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Paper: Thermal Processing and Radial Mixing of Dust: Evidence from Comets and Primitive Chondrites
Volume: 341, Chondrites and the Protoplanetary Disk
Page: 774
Authors: Wooden, D.H.; Harker, D.E.; Brearley, A.J.
Abstract: Cometary nuclei provide evidence for the mixing of high and low temperature materials in the solar nebula. Comets contain abundant amorphous carbon, amorphous Fe-bearing (Fe-Mg) silicates, organic materials (CHON), and volatile ices and gases that have never experienced temperatures above ~1000 K, ~1000 K, ~300 K, and 50—150 K, respectively. Two Oort cloud comets, Hale-Bopp and C/2001 Q4 (NEAT), contain a high mass fraction (~0.7) of Mg-rich silicate crystals. The absence of crystalline silicates in the interstellar medium infers that these cometary silicates are grains of interstellar materials that were processed at high temperature in the solar nebula. Crystals are either condensates (~1450 K) or amorphous silicate grains annealed into crystals (≥1000 K).We review the current scenarios for condensation and for annealing of amorphous silicates into crystals.

Type 3.0 chondrite matrices contain Mg-rich crystalline silicates in varying proportions to amorphous Fe-Mg silicates with highly variable Fe/Mg ratios. It is possible that the Mg-rich crystals in chondrites come from the same inner solar system source as crystals in comets. Mg-rich amorphous silicates are absent from both chondrite matrices and cometary interplanetary dust particles. Given the lack of a reservoir of Mg-rich amorphous silicates to anneal into Mg-rich crystals, we conclude that the Mg-rich crystals in comets and chondrites probably formed as condensates. Condensation of Mg-rich crystals occurs in the hot, inner regions of the young (≤1—3 × 105 yr) protoplanetary disk. Condensation of Mg-rich crystals may have also occured under highly reducing conditions in the chondrite-forming zone. Examining comets and primitive chondrites together supports the concept of a dynamic nebula where radial mixing of high and low temperature materials was prevalent.

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