Origins of Al-rich chondrules: Clues from a compound Al-rich chondrule in the Dar al Gani 978 carbonaceous chondrite

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.