Heating events in the nascent solar system recorded by rare earth element isotopic fractionation in refractory inclusions

Abstract

It is widely accepted that the building blocks of the planets and other inner solar system objects have been vaporized and homogenized at the birth of the solar system. According to this view, solids condensed as the solar nebula cooled, with composition evolving from more refractory phases rich in Ca, Al, and Ti to more volatile phases rich in K and Na. This view has been supported by the observation of the depletions in both the least (Eu, Yb) and most refractory (Gd-Er, Lu) rare earth elements (REEs) in group II calcium-aluminum-rich inclusions (CAIs) [1]. The formation of group II CAIs were thought to represent a snapshot of the condensation sequence, the partial condensate in equilibrium with the solar nebula as it cooled [2,3,4]. However, this view of group II CAI formation was never tested other than through thermodynamic calculations that suffer from large uncertainties due to the paucity of thermodynamic data. To test this paradigm of equilibrium condensation of solar gas, we measured the isotopic compositions of Ce, Nd, Sm, Eu, Gd, Dy, Er, and Yb in 8 CAIs (7 with group II REE patterns). Contrary to expectations for equilibrium condensation, we found significant negative isotopic compositions for the most refractory REEs and more subdued isotopic variations for the least refractory REEs, indicative of disequilibrium formation pathways.