Birth of the presolar nebula: The sequence of condensation revealed in the Allende meteorite

Abstract

This work applies the well-known supernova-trigger hypothesis for solar system formation to explain in detail many properties of the Allende meteorite. The Allende carbonaceous chondrite meteorite is an assemblage of millimetre- to centimetre-sized Ca-Al-rich inclusions (CAI's), fine-grained alkali-rich spinel aggregates, amoeboid olivine aggregates, olivine chondrules and sulfide chondrules set in an extremely fine-grained black matrix. Detailed isotopic, chemical and textural properties show that these components formed in the above order as independent cosmic grains. Some CAI's containmicron-sized metal nuggets in which the normally incompatible refractory (Mo, Re, W) and platinum group (Pt, Os, Ir, Ru) metals are alloyed together in approximately ‘cosmic’ proportions, suggesting that these nuggets also condensed as cosmic grains.

From the consistent pattern of enclosure of earlier components on the above list within later ones, it appears that in the environment where these materials formed, condensation moved inexorably in the direction of increasing olivine and decreasing refractory element and¹⁶O content (from ∼4% excess¹⁶O to ∼‘normal’ terrestrial oxygen isotopic composition). Condensation sequences are all short and incomplete, from which it is concluded that condensing materials were soon separated from the condensing environment and isolated until all were brought together in a final ‘snowstorm’ of fine-grained, olivine crystals constituting the meteorite matrix.

These major properties can be accounted for in a model in which a supernova remnant (SNR) in the ‘snowplow’ phase, whose oxygen was initially pure¹⁶O, pushes into a dark interstellar cloud. In the model, condensation of CAI's begins in the SNR shell when it has been diluted with ∼2500 times its mass of matter from the cloud, which also in part explains the rarity of observed isotopic anomalies in CAI's. The retardation of the SNR by the cloud propels condensed grains ahead toward the cloud under their own momentum. Continuing dilution by the cloud and continuing removal of the most refractory elements in grains can explain the evolving patterns of fractionation and depletion of refractory elements, including REE's, in successive condensates. Features such as rims on CAI's and concentric zonation of fine-grained aggregates can also be satisfied in the model. A presolar origin and a short (∼ 10 000 years) formation time for inclusions in carbonaceous chondrites are major implications of the model.