Molybdenum Isotopes Research Articles

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.

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Molybdenum isotopes in plume-influenced MORBs reveal recycling of ancient anoxic sediments

Attenuation of antimony in groundwater from the Xikuangshan antimony mine, China: Evidence from sulfur and molybdenum isotope study

The Xikuangshan (XKS) mine, an antimony mine that has been active for more than a century, was chosen for a detailed antimony hydrogeochemical study because of the historical antimony contamination in local bedrock aquifers. Hydrochemical data, δ34S values of dissolved SO42−, and δ98Mo values were analyzed to identify the predominant geochemical processes affecting Sb attenuation within the aquifers. Groundwater from the adits at the XKS Sb mine reflects an oxidizing environment and has pH values between 6.81 and 8.89 because of the sufficient acid-neutralizing effect of carbonates. The decrease in Sb concentrations along the groundwater flow direction and the highly coloured deposits in the adits indicate Fe and Mn oxides have a significant capacity for Sb sorption. The δ34S values of the groundwater indicate that reductive dissolution of Fe(III) hydroxides is not the dominant process controlling Sb mobilization. Furthermore, the δ34S values and weak-alkaline groundwater corporately suggest that Sb adsorption on and desorption from Fe/Mn (hydr)oxides may both occur, with adsorption being more significant than desorption with respect to Sb attenuation in groundwater. The δ98Mo values of the groundwater further demonstrate that this adsorption is relatively significant in terms of Sb attenuation, and that competitive adsorption may occur between Sb and As in groundwater at the XKS Sb mine. Overall, the hydrochemistry analysis, especially the S and Mo isotope evidences collectively elucidate that the attenuation of Sb in groundwater from the XKS Sb mine are influenced by several geochemical processes, including: (1) more adsorption on than desorption from Fe/Mn (hydr)oxides; (2) competitive adsorption of arsenic; and (3) dilution with low-Sb groundwaters. This study demonstrates that hydrochemical analyses, particularly S and Mo isotope studies, are useful for tracking Sb attenuation in groundwater from antimony-contaminated mine areas.

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.

Molybdenum isotope variation mechanism and ore-genesis of Niutoushan Pb–Zn sulfide orebodies in the Xiangshan volcanic basin, South China

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.

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

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

Resonance Structure of the Charge-Exchange Strength Function for the Molybdenum Isotopes $${}^{{98,100}}$$Mo

Resonance Structure of the Charge-Exchange Strength Function for the Molybdenum Isotopes $${}^{{98,100}}$$Mo

Extremely light molybdenum isotope signature of sediments in the Mariana Trench

Molybdenum (Mo) isotope signature (δ98/95Mo) of marine sediments is a powerful proxy for constraining marine redox conditions and early diagenetic processes. Oxic pelagic sediments have been increasingly acknowledged as an important sink for Mo in its global cycling, however, the mechanisms controlling Mo enrichment and isotope fractionation during early diagenesis in this environment are not fully understood. In this study, four sedimentary cores were sampled by full-ocean-depth lander system in the Mariana Trench, aiming to explore the processes controlling Mo partitioning in the sediments of the deepest ocean in the world. Sediments from four locations exhibit a wide range of authigenic Mo contents (Moauth, relative to continental crust), varying from 1.98 to 43 μg/g. A positive relation between Mo and Mn contents was identified for these sediments, which suggests that Mo are mainly hosted in Fe-Mn (oxyhydr)oxides. In addition, negative correlations between δ98/95Mo values and Mo/Mn ratios are found at each site. The δ98/95Mo values of the sediments vary from −1.71 ± 0.05 to −0.15 ± 0.04‰, revealing similar trends towards lighter values with burial depth from three sites. However, throughout the sediment core TY41 obtained from Mariana Trench, the δ98/95Mo values of sediment remain nearly constant around −0.55‰. Supported by the abundant Fe (oxyhydr)oxides throughout this site, it is likely the sediment is affected by enhanced fluid exchange across sediment-seawater interface, and the δ98/95Mo values of dissolved Mo in pore water resemble seawater signature. Therefore, the Mo isotopic offset between dissolved Mo and sediment is similar to the expected fractionation during the adsorption to Mn oxides (~ + 3.0‰) and hematite (~ + 2.2‰), which suggests that most of the dissolved Mo is scavenged by Mn oxides and/or hematite. The extremely light δ98/95 Mo values of sediments in deeper parts of the cores coincide with enhanced Mo accumulation. This indicates an even lighter isotope signature of dissolved Mo in pore water during the re-precipitation of Fe-Mn (oxyhydr)oxides upon an isotopic equilibrium for Mo adsorption to metal oxides. Both the patterns of both dissolved oxygen and nitrate suggest that the study sites are located within the nitrate reduction zone and Mn reduction occur at deeper depths below the studied interval. Therefore, the source of the isotopically light dissolved Mo in pore water was explained by the preferential release of isotopically lighter Mo from Fe-Mn (oxyhydr)oxides during the enhanced dissolution at deeper depth. In contrast, at the shallower depths of these cores (expect for core TY41), the higher δ98/95 Mo values towards seawater-sediment interface suggest the dissolved Mo is more impacted by the seawater exchange. This study provides a deeper insight into the diagenetic Mo cycling in trench environments, with Fe-Mn (oxyhydr)oxides rich sediments as an important sink for Mo.

Co-variation systematics of uranium and molybdenum isotopes reveal pathways for descent into euxinia in Mediterranean sapropels

The elemental concentrations and isotope compositions of molybdenum (Mo) and uranium (U) are commonly used for the reconstruction of past global and local redox conditions, and recent studies using both elements have revealed the potential of their paired application. However, such studies have generally focused either on modern marine sediments or on relatively low-resolution reconstructions of deep-time paleo-redox conditions. Here, we present high-resolution profiles (every 0.2-0.3 kyr) of Mo and U elemental and isotope compositions for anoxic organic-rich sediments of Eastern Mediterranean sapropels S5 and S7. The new Mo-U data reveal the processes leading to descent into basinal euxinia in more precise and systematic detail than lower resolution datasets focused on either Mo or U only.During the intensification from anoxic non-sulfidic to persistently euxinic conditions, δ98Mo and δ238U covariation systematics exhibit two stages. We identify the Mo-U isotope signature of the early transition from anoxic non-sulfidic to mildly euxinic conditions in the first stage, characterized by a rise in U isotope ratios (from −0.3‰ to +0.2‰ ±0.05‰) controlled by the depth within the sediment of the uranium reduction-accumulation front. As the water column turns persistently euxinic, δ98Mo values approach the seawater value for both sapropels, but δ238U evolves to different final values in the two sapropels. We interpret these differences as reflecting different redox potentials and/or different degrees of restriction of these two sapropel events, and the more gradual response of U sequestration to redox variation in comparison with threshold behavior of Mo. The findings presented here show temporal patterns in δ98Mo and δ238U on relatively short timescales that suggest the combined use of these proxies at high resolution allows detailed reconstruction of local redox and hydrographic conditions.

High-precision measurements of Mo isotopes by N-TIMS

We have developed a new high-precision analytical method for measuring molybdenum (Mo) isotopes by negative thermal ionization mass spectrometry (N-TIMS). This method represents an improvement over existing methods, thanks to a new chemical separation method that yields a better purification of the Mo fraction. In addition, we have used a multi-dynamic method for data collection with three different magnet settings to avoid being sensitive to drifts in Faraday cup efficiencies. Our method resulted in a more reliable thermal emission of MoO3- for a given run and we have obtained Mo isotope ratios measured on a series of 26 standards with the following reproducibilities (2 SD): ±6.1, ±5.8, ±2.9, and ±9.6 ppm for 94Mo/96Mo, 95Mo/96Mo, 97Mo/96Mo, and 100Mo/96Mo, respectively. This represents an improvement by a factor of 2–3 compared with previously published methods, using N-TIMS or MC-ICPMS. This new analytical method will have applications in the field of cosmochemistry (nucleosynthetic anomalies), environmental geochemistry or nuclear forensics (mass-independent fractionation).

Molybdenum isotope-based redox deviation driven by continental margin euxinia during the early Cambrian

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.

Concentration of Intermediate Molybdenum Isotopes in a Cascade with a Specified Number of Gas Centrifuges in Steps

Concentration of Intermediate Molybdenum Isotopes in a Cascade with a Specified Number of Gas Centrifuges in Steps

Chromium isotope evidence for oxygenation events in the Ediacaran ocean

Pulses of the Ediacaran ocean oxygenation were inferred from strong enrichments of redox-sensitive elements (RSEs; particularly Mo, V, U, Re) and negative pyrite sulfur isotopes (δ34Spy) in black shales of the Doushantuo Formation in South China. These oceanic oxygenation events (OOEs) have been challenged by the lack of comparable RSE enrichments in correlative strata of northwestern Canada. Here we report four positive chromium isotope (δ53Cr) excursions with peak values (+0.79 ± 0.03‰ to +1.45 ± 0.06‰; 2SD) close to the average δ53Cr value of the modern ocean (+1.0 ± 0.3‰) at the intervals of OOEs, which are separated by low δ53Cr values close to that of the bulk silicate Earth (BSE, −0.124 ± 0.101‰). The positive δ53Cr excursions could be explained by episodic input of oxygenated water from the open ocean to the restricted Nanhua basin, or pulses of ocean oxygenation during the Ediacaran-early Cambrian. The two interpretations can explain the majority of the geochemical data available from the Wuhe section, but both have limitations. The first interpretation implies that the open ocean was more oxygenated than the Nanhua basin during the entire Ediacaran-early Cambrian period, which is not supported by the currently available data from other Ediacaran successions. The second interpretation has difficulties to explain the thallium isotope (ε205Tl) data from the same section, pending confirmation of the OOEs from other successions globally. The data from the Wuhe section highlight the complexity of interpreting the redox history of the global ocean from geochemical results of a single section. Nonetheless, the synchronous changes of multiproxy records—including positive δ53Cr excursions, RSE peaks, negative δ34Spy and molybdenum isotope (δ98Mo) excursions—at the same stratigraphic intervals do support a more oxygenated ocean at ~635 Ma, ~580 Ma, ~560 Ma, and early Cambrian, regardless of the degree of restriction of the Nanhua basin from the open ocean.

Palaeoredox reconstruction in the eastern Arabian Sea since the late Miocene: Insights from trace elements and stable isotopes of molybdenum (δ98/95Mo) and tungsten (δ186/184W) at IODP Site U1457 of Laxmi Basin

The present study investigates the oxygenation history of the northeastern Arabian Sea since the late Miocene using redox sensitive elemental and metal stable isotopic signatures in the deep-sea sediments. To achieve this, the sediment core samples collected at Site U1457 (67°55.80′E, 17°9.95′N, water depth 3534 m) of Laxmi Basin in the northeastern Arabian Sea during the International Ocean Discovery Program (IODP) Expedition 355 were analysed for a suite of elemental (Mo, W, U, V, Ba, Cd and P) and stable molybdenum (Mo) isotope (δ 98/95 Mo relative to NIST SRM 3134 lot No. 130418) as well as stable tungsten (W) isotope (δ 186/184 W relative to NIST SRM 3163 lot No. 080331) composition. Sedimentary δ 98/95 Mo values (−0.70‰ to +1.18‰) at IODP Site U1457 in the northeastern Arabian Sea indicated partial authigenic Mo component. In contrast, the sedimentary δ 186/184 W values (−0.02‰ to +0.21‰) were in the range similar to that of lithogenous material suggesting dominance of detrital composition. The study reveals that the water column in the eastern Arabian Sea was oxic during the late Miocene and Pliocene while oxic to suboxic condition prevailed during the Pleistocene. The study also explores that under oxic to suboxic condition with limited particle shuttling, the W isotopes do not undergo significant fractionation, and their isotope ratios reflect the detrital source signature. This study reports the first results on isotopic compositions of Mo and W in sediments of the northeastern Arabian Sea since the late Miocene to investigate the palaeoredox conditions on a million-year time scale. • First results of δ 98/95 Mo and δ 186/184 W in the late Miocene Arabian Sea sediments. • Oxic condition during late Miocene & Pliocene, and suboxic during Pleistocene. • δ 98/95 Mo values (−0.70 to +1.18‰) exhibits partial authigenic component. • δ 186/184 W values (−0.02 to +0.21‰) exhibit dominant detrital control. • Under oxic setting with limited particle shuttling, tungsten isotopes do not fractionate.

Optimization Criterion for Two-Stage Cascade for Iterative Molybdenum Isotopes Recovery by the method of laser-assisted retardation of condensation(SILARC)

In this paper, application of the method of laser-assisted retardation of condensation has been considered for molybdenum isotope separation. Enrichment factor as a function of pressure and temperature of gas flow cold core have been evaluated. Since the maximal value of enrichment factor is not high(~ 1.8), to achieve desired enrichment level in products an iterative procedure is applied. To reduce the amount of wastes and increase performance of the SILARC method, introduction of the second stage into the separation cascade is considered. Relations between molar and isotopic fractions of target isotopologues on the first and second stages of the cascade at each iteration are found. Optimization problem is formulated to evaluate optimal physical parameters of a two-stage cascade for molybdenum isotopes enrichment.

Charged Particles Emission in Fast Neutron Processes on Molybdenum Isotopes

Charged Particles Emission in Fast Neutron Processes on Molybdenum Isotopes

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

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

Molybdenum isotopic constraints on the origin of EM1-type continental intraplate basalts

The origin of enriched mantle 1 (EM1) component in the mantle has long been debated. EM1 source of basalts has been variously attributed to recycled sub-continental lithosphere, recycled lower continental crust, or recycled oceanic crust plus pelagic sediments. Molybdenum stable isotope systematics has the potential to resolve this debate, because of the significant isotopic differences between mantle and crustal reservoirs. Here we present molybdenum isotope data for EM1-type continental intraplate basalts from Nuominhe (NMH), Northeast China. The NMH basalts display significant variations in both δ98/95Mo (98Mo/95Mo ratio relative to NIST SRM 3134, −0.49 to −0.15‰) and Mo/Ce (0.009–0.037), and strong correlations between δ98/95Mo, Mo/Ce and Hf isotope and trace elements. EM1 end-member of the NMH basalts shows the lightest δ98/95Mo and lowest Mo/Ce values. The involvement of continental crust and sub-continental lithospheric mantle cannot account for the light Mo isotopic compositions of the basalts. By contrast, the recycled oceanic crust with pelagic sediments, characterized by low δ98/95Mo and Mo/Ce values, is the satisfactory candidate for the EM1 component in the NMH basalts. Our results also suggest a residual slab reservoir with light δ98/95Mo values (<−0.5‰) in the mantle beneath Northeast China, representing the complementary reservoir to heavy Mo isotope continental crust. Identification of this reservoir makes the Mo isotope system a powerful tracer of crust-mantle interactions.