Episodic formation of refractory inclusions in the Solar System and their presolar heritageOPEN ACCESS 

K.K. Larsen, D. Wielandt, M. Schiller, A.N. Krot, M. Bizzarro

Earth and Planetary Science Letters
Volume 535, 1 April 2020, 116088

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• Mg isotopes show multiple populations of refractory inclusions.
• Formation of the Solar System’s first solids was punctuated and recurrent.
• Episodic formation of refractory solids may be generic to the star-forming process.
• Refractory inclusions formed through thermal processing of presolar material.
• Refractory solids preserve isotope signatures from the proto-Solar molecular cloud.”

“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 26Al to 26Mg decay system, we show that bulk refractory inclusions in CV (Vigarano-type) and CR (Renazzo-type) carbonaceous chondrites captured at least two distinct 26Al-rich (26Al/27Al ratios of ∼5 × 10−5) populations of refractory inclusions characterized by different initial 26Mg/24Mg isotope compositions (μ26Mg0). Another 26Al-poor CAI records an even larger μ26Mg0 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 26Al 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 26Al-rich refractory inclusions incorporated 26Al-rich dust which formed <5 Myr prior to our Sun, whereas 26Al-poor inclusions (such as FUN- and PLAC-type CAIs) incorporated >10 Myr old dust.”

Fig. 2. Disk sketch diagram showing the transport routes and recycling of inner disk refractory material to the accretion regions of the giant planets. 26Al-rich population A inclusions, which represent refractory inclusions from the bulk CV isochron, end up in the accretion region of the CV chondrite parent body close to the orbit of Jupiter, whereas population B inclusions are distributed between the accretion regions of both CV and CR chondrite parent bodies at larger orbital distance. Early-formed 26Al-poor refractory inclusions are transported to larger heliocentric distances to accrete with CH carbonaceous chondrites.