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

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