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The end-Devonian Hangenberg Crisis was one of the biggest Phanerozoic mass extinctions. However, the mechanism(s) that triggered this event is still debated. In this study, multiple geochemical paleoredox proxies (redox-sensitive trace metals [e.g., Mo, U, Re, V] and isotope systems [Mo, U, S, C]) were applied to the Exshaw Formation black shales to infer ocean redox conditions during the Hangenberg Crisis. The measured δ13C org values generally decrease upsection in the Exshaw Formation. The Exshaw black shales have increasing maturity levels from east to west in the Western Canada Sedimentary Basin. Large sulfur isotope fractionations (–15‰ to 65‰) between original seawater sulfate and pyrite is best explained by microbial sulfate reduction during deposition and early diagenesis rather than thermochemical sulfate reduction. Precise Re-Os ages previously reported for the Exshaw Formation suggest that metal geochemical signatures in these shales, including overmature shales, were minimally affected by hydrocarbon maturation and reflected depositional conditions. Both Sr/Ba and TS/TOC ratios (the latter only from immature shales affected minimally by hydrocarbon generation) suggest predominantly brackish and marine settings with significant water exchange between the basin and open ocean. The TOC contents, redox-sensitive trace metal concentrations (Mo, U, Re), and Fe speciation indicate local bottom waters ranged from suboxic (with sulfidic pore waters) to euxinic. The authigenic δ98Mo and δ238U values for the Exshaw shales range from 0.3‰ to 1.1‰, and from –0.23‰ to 0.39‰, respectively. The δ98Mo of the Exshaw Formation may have been offset to lower values than coeval seawater because of a local particulate Fe-Mn shuttle and/or local weakly euxinic bottom waters. Two different correlation patterns (positive correlation and no correlation) of δ238U and U enrichment factors (EF) were observed for Exshaw black shales deposited from locally euxinic bottom waters. The positive correlation group samples (−0.13‰ to 0.23‰) suggest U isotope fractionations (0.4‰ to 0.8‰) between sediments and seawater like those observed in modern euxinic basins. Higher δ238U values (>0.2‰) in the no correlation group suggest even larger U isotope offsets (0.8–1.2‰) from seawater, possibly associated with removal of U into organic floccule layers during deposition. Applying the above Mo and U isotope fractionations to the Exshaw shales, global seawater δ98Mo and δ238U at the Devonian-Carboniferous boundary (DCB) may have been in the range of 1.4‰ to 1.9‰ and −0.9‰ to −0.5‰, respectively. A Mo and U isotope mass balance model suggests a large areal extent of euxinic seafloor (6–10%) at the DCB, which could be pervasive along continental margins and in epicontinental seas during transgression. Therefore, our data support expanded ocean euxinia at the DCB as an important contributor to the Hangenberg Crisis. [ABSTRACT FROM AUTHOR]

Iron (Fe), sulfur (S), and molybdenum (Mo) geochemistry in marine sediments impacted by hydrothermal plumes and/or cold seeps is complex and has not been systematically documented. Here we characterize Fe, S, and Mo diagenesis in sediments between the Minami‐Ensei Knoll hydrothermal field and a cold‐seep site of the middle Okinawa Trough. Results show that distances away from the hydrothermal field and the steep trough slope may significantly affect the transport of hydrothermal Fe. The transformation of hydrothermal reactive Fe to poorly reactive or unreactive Fe‐bearing phyllosilicates decreased the relative fractions of highly reactive Fe (FeHR) in total Fe (FeHR/FeT). Despite this, the standing stocks of Fe oxides in the methane‐free sediments have not been dampened, indicating no net impacts of hydrothermal Fe inputs on the size of Fe oxides. In the methane‐free sediments, low ratios of total reduced inorganic sulfide (TRIS) to total organic carbon (TOC) (TRIS/TOC), highly 34S‐depleted pyrite, and low Mo contents suggest that organoclastic sulfate reduction is at low rates and plays a limited role in carbon cycle. In the cold‐seep sediments, however, intense sulfate reduction coupled to anaerobic methane oxidation significantly elevate TRIS/TOC ratios, Mo enrichment, and isotope compositions of Mo and pyrite‐S. This pathway is expected to be important in carbon cycle in the basin due to the wide occurrence of cold seeps. Our results highlight the important controls of the local extreme depositional/diagenetic conditions on sedimentary S and Mo records, with implications for the reconstruction of paleoredox states of the past earth's surface. Plain Language Summary: Submarine methane‐rich cold seeps and hydrothermal venting exert important influences on the geochemistry of iron, sulfur, and molybdenum. However, the influences of the two extreme environments on the three interlinked elements in seafloor have not been well documented. Here, we used solid‐phase chemistry of four sediment cores located between a hydrothermal field and a cold‐seep site in the middle Okinawa Trough to document the impacts of the two extreme environments on the geochemistry of iron, sulfur, and molybdenum. Results show that hydrothermal Fe signature in the sediments is largely controlled by the distances away from the hydrothermal field and the steep slope of the trough; the hydrothermal Fe inputs have no net impacts on sedimentary iron oxide contents due to simultaneous inactivation of reactive Fe. In the methane‐free sediments, sulfate reduction (SR) is coupled mainly to oxidation of sedimentary organic carbon. In the cold‐seep site, however, SR coupled to anaerobic methane oxidation is the main pathway of carbon cycle, leading to significant enrichments of molybdenum and isotopically heavy pyrite. The differences highlight the important control of local extreme depositional/diagenetic conditions on geochemical records of iron, sulfur, and molybdenum, with implications for the reconstruction of paleoredox states in the past. Key Points: Distances away from the vent, steep basin slope and Fe inactivation control the signature of hydrothermal Fe in the basin sedimentsLow lability of organics limits sulfate reduction (SR) in CH4‐free sediments, while CH4 largely facilitates SR in cold‐seep sedimentsLocal extreme conditions exert key controls on S‐Fe‐Mo diagenesis in the basin [ABSTRACT FROM AUTHOR]

Abstract Iron (Fe), sulfur (S), and molybdenum (Mo) geochemistry in marine sediments impacted by hydrothermal plumes and/or cold seeps is complex and has not been systematically documented. Here we characterize Fe, S, and Mo diagenesis in sediments between the Minami‐Ensei Knoll hydrothermal field and a cold‐seep site of the middle Okinawa Trough. Results show that distances away from the hydrothermal field and the steep trough slope may significantly affect the transport of hydrothermal Fe. The transformation of hydrothermal reactive Fe to poorly reactive or unreactive Fe‐bearing phyllosilicates decreased the relative fractions of highly reactive Fe (FeHR) in total Fe (FeHR/FeT). Despite this, the standing stocks of Fe oxides in the methane‐free sediments have not been dampened, indicating no net impacts of hydrothermal Fe inputs on the size of Fe oxides. In the methane‐free sediments, low ratios of total reduced inorganic sulfide (TRIS) to total organic carbon (TOC) (TRIS/TOC), highly 34S‐depleted pyrite, and low Mo contents suggest that organoclastic sulfate reduction is at low rates and plays a limited role in carbon cycle. In the cold‐seep sediments, however, intense sulfate reduction coupled to anaerobic methane oxidation significantly elevate TRIS/TOC ratios, Mo enrichment, and isotope compositions of Mo and pyrite‐S. This pathway is expected to be important in carbon cycle in the basin due to the wide occurrence of cold seeps. Our results highlight the important controls of the local extreme depositional/diagenetic conditions on sedimentary S and Mo records, with implications for the reconstruction of paleoredox states of the past earth's surface.

Methane seepage has been extensively observed in various continental margin settings. It has profound effects on the marine redox environment and the molybdenum (Mo) cycles in marine sediments. Therefore, there has been much recent attention on the redox-sensitive behavior of Mo in methane seepage environments. However, the characteristics of the Mo isotope composition in the cold-seep system remain poorly understood. In this study, we performed geochemical analyses, including Mo content and isotope composition, on sediment samples (core QDN-MS6) from the "Haima" cold-seep deposit area in the South China Sea. The analysis reveals a significant concentration of authigenic pyrite in the mid-section of QDN-MS6 core (373–403 cm). Moreover, the δ34S value in this interval is notably elevated with high total sulfur/total organic carbon ratio. Additionally, the sediments in the mid-section exhibits substantial enrichment in Mo (enrichment factors of Mo ranging from 5.29 to 39.32). This implies that the sediments in the mid-section are influenced by sulfate-driven anaerobic oxidation of methane. Most notably, the sediments in the mid-section displayed distinct low δ98Mo values (with an average of −0.7‰). After careful consideration, we ruled out the influence of organic matter, an oxic environment, a weakly sulfidic environment, and incomplete removal of thiopolybdate as contributing factors. Based on δ56Fe-Fe/Al ratios, (Mo–U) enrichment factors, and As enrichment factors, we propose that the "benthic Fe-Mn redox shuttle process" is the primary cause of the observed light δ98Mo signatures in sediments. This newly identified mechanism sheds light on Mo isotope cycling in methane seepage environments and enhances our understanding of the Mo isotope cycling process. • δ34S and enrichment of Mo and U indicated sulfate-driven anaerobic oxidation of methane in the 373–403 cm section. • Clear low δ98Mo and δ56Fe characteristics were evident in the sediments of methane seepage. • The reductive dissolution of Fe and Mn (oxy)hydroxides was the main driver of low δ98Mo. • The benthic Fe–Mn redox shuttle process is an important contributor to the Mo source in methane seepage environments. [ABSTRACT FROM AUTHOR]

Oceanic redox changes may be a key environmental trigger for the "Cambrian explosion". However, a hypothesized predominantly oxygenated early Cambrian ocean, inferred from high sedimentary molybdenum (Mo) isotope ratios, conflicts with other evidence for euxinic continental margins. Here, we demonstrate that seawater Mo isotope ratios like those of today's predominantly well-oxygenated oceans can also occur in less oxygenated early Cambrian oceans. New and existing Mo isotope data from early Cambrian black shales in South China span a range of greater than 3‰, which can only be explained by multi-stage fractionation of Mo isotopes. The lowest Mo isotopes ratios likely reflect a Fe-Mn (oxyhydr)oxide shuttle that preferentially transferred the lighter Mo isotopes into weakly euxinic bottom waters where Fe-Mn (oxyhydr)oxide dissolution released isotopically light Mo that was then scavenged by organic matters and sulfide minerals. We use a novel isotope mass balance model to show that Fe-Mn (oxyhydr)oxide shuttle operating on euxinic continental margins would preferentially bury lighter-mass Mo isotopes to such an extent that early Cambrian seawater can become enriched in heavier Mo isotopes at levels comparable to modern seawater. Hence, widespread ocean oxygenation may not have occurred during the early Cambrian as previously thought. Based on this paradigm shift, reconsideration of sedimentary Mo isotope temporal trends suggests a stepwise and protracted oxidation of the oceans until the Late Devonian. Sporadic intervals with Mo isotope ratios spanning over 3‰ and often accompanied by dramatic change in biodiversity (e.g., Ediacaran-Cambrian and Permian-Triassic transitions) probably had euxinic continental margins. Links between fossil records and marine redox structures disclose the importance of spatiotemporal fluctuations in continental margin euxinia on biodiversity changes. [ABSTRACT FROM AUTHOR]

The chemical and lithological heterogeneities of Earth's mantle in oceanic settings have long been attributed to the presence of recycled materials such as subducted crust, oceanic or subcontinental lithosphere mantle and sediment, as evidenced by Mid-Ocean Ridge Basalts (MORB) and Ocean Island Basalts (OIB). However, the extent to which recycled materials contribute to mantle heterogeneity in continental settings remains uncertain. Here we present systematic Sr-Nd-Pb-Mo isotope data for the Pingchuan picritic porphyries and associated mafic rocks (basalts and gabbros) in the central Emeishan Large Igneous Province, southwestern China, to resolve this issue. In contrast to the gabbro samples that show continental crust contamination in variations of δ98/95Mo (relative to NIST SRM 3134, −0.35 ± 0.03 to 0.03 ± 0.02 ‰), the Pingchuan picritic porphyries and basalts show a large range of δ98/95Mo (−0.32 ± 0.01 to −0.15 ± 0.06 ‰) and broadly negative trend between δ98/95Mo and Ce/Mo ratios, which cannot be explained by post-magmatic alteration, crustal contamination, fractional crystallization, olivine accumulation, metasomatism or fluid-activity in the mantle source. They were most likely derived from incongruent melting of a heterogeneous mantle source, i.e., a deep mantle with chondritic-like δ98/95Mo (−0.15 ± 0.01 ‰) containing recycled oceanic crust (+ sediment) with light δ98/95Mo (< −0.32 ‰). The basalts display low δ98/95Mo (−0.32 ± 0.01 to −0.24 ± 0.04 ‰), high 87Sr/86Sr i (0.7046 to 0.7059), TiO 2 , Sm/Yb, Dy/Yb and Ce/Mo, pointing to characteristics of melts produced by MORB-type eclogite (+ sediment) at relatively low degrees of partial melting. With increasing degrees of partial melting, melts produced by peridotite (87Sr/86Sr i = 0.7035 to 0.7043, chondritic-like δ98/95Mo = −0.18 ± 0.03 to −0.15 ± 0.06 ‰) come to dominate, resulting in low TiO 2 , Sm/Yb, Dy/Yb and Ce/Mo of the Pingchuan picritic porphyries. From a global perspective, continental basalts also plot on the negative trend defined by the OIB datasets (δ98/95Mo vs. Ce/Mo). As such, Mo isotope may provide a robust tracer of recycled oceanic crust in shaping mantle heterogeneities that occur in both oceanic and continental settings. • The Pingchuan lavas show broad negative trend between δ98/95Mo and Ce/Mo. • The recycled oceanic crust (+sediment) contributes to the mantle source of Emeishan lavas. • Mo isotope is a powerful tracer of global mantle heterogeneity. [ABSTRACT FROM AUTHOR]

The concentration and isotopic composition of sedimentary molybdenum (Mo) has been used to distinguish different redox environments in modern marine settings and in the geological record. We report Mo concentrations and δ98Mo from porewaters and sediments in three anoxic East Anglian salt marsh pond environments: (1) 'iron-rich' sediments containing high concentrations of dissolved ferrous iron (up to 2 mM), (2) 'sulfide-rich' sediments containing very high concentrations of aqueous sulfide (up to 10 mM) and, (3) sediments that we consider to be intermediate between 'iron-rich' and 'sulfide-rich' conditions. In iron-rich sediments, we suggest that iron speciation and mineralogy controls the concentration and isotopic composition of Mo. Despite similar aqueous sulfide profiles, the intermediate and sulfide-rich pond sediment have different porewater Mo concentrations and δ98Mo. In the sulfide-rich pond sediment, we suggest that the concentration and isotopic composition of Mo is controlled by solubility equilibrium with an Fe-Mo-S mineral species (e.g. FeMoS 4) due to similarities in sediment and porewater δ98Mo throughout the sediment column. In the intermediate pond sediment, we conclude that active breakdown of iron oxides redistributes porewater Mo, observable as a peak of dissolved Mo (>100 ppb), which diffuses within the sedimentary porewaters. The sedimentary δ98Mo is higher in sulfide-rich and intermediate pond sediment (mean = 1.66‰, range = 0.98–1.92‰) than in iron-rich pond sediment (mean = 1.10‰, range = 0.28–1.65‰) with all ponds having sedimentary δ98Mo that is lower than seawater. The maximum sedimentary δ98Mo observed in these anoxic sediments, which is 0.5–0.7‰ lower than seawater, appears to be set by Fe-Mo-S equilibration with ambient thiomolybdate species. We suggest diagenetic overprinting can cause more efficient capture of pond water Mo and causes sediment δ98Mo of originally iron-rich pond sediment to evolve to higher values at progressively higher aqueous sulfide concentrations. [ABSTRACT FROM AUTHOR]

Molybdenum isotopic compositions (δ98/95Mo) of Archean and Paleoproterozoic Fe- and Mn-rich sedimentary rocks have been used to investigate local accumulations of O 2 in an O 2 -lean ocean. Previous studies interpret that these δ98/95Mo values would be a representation of the global minimum for δ98/95Mo of contemporaneous seawater and would, therefore, link to global paleoredox conditions. Here, we present new δ98/95Mo data on modern marine hydrothermal Fe-Mn oxides, for a wide range of Fe/Mn ratios, from five hydrothermal systems. Samples composed mainly of Fe oxides (Fe/Mn > 10) show positive values (δ98/95Mo ∼ +0.7‰), whereas those containing substantial amounts of Mn oxides (Fe/Mn < 10−1) generally exhibit negative values (δ98/95Mo ∼ −0.8 ‰). These δ98/95Mo values are consistent with isotopic fractionations due to the adsorption of seawater Mo onto Fe and Mn oxides, respectively. The aforementioned positive and negative values are connected by a positive correlation between δ98/95Mo values and the Fe/Mn ratios of samples with Fe/Mn ∼ 10−1–101. The positive correlation can be explained by the mixing of δ98/95Mo in Fe and Mn oxides. Based on these data, we propose that the measured δ98/95Mo trends for the modern hydrothermal Fe-Mn oxides can be reproduced using a simple mass-balance calculation with both modern seawater δ98/95Mo and Mo isotopic fractionations due to the adsorption of Mo onto Fe and Mn oxides. By applying this mass-balance calculation to published Fe/Mn and δ98/95Mo data on ancient Fe- and Mn-rich sedimentary rocks, we estimate Archean and Paleoproterozoic seawater δ98/95Mo and their deviations (Δ98/95Mo) from modern seawater δ98/95Mo. The long-term evolution of seawater δ98/95Mo inferred as a result suggests extensive deposition of Fe and Mn oxides at ∼2.3–2.2 Ga and an expansion of euxinic conditions at ∼1.9 Ga, which are broadly consistent with the proposed redox evolution of the ocean–atmosphere system during the Archean and Paleoproterozoic. [ABSTRACT FROM AUTHOR]

The molybdenum (Mo) isotope system is a powerful tool for tracing recycled crustal components in volcanic rocks. Existing Mo isotope data show that intraplate basalts have highly variable Mo isotopes. However, the heterogeneity of Mo isotopes in intraplate basalts was interpreted to be inherited from recycling of different slab components, and its origin remains unclear. Here we synthesize Mo isotope and other geochemical data for global intraplate basalts, which provide insights into Mo recycling in Earth's deep mantle. Global intraplate basalts display highly variable δ98Mo values of −0.84‰ to 0.10‰, suggesting mantle Mo isotopic heterogeneity. Most of the basalts have low δ98Mo values and high Ce/Mo ratios that are similar to those of the oceanic eclogites, indicating that recycled dehydrated oceanic crustal components were involved in their mantle sources. However, some intraplate basalts have heavy Mo isotopes. This reveals that slab components at shallow depths were dragged by the downgoing slab and have carried heavy Mo isotopes into the deep mantle. Significantly, high δ98Mo values of these basalts are also associated with low Ce/Mo ratios, high fluid-mobile element contents, and depleted radiogenic Sr Nd isotopes. Given that recycled basaltic oceanic crust and sediment are generally efficiently dehydrated during subduction and sequester light Mo isotopes, subducted serpentinised lithospheric mantle is a plausible candidate to transfer heavy Mo isotopes into the deep mantle. Thus, Mo isotope systematics of intraplate basalts record complex geochemical fluxes from subducting slab to the deep mantle, in which subducted serpentinites play an important role in the Mo isotope recycling. Our study highlights the effectiveness of Mo isotopes as indicators for geochemical heterogeneity of Earth's deep mantle. [ABSTRACT FROM AUTHOR]

The Mesoproterozoic Era (1,600–1,000 million years ago, Ma) geochemical record is sparse, but, nevertheless, critical in untangling relationships between the evolution of eukaryotic ecosystems and the evolution of Earth‐surface chemistry. The ca. 1,400 Ma Xiamaling Formation has experienced only very low‐grade thermal maturity and has emerged as a promising geochemical archive informing on the interplay between climate, ecosystem organization, and the chemistry of the atmosphere and oceans. Indeed, the geochemical record of portions of the Xiamaling Formation has been used to place minimum constraints on concentrations of atmospheric oxygen as well as possible influences of climate and climate change on water chemistry and sedimentation dynamics. A recent study has argued, however, that some portions of the Xiamaling Formation deposited in a highly restricted environment with only limited value as a geochemical archive. In this contribution, we fully explore these arguments as well as the underlying assumptions surrounding the use of many proxies used for paleo‐environmental reconstructions. In doing so, we pay particular attention to deep‐water oxygen‐minimum zone environments and show that these generate unique geochemical signals that have been underappreciated. These signals, however, are compatible with the geochemical record of those parts of the Xiamaling Formation interpreted as most restricted. Overall, we conclude that the Xiamaling Formation was most likely open to the global ocean throughout its depositional history. More broadly, we show that proper paleo‐environmental reconstructions require an understanding of the biogeochemical signals generated in all relevant modern analogue depositional environments. We also evaluate new data on the δ98Mo of Xiamaling Formation shales, revealing possible unknown pathways of molybdenum sequestration into sediments and concluding, finally, that seawater at that time likely had a δ98Mo value of about 1.1‰. [ABSTRACT FROM AUTHOR]

The IAB iron meteorite complex consists of a main group (MG) and five chemical subgroups (sLL, sLM, sLH, sHL, and sHH). Here, mass-independent Mo and radiogenic 182 W isotope compositions are reported for IAB complex meteorites to evaluate the genetics and chronology, respectively, of the MG and subgroups. Osmium isotopes are used to correct for cosmic ray exposure effects on isotopes of Mo and W. The MG and three subgroups (i.e., sLL, sLM, and sLH), characterized by low Au abundances, have the same Mo isotopic compositions within analytical uncertainty, consistent with a common genetic origin. These meteorites, together with winonaites, are the only cosmochemical materials yet identified with Mo isotopic compositions that are identical to Earth. The Mo isotopic compositions of two subgroups characterized by higher Au abundances (sHL and sHH) are identical to one another within uncertainty, but differ from the low Au subgroups, indicating derivation from genetically distinct materials. The MG has a 182 W, post calcium–aluminum inclusion (CAI) formation model age of 3.4 ± 0.7 Ma. One of the low Au subgroups (sLM) is ∼1.7 Ma younger, whereas the high Au subgroups are ∼1.5–3 Ma older. The new Mo–W data, coupled with chemical data, indicate that the MG and the low Au subgroups formed in different impact-generated melts, some of which evidently formed on a chemically disparate, but genetically identical parent body. The high Au subgroups likely formed via core-formation processes on separate, internally-heated parent bodies from other IAB subgroups. The IAB complex meteorites fall on a linear trend defined by 94 Mo/ 96 Mo vs. 95 Mo/ 96 Mo, along with most other iron meteorite groups. Variation along this line was caused by mixing between at least two nebular components. One component was likely a pure s -process enriched nucleosynthetic carrier, and the other a homogenized nebular component. Sombrerete, currently classified as an sHL iron, has a Mo isotopic composition that is distinct from all IAB complex meteorites analyzed here. Along with group IVB iron meteorites and some ungrouped iron meteorites, it falls on a separate line from other meteorites which may reflect addition of an r -process-enriched component, and it should no longer be classified as a IAB iron. [ABSTRACT FROM AUTHOR]

Niutoushan Pb–Zn sulfide orebodies are found in the western part of the Xiangshan volcanic basin; however, genesis of the mineralization is highly debated. To appropriately constrain the mineralization process and genesis of the Niutoushan Pb–Zn sulfide orebodies present in the volcanic rocks in the Xiangshan volcanic basin, we investigate Mo isotope compositions of sulfide minerals (pyrite and sphalerite) from the Niutoushan Pb–Zn sulfide orebodies and those of the volcanic and subvolcanic rocks (the least-altered porphyritic lava and granitic porphyry rocks) from the Xiangshan volcanic-subvolcanic complex. The δ98Mo values (normalized to NIST3134 of 0.25 ‰) of the Xiangshan volcanic-subvolcanic rocks, including the least-altered granitic porphyry (0.20 ‰–0.26 ‰) and porphyritic lava rocks (0.40 ‰–0.51 ‰), are similar to those of the bulk continental crust (0.1 ‰–0.35 ‰, Willbold and Elliott, 2017). The δ98Mo values of sulfides from the Niutoushan Pb–Zn sulfide orebodies (pyrite: −0.95 ‰ to 0.30 ‰, sphalerite: −0.94 ‰ to −0.59 ‰) are mostly lower than those of the continental crust and span a wide range. After examining potential factors that may cause δ98Mo variation, it is concluded that the wide δ98Mo range of the Niutoushan Pb–Zn sulfide orebodies resulted mainly from the hydrothermal fluid evolution through Rayleigh fractionation in the mineralization process than from changes in other factors of the fluid, such as temperature, redox, and fluid boiling process. Further, based on this conclusion, the δ98Mo value of the initial hydrothermal fluid finally forming the Niutoushan sulfide orebodies was estimated to be 0.46 ‰ by combining the lowest δ98Mo value of the sulfides and the Mo isotope fractionation between the fluid and molybdenite. The initial hydrothermal ore-forming fluid with such a δ98Mo value was most likely derived from the Xiangshan subvolcanic magmas. This study highlights the potential of Mo isotope composition as a novel tool for tracking origin and evolution of ore-forming hydrothermal fluids and metals in case of Pb–Zn sulfide mineralization associated with igneous activities. [Display omitted] • Fluid evolution process controlling δ98Mo variation of Pb–Zn sulfide orebodies • δ98Mo data reveal genesis of Pb–Zn sulfide orebodies in Xiangshan volcanic basin. • Ore-forming fluid and matter of Pb–Zn sulfide ore from Xiangshan subvolcanic magmas • Mo isotope may track sources for other ore deposits besides porphyry-type deposits. [ABSTRACT FROM AUTHOR]

Procedures for the separation, purification, and high-precision analysis of mass-independent isotopic variations in molybdenum (Mo) using negative thermal ionization mass spectrometry are reported. Separation and purification of Mo from silicate and metal matrices are achieved using a two-stage anion exchange chromatographic procedure. Molybdenum is ionized as the MoO 3 − species using a double filament assembly. The MoO 3 − ion beams are collected using Faraday cup detectors equipped with a mixed array of amplifiers utilizing 10 11 and 10 12 Ω resistors, which allows for in situ measurement and correction of oxygen isobars. The long-term external reproducibility of 97 Mo/ 96 Mo, the most precisely measured Mo isotope ratio, is ±5.4 ppm (2SD), based on the repeated analyses of the Alfa Aesar Specpure ® Mo plasma standard and using 98 Mo/ 96 Mo for fractionation correction. The long-term external reproducibilities of 92 Mo/ 96 Mo, 94 Mo/ 96 Mo, 95 Mo/ 96 Mo, and 100 Mo/ 96 Mo are ±107, 37, 23, and 32 ppm (2SD), respectively. With this precision, smaller differences in Mo isotopic compositions can be resolved than have been previously possible. [ABSTRACT FROM AUTHOR]

We present Mo isotopic ratios of molybdenite from five porphyry molybdenum deposits (Chagele, Sharang, Jiru, Qulong, and Zhuonuo) and one quartz-molybdenite vein-type deposit (Jigongcun) along the Gangdese metallogenic belt in the Tibetan Plateau. These deposits represent a sequence of consecutive events of the India-Asia collision at different periods. Additional molybdenite samples from the Henderson Mo deposit (USA), the oceanic subduction-related El Teniente (Chile), and Bingham (USA) porphyry Cu-(Mo) deposits were analyzed for better understanding the controls on the Mo isotope systematics of molybdenite. The results show that molybdenite from Sharang, Jiru, Qulong, and Zhuonuo deposits have similar δMo (∼0 ‰), in agreement with the values of the Henderson Mo deposit (−0.10 ‰). In contrast, samples from the Changle and Jigongcun deposit have δMo of 0.85 ‰ to 0.88 ‰ and −0.48 %, respectively. Molybdenite from the El Teniente and Bingham deposits yields intermediate δMo of 0.27 and 0.46 ‰, respectively. The Mo isotopes, combined with Nd isotope data of the ore-bearing porphyries, indicate that source of the ore-related magmas has fundamental effects on the Mo isotopic compositions of molybdenite. Our study indicates that molybdenite related to crustal-, and mantle-derived magmas has positive or negative δMo values, respectively, whereas molybdenite from porphyries formed by crust-mantle mixing has δMo close to 0 ‰. It is concluded that the Mo isotope composition in the porphyry system is a huge source signature, without relation to the tectonic setting under which the porphyry deposits formed. [ABSTRACT FROM AUTHOR]

A high concentration of molybdenum (Mo) is a striking feature of modern marine hydrothermal manganese (Mn) crusts from both low- and high-temperature hydrothermal systems; however, the origin of that Mo is poorly constrained. In this study, we analyzed Mo isotopic composition (δ 98/95 Mo) in a Mn crust collected from the Ryukyu arc system, and assessed the possible use of δ 98/95 Mo to constrain the origin of Mo in hydrothermal Mn crusts. Along with Mo isotope analysis, we also measured the abundance of major and trace elements and the Re–Os isotopic composition. The Mn crust sample had a high Mn content (42%–47%) and was low in Fe (0.2%–0.5%). Among the manganophile elements (Co, Ni, Cu, Zn, and Mo), only Mo showed high concentrations (> 550 ppm). We also found low contents of Os (~ 35 ppt) with an isotopic composition of 187 Os/ 188 Os = ~ 1.00. These results are consistent with previously reported chemical compositions of hydrothermal Mn crusts. δ 98/95 Mo values in the sample ranged from − 0.56‰ to − 0.66‰, which are ~ 2.7‰ lighter than the present-day seawater δ 98/95 Mo but similar to those in modern hydrogenous Fe–Mn crusts and Mn nodules. Although current data do not preclude the possible contribution of hydrothermally derived Mo, the light δ 98/95 Mo values can be explained by isotope fractionation associated with a change in coordination number during the adsorption of Mo from seawater onto Mn oxides. We suggest that the δ 98/95 Mo data are useful for constraining the source of Mo in hydrothermal Mn crusts. [ABSTRACT FROM AUTHOR]

Little is known about the range or controls on the molybdenum isotopic composition of low-medium temperature hydrothermal ore-forming systems. We present molybdenum isotope data from 12 hydrothermal syndepositional silicalite and carbonaceous slate samples from the Dajiangping pyrite deposit in western Guangdong Province, South China. The δ Mo values from Orebody III range from −0.02‰ to 0.29‰, with an average of 0.18‰. In contrast, the composition values from Orebody IV display a larger variation from −0.70‰ to 0.62‰. However, the five samples from the main ore bed all show strong negative values. This indicates that the significant variation in molybdenum isotopic composition supports different hydrothermal ore-forming metallogenesis and a metallogenic environment between the two orebodies. Orebody III is likely to have been deposited from submarine exhalative hydrothermal fluids under a relatively strong reducing environment and Orebody IV may have also been influenced by hydrothermal superimposition in a more oxidized disequilibrium condition. In addition, the δ Mo values of Orebody IV are clearly negative, together with the values increasing stratigraphically upward in the ore beds, suggesting that the metallogenic environment of Orebody IV is different from the open oceanic systems. There might be dynamic fractionation in this restricted environment. Therefore, the molybdenum isotope can be used as an effective tracer for the ore-forming fluid and metallogenic environment. [ABSTRACT FROM AUTHOR]

Mo isotopes along with Nd isotopes and other geochemical characteristics of the Phanerozoic clastic sediments from the northern margin of the Yangtze block were analyzed by MC-ICP-MS and some other techniques, spanning the period from Cambrian to Jurassic. The δ98Mo values and ϵNd(t) in these sedimets were observed to exhibit a large range of variation (-0.65‰-+1.87‰, -1.46--10.90, respectively). Specifically, the sample from Late Permian Maokou Formation has relatively positive values in both δ98Mo and ϵNd(t) (+0.97‰, -1.46, respectively). These elevated values are proposed to relate to the input of a large quantity of H2S and HS into the contemporaneous seawater by the volcanic eruption of the Emeishan mantle plume. Introduction of these sulfide gases would cause MoO42- to be transferred into MoS42- favoring the preferential deposition of the heavy Mo isotope as MoS42- and thus leaving a positive Mo isotope value. The sample from Late Permian Changxing Formation has a significantly elevated δ98Mo value of +1 .87‰, in association with the relatively negative ϵNd(t) (-10.90) and remain the relatively higher content of both TOC and total sulfur (TS) (1.52%, 2.02%, respectively). This infers the presence of the enhanced primary productivity and high organic burial during the Changxing period. The production of a huge quantity of HS- and H2S by the sulfate-reducing reaction led to the existence of the elevated seawater δ98Mo values during the Changxing period. The preliminary Phanerozoic data reported here allow us to propose that the Mo isotope signature preserved in reductive sediments is related to the regional tectonic settings and the associated environmental conditions, and thus the measurement of δ98Mo promotes our understanding on the evolution of the paleoenvironmental event in the Earth history. [ABSTRACT FROM AUTHOR]

• Significant Mo isotopes fractionation during MORB melt generation and evolution is unlikely. • δ 98 / 95 Mo in MORB and seamount lavas correlate with indices of mantle enrichment. • δ 98 / 95 Mo variations are inherited from mantle heterogeneity. • Recycled oceanic lithosphere generates upper mantle heterogeneity. 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 δ 98 / 95 Mo (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 δ 98 / 95 Mo (∼−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 δ 98 / 95 Mo (∼−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. [ABSTRACT FROM AUTHOR]

Recent geochemical data suggest the occurrence of an O 2 overshoot during the mid-Paleoproterozoic (~2.3–2.0 Ga). This O 2 overshoot appears to be consistent with carbon isotope records that suggest high burial rates of organic carbon during that period, the so-called Lomagundi Event. However, little is known about the changes in the ocean redox conditions associated with the O 2 overshoot. To better understand the mid-Paleoproterozoic ocean chemistry, we investigated the microstructures, major and trace element concentrations, Re-Os and Mo (δ98/95Mo) isotopes, and total organic carbon contents of Mn-ore and phyllite samples from the Nsuta Mn deposit in the Birimian Supergroup of Ghana which were deposited during the O 2 overshoot (at ~2.2 Ga). The Mn-ore samples contain early diagenetic rhodochrosite (Mn carbonate). The trace element compositions and Re-Os isotopes of the Mn-ore samples suggest that the rhodochrosite originated from primary manganese oxides (MnO 2) deposited at ~2.2-Ga. The δ98/95Mo values of the least-altered Mn-ore samples range between −1.10‰ and −0.55‰ (relative to NIST3134), suggesting seawater δ98/95Mo values of 1.85 ± 0.18‰ (1SD) during the O 2 overshoot. Such high seawater δ98/95Mo values can be best explained by enhanced removal of isotopically light Mo through adsorption onto Mn oxides. To form extensive Mn-oxide deposits, bottom seawater with O 2 concentrations of > 10 μM would have expanded at ~2.2 Ga. The oxidizing conditions might have supported the emergence of stem group eukaryotes during the mid-Paleoproterozoic. • Trace elements and Re-Os data suggest that Mn oxides were the primary precipitates of the ~2.2 Ga Nsuta Mn deposit. • δ98/95Mo values of the Nsuta deposit imply an isotopically heavy reservoir of dissolved Mo in the mid-Paleoproterozoic ocean. • δ98/95Mo data suggest extensive deposition of Mn oxides associated with the O 2 overshoot. [ABSTRACT FROM AUTHOR]

The stable isotope ratio of W is a new tracer in oceanographic studies and a new proxy in paleoceanographic studies; however, precise data for modern seawater have not been reported to date. Because the concentration of W in seawater is as low as 49 pmol kg−1, an ~3000-fold preconcentration is necessary prior to measurement by multicollector inductivity coupled plasma mass spectrometry (MC-ICP-MS). For the preconcentration, we investigated solid-phase extraction using chelating resins, namely, NOBIAS Chelate-PA1 with ethylenediaminetriacetic acid groups and TSK-8HQ with 8-hydroxyquinolie groups. We report that TSK-8HQ is useful because the effects of the seawater matrix are minor thermodynamically and kinetically. We present a novel method for analysis of the concentrations and isotope ratios of W and Mo in seawater, consisting of solid phase extraction, chromatographic separation using anion exchange resin AG1 X8, and measurement by MC-ICP-MS. Both W and Mo are quantitatively recovered by this method, which was applied to seawater samples collected from the North Pacific Ocean. The measured concentration of W and the concentration and isotope ratio of Mo are consistent with those in the literature. The isotope ratio of W is found to be uniform throughout the water column in the western North Pacific Ocean; δ186/184W is 0.55 ± 0.12‰ (ave ± 2sd, n = 7) using NIST SRM 3163 as a reference for W. On the basis of this data, we determined that the isotopic difference in δ186/184W is ~0.49‰ between seawater and oxic sediments in the modern ocean. This value accords with the reported experimental data for the isotope fractionation of W during adsorption on manganese and iron (oxyhydr)oxides, suggesting the validity of our data. Unlabelled Image [ABSTRACT FROM AUTHOR]