Abstract:
Fossil biogenic apatites display rare-earth element (REE) patterns which have been proposed to reflect environmental and biological controls on past seawater composition. These patterns can be separated in two groups: (1) patterns that are similar to those of open ocean and epicontinental waters and (2) patterns that exhibit a strong enrichment in intermediate REE (bell-shaped patterns). Because REE trapping in biogenic apatites occurs mostly post-mortem, it is essential to understand the crystal-chemical (inorganic) factors that can influence the fractionation of REE between apatite and water. A model is proposed by which partition coefficients of REE between apatite and water for a substitution mechanism can be extrapolated from mineral/melt partition data to seawater or sedimentary basin conditions. These are compared with available experimental partition coefficients for REE adsorption mechanism and used to discuss REE patterns in fossil biogenic apatites. Calculations of REE patterns for apatites at equilibrium with water show that the bell-shaped patterns in fossil apatites can be explained by fractionation with seawater or continental fluids at low temperature, under crystal-chemical control involving a substitution mechanism and in the context of `extensive' or `late' diagenesis. In that case, the equilibrium composition of the fluid cannot be unequivocally determined from observed REE concentrations in fossil apatites. This type of sample is inappropriate to discuss the past environmental or biological controls on REE concentrations in the ocean. Other sample compositions suggest little to moderate alteration of the original oceanic to freshwater pattern through the adsorption mechanism in the context of `weak' or `early' diagenesis, and may be used with caution for that purpose.