Low temperature aquaeous mobility of rare earth elements; evidence from diagenetic francolite

Diagenetic francolite (carbonate fluor-apatite) occurs as overgrowths on detrital apatite grains in sandstones from the Lower Jurassic Statfjord Formation of the North Sea. The francolite overgrowths are considerably more enriched in the rare-earth elements (up to 1 wt % Ce₂O₃), Sr and F than the detrital cores. These elements were carried in aqueous solution, probably in the form of complex ligands involving organically sourced carbon and halogens. It is possible that reported aberrant neodymium isotope model ages within the Statfjord Formation are the result of mobilization and relative fractionation of Sm and Nd during diagenesis, rather than a result of changes in provenance.