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Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS
Chemical Geology 181 Ž2001. 169–179 www.elsevier.comrlocaterchemgeo
Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS Jan Kosler ˇ a,b,) , Michal Kucera ˇ c , Paul Sylvester b a
b
Department of Geochemistry, Charles UniÕersity, Prague CZ 12843, Czech Republic Department of Earth Sciences, Memorial UniÕersity of Newfoundland, St. John’s, NF, Canada A1B 3X5 c Department of Geological Sciences, UC Santa Barbara, Santa Barbara, CA 93106-9630, USA Received 16 August 2000; accepted 1 March 2001
Abstract Precise measurements of the isotopic composition of Li in low-concentration natural carbonate samples is demonstrated for quadrupole ICPMS. Cool plasma conditions result in high samplerbackground ratios for the measured Li signals even for low concentrations. Quantitative chemical separation of Li prior to the measurement on ICPMS is necessary to avoid the matrix-induced isotopic fractionation and the results have to be externally corrected for mass bias and instrumental fractionation. The isotopic composition of lithium can be measured in samples containing only 5–10 ng Li with a within-run precision better than 0.5‰ Ž2 standard errors of the mean., and a long term reproducibility that is better that 2.1‰ Ž2 standard deviations.. The method has been successfully used to measure Li isotopic composition of 1–10 mg samples of calcium carbonate tests of Pulleniatina obliquiloculata and Globorotalia tumida that were collected from the sediment– seawater interface and have Li concentrations of about 1 ppm. The isotopic composition of Li in studied calcium carbonate tests of P. obliquiloculata is independent of the size of tests and resembles the lithium isotopic composition of modern seawater, suggesting that this species may provide a record of Li isotope variations in past oceans. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma mass spectrometry; Li isotopes; Planktonic foraminifera; Mass bias; Matrix effect
1. Introduction Isotopes of light and heavy elements are used to constrain the mass balance of elements in Earth’s crust, mantle and oceans and to monitor the fluxes between them. The fluxes can then be used to derive
)
Corresponding author. Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NF, Canada A1B 3X5. Fax: q1-709-737-2589. E-mail address: [email protected] ŽJ. Kosler ˇ ..
the cycles of individual elements in nature. The isotopic composition of lithium is potentially a powerful tracer of geochemical processes such as magmatic differentiation, crust–mantle recycling, hydrothermal alteration and continental weathering ŽHuh et al., 1998; Moriguti and Nakamura, 1998a; Zhang et al., 1998; Tomascak et al., 1999b, 2000.. Unlike the fluxes of elements such as C, N, S, H, O, Sr or Pb that have been studied extensively, the behaviour of Li is only poorly understood due to the lack of reliable isotopic data for most geological materials. It has been demonstrated, however, that
0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 1 . 0 0 2 8 0 - 7
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there are systematic differences in Li isotopic composition between geochemical reservoirs and that the natural variations in the isotopic composition of Li are large, with a range of ca. 100‰ ŽHoefs and Sywall, 1997; Huh et al., 1998; Moriguti and Nakamura, 1998a.. As the Earth’s crust and seawater represent significant reservoirs of Li, it is particularly important to understand Li behaviour in the marine environment in order to better constrain the present-day Li fluxes. If a proxy for the Li isotopic composition of the seawater could be developed, it may also be possible to better understand interactions between geochemical reservoirs in the past. Previous studies have shown that the composition of biogenic calcite, especially that of tests of planktonic foraminifera, may be used as a proxy for seawater composition in the past, provided that the effects of potential biological isotopic fractionation among different foraminiferal species are understood. Derived secular variations in seawater composition have been related to long-lasting Žmillion year. tectonic processes Že.g. 87 Srr86 Sr; Hess et al., 1986. and short-lived events such as are the Quaternary glaciations Že.g. 143 Ndr144 Nd; Palmer and Elderfield, 1985; Vance and Burton, 1999.. Such interpretations have been made possible by Ž1. studies of the partitioning of relevant elements and isotopes between seawater and foraminiferal calcite and Ž2. development of analytical techniques capable of precise and accurate isotopic measurements. Detailed study of Li isotopic composition in geological materials has been precluded by the lack of analytical techniques capable of high-sensitivity, precise, and accurate measurements of lithium isotopes. Various instrumental techniques have been employed to measure the isotopic composition of Li in natural samples including atomic emission and absorption spectrometry ŽAESrAAS., neutron activation ŽNA., secondary ion mass spectrometry ŽSIMS., thermal ionisation mass spectrometry ŽTIMS. and inductively coupled plasma ŽICP. mass spectrometry utilising quadrupole, magnetic sector and time-of-flight instruments. Each method has important limitations. AASrAES, NA and SIMS techniques cannot achieve a precision of 1% or better, making them incapable of resolving the detailed variations in Li isotopic composition seen in nature. TIMS suffers from large instrumental fractionation of Li isotopes in its
thermionic source ŽBickle et al., 2000., although several attempts have been made to reduce this fractionation by measuring the isotopes as borate, fluoride or phosphate ions ŽChan, 1987; Green et al., 1988; You and Chan, 1996; Moriguti and Nakamura, 1998b.. The technique also requires relatively largemass Li samples Ž) 100 ng. separated from their matrix prior to analysis, to achieve reproducibility that is useful for geological applications, unless time-consuming and costly procedures are employed that allow to reduce the sample size and still retain a useful precision and reproducibility ŽYou and Chan, 1996; Moriguti and Nakamura, 1998b.. It has been recently demonstrated that accurate and precise determination of Li isotopes in natural samples can be achieved by multi-collector Žmagnetic sector. ICPMS ŽTomascak et al., 1999a,b.. MC ICPMS allows a rapid analysis Ž; 2 samplesrh. of moderate-mass samples Ž40 ng Li. with a reproducibility better than 1.1‰. Although it requires pre-separation of Li on ion exchange columns, it is not as sensitive to matrix contamination of analyte solutions as TIMS. Yet, this technique requires rather expensive instrumentation that is not readily available in most geochemical laboratories. Also, with conventional Faraday detectors there is a significant loss of precision for small-mass samples Ž- 10 ng.. There is consequently much interest in measuring Li isotopes using single detector quadrupole ICPMS. These instruments are more readily available in geochemical laboratories than their multicollector counterparts. Also, the electron multiplier used in quadrupole instruments allows measurement of isotopic ratios in small-mass samples with a precision comparable to TIMS and MC ICPMS. The availability of precise measurements of Li isotopes in smallmass samples is particularly desirable when measuring the Li isotopic composition of foraminiferal tests, where separation of large numbers of tests from marine sediment is limited by time and by the small amount of sediment recovered from deep-sea cores. So far there have been only three attempts to analyse the isotopic composition of Li by quadrupole ICPMS. Sun et al. Ž1987. and Vanhoe et al. Ž1991. could not achieve a precision better than 1%. In contrast, Gregoire et al. Ž1996. were able to make Li ´ isotopic measurements in minerals containing less than 300 ppm Li to a precision better than 0.8‰.
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However, large background count rates on both lithium masses resulted in signalrbackground ratios of 120, precluding measurement of samples containing less than a microgram of Li. Here we report results for measurements of Li isotopes that demonstrate that single detector quadrupole ICPMS is capable of achieving a within-run precision of less than 1‰ and a long-term reproducibility close to 2‰ on 7 Lir6 Li ratio, even on samples containing as little as 10 ng Li. The method, therefore, provides a rapid and affordable means to study Li isotopes in low concentration natural samples.
2. Analytical techniques Similar to other light elements, the isotopic composition of lithium is usually expressed as delta units calculated from ratios of the less abundant to more abundant isotope relative to the composition of a suitable standard. Unlike most other light elements where the light isotope is also the more abundant one, the natural abundances of 6 Li and 7 Li are 7.5%
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and 92.5%, respectively, and the delta values are often calculated from 6 Lir7 Li ratios as d6 Li w ‰ x s
ž
Ž 6 Lir7 Li . Sample y1 Ž 6 Lir7 Li . Standard
/
1000.
This notation has been used in most previous Li isotopic studies although it has caused some confusion because the negative delta values correspond to heavy Li isotopic composition Ži.e. opposite to delta values of other light elements.. We will refer to the isotopic composition of Li in delta units relative to the NBS lithium carbonate standard L-SVEC Ž6 Lir7 Li s 0.0832 " 0.0002; Flesch et al., 1973. and the mean seawater composition ŽTable 1; 6 Lir7 Li s 0.080151 " 0.00008; James and Palmer, 2000a.. 2.1. Instrument parameters and data acquisition The instrument used for this study was a VG Elemental PQ3 ICPMS at Charles University in Prague. Samples were mixed online with 10 ppb Be solution and aspirated to the plasma through an MCN-100 microconcentric nebuliser ŽCetac. and wa-
Table 1 Isotopic composition of Li in planktonic foraminiferal tests from Ontong-Java Plateau and Ceara Rise Total procedural blank and L-SVEC Li standards were measured in the course of the same analysis. Sample weight Žmg.
Li Žppm.
6
Lir 7 Li
"2 sm Ž‰.
d 6 Li Ž‰, L-SVEC.
d 6 Li Ž‰, seawater.
Ontong-JaÕa Plateau (cruise MW 91-9, sample GGC 48) P. obliquiloculata Ž500–700 mm. 6.98 0.94 P. obliquiloculata Ž300–500 mm. 7.42 1.05 G. tumida Ž500–1000 mm. 1.28 0.71
0.08068 0.08074 0.08009
0.30 0.47 0.72
y30.35 y29.54 y37.38
1.18 2.02 y6.08
Ontong-JaÕa Plateau (cruise MW 91-9, sample GGC 8) P. obliquiloculata Ž500–700 mm. 11.11 0.97 P. obliquiloculata Ž300–500 mm. 9.60 0.97 G. tumida Ž500–1000 mm. 2.30 0.78
0.08094 0.08075 0.07919
0.45 0.37 1.52
y27.13 y29.44 y48.15
4.50 2.12 y17.20
Ceara Rise (cruise KN142-2, sample KC 62) G. tumida Ž500–1000 mm. 8.84
0.85
0.08077
0.34
y29.21
2.36
Blank and standards Total procedural blank ŽTPB. L-SVEC Ž1. L-SVEC Ž2.
120 pg Li a 0.01b 0.01b
0.07988 0.08323 0.08315
3.34 0.27 0.54
y39.90 0.36 y0.59
y8.69 32.88 31.90
a b
Total Li contribution in picogram to each sample. Concentration in parts per million in the measured solution.
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ter cooled Scott-type spray chamber at a rate of 0.1 mlrmin. Cool plasma conditions ŽRF power 700 W. were used and the plasma was shielded against the load coil using a Ni ring insert ŽPlasmaScreen. to achieve lower background and to improve the signal stability. The instrument was tuned to a sensitivity of at least 10 7 cpsrppm Li on mass 7 and the corresponding background measured on 2% HNO 3 was less than 10 cps. Data were acquired in peak jumping mode for masses 6, 7 and 9 with 1 point measured per mass unit and a dwell time of 10.24 msrpoint. Quadrupole settling time was set to 1.5 ms and dead time of the electron multiplier was 20 ns. Each analysis consisted of 300 s of measurement. Each sample analysis was bracketed by measurements of a 10-ppb solution of L-SVEC Li standard. The data were processed off-line and, where applicable, the raw counts on masses 6 and 7 were corrected for instrument drift using the 9 Be signal prior to blank subtraction. Li concentrations were calculated by calibration to the L-SVEC standard. For Li isotope measurements, correction for instrument mass bias was made by linear interpolation of 6 Lir7 Li ratios of replicate L-SVEC analyses that bracketed each sample. Experimental errors were propagated through the calculation and the final errors are quoted at 2 s level. 2.2. Instrument mass bias Mass discrimination in ICPMS is mainly caused by Ž1. preferential transmission of ions of isotopes
used for instrument optimisation due to their particular kinetic energies, and Ž2. space charge effects caused by concomitant elements present in the sample matrix ŽBeauchemin et al., 1987; Douglas and Tanner, 1998.. During this study, the instrument was always optimised on both lithium masses and the measured 6 Lir7 Li ratios in L-SVEC differed from the expected value of 0.0832 ŽFig. 1.. There was no systematic variation of measured 6 Lir7 Li ratios with Li concentration in the solution but the measured 6 Lir7 Li values varied from day-to-day depending on the optimal tuning parameters for the instrument on a given day. Concentration-dependent mass discrimination of 6 Li Ži.e. shift to higher 6 Lir7 Li values with increasing Li concentration in the solution. has been reported by Gregoire et al. Ž1996. while Sun et al. ´ Ž1987. reported the opposite trend Ži.e. decreasing 6 Lir7 Li values with increasing Li concentration in the solution.. Although it is difficult to provide a simple explanation of mechanisms causing this effect, during the course of this study, we were able to compensate for the concentration-dependent mass discrimination by selecting instrument operating conditions Želectrostatic lens settings. that, within the error of the measurement, yielded consistent isotopic ratios for a range of Li concentrations Žcf. Fig. 1, Day 3.. We have achieved a stable signal on both Li masses with no significant variations in the fractionation of Li isotopes over the period of 300-s acquisition corresponding to each analysis ŽFig. 2.. For low count rates Žless than 10,000 cps on mass 7, corresponding to 1 ppb Li in the solution or less., the
Fig. 1. Uncorrected 6 Lir7 Li ratios measured in solution of L-SVEC Li carbonate standard with different concentrations at three different lens settings ŽDays 1–3.. Error bars are 2 standard errors of the mean based on 10 repeat measurements.
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Fig. 2. Measured Li isotopic composition of 10 ppb Li L-SVEC standard using two different lens settings ŽA and B.. The data were collected during 300-s acquisitions and each data point corresponds to 6 Lir7 Li ratios integrated over a period of 30 s. Dashed line represents the calculated mean 6 Lir7 Li values.
within-run precision of 6 Lir7 Li measurements was worse than the long-term reproducibility and should, therefore, be used as a measure of analytical error. In order to achieve accurate results, the measured values have to be corrected for instrument mass bias using interpolated 6 Lir7 Li ratios of two bracketing Li standards. For samples where Li is present as a trace component Ži.e. at ppm level., the presence of matrix ions in the plasma may result in formation of a space charge, repelling of lighter ions towards the marginal parts of the plasma and enhancing the signal of heavier ions. For foraminiferal tests, the presence of calcium, magnesium, sodium and potassium matrix ions suppresses the signal on both lithium masses and, due to a large mass difference between 6 Li and 7 Li Ž16.7%., results in fractionation of lithium isotopes towards lower d6 Li values. We have measured a series of 10 ppb solutions of lithium L-SVEC standards containing variable amounts of Ca, Mg, Na and K, added in proportions typical of marine carbonate sample. The results show a massive shift of more than 30‰ towards lower d6 Li values for samples where concentration of matrix elements exceeded the Li content by more than 10 3 Ži.e. typical
of foraminiferal tests that are mostly formed by calcium carbonate and often contain 1 ppm Li or less, Fig. 3.. On the other hand, if the ratio of matrix elementrLi did not exceed 1000, the 6 Lir7 Li ratios were not sensitive to the presence of matrix elements and the variation of d6 Li values was within the limits of long term reproducibility of our measurements. Although the matrix effects on the 6 Lir7 Li ratios can be corrected for by selection of Li isotopic standards that match the matrix composition of the unknown samples, this correction method is difficult to use for analyses of sets of samples with different matrix compositions. Alternatively, the matrix effect on the Li isotopic composition can be removed by chromatographic separation of Li from the matrix. 2.3. Sample preparation and separation of Li from matrix In order to achieve low procedural blanks, we have used acids purified by two-step sub-boiling distillation in quartz and PFA Savillex stills throughout this study. All other chemicals were analytical grade and resistivity of deionised water was at least 18 m V. Samples of foraminiferal tests were sieved
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were again inspected under the microscope to ensure they were sediment-free before they were weighted into screw cap PFA Savillex beakers and dissolved in 0.3 ml 1 M HCl being at 808C for 12 h. Following the evaporation to dryness, the samples were converted to nitrate and dissolved in 0.5 ml 0.67 M HNO 3 made up in 30% methanol. The ion exchange chemistry followed the procedure described in Tomascak et al. Ž1999a., modified for small-volume samples. Exchange columns Ž2 ml polyethylene BioRad. contained 1.8 ml Dowex AG50W-x8 Ž200–400 mesh. cation exchange resin. They were cleaned before use and between samples with large volumes of acid and deionised water and calibrated with solution containing Žin mg. 0.1 Li ŽL-SVEC., 3 Na and K, 5 Mg, 10 Sr and 4000 Ca Ži.e. matching the Li content and matrix composition of carbonate foraminiferal tests. to ensure complete separation of Li from the matrix Ž100% yield.. Proper separation of Li from the matrix is a crucial step in this procedure as failure to achieve complete Li recovery from the columns results in a strong fractionation of Li isotopes ŽFig. 4.. Samples were loaded onto columns in 0.5 ml 0.67 M HNO 3 made up in 30% methanol and eluted with 1 ml deionised water and 4 ml 1 M HNO 3 made up in 80% methanol. The Li fraction was collected in a following 6 ml 1 M HNO 3 elution, dried down and dissolved in 1 ml 2% HNO 3 before the analysis on ICPMS. Total procedural blank in the course of this study was 120 pg Li. Fig. 3. Effect of concentration of matrix elements ŽCa, Mg, Na and K. on the measured Li isotopic composition of 10 ppb Li L-SVEC standard. Error bars are 2 s .
to different size fractions, hand-picked under the microscope and crushed to break-open the chambers that were checked for contamination by sediment material before they were repeatedly ultrasonically cleaned Ž10–15 s, three times. in ethanol. Sediment is potentially a major source of Li contamination in the procedure as it contains much more of the element than do the foraminiferal tests. Each sample consisted of less than 50 foraminiferal tests, containing a total of only 1–10 ng Li. This was followed by 4 min of cleaning in hydrogen peroxide and 10 s in 1% HNO 3 and the reagents were repeatedly decanted with deionised water after each step. The samples
2.4. Within run precision and long term reproducibility The within-run precision of 6 Lir7 Li isotopic ratio measurements expected from counting statistics is 1.3‰ Ž2 s ., assuming 10 7 cps on mass 7rppm Li for a 10-ppb solution with 300 s of acquisition. This can be further improved to a precision better than 0.3‰ by increasing the signalrbackground ratio of the measurement, by using longer counting times for the less abundant Li isotope and by increasing the total time of signal acquisition. In the course of this study, the actual within-run precision, calculated as the standard error of the mean Ž sm ., on a 10-ppb Li L-SVEC standard was always better than 1.5‰, and the most precise measurements were better that 0.5‰, based on 850 integrated 6 Lir7 Li isotopic ratios cal-
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Fig. 4. Fractionation of Li isotopes in aliquots of Li separated in sequential fashion from the L-SVEC standard on Dowex AG50W-x8 cation exchange resin. Complete recovery of Li is attained by collection of the 5–10 ml HNO 3 elution and avoids fractionation of Li isotopes during the collection step.
culated from more than 8500 time slices. However, error calculations based on the large number of isotopic ratio measurements typically made during a single quadrupole ICPMS analysis result in unrealistically low standard error of the mean. A better measure of analytical error is the reproducibility Žexternal precision., expressed as 2 standard deviations of 6 Lir7 Li isotopic ratios measured over a long period of time ŽThirwall 1991..
We have monitored the reproducibility of Li isotopic measurements of L-SVEC standard for more than 6 months and the resulting 2 standard deviations on the average 6 Lir7 Li value of 0.08321 is 0.00018, or 2.1‰ ŽFig. 5.. We, therefore, quote the 2.1‰ as a best estimate of error for isotopic analysis of Li on quadrupole ICPMS. However, given the strong Li isotope fractionation that may occur during the chromatographic separation of Li from the matrix in
Fig. 5. Long term reproducibility of Li isotopic measurements in 10 ppb L-SVEC standard ŽAugust 1999–January 2000.. Open circles represent composition of L-SVEC standard separated from matrix elements on cation exchange resin, dotted lines correspond to 2.1‰ 2 standard deviations reproducibility limits, error bars are 2 standard errors of the mean Žblack square..
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small volume columns ŽFig. 4., this error estimate is valid only if perfect column calibration is maintained.
3. Results and discussion We have used the described method to study the isotopic composition of Li in calcium carbonate tests of planktonic foraminifera. Samples of Pulleniatina obliquiloculata and Globorotalia tumida were separated from top 2 cm of cores collected by giant gravity corer from two sites on the Ontong-Java Plateau Žwestern Pacific, cruise MW91-9, sample GGC8: 2812.54X S, 156857.90X E, water depth 1625 and cruise MW91-9, sample GGC48: 080.47X S, 16180.20X E, water depth 3397 m. and by knight corer from the Ceara Rise Žwestern equatorial Atlantic, cruise KN142-2, sample KC62: 5845.20X N, 43823.90X W, water depth 4344 m.. Li concentrations and isotopic data for different size fractions of the foraminiferal tests are given in Table 1 and Fig. 6. The studied tests contain 0.7–1 ppm Li and their Li isotopic composition varies from y27.13‰ to y48.15‰ relative to L-SVEC. While our limited data set shows a variable Li isotopic composition for
G. tumida, the isotopic compositions of Li in all but one of the studied fractions of P. obliquiloculata plot within the range of Li isotopic composition that has been reported for the present-day seawater ŽFig. 6., which is from y28.8‰ to y32.8‰ ŽYou and Chan, 1996; Chan and Edmond, 1988; Moriguti and Nakamura, 1998b; Tomascak et al., 1999a; James and Palmer, 2000a.. Calcium carbonate foraminiferal tests may preferentially dissolve with increasing water depth ŽBrown and Elderfield, 1996.. The difference in water depth between samples GGC8 and GGC48 apparently had no measurable effect on Li isotopes in P. obliquiloculata, however, as the isotopic compositions of Li in all fractions of this foraminifera species are identical within analytical error. In addition, the composition of P. obliquiloculata is virtually independent of the size of the tests. Consistent results for the two size fractions of P. obliquiloculata from both samples GGC8 and GGC48 also point to good reproducibility of our measurements. Low and restricted Li contents in all studied foraminifera samples Ž0.7–1 ppm., well within the range of concentrations previously reported from foraminiferal tests Ž0.2–2.6 ppm; Delaney et al., 1985; Delaney and Boyle, 1986;You and Chan, 1996; Hoefs and Sywall, 1997., indicate that
Fig. 6. Isotopic composition of Li in planktonic foraminiferal tests of P. obliquiloculata Žopen circles. and G. tumida Žopen squares. from Ontong-Java Plateau ŽGGC48 and GGC8. and Ceara Rise ŽKC62.. Also plotted are compositions of four fractions of P. obliquiloculata from ODP site 130-806B in Ontong-Java Plateau with ages of 804–397 ka ŽYou and Chan, 1996; full circles. and two 10 ppb Li L-SVEC standards and total procedural blank ŽTPB. measured in the course of the analysis. Error bars are 2 s .
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the cleaning procedure adopted in this study was efficient. and that our data are not significantly affected by contamination from the host sediments whose Li content usually exceeds that of the foraminiferal tests by at least one order of magnitude Že.g. You and Chan, 1996; James and Palmer, 2000b.. Given the long residence time of Li in seawater Ž0.3–3 = 10 6 years; Edmond et al., 1979; StoffynEgli and Mackenzie, 1984., secular variations in the Li isotopic composition of the oceans may reflect variable inputs from two major sources: fluvial transport of the products of continental weathering Žfrom y6‰ to y32.2‰; Huh et al., 1998. and high-temperature hydrothermal alteration of ocean floor basalts Žy9‰; Chan et al., 1992, 1993.. To achieve and maintain the mean present-day seawater Li isotopic composition of y31.5‰, additional inputs or sinks that fractionate Li isotopes, such as is the early diagenesis of clay minerals ŽJames and Palmer, 2000b., are required. Thus, if ancient specimens of P. obliquiloculata could be used as proxy for the Li isotopic composition of past oceans, they would provide us with invaluable information about variations in rates of continental weathering, andror sea floor spreading. Not much is known about the Li isotopic systematics in ancient oceans and planktonic foraminifera. Hoefs and Sywall Ž1997. reported differences in isotopic composition of Li between Holocene Žy19‰. and Tertiary Žfrom y42‰ to y2‰. foraminiferal tests, but their study was based on large-mass samples Ž50 mg corresponding to 100–1000 individuals. that contained mixture of several foraminifera species. Variations of ca. 21‰ in Li isotopic composition on much smaller time scale of 407 ka and values as low as y40‰ were reported by You and Chan Ž1996. for samples of P. obliquiloculata from ODP Site 806B on the Ontong-Java Plateau. These previous studies, along with the data presented in this paper, suggest that Ž1. planktonic foraminiferal tests contain Li at low ppm level, Ž2. there are variations in Li isotopic composition between different foraminiferal species and Ž3. there are variations in Li isotopic composition of P. obliquiloculata tests of different ages. Our data indicate that Li isotopic composition of P. obliquiloculata from the sediment–water interface is close to the mean present-day seawater composition and is independent of the test size. However, more study is
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needed to assess possible effects of carbonate dissolution below the carbonate compensation depth ŽBrown and Elderfield, 1996; Martin and Sayles, 1996., winnowing, and spatial–temporal variations before P. obliquiloculata or another species of planktonic foraminifera can be used as a reliable proxy for the Li isotopic composition of seawater. 3.1. Comparison of quadrupole ICPMS with TIMS and MC ICPMS This study presents the first Li isotopic data from small, well characterised samples of individual foraminiferal species. This has been enabled by Ž1. low total procedural blanks, Ž2. the high sensitivity of a quadrupole ICPMS, and Ž3. the large sampler background ratio of the Li signal measured by ICPMS under cool plasma conditions. TIMS techniques are often used for precise and accurate Li isotopic analysis of geological samples but ICPMS techniques offer several advantages. Analysis of Li isotopes by TIMS requires samples that contain at least 100 ng Li, which has been separated completely from the matrix elements prior to the analysis. Although small samples Žca. 2 ng Li. have also been analysed by TIMS ŽYou and Chan, 1996., the procedure is time consuming and costly. The reproducibility of TIMS Li isotopic analyses usually varies between 1–2‰. Magnetic sector multicollector ŽMC. ICPMS provides an alternative to TIMS in that it can achieve a comparable reproducibility of Li isotopic ratios for samples that contain only 40 ng Li ŽTomascak et al., 1999a,b.. Although it still requires separation of Li from the matrix, compared to TIMS, MC ICPMS has a higher level of tolerance for matrix elements and analysis is 5–10 times faster. Unlike in single detector quadrupole ICPMS where precision of isotopic measurements potentially suffers from short term variations of isotopic signals caused by plasma instability Žso-called flicker noise., simultaneous detection of both Li isotopes in MC ICPMS eliminates this problem. In addition, the different shape of ion beam and more even energy distribution of ions in MC ICPMS produce flat-top peaks which allow more precise isotopic measurements. Fast switching between individual masses by a quadrupole mass filter mostly compensates for the instability of Li isotopic signal. Accordingly, the somewhat lower precision
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of quadrupole ICPMS isotopic measurements compared to MC ICPMS may be attributed to the shape of peaks in the quadrupole mass spectrum and small deviations from perfect mass calibration. On the other hand, most quadrupole ICPMS instruments use an electron multiplier detector that is ca. 1000 times more sensitive compared to conventional Faraday detectors that are used for simultaneous isotopic measurements in most MC ICPMS and TIMS instruments. The high sensitivity of quadrupole instruments enables measurements of large Li samples Ž) 40 ng. with a precision that is only 1.5–2 times worse compared to TIMS and MC ICPMS, but, more importantly, it also allows precise measurements of the Li isotopic composition in small samples Žca. 5 ng Li., which cannot be done routinely by MC ICPMS or TIMS. Moreover, as the limiting error in Li isotopic analysis is largely dependent on quantitative Li separation from the matrix elements and the efficiency of instrument mass bias corrections, the somewhat lower precision of quadrupole ICPMS measurements compared to TIMS and MC ICPMS is not an obstacle to most geochemical applications.
Acknowledgements We thank Dan McCorkle ŽWHOI. for providing foraminifera samples for this study and Marie Fayadova´ for laboratory assistance. Jochen Hoefs, Conrad Gregoire and an anonymous referee provided ´ useful comments to the manuscript. This work was supported by Charles University through grants 286r1999B-Geo and CEZ:J13r98:113100005. ICPMS facility at Charles University was funded by PHARE.
References Beauchemin, D., McLaren, J.W., Berman, S.S., 1987. Study of the effects of concomitant elements in inductively coupled plasma mass-spectrometry. Spectrochim. Acta, Part B—At. Spectrosc. 42, 467–490. Bickle, M.J., Chapman, H.J., You, C.F., 2000. Measurement of lithium isotopic ratios as lithium tetraborate ions. Int. J. Mass Spectrom. 202, 273–282.
Brown, S.J., Elderfield, H., 1996. Variations in MgrCa and SrrCa ratios of planktonic foraminifera caused by postdepositional dissolution: evidence of shallow Mg-dependent dissolution. Paleoceanography 11, 543–551. Chan, L.H., 1987. Lithium isotope analysis by thermal ionisation 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., Gillis, K., 1992. Lithium isotopic composition of submarine basalts: implication for the lithium cycle in the oceans. Earth Planet. Sci. Lett. 108, 151–160. Chan, L.H., Edmond, J.M., Thompson, G., 1993. A lithium isotopic study of hot springs and metabasalts from mid-ocean ridge hydrothermal systems. J. Geophys. Res. 98, 9653–9659. Delaney, M.L., Boyle, E.A., 1986. Lithium in foraminiferal shells: implications for high-temperature hydrothermal circulation fluxes and oceanic crustal generation rates. Earth Planet. Sci. Lett. 80, 91–105. Delaney, M.L., Be, ´ A.W.H., Boyle, E.A., 1985. Li, Sr, Mg and Na in foraminiferal calcite shells from laboratory culture, sediment traps, and sediment cores. Geochim. Cosmochim. Acta 49, 1327–1341. Douglas, D.J., Tanner, S.D., 1998. Fundamental considerations in ICPMS. In: Montasser, A. ŽEd.., Inductively Coupled Plasma Mass Spectrometry. Wiley VCH, New York, pp. 615–679. Edmond, J.M., Measures, C., McDuff, R.E., Chan, L.H., Collier, R., Grant, B., Gordon, L.I., Corliss, J.B., 1979. Ridge crest hydrothermal activity and the balance of the major and minor elements in the ocean: the Galapagos data. Earth Planet. Sci. Lett. 46, 1–18. Flesch, G.D., Anderson Jr, A.R., Svec, H.J., 1973. A secondary isotopic standard for 6 Lir7 Li determinations. Int. J. Mass Spectrom. Ion Processes 12, 265–272. Green, L.W., Leppinen, J.J., Elliot, N.L., 1988. Isotopic analysis of lithium as thermal dilithium fluoride ions. Anal. Chem. 60, 34–37. 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. At. Spectrom. 11, 765–772. Hess, J., Bender, M.L., Schilling, J.G., 1986. Evolution of the ratio of strontium-87 to strontium-86 in seawater from Cretaceous to present. Science 231, 979–984. Hoefs, J., Sywall, M., 1997. Lithium isotope composition of Quaternary and Tertiary biogene carbonates and global lithium isotope balance. Geochim. Cosmochim. Acta 61, 2679–2690. Huh, Y., Chan, L.H., Zhang, L., Edmond, J., 1998. Lithium and its isotopes in major world rivers: implications for weathering and the oceanic budget. Geochim. Cosmochim. Acta 62, 2039–2051. James, R.H., Palmer, M.R., 2000a. The lithium isotope composition of international rock standards. Chem. Geol. 166, 319– 326. James, R.H., Palmer, M.R., 2000b. Marine geochemical cycles of
J. Kosler ˇ et al.r Chemical Geology 181 (2001) 169–179 the alkali elements and boron: the role of sediments. Geochim. Cosmochim. Acta 64, 3111–3122. Martin, W.R., Sayles, F.L., 1996. CaCO 3 dissolution in sediments of the Ceara Rise, western equatorial Atlantic. Geochim. Cosmochim. Acta 60, 243–263. 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. Palmer, M.R., Elderfield, H., 1985. Variations in the Nd isotopic composition of foraminifera from Atlantic Ocean sediments. Earth Planet. Sci. Lett. 73, 299–305. Stoffyn-Egli, P., Mackenzie, F.T., 1984. Mass balance of dissolved lithium in the oceans. Geochim. Cosmochim. Acta 48, 859–872. Sun, X.F., Ting, B.T.G., Zeisel, S.H., Janghorbani, M., 1987. Accurate measurement of stable isotopes of lithium by inductively coupled plasma mass-spectrometry. Analyst 112, 1223– 1228. Thirwall, M.F., 1991. Long term reproducibility of multicollector Sr and Nd isotope ratio analyses. Chem. Geol. 94, 85–104 ŽIsot. Geosci. Section.. Tomascak, P.B., Carlson, R.W., Shirey, S.B., 1999a. Accurate and
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precise determination of Li isotopic compositions by multicollector sector ICP-MS. Chem. Geol. 158, 145–154. Tomascak, P.B., Tera, F., Helz, R.T., Walker, R.J., 1999b. The absence of lithium isotope fractionation during basalt differentiation: new measurements by multicollector sector ICP-MS. Geochim. Cosmochim. Acta 63, 907–910. Tomascak, P.B., Ryan, J.G., Defant, M.J., 2000. Lithium isotope evidence for light element decoupling in the Panama subarc mantle. Geology 28, 507–510. Vance, D., Burton, K., 1999. Neodymium isotopes in planktonic foraminifera: a record of the response of continental weathering and ocean circulation rates to climate change. Earth Planet. Sci. Lett. 173, 365–379. Vanhoe, H., Vandecasteele, C., Versieck, J., Dams, R., 1991. Determination of lithium in biological samples by inductively coupled plasma mass-spectrometry. Anal. Chim. Acta 244, 259–267. You, C.F., Chan, L.H., 1996. Precise determination of lithium isotopic composition in low concentration natural samples. Geochim. Cosmochim. Acta 60, 909–915. Zhang, L., Chan, L.H., Gieskes, J.M., 1998. Lithium isotope geochemistry of pore waters from Ocean Drilling Program Sites 918 and 919, Irminger Basin. Geochim. Cosmochim. Acta 62, 2437–2450.
Precise measurement of Li isotopes in planktonic foraminiferal tests by quadrupole ICPMS Jan Kosler ˇ a,b,) , Michal Kucera ˇ c , Paul Sylvester b a
b
Department of Geochemistry, Charles UniÕersity, Prague CZ 12843, Czech Republic Department of Earth Sciences, Memorial UniÕersity of Newfoundland, St. John’s, NF, Canada A1B 3X5 c Department of Geological Sciences, UC Santa Barbara, Santa Barbara, CA 93106-9630, USA Received 16 August 2000; accepted 1 March 2001
Abstract Precise measurements of the isotopic composition of Li in low-concentration natural carbonate samples is demonstrated for quadrupole ICPMS. Cool plasma conditions result in high samplerbackground ratios for the measured Li signals even for low concentrations. Quantitative chemical separation of Li prior to the measurement on ICPMS is necessary to avoid the matrix-induced isotopic fractionation and the results have to be externally corrected for mass bias and instrumental fractionation. The isotopic composition of lithium can be measured in samples containing only 5–10 ng Li with a within-run precision better than 0.5‰ Ž2 standard errors of the mean., and a long term reproducibility that is better that 2.1‰ Ž2 standard deviations.. The method has been successfully used to measure Li isotopic composition of 1–10 mg samples of calcium carbonate tests of Pulleniatina obliquiloculata and Globorotalia tumida that were collected from the sediment– seawater interface and have Li concentrations of about 1 ppm. The isotopic composition of Li in studied calcium carbonate tests of P. obliquiloculata is independent of the size of tests and resembles the lithium isotopic composition of modern seawater, suggesting that this species may provide a record of Li isotope variations in past oceans. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Inductively coupled plasma mass spectrometry; Li isotopes; Planktonic foraminifera; Mass bias; Matrix effect
1. Introduction Isotopes of light and heavy elements are used to constrain the mass balance of elements in Earth’s crust, mantle and oceans and to monitor the fluxes between them. The fluxes can then be used to derive
)
Corresponding author. Department of Earth Sciences, Memorial University of Newfoundland, St. John’s, NF, Canada A1B 3X5. Fax: q1-709-737-2589. E-mail address: [email protected] ŽJ. Kosler ˇ ..
the cycles of individual elements in nature. The isotopic composition of lithium is potentially a powerful tracer of geochemical processes such as magmatic differentiation, crust–mantle recycling, hydrothermal alteration and continental weathering ŽHuh et al., 1998; Moriguti and Nakamura, 1998a; Zhang et al., 1998; Tomascak et al., 1999b, 2000.. Unlike the fluxes of elements such as C, N, S, H, O, Sr or Pb that have been studied extensively, the behaviour of Li is only poorly understood due to the lack of reliable isotopic data for most geological materials. It has been demonstrated, however, that
0009-2541r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 5 4 1 Ž 0 1 . 0 0 2 8 0 - 7
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there are systematic differences in Li isotopic composition between geochemical reservoirs and that the natural variations in the isotopic composition of Li are large, with a range of ca. 100‰ ŽHoefs and Sywall, 1997; Huh et al., 1998; Moriguti and Nakamura, 1998a.. As the Earth’s crust and seawater represent significant reservoirs of Li, it is particularly important to understand Li behaviour in the marine environment in order to better constrain the present-day Li fluxes. If a proxy for the Li isotopic composition of the seawater could be developed, it may also be possible to better understand interactions between geochemical reservoirs in the past. Previous studies have shown that the composition of biogenic calcite, especially that of tests of planktonic foraminifera, may be used as a proxy for seawater composition in the past, provided that the effects of potential biological isotopic fractionation among different foraminiferal species are understood. Derived secular variations in seawater composition have been related to long-lasting Žmillion year. tectonic processes Že.g. 87 Srr86 Sr; Hess et al., 1986. and short-lived events such as are the Quaternary glaciations Že.g. 143 Ndr144 Nd; Palmer and Elderfield, 1985; Vance and Burton, 1999.. Such interpretations have been made possible by Ž1. studies of the partitioning of relevant elements and isotopes between seawater and foraminiferal calcite and Ž2. development of analytical techniques capable of precise and accurate isotopic measurements. Detailed study of Li isotopic composition in geological materials has been precluded by the lack of analytical techniques capable of high-sensitivity, precise, and accurate measurements of lithium isotopes. Various instrumental techniques have been employed to measure the isotopic composition of Li in natural samples including atomic emission and absorption spectrometry ŽAESrAAS., neutron activation ŽNA., secondary ion mass spectrometry ŽSIMS., thermal ionisation mass spectrometry ŽTIMS. and inductively coupled plasma ŽICP. mass spectrometry utilising quadrupole, magnetic sector and time-of-flight instruments. Each method has important limitations. AASrAES, NA and SIMS techniques cannot achieve a precision of 1% or better, making them incapable of resolving the detailed variations in Li isotopic composition seen in nature. TIMS suffers from large instrumental fractionation of Li isotopes in its
thermionic source ŽBickle et al., 2000., although several attempts have been made to reduce this fractionation by measuring the isotopes as borate, fluoride or phosphate ions ŽChan, 1987; Green et al., 1988; You and Chan, 1996; Moriguti and Nakamura, 1998b.. The technique also requires relatively largemass Li samples Ž) 100 ng. separated from their matrix prior to analysis, to achieve reproducibility that is useful for geological applications, unless time-consuming and costly procedures are employed that allow to reduce the sample size and still retain a useful precision and reproducibility ŽYou and Chan, 1996; Moriguti and Nakamura, 1998b.. It has been recently demonstrated that accurate and precise determination of Li isotopes in natural samples can be achieved by multi-collector Žmagnetic sector. ICPMS ŽTomascak et al., 1999a,b.. MC ICPMS allows a rapid analysis Ž; 2 samplesrh. of moderate-mass samples Ž40 ng Li. with a reproducibility better than 1.1‰. Although it requires pre-separation of Li on ion exchange columns, it is not as sensitive to matrix contamination of analyte solutions as TIMS. Yet, this technique requires rather expensive instrumentation that is not readily available in most geochemical laboratories. Also, with conventional Faraday detectors there is a significant loss of precision for small-mass samples Ž- 10 ng.. There is consequently much interest in measuring Li isotopes using single detector quadrupole ICPMS. These instruments are more readily available in geochemical laboratories than their multicollector counterparts. Also, the electron multiplier used in quadrupole instruments allows measurement of isotopic ratios in small-mass samples with a precision comparable to TIMS and MC ICPMS. The availability of precise measurements of Li isotopes in smallmass samples is particularly desirable when measuring the Li isotopic composition of foraminiferal tests, where separation of large numbers of tests from marine sediment is limited by time and by the small amount of sediment recovered from deep-sea cores. So far there have been only three attempts to analyse the isotopic composition of Li by quadrupole ICPMS. Sun et al. Ž1987. and Vanhoe et al. Ž1991. could not achieve a precision better than 1%. In contrast, Gregoire et al. Ž1996. were able to make Li ´ isotopic measurements in minerals containing less than 300 ppm Li to a precision better than 0.8‰.
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However, large background count rates on both lithium masses resulted in signalrbackground ratios of 120, precluding measurement of samples containing less than a microgram of Li. Here we report results for measurements of Li isotopes that demonstrate that single detector quadrupole ICPMS is capable of achieving a within-run precision of less than 1‰ and a long-term reproducibility close to 2‰ on 7 Lir6 Li ratio, even on samples containing as little as 10 ng Li. The method, therefore, provides a rapid and affordable means to study Li isotopes in low concentration natural samples.
2. Analytical techniques Similar to other light elements, the isotopic composition of lithium is usually expressed as delta units calculated from ratios of the less abundant to more abundant isotope relative to the composition of a suitable standard. Unlike most other light elements where the light isotope is also the more abundant one, the natural abundances of 6 Li and 7 Li are 7.5%
171
and 92.5%, respectively, and the delta values are often calculated from 6 Lir7 Li ratios as d6 Li w ‰ x s
ž
Ž 6 Lir7 Li . Sample y1 Ž 6 Lir7 Li . Standard
/
1000.
This notation has been used in most previous Li isotopic studies although it has caused some confusion because the negative delta values correspond to heavy Li isotopic composition Ži.e. opposite to delta values of other light elements.. We will refer to the isotopic composition of Li in delta units relative to the NBS lithium carbonate standard L-SVEC Ž6 Lir7 Li s 0.0832 " 0.0002; Flesch et al., 1973. and the mean seawater composition ŽTable 1; 6 Lir7 Li s 0.080151 " 0.00008; James and Palmer, 2000a.. 2.1. Instrument parameters and data acquisition The instrument used for this study was a VG Elemental PQ3 ICPMS at Charles University in Prague. Samples were mixed online with 10 ppb Be solution and aspirated to the plasma through an MCN-100 microconcentric nebuliser ŽCetac. and wa-
Table 1 Isotopic composition of Li in planktonic foraminiferal tests from Ontong-Java Plateau and Ceara Rise Total procedural blank and L-SVEC Li standards were measured in the course of the same analysis. Sample weight Žmg.
Li Žppm.
6
Lir 7 Li
"2 sm Ž‰.
d 6 Li Ž‰, L-SVEC.
d 6 Li Ž‰, seawater.
Ontong-JaÕa Plateau (cruise MW 91-9, sample GGC 48) P. obliquiloculata Ž500–700 mm. 6.98 0.94 P. obliquiloculata Ž300–500 mm. 7.42 1.05 G. tumida Ž500–1000 mm. 1.28 0.71
0.08068 0.08074 0.08009
0.30 0.47 0.72
y30.35 y29.54 y37.38
1.18 2.02 y6.08
Ontong-JaÕa Plateau (cruise MW 91-9, sample GGC 8) P. obliquiloculata Ž500–700 mm. 11.11 0.97 P. obliquiloculata Ž300–500 mm. 9.60 0.97 G. tumida Ž500–1000 mm. 2.30 0.78
0.08094 0.08075 0.07919
0.45 0.37 1.52
y27.13 y29.44 y48.15
4.50 2.12 y17.20
Ceara Rise (cruise KN142-2, sample KC 62) G. tumida Ž500–1000 mm. 8.84
0.85
0.08077
0.34
y29.21
2.36
Blank and standards Total procedural blank ŽTPB. L-SVEC Ž1. L-SVEC Ž2.
120 pg Li a 0.01b 0.01b
0.07988 0.08323 0.08315
3.34 0.27 0.54
y39.90 0.36 y0.59
y8.69 32.88 31.90
a b
Total Li contribution in picogram to each sample. Concentration in parts per million in the measured solution.
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ter cooled Scott-type spray chamber at a rate of 0.1 mlrmin. Cool plasma conditions ŽRF power 700 W. were used and the plasma was shielded against the load coil using a Ni ring insert ŽPlasmaScreen. to achieve lower background and to improve the signal stability. The instrument was tuned to a sensitivity of at least 10 7 cpsrppm Li on mass 7 and the corresponding background measured on 2% HNO 3 was less than 10 cps. Data were acquired in peak jumping mode for masses 6, 7 and 9 with 1 point measured per mass unit and a dwell time of 10.24 msrpoint. Quadrupole settling time was set to 1.5 ms and dead time of the electron multiplier was 20 ns. Each analysis consisted of 300 s of measurement. Each sample analysis was bracketed by measurements of a 10-ppb solution of L-SVEC Li standard. The data were processed off-line and, where applicable, the raw counts on masses 6 and 7 were corrected for instrument drift using the 9 Be signal prior to blank subtraction. Li concentrations were calculated by calibration to the L-SVEC standard. For Li isotope measurements, correction for instrument mass bias was made by linear interpolation of 6 Lir7 Li ratios of replicate L-SVEC analyses that bracketed each sample. Experimental errors were propagated through the calculation and the final errors are quoted at 2 s level. 2.2. Instrument mass bias Mass discrimination in ICPMS is mainly caused by Ž1. preferential transmission of ions of isotopes
used for instrument optimisation due to their particular kinetic energies, and Ž2. space charge effects caused by concomitant elements present in the sample matrix ŽBeauchemin et al., 1987; Douglas and Tanner, 1998.. During this study, the instrument was always optimised on both lithium masses and the measured 6 Lir7 Li ratios in L-SVEC differed from the expected value of 0.0832 ŽFig. 1.. There was no systematic variation of measured 6 Lir7 Li ratios with Li concentration in the solution but the measured 6 Lir7 Li values varied from day-to-day depending on the optimal tuning parameters for the instrument on a given day. Concentration-dependent mass discrimination of 6 Li Ži.e. shift to higher 6 Lir7 Li values with increasing Li concentration in the solution. has been reported by Gregoire et al. Ž1996. while Sun et al. ´ Ž1987. reported the opposite trend Ži.e. decreasing 6 Lir7 Li values with increasing Li concentration in the solution.. Although it is difficult to provide a simple explanation of mechanisms causing this effect, during the course of this study, we were able to compensate for the concentration-dependent mass discrimination by selecting instrument operating conditions Želectrostatic lens settings. that, within the error of the measurement, yielded consistent isotopic ratios for a range of Li concentrations Žcf. Fig. 1, Day 3.. We have achieved a stable signal on both Li masses with no significant variations in the fractionation of Li isotopes over the period of 300-s acquisition corresponding to each analysis ŽFig. 2.. For low count rates Žless than 10,000 cps on mass 7, corresponding to 1 ppb Li in the solution or less., the
Fig. 1. Uncorrected 6 Lir7 Li ratios measured in solution of L-SVEC Li carbonate standard with different concentrations at three different lens settings ŽDays 1–3.. Error bars are 2 standard errors of the mean based on 10 repeat measurements.
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Fig. 2. Measured Li isotopic composition of 10 ppb Li L-SVEC standard using two different lens settings ŽA and B.. The data were collected during 300-s acquisitions and each data point corresponds to 6 Lir7 Li ratios integrated over a period of 30 s. Dashed line represents the calculated mean 6 Lir7 Li values.
within-run precision of 6 Lir7 Li measurements was worse than the long-term reproducibility and should, therefore, be used as a measure of analytical error. In order to achieve accurate results, the measured values have to be corrected for instrument mass bias using interpolated 6 Lir7 Li ratios of two bracketing Li standards. For samples where Li is present as a trace component Ži.e. at ppm level., the presence of matrix ions in the plasma may result in formation of a space charge, repelling of lighter ions towards the marginal parts of the plasma and enhancing the signal of heavier ions. For foraminiferal tests, the presence of calcium, magnesium, sodium and potassium matrix ions suppresses the signal on both lithium masses and, due to a large mass difference between 6 Li and 7 Li Ž16.7%., results in fractionation of lithium isotopes towards lower d6 Li values. We have measured a series of 10 ppb solutions of lithium L-SVEC standards containing variable amounts of Ca, Mg, Na and K, added in proportions typical of marine carbonate sample. The results show a massive shift of more than 30‰ towards lower d6 Li values for samples where concentration of matrix elements exceeded the Li content by more than 10 3 Ži.e. typical
of foraminiferal tests that are mostly formed by calcium carbonate and often contain 1 ppm Li or less, Fig. 3.. On the other hand, if the ratio of matrix elementrLi did not exceed 1000, the 6 Lir7 Li ratios were not sensitive to the presence of matrix elements and the variation of d6 Li values was within the limits of long term reproducibility of our measurements. Although the matrix effects on the 6 Lir7 Li ratios can be corrected for by selection of Li isotopic standards that match the matrix composition of the unknown samples, this correction method is difficult to use for analyses of sets of samples with different matrix compositions. Alternatively, the matrix effect on the Li isotopic composition can be removed by chromatographic separation of Li from the matrix. 2.3. Sample preparation and separation of Li from matrix In order to achieve low procedural blanks, we have used acids purified by two-step sub-boiling distillation in quartz and PFA Savillex stills throughout this study. All other chemicals were analytical grade and resistivity of deionised water was at least 18 m V. Samples of foraminiferal tests were sieved
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were again inspected under the microscope to ensure they were sediment-free before they were weighted into screw cap PFA Savillex beakers and dissolved in 0.3 ml 1 M HCl being at 808C for 12 h. Following the evaporation to dryness, the samples were converted to nitrate and dissolved in 0.5 ml 0.67 M HNO 3 made up in 30% methanol. The ion exchange chemistry followed the procedure described in Tomascak et al. Ž1999a., modified for small-volume samples. Exchange columns Ž2 ml polyethylene BioRad. contained 1.8 ml Dowex AG50W-x8 Ž200–400 mesh. cation exchange resin. They were cleaned before use and between samples with large volumes of acid and deionised water and calibrated with solution containing Žin mg. 0.1 Li ŽL-SVEC., 3 Na and K, 5 Mg, 10 Sr and 4000 Ca Ži.e. matching the Li content and matrix composition of carbonate foraminiferal tests. to ensure complete separation of Li from the matrix Ž100% yield.. Proper separation of Li from the matrix is a crucial step in this procedure as failure to achieve complete Li recovery from the columns results in a strong fractionation of Li isotopes ŽFig. 4.. Samples were loaded onto columns in 0.5 ml 0.67 M HNO 3 made up in 30% methanol and eluted with 1 ml deionised water and 4 ml 1 M HNO 3 made up in 80% methanol. The Li fraction was collected in a following 6 ml 1 M HNO 3 elution, dried down and dissolved in 1 ml 2% HNO 3 before the analysis on ICPMS. Total procedural blank in the course of this study was 120 pg Li. Fig. 3. Effect of concentration of matrix elements ŽCa, Mg, Na and K. on the measured Li isotopic composition of 10 ppb Li L-SVEC standard. Error bars are 2 s .
to different size fractions, hand-picked under the microscope and crushed to break-open the chambers that were checked for contamination by sediment material before they were repeatedly ultrasonically cleaned Ž10–15 s, three times. in ethanol. Sediment is potentially a major source of Li contamination in the procedure as it contains much more of the element than do the foraminiferal tests. Each sample consisted of less than 50 foraminiferal tests, containing a total of only 1–10 ng Li. This was followed by 4 min of cleaning in hydrogen peroxide and 10 s in 1% HNO 3 and the reagents were repeatedly decanted with deionised water after each step. The samples
2.4. Within run precision and long term reproducibility The within-run precision of 6 Lir7 Li isotopic ratio measurements expected from counting statistics is 1.3‰ Ž2 s ., assuming 10 7 cps on mass 7rppm Li for a 10-ppb solution with 300 s of acquisition. This can be further improved to a precision better than 0.3‰ by increasing the signalrbackground ratio of the measurement, by using longer counting times for the less abundant Li isotope and by increasing the total time of signal acquisition. In the course of this study, the actual within-run precision, calculated as the standard error of the mean Ž sm ., on a 10-ppb Li L-SVEC standard was always better than 1.5‰, and the most precise measurements were better that 0.5‰, based on 850 integrated 6 Lir7 Li isotopic ratios cal-
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175
Fig. 4. Fractionation of Li isotopes in aliquots of Li separated in sequential fashion from the L-SVEC standard on Dowex AG50W-x8 cation exchange resin. Complete recovery of Li is attained by collection of the 5–10 ml HNO 3 elution and avoids fractionation of Li isotopes during the collection step.
culated from more than 8500 time slices. However, error calculations based on the large number of isotopic ratio measurements typically made during a single quadrupole ICPMS analysis result in unrealistically low standard error of the mean. A better measure of analytical error is the reproducibility Žexternal precision., expressed as 2 standard deviations of 6 Lir7 Li isotopic ratios measured over a long period of time ŽThirwall 1991..
We have monitored the reproducibility of Li isotopic measurements of L-SVEC standard for more than 6 months and the resulting 2 standard deviations on the average 6 Lir7 Li value of 0.08321 is 0.00018, or 2.1‰ ŽFig. 5.. We, therefore, quote the 2.1‰ as a best estimate of error for isotopic analysis of Li on quadrupole ICPMS. However, given the strong Li isotope fractionation that may occur during the chromatographic separation of Li from the matrix in
Fig. 5. Long term reproducibility of Li isotopic measurements in 10 ppb L-SVEC standard ŽAugust 1999–January 2000.. Open circles represent composition of L-SVEC standard separated from matrix elements on cation exchange resin, dotted lines correspond to 2.1‰ 2 standard deviations reproducibility limits, error bars are 2 standard errors of the mean Žblack square..
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small volume columns ŽFig. 4., this error estimate is valid only if perfect column calibration is maintained.
3. Results and discussion We have used the described method to study the isotopic composition of Li in calcium carbonate tests of planktonic foraminifera. Samples of Pulleniatina obliquiloculata and Globorotalia tumida were separated from top 2 cm of cores collected by giant gravity corer from two sites on the Ontong-Java Plateau Žwestern Pacific, cruise MW91-9, sample GGC8: 2812.54X S, 156857.90X E, water depth 1625 and cruise MW91-9, sample GGC48: 080.47X S, 16180.20X E, water depth 3397 m. and by knight corer from the Ceara Rise Žwestern equatorial Atlantic, cruise KN142-2, sample KC62: 5845.20X N, 43823.90X W, water depth 4344 m.. Li concentrations and isotopic data for different size fractions of the foraminiferal tests are given in Table 1 and Fig. 6. The studied tests contain 0.7–1 ppm Li and their Li isotopic composition varies from y27.13‰ to y48.15‰ relative to L-SVEC. While our limited data set shows a variable Li isotopic composition for
G. tumida, the isotopic compositions of Li in all but one of the studied fractions of P. obliquiloculata plot within the range of Li isotopic composition that has been reported for the present-day seawater ŽFig. 6., which is from y28.8‰ to y32.8‰ ŽYou and Chan, 1996; Chan and Edmond, 1988; Moriguti and Nakamura, 1998b; Tomascak et al., 1999a; James and Palmer, 2000a.. Calcium carbonate foraminiferal tests may preferentially dissolve with increasing water depth ŽBrown and Elderfield, 1996.. The difference in water depth between samples GGC8 and GGC48 apparently had no measurable effect on Li isotopes in P. obliquiloculata, however, as the isotopic compositions of Li in all fractions of this foraminifera species are identical within analytical error. In addition, the composition of P. obliquiloculata is virtually independent of the size of the tests. Consistent results for the two size fractions of P. obliquiloculata from both samples GGC8 and GGC48 also point to good reproducibility of our measurements. Low and restricted Li contents in all studied foraminifera samples Ž0.7–1 ppm., well within the range of concentrations previously reported from foraminiferal tests Ž0.2–2.6 ppm; Delaney et al., 1985; Delaney and Boyle, 1986;You and Chan, 1996; Hoefs and Sywall, 1997., indicate that
Fig. 6. Isotopic composition of Li in planktonic foraminiferal tests of P. obliquiloculata Žopen circles. and G. tumida Žopen squares. from Ontong-Java Plateau ŽGGC48 and GGC8. and Ceara Rise ŽKC62.. Also plotted are compositions of four fractions of P. obliquiloculata from ODP site 130-806B in Ontong-Java Plateau with ages of 804–397 ka ŽYou and Chan, 1996; full circles. and two 10 ppb Li L-SVEC standards and total procedural blank ŽTPB. measured in the course of the analysis. Error bars are 2 s .
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the cleaning procedure adopted in this study was efficient. and that our data are not significantly affected by contamination from the host sediments whose Li content usually exceeds that of the foraminiferal tests by at least one order of magnitude Že.g. You and Chan, 1996; James and Palmer, 2000b.. Given the long residence time of Li in seawater Ž0.3–3 = 10 6 years; Edmond et al., 1979; StoffynEgli and Mackenzie, 1984., secular variations in the Li isotopic composition of the oceans may reflect variable inputs from two major sources: fluvial transport of the products of continental weathering Žfrom y6‰ to y32.2‰; Huh et al., 1998. and high-temperature hydrothermal alteration of ocean floor basalts Žy9‰; Chan et al., 1992, 1993.. To achieve and maintain the mean present-day seawater Li isotopic composition of y31.5‰, additional inputs or sinks that fractionate Li isotopes, such as is the early diagenesis of clay minerals ŽJames and Palmer, 2000b., are required. Thus, if ancient specimens of P. obliquiloculata could be used as proxy for the Li isotopic composition of past oceans, they would provide us with invaluable information about variations in rates of continental weathering, andror sea floor spreading. Not much is known about the Li isotopic systematics in ancient oceans and planktonic foraminifera. Hoefs and Sywall Ž1997. reported differences in isotopic composition of Li between Holocene Žy19‰. and Tertiary Žfrom y42‰ to y2‰. foraminiferal tests, but their study was based on large-mass samples Ž50 mg corresponding to 100–1000 individuals. that contained mixture of several foraminifera species. Variations of ca. 21‰ in Li isotopic composition on much smaller time scale of 407 ka and values as low as y40‰ were reported by You and Chan Ž1996. for samples of P. obliquiloculata from ODP Site 806B on the Ontong-Java Plateau. These previous studies, along with the data presented in this paper, suggest that Ž1. planktonic foraminiferal tests contain Li at low ppm level, Ž2. there are variations in Li isotopic composition between different foraminiferal species and Ž3. there are variations in Li isotopic composition of P. obliquiloculata tests of different ages. Our data indicate that Li isotopic composition of P. obliquiloculata from the sediment–water interface is close to the mean present-day seawater composition and is independent of the test size. However, more study is
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needed to assess possible effects of carbonate dissolution below the carbonate compensation depth ŽBrown and Elderfield, 1996; Martin and Sayles, 1996., winnowing, and spatial–temporal variations before P. obliquiloculata or another species of planktonic foraminifera can be used as a reliable proxy for the Li isotopic composition of seawater. 3.1. Comparison of quadrupole ICPMS with TIMS and MC ICPMS This study presents the first Li isotopic data from small, well characterised samples of individual foraminiferal species. This has been enabled by Ž1. low total procedural blanks, Ž2. the high sensitivity of a quadrupole ICPMS, and Ž3. the large sampler background ratio of the Li signal measured by ICPMS under cool plasma conditions. TIMS techniques are often used for precise and accurate Li isotopic analysis of geological samples but ICPMS techniques offer several advantages. Analysis of Li isotopes by TIMS requires samples that contain at least 100 ng Li, which has been separated completely from the matrix elements prior to the analysis. Although small samples Žca. 2 ng Li. have also been analysed by TIMS ŽYou and Chan, 1996., the procedure is time consuming and costly. The reproducibility of TIMS Li isotopic analyses usually varies between 1–2‰. Magnetic sector multicollector ŽMC. ICPMS provides an alternative to TIMS in that it can achieve a comparable reproducibility of Li isotopic ratios for samples that contain only 40 ng Li ŽTomascak et al., 1999a,b.. Although it still requires separation of Li from the matrix, compared to TIMS, MC ICPMS has a higher level of tolerance for matrix elements and analysis is 5–10 times faster. Unlike in single detector quadrupole ICPMS where precision of isotopic measurements potentially suffers from short term variations of isotopic signals caused by plasma instability Žso-called flicker noise., simultaneous detection of both Li isotopes in MC ICPMS eliminates this problem. In addition, the different shape of ion beam and more even energy distribution of ions in MC ICPMS produce flat-top peaks which allow more precise isotopic measurements. Fast switching between individual masses by a quadrupole mass filter mostly compensates for the instability of Li isotopic signal. Accordingly, the somewhat lower precision
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of quadrupole ICPMS isotopic measurements compared to MC ICPMS may be attributed to the shape of peaks in the quadrupole mass spectrum and small deviations from perfect mass calibration. On the other hand, most quadrupole ICPMS instruments use an electron multiplier detector that is ca. 1000 times more sensitive compared to conventional Faraday detectors that are used for simultaneous isotopic measurements in most MC ICPMS and TIMS instruments. The high sensitivity of quadrupole instruments enables measurements of large Li samples Ž) 40 ng. with a precision that is only 1.5–2 times worse compared to TIMS and MC ICPMS, but, more importantly, it also allows precise measurements of the Li isotopic composition in small samples Žca. 5 ng Li., which cannot be done routinely by MC ICPMS or TIMS. Moreover, as the limiting error in Li isotopic analysis is largely dependent on quantitative Li separation from the matrix elements and the efficiency of instrument mass bias corrections, the somewhat lower precision of quadrupole ICPMS measurements compared to TIMS and MC ICPMS is not an obstacle to most geochemical applications.
Acknowledgements We thank Dan McCorkle ŽWHOI. for providing foraminifera samples for this study and Marie Fayadova´ for laboratory assistance. Jochen Hoefs, Conrad Gregoire and an anonymous referee provided ´ useful comments to the manuscript. This work was supported by Charles University through grants 286r1999B-Geo and CEZ:J13r98:113100005. ICPMS facility at Charles University was funded by PHARE.
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