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

LetterAMS Radiocarbon Dating of a Charcoal Fragment from the Irosin
Ignimbrite, Sorsogon Province, Southern Luzon, Philippines
Ma. Hannah T. MIRABUENOῌ,ῌῌ, Mitsuru OKUNOῌῌῌ, Toshio NAKAMURAῌῌῌῌ,Eduardo P. LAGUERTAῌῌῌῌῌ, Christopher G. NEWHALLῌῌῌῌῌῌ
and Tetsuo KOBAYASHIῌῌῌῌῌῌῌ
(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 MassSpectrometry) +.C date of charcoal fragment from the Irosin ignimbrite that is distributed in the province ofSorsogon. 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 databaseof widespread tephra deposits in the Philippines.
Key words : Irosin caldera, Irosin ignimbrite, AMS +.C dates, Philippines+. Introduction
The Philippine Institute of Volcanology and Seismology (PHIVOLCS, ,**,) identiﬁes ,, active and ,1
potentially active volcanoes in the Philippine archipelago. Among the active volcanoes, Bulusan volcano(+,῎.0.,῍N, +,.῎*-.*῍E), which is located in the province of Sorsogon in the southern end of the Bicol Arc,southern Luzon (Fig. +) has erupted in recent historictimes. Bulusan volcano is generally known for sudden
occurrence of phreatic type of eruption. The mostrecent volcanic activity involved a series of phreaticeruptions which started from March ,**0 until ,. January ,**1 (http : //www.phivolcs.dost.gov.ph).
This stratovolcano is one of the post-caldera cones ofIrosin caldera. The caldera was formed by the eruption of the Irosin ignimbrite, which is mostly massive,
poorly to moderately sorted, dacitic to rhyolitic pyroclastic ﬂows, and distributed widely around the caldera(Delﬁn et al., +33- ; McDermott et al., ,**/). Previous radiocarbon ( +.C) ages of --,/**ῌ+/* BP,῍-.,***

BP and ῍-0,*** BP (Newhall, unpublished data) for
the ignimbrite plotted within the --ῌ-0 kyr BP range
but near the limit of +.C detection.
In order to reﬁne the chronology of the caldera
formation, we performed +.C dating with accelerator
mass spectrometry (AMS) of charred wood fragments
collected from the Irosin ignimbrite. This paper presents the result of the +.C dating and discusses theeruption age of the ignimbrite. This new age is important not only in understanding the eruptive history of
the Irosin caldera and associated volcanoes such as theactive Bulusan volcano, but also in providing a chronological framework of the volcanism of the Bicol Arc.
,. Outline of the Irosin caldera
The Irosin caldera, together with the active Bulusan
volcano and associated older volcanic centers in various
stages of erosion, comprise the Bulusan Volcanic Complex (BVC). The rim of the caldera forms a semicircle
showing a strong topographic expression and steep gra-῏Graduate School of Science and Engineering, Kago- ῏῏῏῏῏ Mayon Volcano-Ligñon Hill Observatory, Philippine
shima University, +ῌ,+ῌ-/ Korimoto, Kagoshima 23*ῌInstitute of Volcanology and Seismology (PHIVOLCS),**0/, Japan.
Ligñon 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 theAmerica.
῏῏῏῏῏῏῏ Department of Earth and Environmental Science,Philippines, Diliman, Quezon City, Philippines.
῏῏῏Department of Earth System Science, Faculty of SciFaculty of Science, Kagoshima University, +ῌ,+ῌ-/ence, Fukuoka University, 2ῌ+3ῌ+ Nanakuma, JonanKorimoto, 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*/hanah@eniac.sci.kagoshima-u.ac.jp