Correlated isotopic and chemical evidence for condensation origins of olivine in comet 81P/Wild 2 and in AOAs from CV and CO chondrites

Abstract

Magnesium stable isotope ratios and minor element abundances of five olivine particles from comet 81P/Wild 2 were examined by secondary ion mass spectrometry (SIMS). Wild 2 olivine particles exhibit only small variations in δ²⁵Mg values from –1.0 ^+0.4/_–0.5‰ to 0.6 ^+0.5/_–0.6‰ (2σ). This variation can be simply explained by mass-dependent fractionation from Mg isotopic compositions of the Earth and bulk meteorites, suggesting that Wild 2 olivine particles formed in the chondritic reservoir with respect to Mg isotope compositions. We also determined minor element abundances, and O and Mg isotope ratios of olivine grains in amoeboid olivine aggregates (AOAs) from Kaba (CV3.1) and DOM 08006 (CO3.01) carbonaceous chondrites. Our new SIMS minor element data reveal uniform, low FeO contents of ∼0.05 wt% among AOA olivines from DOM 08006, suggesting that AOAs formed at more reducing environments in the solar nebula than previously thought. Furthermore, the SIMS-derived FeO contents of the AOA olivines are consistently lower than those obtained by electron microprobe analyses (∼1 wt% FeO), indicating possible fluorescence from surrounding matrix materials and/or Fe,Ni-metals in AOAs during electron microprobe analyses. For Mg isotopes, AOA olivines show more negative mass-dependent fractionation (–3.8 ± 0.5‰ ≤ δ²⁵Mg ≤ –0.2 ± 0.3‰; 2σ) relative to Wild 2 olivines. Further, these Mg isotope variations are correlated with their host AOA textures. Large negative Mg isotope fractionations in olivine are often observed in pore-rich AOAs, while those in compact AOAs tend to have near-chondritic Mg isotopic compositions. These observations indicate that pore-rich AOAs preserved their gas–solid condensation histories, while compact AOAs experienced thermal processing in the solar nebula after their condensation and aggregation. Importantly, one ¹⁶O-rich Wild 2 LIME olivine particle (T77/F50) shows negative Mg isotope fractionation (δ²⁵Mg = –0.8 ± 0.4‰, δ²⁶Mg = –1.4 ± 0.9‰; 2σ) relative to bulk chondrites. Minor element abundances of T77/F50 are in excellent agreement with those of olivines from pore-rich AOAs in DOM 08006. The observed similarity in O and Mg isotopes, and minor element abundances suggest that T77/F50 formed in an environment similar to AOAs, probably near the proto-Sun, and then was transported to the Kuiper belt, where comet 81P/Wild 2 likely accreted.

Introduction

Particles collected from the Jupiter family comet 81P/Wild 2 (NASA Stardust mission) have provided unique opportunities to study materials from the outer Solar System, likely within the Kuiper Belt (Brownlee et al., 2006, Brownlee et al., 2012, Brownlee, 2014). Wild 2 particles consist of a variety of materials, including presolar grains (McKeegan et al., 2006, Messenger et al., 2009, Leitner et al., 2010, Leitner et al., 2012) and organic matter (Sandford et al., 2006, Sandford, 2008, Matrajt et al., 2008) as well as numerous high temperature materials such as Ca, Al-rich inclusions (CAIs; e.g., McKeegan et al., 2006, Zolensky et al., 2006, Chi et al., 2009, Simon et al., 2008, Joswiak et al., 2012, Joswiak et al., 2017) and ferromagnesian silicates (Nakamura et al., 2008, Bridges et al., 2012, Joswiak et al., 2012, Nakashima et al., 2012, Ogliore et al., 2012, Frank et al., 2014, Gainsforth et al., 2015, Defouilloy et al., 2017). Determining the origins of such particles can shed light on dynamic properties of the solar protoplanetary disk, including transport and processing of comet precursors. For example, the existence of high temperature minerals in Wild 2 suggests that materials originating from the hotter inner part of the solar protoplanetary disk were transported outward to the cooler Kuiper Belt region and eventually accreted to comets (e.g., McKeegan et al., 2006, Ciesla, 2007, Nakamura et al., 2008, Brownlee et al., 2012). In addition, Nakashima et al. (2012) suggested that at least a fraction of the ferromagnesian silicates in comet Wild 2 formed in the outer regions of the asteroid belt, based on the observed similarity of oxygen isotope systematics between Wild 2 particles and chondrules from CR chondrites. Furthermore, Bridges et al. (2012) hypothesize an outer Solar System origin for some Wild 2 silicate particles that have similar oxygen isotope and elemental characteristics when compared to Al-rich chondrules from carbonaceous chondrites. Therefore, at least some Wild 2 silicate particles may also hold information about the physicochemical conditions of the outer Solar System where comets accreted.

In the past decade, oxygen three-isotope ratios of forty-seven Wild 2 silicate particles larger than 2 µm have been acquired by secondary ion mass spectrometry (SIMS) (McKeegan et al., 2006, Nakamura et al., 2008, Nakamura-Messenger et al., 2011, Bridges et al., 2012, Nakashima et al., 2012, Ogliore et al., 2012, Ogliore et al., 2015, Joswiak et al., 2014, Gainsforth et al., 2015, Defouilloy et al., 2017, Chaumard et al., 2018), and they show a bimodal distribution on an oxygen three-isotope diagram (Fig. 1). The mass-independent fractionation of oxygen isotopes among Wild 2 particles are expressed as Δ¹⁷O (=δ¹⁷O − 0.52 × δ¹⁸O, where δ^17,18O = [(^17,18O/¹⁶O)_sample/(^17,18O/¹⁶O)_VSMOW − 1] × 1000; VSMOW = Vienna Standard Mean Ocean Water; Baertschi, 1976), and range from –24‰ to +3‰ for all data with uncertainties smaller than 5‰. Among them, the Δ¹⁷O values of thirty-six Wild 2 particles (≥2 µm) range from –7‰ to +3‰ (McKeegan et al., 2006, Nakamura et al., 2008, Nakashima et al., 2012, Ogliore et al., 2012, Ogliore et al., 2015, Joswiak et al., 2014, Gainsforth et al., 2015, Defouilloy et al., 2017, Chaumard et al., 2018), similar to those of chondrules found in chondrites (Tenner et al., 2018a). In contrast, seven Wild 2 particles show more ¹⁶O-rich oxygen isotopic characteristics (Δ¹⁷O = –24‰ to –20‰). These seven particles are nearly pure forsterite or enstatite, some of which are low-iron, manganese-enriched (LIME) olivines and pyroxenes (Nakashima et al., 2012, Defouilloy et al., 2017, Chaumard et al., 2018). LIME olivines and pyroxenes (hereafter LIME silicates) have MnO/FeO (wt%) ratios much higher than 0.1 and low FeO contents (typically ≤1 wt%). As Mn is not stable as metal at solar nebula conditions, it likely condensed at ∼1100 K, as Mn₂SiO₄ in solid solution with forsterite, and as MnSiO₃ with enstatite (Larimer, 1967, Wai and Wasson, 1977, Lodders, 2003). In contrast, Fe co-condenses as metal with forsterite (if the oxidation state is below the Fe-FeO buffer), meaning that high MnO/FeO ratios observed in LIME silicates suggest a condensation origin from the solar nebula (Klöck et al., 1989, Weisberg et al., 1993, Weisberg et al., 2004, Ebel et al., 2012). LIME silicates have been found in chondritic interplanetary dust particles (IDPs) (Klöck et al., 1989) that are believed to have a cometary origin (e.g., Bradley, 2003 and references therein), implying that LIME silicates may have been abundant nebular condensates and they were present in comet accretion regions. Thus, Wild 2 LIME silicates contain important clues for understanding the formation and transportation of solids in the early Solar System.

LIME silicates are also observed in amoeboid olivine aggregates (AOAs; e.g., Weisberg et al., 2004, Komatsu et al., 2015) that are the most common type of refractory inclusions in carbonaceous chondrites (e.g., Krot et al., 2004a and references therein; Sugiura et al., 2009, Ruzicka et al., 2012, Krot et al., 2014). Based on their irregularly-shaped, porous, and fine-grained textures, AOAs are thought to be aggregates of solids condensed from the solar nebula and appear to have avoided significant melting after aggregation (e.g., Grossman and Steele, 1976, Komatsu et al., 2001, Komatsu et al., 2009, Krot et al., 2004a and references therein; Weisberg et al., 2004, Sugiura et al., 2009, Han and Brearley, 2015). In addition, Mg stable isotope ratios of AOAs are indicative of a condensation origin. δ²⁵Mg values of bulk AOAs (where δ²⁵Mg = [(²⁵Mg/²⁴Mg)_sample/(²⁵Mg/²⁴Mg)_DSM-3 − 1] × 1000; per-mil deviation from Mg reference material DSM-3; Galy et al., 2003) range from –2.47‰ to –0.03‰ (Larsen et al., 2011, Olsen et al., 2011), some of which are negatively fractionated from those of bulk chondrites (δ²⁵Mg = –0.15 ± 0.04‰; Teng et al., 2010). In situ SIMS Mg isotope analyses of AOA olivines also show similar variations in δ²⁵Mg values, as low as ∼–2.8‰ (MacPherson et al., 2012, Ushikubo et al., 2017, Marrocchi et al., 2019a). The light isotope enrichments observed in AOAs may be the result of disequilibrium condensation from solar nebula gas due to kinetic isotope fractionation (Richter, 2004). Note that AOAs from the least metamorphosed chondrites are ¹⁶O-rich (Δ¹⁷O < –20‰; Aléon et al., 2002, Fagan et al., 2004, Krot et al., 2004a, Itoh et al., 2007, Yurimoto et al., 2008 and references therein; Ushikubo et al., 2017, Komatsu et al., 2018). The similarities in oxygen isotope ratios and mineral chemistry (i.e., ¹⁶O-rich oxygen isotope signatures and the occurrence of LIME silicates) suggest a genetic link between Wild 2 LIME silicates and AOAs (Nakashima et al., 2012, Defouilloy et al., 2017). However, unlike AOAs, the petrological context of ¹⁶O-rich Wild 2 particles is not well understood because most particles have been found as single mineral grains. If these particles formed by disequilibrium condensation, their Mg isotope ratios could be enriched in lighter isotopes (Richter, 2004), which could be used to trace condensation processes. Due to small particle sizes (<10 µm), Mg isotope data from Wild 2 particles at sub-‰ precision are currently lacking, but would nonetheless be valuable for testing condensation hypotheses for ¹⁶O-rich Wild 2 particles.

Here, we report Mg three-isotope ratios and minor element concentrations of eight Wild 2 olivine particles (four that are ¹⁶O-rich and four that are ¹⁶O-depleted) in order to better understand their formation processes. To obtain high spatial resolution (≤2 µm) SIMS Mg isotope analyses, we upgraded the WiscSIMS Cameca IMS 1280 with a radio frequency (RF) plasma oxygen ion source (Hertwig et al., 2019) and applied an instrumental bias correction method that combines forsterite endmember values (=Mg/[Mg + Fe] molar %) and secondary ion Mg⁺/Si⁺ ratios (Fukuda et al., 2020). We also conducted SIMS O and Mg isotope analyses, and SIMS minor element analyses of olivine grains in AOAs from Kaba (CV3.1; Bonal et al., 2006) and the least metamorphosed CO chondrite DOM 08006 (classified as CO3.00 from Davidson et al., 2019, while we adopt CO3.01 from Fe,Ni-metal analyses of Tenner et al., 2018b), in order to compare their formation histories with those of Wild 2 particles.