INFLUENCE OF HAVERSIAN SYSTEMS ON INCORPORATION OF RARE EARTH AND TRACE ELEMENTS IN LATE EOCENE BRONTOTHERES

Three associated brontothere (Perissodactyla) bones (metapodial, distal femur, and rib) were collected from an overbank mudstone in the Peanut Peak Member of the Late Eocene Chadron Formation (White River Group) of northwest Nebraska. Laser Ablation-ICP-MS (LA-ICP-MS) was used to measure concentrations of rare earth (REE) and trace elements (TE) in the three bones. Light- (LREE) and middle-REE (MREE) concentrations are highest near the bone surface but decrease rapidly with depth whereas heavy-REE (HREE) concentrations remain fairly constant. REE signatures are LREE-enriched near the bone surface but become MREE-depleted with depth. Sr and Ba concentrations are constant in the bones; however Th is concentrated only near bone surfaces. U concentrations decrease slightly with depth in the rib, but are constant or increase with depth in the other bones. An outer circumferential layer (OCL) is preserved along the outer 1 mm of the rib and femur. Concentrations in the OCL are much lower than in the underlying bone, indicating lower incorporation or greater leaching. Concentration gradients are consistent with a double medium-diffusion (DMD) model, with slow diffusion through lamellar bone and rapid diffusion along open Haversian systems. Concentrations surrounding some Haversian canals in the rib were higher and more LREE-enriched than in surrounding bone, suggesting fast diffusion of undepleted ground water. In the rib and femur, 20 - 30 percent of REE and TE were incorporated through these fast diffusion pathways. Periods of diffusion (fossilization) were calculated from U, La, Ce, Nd, and Gd gradients in the bones. Calculated diffusion periods for U and the REE are similar in the rib, suggesting that REE may also be used to date periods of fossilization. Average diffusion periods are ca. 65 ka (rib), 50 ka (metapodial), and 55 ka (femur). The three bones vary from 1.7 cm (rib) to 8.5 cm (femur) in diameter. These results suggest that the period of diffusion is not influenced by the size of the bone.