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BP and ῍-0,*** BP (Newhall, unpublished data) for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

242

Ma. Hannah T. MIRABUENO, Mitsuru OKUNO, Toshio NAKAMURA, Eduardo P. LAGUERTA,Christopher G. NEWHALL and Tetsuo KOBAYASHI

Fig. +. Map showing the volcanoes of the Bicol Arc(after PHIVOLCS, ,**,). Box approximates thelocation of Bulusan Volcanic Complex (Fig. ,).
Inset shows a map of the Philippines ; box in theinset map shows location of the Bicol Peninsula.

dient in the south but the northern trace is absent. The
northern rim was presumed to be covered by younger
post-caldera deposits (Fig. , : Delﬁn et al., +33-) however, a gravity survey conducted at the BVC conﬁrmed
the absence of the northern trace (Komazawa et al.,,***).
The largest single extrusion of dacite to rhyolite
magmas in the Bicol Arc led to a collapse and formationof the ++ km-wide Irosin caldera (Delﬁn et al., +33-).
The formation of the caldera represents the second stageof the --stage eruptive history of the BVC. Cone-building episode constituted the ﬁrst stage which commenced
at +.+* Ma, and formed the pre-caldera andesitic volcanoes (Delﬁn et al., +33-). Post-caldera volcanism
followed forming the stratovolcanoes, Sharp Peak andthe currently active volcano, Bulusan (Fig. ,).
-. Experimental procedure
At least two types of ignimbrite are recognized in theﬁeld : lower ﬁne and upper coarse ignimbrites. Both units
consist of beige, massive and pumice-rich pyroclastic
ﬂow deposits. The phenocrysts consist of plagioclase,biotite, magnetite and trace amounts of amphibole andquartz. The bulk chemistry of the pumice sample
shows 1/.2 wt.ῌ SiO,, which is consistent with a rhyo-Fig. ,. Map of the Bulusan Volcanic Complex. Shadowed areas show distribution of the Irosin Ignimbrite. The active volcano, Bulusan and othervolcanoes are represented by triangles. Star represents the sampling location of charred wood
fragment (Fig.-). Solid line traces the outline ofIrosin caldera rim (after Delﬁn et al., +33-, McDermott et al., ,**/).

lite composition (Delﬁn et al., +33- ; McDermott et al.,,**/).
The charcoal fragment was embedded in the lower
portion of the ﬁne pyroclastic ﬂow deposit exposed in
Juban, Sorsogon north-northwest of the caldera (Fig.
,). In this outcrop, the ignimbrite is approximately +/meter-thick and consists of many thin ﬂow units especially at the upper horizon (Fig. -).
The charcoal sample was puriﬁed by acid-alkali-acid(AAA) treatments. The pretreated material was oxidized
by heating at 3**ῌ for , hours in a sealed Vycor῍ tube
together with CuO. The produced CO, was reduced
catalytically to graphite on Fe-powder with hydrogengas in a sealed Vycor῍ tube (Kitagawa et al., +33-).
We used a HVEE Tandetron AMS system at Nagoya
University to make +.C measurements of graphite targets
with NIST oxalic acid (HoxII) as standards (Nakamuraet al., ,***). We corrected for carbon isotopic fractionation using the +-C/ +,C ratio (d +-CPDB). To estimate the +.C background level, the +.C age of commercial graphite powder (dead carbon) was also measured
in the same sequence of sample measurements. TheAMS Radiocarbon Dating of a Charcoal Fragment from the Irosin Ignimbrite,Sorsogon Province, Southern Luzon, Philippines
Table +. Result of AMS
ignimbrite.

243

+.

C dating for the Irosin

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*). However, it has been shown at Kikai and Aira calderas inJapan that post-volcanism has started +ῌ- kyr after theignimbrite eruption (Okuno, ,**, ; Okuno and Nakamura, ,**- ; Okuno et al., +331). These studies at Kikaiand Aira calderas underscore the importance in determining the age of the ﬁrst activity in a post-caldera
stage. The new +.C date provides additional data inconstraining the age of post-caldera volcanoes in theBVC. Further, the study on widespread ash-fall deposits associated with Irosin ignimbrite will be useful forestablishing the chronological framework of the volcanism in Luzon, and probably throughout the Philippines.

Fig. -. Photo of the Irosin ignimbrite in Juban, Sorsogon (+,῍.3.-/῎N, +,-῍/1.-/῎E). Charcoal sample
was taken from the lower part of the +/ meter-thickoutcrop of the ignimbrite.

C age was calculated by subtracting the +.C concentration of the background sample.

Acknowledgements
This study was partly supported by a Grant-in-Aidfor Scientiﬁc Research (nos.+/.*-**, and +0-,*+*2)from the Japan Society for the Promotion of Science(JSPS). We thank Hiroyuki Tsutsumi (Kyoto University) and PHIVOLCS for supporting ﬁeld activities.
We also thank Kazutaka Mannen (Hot Springs Research Institute, Kanagawa Prefecture) and Teruki
Oikawa (Geological Survey of Japan, AIST) for reviewing manuscript, Lito Begonia and Joanna Ruvi Ayuson(PHIVOLCS) for their help.

+.

.. Result and discussion
Table + shows the result of AMS dating and its
calibration. The obtained +.C age of -/,3-*ῌ,/* BP(NUTA,ῌ+*13/) is signiﬁcantly younger than ..,01*ῌ
-+* BP (NUTA,ῌ+*123), which is used as the +.C background level. The obtained age is consistent with theages reported by Newhall (unpublished data), thus,
further constrains the eruption age for the ignimbrite tobe -0 kyr BP. Currently, the +.C age calibration curve
IntCal*. (Reimer et al., ,**.) extends back to ,0 cal
kyr BP but no precise calibration curve is available forthe time range, ,0ῌ/* cal kyr BP (van der Plicht et al.,,**.). Thus, we tentatively used the Fairbanks calibration program (Fairbanks et al., ,**/), and obtained
calibrated year of .+,-,3ῌ+03 cal BP.
Catane et al. (,**/) suggested that the post-caldera

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