LA-ICP-MS analysis of rare earth elements in marine reptile bones from the Middle Triassic bonebed (Upper Silesia, S Poland): Impact of long-lasting diagenesis, and factors controlling the uptake

Abstract

The rare earth elements (REE) contained in fossil bones are widely used for provenance and palaeoenvironmental studies. Earlier works suggested that the REE uptake is limited to early diagenesis, but more recent research provided evidence of long-term open-system diffusion in fossil bones. We analysed thin-walled reptile bones, all collected from the same Middle Triassic (~245Ma) marine bonebed, comprising vertebrate remains of likely different ages, to check the influence of the long-term diagenesis on the concentrations of REE, in terms of impact of pore water; the upper parts of the bones are contained in a low-permeability crinoidal limestone, whereas their lower parts have been affected by diagenetic fluids for a long time. We measured REE and other trace element contents in 29 bones using LA-ICP-MS (totally 660 spots in 45 depth profiles). Neither the absolute concentrations of the REE and the REE intra-bone PAAS-normalised patterns, nor the inter-element ratios and the La, Ce and Eu anomalies appear useful to distinguish between the likely older and younger bones in the bonebed. The MREEN enrichment combined with the flat intra-bone distributions of the REE lead to conclusion that REE fractionation and substitution for Ca were the most important processes responsible for the selective uptake. The low-permeability host rock and diagenetic fluids had a significant impact on the trace element uptake and their intra-bone distribution patterns. The lower parts of the bones that likely had a long-lasting contact with the diagenetic fluids reveal a relative enrichment in the REE and their easier diffusion into the bone centres. The ability of fossil bones to incorporate trace elements over a long time-span strongly limits the usage of particularly the older samples for provenance and palaeoenvironmental reconstructions. Additionally, the presented new analyses fill the gap between the available databases of trace element concentrations in fossil bones with regard to Middle Triassic samples.