Isotopic Composition Of Molybdenum Research Articles

Molybdenum isotope composition of the upper mantle and its origin: insight from mid-ocean ridge basalt

Molybdenum isotope composition of the upper mantle and its origin: insight from mid-ocean ridge basalt

Iron and Molybdenum Isotope Application for Tracing Industrial Contamination in a Highly Polluted River

Rivers adjacent to industrial zones usually suffer from severe pollution issues. Industrial wastewater that has undergone sewage treatment processes may be legally discharged into rivers under water quality permits. Previous studies have frequently employed isotopic tracers to identify potential contaminants for pollution control. Conventional radiogenic isotopes utilized in tracing studies cannot discern whether the source is untreated (primary) industrial wastewater, which can have serious impact to the environment. By analyzing the iron (Fe) and molybdenum (Mo) isotopic compositions in original industrial wastewater and treated effluent, this study aims to investigate whether the heavily polluted Agongdian River is contaminated by the untreated wastewater. Based on the results from this study, the original industrial wastewater exhibits higher concentrations of metallic elements and heavier Fe and lighter Mo isotopic compositions, compared to the treated effluent. Consequently, it appears that Agongdian River water indeed exhibits evidence of untreated industrial wastewater. Furthermore, the volume of original industrial wastewater entering the river can be estimated from these results. This research offers a more precise and accurate approach to monitor potential industrial wastewater pollution in natural water bodies, contributing to the goal of environmental protection and sustainable development.

Molybdenum isotopic compositions in volcanic rocks from Northern Izu arc (Izu-Oshima, Niijima, and Kozushima)

Molybdenum isotopic compositions in volcanic rocks from Northern Izu arc (Izu-Oshima, Niijima, and Kozushima)

Contribution of recycled oceanic crust to the extremely light molybdenum isotopic compositions of mid-ocean ridge basalts from the South China Sea

The contribution of recycled oceanic crust to the mantle source of mid-ocean ridge basalts (MORBs) remains a subject of debate, as radiogenic isotopes alone yield ambiguous insights. Here, we present comprehensive data on whole-rock major element, trace element, and Mo–Nd–Hf isotopic compositions for MORB samples from International Ocean Discovery Program Expedition 349 sites U1431E and U1433B in the eastern (ESB) and southwestern (SWB) subbasins, respectively, of the South China Sea (SCS). The δ98/95Mo values of the SCS MORBs exhibit a significant range (from −0.80‰ to +0.05‰), in contrast to the restricted composition observed in MORBs (δ98/95Mo = −0.19‰ ± 0.01‰). The ESB MORBs display extremely light Mo isotopic compositions (δ98/95Mo = −0.80‰ to −0.24‰) with NdHf isotopic compositions similar to those of Pacific MORB, whereas the SWB MORBs show higher δ98/95Mo values (−0.44‰ to +0.05‰) and depleted NdHf isotopic compositions. The δ98/95Mo values of the SCS MORBs are negatively correlated with Ce/Mo ratios and positively correlated with εHf values, suggesting the incorporation of dehydrated slab components into the source of the SCS MORBs. The subbasin-scale mantle heterogeneity in the SCS can be best explained by varying degrees of interaction between a mantle plume and stagnant slabs in the mantle transition zone, as suggested by mantle tomography. The rigid stagnant slab in the mantle transition zone beneath the SWB largely impeded the upwelling of the mantle plume, whereas the slab beneath the ESB was disrupted by the plume and subsequently transported into the upper mantle. This study, therefore, provides compelling evidence for the involvement of recycled oceanic slabs in generating MORB and the influence of an ascending mantle plume on seafloor spreading in a continental marginal sea. Mantle recycling processes in marginal sea basins exhibit distinct characteristics compared to those occurring in open oceans.

Molybdenum isotopes record recycling of subducting sediment in active continental margin, Northeast China

Highly variable molybdenum (Mo) isotope compositions have been reported for mafic arc volcanics above oceanic subduction zones. The heavy Mo isotope signatures of mafic arc magmas are generally attributed to addition of high δ98Mo oceanic crust-derived fluids to the subarc mantle sources. However, subduction zone fluids at subarc depths may occur in the forms of aqueous solution and hydrous melts, and seafloor sediments would also be subducted to subarc depths to produce hydrous melts. If such sediment-derived melts were incorporated into the mantle wedge, it is intriguing what is their effect on the Mo isotope composition of mafic arc magmas. Here we report Mo isotope data for Early Jurassic mafic igneous rocks from Northeast China, which was an active continental margin during subduction of the Paleo-Pacific Plate in the Mesozoic. These rocks have δ98Mo values of −0.26‰ to 0.17‰, which are mostly higher than the normal mantle value of −0.20‰ ± 0.01‰. They also exhibit arc-like trace element compositions, moderate (87Sr/86Sr)i ratios of 0.7042 to 0.7047, and neutral εNd(t) values of −2.8 to 2.2. The high δ98Mo values are associated with low Mo/Ce ratios, enriched SrNd isotopes, and high melt-mobile element contents in the mafic igneous rocks. These geochemical features indicate that the heavy Mo isotope signatures in these rocks were mainly inherited from subducting sediment-derived melts. Subducting Paleo-Pacific oceanic crust-derived aqueous solutions and seafloor sediment-derived hydrous melts would metasomatize the overlying mantle wedge beneath the northeastern China continent, and the sediment-derived melts would have contributed the heavy Mo isotope signatures to the mantle sources of mafic arc magmas in the active continental margin. Therefore, the Mo isotope study of mafic igneous rocks is a powerful means to decipher the recycling of crustal components at convergent plate margins.

Multiple ocean oxygenation events during the Ediacaran Period: Mo isotope evidence from the Nanhua Basin, South China

The Ediacaran Period (ca. 635–539 Ma) was an eventful interval in Earth history, during which a succession of biological and environmental changes, including episodic ocean oxygenation events (OOEs), paved the way for the Cambrian radiations of animal life. To better understand the evolution of ocean redox conditions and to estimate the extent of seafloor oxygenation during this period, we analysed molybdenum (Mo) isotope compositions and redox sensitive element (RSE) concentrations from a continuous, mid-slope section of the Doushantuo Formation (ca. 635–560 Ma) of the Nanhua Basin on the South China Craton. Alongside an updated compilation of published Mo isotope and RSE data, our new data show that three OOEs occurred within a generally anoxic Ediacaran ocean, with the last, particularly extensive event occurring during deposition of Doushantuo Member IV. Here we show how the global balance of redox-related Mo sinks shifted dynamically in response to these transient OOEs, which correlate well with the first appearance of Ediacaran fossil groups and so may have triggered or stimulated biotic innovations and radiations. Moreover, the spatial distribution of the Mo data from multiple sections supports episodic expansion of a euxinic wedge on the slopes of the Nanhua Basin, consistent with pyrite burial as a potential cause of the OOEs.

Serpentinite fluids and slab-melting in the Aleutian arc: Evidence from molybdenum isotopes and boron systematics

Mo isotope compositions have been increasingly used as a source tracer in magmatic systems. Here, we investigate the relative role of subducting oceanic crust, fluid-rich sources (e.g., serpentinite, altered oceanic crust – AOC), subducted sediments and upper mantle in the chemical composition of Aleutian arc magmas with Mo and B systematics. We present elemental and Mo isotope compositions (δ98/95Mo) and B concentrations on Aleutian lavas (n = 59), from Okmok Volcano in the east to the westernmost seamount Piip, showing absence of Mo isotope fractionation during magmatic differentiation. Additionally, we report Mo isotope systematics for serpentinized peridotites from the South-West Indian Ridge (SWIR) (n = 6), AOC (n = 2) and Pacific sediments (DSDP 183, ODP 886) (n = 5) outboard the Aleutian arc. Molybdenum isotope composition and B enrichment (e.g., B/Ce) patterns display a step-function increase along the arc, with low, MORB-like, δ98/95Mo and low B/Ce values in the western section of the arc (B/Ce = 0.15–1.07; δ98/95Mo =−0.38 to +0.01‰), that abruptly increase in the central-eastern volcanoes Korovin, Seguam and Yunaska (B/Ce = 1.20–2.60; δ98/95Mo = +0.03 to +0.30‰) near the intersection of the Amlia Fracture Zone (AFZ) with the trench, but decrease again farther east at Okmok (B/Ce = 0.76 and δ98/95Mo = −0.12‰ on average). These data patterns are interpreted to reflect an along-arc changing source in the Aleutian magmas. AOC and Pacific sediments have predominantly low δ98/95Mo (−0.47 to −0.32 and +0.17 to −1.9‰, respectively), while serpentinites have extremely high δ98/95Mo (up to +1.09‰) and high B/Ce (∼22000). Based on the low δ98/95Mo in sediments and AOC, and lack of correlation between along-arc δ98/95Mo and radiogenic sediment tracers, subducted sediments and AOC do not exert first-order controls on the observed Mo isotope compositions. Rather, low, MORB-like, δ98/95Mo but high Mo enrichments (e.g., Mo/Ce) in the western samples are consistent with slab melting under rutile-bearing eclogitic facies with near absent Mo isotope fractionation from the slab to the arc sources. In turn, the abrupt increase of δ98/95Mo and B/Ce in lavas near the AFZ are best explained by a serpentinite endmember (likely dehydration fluids) at the AFZ that is not evident elsewhere along the arc. Results from this study provide evidence for serpentinites as an additional heavy Mo isotope signature component in subduction zones and demonstrate that high δ98/95Mo coupled with B enrichments are a useful proxy for tracing serpentinite fluids in subduction zones.

Molybdenum isotopic composition of molybdenite and the fertility potential of the Ekomédion U–Mo prospect, SW Cameroon

Molybdenum isotopic composition of molybdenite and the fertility potential of the Ekomédion U–Mo prospect, SW Cameroon

Evaluation of the molybdenum isotope system as a petroleum tracer: The Phosphoria petroleum system, western U.S.A.

The molybdenum (Mo) isotope composition of organic-rich mudrocks is used to infer paleo-ocean redox conditions but the potential of this isotope system as a petroleum tracer is uncertain. In this study, we measured the Mo isotope compositions of crude oils, asphaltene separates, and organic-rich source mudrocks (Meade Peak Member and Retort Member of the Permian Phosphoria Formation) for the Phosphoria petroleum system, western U.S.A. Crude oils from the Bighorn Basin have Mo concentrations of 0.01–0.39 μg/g that are significantly lower than Mo concentrations of 10–443 μg/g for the organic-rich mudrocks (2.4–16.6 wt% total organic carbon; TOC), suggesting inefficient transfer of Mo from source rock kerogen to oils. Asphaltene Mo concentrations are elevated (1.0–3.2 μg/g), indicating that asphaltene is an important host of Mo in the Phosphoria crude oils. However, statistically insignificant correlation between crude oil and asphaltene Mo concentrations as well as differences between the δ98Mo of some crude oils and their asphaltene separates (Δ98Moasphaltene–crude oil between −0.5‰ and +0.1‰) suggests other important hosts for Mo in the non-asphaltene fraction of the Phosphoria oils. The overall range of δ98Mo for fifteen Phosphoria crude oils and six asphaltene separates is 0.5–1.7‰ and 0.8–1.5‰, respectively. Crude oils unaltered by reservoir processes (biodegradation, water washing, thermochemical sulfate reduction) have δ98Mo of 1.1–1.4‰ whereas crude oils mildly affected by biodegradation and/or water washing span a wider range of δ98Mo (1.0–1.7‰). Five crude oils altered by thermochemical sulfate reduction (TSR), as indicated by higher δ34S, have lower δ98Mo (0.5–0.9‰), suggesting that TSR decreases the δ98Mo of crude oils via redox reactions and/or the introduction of external fluids containing isotopically light Mo into the oil reservoir. Biodegradation may cause variable δ98Mo in crude oils via kinetic isotope effects, microbial processes, or the recapture of isotopically distinctive Mo released from biodegraded oils elsewhere. A statistically significant inverse correlation between δ98Mo and δ34S for non-TSR altered oils may reflect a decrease in the δ98Mo of crude oils with increasing thermal maturity, perhaps associated with de-metalation reactions and/or formation of new organo-metal compounds. The δ98Mo of the unaltered crude oils falls within the range of δ98Mo for the Mo- and TOC-rich source rocks of the Retort and Meade Peake Members (1.0–2.0‰), suggesting the Mo isotope system has potential as an oil – source rock tracer. This application will have the most success in petroleum systems whose source rock(s) exhibit a narrow range of distinctive δ98Mo. Our study shows the Mo isotope system has promise as a petroleum tracer and potentially could be useful for studies on petroleum contamination in the environment.

Heavy Mo isotope composition of northern Bataan adakites, Philippines: Evidence for fore-arc subduction erosion?

Abstract The identification of an eroded fore-arc crust component in arc magmas is challenging due to the combined effects of mantle metasomatism and crustal assimilation–fractional crystallization. In this study, molybdenum (Mo) isotope compositions are used in conjunction with Sr-Nd-Hf isotopic and elemental data to identify eroded fore-arc crust components in adakites from the Cuyapo and Balungao volcanoes of the northern Bataan segment of the Luzon arc (Philippines). The Mo isotopic ratios (δ98/95Mo, relative to the NIST SRM 3134 standard) of these adakites increase with increasing εNd (+4.3 to +5.6) and Ba/Nb (206–286). The low δ98/95Mo (−0.36‰ to −0.26‰) in the Cuyapo adakites coupled with low Sr-Nd-Hf isotopic ratios suggests contributions from the residual slab, which lost isotopically heavy Mo during dehydration. Interestingly, the high δ98/95Mo (−0.18‰ to 0.00‰) Balungao adakites have Mo-Sr-Nd-Hf isotopic ratios similar to those of the Luzon basement. Fractionated Nb/Ta (16–18) and high Sr/Y indicate the coexistence of melt with residual rutile and garnet ± amphibole assemblages, corresponding to a source region (>~45 km) below the present Luzon crust (~33 km). This thus suggests an origin of heavy Mo from partial melting of eroded crust in the mantle wedge rather than in the upper-plate crust. Our work not only demonstrates that Mo isotopes may be a potential tracer of eroded crust but also highlights that lavas with combined high δ98/95Mo, εNd, and Ba/Nb emplaced at subduction zones with juvenile arc crust may be a result of subduction erosion.

Molybdenum isotopes in mafic igneous rocks record slabmantle interactions from subarc to postarc depths

Abstract Arc volcanic rocks erupted above oceanic subduction zones generally have heavy molybdenum (Mo) isotopic compositions, which are attributed to contributions from oceanic slab–derived fluids with isotopically heavy Mo at subarc depths. Given that mafic igneous rocks with light Mo isotopic compositions have been rarely identified, it is unclear where light Mo isotope reservoirs reside, as these must exist due to oceanic subduction through geologic time. We present Mo isotope data from Mesozoic–Cenozoic mafic igneous rocks from the eastern North China craton, which were affected by continuous subduction of the oceanic slab. The Mesozoic mafic igneous rocks have island-arc basalt (IAB)–like features and high δ98Mo values of –0.15‰ to +0.09‰. In contrast, the Cenozoic mafic igneous rocks have ocean-island basalt (OIB)–like features and low δ98Mo values of –0.53‰ to –0.19‰. In addition, these two suites of rocks exhibit systematic differences in Sr-Nd isotopic compositions and fluid- and melt-mobile element contents, indicating that their mantle sources contained crustal components derived from different depths. The heavy Mo and weakly enriched Sr-Nd isotopic compositions and IAB-like trace-element signatures were inherited from slab fluids (including oceanic crust–derived aqueous solutions and seafloor sediment–derived hydrous melts) liberated at subarc depths. The light Mo and depleted Sr-Nd isotopic compositions and OIB-like trace-element signatures were inherited from dehydrated slab–derived melts (with rutile breakdown) at postarc depths. Therefore, Mo isotope systematics of mafic igneous rocks are a powerful means with which to trace the geochemical fluxes from subducting oceanic slabs to the mantle wedge at different depths.

Basalt Mo isotope evidence for crustal recycling in continental subduction zone

Highly variable molybdenum (Mo) isotope compositions are common in mafic arc magmas above oceanic subduction zones, but Mo isotopes of mafic magmas related to continental deep subduction have not yet been documented. Here, we report for the first time the Mo isotope composition of mafic igneous rocks above a typical continental subduction zone in east-central China, where the North China Block was subducted by the South China Block in the Triassic. Continental basalts of Early Cretaceous age show negative δ98Mo values of −0.98 to −0.16 ‰, significantly lower than the normal mantle value of −0.20 ± 0.01 ‰. These basalts also exhibit arc-like trace element compositions, high (87Sr/86Sr)i ratios of 0.7050 to 0.7058, negative εNd(t) values of −15.2 to −10.4, and negative εHf(t) values of −18.7 to −7.9. The light Mo isotope signatures are associated with enrichments in not only radiogenic Sr-Nd-Hf isotopes but also melt-mobile incompatible elements in the basalts. This indicates that the mantle source of these basalts would be generated by metasomatic reaction of the mantle wedge peridotite with felsic melts derived from partial melting of the deeply subducted continental crust. Given that continental crust usually exhibits heavy Mo isotope compositions, it would be dehydrated during its subduction to subarc depths. While the dehydration would have released isotopically heavy Mo fluids from the subducting crust, it leaves isotopically light Mo in the residual crust. The dehydrated continental crust underwent partial melting at subarc depths to produce felsic melts with low δ98Mo values, transferring the light Mo isotope signature into the mantle source of the basalts. Therefore, Mo isotopes in mafic igneous rocks are a powerful means to decipher the recycling of crustal components at convergent plate margins. Our study provides the first insight into Mo isotope recycling in continental subduction zones.

The influence of crustal recycling on the molybdenum isotope composition of the Earth's mantle

Several studies have suggested that the Earth's upper mantle is slightly enriched in light molybdenum isotopes relative to bulk Earth, defined by chondrites, but there is no consensus on the presence of this subtle but potentially notable signature. To establish better whether or not the 98Mo/95Mo of Earth's upper mantle is indeed sub-chondritic, we have analysed hand-picked glasses of depleted (i.e. chondrite normalised La/Sm<1) mid-ocean ridge basalts (MORB) from the Pacific, Atlantic and Indian ocean basins. The mean Mo isotope composition of our depleted MORB relative to reference NIST SRM 3134 (δ98/95MoNIST SRM 3134) is −0.22±0.03‰ (95% confidence interval, c.i.) compared to a value of −0.15±0.01‰ (95% c.i.) for bulk Earth. Our high precision analyses of the 234U/238U activity ratios of these samples are within uncertainty of unity, which rules out the effect of possible secondary, sea-floor processes as the dominant cause of their low δ98/95MoNIST SRM 3134. We further report experimental data showing that sulphide liquid has δ98/95MoNIST SRM 3134 0.25±0.01‰ lower than basaltic silicate liquid at 1400°C. This fractionation is too small to significantly alter the Mo isotope composition of basalts relative to their sources during melting or differentiation.Our MORB data show that resolvably sub-chondritic Mo isotope compositions are common in the upper mantle. Moreover, an appropriately weighted average δ98/95MoNIST SRM 3134 of depleted and enriched MORB, taken from this study and the literature, yields an estimated mantle value of −0.20±0.01‰, indicating that the upper mantle as a whole is sub-chondritic. Since prior work demonstrates that core formation will not create a residual silicate reservoir with a sub-chondritic δ98/95MoNIST SRM 3134, we propose that this feature is a result of recycling oceanic crust with low δ98/95MoNIST SRM 3134 because of Mo isotope fractionation during subduction dehydration. Such an origin is in keeping with the sub-chondritic Th/U and low Ce/Pb of the depleted mantle, features which cannot be explained by simple melt extraction. We present mass balance models of the plate tectonic cycle that quantitatively illustrate that the δ98/95MoNIST SRM 3134 of the Earth's mantle can be suitably lowered by such oceanic crustal recycling. Our Mo isotope study adds to the notion that the depleted mantle has been substantially modified by geodynamic cycling of subduction-processed oceanic crust.

Reconciling evidence of oxidative weathering and atmospheric anoxia on Archean Earth.

Evidence continues to emerge for the production and low-level accumulation of molecular oxygen (O2) at Earth’s surface before the Great Oxidation Event. Quantifying this early O2 has proven difficult. Here, we use the distribution and isotopic composition of molybdenum in the ancient sedimentary record to quantify Archean Mo cycling, which allows us to calculate lower limits for atmospheric O2 partial pressures (PO2) and O2 production fluxes during the Archean. We consider two end-member scenarios. First, if O2 was evenly distributed throughout the atmosphere, then PO2 > 10–6.9 present atmospheric level was required for large periods of time during the Archean eon. Alternatively, if O2 accumulation was instead spatially restricted (e.g., occurring only near the sites of O2 production), then O2 production fluxes >0.01 Tmol O2/year were required. Archean O2 levels were vanishingly low according to our calculations but substantially above those predicted for an abiotic Earth system.

The impact of postdepositional alteration on iron- and molybdenum-based redox proxies

Abstract Ratios of (un)reactive iron species, authigenic molybdenum contents (Moauth), and molybdenum isotope compositions (δ98Moauth) in sedimentary rocks are geochemical proxies that are widely used to reconstruct past marine redox states, which have been calibrated in modern marine settings covering oxic to euxinic conditions. However, syn- and postdepositional processes can result in alterations and ambiguities of proxy-derived redox signals that can challenge the validity of paleoreconstructions. We present new data from modern organic-rich sediments of two oxygen minimum zone settings in the Gulf of California and the Peruvian margin. The results show that Mo is fully immobilized shortly after deposition by reaction with hydrogen sulfide (H2S) produced during organoclastic sulfate reduction. Thus, any H2S produced deeper in the sediment (e.g., by sulfate reduction coupled to anaerobic methane oxidation) leaves the initially deposited Mo concentrations and δ98Mo signatures unaltered, which supports the robustness of Mo-based redox proxies. In contrast, the Fe speciation data reveal continued pyritization due to constant exposure of Fe minerals to H2S. Importantly, both Fe bound to oxides and carbonates (highly reactive Fe) and also poorly reactive Fe (e.g., sheet silicates) undergo pyritization during early diagenesis. This process generates Fe-based proxy signatures that falsely imply ferruginous or euxinic conditions.

Molybdenum isotope composition of seep carbonates – Constraints on sediment biogeochemistry in seepage environments

Methane seepage causes significant changes in the local diagenetic environment and impacts the marine sedimentary cycles of sulfur, carbon, and molybdenum (Mo), which are intimately linked to each other. Molybdenum isotopic compositions were determined for authigenic carbonate nodules and bulk sediments of a 230-m long sediment core collected at a methane seep site in the South China Sea. Authigenic carbonate nodules occur at different sediment depths and display low δ13C values around −50‰ revealing that carbon predominantly derived from biogenic methane. Moreover, seep carbonate nodules are substantially enriched in Mo (Al-normalized enrichment factors MoEF between 27 and 70), and display nearly constant δ98Mo values around an average of +1.9‰ (relative to NIST3134 = +0.25‰), close to the Mo isotopic composition of modern seawater (+2.3‰). Linear positive correlations between δ98Mo values and pyrite sulfur contents and Mo contents were identified for seep carbonate samples. This implies that Mo scavenging into pyrite is likely controlled by hydrogen sulfide availability derived from sulfate-driven anaerobic oxidation of methane (SD-AOM). The essentially constant and high δ98Mo values suggest that Mo species were nearly quantitatively incorporated into the authigenic carbonates under highly sulfidic conditions, probably at times when the sulfate-methane transition zones (SMTZ) was situated at shallow sediment depth at times of high seepage activity. In contrast, bulk sediments that exhibit typical marine δ13C values close to 0‰ (versus VPDB), hence devoid of methane-derived seep carbonates, are characterized by low Mo contents (MoEF = ~0.9). Their lower δ98Mo values mostly ranging from +0.2 to +0.8‰, in accord with low Mo contents, likely reflect input of detrital Mo. Bulk sediments bearing seep carbonate nodules display variable δ13C values lower than −5‰ and are characterized by a wide range in Mo contents (MoEF from 1.5 to 45.3) and δ98Mo values (+0.7 to +3.0‰). The high δ98Mo values of some sediment samples, relative to the seep carbonate signature, might indicate an additional accumulation of a 98Mo enriched pool of molybdenum during later SD-AOM at a deeper SMTZ postdating initial seep carbonate formation; such 98Mo enrichment was probably caused by preceding preferential removal of lighter Mo isotopes from pore fluids. Our results indicate that Mo isotope systematics of seep deposits can provide constraints on the dynamics of the biogeochemical cycling of carbon, sulfur, and trace elements in methane seep environments. A potential for authigenic carbonates to archive the Mo isotopic composition of seawater is indicated, but will have to be assessed in future studies.

Molybdenum isotope fractionation at upper-crustal magmatic-hydrothermal conditions

Molybdenum isotopes are an established proxy for paleoredox conditions in low-temperature surface systems. However, the mechanisms behind demonstrated Mo isotope fractionation during igneous and hydrothermal processes at elevated temperatures are still controversial. This study focusses on a comprehensive dataset documenting the late stage magmatic-hydrothermal evolution of Mo isotope systematics in miarolitic cavities and their host granite from a shallow arc-related intrusive system, the Torres del Paine laccolith in Chile. Molybdenum isotopic compositions (δ98MoSRM3134) were measured for (i) granitic bulk with or without petrographic signs of fluid loss, (ii) magmatic-hydrothermal fluids, and (iii) successively crystallised hydrothermal minerals and range from −1.6 to +1.8‰. The observed variability in δ98MoSRM3134 for individual miarolitic cavities approaching closed system conditions are smaller than the overall range in our dataset but still exceed 1.5‰. The Mo isotopic signature of magmatic fluids was directly measured for the first time by bulk dissolution of magmatic fluid inclusion bearing quartz. Absolute values for magmatic-hydrothermal fluids vary between +0.6 to +1.8‰ δ98MoSRM3134, which is significantly heavier than the granitic bulk rock signatures of −0.1 to +0.46‰ δ98MoSRM3134. Hydrothermal minerals in contrast exhibit variably light δ98MoSRM3134 between −1.6 and +0.6‰. Isotopic differences Δ98Mofluid-mineral between fluid and hydrothermal minerals coexisting in the sampled cavities are largest for plagioclase with 1.9–2.2‰ Δ98Mo, and amount to 1.6–1.9‰ Δ98Mo and 1.5–1.9‰ Δ98Mo for alkali feldspar and biotite, respectively. Smaller values of 1.2–1.5‰ Δ98Mofluid-siderite, 0.8–1.9‰ Δ98Mofluid-molybdenite, 0.4–1.2‰ Δ98Mofluid-titanite and 0.4–1.3‰ Δ98Mofluid-allanite are obtained for higher Mo concentration minerals. Given that fluid-mineral pairs coexisted in equilibrium the ranges in Δ98Mofluid-mineral values we report offer first constraints on the extent of hydrothermal Mo isotope fractionation. The magnitude and direction of these values agrees well with fractionation factors calculated based on an ionic bond-strength model for the incorporation of Mo6+ in hydrothermal minerals for crystallisation temperatures in miarolitic cavities (650–450 °C). This implies that significant fractionation effects can arise during hydrothermal processes even without changes in Mo redox state from oxidised fluid.We can summarise the Mo isotope evolution during magmatic-hydrothermal processes as follows: First, Mo is transferred into the fluid phase exsolving from solidifying magma during late stage igneous evolution. The exsolved fluid subsequently precipitates hydrothermal minerals upon cooling, which dominantly incorporate light Mo isotopes (at variable KDMo(fluid-mineral)). With progressive hydrothermal crystallisation, the remaining fluid evolves to increasingly higher δ98MoSRM3134 along with decreasing Mo concentration.Our data demonstrate that large variability in Mo isotopic signatures can be produced solely by primary magmatic-hydrothermal isotope fractionation processes at elevated temperatures. The generated large range in δ98Mo signatures implies that (i) Mo isotopic signatures of evolved samples cannot be employed for tracing sources or precursor processes unless isotopic fractionation during magmatic-hydrothermal stages is quantified and can be corrected for; and (ii) mass balance models for the global Mo cycle used in paleoredox reconstruction need to account for potentially heterogeneous and isotopically fractionated continental contributions from evolved or hydrothermally overprinted rocks.

Molybdenum isotopic evidence for anoxic marine conditions during the end-Permian mass extinction

The end-Permian mass extinction was the most severe biotic crisis of the Phanerozoic. Oceanic anoxia has long been considered one of the main drivers of the biotic crisis. We examined the molybdenum isotope compositions of bulk carbonate samples from two Permian–Triassic boundary sections in southern China: the Meishan Global Stratotype Section and Point and the Dajiang section (an isolated carbonate platform). We also measured the Mo contents of the carbonate phases, oxides, and sulfides of the carbonate samples via sequential extraction to investigate the samples' Mo-bearing minerals. In the Meishan section, from Bed 23 to the middle of Bed 24, the δ98/95Mocorr.Al values decrease slowly from +0.89‰ to +0.35‰, while from the middle of Bed 24 to Bed 25, the δ98/95Mocorr.Al values abruptly increase from +0.35‰ to +1.78‰. In the Dajiang section, samples below the mass extinction boundary have relatively low δ98/95Mocorr.Al values with a narrow variation range (−0.05‰ to +0.46‰; average + 0.19‰); the δ98/95Mocorr.Al values of the five samples directly above the extinction boundary increase abruptly (vary from +1.38‰ to +1.63‰; average of +1.53‰), and the δ98/95Mocorr.Al values of other samples above the mass extinction boundary fluctuate slightly before returning to pre-extinction values. The results of the sequential extraction procedure indicate that carbonate-phase Mo accounts for a small portion of the total Mo (average of 13.6 and 18.4 wt% for the Meishan and Dajiang sections, respectively), oxidized and sulfided Mo are the dominant forms in the carbonate samples. Moreover, the carbonate samples with high δ98/95Mo values tend to have high contents of sulfided Mo. We suggest that marine carbonate sediments record paleo-ocean Mo isotopic signals by a mechanism similar to that of other open-ocean sediments (e.g., shales): higher δ98/95Mo values in the sediments indicate a more reducing sedimentary environment. Based on this assumption, the δ98/95Mo values of the studied sections indicate that during the Permian–Triassic transition, oceanic anoxia coincided with or slightly predated the mass extinction, and the ocean anoxic event lasted for a relatively short time, likely less than 0.061 Myr.

Preliminary exploration of molybdenum isotope fractionation during coprecipitation of molybdate with abiotic and microbial calcite

Molybdenum isotopes in marine carbonates have been proposed to track the redox conditions of oceans through time. Large variations of Mo concentration and isotopic composition were observed in primary biogenic carbonates and shallow-water carbonate sediments from modern oceans. However, it remains unknown whether the abiotic calcium carbonate precipitation process fractionates Mo isotopes and affects Mo isotopic composition in marine carbonates. To better understand carbonate Mo isotope proxy, we conducted abiotic calcite coprecipitation experiment to determine Mo partition coefficient in calcite and Mo isotope fractionation during molybdate incorporation into calcite. We also investigated Mo isotopic compositions in microbial calcite induced by photosynthetic activity of cyanobacteria and diatoms in the surface water of Fayetteville Green Lake (FGL), New York, USA.Our calcite coprecipitation experiment, for the first time, determined the Mo partition coefficient Kd=MoCacalcite/MoO42−CO32−solution = 1.49 × 10−5 in calcite at pH ~8.3. We observed a relatively small Mo isotope fractionation of 0.15 ± 0.06‰ with light Mo isotopes preferentially incorporated into calcite. The Mo isotope fractionation observed in our experiment most likely results from equilibrium isotope fractionation among different Mo species and preferential incorporation of the isotopically light Mo(VI) species (MoO42−) into calcite. Microbial calcite formed in FGL fractionated Mo isotopes in the same direction as abiotic calcite coprecipitation experiment but with significantly larger isotope fractionations (0.82–2.45‰) and significantly higher Mo concentrations (0.04–0.28 ppm vs. the inferred 0.002 ppm Mo from our abiotic calcite coprecipitation experiment). Molybdenum isotopic composition decreased with Mn/Fe ratios from +1.56 to −0.07‰, suggesting that Fe and Mn oxides dominantly control Mo isotope fractionation in these microbial calcite samples. Additionally, isotopically light Mo associated with organic matter (e.g., live or dead microorganisms like cyanobacteria) might also contribute to the light Mo isotopic composition in microbial calcite.The relatively small Mo isotope fractionation (0.15 ± 0.06‰) and Mo partition coefficient (1.49 × 10−5) in our abiotic calcite coprecipitation experiment indicate that calcite precipitation process has negligible effects on Mo isotopic composition in primary biogenic calcite precipitates. In contrast, Mo associated with organic matter, detrital particles, and Fe and Mn oxides, and carbonate diagenesis strongly affect Mo isotopic compositions in calcium carbonate precipitates in nature. Caution should be taken when applying Mo isotopes in ancient carbonates rocks to track seawater redox conditions.

Pulsed oxygenation events drove progressive oxygenation of the early Mesoproterozoic ocean

The Mesoproterozoic era has long been considered a time of relative environmental and biological stasis. However, emerging insight suggests that this period may have been more dynamic than previously considered, both in terms of oxygenation and potential consequences for biological evolution. Nevertheless, our understanding of this immense period of time remains limited. To provide more detailed constraints on oxygenation dynamics, we report a multiproxy geochemical study of an early Mesoproterozoic (∼1600–1540 million years ago, Ma) carbonate-dominated succession from the North China craton. We include inorganic carbon isotope (δ13Ccarb), iron-speciation, and major and trace element data, in addition to molybdenum isotopic compositions (δ98/95Mo). These geochemical data support previous inferences of persistent anoxic and ferruginous deeper water conditions in the earliest Mesoproterozoic ocean, with limited oxygenation of surface waters. However, the behaviour of these redox-sensitive geochemical proxies reveals pulsed oxygenation events, with each event increasing the maximum depth of oxygenation, leading to overall progressive oxygenation of the ocean. During these pulsed oxygenation events we find the lightest Mo isotope signatures ever measured in the rock record, which we attribute to initial drawdown of isotopically light Mo in association with extensive Mn and Fe (oxyhydr)oxide precipitation, followed by diagenetic recycling. However, shallower water sediments deposited after the pulses of deeper water oxygenation more faithfully record the Mo isotopic composition of coeval seawater. For these samples, we utilise a single reservoir Mo cycling model, constrained by an updated estimate of Mesoproterozoic seawater Mo concentration, and scaled using a function associated with differential organic carbon flux between the shelf and basin. When scaled to modern rates of Mo accumulation under variable marine redox conditions, our modelling estimates suggest a minimum oxic seafloor area of ∼30% of the total seafloor area at ∼1540 Ma. It remains unclear whether the oxygenation observed across this ∼60 million year interval represents a progressive transition to a more persistently oxygenated ocean, or whether oceanic oxygen levels fluctuated considerably through the later Mesoproterozoic.