Oxygen isotope systematics of crystalline silicates in comet Wild 2: Comparison to anhydrous
minerals in Ryugu and CI chondrites
Noriko T. Kita1, Mingming Zhang1, Donald E. Brownlee2 and David J. Joswiak2
1
WiscSIMS, Department of Geoscience, University of Wisconsin-Madison2
Department of Astronomy, University of Washington
Initial analyses of Ryugu returned samples indicated that they are similar to CI (Ivuna-type) carbonaceous chondrites,
which mainly consist of minerals that formed by parent body aqueous alteration [e.g., 1-5]. Discovery of CO2 bearing fluidinclusions in Ryugu pyrrhotite suggests that the Ryugu parent body accreted in the outer solar system beyond the CO2 and H2Osnowlines (>3-4 au [2]). Kawasaki et al. [6] conducted oxygen isotope analyses of anhydrous minerals, such as olivine andpyroxene that are extremely rare in Ryugu and CI chondrite Ivuna. They examined the distribution of mass-independent
fractionation factors of oxygen 3-isotopes, expressed by ∆17O (= δ17O – 0.52 × δ18O), in olivine grains from Ryugu and CIchondrites, and found them to be similar to those of comet Wild 2 from the NASA Stardust mission [7-12]. They suggested thatthe formation region of the Ryugu asteroid could have been farther into the outer solar system than most carbonaceous chondrites
and closer to the accretion region of comets.
Here, we summarize oxygen isotope data of olivine and pyroxene from comet Wild 2. To date, ~90 relatively coarsegrained particles (>2 µm to upto 60 µm) have been analyzed for oxygen isotopes, including ~20 from the literature [7, 9-13] and~70 from recent studies [14-16]. These latter studies include more than 50 particles extracted from the longest type B Stardust
track (~17 mm) T227, which contains numerous relatively coarse (>2 µm up to 20 µm) particles in the bulb region in addition
to at least four terminal particles (TP). The largest TP measures 60 µm in diameter and has a porphyritic chondrule-like texture[16]. The majority of Wild 2 particle analyses were conducted at the WiscSIMS IMS 1280 using a 2 µm-sized primary beam [7,9, 12-16]. Oxygen isotope data of individual olivine and pyroxene particles are shown in the plot of ∆17O versus Mg# (molar[Mg]/[Mg+Fe] in Figure 1. This diagram has been used for the studies of meteoritic chondrules to understand the isotope
signatures of precursors and redox states during their formation [e.g., 17]. We also summarize published oxygen isotope analysesof olivine and pyroxene from Ryugu [6, 18] and other CI chondrites [6, 19-20] for comparison (Figure 2).
Wild 2 olivine and pyroxene show a wide range of Mg# ranging from 100 to 50 and are evenly distributed (Figure 1).
The ∆17O values are bimodal where most particles are 16O-poor with ∆17O values ranging from –7‰ to +7‰ and others with
∆17O of ~ –22‰, values similar to CAIs in carbonaceous chondrites. The latter 16O-rich particles are either LIME (low iron andMn-enriched) olivine, nearly pure forsterite, or enstatite, all which are with highest Mg#≥98. They are likely early solar nebula
condensates similar to those in AOAs [9,12]. In addition, rare occurrences of 16O-rich relict olivine have been identified, suchas in the chondrule-like object Gozen-sama, studied by [7]. The 16O-poor particles show a general tendency of a slight increasein ∆17O values with decreasing Mg# (Figure. 1). The majority of Wild 2 particles (60-70%) are FeO-rich (Mg#<90) and manyof them show zero to slightly positive ∆17O values. Similar results have been obtained from Giant Cluster IDP U2-20GCA [21],which is considered to be of cometary origin [22]. They overlap with data from CR chondrite chondrules [16, 23-25], suggestinga genetic relationship between CR chondrites and cometary particles [e.g., 9], though FeO-rich chondrules are uncommon in CR
chondrites and do not show ∆17O values higher than 1‰ [12, 16, 21].
Olivine and pyroxene data from Ryugu and CI chondrites are summarized in Figure 2. Most grains have high Mg#(≥98) with bimodal ∆17O values of ~–5‰ and –22‰ and minor FeO-rich grains show ∆17O values of ≤0‰. Similar to Wild 2
particles, 16O-rich olivine grains are often Mn-rich [6, 18, 20] and may be related to AOAs (e.g., [26]). However, the majorityof 16O-poor olivine and pyroxene in Ryugu and CI chondrites cluster at the ∆17O values of –5‰, which are uncommon among
Wild 2 particles. Ryugu and CI data are more similar to chondrule data in major carbonaceous chondrites, such as CMs and CVs(e.g., [27-28]). Rare chondrule-like objects were identified in Ryugu [29], though they are substantially smaller than typicalchondrules in carbonaceous chondrites.
Several Wild 2 particles are chondrule-like objects containing plagioclase and glass [7, 9-11, 16, 30]. Two of them werestudied for 26Al-26Mg chronology, but do not show any resolvable excess 26Mg [9, 31], suggesting that they formed later than 34 Ma after CAI formation (by assuming homogeneity of 26Al in the solar system). In contrast, the Ryugu and CI chondrite parent
bodies might have accreted much earlier ~2 Ma after CAIs and experienced aqueous alteration by the heat generated from 26Al
decay [2]. Thus, a major difference between anhydrous minerals in comets and CI chondrites could be related to the timing ofaccretion in the outer solar system. Comet Wild 2 contains late-forming (>3 Ma) chondrule-like materials, which may haveformed in the outer solar system [16]. They were not present in the Ryugu and CI chondrite-forming regions at the time of theCI chondrite parent body accretion.

Figure 1. Mg# vs. Δ17O relationship among olivine and pyroxene particles in comet Wild 2 (after Zhang et al. [16]). Data sources: Literature[7, 9-13], T227 [16] and others [14-15]. Data from CR chondrules [17, 23-25] are plotted for comparison (error bars are ignored). Chondrulelike objects are shown as circles [7, 9, 11, 16] including 16O-rich relict olivine data [7, 16].

Figure 2. Mg# vs. Δ17O relationship among olivine and pyroxene grains in Ryugu (K23 [6]; L22 [18]) and CI chondrites Orgueil (P20 [19]),Ivuna (K23, M22 [20]), and Alais (M22). The Wild 2 comet data (Figure 1) and CM and CV chondrite chondrule data [27-28] are shown forcomparison (error bars are ignored). Majority of 16O-poor Ryugu and CI chondrite data show systematically lower ∆17O and higher Mg# thanthose of Wild 2.

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