Ontogenetic variability in crystallography and mosaicity of conodont apatite: Implications for microstructure, paleothermometry and geochemistry

X-ray diffraction data from Silurian conodonts belonging to various developmental stages of the species Dapsilodus obliquicostatus demonstrate changes in crystallography and degree of nanocrystallite ordering (mosaicity) in both hyaline and albid crown tissue. The exclusive use of a single species in this study, combined with systematic testing of each element type at multiple locations, provided insight into microstructural and crystallographic differentiation between element position (Sa, Sb-c, M) as well as between juveniles and adults. A relative increase in the unit cell dimensions of the a-axis/c-axis ratio of nanocrystallites during growth was apparent in areas demonstrating single-crystal behavior but no such relationship was seen in dominantly polycrystalline areas. Systematic variations in mosaicity were identified, with mosaicity (as a proxy for disorder) increasing during growth, as well as along elements from tip to base. These results provide potential insight into the integrity of conodont apatite as a recorder of paleoseawater chemistry, as well as demonstrate the need to consider the influence of ontogeny and element position on the use of conodonts in paleothermometry and geochemical investigations.    

Methods
X-ray diffraction experiments were carried out in a Bruker D8 Quest single crystal X-ray diffractometer equipped with a CMOS area detector that allows diffraction pattern analysis of both single-crystalline and polycrystalline material. Irradiation of samples occurred through a microfocus Mo Kα (λ = 0.7107 Å) X-ray source with a beam diameter of 120 μm, which is suitable to analyze multiple positions along each specimen. Depending on the size of the sample, three to six zones were analyzed along the length of each element. In a typical experiment, conodont elements were coated with mineral oil, then attached on top of MiTeGen Dual-Thickness MicroLoop in a vertical position.  The sample was placed on the three-circle goniometer and the position of the X-ray beam was chosen using the microscope camera. X-ray diffraction data associated with each position were collected in reflection mode by varying the Φ angle while keeping the other angles fixed (2θ=0o, ω=0o and χ=54.74ᵒ). An eight- to eleven-second exposure time was needed for analyzing albid and hyaline material and a 20-30 second exposure time was needed for the basal cavity. Alignment of the diffractrometer was calibrated using the standard YLID crystal provided by Bruker AXS.