DISCOVERY OF A NEW FUN CAI FROM A CV CARBONACEOUS CHONDRITE: EVIDENCE FOR MULTISTAGE THERMAL PROCESSING IN THE PROTOPLANETARY DISK

摘要

We report the mineralogy, petrography, as well as oxygen and magnesium isotope data of a newly identified FUN inclusion from the CV carbonaceous chondrite NWA 779. Variability in the texture of the mineral phases coupled with oxygen isotope data provides evidence for multistage evolution of this inclusion under distinct thermal regimes: slow crystallization of ~(16)O-rich melt accompanied by evaporation, and subsequent remelting in in ~(16)O-poor reservoir during transient heating events, possibly associated with the formation of CV chondrules. The inferred oxygen isotope composition of the precursor material of this inclusion (δ~(17.18)O = -48.4_(-3.0)~(+2.5)‰) is consistent with that observed for CAIs and amoeboid olivine aggregates from least metamorphosed carbonaceous chondrites, suggesting that both FUN and normal CAIs formed in an ~(16)O-rich reservoir with oxygen isotope composition similar to that inferred for the Sun. However, in contrast to normal CAIs, most FUN inclusions show no evidence for live ~(26)A1 at the time of their formation. Based on these observations, we propose that the protosolar molecular cloud was polluted with stellar-derived ~(26)Al prior to its collapse. Thus, FUN CAIs formed at a time when dust inherited from the molecular cloud-including the carrier of ~(26)Al-was still poorly homogenized in the protoplanetary disk.