Abstract
Zinc isotope compositions (δ66Zn) and concentrations were determined for metal samples of 15 iron meteorites across groups IAB, IIAB, and IIIAB. Also analyzed were troilite and other inclusions from the IAB iron Toluca. Furthermore, the first Zn isotope data are presented for metal–silicate partitioning experiments that were conducted at 1.5 GPa and 1650 K. Three partitioning experiments with run durations of between 10 and 60 min provide consistent Zn metal–silicate partition coefficients of ∼0.7 and indicate that Zn isotope fractionation between molten metal and silicate is either small (at less than about ±0.2‰) or absent. Metals from the different iron meteorite groups display distinct ranges in Zn contents, with concentrations of 0.08–0.24 μg/g for IIABs, 0.8–2.5 μg/g for IIIABs, and 12–40 μg/g for IABs. In contrast, all three groups show a similar range of δ66Zn values (reported relative to ‘JMC Lyon Zn’) from +0.5‰ to +3.0‰, with no clear systematic differences between groups. However, distinct linear trends are defined by samples from each group in plots of δ66Zn vs. 1/Zn, and these correlations are supported by literature data. Based on the high Zn concentration and δ66Zn ≈ 0 determined for a chromite-rich inclusion of Toluca, modeling is employed to demonstrate that the Zn trends are best explained by segregation of chromite from the metal phase. This process can account for the observed Zn–δ66Zn–Cr systematics of iron meteorite metals, if Zn is highly compatible in chromite and Zn partitioning is accompanied by isotope fractionation with Δ66Znchr-met≈−1.5‰. Based on these findings, it is likely that the parent bodies of the IAB complex, IIAB and IIIAB iron meteorites featured δ66Zn values of about −1.0 to +0.5‰, similar to the Zn isotope composition inferred for the bulk silicate Earth and results obtained for chondritic meteorites. Together, this implies that most solar system bodies formed with similar bulk Zn isotope compositions despite large differences in Zn contents.
Highlights
Cosmochemical studies reveal a clear preference for compositional investigations of primitive and differentiated stony meteorites because their diverse elemental and isotopic chemistry is more readily exploited to examine early solar system conditions and processes
A similar inference can be made for the IAB metal samples of this study, as these exhibit strong siderophile element correlations that are in agreement with published trends but there are no correlations between siderophile elements and either Zn concentrations or isotope compositions
The experiments demonstrate that Zn has a slight preference for silicate melts, in agreement with previous studies, and indicate that Zn isotope fractionation between molten metal and silicate is either small or absent
Summary
Cosmochemical studies reveal a clear preference for compositional investigations of primitive and differentiated stony meteorites because their diverse elemental and isotopic chemistry is more readily exploited to examine early solar system conditions and processes. Iron meteorites fall into the two categories of magmatic and non-magmatic irons The former are considered to be core fragments from differentiated and subsequently disrupted asteroidal parent bodies. Non-magmatic iron meteorites did not form as a single core in a differentiating asteroid Possible models for their formation range from impact melting on the surface of an undifferentiated parent body (Choi et al, 1995; Wasson and Kallemeyn, 2002) to catastrophic impact, breakup and reassembly of a chondritic asteroid, which experienced partial metal–silicate differentiation (Benedix et al, 2000; Schulz et al, 2009; Theis et al, 2013). Further Zn data were obtained for samples from metal– silicate partitioning experiments that were conducted at 1.5 GPa and 1650 K Together, these results were applied to (i) interrogate the processes responsible for the variable Zn contents and isotope compositions of iron meteorites and (ii) constrain the bulk Zn isotope compositions of the respective asteroid cores and parent bodies
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