Precise measurement of cerium isotope composition in rock samples

Chemical Geology (Isotope Geoscience Section), 94 (1991) I-II Elsevier Science Publishers B.V., Amsterdam

Precise measurement of cerium isotope composition in rock samples Akio Makishima and Eizo Nakamura Institute for Study ofthe Earth's Interior, Okayama University, Misasa, Tottori-ken 682-01, Japan (Received November 13, 1990; revised and accepted June 12, 1991)

ABSTRACT Makishima, A. and Nakamura, E., 1991. Precise measurement of cerium isotope composition in rock samples. Chern. Geol. (Isot. Geosci. Sect.), 94: I-II. A high-precision analytical method for the measurement of 138Ce/142Ce ratios is reported using static multicollection mass spectrometry. This technique reduced the data acquisition time for 2 hr. for 400 ratios and improved analytical reproducibility to ±0.002% (11= 16) and precision to ±0.002-0.003%. The better precision and reproducibility were established collecting a large ion beam [I42Cel60 of (2-7) '10- 11A], short data acquisition time and in situ measurement of 180 /160 ratios during the analysis. To reduce the blank effect to the Ce isotope analysis, the chemical procedure for separation ofCe was refined using a small ion-exchange resin bed column (4 ern length X 3 mm diameter) with which the procedural total blank was lowered to 0.04 ng and the recovery yield of Ce from 20 mg BCR-I was 900/0. In order to confirm the reproducibility of this technique including the chemical procedure, six Ce isotope analyses individually separated from the USGS standard BCR-I, were carried out with an analytical reproducibility of ±0.002%. With these analytical precision and reproducibility and normalization to BCR-I in order to eliminate any inter-laboratory biases, it is now possible to apply the La-Ce isotope system to the terrestrial and extraterrestrial samples combined with other isotope systems, such as Sm-Nd and Rb-Sr.

1. Introduction

Tanaka and Masuda (1982) initially applied the La-Ce isotope system to geochronology and geochemical tracers, and stressed that this system is advantageous in understanding the detailed evolution of light rare-earth elements (LREE) combined with the Sm-Nd system. Furthermore, the La-Ce system is a powerful tracer to investigate the origin of Ce anomaly in LREE patterns observed in manganese nodules, island arc volcanic rocks and meteorites. However, the La-Ce system has not been generally appreciated as a geochemical tracer (e.g., Allegre, 1987) because of difficulties in obtaining the precise isotopic analyses that are required.

The abundance of the radiogenic daughter isotope 138Ce is only 0.25% compared to the abundance of the non-radiogenic Ce isotopes such as 140Ce (88.4%) and 142Ce (11.1%). Furthermore, the parent isotope, 138La is also of low abundance (0.090%) and possesses a long total half-life (9.6'10 10 a) and p-decay partial half-life (2.5'10 11 a) (Masuda et aI., 1988). Hence, in the La-Ce. system, variation in the 138Cejl42Ce ratio is limited and overall variation is extremely small compared to other isotope systems. For these reasons, it has been difficult to determine the 138Cejl42Ce ratio with a comparable relative analytical error with those ofNd and Sr isotope analyses. This is essential if the La-Ce system is used as a geochemical tracer combined with the Sm-Nd or

0168-9622/91/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

2

Rb-Sr system. A long-lived intense and stable ion beam is a prerequisite in obtaining precise isotope analysis in mass spectrometry. For a J 38Ce ion beam current of 3.10- 13 A, a total Ce ion beam of 1.2'10- 10 A is required. Such a large total beam current hastens consumption of Ce sample on the filament in the mass spectrometer. Therefore, a large amount of Ce sample of > I Jig has to be loaded onto the filament. Even if the large sample loading is made for Ce isotope analysis, it takes long time for data acquisition to reduce the analytical error because of the very low abundance of 138Ce. For example, Dickin et al. (1987) collected more than 1000 ratios taking 16-30 hr. for one run. Several years ago, a multicollector mass spectrometer with variable Faraday cups appeared on the commercial market (e.g., Wagner et aI., 1984) and the static multicollection technique has been generally used in Sr, Nd and Pb isotope analyses. In the static multicollection technique, all isotopes concerned are measured simultaneously. Compared with the single-collector peak jumping technique, therefore, the static multicollection technique is advantageous in that: ( I ) data acquisition time is short so that less sample size is available to be analyzed; and (2) time drift correction for ion beam decay is not necessary. The Ce isotope analysis with the static multicollection was pioneered by Allan (1988) . Although he reduced the data acquisition time to 2-4 hr., the long-term precision ofthe analytical data was still poor to apply the La-Ce system to natural samples in which the 138Ce/142Ce ratio varies only from - 3 to + 5 e-units (0.08%). Complete separation of Ce from the other REE is also an important factor to obtain precise Ce isotope composition. Isobaric interferences from elements such as Ba, La, Pr and Nd must be measured for the interference corrections, which cause the error magnification in mass spectrometry. Furthermore, the procedural total blank of Ce in the chemical separation shifts the isotopic composition of the

A. MAKISHIMAAND E. NAKAMURA

sample and consequently enlarges the analytical error. In general, for the separation of Ce from the other REE, a long column for ion-exchange chromatography is used. Dickin et al. (1987) used a 30-cm-IongX2.5-mm-diameter column containing '" 1.5 ml of resin. Such a long column requires a long separation time and a large eluate volume, and increases the procedural Ce blank from reagents and environment. Therefore, it is necessary to overcome these problems in the Ce chemical separation to get precise Ce isotope analysis. We have established the static multicollection technique for the Ce isotope analysis with significant improvements in the analytical precision and reproducibility, and reduced the data acquisition time. We have also improved the chemical procedure suitable for the separation ofCe using a smaller column. In this paper, we describe the detailed method of the Ce isotope analysis for rock samples and discuss the reliability of the Ce isotope data.

2. Analytical techniques 2. J. Cerium isotope standards The Ce standard reagent JMC 304 prepared by Tanaka and Masuda (1982) and the USGS standard rock powder BCR-J were used to evaluate analytical precision, reproducibility and accuracy of the static multicollection method and chemical separation of Ceo Ce isotope ratios ofJMC 304 and BCR-J which have been reported so far are given in Table I with correction factors used in the analyses.

2.2 . Chemical procedure A Ce separation procedure of modified from Tanaka and Masuda (1982) was used in this study. Details of the chemical procedure for REE separation will be described elsewhere (Shibata et aI., in prep.). 20 mg ofBCR-J (containing", I JlgCe) were

3

PRECISE MEASUREMENTS OF Ce ISOTOPE COMPOSITION IN ROCK SAMPLES

TABLEI Ce isotope ratios of JMC 304 and BCR-l Correction factors

JMC304:

Shimizu et at. (1984) Makishirna et at. (1987)

0.0228559 ±0.000000 11 0.0225762 ±0.000000 14

0.01720 0.01688

0.00204 0.002129

7.992 7.947

BCR-l: Tanakaetat. (1987)

0.0228520 ± 0.000000 10

0.01720

0.00204

7.992

decomposed by a mixed acid solution consisting of HF, HCI0 4 , HCl and HN0 3 • The sample solution was evaporated to dryness and HCI0 4 was added. Then the sample was dried, dissolved with 1 ml 4 N HCl and loaded onto a 1 ml AG50W-X12® resin bed (200 to 400 mesh, H + -form) charged in a polypropylene column (5 mm in diameter X 5 em in length).The schematic chromatogram for Fe, Mg, Ca, Sr and REE of this column is shown in Fig. la. Major elements (Fe, Mg, Ca) and Sr were eluted with 10 m12.8 NHCl. Light rareearth elements (Sm, Nd, Pr, Ce and La) were collected by the subsequent addition of 13 ml

(a)

.

4N HCI

6N HCI

HCI

:=:J Fe

~Mg

C==:J Ca I I Sr

REE

I

o

5

10

15

20

ml of eluate

(b) 0.3"1HIBA

0.2"1HIBA

o

Sm

c=:J Nd c:::J Ce c=::J La

6NHCl.

The REE fraction was dried and then dissolved with 0.06 ml of 0.06 N HCl. The solution was subsequently loaded onto a 0.3-ml AG50W-X8® resin bed (200 to 400 mesh, NH 4 + -form) in a Teflon ® column (3 mm in diameter x-t em in length). As shown in the schematic chromatogram of LREE in Fig. 1b, Sm and Nd were eluted with 7 ml of 0.2 M HIBA (a-hydroxyisobutyric acid solution, adjusted pH 4.5 with NH 3 ) . Ce was then recovered with 1.2 ml of0.3 M HIBA. In this elution sequence, Ce was perfectly separated from Ba with this column, because Ba does not make a stable complex with HIBA at this pH. After dryness, one drop of HN0 3 was added to the Ce sample to change into the nitric form followed by evaporation to dryness. In the whole chemical procedure, total blank and recovery

2.8N

o

5 10 ml of eluate

Fig. I. Schematic chromatograms for ion-exchange resin bed columns. a. Chromatogram of Fe, Mg, Ca, Sr and REE for the first column. b. Chromatogram of Sm, Nd, Ce and La for the second column.

yield of Ce were 0.04 ng and 90%, respectively, during this study. 2.3. Mass spectrometry

A Finnigan" MAT261 multi collector mass spectrometer was employed in this study. This mass spectrometer is equipped with four variable Faraday cups and a fixed one. In order to

4

A. MAKISHIMA AND E. NAKAMURA

eliminate the difference in the characteristics between amplifiers connected to individual collectors, gain factors of amplifiers of each collector are measured, supplying a uniform current into the amplifiers. In the ion source section a cold trap with liquid air was used. The Re double filament technique was applied to determine the Ce isotope ratios using oxide ions. One pg of Ce sample was dissolved with 1 pI of pure water and loaded on the evaporation filament with phosphoric acid. The ionization filament current was kept at 4.0 A (2000 K) and the pressure of ion source was maintained at 2.10- 5 Pa. During data acquisition, the evaporation filament current was kept constant at 1.2-1.4 A so that the intensity ofthe Ce ion beam slowly decreased. At the beginning of the measurement, the ion current of 142CeI60was increased to (4-7) .10- 11A. The ion current normally came down to around 2· 10- 11 A at the end of data acquisition after 2 hr. The cup configurations in the Ce isotope analysis are shown in Table II. At position 1, 152( 136Ce I60), 154( 138Ce I60), 158e 42CeI60), 15ge 4JNd 160) and 142CeISO) 160( were collected simultaneously with five collectors, COL 5, COL 4, COL 3, COL 2 and COL 1, respectively. 153e 37Ba160) and 157e 41Pr160) were collected with COL 4 and COL 3 at position 2 for Ba and Pr interference corrections, respectively. Baselines for COL 5, COL 4, COL 3, COL 2 and COL 1 were measured simultaneouslyat 151.5, 153.5, 157.5, 158.5 and 159.5, respectively. In these cup configurations,

140Cel60 was not collected with any collector in order to avoid collector damage and memory effect from an extremely large ion beam of 140Ce160. This consequently allows to increase an operating total ion beam current in the Ce isotope analysis. One isotope measurement run consisted of 400 ratios in 20 blocks. Counting time for one ratio (scan) was 8 s. Baselines and interferences were measured in the baseline position and position 2, respectively, before and after each block. Idling and counting time for the baselines and interferences were 8 and 16 s, respectively. Gain factors of each collector were measured at the beginning of the measurement. The data acquisition time for one Ce isotope analysis was 2 hr.

2.4. Calculation ofcerium isotope ratios In this study, ISO/160 (=R s ) of the sample on the filament was measured in each scan and used for oxygen isotope correction for the Ce isotope ratio calculation in each scan (Masuda et al., 1988). This is denoted as the in situ oxygen isotope correction in this study. In order to obtain R s and 138Ce/142Ce, a circular calculation was performed as described below. Interference corrections of Pr and Nd were made before mass fractionation correction and oxygen isotope correction. 17 0 /160 was fixed to be 0.0003916 (MakishimaetaI., 1987). R s was obtained from 142CeISO/142CeI60, which was calculated based on the intensities of masses 160 and 158: (160)oB5

(1)

TABLE II Cup configurations for Ce isotope analyses Collector

Position J

Position 2

Baseline

COL5 COL 4 COL3 COL 2 Call

152 154 158 159 160

151 153 157 158 159

151.5 153.5 157.5 158.5 159.5

where OBS means an observed intensity. Since 142Cel60 is isobaric to 140CelS0, the 140CelS0 was subtracted from the observed intensity at mass 158. The intensity of 140CelS0 was calculated using a constant 140Ce/142Ce ratio, a mass fractionation factor (F) per one mass unit and R s based on Tanaka and Masuda (1982):

PRECISE MEASUREMENTS OF Ce ISOTOPE COMPOSITION IN ROCK SAMPLES (I40CeISO) = (140Ce/I42Ce)CONSTF 2R s

(2)

where (I40Ce/142Ce>CONST is 7.947 (Makishima et aI., 1987). At the beginning ofthe circular calculation, R s and Fwere assumed to be 0.00211 and 1, respectively. Then the mass fractionation factor, F, was recalculated by the following equation:

F=

//6

(152/158)oBs [

( :::CC:)

NORM

[1

5

is the weighted mean of the measurement runs given by: 1 2 __ [1:(X)0'; )2J / (7) X2 . 1:(1/0';) This estimation of reproducibility is the same as that of the Ce isotope analysis previously used by Shimizu et al. (1984) and Makishima et al. (1987).

]

3. Result and discussion

+(:::~:)CONST RsJ

3.1. Instrumental performance (3)

where (136Ce/I42Ce)NORM is a normalization factor of 0.01688 (Makishima et aI., 1987). Using eqs. 1 and 2 and this newly defined F, the actual R s was calculated. F was then recalculated with eqn. 3. This circular calculation was repeated until R s and F converged to constant value. Mass fractionation correction for the Ce isotope ratio was performed by the following equation using the power law: (154/158>CORR=(154/158) OBSF- 4 (4) where CORR means a mass fractionation corrected ratio. Then 138Ce/I42Ce was obtained using (154/158 >CORR and R s. Analytical precision in a single measurement run is defined as 20'm, and is given by: 20'm=20'/n I/2 (5) where 0' and n are the standard deviation and number of scans (ratios) recorded in one analysis, respectively (Papanastassiou and Wasserburg, 1969). Analytical reproducibility is denoted as "2 sigma", and defined as: I/2 (X;-X)/O';]2J (6) 2 sigma e z x (n-1 )1:(1/0';)2

.

[1:[

where x, and 0'; are the mean and the standard error of each measurement, respectively; and x

Instability of gain factors directly affects the resulting isotope ratios in the static multicollection technique. Reproducibilities of individual gain factors of each collector in the Ce isotope analysis (16 runs) were < ± 0.0007% (20'm). This is significantly small compared with the reproducibility of the obtained 138Ce/ I42Ce ( ± 0.002%, see Table III). Furthermore, there appeared no correlation between gain factor and 138Ce/I42Ce (not shown), indicating that such a small instability in the gain factor does not influence the Ce isotope ratio in the mass spectrometer. Baseline of mass spectrum is an another important factor especially in the Ce isotope analysis, compared with other isotope analyses. Because I36Ce and 13SCe have very small isotopic abundances of 0.19% and 0.25%, respectively, so a small change of the baseline affects both 136Ce/142Ce and 138Ce/I42Ce. Particularly, the large abundance of I40Ce causes a tailing ofthe beam (Dickin et aI., 1987) which leads to overestimation of the baseline. In order to examine the tailing effect to the baseline, baselines at mass numbers from 151.5 to 160.5 were measured by the peak jumping method using COL3. In this experiment, the ion current of I40Ce160 was kept to be 4.4·10- 10 A. Observed results are shown in Fig. 2. Baselines at 155.5 and 156.5 were slightly

6 ~

A. MAKISHIMA AND E. NAKAMURA 20,.---------


Analytical results of 138Ce/ 142Ce and 180 /1 60 ofJMC 304

o ..-

~

Run

.q til

c:

1 2

(l)

£

TABLE III

- 1 0 ~~-

152

156

160

Mass number

Fig. 2. Baselines from mass numbers 151.5 to 160.5. Each baselin e from mass number 151.5 to 160.5 was measured 20 times by COLJ with idling and integration time 4 and 8 s, respectively. The intensities of14°Ce160 and 142Cel60 were 4.4'10- 10 and 5'10- 11 A, respectively, during the measurement. Errors of each background arc given as 2a. Hatched area is the 2a range of the background of COL 3 without ion beam.

higher than other baselines. This may result from the tailing of the largest ion beam of I~OCeI60. Therefore, we measured baselines at 151.5, 153.5, 157.5, 158.5 and 159.5 for peaks of 152, 154, 158, 159 and 160, respectively, in the Cc isotope analysis, so that the tailing effect from the large ion beam to the baselines was cancelled out in this study.

3 4 5 6 7 8 9 10 JJ

12 13 14 15 16 Weighted mean 2 sigma

±2am

18

0.0225675 0.0225680 0.0225686 0.0225690 0.0225689 0.0225694 0.0225681 0.0225696 0.0225685 0.0225682 0.0225695 0.0225696 0.0225670 0.0225686 0.0225668 0.0225672 0.0225685

0.0000005 0.0000004 0.0000004 0.0000005 0.0000005 0.0000005 0.0000004 0.0000005 0.0000004 0.0000005 0.0000004 0.0000004 0.0000006 0.0000005 0.0000004 0.0000006

0.0021104 0.0021137 0.0021156 0.0021052 0.0021063 0.0021020 0.0021183 0.0021182 0.0021083 0.0020922 0.0021233 0.0021158 0.0021089 0.0021151 0.0021069 0.0021285 0.0021110

Mean

0.0000004

l

3.2. Ce isotope measurement ojJMC 304 Ce isotope results with 180 /160 of JMC 304 carried out in this study are given in Table III and Fig. 3. The analytical precision in the standard error (eqn. 5) becomes smaller and better with increasing number of scans. Therefore, the analytical precision must be evaluated using the analyses with the same number of scans. However, the number of scans recorded in the previous Ce isotope analyses is largely variable from 300-400 (Tanaka and Masuda, 1982; Makishima et al., 1987) to 4300 (Dickin et al., 1987) so that it is difficult to simply compare the analytical precision. Standard deviations are calculated from published data using eq. 5 to eliminate the influence of the number of scans (Table IV). The standard deviations in a single run calculated from Dickin (1987a, b) and Dickin et al. ( 1987) are much larger than those of this study in spite of

0 / 160

1J8Ce/142Ce

+•

.

••+ +

,! •

T

2

+;

•

4

6

, ,

8 10 12 14 16 RUN

Fig. 3. Reproducibility of 138Ce/142Ce ofCe reagent, JMC

304. Errors in each measurement are presented in 2am • The weighted mean and reproducibility are 0.0225685 and ± 0.0000004 (± 0.002%), respectively.

TABLE IV Comparison of sta ndard deviations in Ce isotope analyses Number of scans

Precis ion , 2am ( %)

Standard deviat ion.

2a ( %) Dickin (19 87a) 1,400-3.800 ±0.009-0.0018 Dickin (1987b) 1,200-3 ,000 ±0.005-0.014 Dickin et al. (1987) 1,800-4,300 ± 0.008-0.0 17 This work (JMC 304) ±0.004-0.005 400 This work (DCR-I) 180- 400 ±0.004-0.009

±0.45-0.79 ±0.21-0.63 ±0.33-0.81 ±0.07-0 .11 ±0.07-0.12

7

PRECISE MEASUREMENTS OF Ce ISOTOPE COMPOSITION IN ROCK SAMPLES

their comparable analytical precision with ours. This evaluation implies that the analytical precision in our method is improved to at least three times better than those of the previous studies. The weighted mean and analytical reproducibility (2a) of 138Ce/142Ce in 16 runs are 0.0225685 and ± 0.0000004, respectively (Table III). This reproducibility can be rewritten to be ± 0.002%*. Shimizu et aI. ( 1984) and Makishima et aI. (1987) reported reproducibilities of ±0.005% and ±0.006% (12=27) using the same calculation (eqs. 6 and 7). Allan (1988) presented a reproducibility of ± 0.0 17% (12 = 26) in the standard deviation (2a), while the standard deviation of this work is ± 0.008%*. It is concluded that the reproducibility obtained in this work is better by a factor of > 2 compared to those of the previous works. The averages of 18 0 /160 obtained in each run vary from 0.002092 to 0.002129 and the mean is 0.002111 for 16 runs which is consistent with those of previous studies (Wasserburg et aI., 1981; Dickin et aI., 1987; Makishima et aI., 1987). However, the 18 0 /160 of each scan clearly increases with time in a run and actually varies from 0.002105 to 0.002145 in the representative run 11 (Fig. 4). This is clearly inconsistent with that 18 0 /160 does not change during the Ce isotope analysis as re0.00215.-----------, 0.00214 <00 0.00213

00.00212 co

-

0.00211 0.00210 +----_ _- _ _----.-J o 10 20 Block number

Fig. 4. Variation of 18 0 /160 in run II. Each plot represents a mean of 18 0 /160 in each block containing 20 scans. Errors are presented in 2a. "Note that the analytical reproducibility in this paper is ±0.002% defined by eq. 6, otherwise the reproducibility at the 95% confidence level is ±0.008%.

ported by Dickin et aI. (1987) who used a fixed 180 /160 ratio for the oxygen isotope correction. When 18 0 /160 varies from 0.00209 to 0.00213, 138Ce/142Ce of JMC 304 changes from 0.0225635 to 0.0225718 by the oxygen isotope correction. Hence, the change of 18 0 /160 seriously results in poor reproducibility and precision in the Ce isotope analysis. One of the reasons why our Ce isotope results are much better in analytical reproducibility and precision than those of previous studies may be derived from the in situ oxygen isotope correction. Our precise results are also dependent on the high-intensity measurement ofthe Ce ion beam 142 [ Ce I60 of (2-7)'10- 11 A) without collecting 140Cel60 in any Faraday cup collector of the mass spectrometer. Tanaka and Masuda (1982), Dickin et aI. (1987) and Makishima et aI. (1987) only reached intensities of 142Cel60 of (0.5-1).10- 11 , (1-3).10- 11 and 1· 10- 11A, respectively. Our high-intensity measurement consequently reduces the relative effects of baseline drifts and electric noise in the mass spectrometer. The relationship between the analytical precision (2am ) with the total quantity ofelectricity of the 142Ce160 ion beam [(ion beam current) X (data acquisition time per scan) X (number of ratios) ] in the individual runs is shown in Fig. 5. Analytical precision of each run was < ± 0.003% and decreases with increasing quantity of electricity. Change of an0.006 ~------;::;~===::::::;l o , - Individual run 0.005 o Runll 0 S 0.004 0 c: o .20.003 0 •• 0 0" o ~r!~!Jooow .~ 0.002 QJ 0.001

I

-0-

CC)

a:

O+--------~----'

012 Quantity of electricity (xlO- 7 C)

Fig. 5. Variation of error of 138Ce/142Ce with quantity of electricity of 142Ce160 ion beam. The vertical axis is the analytical precision (2am ) . The quantity of electricity is calculated by (ion beam current) X (data acquisition time per scan) X (number of ratios).

8

alytical precision in relation to the quantity of electricity in the representative run (run J J in Table III) is also shown in Fig. 5. The precision decreases exponentially with accumulation of the quantity of electricity and is approximately saturated to ± 0.00 18%at around 1.7-I 0- 7 C. These observations indicate that the analytical precision is mainly dependent on the total quantity of electricity of the Ce ion beam when the ion beam intensity is enough to make the instability of baseline and electric noise negligible, and that more than 1.5- 10- 7 C is necessary when the precision of < ± 0.002% is required. We recalculated 138Ce/142Ce of JMC 304 reported by Shimizu et aI. (1984) and Makishima et a1. (1987), using the correction factors 136Ce/142Ce=0.01688, 140Ce/142Ce=7.947 18 16 0 / 0 = 0.00211, and obtained and 0.0225739 and 0.0225754, respectively (Table I). The differences between these values and our result are +0.0000054 and +0.0000069, equivalent to + 2.4 and + 3.0 e-units, respectively. Although the cause of differences is unclear, these differences are significantly larger than the analytical reproducibility. In the early Nd isotope studies, there also existed up to 4 e-units difference in 143Nd/ 144Nd of the same standard samples in some cases (DePaolo, 1988). Considering the analytical difficulties, the difference in the Ce isotope ratio of JMC 304 may be derived from systematic differences between laboratories. If this is the case, it can be possible to eliminate the inter-laboratory biases by normalizing the obtained result to standard samples.

Six separate Ce isotope analyses of BCR-I were carried out in this study and the results are presented in Table V and Fig. 6. Each Ce sample for the mass spectrometry was independently separated from 20 mg ofBCR-1 rock powder. Therefore, the analytical reproducibility of these analyses includes reproducibili-

A. MAKISHIMA AND E. NAKAMURA

TABLE V

Analytical results ofsix analyses of BCR-] Number of scans

Run ]

2 3 4 5 6 Weighted mean 2 sigma

0.0225662 0.0225650 0.0225649 0.0225650 0.0225645 0.0225654 0.0225652 0.0000004

0.0000008 0.0000005 0.0000006 0.0000010 0.0000007 0.0000004

280 400 400 180 360 400

0.022570,.---------,

c3

0.022568

f

:!

.--'-

0.0225661-----''----__ --1---.1,1 T f T ! ~ 0.022564

o

~

0.022562 0.022560+-~--~~--~-I

1

2

3

4

5

6

RUN Fig. 6. Reproducibility of 138Ce/142Ce of HCR·]. Errors in each measurement are given in 2am • The weighted mean and the reproducibility are 0.0225652 and ±0.0000004 ( ± 0.002%), respectively.

ties in both the mass spectrometry and the chemical procedure. The mean and reproducibility of six runs are 0.0225652 and 0.0000004, respectively, and the standard deviation (20') is ±0.07-0.12% (Table III). The reproducibility and the standard deviation of rock samples are also more than factor of 2 better than those of previous studies. During the measurements, interference peaks of 141Prl60 and 143Nd160 were negligibly low, < 1-10- 14 and < 1-10- 15 A, respectively. This implies that our chemical separation of Ce with a small volume of ion-exchangeresin and eluate is suitable for the Ce isotope analysis and the resulting procedural blank of 0.04 ng does not require the blank correction for the Ce isotopic composition ofBCR-I. There has been so far only one Ce isotope analysis of BCR-l reported by Tanaka et a1. (1987). The recalculated 138Ce/142Ce of Tanaka et a1. (1987) is 0.0225700 based on the same calculations as made for JMC 304. The

PRECIS E MEASUREMENTS OF Ce ISOTOPE

CO~IPOSITION

IN ROCK SA~tPLES

difference in 138Ce/142Ce of BCR-l between Tanaka et al. (1987) and this study is +0.0000048, and larger than the analytical reproducibility. The difference is consistent with the difference of +0.0000054 in JMC 304 between Tanaka et al. (1987) and this study. This consistency indicates that the difference between laboratories can be corrected by standard analyses. In the Nd isotope analysis, La Jolla standard, Johnson Matthey" Nd reagent and BCR1 have been used to evaluate the analytical reliability in different laboratories. We propose that BCR-l can be a Ce isotope standard. This is appropriate as the Ce isotope composition ofBCR-l is very close to the chondritic orbulk Earth value (Tanaka et aI., 1987) as well as the ENd-value is nearly equal to zero (DePaolo and Wasserburg, 1976). In order to eliminate any inter-laboratory biases in Ce isotope analysis including the chemical separation, it is proposed to present the Ce isotopic composition as follows:

9

TABLE VI Comparison of the La-Ce isotope system with the Sm-Nd isotope system and its applicability to study the Ce anomaly (a) Comparison of the La-Ce system with the Sm-Nd system La-Ce system Parent-daughter ratio (R p ) of Leedey chondrite" Change of isotope ratio over I Ga (f units) Analyt ical reproducibility ( e units) Detection lim it in the change of s, over I Ga

0.381

Sm-Nd system 0.321

± 3.9. 2

±o.r' (OS» 0.02 (0.05)

0.003

(b) Detectable Ce anomaly over I Ga for a sample with a flat LREE pattern

This study Previous studies'>

Analytical reproducibility (e units)

Ceanomaly (CeN/CeN*=O)

±0.2 ±0.5

.10>0.06 .10>0.15

4. Applicability of La-Ce isotope system

"Masuda et al. (1973). La= 0.916 ppm , Ce=0.318 ppm , Nd=0.116 ppm , Sm=0.230 ppm . .2)..p Il'La= 2.11.1O- 12 a- I (Masuda et er., 1988), )..EC Il'La=4.44 ·1O- 12 a -I (Sato and Hirose, 1981), ( IJ8 La/ ' 42Ce)=(La/Ce)w,;&!uxO.00811 (Makishima et al., 1981) and f~~luR =0.0225652 (this study) were used. .3)." I41Sm=6.54.10-12 a-I, ( ' 41S m/'''Nd) = (Sm/Nd).';8h1 xO .6049 and ft~lu R =0.512638 (Jacobsen and Wasserburg, 1980) were used. "This study. "Makishima et al. (19 81) . . 6Shimizu ct al. ( 1989).

The applicability of the La-Ce isotope system is evaluated and compared with the SmNd isotope system in Table VI, (a). In Leedey chondrite (Masuda et aI., 1973), the Ce isotope ratio changes 3.9 e-units while the Nd isotope ratio changes 24.9 e-units over I Ga. Since typical reproducibility of the Nd isotope analysis is ± 0.2E, the minimum detectable change of'Sm/Nd ratio (LiR p ) over I Ga is +0.003. In the La-Ce system, analytical reproducibilities of the Ce isotope ratio were larger than the ± 0.5E in previous studies (e .g., Makishima et aI., 1987), so that the detection limit in the change of'La/Ce ratio of I Ga was> +0.05. Such a resolution in the La-Ce system resulted in that the detectable Ce anomaly (CeN/

CeN*=O) over 1 Ga with a flat LREE pattern was 0<0.85 or 0> 1.15; in other words, LiO> 0.15 [Table VI, (b)], where CeN is the chondrite-normalized Ce content and CeN* is an estimate ofCe content provided by a smooth LREE pattern. However, the improved analytical reproducibility ofO.2E in this study can reduce the detection limit of Ce anomaly with 0< 0.994 or 0> 1.004 (LiO> 0.06). This indicates that the applicability of the La-Ce system is largely extended to the understanding of the fine structure of the LREE pattern in the ancient samples. For example, 0 of most Archean sedimentary rocks and of Archean amphibolites and granitoids ranges from 0.93 to 1.13 (Taylor and McLennan, 1985) and from

[(

t38Ce/142Ce) sample

(8)

10

0.84 to 1.06 (Meen, 1990), respecti vely. These Ce anomalies should be "reflected in the Ce isotope ratios, and can be detected with the analytical techniques described in this study. The improvement in the techniques for the Ce isotope analysis reported here with the better overall analytical reproducibility and precision, and the normalization to BCR-J make it now possible to extend the application of the La-Ce system as a geochemical tracer in a similar way in Sr, Nd and Pb isotope systems. Further, precise Ce isotope studies of the terrestrial and extraterrestrial samples with Nd isotope analyses will provide more detailed information on the LREE evolution in the Earth and the solar systems. 5. Conclusions ( 1) A method for the Ce isotope analysis with precision of ± 0.002-0.003% and reproducibility of ± 0.002% has been established with the static multicollection technique reducing the data acquisition time to 2 hr. for 400 ratios. (2) The better precision and reproducibility were obtained by the in situ oxygen isotope correction, the short data acquisition time and the large ion beam [I42Ce160 of (2-7) .10- 1 1

Al. (3) The chemical procedure for the separation of Ce was modified using a small ion-exchange resin bed column with HIBA. The procedural total blank and the recovery yield of Ce were 0.04 ng and 90%, respectively, for 20-mg rock sample. (4) Six Ce isotope analyses individually separated from BCR-J gave an analytical reproducibility of ± 0.002% which is more than factor of2 better than those of previous studies. (5) Since the 138Cejl42Ceratio of BCR-J is very close to the present chondritic value, it is recommended to normalize obtained Ce isotope ratios to that of BCR-J in order to eliminate the inter-laboratory biases in the Ce isotope analysis.

A. MAKISHIMA AND E. NAKAMURA

(6) These techniques developed in this study enables us to utilize the La-Ce isotope system in terrestrial and extraterrestrial samples combined with the Sm-Nd system. This will contribute to the understanding of the detailed LREE evolution in the Earth and the solar systems in future studies. Acknowledgements We are very thankful to S. Akimoto, H. Honma, E. Ito, H. Kagami and K. Nagao (ISEI, Okayama Uni versity) for supporting this work. We are also grateful to Y. Matsui (ISEI) for offering BCR-J. We are grateful to T. Shibata for his technical help, and other people at ISEI. Special thanks go to A. Masuda and H. Shimizu (The University of Tokyo) for offering JMC 304 and encouraging. We also thank journal reviewers, especially M.T. McCulloch, for their helpful reviews. This research is supported by the Monbusho (Ministry of Education, Science and Culture, Japan) International Scientific Research Program to S. Akimoto and by a Grant-in-Aid for Scientific Research from Monbusho to E.N. and E. Ito. References Allan , M.J., 1988. Cerium isotop es: Progress towards a reproducible mass spectrometric technique for high precision 138/136 ratio determinations. Eos (Trans. Am. Geophys, Union), 69: 1502 (abstract). Allegre, C.J., 1987. Isotope geodynamics . Earth Planet Sci. Lett., 86: 175-203. DePaolo, D.J., 1988. Neodymium Isotope Geochemistry. Springer, Berlin, Tokyo, 187 pp. DePaolo, D.J. and Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic models. Geophys. Res. Lett ., 3: 743-746. Dickin, A.P., 1987a. La-Ce dating of Lewisian granulites to constrain the 138La p-decay half-life. Nature (London), 325: 337-338. Dickin, A.P., 1987b. Cerium isotope geochemistry of ocean island basalts. Nature (London) , 326: 283-284. Dickin, A.P., Jones; N.W., Thirlwall, M.E and Thompson, R.N., 1987. A Ce/Nd isotope study ofcrustal contaminat ion processes affecting Palaeocene magmas in Skye, Northwest Scotland. Contrib. Mineral. Petrol., 96: 455-464. Jacobsen, S.B. and Wasserburg, G.J., 1980. Sm-Nd iso-

PRECISE MEASUREMENTS OF Ce ISOTOPE CO:-'IPOSITION IN ROCK SAMPLES

top ic evolution of chondrites. Earth Planet . Sci. Lett., 50: 139-155. Makishima, A., Shimizu, H. and Masuda, A., 1987. Precise measurement of cerium and lanthanum isotope ratios. Mass Spectrosc., 35: 64-72. Masuda, A., Nakamura, N. and Tanaka, T., 1973. Fine structures of mutually normalized rare-earth patterns of chondrites. Geochim. Cosmochim. Acta, 37: 239248. Masuda, A., Shimizu, H., Nakai, S., Makishima, A. and Lahti, S., 1988. 138Lap-decay constant estimated form geochronological studies. Earth Planet. Sci. Lett., 89: 316-322. Meen, J.K., 1990. Negative Ce anomalies in Archean amphibolites and Laramide granitoids, southwestern Montana, U.S.A. Chern. Geol., 81: 191-207. Nier, A.a., 1950. A redetermination of the relative abundances of the isotopes of carbon, nitrogen, oxygen, argon, and potassium. Phys. Rev., 77: 789-793. Papanastassiou, D.A. and Wasserburg, GJ., 1969. Initial strontium isotopic abundances and the resolution of small time differences in the formation of planetary objects . Earth Planet. Sci. Lett., 5: 361-376. Sato, J. and Hirose, T., 1981. Half-life of 138La. Radiochem. Radioanal. Lett., 46: 145-152. Shibata, T. , Makishima, A. and Nakamura, E., in prep. Precise isotope measurement of trace neodymium in silicate minerals. Shimizu, H., Tanaka, T. and Masuda, A., 1984. Meteori-

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tic mCe/ 142Ce ratio and its evolution. Nature (London), 307: 251-252. Shimizu, H., Mori, K. and Masuda, A., 1989. REE, Ba, and Sr abundances and Sr, Nd and Ce isotopic ratios in Hole 504B basalts, ODP Leg 111, Costa Rica Rift. In: K. Becker, H. Sakai et al. (Editors), Proc. Ocean Drill Prog., Sci. Results, Ocean Drill . Prog., College Station, Texas, III: 77-83. Tanaka, T. and Masuda, A., 1982. The La-Ce geochronometer: a new dating method. Nature (London), 300: 515-518. Tanaka, T., Shimizu, H., Kawata, Y. and Masuda , A., 1987. Comb ined La-Ce and Sm-Nd isotope systematics in petrogenetic studies. Nature (London), 327: 113-117. Taylor, S.R. and McLennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford, 312 pp. Wagner, G., Rache, H. and Tuttas, D., 1984. Variable Multicollection. Speed, precision and versatility with the Model 261 thermal ionization mass spectrometer. Finnigan MAT G.m.b.H., Bremen, Tech. Rep. No. 416, 10pp. Wasserburg , GJ., Jacobsen, S.B., DePaolo, D.J., McCulloch , M.T. and Wen, T., 1981. Precise determination of'Sm/Nd ratios, Sm and Nd isotopic abundances in standard solutions. Geochim. Cosmochim. Acta, 45: 2311-2323.