Mo Isotope Variations Research Articles

Systematic Across‐Arc Variations of Molybdenum Isotopes in a Fluid‐Dominated Subduction Zone System

AbstractMass‐dependent Mo isotope variations are a promising new tracer to study magmatic processes in different geological settings. We report the first Mo isotope data for the Kamchatka arc system in the Northwest Pacific, comprising basaltic lavas of a complete Southeast‐Northwest traverse from the volcanic arc front through to the back arc region. The majority of volcanic centers investigated directly override the Hawaii‐Emperor Seamount Chain, which is currently being subducted underneath the arc system. Our Mo isotope data show systematic trends with Ce/Pb, Ce/Mo, Nb/Zr, La/Sm, and 143Nd/144Nd ratios from the volcanic arc front to the back arc. Arc front lavas have higher δ98/95Mo and lower Ce/Pb, Ce/Mo, Nb/Zr, La/Sm compared to back arc lavas. Because the involvement of subducted sediments can be excluded, we attribute the observed variations to a change in the mantle source composition from the arc front to the back arc regions. The isotopic and chemical budget of arc front lavas is dominated by a slab fluid component (high δ98/95Mo, low Ce/Pb, Ce/Mo), whereas mantle‐like Ce/Pb, Ce/Mo, elevated Nb/Zr and La/Sm in the back arc samples suggest an enriched mantle source. Combined δ98/95Mo, Nd, and Pb isotope data in back arc lavas are very similar to those observed for modern ocean island basalts from Hawaii. We thus explore the possibility that the back arc mantle was contaminated by a Hawaii‐type, enriched asthenospheric mantle component from the subducted Hawaii‐Emperor Seamount Chain.

Molybdenum isotope evidence from restricted-basin mudstones for an intermediate extent of oxygenation in the late Ediacaran ocean

The Mo isotope composition of late Ediacaran global seawater and the global extent of ocean oxygenation are still debated due to the complex controls on sedimentary Mo isotope compositions and the rarity with which sediments directly capture global seawater Mo isotope compositions. Deep-water sulfidic sediments from modern severely restricted basins like the Black Sea have the best chance of capturing global seawater Mo isotope compositions. However, few studies have focused on sedimentary Mo isotope variations and their causes in late Ediacaran restricted basins. In this study, we present Mo isotope data for a new section from the Chengkou sub-basin of South China. The sedimentary rocks, mostly organic-rich mudstones, in this restricted basin have authigenic δ98Mo values from −0.01‰ to 1.27‰. Comparison of δ98Mo values with redox-sensitive elemental abundances reveals variation in the extent of Mn-oxide shuttling, related to enhanced hydrothermal activity and the inflow of open-ocean seawater, as the main cause of sedimentary Mo isotope variation in the Chengkou sub-basin. An unambiguous negative correlation between δ98Mo and Mn content points to a greater importance of Mn-oxide shuttling than Fe-oxide shuttling on the sedimentary Mo isotope signatures. Variations in the intensity of Mn-oxide shuttling were likely linked to changes in Mn inputs and the extent of basin restriction.A compilation of late Ediacaran sedimentary Mo isotope data suggests overall limited Mo isotope variation in (semi) restricted late Ediacaran basins, but local factors causing the variation are different and may result in the heaviest Mo isotopic signal in these settings being slightly lighter than that of open seawater. Using the observed negative linear relationships between δ98Mo values and the shuttling-sensitive proxies (V/TOC and Mn), the Mo isotope compositions recorded in the Chengkou sub-basin yield a reliable estimate of 1.3–1.6‰ for the δ98Mo of late Ediacaran global seawater. This value is slightly higher than the δ98Mo estimated for global seawater in the Cryogenian and early-middle Ediacaran, suggesting the extent of ocean oxygenation during the late Ediacaran Period was more similar to the oceans during Mesozoic Oceanic Anoxic Events than to the modern ocean.

Extreme Mo isotope variations recorded in high-SiO2 granites: Insights into magmatic differentiation and melt–fluid interaction

The Mo stable isotope system has been used to trace material recycling during subduction-related processes, but the behavior of Mo isotopes during magmatic evolution (e.g., crystal–melt fractionation and melt–fluid interaction) remains contentious, especially in high-SiO2 granites. This study addresses the issue of Mo isotope variation in high-SiO2 granites by measuring bulk-rock and mineral Mo isotopes of biotite granites (BGs) and garnet-bearing two-mica granites (GBGs) from the well-characterized Zhengga granite pluton (southern Tibet, China). The GBGs have similar Sr–Nd–O isotope compositions to those of the BGs but show higher SiO2 and lower TiO2, MgO, total Fe2O3, and CaO contents, and represent the products of advanced fractionation of the BG magmas. The BGs have lower Mo contents (0.02–0.07 ppm) and higher δ98/95Mo values (−0.54‰ to 0.22‰) compared with the GBGs (0.029–2.121 ppm and −0.97‰ to −0.41‰, respectively). Analysis of major silicate minerals suggests that substantial segregation of biotite and feldspar with high δ98/95Mo values of 0.00‰ to 0.38‰ and −1.06‰ to 0.57‰ (most within −0.58‰ to 0.13‰) could have driven the GBGs and the late-stage crystalline phase of garnet (−1.22‰ to −0.98‰) towards very low δ98/95Mo values. However, the trend of decreasing δ98/95Mo with indices of magma differentiation is not linear: one group of GBGs show increasing Mo contents and decreasing δ98/95Mo values with decreasing Y, Ho, and Dy contents; while the other group display increasing Mo contents and slightly decreasing δ98/95Mo values with respect to the increasing contents of Y, Ho, and Dy. These two contrasting behaviors can be ascribed to further crystal fractionation and melt–fluid interaction in a closed magmatic–hydrothermal system. This is also evidenced by the formation of two types of garnets with different contents of Mo and rare earth elements in these two groups of GBGs. Closed-system fluid saturation is inferred to have driven the silicate melt to be enriched in 98Mo, which limited the decrease in melt δ98/95Mo caused by crystal fractionation. These observations are supported by quantitative geochemical modeling. We conclude that both fractional crystallization and melt–fluid interaction control Mo isotope fractionation in high-SiO2 granites and that Mo isotopes are useful for tracing the evolution of high-SiO2 igneous rocks.

Molybdenum isotopic constraint from Java on slab inputs to subduction zone magmatism

Molybdenum isotope is a diagnostic tracer for crustal and mantle components in arc magmatism. However, the mechanism of Mo isotopic variation in arc magmas is still debated, e.g., input of different subduction components into the mantle wedge versus isotopic fractionation during dehydration of subducted slab. Here we present whole-rock Mo-Sr-Nd-Hf-Pb isotopic data for the Continental Arc Basalt (CAB) and Back Arc Basalt (BAB) from Java, Indonesia, to investigate the role of slab inputs in Mo isotopic variation of the Sunda arc magmatism. The CAB samples have variable K2O contents (0.44–2.49 wt%) and are mainly classified as calc-alkaline series, while the BAB samples are shoshonitic with markedly high K2O contents (2.12–6.90 wt%) relative to the CABs. The Java CABs and BABs have similar Mo isotopic compositions (δ98/95Mo = −0.65 to −0.07‰ and −0.66 to −0.07‰, respectively, relative to NIST SRM3134), suggesting that such a significant Mo isotopic variation should not be caused solely by the isotopic fractionation during the subduction. Instead, δ98/95Mo values of the Java basalts positively correlate with Pb isotopic ratios. This implies that the Mo isotopic variations in the Java arc rocks should result from the metasomatism in the mantle wedge by hybrid agents, including varying proportions of melts from subducted sediments (with heavy Mo isotope) and melts from the subducted altered upper oceanic crust (SAOC) (with light Mo isotope). The light Mo isotope of the Java arc rocks, compared with the Mariana arc basalts, suggests that melts from the SAOC have much lighter Mo isotopic compositions than the components from the lower oceanic crust. Thus, Mo isotope has great potential to distinguish the components from the subducted upper and lower oceanic crust. The Java CABs show along-arc variations in Mo-Sr-Nd-Hf-Pb isotopes, which is related closely with the thermal status of the subducted slab. Upwelling of the asthenosphere due to the slab tearing beneath the Java arc might have enhanced the partial melting of subducted sediments nearby the slab window. The complicated subduction system in the Sunda arc has strongly controlled the geochemical composition of arc magmas, which changes with input of different subduction components into the mantle wedge along arc.

Molybdenum isotope systematics of lavas from the East Pacific Rise: Constraints on the source of enriched mid-ocean ridge basalt

Mid-ocean ridge basalts (MORB) reveal large mantle compositional heterogeneity, whose origin remains debated. Here we present a systematic study of molybdenum isotopes on well-characterized MORB glass samples from the East Pacific Rise (EPR) and near-EPR seamounts. Our analyses show significant Mo isotope variations with δ98/95Mo (relative to NIST SRM3134) ranging from −0.23‰ to −0.06‰. We argue that these Mo isotope variations are not caused by processes of MORB melt generation and evolution but reflect mantle isotopic heterogeneity. Taking together with the literature data, we show that MORB Mo isotope compositions vary systematically with geochemical parameters indicating mantle enrichment. These observations are best explained by two-component mixing between an incompatible element depleted endmember (e.g., low La/Sm, Nb/La, Nb/Zr and Th/Yb, and high Sm/Nd and 143Nd/144Nd) with low δ98/95Mo (∼−0.21‰) and an incompatible element enriched endmember (e.g., high La/Sm, Nb/La, Nb/Zr and Th/Yb, and low Sm/Nd and 143Nd/144Nd) with high δ98/95Mo (∼−0.05‰). The association of heavier Mo isotope compositions with the geochemically more enriched MORB is inconsistent with recycled ocean crust with or without sediment being the enriched endmember. Instead, this is consistent with the enriched endmember being of magmatic origin, most likely lithologies of low-degree melt metasomatic origin dispersed in the more depleted peridotite matrix in the MORB mantle. Thus, with MORB Mo isotope systematics, we confirm that recycled oceanic mantle lithosphere metasomatized by low degree melt plays a key role in the formation of E-MORB source lithologies. Our study also highlights Mo isotopes as an effective tool for studying upper mantle processes.

Effect of Fe–Ti oxides on Mo isotopic variations in lateritic weathering profiles of basalt

This study aimed to improve our understanding of processes fractionating Mo isotopic compositions during extreme weathering of basalt and to identify a long-term surface reservoir for light δ98Mo signatures, which are key to constraining the isotopic composition of riverine inputs to the oceans. A lateritic weathering profile of basalt from Hainan Island, South China, was studied by determining bulk δ98Mo values as well as δ98Mo values of extracted Fe–Mn (hydro) oxides and residual phases, and parent-rock minerals. δ98Mo values relative to NIST SRM 3134 = +0.25‰ in the lateritic weathering profile were in the ranges of −0.28‰ to +0.48‰, with the δ98Mo values of the upper and middle sections being heavier, and those of the lower section lighter than that of the parent basalt. Rainwater input and changing redox conditions are likely to be the essential factors controlling variations in bulk δ98Mo values in the upper and middle sections, respectively, resulting in their heavier δ98Mo values. In contrast, the lighter values in lower section are likely due to the leaching of primary minerals (e.g., pyroxene and plagioclase) with some heavier δ98Mo being released to aqueous media. The Fe–Mn (hydro) oxides phase in the sequential extraction procedure is thus characterized by heavy δ98Mo values in the lateritic weathering profile, which may be responsible for heavy Mo isotopic composition in bulk samples. Light δ98Mo signatures are preserved in the residual phase, where Mo hosted in Fe–Ti oxides is inherited mainly from parent basalt rock, possibly accounting for light Mo isotopic composition in bulk samples. Owing to their high resistance to weathering and chemical erosion, Fe–Ti oxides might be a potential long-term surface reservoir for light δ98Mo signatures. The difference in degrees of Mo isotopic fractionation between basalt and granite lateritic weathering profiles may reflect variations in Mo behavior in parent-rock host minerals during weathering. The results elucidate mechanisms of Mo isotopic fractionation during chemical weathering and improve understanding of the riverine input of Mo isotopes to oceans.

Mo isotopic variations of a Cambrian sedimentary profile in the Huangling area, South China: Evidence for redox environment corresponding to the Cambrian Explosion

The radiation of macroscopic animals in the Early Cambrian, commonly be called as ‘Cambrian explosion’, had a close relationship with Earth oxygenation. These macrofossils are widely preserved within the Early Cambrian in the northern and western Yangtze Craton. In order to show the casual relationship between animal evolution and Earth redox environment, Mo content and isotopic composition analyses have been conducted on dolomites of the Tianzhushan Formation and the black shales of the Shuijingtuo Formation from the lowermost Cambrian profile in the eastern Huangling dome, northern Yangtze Craton. The calcitic dolostones from the Tianzhushan Formation display lower concentrations of Mo contents (0.055–0.666 ppm) than that of the black silty shales from the Shuijingtuo Formation (6.0–172 ppm). This magnitude difference is most likely related to the different lithologies due to Mo enrichment in shales. The Mo isotope values (δ98/95Mo) of the calcitic dolostones for the Tianzhushan Formation vary from −0.73‰ to 0.41‰. In comparison, the black sandy shales from the Shuijingtuo Formation exhibit δ98/95Mo of −1.04–1.84‰, including 5 samples from the earliest Shuijingtuo Formation displaying systematically higher δ98/95Mo values (0.09–1.84‰) than those of the other 5 samples from upper layers (δ98/95Mo = −1.04 to −0.19‰). These geochemical data suggest that the redox environment of the early Cambrian is fluctuating, and the peak oxygenation of the ocean had already reached a level similar to modern environments. According to the Mo concentrations and δ98/95Mo data, the sedimentary succession can be divided into five intervals, which are closely correlated to the episodic evolution of early life in the Cambrian. Together with the available published data from other profiles in the Yangtze Craton, our new data reveal that the higher oxidation environment had a positive effect on the evolution of early life.

Molybdenum record from black shales indicates oscillating atmospheric oxygen levels in the early Paleoproterozoic

The early Paleoproterozoic witnessed Earth9s first major oxygenation, referred to as the Great Oxidation Event or GOE. The GOE began around 2.45 billion years ago (Ga) and progressed over hundreds of millions of years, as evidenced by multiple redox indicators, before coming to an abrupt end by ca. 2.06 Ga. The details of the GOE and the extent of oxygenation are still not resolved, however, and it is not clear how redox conditions across the GOE compare with those during the middle Proterozoic. In order to investigate the evolution of deep-ocean redox conditions during the GOE, we present Mo concentration and isotope data together with Fe speciation values for three key organic matter-rich shale units of the early Paleoproterozoic age (2.32–2.06 Ga). In addition, we present a new graphical representation of modeling suggesting that the oceanic Mo isotope system is highly sensitive to the balance between anoxic/suboxic and euxinic conditions until deep-ocean oxygenation, similar in scale to modern ocean oxygenation, is reached. Our approach indicates rising, yet oscillating atmospheric oxygen at 2.32 Ga, leading to an abrupt increase in Mo supply to the oceans and large Mo isotope variations under non-steady state conditions. The low seawater δ98Mo value based on the ca. 2.32 Ga black shales (0.32 ± 0.58‰) suggests that the oceans were still largely anoxic with locally developed euxinic conditions. Between 2.2 and 2.1 Ga, during the peak of the Lomagundi carbon isotope excursion, we observe higher δ98MoSW values (1.23 ± 0.36‰) together with lower Mo concentrations in euxinic shales ([Mo] = 6.3 ± 9.0 ppm). We suggest that a decrease in the continental Mo input flux in the later part of the GOE was the main cause of this trend. Lower sulfide availability on the continents after protracted sulfide weathering associated with the early stages of the GOE, and efficient Mo removal in poorly oxygenated oceans under weakly euxinic conditions would both have contributed to the contraction of the Mo oceanic reservoir. By ca. 2.06 Ga, the Mo isotope composition of seawater, as inferred from euxinic black shale intervals, became significantly lighter (0.70 ± 0.21‰), reflecting an increased rate of quantitative Mo removal due to the more widespread development of strongly euxinic conditions. Counterintuitively, seawater Mo concentrations recovered, likely due to an increase in the Mo input, which in turn might reflect enhanced weathering of organic carbon-rich shales deposited during the Lomagundi Event.

Molybdenum isotope fractionation during adsorption to organic matter

Organic matter is of emerging interest as a control on molybdenum (Mo) biogeochemistry, and information on isotope fractionation during adsorption to organic matter can improve interpretations of Mo isotope variations in natural settings. Molybdenum isotope fractionation was investigated during adsorption onto insolubilized humic acid (IHA), a surrogate for organic matter, as a function of time (2–170 h) and pH (2–7). For the time series experiment performed at pH 4.2, the average Mo isotope fractionation between the solution and the IHA (Δ98Mosolution-IHA) was 1.39‰ (±0.16‰, 2σ, based on 98Mo/95Mo relative to the NIST 3134 standard) at steady state. For the pH series experiment, Mo adsorption decreased as pH increased from 2.0 to 6.9, and the Δ98Mosolution-IHA increased from 0.82‰ to 1.79‰. We also evaluated natural Mo isotope patterns in precipitation, foliage, organic horizon, surface mineral soil, and bedrock from 12 forested sites in the Oregon Coast Range. The average Mo isotope offset observed between precipitation and organic (O) horizon soil was 2.1‰, with light Mo isotopes adsorbing preferentially to organic matter. Fractionation during adsorption to organic matter is similar in magnitude and direction to prior observations of Mo fractionation during adsorption to Fe- and Mn- (oxyhydr)oxides. Our finding that organic matter influences Mo isotope composition has important implications for the role of organic matter as a driver of trace metal retention and isotopic fractionation.

Experimental dissolution of molybdenum‐sulphides at low oxygen concentrations: A first‐order approximation of late Archean atmospheric conditions

AbstractThe abundance of atmospheric oxygen and its evolution through Earth's history is a highly debated topic. The earliest change of the Mo concentration and isotope composition of marine sediments are interpreted to be linked to the onset of the accumulation of free O2 in Earth's atmosphere. The O2 concentration needed to dissolve significant amounts of Mo in water is not yet quantified, however. We present laboratory experiments on pulverized and surface‐cleaned molybdenite (MoS2) and a hydrothermal breccia enriched in Mo‐bearing sulphides using a glove box setup. Duration of an experiment was 14 days, and first signs of oxidation and subsequent dissolution of Mo compounds start to occur above an atmospheric oxygen concentration of 72 ± 20 ppmv (i.e., 2.6 to 4.6 × 10−4 present atmospheric level (PAL)). This experimentally determined value coincides with published model calculations supporting atmospheric O2 concentrations between 1 × 10−5 to 3 × 10−4 PAL prior to the Great Oxidation Event and sets an upper limit to the molecular oxygen needed to trigger Mo accumulation and Mo isotope variations recorded in sediments. In combination with the published Mo isotope composition of the rock record, this result implies an atmospheric oxygen concentration prior to 2.76 Ga of below 72 ± 20 ppmv.

Mo isotope and trace element patterns of Lower Cambrian black shales in South China: Multi-proxy constraints on the paleoenvironment

The Precambrian/Cambrian boundary marks a critical interval in Earth history. Paleoenvironmental changes of the oceans have been suggested as a possible explanation for some significant events such as the marine biological evolution (“Cambrian explosion”). Here we report a remarkable variation range in trace element, iron speciation and molybdenum isotope signatures of black shales from the Early Cambrian Dingtai profile (Guizhou province) and three synsedimentary polymetallic Ni–Mo–(PGE–Au) sulfide ore deposits (Dazhuliushui and Maluhe in Guizhou province, and Sancha in Hunan province) on the Yangtze Platform, South China. Based on the (isotope) geochemical features, we classify the Dingtai profile into three intervals: interval 1 (0–1m) has extreme metal and TOC enrichment and heavy δ98/95Mo values. This unit has all features of a euxinic environment, and is then followed by interval 2 (1–11m) with variable δ98/95Mo composition. Elevated contents of redox-sensitive elements and their ratios, and relatively high FeHR/FeT ratios (“reactive iron”) suggest anoxic conditions. The overlying interval 3 (11–33m) reveals oxic to suboxic conditions with generally light δ98/95Mo values in the lowermost part and fluctuant δ98/95Mo values in the uppermost part, and relatively low redox-sensitive element contents. We propose a model of a partly closed ocean basin with periodic replenishment together with changing redox conditions for the remarkable variability of δ98/95Mo values. Light δ98/95Mo values indicate restricted seawater circulation dominated by local continental input, periodically replenished by fresh seawater with heavy δ98/95Mo.A global compilation of Mo isotope data from black shales indicates that the Mo isotopic variations have a consistent temporal trend, reflecting the oxygen evolution of the oceans. Relatively light δ98/95Mo values of the Lower Cambrian black shales compared to those of modern euxinic sediments are indicative of more widespread anoxic marine environments during the Early Cambrian.

Mo isotopes in the Lower Cambrian formation of southern China and its implications on paleo-ocean environment

Over the past decade, as one of nontraditional stable isotopes, Mo isotope has developed rapidly and now become an important geochemical proxy to trace paleo-oceanic and atmospheric evolution through geological history. In this paper, Early Cambrian formations in southern China are investigated. The results indicate that δ97/95Mo values of Early Cambrian seawater may have been larger than 1.4%, values that are close to those of the modern ocean. It was also found that the variations in Mo isotope composition in samples from two sections (Huangjiawan and Gezhongwu in Guizhou) were closely related to changes in redox conditions during sedimentary processes. Combining our results with existing data, a preliminary model for the evolution of seawater Mo isotope composition through geological history was provided. It indicated that Mo isotopic variations were generally consistent with the evolution of atmospheric oxygen.

Identification of a matrix effect in the MC-ICP-MS due to sample purification using ion exchange resin: An isotopic case study of molybdenum

Multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) has become the preferred method for precise and accurate measurements of the relative abundances of many radiogenic and stable isotopes in natural materials. Isotopic analyses by MC-ICP-MS require a correction for instrumental mass-dependent isotopic fractionation (“instrumental mass bias”). Two techniques have been used to correct for instrumental mass bias in the MC-ICP-MS: (1) standard-sample bracketing (SSB) or (2) double spiking. SSB is often cited as the preferred method, but it is more susceptible to matrix effects. Here we demonstrate that a matrix effect in the MC-ICP-MS may arise indirectly from the chemical separation and purification of molybdenum using anion exchange resin. The results of our experiments show that a Mo standard passed through a column of anion exchange resin or a Mo standard added to a Mo-free solution that had been collected from anion exchange resin appears to be isotopically lighter than expected from direct analysis of the same standard. Using amounts of Mo similar to what might be expected from most natural samples (∼3 μg per column cut), these offsets span a significant fraction (∼10–60%) of the total known range of mass-dependent Mo isotopic variation in nature. This “column matrix effect” appears to be caused by organic material stripped from the resin. All of our attempts to eliminate or control this column matrix effect have failed, making it difficult (if not impossible) to obtain accurate measurements of mass-dependent Mo isotopic variations in natural materials using the anion exchange resin procedure described in this study and SSB techniques to correct for instrumental mass bias in the MC-ICP-MS. It is currently unknown if this type of column matrix effect will affect measurements of other stable or radiogenic isotopes by MC-ICP-MS when SSB is used to correct for instrumental mass bias.

Mo isotopic variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo

Mo isotopic variations in molybdenite: Vapor transport and Rayleigh fractionation of Mo

Determination of mass-dependent molybdenum isotopic variations by MC-ICP-MS: An evaluation of matrix effects

Several analytical techniques are currently used to determine mass-dependent molybdenum isotopic variations in natural materials using multiple-collector inductively coupled plasma mass spectrometry (MC-ICP-MS), including different methods for the separation of Mo from the sample and the correction for instrumental mass-dependent isotopic fractionation (instrumental mass bias). Both internal (“double-spiking” using two enriched Mo isotopes) and external (“zirconium doping” with standard-sample bracketing) techniques have been used in previous studies to deal with the effects of instrumental mass bias. The results of these studies have indicated that the precision for Mo isotopic analyses of natural (matrix-bearing) samples is a factor of ∼4–7× better using a double spike. Here we present a detailed study of the ability of MC-ICP-MS to determine, both precisely and accurately, the isotopic composition of Mo extracted from molybdenite using a low blank, high yield two-column procedure for Mo separation and a simple standard-sample bracketing approach to correct for instrumental mass bias. Based on analyses of molybdenites, the precision of this technique is shown to be similar to published double-spike data (within a factor of ∼2). All three of the known types of potential matrix effects in the MC-ICP-MS are also evaluated: automatrix effects, matrix effects due to Zr doping and matrix effects due to elements in the sample other than Mo and Zr. Each of these matrix effects is found to be either insignificant or controllable. Analyses of five molybdenites of hydrothermal origin reveal a range in their Mo isotopic composition that is a factor of ∼4 greater than the previous range reported for such samples. More detailed work is required to elucidate the origin of these mass-dependent Mo isotopic variations in molybdenites.