The origin of dark inclusions in Allende: New evidence from lithium isotopes

Abstract

Aqueous and thermal processing of primordial materials occurred prior to and during planet formation in the early solar system. A record of how solid materials were altered at this time is present in the carbonaceous chondrites, which are naturally delivered fragments of primitive asteroids. It has been proposed that some materials, such as the clasts termed "dark inclusions" found in type III chondrites, suggest a sequence of aqueous and thermal events. Lithium isotopes (6Li and 7Li) can reveal the role of liquid water in dark inclusion history. During aqueous alteration, 7Li passes preferentially into solution leaving 6Li behind in the solid phase and, consequently, any relatively extended periods of interaction with 7Li-rich fluids would have left the dark inclusions enriched in the heavier isotope when compared to the meteorite as a whole. Our analyses of lithium isotopes in Allende and its dark inclusions reveal marked isotopic homogeneity and no evidence of greater levels of aqueous alteration in dark inclusion history.