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Fluffy Type A Ca–Al-rich inclusions (CAIs) containing reversely zoned melilite crystals are suggested to be aggregates of direct condensates from solar nebular gas. We conducted an investigation of 26 Al− 26 Mg systematics of a fluffy Type A CAI from Vigarano, named V2-01, with known oxygen isotopic distributions of reversely zoned melilite crystals; we also conducted oxygen isotope measurements of coexisting minerals. Two of six reversely zoned melilite crystals show continuous variations in magnesium isotopic composition, with δ 25 Mg decreasing along the inferred direction of crystal growth, which supports the idea that they originated through condensation. Petrography suggests that the constituent minerals of V2-01 formed in the following order: first spinel and fassaite enclosed by melilite, then reversely zoned melilite crystals, and spinel and diopside in the Wark–Lovering rim. The spinel enclosed by melilite has 16 O-rich compositions (Δ 17 O ∼ −24‰) and on an Al−Mg evolutionary diagram plots along model isochron with an initial value of ( 26 Al/ 27 Al) 0 = (5.6 ± 0.2) × 10 −5 . The fassaite enclosed by melilite crystals shows variable oxygen isotopic compositions (Δ 17 O ∼ −12‰ and −17‰) and plots on an isochron with ( 26 Al/ 27 Al) 0 = (5.6 ± 0.2) × 10 −5 . The oxygen isotopic compositions of reversely zoned melilite showed continuous variations in Δ 17 O along the inferred direction of crystal growth, suggesting that surrounding nebular gas, during the formation of the reversely zoned melilite, changed from 16 O-poor (Δ 17 O values larger than −10‰) to 16 O-rich (Δ 17 O ∼ −25‰). The six reversely zoned melilite crystals show indistinguishable initial 26 Al/ 27 Al values with an average ( 26 Al/ 27 Al) 0 of (4.7 ± 0.3) × 10 −5 , which is clearly distinguishable from the value of enclosed spinel and fassaite, indicating a younger formation age than the enclosed spinel and fassaite. The spinel and diopside from the Wark–Lovering rim show 16 O-rich compositions (Δ 17 O ∼ −23‰) with ( 26 Al/ 27 Al) 0 = (4.5 ± 0.4) × 10 −5 . The values of ( 26 Al/ 27 Al) 0 are consistent with the formation sequence inferred from petrography. The formation period for the V2-01 CAI is estimated to be 0.18 ± 0.07 Myr from the difference in initial 26 Al/ 27 Al values. These data suggest that the oxygen isotopic composition of solar nebular gas surrounding the CAI changed from 16 O-rich to 16 O-poor and back to 16 O-rich during the first ∼0.2 Myr of Solar System formation. [ABSTRACT FROM AUTHOR]

Abstract: Aluminum-rich chondrules are one of the most interesting components of primitive chondrites, because they have characteristics that are similar to both Ca, Al-rich inclusions (CAIs) and ferromagnesian chondrules. However, their precursor and formation history remain poorly constrained, especially with respect to their oxygen isotopic distributions. In this study, we report on the petrography, mineralogy, oxygen isotope ratios, and rare-earth-element compositions of a sapphirine-bearing Al-rich chondrule (SARC) in the ungrouped chondrite Dar al Gani (DaG) 978. The SARC has a complex core-mantle-rim texture; while both the core and the mantle are mainly composed of Al-rich enstatite and anorthite with minor amounts of mesostasis, these regions are distinguished by the presence of Fe-rich spinel and sapphirine in the core and their absence in the mantle. The rim of the SARC consists mainly of Fe-rich olivine, enstatite, and Fe–Ni metal. Spinel and some olivine grains in the SARC are 16O-rich, with Δ17O values down to −20‰ and −23‰, respectively. Enstatite, sapphirine, and most olivine grains have similar Δ17O values (∼ −7‰), which are lower than those of anorthite and the mesostasis (including augite therein) (Δ17O: ∼ −3‰). Mesostasis from both the core and mantle have Group II rare-earth-element (REE) patterns; however, the core mesostasis has higher REE concentrations than the mantle mesostasis. These observations provide a strong indication that the SARC formed by the melting and crystallization of a mixture of materials from Group II CAIs and ferromagnesian chondrules. Both spinel and olivine with 16O-rich features could be of relict origin. The 16O-poor isotopic compositions of most components in Al-rich chondrules can be explained by oxygen isotopic exchange between the melt and 16O-poor nebular gas (Δ17O: ∼ −7‰) during melting in chondrule-forming regions; whereas the anorthite and mesostasis could have experienced further oxygen isotopic exchange with a relatively 16O-poor reservoir (Δ17O: ∼ −3‰) on the parent body, likely during fluid-assisted thermal metamorphism. During the same thermal metamorphism event, spinel, olivine, some enstatite, and the mesostasis experienced Mg–Fe exchange to various extents. [Copyright &y& Elsevier]

Chondrules and calcium-aluminium-rich inclusions (CAIs) are preserved materials from the early history of the Solar System, where they resulted from thermal processing of pre-existing solids during various flash heating episodes which lasted for several million years[SUP1]. CAIs are believed to have formed about two million years before the chondrules[SUP2-5]. Here we report the discovery of a chondrule fragment embedded in a CAI. The chondrule's composition is poor in [SUP16]O, while the CAI has a [SUP16]O-poor melilite (Ca, Mg, Al-Silicate) core surrounded by a [SUP16]O-rich igneous mantle. These observations, when combined with the previously reported CAI-bearing chondrules[SUP6-9], strongly suggest that the formation of chondrules and CAIs overlapped in time and space, and that there were large fluctuations in the oxygen isotopic compositions in the solar nebula probably synchronizing astrophysical pulses. [ABSTRACT FROM AUTHOR]