Geochemical and Isotopic Constraints for Uraninite Formation: Implications for Nuclear Forensic Analyses
This thesis examines nuclear forensic signatures of several uraninite samples from North America (n=14) and the Democratic Republic of Congo (n=1) for enhancing and improving provenance determination methods. Focused ion beam (FIB) coupled with scanning electron microscopy (SEM) analyses were conducted on both pristine and altered sections of uraninite in order to understand its 3-dimensional chemical and mineralogical nature. The latter results were then used to establish Pb as an ideal internal standard for laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses of uraninite. Trace element abundances and isotopic ratios (e.g., Pb, Sr, and U) were determined by utilizing both LA- and solution mode (SM)-ICP-MS and multicollector (MC)-ICP-MS methods. Trace element abundances are attributed to both crystallographic-controlled substitution mechanisms and crustal source(s) involved in the formation of uraninite. Secondary 207Pb-206Pb isochron ages obtained by LA-MC-ICP-MS are equivalent to those both derived by SM-MC-ICP-MS and available in literature for the U ore deposits investigated here; this feature confirms the use of Pb-Pb secondary isochron ages as a new and viable tool in nuclear forensic investigations. 207Pb-206Pb isochron ages also indicate alteration of uraninite occurs relatively soon (within a few million years) after mineralization. Correlation between initial 87Sr/86Sr values and host craton ages suggest uraninite mineralization is influenced by the host rock geology. δ238U and δ234U values indicate that uranium fractionation is the result of multiple mechanisms and that the uraninites investigated here have incurred alteration events within the last ~2 million years, respectively; the latter did not perturb the secondary Pb-Pb isochron ages.