The lithium isotope composition of international rock standards

Chemical Geology 166 Ž2000. 319–326 www.elsevier.comrlocaterchemgeo

The lithium isotope composition of international rock standards Rachael H. James a,) , Martin R. Palmer b a

Department of Earth Sciences, Bristol UniÕersity, Queens Road, Bristol BS8 1RJ, UK T.H. Huxley School, Imperial College, Prince Consort Rd., London SW7 2BP, UK

b

Received 17 March 1999; accepted 28 October 1999

Abstract We present the results of analysis of the Li isotope composition of open ocean seawater, nine international rock standards and a C1 chondrite. In addition, we suggest some modifications of the chemical preparation techniques for analysis of lithium isotopes in low concentration samples that give precise isotope ratios by thermal ionisation mass spectrometry ŽTIMS.. The aim of this study is to provide a benchmark for ensuring the reliability of Li isotope data between different laboratories by TIMS and other isotope ratio techniques, including multi collector-inductively coupled plasma-mass spectrometry and ion probe. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Lithium; Isotope ratios; Standard rocks; Thermal ionization mass spectrometry; Analytical methods

1. Introduction There is a growing interest in lithium isotope studies of both terrestrial and extra-terrestrial materials as the nature of Li isotope geochemistry is such that it has the potential to reveal important information concerning a wide range of processes; for example, riverine geochemistry ŽHuh et al., 1998., hydrothermal activity and alteration of the oceanic crust ŽChan et al., 1992, 1993, 1994; Seyfried et al., 1998., cycling of material between the mantle and the oceanic crust ŽMoriguti and Nakamura, 1998a; Tomascak et al., 1999. and nucleosynthetic processes in the solar system ŽChaussidon and Robert,

)

Corresponding author. Department of Earth Sciences, The Open University, Walton Hall, Milton Keynes MK76AA, UK. Tel.: q44-1908-654-296; fax: q44-1908-655-151. E-mail address: [email protected] ŽR.H. James..

1998.. Although this work has already revealed interesting information, the accuracy of the published data can be difficult to ascertain, as there are no internationally accepted Li isotope values for natural rocks or solutions. Synthetic Li 2 CO 3 standards are available and these are used to define E6 Li and E7 Li values. However, analysis of the Li isotope composition of natural samples generally requires sample processing through ion exchange columns and, in the case of solids, preliminary sample dissolution. Both of these steps have the potential to fractionate the two Li isotopes Ž6 Li and 7 Li. from one another. Hence, there is a need for natural standards of known Li isotope composition, so that investigators can ensure that their data are not affected by analytical artefacts. In addition, efforts to directly measure Li isotope ratios using solid sampling techniques Žlaser ablation and ion probe. are gaining momentum. While these techniques benefit from minimal sample preparation, careful calibration using solid reference

0009-2541r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 9 9 . 0 0 2 1 7 - X

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standards is required, as they are susceptible to matrix-dependent mass fractionation effects. To this end, we present here Li isotope compositions for a number of international standard rocks that span a range of chemical compositions. In addition, we include a discussion of some refinements to the analytical technique for thermal ionisation mass spectrometry ŽTIMS. which can improve the utility of Li isotope studies for low-Li samples.

2. Experimental methods 2.1. Sample dissolution The rock standards were dissolved by closed microwave digestion ŽKemp and Brown, 1990.. Briefly, ; 0.2 g of powdered rock was placed in a Teflon w pressure vessel together with ; 3 ml of sub-boiling distilled ŽSB. HF and ; 3 ml of high purity H 3 ClO4 . The vessels were then sealed using a capping station, and the sample was digested using 100% power for 2 min, followed by 80% power for 30 min. This procedure resulted in complete dissolution of all the samples with no visible residue. The samples were then reduced to near-dryness on a hot plate at 2008C and redissolved in a few milliliters of SB 1% nitric acid. After cooling, they were made up to 20 ml and the Li concentrations determined by inductively coupled plasma-atomic emission spectroscopy ŽICPAES. using a standard additions technique. The Li concentration of a sample of the Orgueil carbonaceous chondrite was determined by isotope dilution ŽID.-TIMS using a 95.3% 6 Li enriched Li 2 CO 3 spike supplied by AEA Technology. The precision of Li concentration analyses by ICP-AES was better than "2% Ž1 s . and better than "0.2% Ž1 s . by IDTIMS. 2.2. Chemical separation It is essential to achieve complete separation of Li from other elements if isotopic analyses are to be performed by TIMS, as the presence of other ions can suppress ionisation efficiency and lead to isotopic fractionation in the mass spectrometer ŽMoriguti and Nakamura, 1993; Hoefs and Sywall, 1997.. Even

if the analyses are performed by multi collector-inductively coupled plasma-mass spectrometry ŽMCICP-MS. ŽTomascak et al., 1999., it is advisable to carry out sample purification as high levels of Na have been shown to induce isotopic fractionation during ICP-MS Li isotope studies ŽGregoire et al., ´ 1996.. In addition, memory effects for Li and other elements are reduced during MC-ICP-MS if the sample matrix is simplified ŽZ. Palacz, personal communication, 1998.. However, care must be taken during the use of ion exchange resins as significant isotope fractionation of Li can take place if the recovery yield of Li is less than 100% ŽMoriguti and Nakamura, 1998b.. Sodium is relatively close to Li in terms of its affinity to sulphonated polystyrene cation-exchange resins. Good separation of the two elements has been achieved using either: Ži. a relatively large resin volume Ž12–98 cm3 . and low concentrations of HCl Ž0.5N. ŽChan, 1987; You and Chan, 1996., or Žii. a four-step procedure using a small resin volume Ž0.1– 1 ml. and an organic-based eluant in the third step Ž30% ethanol–0.5 M HCl. to improve separation of Li from Na ŽMoriguti and Nakamura, 1998b.. The first of these procedures uses a large volume of eluant and is therefore vulnerable to blank contamination, and its reproducibility Ž"0.9‰; 1 s . for Mgand Fe-rich silicate samples with low Li may be marginal for some studies ŽYou and Chan, 1996.. The latter procedure is time consuming, susceptible to cumulative loss of Li and requires a sample size of 250–1000 ng in order to give acceptable reproducibility. In addition, the four-column technique requires the use of organic solvents that can introduce tailing effects and interfere with TIMS analyses. Using the column procedure outlined below, we achieve good separation of Li from Na using a small resin volume Ž2.7 ml. and sample size Ž100 ng Li., which is both rapid Ž- 2 days. and gives reproducible results Žaverage "0.5‰, 1 s . for low-Li samples. However, we have observed that processing of dissolved Mg- and Fe-rich silicate samples with low Li Ž- 4 ppm. causes a shift in the column calibrations, so that a larger volume of eluant must be collected in order to ensure complete recovery of Li. This leads to significant retention of Na in the analyte and necessitates a repetition of the column

R.H. James, M.R. Palmerr Chemical Geology 166 (2000) 319–326

procedure to ensure optimal purification of the sample ŽFig. 1.. Mg- and Fe-rich silicate samples with low Li that were not passed through the column twice yielded stable, but anomalously high 6 Lir7 Li ratios. We believe that this shift occurs because the low-Li samples require processing of up to 75 mg of the solid which contains sufficient cations Žprincipally Fe and Mg. to use up about 20% of the ion exchange capacity of the resin used in this study. Empirical evidence suggests that ion selectivity can only be ensured for sample quantities that do not exceed 0.1 times the quantity of resin Žboth measured in milliequivalents; Rieman and Walton, 1970.. Although increasing the quantity of resin potentially solves this problem, this must be traded off against associated increases in the volume of eluant and the size of the blank.

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Overall, the column procedure adopted in this Y study is as follows. A 14 i.d. Teflon w column was loaded to a height of 8.5 cm with Bio-Rad AGW-X12 200–400 mesh cation exchange resin. A sample solution containing 100 ng of Li was evaporated to near dryness and taken up in 200 ml of 0.2N SB HCl. This was then added to the column that had been pre-treated with 0.2N SB HCl. The cations were then eluted with further volumes of 0.2N SB HCl, with collection of the entire Li fraction which was then evaporated to near dryness under an infrared lamp. 2.3. Blanks Blanks were determined Žusing a small aliquot of the 6 Li 2 CO 3 spike. on several occasions for the column procedure alone and for the combined sample dissolution and column procedure. In both cases, the blanks were indistinguishable Ž0.08–0.15 ng., suggesting that the column procedure was the major source of extraneous Li. As our analyses were performed using 100 ng of Li, the blank correction is not significant at the uncertainty levels presented below. 2.4. Mass spectrometry

Fig. 1. Elution of Li and Na for seawater and chondrite samples resulting from Ža. first pass, and Žb. second pass through the cation exchange column. For a sample containing 100 ng Li, the total cation loading in the first step is 5% of the resin exchange capacity for seawater, and 20% of the resin exchange capacity for chondrite.

The purified sample was taken up in 100 ml SB H 2 O and converted to Li 3 PO4 by addition of 10 ml high purity 0.01M H 3 PO4 , and the solution reacted to dryness on a ; 2008C hot plate for ; 2 h. The sample was then taken up in 3 ml of SB H 2 O and loaded onto one side of a degassed double Re filament. The sample was dried at a filament current of 1 A, which was then increased to 2 A until the phosphoric acid fumes were driven off. Nine samples and one L-SVEC standard were then loaded into a 10-bead turret and placed in a VG 336 TIMS. After the source vacuum reached ; 10y8 Torr, the current to the ionization filament was raised to a temperature of 10008C Žas recorded by an optical pyrometer.. The evaporation filament current was then increased slowly to 0.9–1.0 A until a stable 7 Li beam of 1 = 10y1 1 A was achieved. The 6 Lir7 Li ratio was measured in dynamic mode against a baseline at 5.5 amu using a single Faraday collector.

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R.H. James, M.R. Palmerr Chemical Geology 166 (2000) 319–326

Integration times of 10 s were used for the Li peaks and 4 s for the baseline. In general, 10 blocks of 10 ratios each were collected for each sample, with focusing of the beam between each block. At temperatures less than 8508C, we observed significant fractionation of Li isotopes. However, between 850–12008C, the 6 Lir7 Li ratio reaches a plateau with excellent reproducibility, thus permitting high precision isotope measurements. This finding is similar to that of You and Chan Ž1996. who found significant isotope fractionation at low temperatures, with a plateau at 1000–14008C. In contrast, Moriguti and Nakamura Ž1993, 1998b. did not observe any isotope fractionation between 7508C and 12258C, and they attribute any apparent difference with the results of You and Chan to incomplete formation of Li 3 PO4 by the latter. However, we believe that the relatively small differences in temperature dependence observed by the other two groups and ourselves likely arise from differences in the calibration of the pyrometers and subtle variations between the types of mass spectrometer.

3. Results and discussion 3.1. Reference standards The average 6 Lir7 Li ratio of 25 analyses of 100 ng of the NIST L-SVEC standard following column chemistry is 0.082757 " 28 Ž1 s ; where s is the standard deviation., which is within error of the value of 0.082727 " 34 obtained for loading of 100 ng of this standard without column chemistry. This is in close agreement with values obtained in other laboratories using the phosphate method for isotopic analysis ŽMoriguti and Nakamura, 1993, 1998a, 1998b; You and Chan, 1996.; the mean of these reported determinations Žincluding this study. is 0.082634 " 95 Ž"1.1‰, 1 s .. The reproducibility of our method is comparable to that of You and Chan Ž1996., but a factor of more than two times better than that reported by Moriguti and Nakamura Ž1998b. for a 100-ng sample. Furthermore, reproducibility is better than for Li isotopic analysis of high mass . ŽChan, molecular ions ŽLi 2 BOq and LiNaBOq 2 2 1987; Chan and Edmond, 1988; Chan et al., 1992. which are generally thought to yield more reliable

results as they are less vulnerable to isotopic fractionation and less sensitive to beam focusing. As the q Li 2 BOq 2 and LiNaBO 2 techniques both require a relatively large sample size Ž3 mg Li. and more complicated sample processing, and given the close agreement and good reproducibility in 6 Lir7 Li ratios between different laboratories using Li 3 PO4 techniques, we suggest that this should be the method of choice for analysis of Li isotopes Žparticularly for low-Li samples. by TIMS. 3.2. Solution standard Lithium is a conservative element in seawater and has a residence time that is at least 100 times greater than the mixing time of the oceans ŽStoffyn-Egli and Mackenzie, 1984., hence the Li isotopic composition of modern seawater should be constant. Thus, open ocean water represents a useful solution standard as its chemical composition is such that it requires purification of the Li before it can be introduced into either TIMS or MC-ICP-MS Ži.e., most natural solutions will require processing in a similar manner.; in addition, it is readily available. The average 6 Lir7 Li ratio obtained for seven analyses of open ocean seawater ŽTable 1. in this study is 0.080151 " 56 Ž1 s .. Table 2 lists this result, together with other published E6 Li values. Our E6 Li values Žy31.5 " 0.7‰. are similar to those of Chan and Edmond Ž1988. Žy32.3 " 0.5‰. and You and Chan Ž1996. Žy31.4 " 1.2‰., but are heavier than those of Moriguti and Nakamura Ž1998b. Žy29.1 " 0.3‰.. The entire data set for both the TIMS and MC-ICP-MS analyses yields an average E6 Li value of y31.0 " 0.4‰ Ž"2 sr 'n y 1 ; n s 38.. Excluding the results of Tomascak et al. Ž1999.

Table 1 E6 Li values of seawater obtained in this study 6

Lir 7 Li

0.080087 0.080213 0.080220 0.080073 0.080144 0.080122 0.080200

1s

E 6 Li Ž‰.

1 s Ž‰.

0.000020 0.000016 0.000022 0.000014 0.000024 0.000014 0.000020

y32.3 y30.7 y30.6 y32.4 y31.6 y31.8 y30.9

0.25 0.20 0.27 0.17 0.30 0.17 0.25

R.H. James, M.R. Palmerr Chemical Geology 166 (2000) 319–326 Table 2 Average E6 Li values of seawater Ž1. Chan and Edmond Ž1988., Ž2. You and Chan Ž1996., Ž3. Moriguti and Nakamura Ž1998b., Ž4. Tomascak et al., 1999. nra: Information not available. E 6 Li

1 s Ž‰.

n

Method

Source

y31.5 y32.3 y31.4 y29.1 y30.8

0.7 0.5 1.0 0.3 nra

7 5 6 5 15

TIMS–Li 3 PO4 TIMS–Li 2 B 4 O 7 TIMS–Li 3 PO4 TIMS–Li 3 PO4 MC–ICP–MS

This study 1 2 3 4a

a

This study expresses Li isotope ratios in terms of E7 Li. As no isotope ratio for the L-SVEC standard is reported, then, it is not possible to convert data to E6 Li values directly. Thus, the figure quoted here is calculated assuming that the Li isotope ratio of L-SVEC is the same as that found in our laboratory Ž6 Lir7 Li s 0.082757..

Žgiven the uncertainty in conversion of their data to E6 Li values. yields an average E6 Li value of y31.1 " 0.6‰ Ž"2 sr 'n y 1 ; n s 23.. In view of the levels of analytical uncertainty, we propose a standard E6 Li value of y31.5 " 1.0‰ for seawater. 3.3. Solid standards We have determined the concentrations and E6 Li values of nine international rock standards. The individual analyses are listed in Table 3, together with the results of five analyses of samples of the Orgueil C1 chondrite. The only significant discrepancy between our measured Li concentrations and those proposed, or recommended by Govindaraju Ž1994. is for JR-2, where we obtain a Li concentration that is 11% lower than the proposed value. Our analyses of the K 2 O and MgO content of this sample agree with the recommended values for this standard, suggesting complete sample dissolution. Moriguti and Nakamura Ž1998b. point out that Li can be retained in fluorides, but we did not detect the presence of fluoride deposits in this solution by centrifugation. Three other studies have reported Li isotope data for standard rocks. Repeat analyses of JB-2 by Moriguti and Nakamura Ž1998b. gave an average E6 Li of y4.9 " 0.4‰ Ž1 s ., and 13 analyses of this

323

standard by Tomascak et al. Ž1999. gave an average of y5.1‰ Žno standard deviation was reported; note that conversion of the reported E7 Li value to E6 Li has a negligible effect other than a change of sign.. These values differ from our value for this standard Žy6.8 " 0.1‰; 1 s .. The difference does not appear to be related to incomplete dissolution of the sample as both Moriguti and Nakamura Ž1998b. and ourselves measured Li concentrations Ž8.0 and 8.05, respectively. that are identical to the proposed value of 8.0 ppm ŽGovindaraju, 1994.. Oi et al. Ž1997. have reported Li isotope data for a series of GSJ rock reference materials. Unfortunately, it is difficult to fully evaluate this study as no value was reported for L-SVEC and all the data were referenced to another standard ŽIRM-016.. L-SVEC and IRM-016 have different certified 6 Lir7 Li isotope ratios Ž0.08319 " 0.00021 and 0.08137 " 0.00034, respectively: Flesch et al., 1973; Michiels and de Bievre, 1983., but Gregoire et al. Ž1996. ´ measured similar values for both standards. These authors point out that both standards are derived from the same geological source materials, and further analysis of samples from the same locality also gave similar values. This raises the possibility that both standards have similar isotope ratios, and if this is correct, then we can compare our data with those of Oi et al. Ž1997.. For three standards ŽJR-2, JGb-1 and JG-2; Table 3., the agreement between the two studies is excellent. However, the difference between our study and the E6 Li value of q0.8‰ obtained for JB-2 by Oi et al. Ž1997. is large relative to the offset between our study and those by Moriguti and Nakamura Ž1998b. and Tomascak et al. Ž1999.. There are no published analyses for BHVO-1, but a suite of analyses of basalts from Kilauea by Tomascak et al. Ž1999. yielded an average E6 Li value of y3.8‰ Žrange s y5.8 to y2.5‰. that is close to our value of y5.8 " 0.8‰ Ž1 s .. In this connection, we note that Hawaiian basalts have diverse isotopic compositions. The E6 Li value determined for the Orgueil C1 chondrite Žy3.9 " 0.6‰. is close to the median value of y10 " 10‰ observed by Chaussidon and Robert Ž1998. in an ion probe study of the unequilibrated Semarkona chondrite. It is also similar to the E6 Li value of fresh MORB Žy4.7 " 0.7‰. determined by Chan et al. Ž1992., suggesting that bulk

324

R.H. James, M.R. Palmerr Chemical Geology 166 (2000) 319–326

Table 3 E6 Li values and Li concentrations of selected international rock standards Ž1. Oi et al. Ž1997.; note that these data are referenced to a different isotopic standard ŽIRM-016. and cannot be directly compared to the remaining data that are referenced to the L-SVEC standard. However, as discussed in the text, there is some evidence that both isotope standards have similar Li isotope ratios; Ž2. Moriguti and Nakamura Ž1998b.; Ž3. Tomascak et al. Ž1999.. Standard

Measured Li Žppm.

Reported Li a Žppm.

E 6 Li Ž"2 s . y6.08 " 0.26 y6.51 " 0.26 y4.70 " 0.20 Mean: y5.8 " 0.8‰ Ž1 s . y5.2 " 0.39 y5.7 " 0.27 y7.5 " 0.22 Mean: y6.1 " 1.0‰ Ž1 s . y4.3 " 0.27 y3.3 " 0.18 y3.8 " 0.33 Mean: y3.8 " 0.4‰ Ž1 s . 0.9 " 0.59 1.3 " 0.35 1.5 " 0.25 Mean: 1.2 " 0.2‰ Ž1 s . 0.5 " 0.35 0.5 " 0.26 0.3 " 0.21 Mean: 0.4 " 0.1‰ Ž1 s . y6.0 " 0.41 y4.5 " 0.34 y5.2 " 0.24 Mean: y5.2 " 0.6‰ Ž1 s . 2.2 " 0.30 3.1 " 0.38 2.2 " 0.25 3.2 " 0.23 Mean: 2.7 " 0.5‰ Ž1 s . y1.8 " 0.32 y2.9 " 0.24 y2.3 " 0.31 Mean: y2.3 " 0.4‰ Ž1 s . y6.6 " 0.34 y6.8 " 0.25 y6.9 " 0.27 Mean: y6.8 " 0.1‰ Ž1 s .

BHVO-1 Hawaiian basalt

4.9

4.6 Žp. b

JGb-1 Gabbro

4.2

4.3 Žp.

JR-2 Rhyolite

73.9

83 Žp.

G-2 Granite

31.8

34 Žr.

JG-2 Granite

41.0

43.4 Žp.

SCo-1 Shale

45.8

45 Žr.

UB-N Serpentine

25.2

27 Žr.

DR-N Diorite

39.1

40 Žr.

8.0

8 Žp.

JB-2 Basalt

Orgueil C1 Chondrite

a

1.49

1.49 Žp. c

y3.2 " 0.52 y4.8 " 0.17 y4.0 " 0.39 y3.4 " 0.24 y4.0 " 0.10 Mean: y3.9 " 0.6‰ Ž1 s .

The recommended Žr., or proposed Žp., Li concentrations are taken from Govindaraju Ž1994.. Flanagan et al. Ž1976. report a value of 4.90 ppm Li. c As reported by Anders and Grevesse Ž1989.. b

Other studies

y6.4 " 1.2 Ž1.

y3.8 " 0.4 Ž1.

y0.3 " 1.7 Ž1.

q0.8 " 1.4 Ž1. y4.9 " 0.4 Ž2. y5.1 Ž3.

R.H. James, M.R. Palmerr Chemical Geology 166 (2000) 319–326

Earth has an approximately chondritic Li isotopic composition. Clearly, further studies of rock standards are required by other laboratories measuring Li isotope compositions, before the E6 Li data presented here can be taken as standard values. Nevertheless, we believe that such measurements are important to ensure the reliability of Li isotope geochemical studies, and for the calibration of solid sampling techniques.

4. Conclusions We have presented Li isotope data for nine international rock standards, a C1 chondrite and open ocean seawater. In addition, we have suggested some refinements to analytical techniques that can improve the utility of Li isotope studies for samples with a low Li content. Comparison of our results with others in the literature prompts us to make the following recommendations: Ø The purification of Li for isotopic analysis should be as simple as possible and should ensure complete recovery of Li, and should use the smallest quantity of reagents as possible in order to minimise the possibility of sample contamination. Ø Open ocean seawater should be considered as a standard for Li isotope solution analyses and laboratories carrying out studies of solutions should report a E6 Li value for this standard. We propose a E6 Li value of y31.5 " 1‰ for this standard. Ø Studies of the Li isotopic composition of solid materials should be accompanied by determination of the Li isotope composition of an appropriate international rock standard. Although we believe that the E6 Li values for rock standards reported in this study are reliable, they require ratification.

Acknowledgements This work was supported by NERC through grant BRIDGE 76 to RHJ and by grants from the Royal Society and NERC to MRP. We thank Tony Kemp and Kim Goodman for assistance at various stages of these analyses, and Monica Grady for providing a

325

sample of the Orgueil chondrite. Lui Chan and Marc Chaussidon provided helpful reviews of this manuscript. [PD]

References Anders, E., Grevesse, N., 1989. Abundances of the elements: meteoritic and solar. Geochim. Cosmochim. Acta 53, 197–214. Chan, L.H., 1987. Lithium isotope analysis by thermal ionization mass spectrometry of lithium tetraborate. Anal. Chem. 59, 2662–2665. Chan, L.H., Edmond, J.M., 1988. Variation of lithium isotope compositions in the marine environment — a preliminary report. Geochim. Cosmochim. Acta 52, 1711–1717. Chan, L.H., Edmond, J.M., Thompson, G., 1993. A lithium isotope study of hot springs and metabasalts from mid-ocean ridge hydrothermal systems. J. Geophys. Res. 98, 9653–9659. Chan, L.H., Edmond, J.M., Thompson, G., Gillis, K., 1992. Lithium isotopic composition of submarine basalts — implications for the lithium cycle in the oceans. Earth Planet. Sci. Lett. 108, 151–160. Chan, L.H., Gieskes, J.M., You, C.F., Edmond, J.M., 1994. Lithium isotope geochemistry of sediments and hydrothermal fluids of the Guaymas Basin, Gulf of California. Geochim. Cosmochim. Acta 58, 4443–4454. Chaussidon, M., Robert, F., 1998. Li-7rLi-6 and B-11rB-10 variations in chondrules from the Semarkona unequilibrated chondrite. Earth Planet. Sci. Lett. 164, 577–589. Flanagan, F.J., Wright, T.L., Taylor, S.R., Annel, C.S., Christian, R.C., Dimnin, J.I., 1976. Basalt, BHVO-1 from Kilauea crater, Hawaii. U.S. Geol. Surv. Prof. Pap. 840, 33–40. Flesch, G.D., Anderson, A.R., Svec, H.J., 1973. A secondary isotopic standard for 6 Lir7 Li determinations. Int. J. Mass Spectrom. Ion Processes 12, 265–272. Govindaraju, K., 1994. 1994 complication of working values and sample descriptions for 383 geostandards. Geostandards Newsletter 18, 1–55. Gregoire, D.C., Acheson, B.M., Taylor, R.P., 1996. Measurement ´ of lithium isotope ratios by inductively coupled plasma mass spectrometry: application to geological materials. J. Anal. Atom. Spectrom. 11, 765–772. Hoefs, J., Sywall, M., 1997. Lithium isotope composition of Quaternary and Tertiary biogenic carbonates and a global lithium isotope balance. Geochim. Cosmochim. Acta 61, 2679–2690. Huh, Y., Chan, L.H., Zhang, L., Edmond, J.M., 1998. Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget. Geochim. Cosmochim. Acta 62, 2039–2051. Kemp, A.J., Brown, C.J., 1990. Microwave digestion of carbonate rock samples for chemical analysis. Analyst 115, 1197–1199. Michiels, E., de Bievre, P., 1983. Absolute isotopic composition and the atomic weight of a natural sample of lithium. Int. J. Mass Spectrom. Ion Processes 49, 265–274. Moriguti, T., Nakamura, E., 1993. Precise lithium isotopic analy-

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sis by thermal ionization mass spectrometry using lithium phosphate as an ion-source material. Proc. Jpn. Acad., Ser. B: Phys. Biol. Sci. 69, 123–128. Moriguti, T., Nakamura, E., 1998a. Across-arc variation of Li isotopes in lavas and implications for crustrmantle recycling at subduction zones. Earth Planet. Sci. Lett. 163, 167–174. Moriguti, T., Nakamura, E., 1998b. High-yield lithium separation and the precise isotopic analysis of natural rock and aqueous samples. Chem. Geol. 145, 91–104. Oi, T., Odagiri, T., Nomura, M., 1997. Extraction of lithium from GSJ rock reference samples and determination of their lithium isotopic compositions. Anal. Chim. Acta 340, 221–225. Rieman, W., Walton, H.F., 1970. In: Ion Exchange in Analytical Chemistry. Int. Ser. Monogr. Anal. Chem. 38 Pergamon Press, 295 pp.

Seyfried, W.E., Chen, X., Chan, L.H., 1998. Trace element mobility and lithium isotope exchange during hydrothermal alteration of seafloor weathered basalt: an experimental study at 350 degrees C, 500 bars. Geochim. Cosmochim. Acta 62, 949–960. Stoffyn-Egli, P., Mackenzie, F.T., 1984. Mass balance of dissolved lithium in the oceans. Geochim. Cosmochim. Acta 48, 859–872. Tomascak, P.B., Tera, F., Helz, R.T., Walker, R.J., 1999. The absence of lithium isotope fractionation during basalt differentiation: new measurements by multicollector sector ICP-MS. Geochim. Cosmochim. Acta 63, 907–910. You, C.F., Chan, L.H., 1996. Precise determination of lithium isotopic composition in low concentration natural samples. Geochim. Cosmochim. Acta 60, 909–915.