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.