Geochemical Journal, Vol. 36, pp. 465 to 473, 2002
238
U/230Th disequilibrium measurement for volcanic standard rock
samples using a multiple-collector ICPMS
SATORU FUKUDA * and SHUN’ICHI NAKAI
Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
(Received November 26, 2001; Accepted April 22, 2002)
The radioactive disequilibrium measurements between 238U and 230Th with a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICPMS) were applied to igneous standard rocks distributed by Geological Survey of Japan and Geological Survey of the United States. We modified sample
dissolution and Th separation method, as well as Th isotope analysis by MC-ICPMS described by Nakai et
al. (2001). High sensitivity of our ICP-MS enabled to analyze less than 10 ng Th. The abundances of U and
Th were determined by an isotope dilution method, using a 235U depleted uranium reagent and 230Th prepared by milking of natural U as spikes.
We can determine 238U/ 232Th abundance ratio with a precision of about 2%.
This procedure was applied to ( 238U/230Th) radioactive disequilibrium measurements of igneous standard rock samples. Samples older than 350 Ka were found to attain secular equilibrium, indicating the
accuracy of our analysis. On the other hand, young igneous standard rocks were found enriched in 238 U
relative to 230 Th, (238U/230Th) activity ratio > 1, which is often observed for igneous rocks from subduction
zones.
a high first ionization potential (5.95 eV) and is
difficult to ionize with the thermal ionization. The
isotope analysis of Th by TIMS usually consumes
about 100 ng of Th. Since 1990’s, multiple-collector inductively coupled plasma mass
spectrometry (MC-ICPMS) has been introduced
into the field of isotope geochemistry. The advantage of MC-ICPMS is its high ionization efficiency
that enables analysis of elements with a high ionization potential, such as W, Fe and Th (Halliday
et al., 1998; Luo et al., 1997). ICP-MS reduced
the required amount of Th for determination of
its isotope ratio to less than 10 ng (Nakai et al.,
2001).
In recent radioactive disequilibrium studies,
the determinations of U and Th abundances are
carried out by isotope dilution analysis. In general, artificially produced nuclides, 229Th and 236U
(Turner et al., 1997), or highly enriched 230Th and
235
U (Joannon et al., 1997) are used as spikes.
I NTRODUCTION
The 230Th is one of the radioactive nuclides in
the 238U decay series. The radioactive disequilibrium between 238U and 230Th results from chemical fractionation between U and Th and it returns
to equilibrium with a time scale of the half-life of
the daughter nuclide. The 230Th has a half-life of
7.52 × 10 4 yr and can be used to investigate the
timing of chemical fractionation between U and
Th for the time scale of ten to several hundred
thousand years. The radioactivity disequilibrium
dating has been applied to volcanology,
paleoenvironmental studies and archaeology and
so on in late Quaternary (Ivanovich and Harmon,
1992).
Thermal Ionization Mass Spectrometry (TIMS)
has been used for 238U/230Th disequilibrium measurements since 1980’s (Goldstein et al., 1989) and
provides the most accurate data. Th, however, has
*Corresponding author (e-mail: [email protected])
465
466
S. Fukuda and S. Nakai
These spikes are appropriate for accurate analysis. However, purchase of these materials requires
special permission from the government in Japan.
Nakai et al. (2001) determined the abundances of
U and Th by using an ICP-MS with a quadrupole
mass analyzer. They have estimated errors for U/
Th concentration ratio about 5%. The large errors
for U/Th concentration ratio give rise to large dating errors. In this study, we prepared moderately
enriched 230Th spike from natural U solution, and
used a U reagent depleted in 235U as an U spike.
We will report the precision of isotope dilution
analysis using the in-house prepared spikes and
the results of radioactive disequilibrium analysis
for standard rock samples distributed by Geological Survey of Japan and by Geological Survey of
the United States.
EXPERIMENTAL
Reagents and samples
All experiments were carried out in a clean
room under condition of class 1000. Highly purified acids, hydrochloric acid (Kanto, 18078-1B),
nitric acid (Kanto, 28163-1B), hydrofluoric acid
(Kanto, 18083-1B), and perchloric acid (Kanto,
32059-1B) were used without further purification.
Boric acid (Merck, Suprapur) was used as obtained. The water used in this study is 18.3 MΩgrade, which was prepared by a Millipore purification system.
Eight standard rock samples distributed by
Geological Survey of Japan (Ando and Shibata,
1988) and an AGV-1 distributed by Geological
Survey of the United States (Flanagan, 1973) were
analyzed for 238U/230Th radioactive disequilibrium
measurements. Five samples among nine, JB-1,
JA-2, JR-1, JR-2 and AGV-1 are older than 350
Ka and they are expected to attain radioactive
equilibrium.
Preparation of U and Th spike solution
The abundance measurements of U and Th
were carried out by isotope dilution analysis. A
reagent of depleted U (U(NO3)4, Fluka chemika,
94270), was used as an U spike. The 230Th spike
was prepared by separation of 230Th from natural
U. This separation method is the same as that used
for Th separation from rock samples. The natural
U of ca. 50 mg was dissolved in a Teflon beaker
with 1 ml of 7N HNO3. This sample solution was
loaded onto the column with 1 ml of anion exchange resin (BioRad AGX1-8). The U was eluted
with 12.5 ml of 7N HNO3, and the Th was collected with 5 ml of 9N HCl. The 230Th spike solution is treated twice to ensure complete separation from U. The two solutions, depleted U and
purified 230Th solution were mixed, and the isotope ratios of 235U/238U and 230Th/232Th and the
abundances of U and Th were measured.
Isotopic composition of U in the mixed spike
was determined by static-multi analysis using a
sector ICP-MS, IsoProbe, described later. A mass
fractionation correction was made using a natural
U solution. Isotopic composition of Th was determined by peak jump analysis using a Daly multiplier with IsoProbe. A mass fractionation correction was also made using a natural U solution. The
isotope ratios of 235 U/ 238U and 230Th/ 232Th are
0.002339 ± 0.40% (n = 6, 2σ) and 0.04180 ± 1.6%
(n = 10, 2σ ), respectively (Table 1 and Fig. 1).
The data whose deviations from the average are
larger than 2σ are excluded from calculations. The
abundances of U and Th were measured by inverse
isotope dilution using standard solutions, whose
isotope ratio and abundance were known. The
concentrations of U and Th are 2.47 ± 0.44% (n =
5, 2 σ) and 0.0215 ± 0.75% (n = 5, 2σ), respectively.
Sample decomposition
We modified an analytical scheme described
in Nakai et al. (2001). Rock powders of 50 to 100
mg were weighed into screw cap Savillex beakers. Rock samples were digested sequentially by
HF (48w%, 1.0 ml)/HClO 4 (30w%, 0.3 ml) and
HCl (30w%, 0.5 ml)/H 3BO3 (2.0w%, 0.5 ml). The
thorium fluoride that remained after HF/HClO 4
acid attack was dissolved by HCl/H 3BO3 (Turner
et al., 1996). The solution was dried down and redissolved with 10 ml of 2w% HNO3, which was
completely clear. This HNO3 solution was divided
238
U/ 230Th disequilibrium measurement for volcanic standard rock samples
467
Table 1. Results of U and Th isotopic analysis for spike
solution
No.
[ 2 3 5 U/2 3 8 U] iso to p e ratio *
[ 2 3 0 Th/2 3 2 Th] iso to p e ratio *
1
2
3
4
5
6
7
8
9
10
0.0023396 ± 0.0000019
0.0023386 ± 0.0000016
0.0023440 ± 0.0000015
0.0023322 ± 0.0000017
0.0023344 ± 0.0000019
0.0023424 ± 0.0000015
0.04244 ± 0.00034
0.04221 ± 0.00027
0.04189 ± 0.00028
0.04166 ± 0.00038
0.04158 ± 0.00038
0.04134 ± 0.00022
0.04150 ± 0.00030
0.04201 ± 0.00025
0.04184 ± 0.00034
0.04157 ± 0.00027
mean
0.0023386 ± 0.0000091
0.04180 ± 0.00068
(a)
*Errors are 2 σ.
into two aliquots with weight, one of which is used
for isotope dilution analysis and another for isotope ratio measurements, with the proportion of
one to four, respectively. More amounts of solutions are required for the isotope ratio measurements.
Chemical separation
The U and Th in solutions were separated from
rock matrix by anion exchange resin (BioRad
AGX1-8). We put 1 ml of resin into heat-shrinkable FEP columns with an i.d. of 8 mm, a length
of 80 mm and a polyethylene frit. Resin was
cleaned with 9N HCl and Milli-Q water
sequentially and was conditioned with 7N HNO3.
The spiked and un-spiked sample solutions after
dissolution were dried and re-dissolved with 1 ml
of 7N HNO3 and then loaded on to the resin. Almost all of the elements except U and Th were
eluted with 2.5 ml of 7N HNO3. U was then eluted
by the addition of 10 ml of 7N HNO3. Finally, Th
was collected with 5 ml of 9N HCl. Recovered
solutions were dried down and dissolved in 2%
HNO3 for ICP analysis. This separation method
was calibrated using JB-1 and JB-2. The U and
Th fractions from the two samples were analyzed
using an ICP-MS with a quadrupole mass analyzer
(PQ3, ThermoElemental, Winsford, UK), the recoveries of U and Th were calculated to 80–90%.
This separation protocol can handle a sample of
(b)
Fig. 1. Results of isotopic analysis of spike solution.
(a) Results of [235U/238U]isotope ratio. (b) Results of [230Th/
232
Th]isotope ratio. Errors are 2σ.
up to 80 mg. The blanks of U and Th for the total
chemical procedure are 40 and 5 pg, respectively,
which were determined by measurements using
the PQ3.
Instrumentation and data correction
We used a hexapole-interfaced multiple-collector Inductively Coupled Plasma Mass
Spectrometer (IsoProbe, MICROMASS, Manchester, UK) for U and Th isotope dilution analysis and Th isotope measurements. Nakai et al.
(2001) described this instrument and a method for
mass fractionation correction and only a brief summary is given here.
IsoProbe is a multi-collector magnetic sector
mass spectrometer with an ICP ion source, and
uses a collision cell instead of an energy analyzer.
The collision cell is applied to reduce energy
spread of ions from the plasma ion source. This
instrument is equipped with eight Faraday collectors for the simultaneous acquisition of multiple
isotopes. A Daly detector is situated behind the
468
S. Fukuda and S. Nakai
axial Faraday. Ions passing onto this collector can
be energy filtered using a WARP (Wide Angle
Retarding Potential) filter. This filter increases the
abundance sensitivity, high enough to cut the tail
of large 232Th for 230Th acquisition. The abundance
sensitivity is 74 ppb, determined by measuring the
contribution of tailing of mass 238 on mass 237
using a uranium standard solution. This abundance
sensitivity is high enough for thorium isotope
analysis on typical volcanic rock samples (230Th/
232
Th > 1 × 10–6). Faraday cups and electrometers
with 10 11 Ω feed-back resistors were used for acquisition of 232Th, 235U and 238U. The gains of the
amplifier associated with each Faraday collector
were calibrated with respect to axial Faraday collector. No correction for collector efficiency was
made in this study.
All measurements were performed with a
desolvating nebulizer (Aridus, Cetac, Omaha,
Nebraska, USA). Sample uptake rate was about
50 µl/min. A sample solution of 0.6 ml was required for acquisition of 50 ratios including time
for stabilizing signals.
Measurements of U and Th isotope ratio were
performed by a multi-static mode. For U analysis, 234U was measured by a Daly and 235U and
238
U were measured by Faraday collectors. For Th
analysis, 230Th and 232Th were measured by a Daly
and a Faraday collector, respectively.
Mass fractionation introduced by an ICP ion
source is about an order of magnitude larger than
that of TIMS. In addition, a Daly/Faraday gain
factor is required to obtain a thorium isotope ratio. We used our in-house natural U standard for
the two corrections. The isotope ratios of 234U/
238
U and 235U/238U of this natural U standard were
assumed to be 0.0000554 and 0.00725, respectively. Mass fractionation for isotope dilution
analysis of U was estimated by measuring [ 235U/
238
U]isotope ratio of the standard solution before and
after each sample measurement. For Th isotopic
measurements, mass fractionation corrected [234U/
238
U]isotope ratio of the natural U standard solution
was used to determine a relative Daly-Faraday
gain factor. In addition, mass fractionation effect
for Th was corrected by measuring [ 235 U/
238
U]isotope ratio of the standard solution before and
after each sample measurement. It is known that
a mass bias factor of ICP-MS depends on mass
number of the nuclides to be analyzed, and elements of similar atomic mass number exhibit similar mass bias factor (Hirata, 1996). Therefore, we
can make a mass fractionation correction for Th
isotope measurement using the factor determined
by the U standard.
RESULTS AND DISCUSSION
Measurement of U isotope ratio
We routinely measured [235 U/ 238U] isotope ratio
and [ 234U/238U] isotope ratio of a 30 ppb natural U
standard solution for mass fractionation correction and to determine Daly/Faraday gain factor.
The 30 ppb standard solution gave about 4.5 ×
10 –11 A signal for 238U.
Repeated analysis during a day gave the results for [235U/238U]isotope ratio and [234U/238U]isotope
ratio of 0.007173 ± 0.12%(2 σ ), 0.0000437 ±
2.4%(2 σ ), respectively. Mass fractionation and
Daly/Faraday Gain factor corrections were made
using the results of the U standard solution before
and after a sample measurement. The variations
of the two ratios adjacent to measurements were
smaller than the variations during whole day.
When the variation was large, the data was rejected
and the sample was re-analyzed.
Fig. 2. Results of Th isotopic analysis of a standard
rock JR-1. Each symbols, 䉬, 䊐, 䉱, 䉫 and 䊏 show the
results for 1 ppb, 5 ppb, 10 ppb, 30 ppb and 50 ppb,
respectively. Error for each analysis and mean are 2σ
and 2 σmean, respectively.
238
U/ 230Th disequilibrium measurement for volcanic standard rock samples
Measurement of Th isotope ratio
A 50 ppb solution was usually used for Th isotope analysis. This concentration gave about
8.0 × 10–11 A for 232Th, 1.1 × 10–16 –3.2 × 10–16 A
for 230 Th. The background for the mass number
of 230 was about 1.6 × 10–18–3.2 × 10–18 A, and
counted before measurements for 60 seconds and
it was subtracted from the intensity of 230Th of a
sample. Fifty ratios of [232Th/230Th]isotope ratio were
acquired in 12 minutes that consume 30 ng of Th.
Table 2 and Fig. 2 show the results of [ 232Th/
230
Th]isotope ratio analysis for JR-1 solutions with
different concentrations. Internal precision of an
individual measurement and repeatability for 50
ppb solution within a day is about 0.3% and 0.5%,
respectively. We analyzed 1, 5, 10 and 30 ppb solutions as well, to investigate the required Th concentration for accurate analysis. The error associated with the mean values for each concentration
was 2σ.
The results from 5, 10 and 30 ppb analysis were
consistent with that for the 50 ppb solution although internal precision and repeatability were
slightly deteriorated. The precision of 1 ppb analysis is low due to weak 230Th signal compared with
the background. This indicates that we can reduce
the concentration of sample solution down to 5
ppb, thus we can measure [232Th/230Th]isotope ratio
using 3 ng Th. The Th isotope ratios of JB-1 and
AGV-1 (Table 4) are in good agreements with the
TIMS measurement by Reid and Ramos (1996),
confirming the accuracy of the present method.
The precision of isotope dilution analysis
The precision of abundance determinations by
isotope dilution method is influenced by an enrichment factor of the spike and spike to sample
ratio. The relation between precision of an abundance determination (|dC/C|) with that of an isotope ratio measurement (|dR/R|) is described by
Table 2. Results of Th isotopic analysis for a standard rock JR-1
No.
Intensity of 2 3 2 Th (×10– 1 1 A)
[ 2 3 2 Th/2 3 0 Th] iso to p e ratio *
50 ppb
1
2
3
mean
5.73
5.90
5.91
183900 ± 600
184700 ± 700
184700 ± 600
184400 ± 920
30 ppb
1
2
3
mean
3.66
3.74
3.79
184500 ± 700
184600 ± 800
185000 ± 800
184700 ± 530
10 ppb
1
2
3
4
mean
1.35
1.26
1.28
1.38
184300 ± 1300
184200 ± 1000
186100 ± 800
185500 ± 1400
185000 ± 1860
5 ppb
1
2
3
4
mean
0.68
0.67
0.67
0.70
186100 ± 2000
185800 ± 1900
184000 ± 1800
185700 ± 1600
185400 ± 1900
1 ppb
1
2
3
mean
0.15
0.16
0.15
184900 ± 4100
183500 ± 3600
185400 ± 3400
184600 ± 2000
Th concentration
*Errors are 2 σ.
469
470
S. Fukuda and S. Nakai
the following equation (Colby et al., 1981).
dC
dR
=F
C
R
(1)
where F is an error multiplication factor. When
enriched 235U or artificially produced 236U is used
as a spike, the F value is close to unity. However,
in Japan, permission from the government is required to handle these U spikes. The F value, when
depleted U is used as a spike, is larger than that
when an enriched isotope or an artificially produced isotope is used. The F value can be theoretically calculated by the following equation:
F=
( Rsp − Rn )R .
( R − Rn )( Rsp − R)
(2 )
In the case of U, Rn is [235U/ 238U]isotope ratio of the
sample, Rsp is [235U/238U] isotope ratio of our spike,
respectively, and R is [235U/ 238U] isotope ratio of a
spike-sample mixed solution. The F value changes
according to the proportion of sample and spike
in the mixture. The analysis of our in-house spike
solution is optimized when the Ms/Mn value is
1.75, where Mn and Ms are amounts of uranium
for sample and spike in the mixture, respectively.
The F value then is still rather large around 3.6.
On the other hand, the F values of Th isotope dilution using our in-house spike solution are closer
to unity for large range of the Ms/Mn from 0.001
to 0.1.
The errors for quantitative analysis by isotope
dilution are estimated from the value F and precision of isotope analyses. The results of five analyses on JB-2 are shown in Table 3. The F value for
U is large, and that for Th is small. On the other
hand, the precision of [235U/238U]isotope ratio measurements is better than that of [232Th/230Th] isotope
ratio measurements. This is due to instability of
Daly/Faraday Gain factor. Accordingly, the error
for U determination is similar to that of Th. For
precise measurements, the amount of spike to be
added to a sample is important. We estimated the
concentration of uranium in samples by using an
ICP-MS PQ3 prior to the dilution analyses. In
Table 3, each error associated with U and Th abundances was calculated with Eq. (1). The errors
associated with mean values of U and Th abundances and U/Th abundance ratio were 2 σ. The
error of the mean value of U abundance is more
than three times of errors associated with each
measurement. The errors of the mean values for
Th and U/Th abundance ratio are comparable to
that of each measurement. The reproducibility of
our analytical method can be estimated by the
experiments on JB-2 and the error of ( 238 U/
232
Th)activity ratio was estimated to be less than 2.3%,
although we cannot evaluate if the problem of
sample heterogeneity may reduce the repeatability.
The error of (230Th/232Th)activity ratios was estimated
as 0.5–2% by the stability of the Daly/Faraday
Gain factor from measurements of the natural uranium standard solution before and after sample
analysis.
Table 3. The results of JB-2 analysis by isotope dilution
*Ms/Mn is the ratio of spike (Ms) and sample (Mn) in mixed solution.
**Errors are 2σ .
238
U/ 230Th disequilibrium measurement for volcanic standard rock samples
238
U/230Th measurement results from standard rock
samples
Nine geological standard samples were
analyzed for 238U/230Th disequilibrium measurements. The results are shown in Table 4 and Fig.
3. The radioactivity ratios were calculated from
isotopic abundances with the following decay constants, λ 230 Th = 9.195 × 10–6 yr–1 (Cheng et al.,
2000), λ 232 Th = 4.933 × 10 –11 yr –1 , λ 238 U =
1.551 × 10–10 yr –1 (Firestone, 1999).
The radioactive disequilibrium between 238U
and 230Th results from chemical fractionation between U and Th. The radioactive disequilibrium
will return to secular equilibrium by 350 Ka after
the fractionation. Thus, the volcanic rock sample
whose eruption time is more than 350 Ka, is expected to attain secular equilibrium, whether or
not it had attained disequilibrium at the eruption
time. The volcanic rocks in secular equilibrium
should have equal ( 238 U/ 232 Th) activity ratio and
( 230 Th/ 232Th) activity ratio in a ( 230 Th/ 232 Th) activity
238
U/ 232Th)activity ratio diagram such as
ratios versus (
Fig. 3. They should fall on a line with a slope of
unity, called the “equiline” (Dickin, 1995).
Table 4. Results of
U (ppm)
238
471
The eruption ages of nine geological standard
samples are also shown in Table 3. The eruption
ages of AGV-1 and JA-1 have not been reported.
However, the K-Ar ages of andesite samples from
Guano Valley, Oregon, USA, where AGV-1 was
collected, was dated to be 14–17 Ma (Sawlan et
al., 1995). The age of andesite, which was collected from the same lava as JA-1, was similarly
determined by the K-Ar dating method and found
to be 180–270 Ka (Hirata, 1999).
Four of the nine samples, JA-2, JB-1, JR-1,
JR-2 are older than 350 Ka and they should attain
radioactivity equilibrium between 230Th and 238U.
Reid (1995) in addition, reported that AGV-1 attained radioactive equilibrium, when analysed
using TIMS. Thus, these five samples should be
plotted on the equiline in Fig. 3. In the results
obtained, these samples are plotted on the equiline
within the error range. The data of first analysis
on JA-2 did not fall on this line. It is likely that
thorium fluoride was not decomposed thoroughly
before spike addition and resulted in the deviation. The data for JA-2, treated with HCl/H3BO3
after HF/HClO4 attack, plotted well on the equi-
U/230 Th analysis for nine standard rocks
Th (ppm)
( 2 3 8 U/2 3 2 Th) activ ity ratio *
[ 2 3 2 Th/2 3 0 Th] iso to p e ratio **
( 2 3 0 Th/2 3 2 Th) activ ity ratio
6.29
4.88
9.16
26.5
31.7
0.966
1.438
0.577
1.017
1.052
201500 ± 2820
133200 ± 1680
338100 ± 4730
180600 ± 1440
172000 ± 880
0.925 ± 0.013
1.398 ± 0.018
0.558 ± 0.008
1.032 ± 0.008
1.083 ± 0.006
—a)
14.1 Ma b )
7.59 Ma b )
0.81 Ma b )
0.60 Ma b )
0.74
3.17
0.22
1.27
1.504
0.980
1.896
1.159
136700 ± 450
200800 ± 2100
152900 ± 1960
174700 ± 2880
1.363 ± 0.004
0.928 ± 0.010
1.219 ± 0.016
1.067 ± 0.018
0.18-0.27 Ma c)
1783d )
1950-1951d )
864d )
133000 ± 2020
1.402 ± 0.021
14.1 Ma b )
Eruption age ***
equilibrium samples
AGV-1
JA-2
JB-1
JR-1
JR-2
1.94
2.25
1.69
8.64
10.7
disequilibrium samples
JA-1
JA-3
JB-2
JB-3
0.35
1.00
0.16
0.47
equilibrium sample (without HCl/H3 BO3 treatment)
JA-2
2.39
4.95
1.509
*Errors for (238U/ 232Th) ratio are estimated as 2.3% (2σ ) from five analysis of JB-2.
**Isotope ratio. Reid and Ramos (1996) reported 232Th/ 230Th ratio of JB-1 and AGV-1 measured by TIMS, 337090 ± 2.2% (n =
3) and 202150 ± 1.2% (n = 3), respectively.
***References; a) Sawlan et al. (1995) and Reid (1995), b) Uchiumi et al. (1989), c) Hirata (1999), d) Ando and Shibata
(1988).
472
S. Fukuda and S. Nakai
Fig. 3. (230Th/ 232Th) versus (238U/232 Th) diagram for nine standard rock samples analyzed in this study. The bold
line is the equiline (see text). The five samples older than 350 Ka (filled circles) are plotted on the equiline. The
open circle is JA-2 data which was not treated with HCl/H3BO 3. The other four samples younger than 350 Ka
(open square) were plotted to the right of the equiline.
librium line. In following analyses, we used HCl/
H3BO3 attacks for ( 238U/230Th) activity ratio measurements. The accuracy of our analysis was proved,
by that the five rock samples older than 350 Ka
attained secular equilibrium in Fig. 3.
Four samples, JA-1, JA-3, JB-2 and JB-3 are
younger than 350 Ka. These samples are the products of island arc volcanisms. It has been reported
that young volcanic rocks from island arcs have
radioactive disequilibrium between 238 U and
230
Th. In many cases, they are enriched in 238U
relative to 230Th, (238U/230Th)activity ratio > 1 (Turner
et al., 2000). The disequilibrium was considered
to result from the following process. Island arc
volcanisms are triggered by addition of fluids from
subducting slab and overlying sediment, which
lowers the solidus temperature of wedge mantle.
The release of the fluids results in enrichment of
U since it is more soluble relative to Th. Four
young standard rock samples analyzed here have
similar characteristic to the study of other arcs.
The chronological significance of the disequilibrium of the samples will be discussed in coming
report.
CONCLUDING REMARKS
We have developed a technique for the (238U/
230
Th) activity ratio measurements for volcanic rock
samples using a MC-ICPMS. The required amount
of Th for isotope ratio measurement was reduced
to 3 ng. The abundances of U and Th were
analyzed by isotope dilution using in-house prepared spike solution. The error for U/Th concentration ratio is estimated at about 2%. The accuracy of this measurement method was certified by
the results of the analysis on the standard rock
samples that should attain radioactive equilibrium.
Acknowledgments—We are grateful to Prof. Setsuya
Nakada for supporting this work. We thank Dr. Zenon
Palacz for advice on analysis. We would like to thank
Dr. Yu Vin Sahoo, Dr. Keiji Misawa and Dr. Noriko
Kita for improving this paper. Thanks are due to Prof.
Ichiro Kaneoka, Dr. Kozo Uto and Dr. Yukiko Hirata
for information of eruption ages to igneous standard
rock samples, and due to Dr. Sarat Kumar Sahoo for
information of decay constants. We especially thank
Dr. Yoshiro Nishio, Dr. Takeshi Hanyu, Dr. Junji
Yamamoto, Mr. Yasunobu Maeda and Dr. Yuji Orihashi
for advices to experimental technique. This research
was partly supported by JSPS Post-doctoral Fellowship
238
U/ 230Th disequilibrium measurement for volcanic standard rock samples
for Foreign Researchers in Japan and a grant-in-aid for
scientific research to SN from the Ministry of Education, Culture, Sports, Science and Technology of
Japan. This research was also partly supported by Unzen
Scientific Drilling Project.
REFERENCES
Ando, A. and Shibata, K. (1988) Isotopic data and rare
gas compositions of GSJ rock reference samples, “Igneous rock series”, 1988. Geochem. J. 29, 91–95.
Cheng, H., Edwards, R. L., Hoff, J., Gallup, C. D.,
Richards, D. A. and Asmerom, Y. (2000) The halflives of uranium-234 and thorium-230. Chem. Geol.
169, 17–33.
Colby, B. N., Rosecrance, A. E. and Colby, M. E. (1981)
Measurement parameter selection for Quantitative
isotope dilution gas chromatography/mass
spectrometry. Anal. Chem. 53, 1907–1911.
Dickin, A. P. (1995) Radiogenic Isotope Geology. 330–
359, Cambridge Univ. Press.
Firestone, R. B. (1999) Table of Isotopes. 8th ed., 206–
211, Oxford Sci. Public.
Flanagan, F. J. (1973) 1972 values for international
geochemical reference samples. Geochim.
Cosmochim. Acta 37, 1189–1200.
Goldstein, S. J., Murrell, M. T. and Janecky, D. R.
(1989) Th and U isotopic systematics of basalts from
the Juan de Fuca and Gorda Ridges by mass
spectrometry. Earth Planet. Sci. Lett. 96, 134–146.
Halliday, A. N., Lee, D. C., Christensen, J. N.,
Rehkämper, M., Yi, W., Luo, X., Hall, C. M.,
Ballentine, C. J., Pettke, T. P. and Stirling, C. (1998)
Applications of multiple collector-ICPMS to
cosmochemistry,
geochemistry,
and
paleoceanography. Geochim. Cosmochim. Acta 62,
919–940.
Hirata, T. (1996) Lead isotopic analyses of NIST standard reference materials using multiple collector inductively coupled plasma mass spectrometry coupled
with a modified external correction method for mass
discrimination effect. Analyst 121, 1407–1411.
Hirata, Y. (1999) Developmental history of
Hakonevolcano, Japan. Res. Rep. Kanagawa Prefect.
Mus. Nat. Hist. 9, 153–178. (in Japanese with English abstract).
Ivanovich, M. and Harmon, H. S. (eds.) (1992) Ura-
473
nium-Series Disequilibrium. Oxford Sci. Public.
Joannon, S., Telouk, P. and Pin, C. (1997) Determination of U and Th at ultra-trace levels by isotope dilution inductively coupled plasma mass spectrometry
using a geyser-type ultrasonic nebulizer: application
to geological samples. Spectrochim. Acta 52, 1783–
1789.
Luo, X., Rehkämper, M., Lee, D. C. and Halliday, A.
N. (1997) High precision 230Th/232 Th and 234U/238 U
measurements using energy-filtered ICP magnetic
sector multiple collector mass spectrometry. Int. J.
Mass Spectrom. Ion Processes 171, 105–117.
Nakai, S., Fukuda, S. and Nakada, S. (2001) Thorium
isotopic measurements for silicate rock samples by
multi-collector Inductively Coupled MassSpectrometer. Analyst 126, 1707–1710.
Reid, M. R. (1995) Processes of mantle enrichment and
magnetic differentiation in the eastern Snake River
Plain: Th isotope evidence. Earth Planet. Sci. Lett.
131, 239–254.
Reid, M. R. and Ramos, F. C. (1996) Chemical dynamics of enriched mantle in the southwestern United
States: Thorium isotope evidence. Earth Planet. Sci.
Lett. 138, 67–81.
Sawlan, M. G., King, H. D. and Plouff, D. (1995) Mineral resources of the Spaulding wilderness study area,
Lake and Harney Countries, Oregon. U.S. Geol. Surv.
Bull. 1738, E1–E18.
Turner, S., Howkesworth, C., van Calsteren, P., Heath,
E., Macdonald, R. and Black, S. (1996) U-series isotopes and destructive plate margin magma genesis
in the Lesser Antilles. Earth Planet. Sci. Lett. 142,
191–207.
Turner, S., Howkesworth, C., Rogers, N., Bartlett, J.,
Worthibton, T., Hergt, J., Pearce, J. and Smith, I.
(1997) 238U-230Th disequilibria, magma petrogenesis,
and flux rates beneath the depleted Tonga-Kermadec
island arc. Geochim. Cosmochim. Acta 61, 4855–
4884.
Turner, S., George, R. M. M., Evans, P. J.,
Howkesworth, C. and Zellmer, G. F. (2000) Timescales of magma formation, ascent and storage beneath subduction-zone volcanoes. Phil. Trans. R. Soc.
Lond. A 358, 1443–1464.
Uchiumi, S., Uto, K. and Shibata, K. (1989) K-Ar ages
of rock reference materials. Mass Spectroscopy 37,
375–381 (in Japanese).