Nickel isotopes in iron meteorites–nucleosynthetic anomalies in sulphides with no effects in metals and no trace of 60Fe

Earth and Planetary Science Letters 242 (2006) 16 – 25 www.elsevier.com/locate/epsl

Nickel isotopes in iron meteorites–nucleosynthetic anomalies in sulphides with no effects in metals and no trace of 60Fe Ghylaine Quitte´ a,*, Matthias Meier a, Christopher Latkoczy b, Alex N. Halliday c, Detlef Gu¨nther b a

Institut fu¨r Isotopengeologie und Mineralische Rohstoffe, ETH Zentrum, Sonneggstrasse 5, CH-8092 Zurich, Switzerland Laboratory of Inorganic Chemistry, ETH Ho¨nggerberg HCI, Wolfgang-Pauli Strasse 10, CH-8093 Zurich, Switzerland c Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, United Kingdom

b

Received 8 October 2005; received in revised form 25 November 2005; accepted 28 November 2005 Available online 17 January 2006 Editor: R.W. Carlson

Abstract Iron-60 decays to 60Ni with a half-life of 1.49 Myrs such that Ni isotopic studies of iron meteorites have the potential to provide powerful new constraints on the energy budgets and time-scales of planetesimal melting, differentiation and core formation. We report high-resolution MC–ICPMS Ni isotope compositions for the Fe–Ni metal phase from 33 iron meteorites as well as for 10 coexisting sulphides. The isotopic composition of every metal sample is indistinguishable from that of the standard within uncertainties, whereas several sulphides show an excess of 61Ni correlated with a deficit in 60Ni. These latter effects are not explicable by currently known analytical artefacts. Nor can they be readily explained by spallation reactions or radioactive decay. Based on our sampling they seem more prevalent in, but not exclusive to, non-magmatic iron meteorites and could reflect admixing of less than 0.4 ppm pure s-process component into bnormalQ Ni on meteorite parent bodies. Sulphides do not show the excess 60Ni expected from their high Fe/Ni ratios if they formed within the first few million years of the solar system. The data provide evidence that sulphides in iron meteorites crystallized more than 10 Myrs after the start of the solar system. D 2005 Elsevier B.V. All rights reserved. Keywords: extinct radioactivities;

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Fe; iron meteorites; chronology; early solar system; nucleosynthetic anomaly; s-process

1. Introduction Iron meteorites can be divided into two categories termed the magmatic (IIA, IIB, IIIA, IIIB, IVA, IVB, IID, IIIF) and non-magmatic groups (IAB, IIICD, IIE). Magmatic irons are thought to represent the cores of

* Corresponding author. Tel.: +41 1 632 78 39; fax: +41 1 632 11 79. E-mail address: [email protected] (G. Quitte´). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.11.053

differentiated planetesimals (e.g. [1]). According to the generally accepted view or bstandard modelQ of terrestrial planet formation, the first small bodies of the solar system were of roughly chondritic composition. As they grew, the heat provided by short-lived radionuclides, in particular 26Al and 60Fe, together with energy from accretion and impacts, caused melting and differentiation including the segregation of metallic cores. Recent studies have demonstrated that magmatic iron meteorites formed within a few million years of the start of the solar system. Some are probably as old as the calcium

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aluminium-rich refractory inclusions (CAIs) found in chondrites [2–4], widely considered to be the oldest objects yet identified that formed in the solar system. Short-lived chronometers are well suited for dating such early events. Excess 107Ag and 53Cr, the respective decay products of 107Pd (t 1/2 = 6.5 Myr) and 53 Mn (t 1/2 = 3.7 Myr), have already been detected in iron meteorites [5,6], indicating that 107Pd and 53Mn were extant when irons formed. Similarly, anomalies of 60 Ni due to the in situ decay of 60Fe (t 1/2 = 1.49 Myr) are expected if iron meteorites formed very early. Iron60 is of particular interest because of its potential role as a heat source. Moynier et al. [7,8] reported evidence of a small excess of 60Ni (up to 150 ppm) in some iron meteorites. However, an independent study by Cook et al. [9,10] could not confirm these first results. They found no anomaly in any sample, including meteorites already studied by Moynier et al. Here, we report on the determination of the Ni isotopic compositions of the metal phases of 33 iron meteorites from different magmatic and non-magmatic groups, most of which have already been used for measuring W isotopes in the same aliquot [4], and for 10 sulphides. 2. Analytical procedures Pieces of iron meteorite were cut with a diamond saw. In most cases these pieces weighed between 0.7 and 2 g for W isotopic analysis. In a few cases, when the sample was only used for Ni and Fe analysis, much smaller chips were cut. The rusty parts and fusion crust were removed by polishing the sample with a small diamond saw blade (dentist tool). The sample was rinsed with acetone in an ultrasonic bath, washed in 16 N HNO3 for 24 to 48 h at room temperature, and then in hot water for 20 min. In order to eliminate any possible terrestrial contamination the external part of the sample was then dissolved in several steps in a HNO3–H2O mixture (7/3 vol./vol.). If necessary, a few drops of 6 N HCl were added to start the reaction. Finally, the innermost part of the sample was digested in a HF–HNO3–HCl mixture (0.5/1/15) for combined W, Ni and Fe analyses or in a HNO3/HCl mixture for Ni and Fe analysis alone. In the case of the combined study, an aliquot of about 0.06% was taken for Ni and Fe after digestion, whereas the rest was processed through the W chemical separation procedure. A small aliquot of the Ni–Fe fraction was kept for the further determination of the Fe/Ni concentration ratio. The rest of the solution was dried, taken up in 2 mL of 6 N HCl and loaded on a 1.8-mL column filled with AG1-X8 resin (Biorad, 200–400 mesh) previously cleaned and

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conditioned in 6 N HCl. As Ni is not fixed on this resin in HCl, it eluted immediately, together with Mn, whereas Fe and Zn are adsorbed on the resin [11]. We collected Ni in the loading fraction and two more milliliters of 6 N HCl. Iron and nickel are the two main elements in iron meteorites and this ion exchange procedure facilitated their efficient separation. Manganese may be present in the Ni fraction, but this is not a major element in iron meteorites and it is responsible for no interference on Ni masses. Chromium and Co are only slightly retained on the resin and partially follow Ni during the chemical separation. However, Cr can only interfere with Ni on mass 62 by combination with carbon (50Cr12C), and Co when associated with H. They are responsible for no interference on mass 61. The most important elements in term of potential interferences when measuring Ni isotopes are Cd, Sn, Te and Xe because these elements can create doubly charged ions at masses 58, 60, 61 and 62. Xenon may come from the Ar supply. Cadmium and Sn have a distribution coefficient higher than 100 in 6 N HCl for the resin used and Te is strongly adsorbed. These elements are therefore efficiently separated from Ni during the ion exchange procedure. The Ni fraction was sufficiently pure after this single column to be measured directly using the MC–ICPMS, except in a few cases where the Zn content was too high. Since Zn is responsible for an isobaric interference with Ni at mass 64, it has to be removed before the analysis. To do so, these samples were dried again, taken up in 2 mL of 2 N HCl and loaded on a 1.8-mL column filled with AG1-X8 resin. Nickel was collected in the loading fraction together with 2 more milliliters of 2 N HCl, whereas Zn was retained on the resin. The yield of the whole procedure was 100% within error and the total procedural blank for Ni was typically b 0.5 ng which is insignificant compared to the amount of Ni analyzed. All Ni isotope measurements were performed using the large geometry high resolution MC–ICPMS Nu1700 at the ETH Zurich. This instrument is equipped with 16 Faraday cups and the 17% mass dispersion permits simultaneous measurement of all Ni isotopes as well as 57Fe and 66Zn for interference correction. In addition to Zn, an unknown interference occurs on mass 64 (low mass side) so that up to now we have not been able to get highly reproducible 64Ni data. This is not a major problem in the framework of the present project which is mainly concerned with the search for radiogenic 60Ni. The MC–ICPMS was run at a mass resolution m/Dm of about 2600 to resolve the 40Ar18O+ and 40 Ar20Ne+, 38Ar22Ne+ interferences on masses 58 and 60 respectively. Sulphide sample solutions as well as

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some metal sample solutions were scanned to check for other possible interferences in their Ni mass spectra. Cadmium and Te are not detected in any sample and the Sn and Xe signals represent less than 0.06% of the Ni signal for all samples. Therefore, doubly charged species of these elements can be ignored. At a high mass resolution m/Dm of about 8000, no interference is

resolved on masses 58, 60, 61 and 62 except ArNe and ArO. Besides, metal solutions yield similar mass spectra over the mass range of interest as sulphide solutions. Therefore, we conclude that the measured 60 Ni/58Ni, 61Ni/58Ni and 62Ni/58Ni ratios are not affected by uncorrected interferences with the mass resolution selected.

Table 1 Nickel isotopic data and Fe/Ni elemental ratios in iron meteorite metals and sulphides Group

Samples

n

60

61

IC

Arispe Gnady Benett County Coahuila Gressk Negrillos Mount Joy Sikhote Alin Carbo Carbo D Carbo G Carbo V Carbo Y Hrashina Henbury Merceditas Bear Creek Chupaderos Grant Clark County Nelson County Duel Hill Gibeon Yanhuitlan Chinga Tawallah Valley Tlacotepec Caddo County Canyon Diablo Odessa Toluca Watson Seesla¨gen

6 2 12 6 9 9 6 6 8 6 6 7 6 6 6 10 9 9 16 9 9 7 16 10 9 8 17 8 8 6 8 12 12

0.385193 F 4 0.385183 F 17 0.385188 F 4 0.385179 F 7 0.385174 F 4 0.385185 F 6 0.385190 F 8 0.385195 F 9 0.385184 F 14 0.385174 F 7 0.385177 F 8 0.385175 F 5 0.385181 F 6 0.385188 F 13 0.385190 F 16 0.385180 F 6 0.385185 F 9 0.385180 F 7 0.385180 F 5 0.385184 F 10 0.385173 F 9 0.385185 F 9 0.385177 F 3 0.385194 F 12 0.385186 F 6 0.385182 F 8 0.385192 F 10 0.385185 F 15 0.385174 F 9 0.385184 F 4 0.385190 F 8 0.385179 F 4 0.385181 F 7

0.0167429 F 5 0.0167458 F 16 0.0167424 F 9 0.0167430 F 7 0.0167418 F 6 0.0167429 F 12 0.0167433 F 19 0.0167435 F 4 0.0167434 F 10 0.0167432 F 11 0.0167429 F 10 0.0167429 F 3 0.0167435 F 5 0.0167428 F 7 0.0167435 F 19 0.0167421 F 7 0.0167428 F 9 0.0167426 F 9 0.0167420 F 7 0.0167425 F 12 0.0167431 F15 0.0167419 F 10 0.0167421 F 9 0.0167441 F 9 0.0167441 F 21 0.0167437 F 11 0.0167435 F 10 0.0167449 F 22 0.0167420 F 13 0.0167427 F 4 0.0167434 F 16 0.0167427 F 6 0.0167427 F 7

15.2 F 0.2 22.7 F 0.8 22.7 F 0.1 21.8 F 1.0 4.7 F 0.2 22.8 F 0.4 19.6 F 0.7 19.7 F 0.7 11.1 F 0.1 10.9 F 0.2 10.2 F 0.2 10.4 F 0.2 9.9 F 0.2 11.2 F 0.4 16.4 F 0.3 11.8 F 0.9 22.5 F 0.4 12.1 F 0.6 12.5 F 0.5 17.2 F 0.3 17.1 F 0.5 11.8 F 0.2 14.9 F 0.2 15.4 F 0.1 5.1 F 0.6 5.9 F 0.1 5.8 F 0.3 10.9 F 0.4 18.6 F 0.1 13.3 F 0.5 14.4 F 0.4 13.2 F 0.8 18.3 F 0.1

0.30 F 0.10 0.02 F 0.44 0.16 F 0.12 0.08 F 0.18 0.19 F 0.11 0.07 F 0.17 0.20 F 0.22 0.33 F 0.25 0.05 F 0.37 0.22 F 0.19 0.13 F 0.21 0.18 F 0.14 0.01 F 0.15 0.17 F 0.33 0.21 F 0.43 0.05 F 0.16 0.08 F 0.24 0.03 F 0.18 0.04 F 0.13 0.05 F 0.27 0.23 F 0.23 0.09 F 0.24 0.11 F 0.07 0.30 F 0.32 0.12 F 0.17 0.01 F 0.21 0.27 F 0.26 0.09 F 0.38 0.19 F 0.23 0.05 F 0.11 0.20 F 0.20 0.08 F 0.10 0.02 F 0.20

0.06 F 0.30 1.70 F 0.96 0.34 F 0.55 0.02 F 0.43 0.72 F 0.39 0.03 F 0.72 0.18 F 1.12 0.27 F 0.27 0.26 F 0.60 0.12 F 0.68 0.04 F 0.62 0.06 F 0.17 0.33 F 0.30 0.15 F 0.40 0.30 F 1.12 0.55 F 0.44 0.13 F 0.52 0.21 F 0.55 0.61 F 0.45 0.28 F 0.71 0.05 F 0.91 0.65 F 0.62 0.51 F 0.55 0.68 F 0.57 0.67 F 1.23 0.44 F 0.67 0.32 F 0.63 1.12 F 1.30 0.59 F 0.79 0.15 F 0.27 0.26 F 0.98 0.21 F 0.36 0.15 F 0.44

6 3 5 6 6 2 2 6 3 4

0.385184 F 2 0.385088 F 8 0.385134 F 16 0.385189 F 5 0.385187 F 7 0.385021 F14 0.385082 F 5 0.385169 F 17 0.385085 F 13 0.385125 F 19

0.0167433 F 6 0.0167630 F 16 0.0167531 F10 0.0167436 F 12 0.0167443 F 18 0.0167718 F 20 0.0167574 F 45 0.0167416 F 9 0.0167618 F 37 0.0167549 F 23

25.3 F 2.3 1158 F 69 42.4 F 5.1 12.9 F 1.1 6.5 F 0.3 3084 F 189 3286 F 106 3908 F 69 12.9 F 0.4 2487 F 8

0.05 F 0.06 2.42 F 0.22 1.25 F 0.42 0.18 F 0.14 0.14 F 0.20 4.17 F 0.36 2.60 F 0.14 0.34 F 0.44 2.52 F 0.34 1.47 F 0.49

0.15 F 0.39 11.9 F 1.0 6.02 F 0.58 0.38 F 0.72 0.81 F1.10 17.2 F 1.2 8.6 F 2.7 0.81 F 0.53 11.2 F 2.2 7.1 F1.4

IIA

IIB IID

IIIA IIIB

IIIF IVA

IVB

IAB

IIE IIICD IIA IIIB IVA IAB

IIICD

Gressk – Sulphide North Chile – Sulphide Bear Creek – Sulphide Gibeon – Sulphide Sao Julio de Moreira – Sulphide Odessa – Sulphide 1 Odessa – Sulphide 2 Toluca – Sulphide 1 Toluca – Sulphide 2 Seesla¨gen – Sulphide

Ni/58Ni F 2r

Ni/58Ni F 2r

56

Fe/58Ni F 2r

e 60 F 2r

e 61 F 2r

The number of runs for each sample is indicated by bnQ. The uncertainties for the 60 Ni/58 Ni and 61 Ni/58 Ni ratios refer to the last significant digits. The Fe and Ni concentrations in the sample cannot be determined with precision because an aliquot has been taken for Fe isotopic measurements without weighing. Therefore, only the Fe/Ni ratio can be measured precisely. The e values are calculated as the deviation relative to the standard expressed in parts per 104 . Carbo D, G, V and Y indicate which bar has been sampled.

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The eluted Ni fraction was evaporated to dryness and re-dissolved in 0.1 N HCl, adjusting the Ni concentration of the solution to about 200 ppb. A 200 ppb Ni standard solution was run with a typical total ion beam intensity of about 10 10A and replicated measurements of the standard solution yielded 60Ni/58Ni = 0.3851818 F 0.0000009 and 61Ni/58Ni = 0.016743 F 0.000002 (n = 544) where the uncertainties correspond to the standard errors (2r) over 6 months. The external reproducibility over one session is F 0.3e and F0.6e for the 60Ni/58Ni and 61Ni/58Ni ratios, respectively. All ratios are normalized to 62Ni/58Ni = 0.05338858 [12] and corrected for mass fractionation using an exponential law. The data for the samples are expressed in e units, where e is the isotopic composition of the sample relative to the average of the two bracketing standards in parts per 104. Most samples were run at least 6 times in different sessions (see Table 1) and each data point on Fig. 1 represents the mean of multiple standard-sample brackets. The uncertainty quoted for a sample is the standard error for all the relevant runs. The Fe/Ni ratios were determined using a double focusing sector field ICPMS (Element 2, Thermo Electron Corporation). A pre-defined mass resolution of m/ Dm = 4000 was used. The aliquots kept for the concentration measurements were dried and taken up in 0.1% HNO3. Rhodium was added as an internal standard element with a concentration of 5 ppb. Nickel-60 and

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Fe were chosen for determining the concentrations because these isotopes are free of interferences at the mass resolution used, but 61Ni and 54Fe were also always monitored to check for consistency. The Fe and Ni concentrations were calculated using calibration curves obtained with single-element external standards. An aliquot for Fe isotopic analysis was taken from the Fe, Ni solution after digestion but was not accurately weighed. Therefore, the Fe/Ni ratios of the solutions were determined precisely but not the Fe and Ni concentrations. Each aliquot was analyzed more than 6 times over different sessions and the uncertainty represents the 2r standard deviation for all measurements. 3. Results The 60Ni/58Ni ratio of the metal from all iron meteorites analysed in the present work is identical to the standard within error (Table 1 and Fig. 1), in agreement with recent data by Cook et al. [9,10]. Moynier et al. [7,8] detected an excess of 60Ni in Canyon Diablo, Casas Grande and Toluca. We cannot confirm these results, although the reason for the discrepancy currently is unclear. Canyon Diablo yields an e 60 value of 0.19 F 0.23 consistent with the value of 0.04 F 0.30 measured by Cook et al. [9]. Toluca yields a value of 0.20 F 0.20. The Fe/Ni ratios vary from 3.5 to 16.9 in the metal and are lower than the Fe/Ni of the solar

Fig. 1. Nickel isotopic composition of the metal from iron meteorites in a 60Ni/58Ni vs. 56Fe/58Ni diagram. The different groups are represented by different symbols. Magmatic irons are shown with black symbols and non-magmatic irons are reported with white symbols. The horizontal dashed line corresponds to the value of the terrestrial standard. All metals have the same isotopic composition as the standard within uncertainty (the gray band stands for the external reproducibility of the standard in 2r) and there is no correlation between the isotopic composition of the samples and their Fe/Ni ratio.

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system because Ni is more siderophile than Fe and has a strong affinity for the nascent core. Most iron meteorite groups display limited variability in Fe/Ni ratio, but there is no relationship between the isotopic composition of the samples and the group to which they belong. Magmatic irons yield an average e 60 = 0.04 F 0.33 (2r) and non-magmatic irons yield an average e 60 = 0.01 F 0.27 (2r). Thus, magmatic and non-magmatic groups cannot be distinguished, although nonmagmatic groups yield W isotopic compositions that indicate that they formed later or were disturbed by a secondary event [4,13]. The e 61 values for the metal are also zero within uncertainty facilitating the use of 61 Ni/58Ni instead of 62Ni/58Ni ratio for internal normalization. This procedure leaves the e 60 values at zero. Thus, the metal phase of iron meteorites shows no nucleosynthetic anomalies for Ni. In contrast to the metals, sulphides yield e 60 values between 4.17 F 0.36 and + 0.18 F 0.14 and e 61 values from 0.81 F 0.53 to + 17.23 F 1.20. Most sulphides are characterised by an excess of 61Ni and a correlated deficit of 60Ni (Fig. 2). Sulphides generally have much higher Fe/Ni than the metals. The elemental ratio spans from 4.9 F 0.2 in the sulphide from Sao Julio de Moreira to 2900 F 51 in Toluca sulphide. Because of the high Fe/Ni ratio in many sulphides, these are expected

to show an excess of 60Ni if they formed early in the solar system when 60Fe was extant (Fig. 3a). Instead a deficit is sometimes present that appears to relate to excess 61Ni. 4. Discussion The causes of the Ni isotopic effects in the sulphides are unclear. A 61Ni excess can be explained either by spallation reactions due to interaction with cosmic rays, or by nucleosynthetic processes. Iron meteorites have typical exposure ages longer than 300 Myrs [14,16] so that cosmogenic effects may not be negligible. Spallation reactions can convert 60Ni to 61 Ni, which in turn produces 62Ni. 62Ni and 64Ni are consumed and are relevant targets for the production of 60 Fe during exposure to cosmic rays. Similarly, 58Ni is also consumed to produce 59Ni. To estimate the relative cosmogenic effects on different isotopes, the cross sections and the relative abundance of the isotopes have to be considered. The neutron capture cross sections are low for all Ni isotopes (4.6, 2.9, 2.5 and 14.5 barns for 58 Ni, 60Ni, 61Ni and 62Ni, respectively) and the three heavy isotopes of Ni are of minor abundance (58Ni = 68.08%, 60Ni = 26.22%, 61Ni = 1.14%, 62Ni = 3.63% and 64 Ni = 0.93%). Because of the small cross sections, cos-

Fig. 2. Nickel isotopic composition of the sulphides from iron meteorites. The 60Ni/58Ni and 61Ni/58Ni ratios are reported using the e notation (e = [(iNi/58Ni)sample/(iNi/58Ni)standard 1]  10 000, i = 60 or 61). Sulphides show 61Ni-excesses that correlate with 60Ni-deficits. This correlation is interpreted as a mixing between a pure s-process component and terrestrial-like nickel. BC S: Bear Creek Sulphide, GIB S: Gibeon Sulphide, GRE S: Gressk Sulphide, NCH S: North Chile Sulphide, ODE S1, S2: Odessa Sulphides 1 and 2, SJdM S: Sao Julio de Moreira Sulphide, SEE S: Seesla¨gen Sulphide, TOL S1, S2: Toluca Sulphides 1 and 2.

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Fig. 3. Comparison between the nickel isotopic composition measured in iron meteorites and the expected composition if they formed at a given time after the start of the solar system. The point bCHQ stands for the chondritic mean value. Isochrons have been calculated assuming that irons formed from a chondritic reservoir and that the Fe–Ni system closed in these meteorites 0.5, 1, 2, 3, 4, 5 and 10 Myrs after the start of the solar system. A value of 10 6 is taken for the initial 60Fe/56Fe ratio of the solar system [23].

mogenic effects in Ni at a detectable level are unlikely, but any effect on 60Ni should be accompanied by an opposite effect on 61Ni that is about 3.2 times greater if the normalization ratio is ignored. Taking into account the effect on the normalization ratio, cosmogenic effects would result in a positive correlation between e 61 and e 60 and a slope of +1.5. This is not what is observed in sulphides (Fig. 2). The only

target able to produce Ni isotopes by spallation reactions is Ni itself. Therefore, the chemical composition of the sample plays no role and cosmogenic effects should be present not only in sulphides but also in the metal phase; this is not the case. Different samples from the Carbo iron meteorite yield the same Ni isotopic composition within uncertainty (Table 1) whereas clear evidence of significant cosmogenic

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effects on tungsten isotopes has been found for these samples [15]. The exposure age of a meteorite is a good proxy to evaluate the intensity of the cosmogenic effects. We plotted the Ni isotopic composition of the samples against the exposure age. There is a correlation neither for the metals nor for the sulphides (Fig. 4). Thus, cosmogenic effects do not account for the observed e 61–e 60 correlation in sulphides. Dauphas et al. [17] demonstrated that iron meteorites carry Mo and Ru nucleosynthetic anomalies and clearly showed that the isotopic composition of irons plot on a mixing line between a pure s-process component and a

terrestrial component for both Ru and Mo. Interestingly, Ni is also over-produced by the s-process. Therefore, we calculated the theoretical mixing line for Ni in a three-isotope plot. For the Mo composition of the sprocess component, Dauphas et al. [17] used the Mo isotopic ratios measured in presolar mainstream SiC grains condensed in the envelopes of AGB stars that carry the s-process signature. Unfortunately, no Ni data exist for such grains. We used the Ni predictions for the envelope mixture of an AGB star as inferred from a recent model by Gallino and collaborators. The ratios are as follows: 60Ni/58Ni = 1.03, 61Ni/58Ni = 1.27 and

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Fig. 4. Nickel isotopic composition of iron meteorites as a function of their exposure age. The 60Ni/58Ni and 61Ni/58Ni ratios are reported using the e notation. Exposure ages are from [14] and [16]. No correlation is observed.

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Ni/58Ni = 1.12 (Gallino, personal communication). Using the same equations as in [17], the s-process compositions (qiNi ) normalized to terrestrial composition are 1.67, 74.85, and 19.98 for the Ni isotopes 60, 61 and 62 respectively. We calculate a slope of 7.20 for the mixing line between terrestrial-like nickel and a s-process component in the e 61 vs. e 60 diagram. If we consider instead the pure s-process component in AGB stars and not the Ni isotopic composition once diluted in the envelope with material with initial composition – as indeed is the composition of the gas out of which SiC grains condense – the Ni ratios are: 60Ni/58Ni = 8.9, 61 Ni/58Ni = 70 and 62Ni/58Ni = 46 [18]. The normalized s-process compositions (qiNi ) are then 22.10, 4179.55, and 860.61 for the Ni isotopes 60, 61 and 62, respectively, and the slope is 8.66 in the e 61 vs. e 60 diagram. The best-fit line going through our experimental data yields a slope of 4.2, lower than the theoretical value. However, cross-sections for Ni isotopes are given with an uncertainty of 20% (2r) and recent studies tend to show that the theoretical estimate for 62Ni may be wrong by a factor of 2 [19], so that the results inferred from models of stellar nucleosynthesis have large uncertainties (easily up to a factor of 2) due to error propagation. Taking these uncertainties into account, the theoretical estimate is broadly consistent with our data. Experimental Ni data for presolar SiC grains would be required to do more precise calculations. Large isotopic anomalies for a non-radiogenic isotope, such as those observed for 61Ni in sulphides (up to 20 e 61) are difficult to interpret except as nucleosynthetic anomalies. Therefore, it seems that the Ni isotopic composition of some sulphides results from a mixing between a pure s-process component and terrestrial-like Ni. A mixing equation indicates that the pure s-process component contributes about 4  10 7 (or 0.4 ppm) of the total nickel present in Odessa S1 sulphide and represents an even lower fraction in the other sulphides. The metal phases show neither a 61Ni-excess, nor a 60 Ni-deficit. This might seem surprising given that the metal and the sulphide are usually considered to have formed within a short time interval from a common melt pool. The presence of this nucleosynthetic isotopic heterogeneity within an iron meteorite is therefore unexpected. If the coexisting sulphides and metals all form by exsolving from S-rich metallic Fe liquids, then each sample of the meteorite should yield the same composition. This is not observed. Instead, different sulphides from the same meteorite, even with similar chemical composition (at least similar Fe/Ni ratio) such as those in Odessa, can have very different

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isotopic compositions that are different again from the metal. This provides evidence that the heterogeneity is caused by an admixed trace component of extreme isotopic composition. This exotic component must either be present to varying levels within a given meteorite or be diluted to differing degrees by bnormalQ Ni. The nucleosynthetic anomaly might not be detectable in the metal simply because it is strongly diluted by normal Ni, whereas it can be resolved more readily when admixed into the more Ni-poor sulphides. But such a model then raises issues about how such sulphides could form. There is a hint that the effects are more prevalent in non-magmatic groups. Large effects are found in Odessa and Toluca (both IAB) and Seesla¨gen (IIICD) (Fig. 2). Therefore, a plausible model would be addition of presolar grains contributed from a chondritic impactor that produced low temperature sulphide-bearing melt pools. Two magmatic irons – North Chile (IIA) and Bear Creek (IIIB) – also display strong effects. This requires that some sulphides in primitive planetesimal cores also are capable of preferentially carrying the exotic Ni, which is difficult to explain unless sulphide segregates as a late addition to planetesimal core material, separate from the metal, which seems unlikely. An alternative explanation is that presolar grains were present as a background undissolved component both in the metal liquid and in the S-rich melts. As far as we know, no data exist about the survival of presolar grains in metallic melts at a temperature of about 1000 8C. A silicate presolar grain with GEMS-like (Glass with embedded metal and sulphides) composition has been found embedded in Fe-metal in the matrix of the LL3.1 Bishunpur chondrite [20], but the Fe-metal envelope of this grain probably condensed directly from the solar nebula. Therefore, this grain does not characterize the survival of presolar grains during magmatic processes. However, presolar grains might be resistant to high temperature processes (metal-silicate segregation occurs at about 1230 8C and the Fe–FeS eutectic is at 987 8C) with the formation of a protective silica surface layer [21,22]. A clear test of this hypothesis would be that presolar grains can be isolated from iron meteorites. The uncertainties on the Ni isotopic composition of the pure s-process component are too large to disentangle the nucleosynthetic effects from small 60Ni-excesses caused by in situ decay of 60Fe. However, if sulphides formed less than 3 Myrs after the start of the solar system, the expected 60Ni-excess would be very large (several tens of e units). Such radiogenic effects should be detectable, even if they are superimposed on nucle-

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osynthetic anomalies. This is not the case. Therefore the Fe–Ni isotopic system must have closed to diffusion in the sulphides more than 10 Myrs after the start of the solar system. This is the minimum time interval required to explain radiogenic anomalies smaller than 1 e 60 (Fig. 3a). The slight scattering of the data from the best-fit line (Fig. 2) may however be explained by small variations in radiogenic Ni. The minimum time interval Dt min required between the start of the solar system and the crystallization of each metal can be inferred from its Fe/Ni ratio and the initial 60Fe/56Fe of the solar system, taken at 10 6 [23]. The closure of the Fe–Ni isotopic system occurred in the metal at least 0.4 Myr after the start of the solar system (Fig. 3b). For all samples except Canyon Diablo and the IIIFs (Clark County and Nelson County), the Fe–Ni model age inferred from Dt min is higher than the Hf–W age corrected for cosmogenic effects [4]. This is expected for the following reasons: (1) the Hf–W system involves one lithophile element (Hf) and one siderophile element (W) so that it dates the metal–silicate differentiation or in other words the metal segregation whereas the Fe–Ni chronometer involves two siderophile elements and thus dates the crystallisation of the core and (2) the closure temperature of Fe–Ni is lower than the closure temperature of the Hf–W system (respectively about 400–500 8C and 800–900 8C). Cooling rates for iron meteorites are not well defined for all groups and vary strongly among the different groups, from 3 8C/Myrs (IIABs and IIIABs [24,25]) to more than 300 8C/Myrs (IABs, IVAs and IVBs [24,26]). The meteorite needs about 130 Myrs or 1.3 Myrs to cool down from 800 to 400 8C, with a cooling rate of respectively 3 8C/Myr and 300 8C/Myr. As the Fe–Ni system closed in some iron meteorites less than 1 Myr after the metal–silicate segregation, our data suggest that the cooling rate in these samples was very high (400 8C/Myr), whereas longer time intervals between core segregation and its crystallisation can be easily explained by much lower cooling rates. In this latter case, 60Fe had completely decayed when the Fe–Ni closure temperature was reached. Like the Fe–Ni system, the Re–Os chronometer also involves two siderophile elements and dates the crystallisation of the metal. The Fe–Ni results are consistent with the Re–Os ages measured for different iron meteorites [27,28]. 5. Conclusion No 60Ni anomaly from the in situ decay of the shortlived 60Fe radionuclide has been found in the metal of

more than 30 iron meteorites. If the uncertainties on the measurements are considered, no clear chronological conclusion can be drawn: some irons may have crystallised as soon as 0.4 Myrs after the start of the solar system whereas others probably cooled through the closure temperature of the Fe–Ni system much later. To better constrain the time scale, we also analysed sulphides because they have much higher Fe/Ni ratios and thus are expected to show a large excess of radiogenic 60Ni. Several sulphides appear to be in fact depleted in 60Ni and enriched in 61Ni. Both anomalies are correlated, which can be explained as a mixing between a pure s-process component and terrestriallike Ni. This nucleosynthetic anomaly cannot be corrected with high precision and prevents detection of the expected 60Ni excess, if any, in sulphides. The Fe–Ni system must have closed in sulphides at least 10 Myrs after the start of the solar system to explain the data. Acknowledgements We thank the Smithsonian Institution of Washington, the National History Museum of London, and the National History Museum of Vienna for providing the samples, Agne`s Markowski, Sune Nielsen, Helen Williams and Sarah Woodland for sharing their sample solutions after dissolution, Philipp Heck, Peter Hoppe, Ingo Leya, Rainer Wieler for fruitful discussions. We particularly thank Roberto Gallino for kindly sending us his most recent results of new model calculations and for numerous discussions. Comments from two anonymous reviewers and from the editor were appreciated and helped improve the manuscript. This work was supported by the Swiss National Science Foundation and the ETH. References [1] E.R.D. Scott, Chemical fractionation in iron meteorites and its interpretation, Geochim. Cosmochim. Acta 36 (1972) 1205 – 1236. [2] T. Kleine, K. Mezger, H. Palme, E. Scherer, C. Muenker, Early core formation in asteroids and late accretion of chondrite parent bodies: evidence from 182Hf–182W in CAIs, metal-rich chondrites and iron meteorites, Geochim. Cosmochim. Acta 69 (2006) 5805 – 5818. [3] D.-C. Lee, Protracted core formation in asteroids: evidence from high precision W isotopic data, Earth Planet. Sci. Lett. 237 (2005) 21 – 32. [4] A. Markowski, G. Quitte´, A.N. Halliday, T. Kleine, Tungsten isotopic composition of iron meteorites: chronological constraints vs. cosmogenic effects, Earth Planet. Sci. Lett. 242 (2005) 1 – 15. [5] J.H. Chen, G.J. Wasserburg, Live 107Pd in the early solar system and implications for planetary evolution, in: A. Basu, S.R. Hart

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