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Abstract The stable isotope ratio of oxygen (δ¹⁸O) stored in otoliths is a common and reliable tool for studying life history events including natal origin and migratory behavior of marine fish species. This approach is useful particularly in cases where traditional tagging is impossible, such as in early larval stages. However, small amounts of available otolith material have historically precluded analysis of δ¹⁸O in individual larval otoliths by traditional isotope ratio mass spectrometer (IRMS). The use of secondary ion mass spectrometer (SIMS) resolves this challenge, allowing users to obtain δ¹⁸O signatures in otoliths at the scale of tens of microns. This paper presents a method for the preparation of larval fish otoliths compatible with SIMS δ¹⁸O analysis through placement of larval otoliths (~ 20 to ~ 150 μm diameter) on a single epoxy mount and use of a Cesium ion beam,¹³³Cs⁺, of 12 μm in diameter with 1 μm penetration depth to excite source larval otolith material. In this pilot study, otoliths from Atlantic bluefin tuna (Thunnus thynnus) larvae were used to assess the efficacy of generating quality δ¹⁸O data using the newly developed preparation method for establishing spawning site baselines. The applied methods led to usable δ¹⁸O signatures in > 90% of the otoliths selected for analysis. Challenges and unanticipated considerations that became apparent during the use of this method include the inverse relationship between¹⁶OH⁻/¹⁶O⁻, an indicator of hydrogen content, and δ¹⁸O, resulting in a limitation of using otoliths from preflexion larvae, as well as a statistically significant difference in δ¹⁸O (−0.17‰) after storage in Type B immersion oil. 

Abstract Comet 81P/Wild 2 is a ∼4.5 km-sized primordial object that almost has not been modified by internal heating by²⁶Al decay. Its nucleus could have been formed by hierarchical agglomeration or gravitational collapse of pebble swarms concentrated by streaming instability. To shed light on the cometesimal formation mechanism from laboratory sample analysis, we reexamined the²⁶Al–²⁶Mg isotope systematics of the plagioclase-bearing fragment, Pyxie (from Wild 2 track 81), with significantly improved analytical precision. The revised upper limit of the initial (²⁶Al/²⁷Al)₀of Pyxie is ≤1.5 × 10⁻⁶, 2 times smaller than those estimated from other Wild 2 fragments. Assuming homogenous distribution of²⁶Al in the early solar system, the minimum crystallization age of Pyxie is estimated to be >3.6 Ma after calcium–aluminum-rich inclusions. Additional petrologic examination demonstrated that it is a chondrule fragment formed in disk environments enriched in moderately volatile elements comparable to the Si-rich rim of CR chondrules before accreting by comet Wild 2. The late accretion of the Wild 2 nucleus with most silicates likely from a common source are not favored by the hierarchical agglomeration model that considers early and continuous accretion. Instead, the results are more in line with comet formation by gentle gravitational collapse of pebbles when the²⁶Al abundance is extremely low (²⁶Al/²⁷Al ≤ 1.5 × 10⁻⁶) before gas dispersal.