Rare earth elements in chondritic phosphates—implications for 244Pu chronology

Abstract

In stony meteorites of the chondrite class, phosphates (comprising about 0.6% of the whole) act as host for rare-earth elements (REE) and for Pu which was present at the time of formation. For the purpose of Pu–Xe dating it has been suggested that, in the absence of a stable Pu isotope, measured Xe abundances may be related to REE concentrations, on the grounds of presumed similarity in the geochemical behaviour of Pu and REE1. There are two phosphate phases in chondrites, apatite and merrillite (or whitlockite), which are difficult to separate from each other for analysis. Recent results based on grain-size discrimination2 indicate similar REE concentrations in both phosphates, whereas Pu favours merrillite3. Direct in situ analyses of individual grains by the ion-probe technique, however, show REE concentrations to be about five times higher in merrillite. The use of REE in the role of Pu analogue thus appears a better prospect than suggested by the previous indirect evidence.