimer, R.W., Remmele, Kyushu, SW Japan, for the last -*,*** years. The Quater- S., Richards, D.A., Southon, J.R., Stuiver, M. and nary Research (Japan), .+, ,,/ῌ,-0 (in Japanese with Weyhenmeyer, C.E. (,**.) NotCal*. ῐ comparison/ English abstract). calibration +.C records ,0ῌ/* cal kyr BP. Radiocarbon, Okuno, M. and Nakamura, T. (,**-) Radiocarbon dating .0, +,,/ῌ+,-2. of tephra layers: recent progress in Japan. Quaternary (Editiorial handling Osamu Ishizuka) International, +*/, .3ῌ/0.

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