Molybdenum Isotopes Research Articles

Molybdenum isotope systematics in cumulate rock of the 2.8 Windimurra layered intrusion: A test for igneous differentiation and the composition of the Archean mantle

Molybdenum isotopes (reported as δ98Mo relative to NIST-3134) show resolvable isotope differences in igneous rocks with the continental crust being markedly heavier in isotope composition than mid-ocean ridge lavas, lunar basalts or the Earth’s mantle. The tholeiitic differentiation series at the intra-plate Hekla volcano (Iceland) shows no resolvable Mo isotope differences from basaltic to rhyolitic compositions. In contrast, convergent margin lavas show a transition from isotopically lighter mantle to heavy continental crust, suggesting that subduction processes drive continental crust towards heavier values. Archean komatiitic lavas, presumed probes of the Archean mantle, have Mo isotope values identical to modern depleted mantle, raising the questions if and how the Mo isotope crust-mantle disparity developed so early in Earth’s history. Here we present new Mo isotope data for a set of cumulate rocks from the Upper Zone of late Archean (2.8 Ga) Windimurra Igneous Complex, a mafic/ultramafic layered intrusion. The intrusion is not subduction related and contains no apparent primary hydrous minerals. We tested the effect of crystal fractionation on Mo isotopes in relatively dry melt along a tholeiitic liquid line of descent by using the cumulate effect of normally anhydrous minerals in the layered intrusion. Near mono-mineralic olivine-pyroxene-rich, feldspar-rich and Fe-Ti-rich oxides show small variations (~0.15‰) in Mo isotope signatures. This is consequently to predominantly isotopically light Fe-Ti-oxide-rich and isotopically heavier feldspar-rich rocks, respectively. This is suggesting minor Mo isotope fractionation, even in dry, tholeiitic systems, which however, counterbalance each other and thus potentially remain undetected. On average, the Windimurra mantle source is indistinguishable, or slightly isotopically lighter than the Mo isotope signature of komatiites. This is reinforcing an isotopically light Mo isotope signature of Archean mantle sources of high-degree mantle melts and is extending these signatures to predominantly mafic Archean crust. It remains to be tested if Archean felsic crust resembles modern continental crust in its heavy isotope values and to which extend the mantle was already isotopically depleted in Mo isotopes at Mesoarchean time.

Development of Molybdenum Isotopic Certified Reference Materials with SI‐traceable Property Values

Molybdenum isotopes have been widely studied and applied to various fields of Earth and planetary sciences in recent years. Inspired by the increasing demand for reliable and comparable measurement data, three molybdenum isotopic reference materials have been developed by the National Institute of Metrology (China), including one natural‐abundance iCRM (GBW 04504) and two isotopically enriched iCRMs of 97Mo (GBW 04505) and 100Mo (GBW 04506), respectively. The property values of isotope amount ratios and isotopic abundances of these iCRMs have been certified by fully calibrated mass spectrometry using a multi‐collector inductively coupled plasma‐mass spectrometer. The instrumental mass bias correction was performed using five synthetic isotope mixtures, which were gravimetrically prepared by all seven high‐purity molybdenum isotope materials. The determined absolute molybdenum isotopic compositions of GBW 04504, 04505 and 04506 are thus traceable to the SI units kilogram and mole. On the basis of the assessment in ISO Guide 35: 2017, these iCRMs exhibited sufficient homogeneity and stability for the application of molybdenum isotope‐ratio determinations. The δ98/95MoNIST SRM 3134 value of GBW 04504 was measured as −0.31 ± 0.07‰ (1sm) relative to the delta zero reference material NIST SRM 3134 (Lot No. 130418).

Geochemistry and molybdenum isotopes of the basal Datangpo Formation: Implications for ocean-redox conditions and organic matter accumulation during the Cryogenian interglaciation

The basal Datangpo Formation in South China deposited in the Cryogenian interglacial period has gained increasing interest with the well development of organic-rich black shale and its hosted manganese (Mn) deposits (Mn carbonate), which contain important clues to the paleoclimate and ocean redox of this critical interval in Earth's history. The depositional environment of Mn deposits is still uncertain, as suggested by different suites of redox-indicating parameters. In addition, the Mn deposits are notable for their enrichment in organic matter (OM), despite the fact that the diagenetic origin for the Mn deposits would result in large consumption of OM. To gain insight into these issues, we present the geochemical and molybdenum (Mo) isotopic data of black shale and Mn carbonate samples from a new section. The Datangpo Mn carbonates are characterized by low Mo content (mostly <3.2 ppm) and slightly light Mo isotopic composition (mostly from +0.07‰ to +0.52‰), suggesting that Mo, once adsorbed in Mn-oxide particles, was released in suboxic or anoxic (non-sulfidic) pore water due to Mn-oxide reduction and then diffused back into the water column. Hence the extremely light Mo isotopic signatures might be rarely preserved in sediments. The Mn carbonate formation accompanied by OM re-mineralization within sediments may have caused lower total organic carbon (TOC) content relative to that in black shales. However, elevated nutrient input related with hydrothermal fluid activity probably enhanced primary productivity during the Mn deposition period. It is evidenced by: (1) the Mn-TOC-total inorganic carbon covariation displayed by the Datangpo section and (2) a positive correlation between TOC and Mn content when Mn content is higher than 10% in a large compiled dataset. A compilation of Mo concentration and Mo isotope data suggests that widespread anoxia prevailed during the Cryogenian interglaciation. Instead, Neoproterozoic ocean oxygenation was postponed to the Ediacaran.

Production of highly concentrated intermediate molybdenum isotopes in optimal cascade with two additional product flows

The problem of highly concentrated molybdenum isotopes production in multiflow cascades is considered. A method of the cascade with two additional product flows optimization has been developed. Two components intermediate in weight are concentrated simultaneously in those flows. The optimization problem is solved using a variation of the partial flow cuts of the cascade stages with large separation coefficients. The optimization criterion is the minimum of the stage total feed flow while ensuring a given concentration of isotopes. A computational experiment was carried out to separate a mixture of molybdenum hexafluoride. The experiment demonstrated the features of intermediate components concentrating in additional product flows of the cascade.

Molybdenum Isotope Constraints on the Origin of Vanadium Hyper-Enrichments in Ediacaran–Phanerozoic Marine Mudrocks

Vanadium is an important redox-sensitive trace metal for paleoenvironmental reconstructions. Modern organic-rich sediments persistently contain sediment V enrichments &lt;500 μg/g, but many ancient marine organic-rich mudrocks record enrichments &gt;500 μg/g. Previous studies propose that ancient V enrichments of these magnitudes (“V hyper-enrichments”) were deposited from hyper-sulfidic bottom-waters with higher H2S levels (≥10 mM) than observed in modern euxinic basins. To test the importance of hyper-sulfidic conditions for generating V hyper-enrichments, we compare V concentrations with Mo isotope (δ98Mo) compositions from mudrock samples ranging in age from Ediacaran to Pleistocene. In the modern ocean, sediments deposited from strongly euxinic bottom waters ([H2S]aq &gt; 11 μM) closely record global seawater δ98Mo because conversion of molybdate to tri- and tetra-thiomolybdate is quantitative. By contrast, large Mo isotope fractionations occur during Mo adsorption to Fe-Mn particulates or because of incomplete formation of the most sulfidic thiomolybdates in weakly euxinic settings ([H2S]aq &lt; 11 μM), which both favor removal of lighter-mass Mo isotopes to sediments. We find multiple examples when mudrocks with V hyper-enrichments are associated with a wide range of δ98Mo for a single time interval, including values at or below oceanic input δ98Mo (0.3–0.7‰). This observation suggests significant isotopic offset from reasonable seawater values (typically ≥1.0‰). Thus, we conclude that hyper-sulfidic conditions were not responsible for many V hyper-enrichments in Ediacaran–Phanerozoic mudrocks. Instead, sediment V hyper-enrichments can be explained by high Fe-Mn particulate fluxes to weakly euxinic sediments or by moderately restricted euxinic settings with strongly euxinic ([H2S]aq &gt; 11 μM but not necessarily &gt; 10 mM) or weakly euxinic (with slow clastic sedimentation rates and high organic carbon fluxes) bottom waters where vigorous water exchange provides a continuous V supply from the open ocean.

Astronomical context of Solar System formation from molybdenum isotopes in meteorite inclusions.

Calcium-aluminum-rich inclusions (CAIs) in meteorites are the first solids to have formed in the Solar System, defining the epoch of its birth on an absolute time scale. This provides a link between astronomical observations of star formation and cosmochemical studies of Solar System formation. We show that the distinct molybdenum isotopic compositions of CAIs cover almost the entire compositional range of material that formed in the protoplanetary disk. We propose that CAIs formed while the Sun was in transition from the protostellar to pre-main sequence (T Tauri) phase of star formation, placing Solar System formation within an astronomical context. Our results imply that the bulk of the material that formed the Sun and Solar System accreted within the CAI-forming epoch, which lasted less than 200,000 years.

Timing Solar System formation

Cosmochemistry The oldest solids that formed in the Solar System are calcium-aluminium–rich inclusions (CAIs), small metallic droplets that were later incorporated into meteorites. The ages of CAIs are conventionally taken as the age of the Solar System, but which exact moment in star formation they correspond to has been unclear. Brennecka et al. measured molybdenum isotope ratios in CAIs and found a wide range of origins in both the inner and outer Solar System. They propose that CAIs formed from heterogeneous material accreting from the presolar nebula and that the ages of CAIs coincide with the Sun's transition from a protostar to a pre–main sequence star. Science , this issue p. [837][1] [1]: /lookup/doi/10.1126/science.aaz8482

SUPERCRITICAL FLUID EXTRACTION OF MOLYBDENUM COMPLEXES AND ITS ISOTOPES WITH CARBON DIOXIDE

Separation of molybdenum complexes and its isotopes by supercritical fluid extraction (SFE) with carbon dioxide was experimentally investigated. The type of modifier, which provides increasing the efficiency of the metal and its isotopes separation, was determined. Extraction of molybdenum isotope complexes was carried out in the updated SFE-U unit at a constant value of the initial pressure P = 20 MPa and constant temperatures of the upper (T1 = 35 ⁰C) and bottom (T2 = 45 ⁰C) flanges of the extraction chamber. The exposure time of the extract and the level along the extraction chamber height, from which the eluent was sampled, were set in the experiments. It is shown, that at a certain sampling height and at certain exposure times, change in the natural content of Mo isotopes in the extracts takes place.

Cross-sections for , and reactions on molybdenum isotopes in the neutron energy range of 13 to 15 MeV * * Supported by the National Natural Science Foundation of China (11875016, 11565012, 11165007)

Given the insufficient cross-sectional data regarding the 14-MeV-neutron experiment of molybdenum, the vital fusion reactor structural material, and the significant heterogeneities among the reported values, this study examined the (n,2n), (n,α), (n,p), (n,d), and (n,t) reaction cross sections in molybdenum isotopes based on the neutrons produced via a T(d,n)4He reaction carried out in the Pd-300 Neutron Generator at the China Academy of Engineering Physics (CAEP). A high-resolution gamma-ray spectrometer, which was equipped with a coaxial high-purity germanium detector, was used to measure the product nuclear gamma activities. In addition, 27Al(n,α)24Na and 93Nb(n,2n)92mNb reactions were utilized as the neutron fluence standards. The experimental 92Mo(n,2n)91Mo, 94Mo(n,2n)93mMo, 100Mo(n,2n)99Mo, 98Mo(n,α)95Zr, 100Mo(n,α)97Zr, 92Mo(n,p)92mNb, 96Mo(n,p)96Nb, 97Mo(n,p)97Nb, 98Mo(n,p)98mNb, 92Mo(n,d)91mNb, and 92Mo(n,t)90Nb reaction cross sections were acquired within the 13–15 MeV neutron energy range. Thereafter, we compared and analyzed these obtained cross sections based on the existing IAEA-EXFOR database-derived experimental data, together with evaluation results corresponding to ENDF/B-VIII.0, JEFF-3.3, BROND-3.1, and CENDL-3.1 and the theoretical outcomes acquired through TALYS-1.95 and EMPIRE-3.2.3 (nuclear-reaction modeling tools).

Molybdenum isotope ratios in Izu arc basalts: The control of subduction zone fluids on compositional variations in arc volcanic systems

Molybdenum isotope variations in mafic arc lavas have mainly been attributed to the influence of slab-derived components, such as subducted sediment melts and aqueous fluids. The latter have been hypothesised to fractionate Mo isotopes through interaction with the oceanic crust and carry an isotopically heavy signal that is transferred to the source of arc magmas. Thus, understanding Mo isotope systematics in subduction zones requires characterising the Mo isotope composition of slab-derived fluids and their influence on the Mo isotope budget of arc magmas. However, Mo isotope data reported to date show a considerable influence from subducted sediments that complicate accurate constraints being placed on the fluid contribution. We present Mo isotope data for mafic lavas from the Izu arc, a highly depleted oceanic island arc whose magma compositions show a dominant control from slab-derived fluids. The lavas from the Izu volcanic front are isotopically heavier than MORB and the depleted mantle. Their δ98/95Mo (the relative difference in measured 98Mo/95Mo to NIST 3134) systematically varies with indicators for fluid-mobile element enrichment, suggesting that slab-derived fluids in the Izu arc have heavy Mo isotope compositions. Additionally, co-variations with radiogenic 143Nd/144Nd and 176Hf/177Hf point to a relationship between the addition of aqueous fluids and compositional heterogeneity of the sub-arc mantle. We present mass balance models that show that the influence of subduction zone fluids on the trace element pattern of arc magmas is more dominant when these are added to a more depleted and refractory sub-arc mantle, which preferentially melts due to a relatively higher fluid flux. The mass balance of Mo in the Izu arc predicts a light Mo isotope composition for the residual oceanic crust as a result of the preferential removal of isotopically heavy Mo during slab dehydration, consistent with previous suggestions for the Mariana arc and isotopically light Mo previously reported for eclogites.

Compressional-mode resonances in the molybdenum isotopes: Emergence of softness in open-shell nuclei near A = 90

“Why are the tin isotopes soft?” has remained, for the past decade, an open problem in nuclear structure physics: models which reproduce the isoscalar giant monopole resonance (ISGMR) in the “doubly-closed shell” nuclei, 90Zr and 208Pb, overestimate the ISGMR energies of the open-shell tin and cadmium nuclei, by as much as 1 MeV. In an effort to shed some light onto this problem, we present results of detailed studies of the ISGMR in the molybdenum nuclei, with the goal of elucidating where–and how–the softness manifests itself between 90Zr and the cadmium and tin isotopes. The experiment was conducted using the 94,96,98,100Mo(α,α′) reaction at Eα=386 MeV. A comparison of the results with relativistic, self-consistent Random-Phase Approximation calculations indicates that the ISGMR response begins to show softness in the molybdenum isotopes beginning with A=92.

Isotopic Evolution of the Inner Solar System Inferred from Molybdenum Isotopes in Meteorites

Abstract The fundamentally different isotopic compositions of non-carbonaceous (NC) and carbonaceous (CC) meteorites reveal the presence of two distinct reservoirs in the solar protoplanetary disk that were likely separated by Jupiter. However, the extent of material exchange between these reservoirs, and how this affected the composition of the inner disk, are not known. Here we show that NC meteorites display broadly correlated isotopic variations for Mo, Ti, Cr, and Ni, indicating the addition of isotopically distinct material to the inner disk. The added material resembles bulk CC meteorites and Ca–Al-rich inclusions in terms of its enrichment in neutron-rich isotopes, but unlike the latter materials is also enriched in s-process nuclides. The comparison of the isotopic composition of NC meteorites with the accretion ages of their parent bodies reveals that the isotopic variations within the inner disk do not reflect a continuous compositional change through the addition of CC dust, indicating an efficient separation of the NC and CC reservoirs and limited exchange of material between the inner and outer disk. Instead, the isotopic variations among NC meteorites more likely record a rapidly changing composition of the disk during infall from the Sun’s parental molecular cloud, where each planetesimal locks the instant composition of the disk when it forms. A corollary of this model is that late-formed planetesimals in the inner disk predominantly accreted from secondary dust that was produced by collisions among pre-existing NC planetesimals.

Molybdenum Isotope Separation in a Cascade with a Specified Number of Gas Centrifuges in Stages

A method of calculating and optimizing a multi-flow cascade with a specified number of gas centrifuges in stages on cuts of their partial flows were developed. A special feature of the method is an iterative fitting of the separation coefficients of the gas centrifuges that depend on the feed flow and the division factor of the flow. A computational experiment was conducted on the separation of molybdenum hexafluoride and concentration of the isotopes 98Mo in an additional product and 100Mo in the waste of the cascade. The basic factors that affect the efficiency in concentrating the components and are associated with the cascade feed and the types of stages in terms of the number of gas centrifuges are determined. It is shown that the feed flows and the separation coefficients of the stages of such cascades differ significantly from those of rectangular and rectangular-sectioned cascades.

Positive cerium anomalies imply pre-GOE redox stratification and manganese oxidation in Paleoproterozoic shallow marine environments

The Paleoproterozoic Koegas Subgroup (Transvaal Supergroup, South Africa) was deposited in the immediate prelude to the Great Oxidation Event (GOE), and can therefore shed light on oceanic paleoredox conditions just before atmospheric oxidation. Manganese enrichments of ~16 wt% in diagenetic kutnahorite horizons suggest that Mn2+ oxidation occurred, either by free O2 or by an ancient photosystem. Iron and molybdenum isotope trends also support the existence of a Mn4+ oxide sediment flux, suggesting that the Koegas basin may have been redox stratified. Evidence from detrital and authigenic pyrite with mass-independently fractionated sulfur isotopes, however, suggests that the atmosphere was devoid of oxygen. To resolve this contradiction, this paper presents new constraints on pathways of Mn2+ oxidation from field, petrographic, stable isotope, and rare earth element and yttrium (REYSN) analysis of stromatolitic carbonates from the upper Koegas Subgroup. Ferroan dolostones and limestones preserve marine REYSN arrays with positive CeSN anomalies. These differences are explained by a redox stratified basin, whereby Mn2+ and Ce3+ are oxidized at a redoxcline and Ce is adsorped onto sinking Mn oxide particles. Mn oxide particles and a negative Ce anomaly from the oxidized upper water column are transferred into carbonates accumulating above the redoxcline. Diagenetic fluids later reduce the Mn oxides to kutnahorite. Below the redoxcline, reduction of Mn oxides enriches carbonates in Mn and a positive Ce anomaly. This contribution adds evidence for the development of oxygen oases and redox-stratified basins before the GOE. Redox stratification was best developed during transgressions. During regressions, a deltaic system prograded into the Koegas Basin. High sedimentation rates likely allowed for preservation of detrital pyrite only in the deltaic sandstones, thus explaining the contradictory geochemical evidence. No previously unknown ancient photosystem of Mn oxidation is required to explain Mn oxidation.

High-precision molybdenum isotope analysis of low-Mo igneous rock samples by MC–ICP–MS

Recent advances in analytical techniques have achieved high-precision (2σ ≤ 0.06‰) determinations of Mo isotopic ratios in silicate rocks but few such data are as yet available, especially for low-Mo (<50 ppb) rocks such as leucogranites. Here we present two low-blank, double-spike, high-yield, ion-exchange chromatographic methods (single- and two-column) for precise determination of Mo isotopic ratios in low-Mo rock samples (10–500 ppb) using multicollector–inductively coupled plasma–mass spectrometry (MC–ICP–MS). The two-column technique involved AG 1-X8 anion-exchange (100–200 mesh) and BPHA chelatin resins, with the latter also used in the single-column procedure. Two USGS reference materials (basalt BIR-1a and diabase W-2a) were analyzed for quality-control purposes. Their δ98/95Mo values determined using the two methods are consistent within uncertainties, and with those determined by previously published methods. We provide the first Mo isotopic data for the reference materials of dunite DTS-2b (USGS), GPt-3 (NRCG) and peridotite WPR-1 (CCRMP). δ98/95Mo values for DTS-2b and WPR-1 by using two-column procedure are in good agreement with those obtained by single-column separation. Repeated analyses of these standards indicate a long-term external reproducibility of better than ±0.05‰. However, the δ98/95Mo values of GPt-3 show poor reproducibility due to either the sample heterogeneity or existing undigested minerals (e.g., spinel). The methods were applied to leucogranites with low Mo contents of 10–74 ppb from the Himalaya orogen and southern Lhasa terrane of Tibet. The isotopic analyses yielded δ98/95Mo values of −0.2‰ to 0.34‰ and −0.41‰ to −0.68‰ (against NIST SRM 3134), respectively, within the range of values for igneous crust. Their contrasting Mo isotopic compositions may indicate compositional diversity in their sources or differences in magma evolution.

Molybdenum isotopic behavior during intense weathering of basalt on Hainan Island, South China

The behavior of molybdenum (Mo) isotopes during chemical weathering was investigated through analysis of saprolite and soil samples from two weathering profiles developed on Neogene basalts of Hainan Island in southern China. The Penglai and Nanyang profiles exhibit strong (mean ∼60 wt.% kaolinite) and extreme (mean ∼85 wt.% kaolinite) degrees of weathering, respectively. Our results show that light Mo isotopes are selectively retained in residual minerals, including kaolinite and hematite, whereas heavy Mo isotopes are preferentially leached out. Relative to the Mo-isotopic composition of the underlying unaltered parent basalt –0.38 ± 0.05 ‰ (2σ); vs. NIST 3134), the δ98/95Mo values of the overlying saprolite and surface soil at Penglai are modestly lower (–0.81 to –0.39‰) and higher (–0.35 to –0.12‰), respectively. In this profile, both δ98/95Mo and total organic carbon (TOC) decrease with depth, and their positive correlation in the soil layer (r = +0.99, p(α) < 0.01, n = 6) implies that heavier Mo isotopes are fractionated into organic matter. In the more intensely weathered Nanyang profile, the δ98/95Mo of the underlying unaltered parent basalt is lower –0.56 ± 0.05‰ (2σ) than that at Penglai, and the weathered samples show a larger range of relatively lower values (–1.39 to –0.33‰), which may reflect adsorption of Mo by Fe oxides. Throughout the weathering profile, Fe2O3 correlates positively with Mo concentration (r = +0.86, p(α) < 0.01, n = 32) but negatively with δ98/95Mo (r = –0.69, p(α) < 0.01, n = 32), implying that adsorption onto Fe-oxides played a key role in Mo mobility and isotopic fractionation. Extremely negative δ98/95Mo values (to –1.39‰) in the middle saprolite were likely linked to hydrothermal activity, as indicated by unusual enrichments of trace metals (Ni, Cu, Mo), low Hf/Y ratios, and a positive Eu/Eu* anomaly. Furthermore, the overlying soil layers of both profiles are characterized by enrichment of heavier Mo isotopes and higher Mo concentrations relative to their parent rocks. This may have been caused by additional inputs of Mo from atmospheric precipitation (average δ98/95Mo = +0.94 ± 0.14‰, 1σ, n = 3). Based on mass balance modeling of Mo isotopes and concentrations through the integrated profile, we infer Mo inputs from volcanic ash in the basalt-derived soils of northern Hainan Island. This study highlights the range of potential factors controlling Mo retention and isotopic fractionation as a function of variable weathering intensity, which may help to explain the large range of Mo isotopic compositions observed in river catchments at a global scale.

Molybdenum isotope fractionation in uranium oxides and during key processes of the nuclear fuel cycle: Towards a new nuclear forensic tool

The ability to identify the sources of nuclear materials using various analytical methods has become a major target of nuclear forensics science. However, despite the emergence of novel tools developed in recent years, it has become obvious that accurate identification of sources requires multiple tools. Here, we have developed a new isotope tool based on Mo isotopes that could be used in nuclear forensics. Molybdenum is a trace element that is found in significant levels in uranium ores (0.5–4800 ppm), uranium minerals (0.02–6000 ppm) and uranium ore concentrates (UOCs, from 0.7 to 1400 ppm). The Mo isotope composition, reported as δ98Mo was analyzed in these materials and shown to have a significant variability from −2.62 to +0.30‰ in uranium ores and from −3.50 to +0.46‰ in UOCs. In uranium ores, the largest Mo isotope fractionation is found in deposits of sedimentary origin while the few ores with a magmatic origin show limited Mo isotope fractionation. Thus, the Mo isotope ratios may ultimately be used to distinguish high temperature from low temperature uranium ores. The δ98Mo values in UOCs also show a significant range that not only originate from variations in ore compositions but can also be attributed to ore processing such as leaching, solvent extraction, resin extraction and UOC precipitation. Laboratory experiments based on existing protocols of uranium ore separation demonstrate the existence of sizeable Mo isotope fractionation associated with these processes. Experiments with solvent extraction and precipitation of ammonium diuranate and peruranate show that the refined uranium fraction is enriched in light Mo isotopes in both cases. In the case of a uranium ore (SOMAÏR, Niger) and associated UOC, the observed fractionation can possibly be assigned to the combined effect of solvent extraction and precipitation, although Mo isotope variability in ores and UOCs may make such a diagnostic difficult. Ultimately, these experiments can be used to reconstruct the Mo isotope composition of the ore, if there is sufficient knowledge about the ore processing flowsheet or to provide information about the type of uranium ore. Alternatively, it could be used to identify the process used for manufacturing a given UOC, if one can make an estimate about the composition of the ore, probably in conjunction with other tools. Our study demonstrates the potential of Mo isotopes as a useful tool in the field of nuclear forensics.

Molybdenum isotope and trace metal signals in an iron-rich Mesoproterozoic ocean: A snapshot from the Vindhyan Basin, India

Fundamental questions persist regarding the redox structure and trace metal content of the Mesoproterozoic oceans. Multiple lines of evidence suggest more widespread anoxia in the deep oceans compared to today, and iron speciation indicates that anoxia was largely accompanied by dissolved ferrous iron (ferruginous conditions) rather than free sulfide (euxinia). Still, exceptions exist—euxinic conditions have been reported from some ocean margin and epeiric sea settings, and oxic conditions were reported in one deeper water environment and are also known from shallow waters. Constraining the temporal evolution of Mesoproterozoic marine redox structure is critical because it likely governed redox-sensitive trace metal availability, which in turn played a significant role in marine diazotrophy and the evolution of early eukaryotes. Here, we present a new, multi-proxy geochemical dataset from the ~1.2 Ga Bijaygarh Shale (Kaimur Group, Vindhyan Basin, India) emphasizing total organic carbon, iron speciation, and trace metal concentrations, as well as sulfur, nitrogen, and molybdenum isotopes. This unit was deposited in an open shelf setting near or just below storm wave base. Taken together, our data provide a unique snapshot of a biologically important shallow shelf setting during the Mesoproterozoic Era, which includes: 1) locally ferruginous waters below the zone of wave mixing, 2) muted enrichment of trace metals sensitive to general anoxia (e.g., chromium) and variable enrichment of trace metals sensitive to euxinia (e.g., molybdenum and, to a lesser extent, vanadium), 3) general sulfate limitation, and 4) nitrogen fixation by molybdenum-nitrogenase and a dominantly anaerobic nitrogen cycle in offshore settings. Differential patterns of trace metal enrichment are consistent with data from other basins and suggest a largely anoxic ocean with limited euxinia during the Mesoproterozoic Era. Our new molybdenum isotope data—the first such data from unambiguously marine shales deposited between 1.4 and 0.75 Ga—record values up to +1.18 ± 0.12‰ that are analogous to data from other Mesoproterozoic shale units. Ultimately, this study provides a broad, multi-proxy perspective on the redox conditions that accompanied early eukaryotic evolution.

Unravelling the controls on the molybdenum isotope ratios of river waters

The molybdenum (Mo) isotope ratios (δ98/95Mo) of river waters control the δ98/95Mo values of seawater and impact on the use of Mo isotope ratios as a proxy of past redox conditions. The δ98/95Mo values of river waters vary by more than 2 ‰, yet the relative roles of lithology versus fractionation during weathering remain contested. Here, we combine measurements from river waters (δ98/95Modiss), river bed materials (δ98/95MoBM) and soils from locations with contrasting lithology. The δ98/95Mo values of river bed materials (δ98/95MoBM), set by rock type, vary by ~1 ‰ between rivers in New Zealand, the Mackenzie Basin, and Iceland. However, the difference between dissolved and solid phase Mo isotopes (Δ98/95Modiss-BM) varies from +0.3 ‰ to +1.0 ‰. We estimate Mo removal from solution using the mobile trace element rhenium and find that it correlates with Δ98/95Modiss-BM across the sample set. The adsorption of Mo to Fe-Mn-(oxyhydr)oxides can explain the observed fractionation. Together, the amount of Mo released through dissolution and taken up by (oxyhydr)oxide formation on land may cause changes in the δ98/95Mo values of rivers, driving long term changes in the Mo isotope ratios of seawater.

Molybdenum isotope fractionation in glacial diamictites tracks the onset of oxidative weathering of the continental crust

Molybdenum isotopes in twenty-four composites of glacial diamictites spanning depositional ages of 2900 to 300 Ma show a systematic shift to lighter compositions and a decrease in Mo concentration over time. The diamictites fall into three age groups relative to the Great Oxidation Event (GOE): pre-GOE (2.43 – 2.90 Ga), syn-GOE (2.20 – 2.39 Ga), and post-GOE (0.33 – 0.75 Ga). Pre-GOE composites have an average δ98MoNIST3134 of +0.03‰ (± 0.18‰), syn-GOE composites average −0.29‰ (± 0.60‰), and post-GOE composites average −0.45‰ (± 0.51‰). These groups are statistically different at p=0.05. We use the pre-GOE data to estimate the average Archean upper continental crust (UCC) δ98Mo signature as +0.03 ± 0.18‰ (2σ), which falls within the range of previous estimates of modern igneous rocks. As the diamictites represent a mixture of igneous and weathered crust, the shift to lighter Mo values over time likely reflects Mo isotope fractionation during oxidative weathering and increased retention of light Mo isotopes in weathered regolith and soils. We hypothesize that this fractionation is due to the mobilization of oxidized Mo following the GOE, and subsequent adsorption of light Mo onto Fe-Mn oxides and/or organic matter in weathered regolith. We conclude that Mo isotopes in continental weathering products record the rise of atmospheric oxygen and onset of oxidative weathering. As the regolith formed under oxidative conditions is isotopically lighter than average continental igneous rocks, mass balance dictates that Mo isotope fractionation during oxidative weathering should result in isotopically heavy groundwater and river water, which is observed in modern systems.