SYSTEMATIC ASSESSMENT OF CRYSTALLINE STRUCTURE ALONG THE LENGTH OF CONICAL CONODONT MICROFOSSILS USING MICROFOCUS X-RAY DIFFRACTION

Conodonts are agnathan chordates whose teeth-like microfossils bear important chemical and thermal information regarding the ancient ocean and related sedimentary basins. These microfossils are routinely used for petroleum exploration to access thermal history of geologic formation through the Color Alteration Index (CAI). CAI is a common qualitative method that relies on an optical assessment of the fossil color, but analysis of chemical and structural changes could be used to create a more quantitative metric. Although a handful of elemental and spectroscopic studies have been carried out on conodont microfossils, a systematic investigation on their crystallinity and composite structure is lacking. This type of detailed investigation is necessary to determine the level of consistency in natural samples and establish a complete chemical picture for changes in CAI values. In the present study, we analyzed 10 coniform conodont specimens (S_a, S_b-c and M elements) from Dapsilodus obliquicostatus that all display low CAI values. A Bruker D8Quest single crystal X-ray diffractometer equipped with 120 µm X-ray beam and CMOS area detector was used to analyze different sections along the length of each fossil. We observed variability of crystalline structure ranging from single crystalline to polycrystalline regions in different sections of the fossils. Albid and hyaline material of juvenile S_a and S_b-c elements were found to have more ordered single crystalline structure than ontogenetically older elements. Unit cell parameters of the highly crystalline regions were smaller than natural apatite crystals, whereas the mosaicity value was larger. The basal cavities of all elements display Debye rings, which is characteristic of polycrystalline material. Powder X-ray diffraction pattern calculated from Debye ring matches with synthesized polycrystalline apatite. Furthermore, texturing was observed in the diffraction pattern of basal cavities due to preferred crystallographic orientation. Psi scans of the diffraction pattern were utilized to relate these features to specific crystallographic planes, revealing the orientation of the coherent nanodomains within the conical elements.