Alpha particle distribution and uranium mechanisms of accumulation in fossilised shells of ammonites and bivalves

Abstract

The uranium concentration and distribution in fossils (ammonite and bivalve specimens) were studied. Fossil samples were cut perpendicularly and thin sections were prepared. The chemical and mineralogical compositions of fossils were analysed using scanning electronic microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS). The distribution of alpha emitters in the area of fossils was registered using CR-39 detectors. Alpha particle emitters were almost evenly distributed in all analysed fossils. We do not observe tracks concentrated in specific regions, which may indicate the absence of highly radioactive mineral grains. The uniformly distributed alpha tracks correlated with areas of mineral composition dominated by apatite, $Ca_{5}(PO_{4})_{3}(Cl/F/OH)$. The correlation between phosphorous content and alpha tracks suggests that this element was crucial in absorbing radionuclides, presumably uranium or other alpha particle emitters, uranium progenies. However, upon analysing the chemical composition on thin sections of fossils, uranium was not detected, likely due to its concentrations being below the detection limits of EDS. Areas on the thin sections devoid of alpha tracks on CR-39 detectors were associated with empty voids in thin sections, ooids partially composed of $FeS_{2}$ (pyrite framboids) or iron oxides $F_{2}O_{3}$ (hematite), phosphorus-free regions, or other areas where crushed fragments of shells composed of calcium carbonate (aragonite) partially filled internal casts. The interaction of elements presented in fossil structures with uranium can depend on various factors, such as the pH of the pore fluids, redox conditions, and the specific mineralogy of the sediments. Our research indicates that the increased radioactivity registered in some fossils is connected with phosphorous content. Small amounts of uranium are disseminated in calcium phosphate (various apatite forms). The uranium concentrations smaller than the detection limit of EDS can be successfully registered using passive track detectors.