Integrated crystallographical indicators and REE–HFSE geochemistry in conodont bioapatite: evidence for decoupled diagenetic signals

Conodont bioapatite is widely used as an archive of palaeoenvironmental and diagenetic information, yet the extent to which crystallographic parameters and trace-element compositions record the same post-mortem processes remains poorly constrained. Here, X-ray microdiffraction (μ-XRD) crystallographic data and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace-element analyses are integrated for latest Famennian conodont elements from Kowala and Dzikowiec, Poland, and Corona Mizziu, Sardinia, representing different palaeogeographical settings and Colour Alteration Index (CAI) values from 1.5 to 5. The investigated material includes the genera Palmatolepis , Branmehla , and Bispathodus , with μ-XRD analyses performed on basal and denticle microdomains. Unit-cell parameters show a narrow range across all samples, with no systematic differences between element microdomains, genera, localities, or CAI groups. Crystallinity indices and crystallite-size estimates show limited variability and do not define distinct crystallographical clusters. In contrast, Rare Earth Elements (REE) and High Field Strength Elements (HFSE) compositions are highly variable. Y/Ho ratios, positive Y–REE covariation, relationships between normalised Pr, Gd and Yb values and normalised La, middle REE (MREE) and heavy REE (HREE) enrichment, and Pb U clustering provide convergent evidence that the trace-element budget of the analysed conodont apatite was substantially modified during diagenesis. This diagenetic imprint is not uniform: the three CAI/locality groups differ most clearly in MREE and HREE behaviour, demonstrating that CAI alone cannot account for the observed REE–HFSE fractionation and that local diagenetic conditions exerted a major control. The comparison between geochemistry and μ-XRD therefore reveals a partial decoupling between trace-element fractionation and apatite lattice parameters. These results suggest that conodont trace-element budgets were modified mainly by local pore-water and sedimentary conditions, whereas the crystallographical structure of the apatite lattice remained comparatively stable. Integrated chemical, crystallographical, and microtextural screening is therefore essential before using conodont trace-element compositions as direct palaeoenvironmental proxies.