REE AND U ZONING IN FOSSIL TEETH

Laser ablation, ICP-MS analyses were collected across dentine-enamel interfaces from typical fossil teeth ranging in age from 25 ka to 33 Ma; these data reveal preserved diffusion profiles in REE and U that formed on time scales of 1-1000 years, irrespective of the age of the fossil. 1.5-3 mm long line traverses were collected with 5-12 µm spatial resolution, and included major and trace elements common in apatite (e.g., P, Ca, Fe, Sr, Ba, REE, Pb, U). Normalization of raw counts to constant Ca or P quantified trace element concentrations to ~±10% (relative). REE and U concentrations in dentine are essentially constant and as high as a few thousand and few hundred ppm respectively; in enamel they decrease away from the dentine-enamel interface in an error-function manner to tens or hundreds of ppb within a few tens of microns. These trends are not analytical artifacts, and instead are preserved diffusion profiles, with Dt values ranging from 2x10^-7 to 1x10^-5 cm².

Effective diffusion coefficients are the product of the tracer diffusion rate (c. 1x10^-8 cm²/sec at typical soil temperatures) and the fluid-apatite distribution coefficient (c. 2x10^-4 for U and 1x10^-6 for REE), i.e., D_eff(U) ~2x10^-14 cm²/sec and D_eff(REE) ~ 1x10^-16 cm²/sec. These effective coefficients imply minimum times for formation of the profiles of ~1 to a few tens of years for U, and a few hundred to ~1000 years for REE. Differences between U and REE results can be reconciled by uncertainties in partition coefficients. The inferred timescales are inconsistent with (a) U-series dating of archeological bone, which implies timescales of  several tens of kyr; such slow rates are precluded by the youth of some fossils analyzed, and (b) unusual preservation of fossilized soft tissues, which appears to require only days or weeks; such fast rates are precluded in enamel by sluggish diffusion coefficients. High porosity in dentine and bone could reduce timescales for fossilization of these tissues to less than 1 year. Different materials must undergo trace element uptake at different rates, and these data suggest that typical fossil teeth integrate at minimum many years to perhaps 1000 years of diagenetic chemical information.