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Nickel Isotopic Compositions in Pallasites and Iron Meteorites

We report Ni isotope ratio results for Ni from metal in iron meteorites and from metal and olivine in pallasites. The investigation of pallasites was undertaken because (1) previous work on pallasites showed evidence for in situ decay of Mn (t1⁄2 = 3.7 Ma) in olivines [1,2] and of Pd (t1⁄2 =6.5 Ma) in metal [3]; (2) evidence for the in situ decay of Fe (t1⁄2 = 1.5 Ma) in eucrites [4,5]; (3) possible early formation of pallasites from Re-Os data [6]; and (4) very high Fe/Ni ratios in pallasite olivines [5,7]. Recently, we reported Ni isotopic data, for Ni, on (1) FeNi metal and sulfides in different groups of iron meteorites, (2) sulfides and a whole rock sample of the St. Séverin chondrite, and (3) chondrules from the Chainpur chondrite [8]. No evidence was found for resolved radiogenic or general Ni isotope anomalies at the resolution levels of 0.2 εu and 0.5 εu (εu = 0.01%) for Ni/Ni and Ni/Ni, respectively. From the Fe/Ni ratios and εNi values, we obtained upper limits for the initial value of (Fe/Fe)0 of a) <2.7±10 for Chainpur chondrules, b) <10 for the St. Séverin sulfide, and c) <4x10 for sulfides from iron meteorites. The purpose of this study is to identify potential general isotope anomalies in Ni and direct evidence for the in situ decay of Fe in pallasites and iron meteorites. The pallasite olivine samples we analyzed were prepared by G. Srinivasan, free from rust and impurities as described in Papanastassiou et al. (1997) [7]. We followed the Ni analytical procedures developed in Chen et al. (2009) [8]. The results are shown in Table 1 and indicate that the εNi and εNi values of all pallasite samples (FeNi metals and olivines) are, respectively, within 0.2 εu and 0.4 εu of terrestrial standards. From the Fe/Ni ratios and error limits of εNi, we calculated bounds on the values of initial (Fe/Fe)0 for the meteorite parent bodies (Table 1). In an εNi versus Fe/Ni diagram (Fig. 1), for pallasites olivines, we show the inferred initial (Fe/Fe)0 for Salta is < 10 and for Brenham and for Thiel Mountain <8x10 based on the observed Ni/Ni and extremely high Fe/Ni (6600 to 17000). The Fe/Ni for the pallasite metals are slightly lower than that of Orgueil (Fig. 2), but the pallasite metals do not show a significant deficit in εNi. The new high precision results show no excess in Ni and agree with the less precise and lower Fe/Ni data reported previously for Omolon olivines and Thiel Mountain olivines [2, 7]. The pallasite Ni data suggest no evidence for the in situ decay of Fe in the meteorite parent bodies or a long interval between the injection of Fe and formation of the pallasite parent bodies. Alternatively, the pallasites cooled over a sufficiently long time (more than 10 Ma) for the Ni in the olivine to equilibrate with the FeNi. Previously, we reported excess Ag in metal of Brenham (εAg = 52±7). The inferred (Pd/Pd)0 for Brenham, (1.1±0.14)x10 suggests a formation time of ~7 Ma after formation of Gibeon (VIA iron). This isolation time is long enough to prevent any resolvable effect to be detected within our current precision for εNi. Previously published Ni isotopic data for pallasite metals show either normal values (Eagle Station, Albin, Brenham and Molong in [9] and Molong in [10]) or anomalous values (Admire, Esquel and Brahin in [11]). So far we have analyzed only one (Brenham) of the same meteorite samples reported in these studies. With the precision of our current data, we also do not support nor exclude [8] the conclusions based on the Ni isotopic anomalies on iron meteorite metal reported by Bizzarro et al. (2007)[11]. The variations of εNi versus Fe/Ni for the FeNi metal samples (data from [8]) from iron meteorites are shown in Fig. 3. The dashed lines represent Fe/Ni