A long duration of the 16O-rich reservoir in the solar nebula, as recorded in fine-grained refractory inclusions from the least metamorphosed carbonaceous chondrites. (15th March 2017)

• Record Type: Journal Article
• Authors: Ushikubo, Takayuki; Tenner, Travis J.; Hiyagon, Hajime; Kita, Noriko T.
• Is Part Of: Geochimica et cosmochimica acta. Volume 201(2017)
• Page Start: 103
• Page End: 122
• Publication Date: 2017-03-15
• Subjects: Oxygen isotope -- Al–Mg systematics -- Refractory inclusion -- Solar nebula -- SIMS -- Acfer 094; Geochemistry -- Periodicals; Meteorites -- Periodicals; Géochimie -- Périodiques; Météorites -- Périodiques; Geochemie; Astrochemie; Electronic journals; 551.905
• DOI: 10.1016/j.gca.2016.08.032
• Languages: English
• ISSNs: 0016-7037

Abstract

Abstract: Oxygen isotope ratios and corresponding 26 Al– 26 Mg isotope systematics of refractory inclusions from the least metamorphosed carbonaceous chondrites, Acfer 094 (C-ungrouped 3.00) and Yamato 81020 (CO3.05), were measured with an ion microprobe. Most of the samples are fine-grained refractory inclusions which are considered as condensates from high temperature solar nebular gas. The refractory inclusions consistently exhibit 16 O-enriched signatures among their interior phases (spinel, melilite, and high-Ca pyroxene), as well as phases within their rim structures (spinel, high-Ca pyroxene, and adjacent anorthite). This observation indicates that aggregated refractory condensates and the formation of rim structures occurred in the same 16 O-rich environment. Evidence for mass-dependent isotopic fractionation in oxygen and magnesium, which would indicate a later flash heating process, was not observed in rims. All oxygen isotope data from fine-grained CAIs are distributed between the Carbonaceous Chondrite Anhydrous Mineral (CCAM) line and the Primitive Chondrule Mineral (PCM) regression line based on oxygen isotope data from the Acfer 094 chondrules. The inferred initial 26 Al/ 27 Al ratios, ( 26 Al/ 27 Al)0, of spinel-melilite-rich CAIs are (4.08 ± 0.75) × 10 −5 to (5.05 ± 0.18) × 10 −5 (errors are 2 σ ), which are slightly lower than the canonical value of 5.25 × 10 −5 . As there is no petrologic evidence for re-melting after condensation, the lower ( 26 Al/ 27 Al)0 values of these CAIs indicate either they formed up to ∼0.3 Ma after canonical CAIs or they formed before 26 Al was homogeneously distributed in the solar nebula. A pyroxene-anorthite-rich CAI, G92, has an 16 O-rich signature like other CAIs but also has an order-of-magnitude less 26 Mg-excess in anorthite, corresponding to a ( 26 Al/ 27 Al)0 of (5.21 ± 0.54) × 10 −6 . As there is no evidence for a later Mg isotopic disturbance, G92 anorthite is interpreted to have formed by interaction with 16 O-rich nebular gas at 2–3 Ma after CAI formation. With the observation that 16 O-rich refractory inclusions, relatively 16 O-poor chondrules, and extremely 16 O-poor cosmic symplectites within Acfer 094 all plot on the PCM line, it suggests that 16 O-rich nebular gas and extremely 16 O-poor primordial volatiles represent mass-independent fractionated endmembers in the early solar system and that the PCM line represents a mixing line of these two endmembers.