Molybdenum Isotopes Research Articles

Molybdenum isotope heterogeneity of metal sulfides from magmatic hydrothermal systems

The sensitivity of molybdenum (Mo) stable isotopes to redox changes enables innovative geochemical means of tracing the evolution of medium- to high-temperature magmatic hydrothermal processes. However, the Mo isotope geochemical behaviors and fractionation mechanisms in magmatic-hydrothermal systems are still unclear. This study performed high-precision Mo isotope research on well-characterized hydrothermal metal sulfides (pyrite, chalcopyrite, molybdenite and pyrrhotite) from different metallogenic stages and the ore-forming porphyries and surrounding rocks from the giant Pulang porphyry copper deposit in Yunnan, Southwest China, to better understand Mo isotope behaviors in magmatic hydrothermal systems. The results show that compared to the high-temperature magmatic system with homogeneous Mo concentrations (1.44 ppm to 11.3 ppm) and δ98Mo values (−0.41‰ ± 0.05 to −0.08‰ ± 0.03), medium- to high-temperature hydrothermal stage molybdenite displays slightly lighter δ98Mo values (−0.90‰ ± 0.04 to 0.08‰ ± 0.04), and pyrite, chalcopyrite and pyrrhotite, which have low Mo concentrations (0.13 ppm to 28.5 ppm), feature significant δ98Mo variations from −0.34 ± 0.04‰ to 3.01 ± 0.03‰. The absence of fractionated but low δ98Mo values in the magmatic system, combined with the lack of any correlation between the δ98Mo and geochemical index values (e.g., Co/Ni value) of the metal sulfides, implies that the Mo isotope signatures of sulfides in the hydrothermal stage were not inherited from component signatures of the magmatic phase or contamination of surrounding rocks. Our study proposes that heterogeneous partitioning and transportation of heavy/light Mo isotopes into progressively precipitated metal sulfides as a result of changes in the redox-driven metallogenic setting and differential geochemical behaviors of Mo species contribute to the observed δ98Mo variations and that the heavier and strongly variable Mo isotopic signatures correspond to the dominant metallogenic stage of the deposit. Such fractionation may be explained by the Rayleigh fractionation model, whereby 95Mo is preferentially partitioned into isotopically light molybdenite, and the subsequent precipitation of pyrite, chalcopyrite and pyrrhotite from the residual 98Mo-rich ore-forming fluids manifests heavier isotopic compositions. We therefore highlight that the Mo isotopes of metal sulfides provide valuable insights into the precipitation of metal-rich fluids, the identification of favorable mineralization zones, and the ability to trace the evolution of sophisticated magmatic-hydrothermal systems.

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.

Photoproton reactions on molybdenum

The yields of (γ, p)-reactions for bremsstrahlung γ-quanta with boundary energies of 20, 40, and 55 MeV on a natural mixture of molybdenum isotopes targets were measured as well as the activity of the resulting 91m, 95mg, 96, 97Nb isotopes. The weighted average yields of these isotopes formed in (γ, p)-reactions on molybdenum were measured for the first time. The experimental values were compared with the theoretical yields calculated using the TALYS1.96 program code and the method of the consideration of the giant dipole resonance isospin splitting.

Obtaining Highly Concentrated Molybdenum Isotopes According to a Multistage Scheme in a Cascade of Gas Centrifuges

Obtaining Highly Concentrated Molybdenum Isotopes According to a Multistage Scheme in a Cascade of Gas Centrifuges

Simultaneous Concentration of Intermediate Molybdenum Isotopes in a Cascade with a Given Number of Gas Centrifuges in Stages

Simultaneous Concentration of Intermediate Molybdenum Isotopes in a Cascade with a Given Number of Gas Centrifuges in Stages

Extensive sea-floor oxygenation during the early Mesoproterozoic

The Mesoproterozoic ocean was previously proposed to be characterized by low oxygen concentrations that may have resulted in a stasis in eukaryotic evolution. Recent studies reveal several pluses of oxygenation during this period that may have facilitated eukaryote evolution. However, few quantitative constraints on the global oxygenation events have hindered the comprehensive understanding of interactions between eukaryotes and environments. Here we present new molybdenum isotope (δ98/95Mo, simplified as δ98Mo) data coupled with sulfur and carbon isotope compositions and trace element contents from the carbonates and mudstones of the Gaoyuzhuang Formation (ca. 1.60–1.54 Ga) in two-well preserved sections in the North China Craton. The δ98Mo data from the two sections show similar trends in stratigraphic variation, with a prominent positive excursion in the Gaoyuzhuang Member III and peak values reaching up to 2.03‰ and 1.92‰, respectively. We developed a revised isotope mass balance model with high manganese (Mn) (hydr)oxide burial rate due to higher manganese concentration in the Mesoproterozoic oceans. Model calculation suggests that at least more than half of global seafloor (>54%) was covered with oxygenated seawater during the deposition of the Gaoyuzhuang Member III. Our estimation points to a significant oxygenation event in the early Mesoproterozoic with magnitude and extent much higher than previously thought. Further, a simple mass-balance calculation suggests that the atmospheric oxygen level was at least > 4% PAL (present atmospheric level) and likely up to 30% PAL at ca. 1.57 Ga. This oxygenation event, together with other environmental and/or developmental factors, may have facilitated a short and rapid evolution of eukaryotes in the early Mesoproterozoic.

Boron and Molybdenum Isotope Evidence for Source‐Controlled Compositional Diversity of Cenozoic Granites in the Eastern Tethyan Himalaya

AbstractThe origins of Cenozoic granites in the Himalaya are key to understanding the evolution of the Himalayan orogen. However, it is unclear whether these granites represent primary melts, and the nature of their magma source is controversial. Here, we present a systematic element and Sr–Nd–B–Mo isotope study of Cenozoic granites from the Yardoi area in the eastern Tethyan Himalaya, China. These granites can be divided into two groups: mid‐Eocene porphyritic two‐mica granites with low SiO2 contents (65.9−69.6 wt.%) and adakitic geochemical signatures, and mid‐Eocene to Miocene equigranular granites with high SiO2 contents (71.6−75.5 wt.%). The high‐SiO2 granites (HSG) have similar Sr−Nd isotope compositions to the low‐SiO2 granites (LSG), but they have distinct δ11B values of −19.4‰ to −11.4‰ and −10.6‰ to −6.89‰. This indicates that the two groups have different sources, with the LSG derived by partial melting dominantly of metamafic rocks at thickened lower crustal conditions, and the HSG generated by partial melting of the mid‐crust metasedimentary rocks with less enriched Nd isotope compositions. The δ98/95Mo of the LSG and HSG are highly variable with values of −0.68‰ to 0.12‰ and −1.13‰ to 0.46‰, respectively. δ11B values of the HSG correlate positively with δ98/95Mo and Sr/Y values and correlate negatively with K2O, Rb, Zr, and Rb/Sr, reflecting the addition of external metamorphic fluids during anatexis of the metapelites. The B–Mo isotope data robustly suggest source‐controlled compositional diversity of the Himalayan granites, which could provide clues to the physical and geochemical responses during the evolution of a large orogen.

Uranium and molybdenum isotope evidence for globally extensive marine euxinia on continental margins and in epicontinental seas during the Devonian-Carboniferous Hangenberg Crisis

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 δ13Corg 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.

Molybdenum Isotopic Fingerprints in Taiwan Rivers: Natural versus Anthropogenic Sources

Two rivers from Taiwan with different source inputs, the Danshuei (polluted) and Liwu (non-polluted), were selected to study the behavior of molybdenum (Mo) isotopes during weathering and riverine transport. In the Danshuei River, δ98/95Mo ranges from 0.83‰ to 1.50‰ (wet season) and 0.54‰ to 1.25‰ (dry season). With a few exceptions, δ98/95Mo in the Danshuei River is lighter during the wet season, while, in contrast, heavier in the dry season. In the Liwu River, δ98/95Mo varies from 0.54‰ to 1.30‰ and gets heavier along the mainstream. Using the MixSIAR model, three Mo sources are identified in the Danshuei River: seawater intrusion, rock–water interaction, and anthropogenic inputs. Seawater intrusion can explain the heavy δ98/95Mo downstream signal during the wet season contributing 14–39% from the MixSIAR model. However, the lighter δ98/95Mo signal during the dry season is most likely due to anthropogenic inputs in the middle and lower reaches of the Danshuei River contributing 75–98%. In the Liwu River, dissolved Mo isotopes correlate with SO42−/Na and (Sr/Na) ×1000 ratio, suggesting that pyrite oxidation coupled with carbonate weathering governs the heavy δ98/95Mo signature, with sequestration of light δ98/95Mo into secondary mineral phases in bedload sediments. Furthermore, these results have important implications for riverine Mo sources to the ocean, controlled by anthropogenic activity and weathering processes.

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.

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.

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.

Investigating the E2 nuclear resonance effect in kaonic atoms

The nuclear E2 resonance effect occurs when an atomic de-excitation energy is closely matched by a nuclear excitation energy. It produces an attenuation of some of the atomic X-ray lines in the resonant isotope target. Investigating the nuclear E2 resonance effect in kaonic atoms, important information about kaon-nucleus strong interaction can be provided. The only nuclear resonance effect was measured by G. L. Goldfrey, G- K. Lum and C. E. Wiegand at Lawrence Berkeley Laboratory, in 1975. The nuclear E2 resonance effect was observed in 25 hours of data taking, not enough to provide a conclusive result. In four kaonic Molybdenum isotopes (, , and ), the nuclear E2 resonance effect is expected at the same transition, with similar energy values. The KAMEO (Kaonic Atoms Measuring nuclear resonance Effects Observables) experiment plans to study the E2 nuclear resonance effect in kaonic Molybdenum isotopes at the DAΦNE e+e− collider, during the SIDDHARTA-2 experiment. The experimental strategy consists of exposing four solid strip targets, each enriched with one Molybdenum isotope, to negatively charged kaons, using a germanium detector for X-ray transition measurements. A further exposure of a non-resonant isotope solid strip target will be used as reference for standard non-resonant transitions.

Double Spike-Standard Addition Technique and Its Application in Measuring Isotopes.

Double spike (DS) method has been extensively used in determining stable isotope ratios of many elements. However, challenges remain in obtaining high-precision isotope data for ultra-trace elements owing to the limitations of instrumental signal-to-noise ratios and the systematics of precision of DS-based measurements. Here, the DS-standard addition (SA) (DSSA) technique is proposed to improve measurements of isotope compositions of ultra-trace elements in natural samples. According to the U-shaped relationship between DS measurement uncertainty and the spike/sample ratio, theoretical equations and an error propagation model (EPM) were constructed comprehensively. In our method, a spiked secondary standard solution with a high, precisely known spike/sample ratio is mixed with samples such that the mixtures have spike/sample ratios within the optimal range. The abundances of the samples relative to the added standards (sample fraction; fspl) and the samples' isotope ratios can then be obtained exactly using a standard DS data reduction routine and the isotope binary mixing model. The accuracy and precision of the DSSA approach were verified by measurements of cadmium and molybdenum isotopes at as low as 5 ng levels. Compared with traditional DS measurements, the sample size for isotope analysis is reduced to 1/6-1/5 of the original with no loss of measurement precision. The optimal mixing range fspl = 0.15-0.5 is recommended. The DSSA method can be extended to isotope measurement of more than 33 elements where the DS method is applicable, especially for the ultra-trace elements such as platinum group and rare earth element isotopes.

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.

Investigating the E2 Nuclear Resonance Effects in Kaonic Atoms: The KAMEO Proposal

The E2 nuclear resonance effect in kaonic atoms occurs when the energy of atomic de-excitation closely matches the energy of nuclear excitation, leading to the attenuation of some X-ray lines in the resonant isotope target. This phenomenon provides crucial information on the strong interaction between kaons and nuclei. The only nuclear E2 resonance effect observed so far was in the K− −9842Mo isotope, measured by G. L. Goldfrey, G-K. Lum, and C. E. Wiegand at Lawrence Berkeley Laboratory in 1975. However, the 25 hours of data taking were not sufficient to yield conclusive results. In four kaonic Molybdenum isotopes (9442Mo, 9642Mo, 9842and Mo, and 10042Mo), the nuclear E2 resonance effect is expected to occur at the same transition with similar energy values. To investigate this, the KAMEO (Kaonic Atoms Measuring Nuclear Resonance Effects Observables) experiment plans to conduct research on kaonic Molybdenum isotopes at the DAΦNE e+e− collider during the SIDDHARTA-2 experiment. The experimental strategy involves exposing four solid strip targets, each enriched with one Molybdenum isotope, to negatively charged kaons and using a germanium detector to measure X-ray transitions. In addition, a non-resonant 9242Mo isotope solid strip target will be used as a reference for standard non-resonant transitions.

The paleoenvironmental implications of pre-Great Oxidation Event manganese deposition in the Mesoarchean Ijzermijn Iron Formation Bed, Mozaan Group, Pongola Supergroup, South Africa

The Ijzermijn Iron Formation Bed occurs in the Vlakhoek Member of the basal Sinqeni Formation of the Mesoarchean Mozaan Group of the Pongola Supergroup of southern Africa. It represents the oldest known supracratonic Superior-type iron formation that has significant manganese enrichments. Lithostratigraphic correlation between the Mozaan Group and the Witwatersrand Supergroup indicates that there was a larger Witwatersrand-Mozaan depositional basin on the Kaapvaal Craton during the Mesoarchean. Detailed stratigraphy, mineralogy, petrography, carbonate mineral chemistry, major and trace element geochemistry and carbonate oxygen and carbon isotopes are presented for two drill cores intersecting the Ijzermijn Iron Formation Bed. Drill core PMH 24, which represents a more coast-distal depositional setting, is characterized by manganese-rich siderite, higher Fe2O3 and lower MnO and Al2O3 contents, more prominent europium anomalies, lower GdSN/YbSN values and less depleted δ13CPDB values. Drill core DDN 1, which represents a more coast-proximal depositional setting, is characterized by iron-rich rhodochrosite, lower Fe2O3 and higher MnO and Al2O3 contents, less prominent europium anomalies, higher GdSN/YbSN values and more depleted δ13CPDB values. The presence of early diagenetic magnetite, hematite and 13C-depleted carbonates indicate the iron and manganese, which were sourced from hydrothermal plumes, were deposited by oxidation, with subsequent diagenetic reduction by organic carbon. The most efficient oxidative mechanism would have been biological oxidation. The increase in manganese contents towards the paleocoastline further supports manganese oxidation as it was redox buffered by Fe2+ in the water column and could only be deposited and preserved once Fe2+ had been removed through oxidation further away from the distal hydrothermal plume source. This is supported by previously published iron isotope data, where δ56/54Fe values are more depleted closer to the paleocoastline. Deposition of iron and manganese was restricted to below the photic zone. This implies that microaerophilic chemolithoautotrophs were responsible for iron and manganese oxidation. This requirement of free oxygen in the Ijzermijn Iron Formation Bed has been supported by earlier molybdenum isotope studies. The presented work therefore further supports the idea of pre-Great Oxidation Event marine oxygen oases that did not lead to any appreciable build-up of oxygen in the atmosphere.

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.

Extreme climate changes influenced early life evolution at ∼ 1.4 Ga: Implications from shales of the Xiamaling Formation, northern North China Craton

The Mesoproterozoic Era (ca.1.6–1.0 Ga) witnessed a long-term evolution stasis of eukaryote, which is usually ascribed to the persistent low atmospheric oxygen levels. However, several oxygenation processes were identified on different continents during the Mesoproterozoic Era, implying that the oxygen levels may not be the only factor influencing the evolution of early-life during the early Mesoproterozoic Era. Here, we report the molybdenum isotopes, redox sensitive elements, nutrient elements and total organic carbon (TOC) content of black and green shales from the ∼ 1.4 Ga Xiamaling Formation in the northern part of the North China Craton (NCC), which has been affected by Milankovitch-style cycle, orbital forcing of extreme change of climate ∼ 1.4 billion years ago. The highest δ98Mo of the green shale reaches 1.56 ± 0.04 ‰ (modern ocean: 2.34 ‰), which may also confirm that it was in an oxygenation state at that time. In contrast, the black shale has lower δ98Mo values (1.02 ± 0.07 ‰) than that of green shale, indicating a less oxidized environment. Moreover, the TOC and nutrient elements (e.g., P, Cu, Ni) contents of the black shale are higher than those of green shale, which results from an increased nutrient input caused by stronger weathering and river runoff during a wet period. We conclude that the marine environment was persistently oxidative during the deposition of the black and green shales, while the TOC contents of the black and green shales are mainly controlled by the concentration of the nutrient elements that are constrained by the climatic conditions. Even though in oxic aquatic environment, the unstable nutrient supply would hinder the early life evolution, our findings imply vice versa. Hence, these results imply that the extreme climate change leading to unsustainable supply of nutrients, may have largely influenced emergence and diversification of animals in the Mesoproterozoic Era.

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.