Crystal-chemical controls on rare-earth element concentrations in fossil biogenic apatites and implications for paleoenvironmental reconstructions

Fossil biogenic apatites display rare-earth element (REE) patterns which ha ve been proposed to reflect environmental and biological controls on past s eawater composition. These patterns can be separated in two groups: (1) pat terns that are similar to those of open ocean and epicontinental waters and (2) patterns that exhibit a strong enrichment in intermediate REE (bell-sh aped patterns). Because REE trapping in biogenic apatites occurs mostly pos t-mortem, it is essential to understand the crystal-chemical (inorganic) fa ctors that can influence the fractionation of REE between apatite and water . A model is proposed by which partition coefficients of REE between apatit e and water for a substitution mechanism can be extrapolated from mineral/m elt partition data to seawater or sedimentary basin conditions. These are c ompared with available experimental partition coefficients for REE adsorpti on mechanism and used to discuss REE patterns in fossil biogenic apatites. Calculations of REE patterns for apatites at equilibrium with water show th at the bell-shaped patterns in fossil apatites can be explained by fraction ation 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 concentr ations 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 t he original oceanic to freshwater pattern through the adsorption mechanism in the context of 'weak' or 'early' diagenesis, and may be used with cautio n for that purpose. (C) 1999 Elsevier Science B.V. All rights reserved.