Contribution of Ryugu-like material to Earth's volatile inventory by Cu and Zn isotopic analysis

Abstract

Initial analyses showed that asteroid Ryugu's composition is close to CI (Ivuna-like) carbonaceous chondrites -the chemically most primitive meteorites, characterized by near-solar abundances for most elements. However, some isotopic signatures (e.g., Ti, Cr) overlap with other carbonaceous chondrite (CC) groups, so the details of the link between Ryugu and the CI chondrites are not fully clear yet. Here we show that Ryugu and CI chondrites have the same zinc and copper isotopic composition. As the various chondrite groups have very distinct Zn and Cu isotopic signatures, our results point at a common genetic heritage between Ryugu and CI chondrites, ruling out any affinity with other CC groups. Since Ryugu's pristine samples match the solar elemental composition for many elements, their Zn and Cu isotopic compositions likely represent the best estimates of the solar composition. Earth's mass-independent Zn isotopic composition is intermediate between Ryugu/CC and non-carbonaceous chondrites, suggesting a contribution of Ryugu-like material to Earth's budgets of Zn and other moderately volatile elements.

Keywords: CI chondrites; Cu isotopes; Hayabusa2; Ryugu; Zn isotopes; volatile elements.

Figure 1. Zinc and copper elemental and isotopic compositions for Ryugu and carbonaceous chondrites samples.

(a) δ⁶⁶Zn vs Mg/Zn, (b) δ⁶⁵Cu vs Mg/Cu and (c) Zn/Mg vs Cu/Mg that we measured for the Ryugu samples (diamonds) and carbonaceous chondrites (large circles with abbreviations: Or=Orgueil, Als=Alais, Tag=Tagish Lake, Tar=Tarda, Mur=Murchison, All=Allende). Small circles are from the literature [17–21] for Zn and Cu isotope compositions (and references therein for major and trace element compositions). The color identifies the type of chondrite as described in the legend. The purple star in panel c represents the CI chondrite composition from [1]. Data are presented as mean values with 2SD error bars, reported in Table 1.

Figure 2. δ⁶⁶Zn vs δ⁶⁵Cu for Ryugu samples and carbonaceous chondrites.

Literature data are from [17,19]. Same symbols as in Figure 1 for the samples analyzed in this study. Other chondrite groups from the literature are reported directly on the figure. Data are presented as mean values with 2SD error bars, reported in Table 1. For clarity, only the error bars of our measurements are displayed. Error bars for literature data are not shown.

Figure 3. δ⁶⁶Zn (this study) vs ε⁵⁴Cr [5,30,53–55] for Ryugu samples and carbonaceous chondrites.

Literature data are from [17,18,20] for Zn isotope compositions, and from [56] for Cr isotope compositions. The dark and light blue shaded areas correspond to the ε⁵⁴Cr ranges for site A and site C, respectively, from [6]. Same symbols as in Figure 1 for the samples analyzed in this study. Other chondrite groups from the literature are reported directly on the figure. Data are presented as mean values with 2SD error bars, reported in Table 1.

Figure 4. Variations of ε⁶⁶Zn among different groups of meteorites.

For comparison purposes, only Ryugu (diamond) and CI (purple circles) samples measured in this study are represented here. Literature data for carbonaceous chondrites ([28] (large symbols), [29] (small symbols)), ordinary chondrites [28,29], enstatite chondrites [28,29], NC and CC iron chondrites [29], ureilites [29] are shown with gray symbols. Bulk Silicate Earth: +0.015 ± 0.075 ‱, 2SE, n = 4 [28] and –0.07 ± 0.013 ‱, 2SE, n = 3 [29]). Data are presented as mean values with 2SE error bars, reported in Table 1.