Larsen K K, Wielandta D, Schillera M, Krot A N, Bizzarro M
Centre for Star and Planet Formation, Natural History Museum of Denmark, University of Copenhagen, Copenhagen DK-1350, Denmark.
Hawai'i Institute of Geophysics and Planetology, University of Hawai'i at Manoa, HI 96822, USA.
Earth Planet Sci Lett. 2020 Jan 28;535:116088. doi: 10.1016/j.epsl.2020.116088. eCollection 2020 Apr 1.
Refractory inclusions [Ca-Al-rich Inclusions (CAIs) and Amoeboid Olivine Aggregates (AOAs)] in primitive meteorites are the oldest Solar System solids. They formed in the hot inner protoplanetary disk and, as such, provide insights into the earliest disk dynamics and physicochemical processing of the dust and gas that accreted to form the Sun and its planetary system. Using the short-lived Al to Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct Al-rich (Al/Al ratios of ~5 × 10) populations of refractory inclusions characterized by different initial Mg/Mg isotope compositions (μMg*). Another Al-poor CAI records an even larger μMg* deficit. This suggests that formation of refractory inclusions was punctuated and recurrent, possibly associated with episodic outbursts from the accreting proto-Sun lasting as short as <8000 yr. Our results support a model in which refractory inclusions formed close to the hot proto-Sun and were subsequently redistributed to the outer disk, beyond the orbit of Jupiter, plausibly via stellar outflows with progressively decreasing transport efficiency. We show that the magnesium isotope signatures in refractory inclusions mirrors the presolar grain record, demonstrating a mutual exclusivity between Al enrichments and large nucleosynthetic Mg isotope effects. This suggests that refractory inclusions formed by incomplete thermal processing of presolar dust, thereby inheriting a diluted signature of their isotope systematics. As such, they record snapshots in the progressive sublimation of isotopically anomalous presolar carriers through selective thermal processing of young dust components from the proto-Solar molecular cloud. We infer that Al-rich refractory inclusions incorporated Al-rich dust which formed <5 Myr prior to our Sun, whereas Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.
原始陨石中的难熔包体[富钙铝包体（CAIs）和似阿米巴状橄榄石集合体（AOAs）]是太阳系中最古老的固体物质。它们形成于炽热的内原行星盘，因此为了解最早的盘动力学以及吸积形成太阳及其行星系统的尘埃和气体的物理化学过程提供了线索。利用短寿命的铝镁衰变系统，我们发现CV（维加诺型）和CR（雷纳佐型）碳质球粒陨石中的整体难熔包体捕获了至少两个不同的富铝（铝/铝比约为5×10）难熔包体群体，其特征是具有不同的初始镁/镁同位素组成（μMg*）。另一个贫铝CAI记录了更大的μMg*亏损。这表明难熔包体的形成是间断且反复的，可能与吸积原太阳的 episodic 爆发有关，爆发持续时间短至<8000年。我们的结果支持一个模型，即难熔包体在靠近炽热原太阳的地方形成，随后可能通过恒星外流以逐渐降低的传输效率重新分布到外盘，即木星轨道之外。我们表明难熔包体中的镁同位素特征反映了前太阳颗粒记录，证明了铝富集和大的核合成镁同位素效应之间的相互排斥性。这表明难熔包体是由前太阳尘埃的不完全热处理形成的，从而继承了其同位素系统的稀释特征。因此，它们记录了通过对原太阳分子云年轻尘埃成分的选择性热处理，同位素异常的前太阳载体逐渐升华的快照。我们推断富铝难熔包体包含了在我们太阳形成前<5百万年形成的富铝尘埃，而贫铝包体（如FUN型和PLAC型CAIs）包含了>10百万年的古老尘埃。
