Identification of mass-independent Mo isotope anomalies in natural and industrially processed samples. Proof of concept for uranium provenance in nuclear forensics

Abstract

Mass-independent isotope fractionation (MIF) has only been documented for a limited number of heavy elements (such as Hg, Tl and U) and has thus received limited attention for intermediate mass elements such as Mo. Conversely, mass-dependent isotope fractionation (MDF) of various isotope systems has become one of the most common tools in geochemistry as a means of deciphering processes that have shaped our planet. MDF signatures have provided fruitful insights by establishing a correspondence between a given process and specific isotope ratios. However, they are hampered by some limitations, notably because several processes can yield the same isotope signature or because superimposed effects can complicate interpretations. In this study, we identified Mo isotope mass-independent signatures resulting from, first, nuclear field shift effects, and second, fissiogenic isotopic anomalies in natural samples of uranium ores, as well as in industrially produced uranium ore concentrates. Furthermore, most MDF signatures yield residual anomalies after correction of instrumental mass bias using an exponential law, leading to a third signature (though an apparent MIF). The three types of mass-independent signatures were successfully deconvoluted and result in a powerful tracer that is shown to discriminate the origin of several ore deposits, from studying the isotope composition of uranium ores or uranium concentrates. These isotope effects discovered in natural uranium ores are produced by generic geochemical processes that have been described in many other environments. Thus, the identification of these effects opens the possibility of extending the search for mass-independent isotope signatures in natural samples with numerous isotope tracers in a broad number of applications such as environmental geochemistry or nuclear forensics.