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

Caroline Fitoussi, Eric Pili, Elsa Yobregat, Mathieu Touboul, Cécile Gardin

► To cite this version:

Caroline Fitoussi, Eric Pili, Elsa Yobregat, Mathieu Touboul, Cécile Gardin. Identification of mass-independent Mo isotope anomalies in natural and industrially processed samples. Proof of concept for uranium provenance in nuclear forensics. Earth and Planetary Science Letters, 2025, 666, pp.119459. 10.1016/j.epsl.2025.119459. hal-05373763

HAL Id: hal-05373763

https://hal.science/hal-05373763v1

Submitted on 19 Nov 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
<figure><img src="image_2.png" /></figure>

<figure><img src="image_3.png" /></figure>

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

Caroline Fitoussi<sup>a, *</sup>, Eric Pili<sup>b, †</sup>, Elsa Yobregat<sup>a, ‡</sup>, Mathieu Touboul<sup>a</sup>, Cécile Gardin<sup>a</sup>

## ARTICLE INFO

Editor: Dr Fang-Zhen Teng

**Keywords:**

Mo isotopes

Mass-independent isotope signature

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.

## 1. Introduction

Stable isotope tracers have been extensively developed and used to provide insight into past or recent geochemical processes that have left specific isotope signatures to geological samples. The search for these signatures has notably aimed at reconstructing the geological history or provenance of a particular sample. Bigeleisen and Mayer (1947) and Bigeleisen (1949) originally described the theory of mass-dependent isotope fractionation by determining the expressions of isotope fractionation factors associated with reactions under equilibrium and kinetic conditions. Quantification of MDF using isotope fractionation factors has opened up the field for identification of numerous geochemical processes. Provided these parameters are known, it is

possible to assign the observed isotope composition to a particular process. However, it is not always possible to design such models because (i) isotope fractionation factors between species, as well as reaction progress, or relative fractions of the various phases, are often unknown in natural environments; (ii) the magnitude of isotope fractionation will depend on the law that governed the isotope fractionation; (iii) the observed isotope fractionation can represent the net effect of more than one process, making it even more difficult to interpret.

This problem is particularly acute in (but not restricted to) low temperature natural environments where reaction kinetics are slow. In addition, the number of chemical species can potentially be very large. In this case, the interpretations can be ambiguous. For example, Mo isotopes in uranium ores and Uranium Ore Concentrates (UOCs) have

<footnote><sup>a</sup> LGLTPE, Ecole Normale Supérieure de Lyon, UCBL and CNRS, 46 Allée d'Italie 69364 Lyon cedex 07, France</footnote>

<footnote><sup>b</sup> CEA, DAM, DIF, F-91297 Arpajon, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: caroline.fitoussi@ens-lyon.fr (C. Fitoussi).</footnote>
shown the existence of large Mo isotope fractionation that cannot be unambiguously attributed to a specific geochemical process, which makes the interpretation in terms of geological provenance or identification of specific type of uranium deposit based on δ⁹⁸Mo values difficult (Migeon et al., 2020; Rolison et al., 2019).

This issue associated with MDF signatures is not restricted to the field of nuclear forensics and has also been problematic in other subfields of geochemistry. As an example, molybdenum isotopes have also been used as a widely used paleoredox proxy to infer the level of O₂ in the ocean-atmosphere system (Siebert et al., 2003; Kendall et al., 2017) thanks to a wealth of data on the modern oceans combined with experimental work. The main Mo sinks in the oceans are (i) oxic sediments (known to fractionate Mo isotopes during adsorption onto Mn and Fe(III) oxides (Siebert et al., 2003; Barling and Anbar, 2004; Goldberg et al., 2009), (ii) restricted euxinic basins where there is no Mo isotope fractionation (Helz et al., 1996; Erickson et al., 2000), and (iii) an intermediate sink termed 'sulfidic at depth' (Kendall et al., 2017). In that case, Mo is trapped within sediments in mildly reducing conditions, possibly associated with the formation of thiomolybdates, reaction with the organic matter, or adsorption onto {Fe,Mn} oxyhydroxides (Phillips and Xu, 2021). The issue is that this sink was not constant in the past (Duan et al., 2010) and can result in variable Mo isotope fractionation (Dickson 2017), hence Mo isotope variations in the oceans cannot be solely attributed to the presence of an oxic Mo sink (Nägler et al., 2011). This makes the interpretations of Mo isotope analyses as a redox proxy difficult to interpret quantitatively in terms of fO₂ level, especially when considering redox changes over short time intervals (Reinhard et al., 2013).

In summary, the Mo mass-dependent isotope fractionation in uranium ore deposits or in sediments are driven by the same generic processes (e.g. association with sulfides or reducing organic matter) and are a source of ambiguity in the interpretations. In this respect, the introduction of novel Mo isotope signatures, that could overcome such ambiguities and help distinguish between different hypotheses by being more specific to a given reaction or process, would be highly desirable. In this study, we have explored such a possibility by searching for high-precision mass-independent Mo isotope signatures in uranium ores and uranium ore concentrates.

## 2. Samples

### 2.1. U ore samples

Samples of uranium ores from different geographic locations (Supplementary Fig. 1) as well as from different ages and types of ore formation were selected. Additionally, we also collected samples of uranium ore concentrates deriving from the processing plants receiving the ores.

Eight samples came from Niger, from two mining districts, each associated with a separate ore body of similar origin and exploited by one mining company (SOMAÏR, COMINAK) using several mines. Each company operates its own processing plant. The samples were provided by Orano, France. These deposits were formed in sandstones containing organic matter, providing a reducing environment that immobilized uranium mostly as pitchblende, and have recently been dated with U-Pb, yielding an age of 119 Ma (Mamadou et al., 2022). From SOMAÏR, ore sample TP38 came from the Tamgak mine, while the origin mine for ore sample SOM 1 is unknown. Samples SOM 3979 and SOM 3218 are sodium diuranate precipitates at the SOMAÏR plant. From COMINAK, samples TP16 and TP20 came from the Ebba North mine. Samples AKO 13028 and AKO 14086 are magnesium diuranate UOCs from the COMINAK plant (IAEA, 1993).

The Mongolia-1 sample comes from Dulaan Uul, a roll-front uranium deposit (de Boissezon et al., 2020). The deposition of uranium is attributed to groundwater circulation in tabular sandstone where uranium was trapped as uraninite and coffinite due to the presence of

organic matter formed in an anoxic environment (Rallakis et al., 2019). This sample was provided by Orano, France.

Orano Resources Canada supplied the ore feed for the McClean Lake mill. It consists primarily of uranium ore from the Cigar Lake mine, potentially blended with ore from the former McClean Lake mine to maintain concentrations of U, Mo, As, and Fe within specified ranges. The Cigar Lake and McClean Lake deposits are of the unconformity-type. Hoeve and Quirt (1987) proposed a qualitative model for uranium accumulation in the Athabasca Basin. The uranium deposits were influenced by hydrothermal circulations and are associated with the presence of graphitic (reducing) gneisses (Tourigny et al., 2007). The Cigar Lake deposit began about 1.3 Gyr ago (Cumming and Krstic, 1992; Philippe et al., 2011).

The South African uranium ore sample (VRM 73/22) comes from a quartz-pebble conglomerate from the Klerksdorp locality. This is a mixed gold and uranium sedimentary deposit ~ 3 Gyr-old (Nwaila et al., 2020). It contains detrital uraninite that was preserved during transport and accumulation thanks to the low oxygen fugacity of the atmosphere (Frimmel, 2005). This sample was provided by JRC Karlsruhe. The ammonium diuranate UOCs, ADU slurry 12851-05 and ADU powder 12851-08 are, respectively, a slurry precipitate and a dry precipitate from the NUFCOR plant (Marks et al., 2015), which processes uranium from the Klerksdorp area. The collection of these UOC samples was arranged by the RSA Support Program to the IAEA. JRC Karlsruhe received a subset of these samples (Varga et al., 2017) and shared a subset for this study.

The Jordan-1 uranium ore sample is a phosphorite ore deposited in a marine environment on the margin of the Tethys Ocean. It is thought to have formed in a former zone of upwelling (Abed, 1989). The sediment is rich in organic matter, and uranium is associated with apatite. This sample was provided by Orano, France.

### 2.2. Analytical methods

Depending on the Mo concentration, 100 mg to 5 g of sample powder were dissolved in closed Savillex™ Teflon beakers. All the samples were pre-treated using diluted HNO₃ for 24 h to avoid a strong oxidation reaction upon the addition of concentrated HNO₃ which would cause projections of uraniferous materials. Dissolution was achieved using a 1:1 mixture of concentrated HF-HNO₃ at 150 °C for 48 h. Samples were dried and fluorides were removed by repeated treatments in aqua regia and concentrated HCl. Samples with high organic matter contents were treated with concentrated HNO₃ and 30 % H₂O₂.

For Jordan_1 (phosphorite) and Mongolia-1 samples, fluorides proved difficult to redissolve after the digestion. We separated them by centrifugation and treated them separately in concentrated HCl, which was more efficient to convert all fluorides to chlorides. The Canadian ore sample McCl1 had sub millimetric grains that did not dissolve in either concentrated HNO₃-HF mixture or aqua regia. We separated them by centrifugation, then dissolved them separately using a concentrated HCl-HF mixture. For UOC, the diluted nitric acid pre-treatment was sufficient to dissolve the whole powder except for ADU slurry 12851-05 which required the addition of 100 µL of concentrated HF to ensure complete dissolution. UOC were then treated once in aqua regia. Ore samples Jordan_1, Mongolia_1, Niger TP20 and UOC samples AKO 13028 and AKO 14086 had a hydrophilic behavior relative to the Teflon beaker after complete dissolution, which we attributed to the presence of organic matter from the samples themselves. They were treated with concentrated HNO₃ and 30 % H₂O₂ until no more color change or bubbling occurred upon further addition of 30 % H₂O₂. Those samples were then converted to chloride using concentrated HCl. Complete dissolution of all samples was achieved eventually. Finally, all samples were dissolved in 5 M HCl for the chemical separation process.

The protocol used to purify Mo consists of four steps and is based on the procedure described in Yobregat et al. (2022). We modified the first step in order to separate some elements for other studies. First, samples
are loaded on 2 mL of anionic resin AG1-X8 (100–200 mesh size). Matrix elements and REE are eluted in 10 mL of 5 M HCl, then Mo along with U and Fe are eluted in 25 mL of 0.6 M HCl. Then Mo fractions are converted to fluorides and processed through two columns filled with anionic resin (AG1-X8, 200–400 mesh size). Remaining matrix elements and interfering elements (Ru, Zr) are removed by successive additions of HF and HCl-HF mixtures. Then, Mo is collected using either a HNO3-HF mixture or 0.6 M HCl. Finally, Mo fractions are loaded on a 1 mL cationic column in 0.6 M HCl to remove U. Purity and yield were evaluated by Q-ICP-MS at the end of the separation protocol (Yobregat et al., 2022). Purified Mo samples were treated several times by small quantities of aqua regia to remove any organics. In order to check for any mass-independent effect that could arise during the chemical separation, we processed the same sample twice through the dissolution and chemical procedure and this yielded the same µ'Mo values in both cases (Yobregat et al., 2022). This proves that the chemical processing is not responsible for the observed MIF.

The yields of the Mo chemical separation were determined to be better than 80 % for the four-column separation method, which limits the potential MDF fractionation that could have taken place during that procedure. Furthermore, by analyzing a Mo NIST standard that had processed through the chemical separation twice, we have found out that no new MDF anomalies were produced, as the results after the first and second chemical separation were identical (Yobregat et al., 2022). This proves that this procedure does not introduce any MIF anomalies due to incomplete chemistry yields.

The Mo isotope compositions were measured in negative mode on a Thermo Fisher Triton Plus™ at the Laboratoire de Géologie de Lyon. Purified Mo samples were dissolved in 6 M HCl to obtain a concentration of 1 µg/µl, then 2 µg of Mo were loaded on degassed Re zone-refined filaments in a double assembly configuration. Lanthanum nitrate was added to the filaments as an activator. Data acquisition used a 3-lines multidynamic mode and amplifiers rotation (Yobregat et al., 2022). The Mo isotope data obtained over the course of this study did not show any residual trends when plotting one µ value versus another within a single mass spectrometry multidynamic runs or runs of the same sample. The only evaporating species in the TIMS source is MoO₃ and there is thus no reason to observe residual trends as has been reported for Cr

isotopes measurements in Bourdon and Fitoussi (2020). Oxygen composition was monitored on-line using a 10¹² Ω resistor to correct isobaric interferences. Our correction of isobaric interferences from Mo trioxide uses an oxygen isotopic composition determined by measurement of the isobaric interference at mass 150. As previously shown (Worsham et al., 2016; Bermingham et al., 2024), this method allows an appropriate correction of isobaric interferences, which accounts for the variations of the O isotope composition during each measurement. Mass bias is corrected using the normalization ratio ⁹⁸Mo/⁹⁶Mo = 1.453171 (Lu and Masuda, 1994). Isotope compositions are reported using the µ notation which expresses the deviation relative to the NIST 3134 standard isotope composition.

## 3. Results

The Mo isotope compositions of various types of uranium-rich samples analyzed in this study are reported in Table 1.

All samples show resolvable mass-independent isotope variations that are outside the uncertainties reported for standard measurements, with the largest deviations observed for samples from Niger uranium deposits. The presence of such large Mo mass-independent fractionation in terrestrial samples has so far not been reported in the literature.

There is no noticeable difference between the uranium ores and the UOCs produced from the ores from a given locality, as shown particularly for the South Africa and Niger samples (Fig. 1). In what follows, we first discuss the data reduction scheme used to identify MIF signatures and then discuss the processes responsible for them.

## 4. Discussion

### 4.1. Data processing and deconvolution method

#### 4.1.1. Apparent MIF (residual MDF)

When investigating mass-independent isotope signatures, the common practice is to correct measured isotope fractionation for mass-dependent isotope fractionation using the exponential law. This is most appropriate to correct for instrumental mass bias in TIMS or MC-ICPMS techniques (Russell et al., 1978; Maréchal et al., 1999).

<caption>**Table 1**</caption>

Multidynamic Mo isotopic data (in ppm, ± 2SE) of U ores and Uranium Ore Concentrates (UOCs). For samples with mass dependent data we worked with the same powders as those used by Migeon et al. (2020).

<table><thead><tr><th></th><th>µ<sup>92</sup>Mo</th><th>µ<sup>94</sup>Mo</th><th>µ<sup>95</sup>Mo</th><th>µ<sup>97</sup>Mo</th><th>µ<sup>100</sup>Mo</th><th>n</th><th>δ<sup>98</sup>Mo<sup>i</sup></th><th>Mo/U<sup>i</sup></th></tr></thead><tbody><tr><td>Niger, U ores</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cominak:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TP16</td><td>275 ± 30</td><td>91 ± 8</td><td>30 ± 6</td><td>-25 ± 3</td><td>111 ± 7</td><td>1</td><td>-2.62 ± 0.02</td><td>0.0033</td></tr><tr><td>TP20</td><td>176 ± 30</td><td>54 ± 6</td><td>14 ± 5</td><td>-18 ± 3</td><td>65 ± 6</td><td>1</td><td>-2.62 ± 0.02</td><td>0.623</td></tr><tr><td>Somair:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>TP38</td><td>172 ± 30</td><td>55 ± 8</td><td>22 ± 7</td><td>-14 ± 3</td><td>62 ± 7</td><td>3</td><td>-1.39 ± 0.02</td><td>0.044</td></tr><tr><td>SOM 1</td><td>202 ± 30</td><td>61 ± 7</td><td>18 ± 6</td><td>-18 ± 3</td><td>73 ± 7</td><td>1</td><td>-2.07 ± 0.02</td><td>0.23</td></tr><tr><td>Niger, UOCs</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Cominak:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>AKO 13028</td><td>188 ± 30</td><td>58 ± 3</td><td>20 ± 4</td><td>-15 ± 5</td><td>65 ± 2</td><td>3</td><td>-1.83 ± 0.02</td><td></td></tr><tr><td>AKO 14086</td><td>210 ± 30</td><td>64 ± 7</td><td>21 ± 5</td><td>-19 ± 5</td><td>80 ± 10</td><td>3</td><td>-2.44 ± 0.03</td><td>1.2 × 10<sup>-3</sup></td></tr><tr><td>Somair:</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SOM 3218</td><td>266 ± 30</td><td>85 ± 4</td><td>28 ± 6</td><td>-17 ± 1</td><td>92 ± 2</td><td>2</td><td>-3.50 ± 0.04</td><td>8.0 × 10<sup>-4</sup></td></tr><tr><td>SOM 3979</td><td>218 ± 30</td><td>66 ± 5</td><td>22 ± 2</td><td>-16 ± 1</td><td>79 ± 13</td><td>2</td><td>-3.16 ± 0.03</td><td></td></tr><tr><td>Mongolia, U ore</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Mongolia_1</td><td>124 ± 30</td><td>37 ± 7</td><td>14 ± 2</td><td>-9 ± 5</td><td>4 ± 4</td><td>2</td><td>-2.18 ± 0.06</td><td>0.0036</td></tr><tr><td>Jordan, U ore</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Jordan_1</td><td>74 ± 30</td><td>22 ± 5</td><td>12 ± 4</td><td>-6 ± 2</td><td>30 ± 5</td><td>1</td><td>-1.55 ± 0.02</td><td>0.18</td></tr><tr><td>South Africa, U ore</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>VRM 73/22</td><td>-8 ± 30</td><td>2 ± 4</td><td>18 ± 3</td><td>15 ± 2</td><td>-4 ± 3</td><td>1</td><td></td><td>0.0030</td></tr><tr><td>South Africa, UOCs</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>ADU slurry 12851-05</td><td>0 ± 30</td><td>-2 ± 7</td><td>18 ± 6</td><td>14 ± 3</td><td>-1 ± 7</td><td>1</td><td></td><td></td></tr><tr><td>ADU powder 12851-08</td><td>-4 ± 30</td><td>3 ± 6</td><td>22 ± 5</td><td>14 ± 3</td><td>6 ± 5</td><td>1</td><td></td><td></td></tr><tr><td>Canada, U ore</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>McCl1</td><td>21 ± 11</td><td>4 ± 7</td><td>10 ± 5</td><td>9 ± 3</td><td>6 ± 6</td><td>1</td><td></td><td>0.0026</td></tr></tbody></table>

<footnote>i from Migeon et al. (2020).</footnote>
<figure><img src="image_4.png" /><figcaption>Fig. 1. Comparison of Mo isotopic data of U ores and UOCs from a given locality (data from Table 1, errors bars are 2SE in ppm).</figcaption></figure>

However, this law is mathematically distinct from mass fractionation laws built to describe natural processes, which are potentially governed by equilibrium mass fractionation or by Rayleigh distillation under kinetic or equilibrium conditions (e.g. Young et al., 2002; Wombacher and Rehkämper, 2003; Wombacher et al., 2004). Therefore, the data corrected using the exponential law expressed as $\mu^iMo$ values could exhibit remaining deviations that depend on the magnitude of the MDF (Wombacher et al., 2008; Carlson et al., 2014; Budde et al., 2023; Bermingham et al., 2024). In this case, the MDF-corrected Mo isotope data, using the exponential law, will exhibit an ‘apparent’ mass-independent fractionation (MIF), $\mu^iMo_{app}$, actually reflecting non-zero residues of corrected natural MDF with a distinctive parabolic shape as a function of the isotope mass (Fig. 2).

One remarkable feature of these residual anomalies is that their ratios are quasi constant, regardless of the MDF process considered. If one calculates the ratio of two $\mu^iMo_{app}$ values, for the different mass fractionation laws, one notices that all of these ratios are almost constant (see Supplementary Table 2). In $\mu^iMo_{app}/\mu^iMo_{app}$ plots, this translates into slopes of lines that are indistinguishable given the uncertainty of the $\mu^iMo_{app}$ measurements, even for high precision TIMS data (Yobregat et al., 2022). Fig. 2 shows an example of such plot for the two isotopes ($^{95}Mo$ and $^{97}Mo$) measured with the highest precision. Thus, while different fractionation laws govern a variety of processes, their isotope pattern, reported as any pair of $\mu^iMo_{app}/\mu^iMo_{app}$, remains unchanged such that:

$$ \left( \frac{\mu^iMo_{app}}{\mu^jMo_{app}} \right)_{process 1} = \left( \frac{\mu^iMo_{app}}{\mu^jMo_{app}} \right)_{process 2} = k \qquad (1) $$

<figure><img src="image_5.png" /><figcaption>Fig. 2. ‘Apparent’ MIF ($\mu^iMo_{app}$) resulting from residual anomalies for various MDF laws (see Supplementary text), after MDF correction using the exponential law. (a) Mo isotope pattern of an Apparent MIF. (b) $\mu^{97}Mo_{app}$ versus $\mu^{95}Mo_{app}$ of an Apparent MIF calculated by using various laws governing MDF: batch equilibrium (purple line), Rayleigh (black line), Rayleigh kinetic (blue line), power law (green line) and linear law (red line). The calculated slopes differ by no more than $10^{-2}$, which translates into extremely small differences in $\mu^iMo$ values (< 1 ppm in all cases, which is smaller than the analytical uncertainty of the TIMS Mo isotope measurement represented by the colored envelope).</figcaption></figure>

where k is a constant value specific to a pair of isotope ratios (corresponding to each column of Supplementary Table 2). In other words, this means that all the isotope patterns for these MDF laws have identical shapes and can be deduced from each other by multiplying each $\mu^iMo_{app}$ value by a constant value.

This property can thus be used to define a unique isotope pattern for the ‘apparent MIF’ (Fig. 2, Supplementary Fig. 2) without having a precise knowledge about the mass-dependent process (and associated law) that affected the samples.

## 4.2. Fissiogenic component

For uranium-rich samples, mass-independent Mo isotope anomalies can result from the production of fissiogenic Mo (Masuda et al., 1993; Wieser and De Laeter, 2004). These authors showed that the spontaneous fission of uranium could produce measurable quantities of fissiogenic $^{95}Mo$, $^{97}Mo$, $^{98}Mo$, and $^{100}Mo$ in zircon or uraninite. In this case, it is possible to directly calculate the effect of spontaneous fission of $^{235}U$ and $^{238}U$ or neutron-induced fission of $^{235}U$ on the abundances of Mo isotopes based on the equation derived in the Supplementary Materials. The corresponding predicted pattern is highly specific (Fig. 3). The magnitude of this effect depends on the timescale during which fission took place for a given initial U/Mo ratio (Table 1).

## 4.3. NFS effect

A third possibility for producing Mo isotope anomalies is a MIF called

<figure><img src="image_6.png" /></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 3. Calculated pattern of Mo fissiogenic anomalies from spontaneous fission of $^{238}$U (main channel), for which instrumental isotope fractionation has been corrected using the exponential law and normalized to $^{98}$Mo/$^{96}$Mo.</figcaption></figure>

Nuclear Field Shift (NFS) effect that has been studied theoretically (Bigeleisen, 1996; Schauble, 2007, Schauble, 2013; Abe et al., 2014). It has been shown that small differences in the distribution of the nuclear charge inside the nucleus could lead to the NFS effect which can modify the distribution of isotopes among various chemical species. This effect arises from the modification of electron energies, specifically those of electrons that have a non-zero density of probability of being in the nucleus. During a chemical reaction, if there is a difference in the electron density at the nucleus between the reagent and the product, the pattern of isotope fractionation between reagent and product will directly depend on the difference in nuclear charge radius of the various isotopes. More specifically for a given compound A containing an isotope i, the energy shift in electron energy associated with this compound is equal to:

$$ \delta E = C |\psi_A(0)|^2 \delta\langle r_i^2 \rangle $$

where C is a constant, $|\psi_A(0)|^2$ is the density of electrons at the nucleus and $\delta\langle r_i^2 \rangle$ is the relative mean square nuclear charge radius (King, 1984). The equation directly links the energy of electrons with the electron density at the nucleus. The mean square nuclear charge radius is a parameter that characterizes the nuclear charge distribution within the nucleus and is defined by the following equation:

$$ \langle r^2 \rangle = \frac{\int_{0}^{+\infty} 4\pi r^4 \rho(r) dr}{\int_{0}^{+\infty} 4\pi r^2 \rho(r) dr} $$

where $\rho(r)$ is the nuclear charge density (King, 1984).

After correcting the Mo isotope data using the exponential law, this effect translates into isotope anomalies with the characteristic pattern shown in Fig. 4. The NFS effect was shown to produce mass-independent fractionation in numerous laboratory experiments involving solvent extraction (e.g. Fujii et al., 2009 and references therein) but was also

described in natural samples for Thallium (Tl) isotopes (Nielsen et al., 2017) and U isotopes (Stirling et al., 2007; Andersen et al., 2017). However, since these two isotope systems have only two isotopes measured at high precision ($^{203}$Tl and $^{205}$Tl, and $^{235}$U and $^{238}$U, respectively), it is not possible to rigorously distinguish mass-dependent (in particular kinetic isotope fractionation for such heavy masses) from mass-independent variations (Stylo et al., 2015; Them et al., 2018).

The expression of $\mu^1Mo$ values accounting for the contribution of residual MDF, NFS, and fission effects can simply be written as:

$$ \mu^1Mo = \mu^1Mo_{app} + \mu^1Mo_{NFS} + \mu^1Mo_{Fiss} = a \times \mu_{app}^i + b \times \mu_{NFS}^i + c \times \mu_{Fiss}^i \quad (2) $$

where a, b, and c are parameters that depend on the magnitude of isotope fractionation due to the apparent MIF, the NFS effect and the fission yield of isotope i, respectively. As the latter process always adds isotopes to the system, the value of c will always be positive (unlike parameters a and b). Owing to the identical patterns for all the MDF (Fig. 2, Supplementary Fig. 2, Supplementary Tables 1 and 2), the $\mu_{app}^i$ values used for the deconvolution are valid for all these laws.

For any sample, the only variables describing the MIF signature will thus be parameters a, b, and c. So, in principle, since each measurement leads to four $\mu^1Mo$ values, it is possible to calculate the possible sets of values for the parameters {a, b, c} for each sample using numerical methods. We used a MonteCarlo method to derive the values of {a, b, c} triplets consistent with the four isotope ratios measured in each sample. A triplet is considered a solution whenever it satisfies Eq. (2) for all four ratios (i = 94, 95, 97, 100) within error (the $\mu^{92}Mo$ isotope data, measured in static mode, was not used in the MonteCarlo inversions because it would have a disproportionate weight in the inversion, as it is not measured with the same level of precision as the other isotopes. However, we systematically checked a posteriori that all the solutions were always consistent with the $^{92}$Mo data within error). In other words, each of these ratios should be such that:

$$ |\mu_{calc}^i - \mu_{meas}^i| < \sigma_i $$

where $\mu_{calc}^i$ and $\mu_{meas}^i$ represent the modeled ratio calculated using Eq. (2) and the measured ratio, respectively; $\sigma_i$ is the analytical uncertainty associated with the measured ratio. For each value of a, b, and c, we explored an initially broad range of parameters that was iteratively narrowed down to increase the density of solutions. In addition, in order to give an overall score for each solution, we also calculated the best-fit solution that satisfies the maximum likelihood principle corresponding to the minimum of the Root Mean Squared (RMS) deviation weighted by the analytical uncertainties:

<figure><img src="image_8.png" /><figcaption>Fig. 4. Mo isotope pattern produced by the nuclear field shift (NFS) effect for which instrumental isotope fractionation has been corrected using the exponential law and normalized to $^{98}$Mo/$^{96}$Mo.</figcaption></figure>
$$RMS = \sqrt{\sum_{i=1}^{4} \left( \frac{\mu_{calc}^{i} - \mu_{meas}^{i}}{\sigma_{i}} \right)^{2}} \quad (3)$$

where $\mu_{calc}^i$ and $\mu_{meas}^i$ represent the calculated and the measured $\mu^i$Mo values for each isotope ratio, respectively, and $\sigma_i$ is the analytical uncertainty of $\mu_{meas}^i$.

The results of this deconvolution method are reported in Supplementary Table 5, where the NFS, apparent MIF, and fissiogenic anomaly are also reported in terms of calculated $\mu_{calc}^i$ values isolating the contribution of each effect. The values of {a, b, c} triplets corresponding to the best RMS values, as calculated with Eq. (3), are also reported in Supplementary Table 5.

## 4.4. Detection and origin of mass-independent signatures

Using the deconvolution method described above, we calculated the contribution of each type of fractionation (Supplementary Table 5). The results are reported as $\mu^iMo_{app}$, $\mu^iMo_{NFS}$, and $\mu^iMo_{Fiss}$, as explained above.

Although the ‘apparent MIF’ often represents the largest contribution, there are also some clear NFS signatures for all the samples from Niger, uranium ores and UOCs (Fig. 5, Supplementary Table 5, Figs. 5 and 6).

The fissiogenic signature was also detected in the ~3 Gyr-old samples from South Africa (Witwatersrand uranium and gold deposits) which are also characterized by a high U/Mo ratio (Fig. 5, Supplementary Table 5). All the South Africa samples carry a similar fissiogenic signature regardless of whether they represent a uranium ore (VRM 73/22), or uranium ore concentrates (ADU slurry and ADU powder).

Similarly, the U ore sample from McClean Lake (Canada), also exhibits a resolvable fissiogenic signature (Fig. 5, Supplementary Table 5). The Mongolia and Jordan uranium ore samples are characterized by a MIF with a dominant contribution from $\mu^iMo_{app}$, reflecting that the exponential law imperfectly corrects for natural mass-dependent isotope signatures therefore governed by other laws.

## 4.5. Origin of mass-independent fractionation signatures

A notable feature of our results is that the different uranium geographic origins, which includes a variety of deposit types, can be distinguished by the contributions of the mass-independent isotope signatures that add up to form the resulting measured MIF, according to the presence of (i) apparent MIF, (ii) NFS, and (iii) fissiogenic Mo anomaly.

Our results obtained for uranium ores and uranium ore concentrates from Niger show well-resolved mass-independent isotope anomalies that can be attributed to NFS. For Mo isotopes, this effect has been predicted theoretically (e.g. Bigeleisen, 1996) or described experimentally (Fujii et al., 2009) but up to now, it had not been observed in natural samples.

From a theoretical viewpoint, the NFS finds its origin in the presence of non-linearities in the increase in nuclear charge radius coupled with differences in electron densities at the nucleus between the molecular species involved in the geochemical processes of ore formation (Bigeleisen, 1996; Schauble, 2007). For Mo isotopes, neither the species, nor the variations in electron densities at the Mo nucleus are known, so it is not yet possible to fully interpret our NFS data. Yet, a comparison of the data obtained for the samples Niger SOM 1 and Mongolia-1 (Fig. 6), which are both sedimentary deposits with a sandstone lithology (Pagel

<figure><img src="image_9.png" /><figcaption>Fig. 5. Mo isotope signatures plotted in a $\mu^{97}$Mo vs $\mu^{100}$Mo diagram, showing examples for 4 samples of the measured (yellow square) and the associated deconvoluted MIF anomalies (apparent MIF in blue, NFS in red, and fissiogenic in green). For each contribution, all possible solutions are represented with colored dots with a larger symbol for higher likelihood (i.e. smaller associated RMS, see text). (a) TP20 (U ore from Niger) has a significant contribution from the NFS effect; (b) McClean Lake 1 (U ore from Canada) and (c) South African U ores have significant contributions from fissiogenic U, even for the solutions that are the least likely (see also Supplementary Fig. 3). The South African UOC (d) shows a signature similar to the U ore (c). The purple fields represent all the solutions obtained by MonteCarlo simulations.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Mo isotope signatures plotted in a µ⁹⁷Mo vs µ⁹⁵Mo diagram, showing examples of the measured (yellow square) and the associated deconvoluted MIF anomalies (apparent MIF in blue, NFS in red, and fissiogenic in green). For each contribution, all the possible solutions are represented with colored dots with a larger symbol for higher likelihood (i.e. smaller associated RMS, see text). Notably, for these samples, one can see that U ore (a) and UOC (b) from Niger have a significant contribution from NFS, even for the solutions that are the least likely, unlike for U ore samples from Mongolia (c) and Jordan (d), see also Supplementary Fig. 3. The purple fields represent all the solutions obtained by MonteCarlo simulations.</figcaption></figure>

et al., 2005; Rallakis, 2019), and which exhibit close δ⁹⁸Mo values (see Migeon et al., 2020), clearly show distinct mass-independent isotope signatures (Table 1 and Supplementary Table 5), both in terms of apparent MIF and NFS. This suggests that MIF signatures have the potential to discriminate geochemical processes that otherwise have an identical MDF signature (associated with the same isotopic pattern) represented by their δ⁹⁸Mo values. The various mass-dependent fractionation laws (Supplementary Table 1) will not produce the same magnitude in apparent MIF values for a given δ⁹⁸Mo value. Thus, one does not necessarily expect a correlation between δ⁹⁸Mo and apparent MIF values. This feature may explain the dispersion in µ¹Mo versus δ⁹⁸Mo diagram observed by Bermingham et al. (2024) in a couple of terrestrial samples. The apparent MIF in the Mongolia ore, that is smaller than for SOM-1 (from Niger), could be explained by a component of kinetic isotope fractionation in the Mongolia ore. In addition, the NFS effect is also smaller in the Mongolia-1 sample (if there is any; indeed a representation of all possible solutions shows that µ¹MoNFS = 0 belongs to the array of solutions, see Supplementary Fig. 3) compared with the TP20 Niger sample, which might again reflect a difference in the process leading to this NFS. One may speculate that this could be associated with a kinetic NFS that may also have led to a smaller residual MIF after MDF correction (using the exponential law) but it could also reflect a difference in the species involved in the respective natural environment during the chemical reactions associated with isotope fractionation. To fully understand these results will require experimental work reproducing the conditions of Mo precipitation in the context of U ore

deposits. Nevertheless, this shows that our new MIF signatures provide an important complementary isotope tool to MDF that could be used to examine and discriminate natural processes.

More specifically, the origin of NFS anomaly found in all the uranium ores from Niger are difficult to interpret due to the lack of precise constraints on the process of Mo precipitation in this geochemical setting. The low δ⁹⁸Mo found in the uranium ores were tentatively interpreted as due to Mo reduction and formation of sulfide species that should be enriched in light isotopes relative to molybdate (the most common form of Mo in oxidized solutions, Migeon et al., 2020). It is possible that this change, both in redox state and in ligand from O to S, was the cause for the observed NFS anomaly in these samples. Knyazev and Myasoedov (2001) had pointed out that the largest change in electron density at the nucleus is expected to be associated with redox change, so a redox reaction is likely to produce the largest NFS. However, there is no experimental or theoretical work to confirm that the associated change in electron density at the nucleus for Mo is of the right magnitude. A second important observation that may confirm this tentative explanation, particularly relevant to the Niger samples, is that the NFS (represented by µ¹MoNFS, Supplementary Table 5) and MDF signatures (as reported in Migeon et al., 2020) appear to be coupled (Fig. 7). According to theoretical considerations, this is not particularly surprising since changes in redox state (Mo (IV) to Mo (VI) in our case) are also expected to produce the largest MDF (Schauble, 2004).

Interestingly, two of the UOC samples from the Somäir uranium deposit plot off the correlation (Fig. 7). This effect can be attributed to
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** Correlation between the NFS contribution to the MIF signature and the MDF of Niger samples. The µNFS data are shown for the best RMS values. The uncertainties of the delta values are smaller than the symbol. Note that the two UOC samples from Somair mill are shifted to lower values as shown by the blue arrow. This can be attributed to an additional MDF during ore processing that did not induce any further NFS effect.</figcaption></figure>

several differences between the Cominak and Somair ore processing plants. First, the final precipitation as a diuranate salt at Somair is in the form of Na₂U₂O₇ (SDU) rather than MgU₂O₇ (MDU) at Cominak. It has been shown that the average bond length of Mo-O in sodium molybdate is longer than that of magnesium molybdate (Matsumoto et al., 1975; Hardcastle and Wachs, 1990). This would be consistent with an enrichment in light Mo isotopes in the SDU compared to the MDU (which is observed). Another difference lies in the methods used for the removal of Mo in the two plants. In the Cominak plant, Mo is removed by solvent extraction, while in the Somair plant it is done by washing during filtration (ORANO, pers. comm.).

It is well known that uranium ores can be rather heterogeneous, chemically but also isotopically (e.g. Migeon et al., 2020). The heterogeneity of ores is obvious in Niger samples (see Mo/U in Table 1) but UOCs should be significantly less heterogeneous because in most uranium mills there is a blending of all ores so as to obtain a relatively constant composition favorable for reproducible industrial operations.

The samples from Canada and South Africa are both characterized by well resolved and similar fissiogenic anomalies. These samples are much older (South Africa ~3 Gyr, Canada ~1.3 Gyr), and have evolved with a high enough U/Mo ratio to generate these signatures. These original Mo isotope anomalies, due to the addition of fissiogenic ⁹⁵Mo, ⁹⁷Mo, ⁹⁸Mo, and ¹⁰⁰Mo produced by the natural spontaneous fission of ²³⁸U and ²³⁵U, offer a clear discriminating tool in the context of nuclear forensics (Figs. 5 and 6, Supplementary Table 5).

Future work will extend our approach to search for specific mass-independent isotope fractionation and anomalies in other isotope systems in order to provide multidimensional constraints that will further increase the level of discrimination between localities.

An important feature of our results, which is particularly relevant to a nuclear forensics application, is that the ore processing does not erase the NFS effect or fissiogenic anomaly. For example, the isotope signature left by the presence of fissiogenic Mo will be preserved through ore processing steps, as shown by samples from South Africa starting from the ore (VRM 73/22) to the precipitates (ADU slurry and ADU powder). The same observation can be made for NFS effect which is preserved in all the uranium ore concentrates from Niger with a similar magnitude to those of the uranium ores, suggesting that these MIF signatures were mainly generated during ore formation. It is thus important to

emphasize that the NFS and fissiogenic signatures are preserved after ore processing, which is not the case of mass-dependent signature (or apparent MIF). As shown in the studies of Migeon et al. (2018, 2020), ore processing can lead to Mo isotope fractionation during leaching, solvent extraction or uranate precipitation. This feature is a limitation in using only Mo isotope MDF signatures for provenance studies on UOCs in the framework of nuclear forensics. Conversely, there is a clear incitation to search for MIF signatures.

## 5. Applications of Mo MIF to geochemistry

The nuclear field shift effect has already been documented with other heavy isotope systems, as reviewed in Moynier et al. (2013) for Hg, Tl and U isotopes. For example, uranium isotopes have been used to study bioreduction mechanisms of uranium (e.g. Sato et al., 2021; Stylo et al., 2015) or the oxygenation of the oceans over time (e.g. Tissot and Dauphas, 2015). For uranium isotopes, it has been shown theoretically that the largest part of the observed ²³⁵U-²³⁸U fractionation can be ascribed to an NFS effect. Similarly, for Tl isotopes, it is estimated that 75 % of the fractionation between ²⁰³Tl and ²⁰⁵Tl is due to the NFS effect (Moynier et al., 2013). However, owing to the presence of only two isotopes for these two systems, it is difficult to discriminate pure MDF from the NFS effect. In contrast, Mo isotopes have more than two isotopes, thus, despite the smaller effect (due to a lower mass), a specific isotope pattern is associated to the NFS effect, making it easier to identify in this case. Similarly to Hg, Tl and U, one can envisage numerous applications of the mass-independent signatures of Mo isotopes in geochemistry beyond the study of uranium ores and ore concentrates for nuclear forensics. Our observations of MIF signatures in natural samples indicate that such signatures are likely to be found in other geological settings, as the NFS signatures originating from generic geochemical processes are not specific to the formation of uranium ores. For example, the NFS effect could be used to better understand Mo pathways in sediments as discussed in the introduction, where the complex interplay of redox and adsorption processes control Mo isotope fractionation (e.g. Siebert et al., 2003; Dickson et al., 2017). If the isotope fractionation resulting from the NFS effect is indeed dominated by redox effects, it may be useful to discriminate redox driven fractionation from other processes that have been identified in sedimentary sinks (e.g. Kendall et al., 2017). Similarly, the NFS effect for Mo isotopes could be used to decipher processes associated with Mo ore formation (e.g. Li et al., 2019). The NFS effect is likely to be detected in those environments, as redox processes are often called upon for interpreting observations (Greber et al., 2014; Shafiei et al., 2015). Last, as Mo is often associated with organic matter, it has been used to trace source rocks in oil deposits (Ventura et al., 2015; Kendall et al., 2023). Thus, this may become an additional application of mass-independent fractionation of Mo isotopes. Overall, this suggests that there could be numerous applications of mass-independent signatures of Mo in geochemistry that largely extend beyond uranium ore deposits.

## 6. Conclusions

Our study demonstrates that when taking sufficient care in the quantification of the multiple stable isotopes of Mo with an uncertainty in the 1–10 ppm range, it is possible to detect unambiguously the presence of MIF signatures in natural samples. Given the small magnitude of these fractionations and anomalies, it is not surprising that they have not been reported so far in the literature. As shown above, this type of signature has a strong potential for nuclear forensics. The original MIF signatures, indicative of the ore provenance, contributed from NFS effect and from fissiogenic origin, are preserved after ore processing. The detection of Mo MIF signatures in our natural samples shows that they represent a powerful new tracer for a broad range of geochemical applications, and with the ability to discriminate processes that are still unresolved at the present time when only considering MDF.
## Data and materials availability

All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials.

### CRediT authorship contribution statement

**Caroline Fitoussi:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **Eric Pili:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **Elsa Yobregat:** Writing – review & editing, Methodology, Investigation, Formal analysis. **Mathieu Touboul:** Writing – review & editing, Methodology, Investigation, Formal analysis, Data curation. **Cécile Gardin:** Writing – review & editing, Formal analysis, Data curation.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

The South African samples were provided by the Joint Research Centre Karlsruhe (JRC Karlsruhe); the sample series comes from the EC Support Program to the IAEA (EC A 1753) while the uranium ore VRM 73/22 was kindly given by Zolt Varga and colleagues. We thank Orano France and particularly Marc Brouand for providing samples from Niger, Mongolia, and Jordan. We thank Orano Canada for providing samples from the McClean Lake mill. This study was funded in the framework of a CEA-ENS Lyon-CNRS-UCBL collaboration and by the Labex LIO (10-LABX-0066). We thank Katherine Bermingham and an anonymous reviewer for their comments that helped improve the manuscript. We are grateful to Christophe Moulin of the CEA for his enthusiasm for this project and his encouragement to see it through.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.epsl.2025.119459.

## Data availability

Data will be made available on request.

## References

* Abe, M., Hada, M., Suzuki, T., Fujii, Y., Hirao, K., 2014. Theoretical study of isotope enrichment caused by nuclear volume effect. J. Comput. Chem. Jpn. 13, 92–104.

* Abed, A.M., 1989. On the genesis of the phosphorite-chert association of the Amman Formation in the Tel es Sur area. Ruseifa, Jordan. Sci. Géol., Bull. 42, 141–153.

* Andersen, M.B., Stirling, C.H., Weyer, S., 2017. Uranium isotope fractionation. Rev. Min. Geochem. 82, 799–850.

* Barling, J., Anbar, A.D., 2004. Molybdenum isotope fractionation during adsorption by manganese oxides. Earth Planet. Sci. Lett. 217, 315–329.

* Bigeleisen, J., Mayer, M.G., 1947. Calculation of equilibrium constants for isotopic exchange reactions. The J. Chem. Phys. 15, 261–267.

* Bigeleisen, J., 1949. The relative reaction velocities of isotopic molecules. The J. Chem. Phys. 17, 675–678.

* Bigeleisen, J., 1996. Nuclear size and shape effects in chemical reactions. Isotope chemistry of the heavy elements. J. Am. Chem. Soc. 118, 3676–3680.

* Bourdon, B., Fitoussi, C., 2020. Isotope fractionation during condensation and evaporation during planet formation processes. ACS Earth Space Chem. 4 (8), 1408–1423.

* Budde, G., Tissot, F.L., Kleine, T., Marquez, R.T., 2023. Spurious molybdenum isotope anomalies resulting from non-exponential mass fractionation. Geochemistry 83, 126007.

* Bermingham, K.R., Tornabene, H.A., Walker, R.J., Godfrey, L.V., Meyer, B.S., Piccoli, P., Mojszis, S.J., 2024. The non-carbonaceous nature of Earth’s late-stage accretion. Geochim. Cosmochim. Acta 392, 38–51.

* Carlson, R., Heinrich, D., Turekian, K., 2014. Thermal ionization mass spectrometry. Treatise on Geochem 15, 337–354.

* Cumming, G.L., Krstic, D., 1992. The age of unconformity-related uranium mineralization in the Athabasca Basin, northern Saskatchewan. Can. J. Earth Sci. 29, 1623–1639.

* De Boiszon, H., Levy, L., Jakymiw, C., Distinguin, M., Guerin, F., Descostes, M., 2020. Modeling uranium and $^{226}$Ra mobility during and after an acidic in situ recovery test (Dulaan Uul, Mongolia). J. Contaminant Hydrology 235, 103711.

* Dickson, A.J., 2017. A molybdenum-isotope perspective on Phanerozoic deoxygenation events. Nat. Geosci. 10, 721–726.

* Duan, Y., Anbar, A.D., Arnold, G.L., Lyons, T.W., Gordon, G.W., Kendall, B., 2010. Molybdenum isotope evidence for mild environmental oxygenation before the Great Oxidation Event. Geochim. Cosmochim. Acta 74, 6655–6668.

* Erickson, B.E., Helz, G.R., 2000. Molybdenum (VI) speciation in sulfidic waters: stability and lability of thiomolybdates. Geochim. Cosmochim. Acta 64, 1149–1158.

* Frimmel, H.E., 2005. Archaean atmospheric evolution: evidence from the Witwatersrand gold fields. South Africa. Earth Sci. Rev. 70, 1–46.

* Fujii, T., Moynier, F., Albarède, F., 2009. The nuclear field shift effect in chemical exchange reactions. Chem. Geol. 267, 139–156.

* Goldberg, T., Archer, C., Vance, D., Poulton, S.W., 2009. Mo isotope fractionation during adsorption to Fe (oxyhydr) oxides. Geochim. Cosmochim. Acta 73 (21), 6502–6516.

* Greber, N.D., Pettke, T., Nägler, T.F., 2014. Magmatic-hydrothermal molybdenum isotope fractionation and its relevance to the igneous crustal signature. Lithos 190, 104–110.

* Hardcastle, F.D., Wachs, I.E., 1990. Determination of molybdenum-oxygen bond distances and bond orders by raman spectroscopy. J. Raman Spectrosc. 21, 683–691.

* Helz, G.R., Miller, C.V., Charnock, J.M., Mosselmans, J.F.W., Pattrick, R.A.D., Garner, C. D., Vaughan, D.J., 1996. Mechanism of molybdenum removal from the sea and its concentration in black shales: EXAFS evidence. Geochim. Cosmochim. Acta 60, 3631–3642.

* Hoeve, J., Quirt, D., 1987. A stationary redox front as a critical factor in the formation of high-grade, unconformity-type uranium ores in the Athabasca basin, Saskatchewan, Canada. Bull. Mineral. 110, 157–171.

* IAEA, 1993. UraniumExtraction Technology. Technical reports series n°359. International Atomic Energy Agency, Vienna, p. 380.

* Kendall, B., Dahl, T.W., Anbar, A.D., 2017. The stable isotope geochemistry of molybdenum. Rev. Mineral. Geochem. 82, 683–732.

* Kendall, B., Wang, S., Lillis, P., Xing, L., Zheng, W., Zhu, C., 2023. Evaluation of the molybdenum isotope system as a petroleum tracer: the Phosphoria petroleum system, western USA. Chem. Geol. 617, 121244.

* King, W.H., 1984. Isotope Shifts in Atomic Spectra (Physics of Atoms and Molecules). Plenum, New York.

* Knyazev, D.A., Myasoedov, N.F., 2001. Specific effects of heavy nuclei in chemical equilibrium. Sep. Sci. Technol. 36, 1677–1696.

* Li, Y., McCoy-West, A.J., Zhang, S., Selby, D., Burton, K.W., Horan, K., 2019. Controlling mechanisms for molybdenum isotope fractionation in porphyry deposits: the Qulong example. Econ. Geol. 114 (5), 981–992.

* Lu, Q., Masuda, A., 1994. The isotopic composition and atomic weight of molybdenum. Int. J. Mass Spectrom. 130, 65–72.

* Mamadou, M.M., Cathelineau, M., Deloule, E., Reisberg, L., Cardon, O., Vallance, J., Brouand, M., 2022. The Tim Mersoï basin uranium deposits (northern Niger): geochronology and genetic model. Ore Geol. Rev. 145, 104905.

* Maréchal, C.N., Télouk, P., Albarède, F., 1999. Precise analysis of copper and zinc isotopic compositions by plasma-source mass spectrometry. Chem. Geol. 156, 251–273.

* Marks, N.E., Borg, L.E., Eppich, G.R., Gaffney, A.M., Genneti, V.G., Hutcheon, I.D., Kristo, M.J., Lindvall, R.E., Ramon, C., Robel, M., Roberts, S.K., Schorzman, K.C., Sharp, M.A., Singleton, M.J., Williams, R.W., 2015. Technical report on the behavior of trace elements, stable isotopes, and radiogenic isotopes during the processing of uranium ore to uranium ore concentrate, (LLNL, United States). https://doi.org/10.2172/1243009.

* Masuda, A., Takahashi, K., Kawashima, A., 1993. A spike of $^{98}_{40}\text{Mo}$ in yields of uranium spontaneous fission an extension of shell structure effect. Proc. Jpn. Acad. Ser. B 69, 155–160.

* Matsumoto, K., Kobayashi, A., Sasaki, Y., 1975. The crystal structure of sodium molybdate dihydrate, $Na_2MoO_4 \cdot 2H_2O$. Bull. Chem. Soc. Jpn. 48, 1009–1013.

* Migeon, V., Bourdon, B., Pili, E., Fitoussi, C., 2018. Molybdenum isotope fractionation during acid leaching of a granitic uranium ore. Geochim. Cosmochim. Acta 231, 30–49.

* Migeon, V., Fitoussi, C., Pili, E., Bourdon, B., 2020. Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: towards a new nuclear forensic tool. Geochim. Cosmochim. Acta 279, 238–257.

* Moynier, F., Fujii, T., Brennecka, G.A., Nielsen, S.G., 2013. Nuclear field shift in natural environments. Comptes Rendus. Géoscience 345 (3), 150–159.

* Nägler, T.F., Neubert, N., Böttcher, M.E., Dellwig, O., Schnetger, B., 2011. Molybdenum isotope fractionation in pelagic euxinia: evidence from the modern Black and Baltic Seas. Chem. Geol. 289, 1–11.

* Nielsen, S.G., Rehkämper, M., Prytulak, J., 2017. Investigation and application of thallium isotope fractionation. Rev. Mineral. Geochem. 82, 759–798.
Nwaila, G.T., Durrheim, R.J., Jolayemi, O.O., Maselela, H.K., Jakaitė, L., Burnett, M., Zhang, S.E., 2020. Significance of granite-greenstone terranes in the formation of Witwatersrand-type gold mineralisation - A case study of the Neoarchaean Black Reef Formation, South Africa. Ore Geol. Rev. 121, 103572.

Pagel, M., Cavellec, S., Forbes, P., Gerbaud, O., Vergely, P., Wagani, I., Mathieu, R., 2005. Uranium deposits in the Arlit area (Niger). In: Mineral Deposit Research: Meeting the Global Challenge: Proceedings of the Eighth Biennial SGA Meeting Beijing, China, 18-21 August 2005. Springer, pp. 303–305.

Philippe, S., Lancelot, J.R., Clauer, N., Pacquet, A., 2011. Formation and evolution of the Cigar Lake uranium deposit based on U-Pb and K-Ar isotope systematics. Can. J. Earth Sci. 30, 720–730.

Phillips, R., Xu, J., 2021. A critical review of molybdenum sequestration mechanisms under euxinic conditions: implications for the precision of molybdenum paleoredox proxies. Earth Sci. Rev. 221, 103799.

Rallakis, D., 2019. The Metallogenic System and the Redox Trap in the Roll-Front Type Uranium Deposit of Zoovch Ovoo, Mongolia. Role of the C and S phases. Université de Lorraine. PhD thesis.

Rallakis, D., Michels, R., Brouand, M., Parize, O., Cathelineau, M., 2019. The role of organic matter on uranium precipitation in Zoovch Ovoo, Mongolia. Minerals 9, 310.

Reinhard, C.T., Planavsky, N.J., Robbins, L.J., Partin, C.A., Gill, B.C., Lalonde, S.V., Bekker, A., Konhauser, K.O., Lyons, T.W., 2013. Proterozoic ocean redox and biogeochemical stasis. Proc. Natl Acad. Sci. 110, 5357–5362.

Rolison, J.M., Druce, M., Shollenberger, Q.R., Kayzar-Boggs, T.M., Lindvall, R.E., Wimpenny, J., 2019. Molybdenum isotope compositions of uranium ore concentrates by double spike MC-ICP-MS. Appl. Geochem. 103, 97–105.

Russell, W., Papanastassiou, D., Tombrello, T., 1978. Ca isotope fractionation on the earth and other solar system materials. Geochim. Cosmochim. Acta 42, 1075–1090.

Sato, A., Bernier-Latmani, R., Hada, M., Abe, M., 2021. Ab initio and steady-state models for uranium isotope fractionation in multi-step biotic and abiotic reduction. Geochim. Cosmochim. Acta 307, 212–227.

Schauble, E.A., 2004. Applying stable isotope fractionation theory to new systems. Rev. Mineral. Geochem. 55, 65–111.

Schauble, E.A., 2007. Role of nuclear volume in driving equilibrium stable isotope fractionation of mercury, thallium, and other very heavy elements. Geochim. Cosmochim. Acta 71, 2170–2218.

Schauble, E.A., 2013. Modeling nuclear volume isotope effects in crystals. Proc. Natl. Acad. Sci 110, 17714–17717.

Shafiei, B., Shamanian, G., Mathur, R., Mirnejad, H., 2015. Mo isotope fractionation during hydrothermal evolution of porphyry Cu systems. Miner. Deposita 50, 281–291.

Siebert, C., Nägler, T.F., von Blanckenburg, F., Kramers, J.D., 2003. Molybdenum isotope records as a potential new proxy for paleoceanography. Earth Planet. Sci. Lett. 211 (1–2), 159–171.

Stirling, C.H., Andersen, M.B., Potter, E.K., Halliday, A.N., 2007. Low-temperature isotopic fractionation of uranium. Earth Planet. Sci. Lett. 264, 208–225.

Stylo, M., Neubert, N., Wang, Y., Monga, N., Romaniello, S.J., Weyer, S., Berlier-Latmani, R., 2015. Uranium isotopes fingerprint biotic reduction. Proc. Natl. Acad. Sci. 112, 5619–5624.

Them, T.R., Gill, B.C., Caruthers, A.H., Gerhardt, A.M., Gröcke, D.R., Lyons, T.W., Marroquin, S.M., Nielsen, S.G., Trabucho Alexandre, J.P., Owens, J.D., 2018. Thallium isotopes reveal protracted anoxia during the Toarcian (early Jurassic) associated with volcanism, carbon burial, and mass extinction. Proc. Natl. Acad. Sci. 115, 6596–6601.

Tissot, F.L., Dauphas, N., 2015. Uranium isotopic compositions of the crust and ocean: age corrections, U budget and global extent of modern anoxia. Geochim. Cosmochim. Acta 167, 113–143.

Tourigny, G., Quirt, D.H., Wilson, N.S.F., Wilson, S., Breton, G., Portella, P., 2007. Geological and structural features of the Sur C uranium deposit, McClean Lake area. Saskatchewan. Bull. Geol. Surv. Can. 588, 229–247.

Varga, Z., Krajko, J., Penkin, M., Novak, M., Eke, Z., Wallenius, M., Mayer, K., 2017. Identification of uranium signatures relevant for nuclear safeguards and forensics. J. Radioanal. Nucl. Chem. 312, 639–654.

Ventura, G.T., Gall, L., Siebert, C., Prytulak, J., Szatmari, P., Hürlimann, M., Halliday, A. N., 2015. The stable isotope composition of vanadium, nickel, and molybdenum in crude oils. Applied Geochemistry, 59, 104–117.

Wieser, M., De Laeter, J., 2004. Spontaneous fission yields for <sup>238</sup>U in molybdenum measured in archaean zircons. J. Radioanal. Nucl. Chem. 261, 95–100.

Wombacher, F., Rehkämper, M., 2003. Investigation of the mass discrimination of multiple collector ICP-MS using neodymium isotopes and the generalised power law. J. Anal. At. Spectrom. 18, 1371–1375.

Wombacher, F., Rehkämper, M., Mezger, K., 2004. Determination of the mass-dependence of cadmium isotope fractionation during evaporation. Geochim. Cosmochim. Acta 68, 2349–2357.

Wombacher, F., Rehkämper, M., Mezger, K., Bischoff, A., Münker, C., 2008. Cadmium stable isotope cosmochemistry. Geochim. Cosmochim. Acta 72, 646–667.

Worsham, E.A., Walker, R.J., Bermingham, K.R., 2016. High-precision molybdenum isotope analysis by negative thermal ionization mass spectrometry. Int. J. Mass Spectrom. 407, 51–61.

Yobregat, E., Fitoussi, C., Pili, E., Touboul, M., 2022. High-precision measurements of Mo isotopes by N-TIMS. Int. J. Mass Spectrom. 476, 116846.

Young, E.D., Galy, A., Nagahara, H., 2002. Kinetic and equilibrium mass-dependent isotope fractionation laws in nature and their geochemical and cosmochemical significance. Geochim. Cosmochim. Acta 66, 1095–1104.