Crystallinity and diagenesis of sedimentary apatites

Abstract

The crystallinity of sedimentary apatites was determined by Fourier transform infrared spectroscopy (FT-IR) using the splitting of a triply degenerate antisymmetric bending vibration of orthophosphate. The crystallinity indices of Recent marine apatites are low (3.0–3.6) while those of onland ancient apatites are high (4.5–7.8), indicating post-depositional recrystallization. The infrared spectra reveal that recrystallization is associated with a decrease in carbonate content substituting for PO 4 3− and an increase in fluoride order within the apatite structure. The relationship between the crystallinity index and PO 4 3− δ 18O suggests alteration of the primary isotopic composition by exchange reactions between PO 4 3− oxygens and surrounding waters. The Monterey samples have a large range of crystallinity index that reflects a set of complex and highly variable diagenetic conditions. This demonstrates the use of FT-IR criteria for differentiating between pristine and altered apatites and, as a consequence, for relating geochemical markers to formation or diagenetic environments. It is suggested that only those samples that have low crystallinity indices (C. I. < 3.8) should be considered as pristine apatite. Spectra of fish remains indicate that differences in rare earth element (REE) patterns correspond to variations in crystallinity, carbonate content and F order in the apatite lattice. The fact that crystallinity is not correlated with geologic age suggests that environmental factors, such as accumulation rate and pore water chemistry, govern the recrystallization process. In general, Sr content decreases and δ 18Op exhibits high variability with increasing crystallinity.