Heterogeneous rare earth element (REE) patterns and concentrations in a fossil bone: Implications for the use of REE in vertebrate taphonomy and fossilization history

摘要

A bone fragment (CGDQ-3) of Falcarius utahensis, a therizinosaur from the Early Cretaceous Cedar Mountain Formation, Utah, contained within a carbonate nodule, was analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) in order to investigate REE variability within a thin-walled phalanx. Previous studies have found depth-related REE pattern variations; however, in CGDQ-3 variation occurs along the circumference of the bone. NASC-normalized REE patterns and concentrations vary between two apparent end members. A light-REE enriched (LREE) pattern, similar to solution ICP-MS analysis of this bone, characterizes approximately two-thirds of the bone fragment. Total REE concentrations are high and do not vary significantly from the periosteal surface to the medullar surface. Conversely, the remaining one-third of the bone has REE patterns that are MREE-depleted and low in total REE concentrations. REE concentrations in this part of the bone do not vary significantly from the periosteal to the medullar surface. A positive Ce anomaly is found throughout the entire bone, and is greatest within the LREE-enriched portion of the bone. This, in combination with the LREE-enrichment, suggests that the bone fossilized under reducing conditions. The distance between the LREE-enriched and MREE-depleted regions is less than 1 mm. Isotopic and petrographic analyses of the bone and surrounding carbonate matrix suggest the REE patterns in the bone were the result of partial fossilization/incomplete filling of micro-pore spaces around bone crystallites in an environment with changing redox conditions. The lower, MREE-depleted part of the bone fossilized contemporaneously with a pendant cement that formed on the underside of the bone in the vadose zone. Formation of the pendant cement restricted water flow through the bone, isolating the lower portion, which incorporated a MREE-depleted pattern. The upper part of the bone (LREE-enriched side) fossilized under more reducing conditions than the lower part. This resulted in reductive dissolution of minerals (such as hydrous ferric oxides and manganese-oxides) which were LREE-enriched with positive Ce anomalies. These REE were then incorporated into the upper part of the bone. This likely occurred after the bone entered the saturated zone (below the water table). The thinness of the bone and presence of a pendant cement helped facilitate partial fossilization of the bone, preserving the REE signature of an earlier fluid. As demonstrated by this case study, differences in REE patterns within a fossil may record changes in geochemical conditions during fossilization, particularly, when bones are encased in a material that reduces fluid permeability, such as micritic vadose calcite. Analysis of such bones may offer the opportunity to decipher complex fluid histories that occur during fossilization. LA-ICP-MS should be utilized to confirm solution ICP-MS analysis if it yields high REE variability, prior to an interpretation of reworking or time-averaging.