Journal of Nuclear and Radiochemical Sciences, Vol. 2, No.1-2, pp. N1-N3, 2001

Notes
Activity Ratios of Uranium Isotopes in Volcanic Rocks from
Izu-Mariana Island-Arc Volcanoes, JapanJun Sato* and Masato Endo†
Department of Industrial Chemistry, School of Science and Technology, Meiji University,
Higashi-mita, Tama-ku, Kawasaki, Kanagawa 214-8571, Japan
Received: November 19, 2000; In Final Form: June 13, 2001
The 234U/238U activity ratios of 45 products from 6 volcanoes of Izu-Mariana island-arc ranged from 0.96 to 1.04,
representing a Gaussian distribution with the average value of 0.997 and the standard deviation of 0.014. The
result indicates that 234U is in radioactive equilibrium with 238U in the erupting magmas from island-arc volcanoes.

1. Introduction

2. Experimentals

Radioactive disequilibrium among U-series nuclides in
erupting magmas has been investigated with products fromvolcanoes of wide localities in the world. 1, 2 As for nonisotopic members, the disequilibria of 230Th < 238U and 230Th >238
U have been observed with the volcanoes locating along the
subducting zone and on the hot-spot or along the mid-oceanic
ridge, respectively.1−7
As 234U is an isotopic daughter of 238U, the deviation of the234
U/238U activity ratio from unity due to chemical processes,
including magmatic evolution, is far less probable than thecase of such non-isotopic combinations as 238U-230Th.
The disequilibrium between 238U and 234U has been observedwith such geochemical samples as underground waters, whichis inferred to be raised mainly due to α -recoil phenomena
within the aquifer, suggesting that the contact of liquid phasewith solid phase can give rise to the disequilibrium in theliquid phase. Liquid magma interacts with surrounding solidmaterials for the long period from its generation to the eruption. Nishimura1 compiled 234U/238U activity ratios in recent
volcanic rocks and reported that the activity ratios ranged from
0.8 to 1.2. The 234 U/ 238 U activity ratios observed with thesubduction-related volcanic products form a group within aclose range around unity, ranging from 0.94 ± 0.06 to 1.06 ±0.10 (uncertainties are expressed in 2 σ),2 indicating that they
are almost in equilibrium, although some of them may besuggesting the possibility of a slight deviation from unity.
However, as the number of samples in the majority of observations with the volcanoes locating in respective region islimited, it is considered not to be sufficient enough for the
discussion on the slight disequilibrium, if any, between 238U
and 234U in erupting magmas from the particular region interested. As for Izu-Mariana island-arc, Japan, two observations
on 234U/238U activity ratio were reported: 1.01 ± 0.10 for Fuji
and 0.95 ± 0.10 for Izu-Oshima (uncertainties are expressed in2 σ).2The present report deals with the observation of activity
ratios of 234U/238U in the 45 volcanic rocks erupted recently
from 6 volcanoes located along Izu-Mariana island-arc. The
observed 234U/238U activity ratios were treated statistically to
discuss on the radioactive equilibrium between 238U and 234U.

2.1. Sample Description. Observations were carried out
with 45 volcanic rock samples. They are the products from
Miyake-jima (1983), Nii-jima (886), Kozu-shima (838), IzuOshima (1986), Fuji (1707) and Asama (1108 and 1783) volcanoes which are locating along Izu-Mariana island-arc as
shown in Figure 1. Figure 1 contains the depth of the subducting plate which is corresponding to the depth of the magmatic
generation of respective volcano. The products from IzuMariana island-arc volcanes are of a wide variety of rock types
ranging from basalt (for Miyake-jima and Izu-Oshima) to rhyolite (for Kozu-shima and Niijima), volcanic rocks formed in
the early and late stage of the crystal differentiation of magmatic evolution, respectively. The rock type of the productfrom Asama is andesite which was formed in the intermediatestage of the evolution.
The products from Hawaiian island on the hot-spot andthose from Iceland along the mid-oceanic ridge are also
employed in the present mesurement for comparison.
The samples analyzed are listed in Table 1, and theirdetailed descriptions are as follows.
1) Miyake-jima (1983); Five samples were taken from Ako andAwabe lava flows and one sample was collected from thescoria cone formed at Nippana.
2) Niijima (886); The 886 eruption of Niijima produced 3 eruptive units: My1 and My2 which consist of pumiceous volcanicash, lapilli, and volcanic brocks (biotite rhyolitic), and My3
which is biotite rhyolitic lava and Shiromama layer of My1.

*Corresponding author. E-mail: jsato@isc.meiji.ac.jp. FAX: +8144-934-7906.
†Present address: Canon Inc., 3-30-2 Shimo-maruko, Ota-ku,
Tokyo 146-8501, Japan.

Figure 1. Localities of 6 volcanoes along Izu-Mariana island-arc.
Lines on the map represent the depth of the subducting plate.

© 2001 The Japan Society of Nuclear and Radiochemical Sciences
Published on Web 12/05/2001

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Sato

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

TABLE 1: Vocanic Products from Izu-Mariana Island-Arc Volcanoes and Their 234U/238U Activity Ratios. Those from Hawaiian
Islands and Iceland are also Included for Comparison. Errors on the Activity Ratios are the 1σ Uncertainties from Counting
Statistics
Volcano (Eruption year)
Niijima volcano (886)Sample identificaftion

Rock type

Activity ratio
234
U/238UNJM96052602 (My1)
NJM96052605 (Shiromama)
NJM96052607 (My3)NJM96052611 (My2)rhyoliterhyoliterhyoliterhyolite

KZS96052702 (Tj2)
KZS96052704 (Tj2)
KZS96052706 (Tj3)
KZS96052710 (Tj4)
KZS96052711 (Tj1)rhyoliterhyoliterhyoliterhyoliterhyolite

ASM840817-6
ASM850523-1
OND 17
OND 25
OND 33
OND 42
OND 45
OND 54

andesite
andesite
andesite
andesite
andesite
andesite
andesite
andesite

MYK97052504 (Ako)
MYK97052505 (Awabe)
MYK97052602 (Ako)
MYK97052507 (Ako)
MYK97052509 (Awabe)
MYK97052510 (Nippana)basaltbasaltbasaltbasaltbasaltbasalt

OSM98062101 (LBI)
OSM98062102 (LBII)OSM98062104 (LBIII)
OSM98062106 (LC)OSM86122007 (LA)basaltbasaltbasaltbasaltbasaltHoei 1
Hoei 2
Hoei 3
Hoei 4
Hoei 5
Hoei 6
Hoei 7
Hoei 8
Hoei 9Hoei 10
Hoei 11
Hoei 12
Hoei 13
Hoei 14
Hoei 15
Hoei 16
Hoei 17

daciteandesite
andesite
andesite
andesitebasaltbasaltbasaltbasaltbasaltbasaltbasaltbasaltbasaltbasaltbasaltbasalt

TF79071901
TF79071905

basalttrachyte

TF80062701
TF80062002

basaltbasalt0.99 ± 0.01
0.99 ± 0.01
1.00 ± 0.01
1.01 ± 0.01
1.00 ± 0.01
0.98 ± 0.01
0.99 ± 0.01
1.00 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.99 ± 0.01
1.03 ± 0.01
1.00 ± 0.01
0.98 ± 0.01
1.00 ± 0.01
1.00 ± 0.01
0.99 ± 0.01
1.00 ± 0.02
1.00 ± 0.01
0.98 ± 0.01
1.00 ± 0.01
0.99 ± 0.01
0.98 ± 0.01
1.01 ± 0.01
0.99 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.99 ± 0.01
1.00 ± 0.01
0.99 ± 0.01
0.99 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.98 ± 0.01
1.01 ± 0.01
1.01 ± 0.01
1.00 ± 0.01
0.99 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.96 ± 0.01
0.99 ± 0.01
1.01 ± 0.01
0.99 ± 0.01
1.00 ± 0.01
1.04 ± 0.01
0.99 ± 0.01
0.99 ± 0.01
1.00 ± 0.02
1.01 ± 0.01
0.97 ± 0.01
0.99 ± 0.02
0.97 ± 0.01
0.99 ± 0.01
0.98 ± 0.01

(Av.)
Kozu-shima volcano (838)(Av.)
Asama volcano (1108)
Asama volcano (1783)(Av.)
Miyake-jima volcano (1983)(Av.)
Izu-Oshima volcano (1986)(Av.)
Fuji volcano (1707)(Av.)
Hawaii(Av.)
Iceland(Av.)

Activity Ratios of Uranium Isotopes in Volcanic Rocks

3) Kozu-shima (838); The 838 eruption of Kozu-shima
produced 4 eruptive units: Tj 1 and Tj 2 which consist of
pumiceous volcanic ash, lapilli, and volcanic brocks (biotite
rhyolitic), and Tj3 which is biotite rhyolitic lava and Tj4 which
is loose volcanic breccia of biotite rhyolite.
4) Izu-Oshima (1986); The 1986 eruption formed 3 lava flows
of LA, LB, and LC. The lava flow LB was devided into 3
branches (LB I , II, and III).
5) Fuji (1707); The 1707 eruption of Fuji started with the eruption of dacitic pumice fall followed by andesitic and basaltic
scoria fall, suggesting that the eruption has proceeded along
the zoning layer formed in the magmatic reservoir. The air-falltephra has formed 17 layers at Midono, Gotemba, and samples
were collected from respective fall units.
6) Asama (1108 and 1783); Two samples from the 1108 eruption of Asama were collected from "B" scoria fall layer, and 6
lava samples were taken from Onioshidashi lava flow of the1783 eruption.
2.2. Chemical Separation and Radioactivity Measurement.
The solvent extraction procedure using CH 3 CO 2 C 2 H 5 was
widely employed in isolation and purification of U from sample solution. However, as Al(NO3)3 · 9H2O, which is added ina large amount as the salting-out material in the procedure,contains ppb-levels of U impurity (0.9−12.1 ppb) in radioactive
disequilibrium (the 234U/238U activity ratios were observed torange from 1.22 ± 0.07 to 4.29 ± 0.18),8 the U impurity in thereagent can constitute a serious obstacle to the determinationof U in minute concentration and to the measurement of
234
U/238U activity ratios in volcanic rocks.
An application of U/TEVA·Spec resin, an extraction chromatograph material supplied by Eichrom Industries, Inc.,
U.S.A., to the separation procedure of U is the main improvement in this work. The extracting material is (C5H11O)2C5H11PO,
and it extracts hexavalent U and tetravalent actinides from
2MHNO3 aqueous solution and releases them into 0.02 MHNO3 aqueous media.
From 30 to 60 g of volcanic rocks were decomposed by themixed acid of HF-HClO4-HNO3. A large amount of Fe waseliminated by the solvent extraction into [(CH3)2CH]2O fromsample solution adjusted to 8 MHCl. After U fraction was
separated by an anion exchange resin (Dowex 1-X8 Cl-form),
U was isolated successfully by use of the U/TEVA · Spec resin.
No detectable amount of U was observed in the blank test
performed prior to the rock analysis.
Uranium was electrodeposited from the electrobath of amixed solution of 0.02 MHNO3 and 0.2 M(NH4)2C2O4 (pH =2) onto a stainless steel planchet for α-ray counting. Activity
ratios of 234U/238U were obtained by α-ray spectrometry using a
900 mm2 Si detector (ORTEC) coupled with an MCA system(Laboratory Equipment; ADC 2201A).

J. Nucl. Radiochem. Sci., Vol. 2, No. 1-2, 2001

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Figure 2. Histogram of 234U/238U activity ratios of the present 45
volcanic rock samples from Izu-Mariana island-arc volcanoes, Japan.
The whole data represent a Gaussian distribution with the averagevalue of 0.997 and the standard deviation of 0.014.

ratios of volcanic products from each volcano ranges from
0.99 to 1.00. The histogram for the distribution of wholeobserved values are shown in Figure 2.
The histogram represents a Gaussian distribution having the
average value of 0.997 and the standard deviation of 0.014
which is corresponding to the statistical errors of the individual
measurements, indicating that the 234U is in radioactive equilibrium with 238U: the degree of the possible disequilibrium in themagma due to the crystal differentiation and/or the interaction
with surrounding solid phase materials is estimated to besmaller than ± 0.01 which is corresponding to the detection limitof the present measurement.
Table 1 also contains the 2 3 4 U/ 2 3 8 U activity ratios forvolcanic rock samples from Hawaiian islands and Iceland.
The values of magmas from the hot-spot and mid-oceanic
ridge ranged from 0.97 to 1.01 which is in a good agreementwith the reported value6 and is in the range of the Izu-Mariana
island-arc volcanes. Uranium-234 in those magmas are also
regarded to be in radioactive equilibrium with 238U, though thenumber of samples are limited.
Acknowledgement. The authors gratefully acknowledge
Dr. T. Fukuoka, Rissho University, for offering the volcanic
rock samples from Hawaiian islands and Iceland. They are alsoindebted to Mr. S. Kashiwaba, New Tokyo International
Airport Authority, Mr. H. Takahashi, University of Tokyo, andProf. T. Nakamura, Meiji University, for their co-operations.
The present work was partly supported by a 1997 - 1998 grantin-aid from Meiji University.

3. Result and Discussion
Table 1 contains the 234U/238U activity ratios for the volcanic
rock samples analyzed. Measurements were carried out so that
all of the statistical errors (1σ) accompanying respective
measurement were arranged to be ± 0.01, because it is convinient
to compare the distribution of the set of individual data withthe result of the statistical treatment of the whole data.
The 234 U/ 238 U activity ratios of rhyolitic magmas from
Niijima and Kozu-shima varies from 0.98 to 1.01, those ofandesitic magma from Asama from 0.98 to 1.03, and those ofbasaltic magma from Miyake-jima and Izu-Oshima from 0.98
to 1.01. The activity ratio along the fall units of sedimentation
layer formed by the 1707 eruption of Fuji ranges from 0.98 to
1.04. The observed activity ratio showed no connection eitherwith the rock type or with the order of eruptive sequence.
While the 234U/238U activity ratio of the 45 samples ranges
from 0.96 to 1.04, the average value of the 234U/238U activity

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