CHARACTERIZATION OF RARE EARTH ELEMENTS IN FOSSILIZED ENAMEL BIOAPATITE FOR ATOM PROBE TOMOGRAPHY

We investigated the bulk distributions of rare earth elements (REE) in fossilized tooth enamel using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) to 1) determine concentrations and distributions of REE in tooth enamel and 2) screen enamel samples for further analysis using atom probe tomography (APT). Tooth enamel consists of oriented hydroxylapatite [Ca₅(PO₄)₃(OH)] crystallites – termed bioapatite – that are nanometer-scale in cross-section and up to ~1 µm in length. During fossilization, REE are somehow incorporated into enamel in concentrations ranging up to 100’s of ppm. However, the REE uptake mechanisms are poorly understood and could include volume diffusion, adsorption onto biogenic crystallite surfaces, growth of secondary apatite crystallites, authigenic growth over original bioapatite crystallites (seeding), or a combination of these mechanisms. The atomic scale resolution of APT will allow us to differentiate between these mechanisms by direct imaging, but REE concentrations must be high enough to allow for their detection. We measured concentration profiles of REE in fossilized enamel of various geologic ages using LA-ICP-MS and identified fossils with enrichments of middle REE (MREE: Sm-Dy) and heavy REE (HREE: Ho-Lu). REE in fossil enamel are zoned, with highest concentrations on tooth surfaces, decreasing inwards. Although more variable concentration profiles are sometimes observed, most decreases are either quasi-exponential or take the form of an error function. Such profiles have been reported in numerous other studies and are representative of typical fossilization. Enrichments of MREE vs. HREE could be indicative of different uptake mechanisms in enamel such as seeding vs. adsorption onto crystallite surfaces, respectively. Diverse fossilized enamel specimens were identified with high enough REE concentrations (>50 ppm) for detection by APT. Measurements of REE distributions from these samples using APT are forthcoming.