Distribution and rare earth element abundances of apatite in Bennu samples determined by synchrotron X‐ray fluorescence spectroscopy

Abstract Apatite is often the most rare earth element (REE)‐enriched mineral in chondritic meteorites. REE enrichment and fractionation levels in apatite grains can give insight into the aqueous fluids in which they formed and the conditions prevailing within the host body. In this study, we applied synchrotron X‐ray fluorescence (SXRF) spectroscopy to investigate the distribution and REE enrichments of 34 apatite grains in five rock fragments from asteroid (101955) Bennu, collected and brought to Earth by NASA's OSIRIS‐REx mission. We observe a strongly heterogeneous apatite distribution with frequent occurrence of clusters containing three to nine apatite grains. SXRF results reveal a wide grain‐to‐grain range of REE enrichment levels, at 15–900 times those of bulk CI chondrites. Of the 34 measured apatite grains, four are Mn‐enriched (Mn > 1.26 wt%) and these show the lowest REE concentrations (15–65 times CI). Little light/heavy REE fractionation is observed in the Bennu apatite grains, in contrast to the heavy REE enrichment observed in apatite grains in samples from asteroid Ryugu. Our results suggest fluid circulation in Bennu's parent asteroid involved apatite precipitation in cavities and local variations in water‐rock interaction leading to local changes in , and fluid chemistry. Given the dynamically evolving fluid conditions and the enhanced peripheral luminescence and oscillatory zoning observed in some apatite grains, an absence of REE fractionation during apatite formation appears unlikely. We postulate that, as fluid chemistry evolved to higher alkalinity and salinity, sequential complexation resulted in early incorporation of light REEs into precipitating apatite grains, subsequently balanced by later adsorption of heavy REEs.