Rare-earth geochemistry of phosphorites

The abundance of rare-earth elements (REE's) in francolite increases with increasing age as a result of post-depositional enrichment. Abundances are higher in pelletal samples than in non-pelletal ones because the higher surface/mass ratio of pelletal francolite promotes enrichment during diagenesis. Onshore francolite has higher REE abundances than that found offshore. Distribution patterns in francolite may be similar to those reported for open-ocean water or shale, depending upon the dominant source of the REE's: different patterns result from weathering and deep-burial diagenetic modification of the original patterns and abundances. Negative Ce anomalies occur in francolites containing structural sulphur that is isotopically indistinguishable from seawater of the same age; zero or positive anomalies occur in francolites which contain sulphur that is isotopically lighter than seawater.