22-8 New Constraints on Trace Element Diffusion Rates in Tooth Enamel Bioapatite

Session: Working Up an Apatite: Teeth as Paleo -Ecological and -Climatological Archives

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Abstract:

Diffusion experiments were performed at 23°C and at 90°C on human and mammoth tooth enamel to determine diffusivities of select rare earth elements (REE) (Sc, Y, La, Pr, Gd, Yb) and other trace elements (Ba, Sr, Pb, and U). Enamel consists of nanometer-scale crystals up to ~1 µm in length of carbonated hydroxylapatite [Ca₅(PO₄, CO₃)(OH, CO₃)], termed bioapatite. During fossilization, REE and many other trace elements partition strongly into bioapatite, forming either simple profiles consistent with diffusion-adsorption or diffusion-recrystallization mechanisms, or tailed distributions consistent with double-medium diffusion with competing “slow” and “fast” diffusion pathways. These profiles have been used to determine durations of fossilization, yet these interpretations rely on knowledge of the diffusivities of different trace elements, which are not well constrained. We performed diffusion experiments using doped pH-neutral solutions in a commercial sous vide for one week on mammoth enamel at 90°C and for several months at 23°C on human and mammoth enamel. Concentrations were measured at Arizona State University using a Cameca IMS 6f ion probe operating in depth profile mode. All trace elements except Ba and Sr show “in-diffusion” (concentrations highest on the surface and decreasing inward); Ba and Sr show “out-diffusion” (concentrations lowest on the surface and increasing inward). All trace element profiles show tailed profiles at depth, which could be consistent with double-medium-type diffusion. Interpretations of “slow” diffusion are complex because they represent a combination of intra-crystalline diffusion and surface ion mixing during surface sputtering. However, “fast” diffusion is interpreted to be inter-crystalline or grain-boundary diffusion, which is expressed at depth in a concentration profile. Calculated “fast” diffusivities in enamel were about 10^-13 cm²/s for REE and U, 10^-14 cm²/s for Pb, and 10^-12 cm²/s for Ba and Sr at 90°C, and 2-3 orders of magnitude slower at 23°C. These diffusion rates are about 2 orders of magnitude slower than in bone. They are consistent with past inferences of typical diffusion rates in enamel, and confirm greater resistance of enamel to trace element alteration during fossilization compared to bone. Future experiments at 70°C, 52°C, and 36°C will refine the Arrhenius relationship in mammoth enamel and determine the activation energies for many relevant trace elements in the geo- and health sciences.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-6747

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