Can crystallinity be used to determine the degree of chemical alteration of biogenic apatites?

A new Raman spectroscopic technique has been refined to more efficiently determine the crystallinity indeces of biogenic apatites. We investigate the possible relationships between the structure (crystallinity) and geochemistry (rare earth element (REE), δ18O) of biogenic apatites. A selection of phosphatic remains dated from present to about 510 Ma (Late Cambrian), for most of which either the oxygen isotope compositions or REE patterns are available, has been characterized for crystallinity using Raman spectroscopy. We define a new crystallinity index (CIRaman) from the ratio of the full width at half maximum (FWHM) of the intense peak of the PO4 symmetric stretching mode in the sample and a reference magmatic apatite. In order to compare our crystallinity index with CI used in previously published studies, we also analyzed part of our sample set with FT-IR spectroscopy and X-ray diffractometry. A detailed study of natural samples demonstrates that crystallinity index is a poor criterion for determining if a sample has been altered since deposition. This result is based on three major observations: (1) independently of the CIRaman, the original geochemical signatures of the biogenic apatites can be preserved over a long period, (2) strong geochemical perturbations (lowering of δ18O values and of La/Sm ratios) may occur without detectable recrystallization, and (3) alteration by heating, marked by the transformation of organic matter into graphite, produces REE fractionations and limited oxygen isotope exchange with crustal aqueous fluids.