Anal. Chem. 2006, 78, 8477-8484
High Precision Measurements of Non-Mass-Dependent Effects in Nickel Isotopes in Meteoritic Metal via Multicollector ICPMS David L. Cook,*,†,‡,§ Meenakshi Wadhwa,†,‡,§ Philip E. Janney,§ Nicolas Dauphas,†,‡,⊥ Robert N. Clayton,†,‡,⊥ and Andrew M. Davis†,‡,⊥
Department of the Geophysical Sciences, The University of Chicago, 5734 South Ellis Avenue, Chicago, Illinois 60637, Chicago Center for Cosmochemistry, 5640 South Ellis Avenue, Chicago, Illinois 60637, Department of Geology, The Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605, and Enrico Fermi Institute, 5640 South Ellis Avenue, Chicago, Illinois 60637.
We measured the Ni isotopic composition of metal from a variety of meteorite groups to search for variations in the 60Ni abundance from the decay of the short-lived nuclide 60Fe (t1/2 ) 1.49 My) and for possible nucleosynthetic effects in the other stable isotopes of Ni. We developed a high-yield Ni separation procedure based on a combination of anion and cation exchange chromatography. Nickel isotopes were measured on a single-focusing, multicollector, inductively coupled mass spectrometer (MC-ICPMS). The external precision on the massbias-corrected 60Ni/58Ni ratio ((0.15 E; 2σ) is comparable to similar studies using double-focusing MC-ICPMS. We report the first high-precision data for 64Ni, the least abundant Ni isotope, obtained via MC-ICPMS. The external precision on the mass-bias-corrected 64Ni/58Ni ratio ((1.5 E; 2σ) is better than previous studies using thermal ionization mass spectrometry. No resolvable excesses relative to a terrestrial standard in the mass-bias-corrected 60Ni/58Ni ratio were detected in any meteoritic metal samples. However, resolvable deficits in this ratio were measured in the metal from several unequilibrated chondrites, implying a 60Fe/56Fe ratio of ∼1 × 10-6 at the time of Fe/Ni fractionation in chondritic metal. A 60Fe/56Fe ratio of (4.6 ( 3.3) × 10-7 is inferred at the time of Fe/ Ni fractionation on the parent bodies of magmatic iron meteorites and pallasites. No clearly resolvable non-massdependent anomalies were detected in the other stable isotopes of Ni in the samples investigated here, indicating that the Ni isotopic composition in the early solar system was homogeneous (at least at the level of precision reported here) at the time of meteoritic metal formation. Meteorites provide a record of events and processes that occurred during the formation and early evolution of the solar system. Various phases in undifferentiated meteorites (e.g., ordinary and carbonaceous chondrites) contain information on * Towhomcorrespondenceshouldbeaddressed.E-mail:
[email protected]. † The University of Chicago. ‡ Chicago Center for Cosmochemistry. § The Field Museum. ⊥ Enrico Fermi Institute. 10.1021/ac061285m CCC: $33.50 Published on Web 11/15/2006
© 2006 American Chemical Society
nebular processes, such as condensation, and on parent body processes, such as accretion, metamorphism, and aqueous alteration. These primitive meteorites may also provide a record of the precursor materials (e.g., presolar dust grains) involved in the formation of the solar nebula. Differentiated meteorites (e.g., eucrites, pallasites, and iron meteorites) record diverse parent body processes, such as silicate differentiation and core formation. Constraining the timing of these processes is critical for unraveling early solar system history. The isotopic record left by the former presence of short-lived, now extinct, radionuclides in meteorites can provide chronometric data for this early period. Chronometry based on a number of extinct radionuclides has been applied to meteoritic materials.1,2 The precision of ages based on short-lived chronometers is typically better than 1 My, which makes them ideally suited to studying the timing of early solar system processes.1,2 Unambiguous evidence of excesses in radiogenic 60Ni from the decay of 60Fe (t1/2 ) 1.49 My) was first reported in bulk eucrite samples.3,4 Since then, analyses of various phases in unequilibrated chondrites have revealed 60Ni excesses in sulfides, oxides, and silicates.5-8 Recently, 60Ni excesses of up to 1.5 were reported for Fe-Ni metal from several iron meteorites9 and ordinary chondrites;10 these were the first reports of excess 60Ni in meteoritic metal. The metal phase in meteorites may have been subjected to both nebular and parent body processes, such as condensation, oxidation, melting, metal-silicate segregation, and core crystallization.11 To date, only the 107Pd-107Ag 12,13 (t1/2 ) 6.5 (1) McKeegan, K. D.; Davis, A. M. In Treatise on Geochemistry; Davis, A. M., Ed.; Elsevier Pergamon: San Diego, CA, 2004; Vol. 1, pp 431-460. (2) Gilmour, J. D. Space Sci. Rev. 2000, 92, 123-132. (3) Shukolyukov, A.; Lugmair, G. W. Science 1993, 259, 1138-1142. (4) Shukolyukov, A.; Lugmair, G. W. Earth Planet Sci. Lett. 1993, 119, 159166. (5) Tachibana, S.; Huss, G. R. Astrophys. J. 2003, 588, L41-L44. (6) Guan, Y.; Huss, G. R.; Leshin, L. A.; MacPherson, G. J. Meteorit. Planet. Sci. 2003, 38, A138. (7) Mostefaoui, S.; Lugmair, G. W.; Hoppe, P. Astrophys. J. 2005, 211, 271277. (8) Tachibana, S.; Huss, G. R.; Kita, N. T.; Shimoda, G.; Morishita, Y. Astrophys. J. 2006, 639, L87-L90. (9) Moynier, F.; Telouk, P.; Blichert-Toft, J.; Albare`de, F. Lunar Planet Sci. 2004, 35, no. 1286. (10) Moynier, F.; Blichert-Toft; J.; Telouk; P.; Albare`de, F. Lunar Planet Sci. 2005, 36, no. 1593.
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My) and 182Hf-182W 14-17 (t1/2 ) 8.9 My) chronometers have been applied toward obtaining high-resolution chronology of metal samples. The findings of Moynier et al.9,10 suggested that the 60Fe-60Ni chronometer may add a tool for deciphering the history of meteoritic metal. The resolution of small time differences and of non-massdependent isotopic anomalies (possibly of nucleosynthetic origin) using short-lived chronometers requires isotope measurements of high precision. Nickel isotopes have been measured using secondary ion mass spectrometry (SIMS),5-8 thermal ionization mass spectrometry (TIMS),3,4,18-20 and multicollector inductively coupled plasma mass spectrometry (MC-ICPMS).9,10,21 Analyses using SIMS can provide high spatial resolution (e.g., 5 µm) but suffer from relatively poor precision.5-8 Isobaric interferences on 58Ni by 58Fe and on 64Ni by 64Zn in SIMS analyses make it difficult to measure these Ni isotopes. Thus, the full complement of Ni isotopes (58Ni, 60Ni, 61Ni, 62Ni, 64Ni) cannot be easily determined using this technique. Analysis via chemical separation and purification of Ni followed by TIMS allows for the measurement of all five Ni isotopes and for the correction of 58Fe and 64Zn interferences. The typical precision of the isotope ratio measurements by this technique is better than by SIMS: a precision on 60Ni/58Ni, the ratio used for chronology, as good as ( 0.6 (2σ) has been achieved.3 However, precisions on the other Ni isotope ratios are on the order of at least several epsilon units.18-20 Like TIMS, MC-ICPMS also provides the ability to measure all Ni isotopes and correct for interferences from 58Fe and 64Zn. Additionally, significantly better precision on all Ni isotope ratios may be possible. Thus far, high-precision Ni isotope ratio measurements have been made only on double-focusing MC-ICPMS instruments.9,10,21 We report the first highly precise and accurate measurements of Ni isotopes using a single-focusing MC-ICPMS. This instrument employs a hexapole collision cell to thermalize ions and remove interfering polyatomic species. The external precision of Ni isotope ratio measurements with this technique is comparable to or better than that of prior work.9,10,21 On the basis of high precision analyses of Ni isotopes in metal from a variety of undifferentiated and differentiated meteorites using this technique, we present implications for the initial abundance of 60Fe and for the degree of Ni isotopic homogeneity in the early solar system. EXPERIMENTAL METHODS Samples and Sample Preparation. Samples were chosen to represent a wide variety of meteorite classes,22 including magmatic (IIA, IIB, IID, IIIA, IIIB, IVA, and IVB) and nonmagmatic (IAB and IIICD) iron meteorites, pallasites (main group and (11) Kelly, W. R.; Larimer, J. W. Geochim. Cosmochim. Acta 1977, 41, 93-111. (12) Chen, J. H.; Wasserburg, G. J. Geophys. Monogr. 1996, 95, 1-20. (13) Carlson, C. W.; Hauri, E. H. Geochim. Cosmochim. Acta 2001, 65, 18391848. (14) Lee, D.-C.; Halliday, A. N. Science 1996, 274, 1876-1879. (15) Horan, M. F.; Smoliar, M. I.; Walker, R. J. Geochim. Cosmochim. Acta 1998, 62, 545-554. (16) Lee, D. C. Earth Planet Sci. Lett. 2005, 237, 21-32. (17) Markowski, A.; Quitte´, G; Halliday, A. N.; Kleine, T. Earth Planet Sci. Lett. 2006, 242, 1-15. (18) Morand, P.; Alle`gre, C. J. Earth Planet Sci. Lett. 1983, 63, 167-176. (19) Shimamura, T.; Lugmair, G. W. Earth Planet Sci. Lett. 1983, 63, 177-188. (20) Birck, J. L.; Lugmair, G. W. Earth Planet Sci. Lett. 1988, 90, 131-143. (21) Quitte´, G; Meier, M.; Latkoczy, C.; Halliday, A. N.; Gu ¨ nther, D. Earth Planet Sci. Lett. 2006, 242, 16-25.
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Eagle Station), and chondrites (LL, H, EH, CR, and CBa). Small pieces ( 227), whereas the interfering elements Fe and Zn do not (Kd < 1).25 One advantage of this method is that it removes both ferrous and ferric Fe,26 whereas only ferric Fe is removed by the anion column procedure.24 Furthermore, several elements present in meteoritic metal that may not be effectively separated (e.g., Co, Cu), or are not separated at all (e.g., Mn), on the anion column24 are separated on the cation column.25 It is important to cap the column once reagents containing acetone are introduced in order to prevent acetone evaporation, which could lead to a change in the partitioning behavior of the elements.25 A third anion exchange column was employed for some samples to remove any Ti that may be present. Titanium, being a lithophile element, is not expected to be present in any significant amounts in metallic samples. However, metal samples containing inclusions of silicate (i.e., pallasites) or hosted in a silicate matrix (i.e., chondrites) could include a minor amount of silicate that, although it would not significantly affect the Ni (which is a major element in the metal but only a trace element in the silicates), could contribute some Ti. Interferences on 62Ni and 64Ni by TiO species can occur. Because it is not possible to easily monitor and correct for interferences from TiO species during the isotopic analysis, it is imperative that the sample solutions be free from Ti. Therefore, the metal from the pallasites Albin and Molong and all chondrite samples were subjected to a third column exchange separation. Disposable Bio-Rad Poly-Prep columns were packed with 1 mL of AG1-X8 anion resin (200-400 mesh). The columns were washed and conditioned with the following: 10 mL of H2O, 5 mL of 1 M HNO3, 10 mL of H2O, 10 mL of 0.4 M HCl, 5 mL of H2O, and 5 mL of an 0.5 M HF/1 M HCl mixture. The sample was loaded in 0.2 mL of 0.5 M HF/1 M HCl, and Ni was eluted with an additional 3 mL of 0.5 M HF/1 M HCl in a sequence of 0.5, 0.5, 1.0, and1.0 mL. Tests showed that under these conditions, Ni was quantitatively recovered, and Ti was fully retained by the resin. The total procedural blank (≈3 ng) is insignificant compared to the amount of Ni in the samples. Iron/Ni Elemental Ratio Measurements. The Fe/Ni ratios were determined in sample solutions using a Varian ICPMS instrument equipped with a quadrupole mass analyzer at the Field Museum. An aliquot of digested but chemically unprocessed sample was dried and then dissolved in 3% HNO3. A known amount of a manganese concentration standard was added to the sample solution and served as an internal standard. The isotopes 55Mn, 57Fe, and 60Ni were measured, and Fe and Ni concentrations were calculated using calibration curves obtained with external standards. Each aliquot was measured three times, and the uncertainty represents the 2σ standard deviation of all measurements. Nickel Isotopic Measurements. Nickel isotopic measurements were performed at the Isotope Geochemistry Laboratory of the Field Museum on a Micromass (now GV Instruments) IsoProbe MC-ICPMS. This instrument has nine Faraday collectors;
Table 1. The Ni Isotopic Compositions of Chemically Processed Aliquots of the Ni Isotopic Standard SRM 986 and of Terrestrial Josephinite sample
60 ( 2σ
61 ( 2σ
64 ( 2σ
n
SRM no. 1 SRM no. 2 SRM no. 3 Josephinite no. 1 Josephinite no. 2
2-Column Chemistry -0.05 ( 0.27 0.19 ( 1.12 -0.06 ( 0.16 -0.67 ( 2.21 -0.04 ( 0.40 0.05 ( 1.74 -0.15 ( 0.11 0.58 ( 0.72 0.16 ( 0.29 0.27 ( 0.72
2.3 ( 4.4 1.4 ( 1.7 0.4 ( 1.6 0.4 ( 2.0 2.6 ( 2.0
5 5 5 9 5
SRM no. 1 SRM no. 2 SRM no. 3 Josephinite no. 1 Josephinite no. 2
3-Column Chemistry -0.01 ( 0.43 -0.64 ( 1.16 -0.01 ( 0.20 -0.60 ( 0.87 0.12 ( 0.14 0.17 ( 1.89 -0.08 ( 0.06 0.71 ( 0.48 -0.12 ( 0.06 -0.05 ( 0.37
0.0 ( 3.2 1.3 ( 2.0 0.0 ( 2.6 0.6 ( 0.7 0.2 ( 0.6
5 5 5 14 13
thus, all Ni isotopes can be measured simultaneously, and 57Fe and 66Zn can be monitored and used to correct for isobaric interferences on 58Ni from 58Fe (0.28 atom % of total Fe) and on 64Ni from 64Zn (49.18 atom % of total Zn). The sample solution (1 ppm in 3 wt % HNO3) was introduced through a PFA Teflon nebulizer (100 µL/min) in a Cetac Aridus desolvating system using a PFA spray chamber heated to 95 °C. Argon and N2 were introduced into the desolvating system at approximately 3 L min-1 and 35 µL min-1, respectively. Argon was introduced into the collision cell at 1.8 mL min-1 as a thermalizing collision gas. The instrument was optimized to obtain a signal of g6.0 V on 58Ni when running a 1 ppm Ni solution. This generates a signal of g100 mV on 64Ni, the least abundant Ni isotope (0.925%).27 Sampler and skimmer cones made of Ni were used because they were found to provide better signal stability than Al cones. Despite the use of Ni cones, the background signal is negligibly small (1-3 mV on 58Ni) compared to the sample signal. Samples were measured via the standard-sample bracketing technique using the NIST SRM 986 as the Ni isotope standard. SRM 986 is the only commercially available Ni standard with a certified isotopic composition.27 Samples were corrected for mass bias using an exponential law and 62Ni/58Ni ≡ 0.053388.27 The analytical protocol consisted of alternating between standard and sample solutions, with each being measured for 200 s (admittance delay 2 min). Each 200-s measurement (consisting of 20 cycles of 10-s integrations) was preceded by 4 min of washout and an on-peak blank measurement consisting of a 60-s integration measurement while aspirating a clean 3 wt % HNO3 solution. Each reported datum comprises the mean of a minimum of five repeat measurements. Precision and Accuracy. All isotope ratio data (Tables 1 and 2) are reported in units, given as i ) [(Rsample - Rstandard)/Rstandard] × 104, where R is the mass-bias-corrected iNi/58Ni ratio (i ) 60, 61, or 64). Internal precisions for individual samples represent the standard error of the mean (2σm). The external precision was determined by repeated analyses of an Aesar Ni concentration standard over a 24-month period. The mean values of the isotopic ratios from each analysis of the Aesar Ni solution were calculated from five repeat measurements. These mean values for all ratios are identical to SRM 986 within uncertainty. Figure 1 shows these data for the mass-bias-corrected 60Ni/58Ni ratio in epsilon units (27) Gramlich, J. W.; Machlan, L. A.; Barnes, I. L.; Paulsen, P. J. J. Res. Natl. Bur. Stand. (US) 1989, 94, 347-356.
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Table 2. The
56Fe/58Ni
sample
Ratios and Ni Isotopic Compositions of Meteoritic Metal group
56Fe/58Ni
( 2σ
60 ( 2σ
61 ( 2σ
64 ( 2σ
n
Renazzo Gujba Semarkona Bishunpur Forest Vale Indarch
CR CBa LL 3.0 LL 3.1 H4 EH4
21.9 ( 0.3 22.0 ( 0.2 14.4 ( 0.2 18.4 ( 0.1 16.4 ( 0.3 18.3 ( 0.5
-0.23 ( 0.10 -0.23 ( 0.08 -0.25 ( 0.10 0.01 ( 0.25 0.07 ( 0.30 -0.13 ( 0.20
0.31 ( 0.54 -0.33 ( 0.33 0.98 ( 0.23 0.32 ( 1.26 -0.05 ( 1.86 0.47 ( 1.29
0.1 ( 0.8 -0.2 ( 0.7 -0.3 ( 0.7 -0.7 ( 1.7 0.0 ( 2.0 0.3 ( 4.0
14 13 10 5 5 5
Eagle Station Albin Brenham Molong
PES PMG PMG PMG
7.16 ( 0.09 14.3 ( 0.2 10.3 ( 0.1 11.4 ( 0.1
-0.12 ( 0.12 0.04 ( 0.17 -0.02 ( 0.26 -0.06 ( 0.13
0.39 ( 0.62 -0.53 ( 0.70 0.23 ( 1.22 0.17 ( 0.76
2.0 ( 1.6 0.8 ( 1.3 1.8 ( 4.1 0.0 ( 1.9
5 9 5 5
Coahuila Santa Luzia Carbo Bella Roca Casas Grandes Henbury Gibeon Yanhuitlan Cape of Good Hope Hoba Tlacotepec
IIAB IIAB IID IIIAB IIIAB IIIAB IVA IVA IVB IVB IVB
23.1 ( 0.1 19.9 ( 0.3 12.6 ( 0.2 12.4 ( 0.2 17.2 ( 0.3 17.5 ( 0.2 15.9 ( 0.2 16.9 ( 0.3 7.27 ( 0.10 7.01 ( 0.07 7.17 ( 0.03
0.00 ( 0.14 0.05 ( 0.10 -0.08 ( 0.25 0.03 ( 0.17 0.02 ( 0.09 -0.01 ( 0.12 -0.13 ( 0.19 0.00 ( 0.36 -0.13 ( 0.39 -0.21 ( 0.07 -0.18 ( 0.20
0.41 ( 1.45 0.06 ( 0.55 -0.07 ( 1.06 0.26 ( 1.23 0.00 ( 0.36 -0.04 ( 0.35 -0.02 ( 1.86 0.82 ( 0.95 0.11 ( 2.30 -0.21 ( 0.44 -0.15 ( 0.76
1.0 ( 3.4 1.5 ( 1.0 -0.3 ( 2.1 -1.0 ( 0.7 1.4 ( 1.1 -0.4 ( 0.6 2.2 ( 1.7 -0.1 ( 2.9 1.9 ( 1.2 -0.1 ( 0.7 0.3 ( 0.9
5 10 5 8 14 14 5 5 5 13 9
Canyon Diablo Toluca Dayton Mundrabilla
IAB IAB IIICD IIICD
18.6 ( 0.3 19.7 ( 0.3 7.08 ( 0.05 16.2 ( 0.4
0.06 ( 0.07 -0.06 ( 0.09 0.08 ( 0.21 -0.12 ( 0.08
0.14 ( 0.58 -0.02 ( 0.34 -0.13 ( 0.75 -0.01 ( 0.47
1.9 ( 0.8 0.3 ( 0.7 2.3 ( 3.2 0.2 ( 0.5
19 15 5 15
Figure 1. 60 values for repeated analyses of an Aesar Ni solution over the course of a 24-month period. Each datum represents the mean of five repeat measurements performed during a single analysis session. The individual error bars are 2σm errors, based on the five repeat measurements for each datum. The external precision is the standard deviation (2σ) based on all of the data plotted here and is shown by the two dashed lines ((0.15 ).
(60); the external precision (2σ) is (0.15 . The external precisions for the 61 and 64 values are (0.85 and (1.5 , respectively (Figures S-1 and S-2). Gramlich et al.28 measured terrestrial sulfides and metals and showed that the mass-bias-corrected Ni isotopic compositions of terrestrial samples do not deviate from those of SRM 986. Three aliquots of SRM 986 were processed using our Ni separation chemistry, both with and without the final cleanup column for Ti. Two samples of terrestrial josephinite were also processed using both procedures. As expected, the mean values for all ratios are identical to SRM 986 within uncertainty (Table 1). Figure 2 shows these data for the mass-bias-corrected 60Ni/58Ni ratios in epsilon (28) Gramlich, J. W.; Beary, E. S.; Machlan, L. A.; Barnes, I. L. J. Res. Natl. Bur. Stand. (US) 1989, 94, 357-362.
8480 Analytical Chemistry, Vol. 78, No. 24, December 15, 2006
Figure 2. 60 values for SRM 986 (circles) and terrestrial josephinite (triangles) processed through Ni separation chemistry; filled symbols included the Ti cleanup column. Plotted errors are 2σm; the external precision (2σ) is shown by the two dashed lines ((0.15 ). Smaller error bars on some josephinite samples reflect a larger number of repeat measurements (Table 1).
units (60). These data confirm that our Ni separation chemistry for metallic samples does not introduce any analytical artifacts and demonstrate our ability to precisely and accurately measure the Ni isotopic compositions of such samples. Effects of Fe and Zn. As previously noted, Fe and Zn isotopes interfere with Ni isotopes, and these interferences must be corrected for when present. To test our ability to correct for these interferences, we doped aliquots of SRM 986 with varying concentrations of Fe and Zn (four aliquots each). Samples were corrected for Fe and Zn interferences using 58Fe/57Fe ≡ 0.1330 and 64Zn/66Zn ≡ 1.7698. Because all Ni isotopes are normalized to 58Ni, an interference from 58Fe has the potential to affect all of the measured Ni ratios. Figure 3 shows the interference-corrected 60 values for the SRM 986 aliquots doped with Fe as well as an undoped aliquot. These data show that the Fe interference
Figure 3. 60 vs the Fe/Ni elemental ratio. Data are from analyses of four aliquots of SRM 986 doped with varying amounts of Fe and one undoped aliquot (Fe/Ni ≈ 10-4). Plotted errors are 2σm; the external precision (2σ) is shown by the two dashed lines ((0.15 ).
Figure 5. 60 vs the ratio of the sample-to-standard signal intensity. Data are from analyses of five aliquots of SRM 986 diluted to various concentrations and one undiluted aliquot (ratio ) 1). Plotted errors are 2σm; the external precision (2σ) is shown by the two dashed lines ((0.15 ).
the Aesar solutions differed from the standard by 10% or less. All measured sample solutions had signal intensities within 15% of the standard signal intensity, and 95% of those were within 10%. These discrepancies in standard-sample concentration matching are well within the range investigated and do not affect our measurements.
Figure 4. 64 vs the Zn/Ni elemental ratio. Data are from analyses of four aliquots of SRM 986 doped with varying amounts of Zn and one undoped aliquot (Zn/Ni ≈ 10-5). Note the scale break on the y axis. Plotted errors are 2σm; the external precision (2σ) is shown by the two dashed lines ((1.5 ).
correction is effective, even at high Fe concentrations (i.e., Fe/ Ni ) 0.1). Additionally, no resolvable effects were observed on 61 or 64 due to the presence of Fe. Figure 4 shows the interference-corrected 64 values for the SRM 986 aliquots doped with Zn as well as an undoped aliquot. These data show that the Zn interference correction is not effective for Zn/Ni g 0.01 and illustrate the importance of separating Ni from Zn during chemical processing. For all the samples processed through Ni separation chemistry, the Fe/Ni and Zn/Ni ratios (
