Angrite Meteorites as Fragments of First-Generation Planetesimals in the Early Solar System

A petrographic thin section micrograph of an angrite primarily composed of augite, olivine, and plagioclase.
Credit: Meteorite Times Magazine.

Angrites are olivine-bearing, basaltic meteorites that originate from a volatile-depleted planetesimal. Previous work on angrites suggests that their parent body accreted within the first 0.5 to 1 Myr of solar system evolution and differentiated via extensive melting and crystallization of igneous rocks within ~2 Myr after the first solids formed (~4.57 billion years ago). Petrologic studies indicate that angrites experienced little physical or chemical alteration after crystallizing. Consequently, angrites are the oldest known igneous rocks, preserving records of early solar system chronology and planetary formation, melting, and differentiation.

François Tissot from the Massachusetts Institute of Technology and his team carried out a series of crystallization experiments under varying conditions and compared their results with previous petrologic data for the 37 known angrite specimens to better understand their crystallization history. They observed two distinct compositional groups among a subset of angrites called quenched (or volcanic) angrites, representing melt compositions rather than mixtures of melts plus accumulated crystals (cumulate angrites). Their results revealed that one group, which was highly reduced, was more primitive and could have given rise to the other, which was more oxidized, by fractional crystallization. This implies the changes in oxidizing conditions during core formation. Tissot and co-authors argued that the bulk composition of the more primitive group of angrites was a direct result of nebular (rather than parent body) processes at high temperatures, implying that the primordial parent body was both volatile-depleted and highly reduced. These constraints place a lower limit on the radius of the parent body, ~600 – 770 kilometers (three times larger than Vesta).

The authors proposed that angrites represent fragments of first-generation planetesimals (ancestors of rocky planets) enriched in high-temperature nebular materials that formed within the innermost solar system (< 1 AU). This study opens a window into our understanding of processing during early solar system evolution and the birth of terrestrial planets. READ MORE