Mo Isotope Fractionation Research Articles

Ion-Specific Isotopic Fractionation of Molybdenum during Diffusion in Aqueous Solutions

Experiments modeling diffusion of Mo in aqueous solutions have been performed and, using multicollector ICP-MS, the ratios of the diffusivities of Mo isotopes, D97Mo/ D95Mo, in aqueous solutions have been determined. Diffusion of MoO42- ions in solution was concomitant with Mo isotopic fractionation with D97Md/D95Mo = 0.99988+/-0.00004 (2sigma for n = 3). In contrast, during diffusion of Mo polyanions, such as Mo70246- and Mo8O264-, no measurable isotope fractionation has been found with D97Mo/D95MO = 1.00000 +/- 0.00002 (2sigma for n = 3). These results indicate the need for due consideration to Mo speciation when attempting to interpret the role of diffusive fluxes in the formation of Mo isotopic signatures in nature. They also raise the possibility that the various chemical forms of other transition metals may be characterized by species-specific isotopic fractionation effects during physicochemical reactions.

Isotope fractionation during microbial metal uptake measured by MC-ICP-MS

High-precision isotopic analyses by MC-ICP-MS were used to investigate the mass-dependent fractionation of Mo and Fe isotopes during bacterial metal assimilation in experiments with Azotobacter vinelandii. A. vinelandii is a diazotroph with high demand for both Mo and Fe during nitrogen fixation. Our results demonstrate that the growth medium became progressively enriched in heavier isotopes of Mo during bacterial growth, indicating preferential assimilation of lighter isotopes. In contrast, for Fe, the medium become isotopically lighter as Fe was removed from solution. The experimental data can be interpreted in terms of Rayleigh fractionation, yielding fractionation factors of 0.9997 and 1.0011 for Mo and Fe, respectively. Hence, we infer Δ97/95Mocells-medium = –0.3‰ and Δ56/54Fecells-medium = 1.1‰. Fractionation of Mo isotopes could result from simple kinetic effects during assimilation, but may also be affected by complexation with high-affinity metal binding ligands. Kinetic effects cannot easily account for the sense of Fe isotope fractionation, and so equilibrium effects, possibly between different Fe complexes, are implied. Adsorption of Mo and Fe onto cell surfaces may also play a role and requires further examination. Isotope fractionation studies using MC-ICP-MS may provide new constraints on the processes by which microbes extract metals from their surroundings, ultimately yielding insights into the mechanisms of metal assimilation into the metallome.

Mass‐Independent Isotope Fractionation of Molybdenum and Ruthenium and the Origin of Isotopic Anomalies in Murchison

Dauphas et al.'s model for the nucleosynthetic origin of Mo and Ru anomalies in meteorites leaves the case of Murchison (CM2) unexplained. We explore the possibility that such a discrepancy is due to mass-independent effects controlled by nuclear field shift with, in particular, staggering between odd and even masses. We first demonstrate the existence of such mass-independent fractionation of Mo and Ru isotopes by chemical exchange of Mo and Ru between DC18C6 crown ether and aqueous solutions. Our results fit the nuclear field shift theory of Bigeleisen. We then review the correlation between the mean-square charge radius (which controls the nuclear field shift) and the isotopic anomalies found in an Allende CAI and in Murchison. Although Mo and Ru in the Allende CAI show a clear indication of nucleosynthetic components, the mass-independent anomalies observed in Murchison show a strong correlation with the nuclear charge distribution. We therefore argue that some isotopic anomalies observed in meteorites may be due to nuclear field shift rather than nucleosynthetic processes. Such effects are temperature dependent and may represent either genuine nebular processes or analytical artifacts. This new interpretation may help assess the existence of anomalies due to the extinct isotopes 97Tc and 98Tc.

Temperature dependence of Mo isotope fractionation during adsorption to δ-MnO 2: Implications for the paleoredox proxy

Temperature dependence of Mo isotope fractionation during adsorption to δ-MnO 2: Implications for the paleoredox proxy

Sedimentary Mo isotope record across the Holocene fresh–brackish water transition of the Black Sea

Mo isotope data on Black Sea sediments spanning the transition from Pleistocene oxic–limnic conditions to the prevailing anoxic marine conditions are presented. Samples were taken from a gravity core collected at a water depth of 396 m. Samples deposited under oxic bottom water conditions range from δ 98/95Mo MOMO − 2.2‰ to − 1.95‰ (MOMO = Mean Ocean Molybdenum) while samples deposited under anoxic bottom water conditions range from δ 98/95Mo MOMO − 1‰ to − 0.54‰. The change of sedimentary environment is also recorded in the Mo contents increasing from oxic to anoxic sediments. The Mo isotopic composition and invariably low Mo content of the oxic sediments deposited under oxic bottom water conditions are compatible with a pure detrital origin of the Mo, irrespective of whether the deposits are of limnic or brackish origin. Mo content and isotopic compositions are identical above and below a sulfidisation front, which originates from the diffusion of sulfur species and in-situ microbial activity after establishment of brackish bottom water conditions. Further, no signal of the overlaying sapropels is seen in the underlying sediments. Thus, transport of sulfur species has not mobilised Mo during diagenesis. The δ 98/95Mo MOMO values of anoxic samples indicate seawater as the dominant source of Mo. However, even the heaviest Mo value of the anoxic period recorded in this core is δ 98/95Mo MOMO = − 0.5‰, with an average of − 0.7‰ for all anoxic sediments, i.e. 0.7‰ lighter than seawater. All samples can be explained qualitatively as three component mixtures of detrital, dissolved riverine and marine Mo. For the lower units a mass balance model can be successfully applied. For the youngest unit mixing models do not yield satisfactory results given present day water fluxes. It is therefore likely that additional Mo isotope fractionation effects are involved. First order modelling suggests that the lighter δ 98/95Mo MOMO values of the most recent samples reflect the presence of some Mo remaining dissolved as MoO 4 2− in a larger part of water column above the core depth, thus allowing for a preservation of a net fractionation between MoO 4 2− and MoS 4 2−. This hypothesis is supported by the fact that the H 2S concentration critical for the MoO 4 2− ↔ MoS 4 2− chemical switch is found at about 400 m water depth in the present Black Sea, close to the depth at the sampling site. At greater depth, increased H 2S concentrations lead to almost complete Mo removal, erasing the fractionation signal. Difference between Unit I samples from this study, and those from earlier publications (with samples taken at greater depth) may thus merely reflect the fraction of Mo scavenged at different depths. The degree of Mo scavenging in fossil black shales and the continental Mo contribution are both difficult to constrain. Therefore, black shales from restricted semi-enclosed basins may not document the Mo isotopic composition of coeval ocean waters. However, oceanic Mo is a dominant Mo source in these basins, and anoxic sediments give reliable minimum values for coeval ocean water Mo isotopic compositions.

PGE, Re-Os, and Mo isotope systematics in Archean and early Proterozoic sedimentary systems as proxies for redox conditions of the early Earth

Re-Os data and PGE concentrations as well as Mo concentrations and isotope data are reported for suites of fine clastic sediments and black shales from the Barberton Greenstone Belt, South Africa (Fig Tree and Moodies Groups, 3.25–3.15 Ga), the Belingwe Greenstone Belt, Zimbabwe (Manjeri Formation, ca. 2.7 Ga) and shales from the Witwatersrand, Ventersdorp and Transvaal Supergroups, South Africa ranging from 2.95 to 2.2 Ga. Moderately oxidizing conditions are required to mobilize Re and Mo in the environment, Mo fractionation only occurs in solution, and these parameters thus have potential use as paleoredox proxies for the early Earth. PGE + Re abundance patterns of Barberton Greenstone Belt sediments are uniform and very similar in shape to those of komatiites. This indicates (1) that the PGE came from a source of predominantly ultramafic composition and, (2) that PGE were transported and deposited essentially in particulate form. Sediments from the younger Belingwe Greenstone Belt show more fractionated PGE + Re patterns and have Re/Os ratios 10 to 100× higher than those of Barberton sediments. Their PGE abundance patterns and Re/Os ratios are intermediate between those of the mid-Archean shales and Neoproterozoic to Recent black shales. They reflect scavenging of Re from solution in the sedimentary environment. δ 98/95Mo values of black shales of all ages correlate with their concentrations. The Barberton Greenstone Belt samples have ∼1–3 ppm Mo, similar to a granitoid-basaltic source. This Mo has δ 98/95Mo between −1.9 and −2.4‰ relative to present day mean ocean water molybdenum, MOMO and is thus not isotopically fractionated relative to such a source. Similar to the PGE this indicates transport in solid form. Sediments from the Belingwe Greenstone Belt show in part enhanced Mo concentrations (up to 6 ppm) and Mo isotope fractionation (δ 98/95Mo up to −1.4‰ relative to MOMO). The combined PGE + Re and Mo data show mainly reducing conditions in the mid-Archean and suggest that by 2.7 Ga, the atmosphere and oceans had become more oxidizing. Substantially younger samples from the Transvaal Supergroup (to ca. 2.2 Ga) surprisingly have mainly low Mo concentrations (around 1 ppm) and show no significant Mo isotope fractionation relative to the continental source. Among possible explanations for this are a return to reducing atmospheric conditions after 2.7 Ga, reservoir effects, or Mo removal by sulfide precipitation following sulfate reduction in early Proterozoic oceans.

Molybdenum isotope fractionation during adsorption by manganese oxides

The isotopic composition of Mo (δ97/95Mo) in seawater is ∼2‰ heavier than Mo in marine ferromanganese crusts and nodules [Barling et al., Earth Planet. Sci. Lett. 193 (2001) 447–457; Siebert et al., Earth Planet. Sci. Lett. 211 (2003) 159–171]. To explore this phenomenon, we have conducted an experimental investigation into the mass-dependent fractionation of Mo isotopes during adsorption onto Mn oxyhydroxide. Two series of experiments were carried out: a ‘time series’, in which adsorption proceeded for 2–96 h; and a ‘pH series’ in which pH varied from 6.5 to 8.5. The extent of Mo adsorption by Mn oxyhydroxides decreases with increasing pH, a trend typical of anion adsorption, and takes 48 h to reach steady-state. Lighter Mo isotopes are preferentially adsorbed. Experimentally determined fractionation factors (αsoln–MnOx) exhibit no systematic variation with either time or experimental pH. The mean αsoln–MnOx for all experiments is 1.0018±0.0005 (2 S.D.). Comparison of the Mo isotopic data for experimental solutions and Mo adsorbed to Mn oxyhydroxide with predictions for ‘closed system’ equilibrium and Rayleigh fractionation models indicates that isotope fractionation occurs as a result of ‘closed system’ equilibrium exchange between dissolved and adsorbed Mo. The isotopic offset between dissolved and adsorbed Mo is comparable to that observed between Mo in seawater and Mo in ferromanganese nodules and crusts. It is therefore likely that adsorption of Mo to Mn oxyhydroxides is a significant factor in the fractionation of Mo isotopes in the oceans.

Molybdenum isotope records as a potential new proxy for paleoceanography

New high-precision isotope ratios of dissolved Mo in seawater from different ocean basins and depths show a homogeneous isotope composition (‘mean ocean water 98Mo/95Mo’ (MOMO)), as expected from its long ocean residence time (800 kyr). This composition appears to have been constant for the past 60 Myr at a 1–3 Myr time resolution as indicated from thick sections of Fe–Mn crusts from the Atlantic and Pacific. These records yield a constant offset from MOMO (average of −3.1 and −2.9‰). They are similar to our new data on recent oxic Mo sinks: pelagic sediments and six Fe–Mn crust surface layers range from −2.7 to −2.9‰ and −2.7 to −3.1‰, respectively. Recent suboxic Mo sinks from open ocean basins display heavier and more variable isotope ratios (−0.7 to −1.6‰ relative to MOMO). Crustal Mo sources were characterized by measuring two granites (and a mild acid leach of one granite), seven volcanic rocks and two clastic sediments. All show a narrow range of compositions (−2.0 to −2.3‰). These data indicate that isotope fractionation by chemical weathering and magmatic processes is insignificant on a global scale. They therefore represent good estimates of the composition of dissolved Mo input to the oceans and that of the average continental crust. Thus, the Mo input into the oceans appears to be distributed into lighter oxic sinks and heavier reducing sinks. This is consistent with steady-state conditions in the modern ocean. The constant isotope offset between oxic sediments and seawater suggests that the relative amounts of oxic and reducing Mo removal fluxes have not varied by more than 10% over the last 60 Myr. An equilibrium fractionation process is proposed assuming that Mo isotope fractionation occurs between (dominant) MoO42− and (minor) Mo(OH)6 species in solution, of which the latter is preferentially scavenged.

Natural mass-dependent variations in the isotopic composition of molybdenum

We present the first observations of natural mass-dependent fractionation of the isotopic composition of molybdenum (Mo), using multi-collector inductively coupled plasma mass spectrometry. Variations in the isotopic composition of Mo are reported as δ 97/95Mo (=(( 97Mo/ 95Mo) sample/( 97Mo/ 95Mo) standard−1)×1000‰). External analytical precision of δ 97/95Mo is <±0.25‰ (2σ) on natural samples. Our data demonstrate a clear offset of >1‰ between sediments deposited under anoxic conditions (δ 97/95Mo=+1.02 to +1.52‰ relative to our in-house standard) and ferromanganese nodules (δ 97/95Mo=−0.63 to −0.42‰). δ 97/95Mo of Pacific Ocean seawater (δ 97/95Mo=+1.48‰) lies within the range of values for anoxic sediments, closest to modern Black Sea anoxic sediments. Molybdenites from continental ore deposits have intermediate δ 97/95Mo ranging from −0.26 to +0.09‰. Variations in the abundances of 92Mo, 95Mo, 96Mo, 97Mo and 98Mo are consistent with mass-dependent fractionation. A sporadic unidentified interference occurs at mass 94 and 100Mo is not measured. We hypothesize that the δ 97/95Mo offset between anoxic sediments and ferromanganese nodules results from Mo isotope fractionation during inefficient scavenging of Mo from seawater by Mn oxides under oxic conditions. The similarity in δ 97/95Mo of anoxic sediments and seawater is consistent with the very efficient removal of Mo from seawater under anoxic conditions in the presence of H 2S. The data can be interpreted in terms of a steady-state mass balance between the Mo flux into the oceans from the continents and the Mo flux out of the oceans into oxic and anoxic sediments. Such an interpretation is quantitatively consistent with existing estimates of the removal fluxes of Mo to anoxic and oxic sediments. These findings suggest that δ 97/95Mo in seawater may co-vary with changes in the relative proportions of anoxic and oxic sedimentation in the oceans, and that this variation may be recorded in δ 97/95Mo of anoxic sediments. Hence, the Mo isotope system may be useful in paleoredox investigations.

Determination of molybdenum isotope fractionation by double‐spike multicollector inductively coupled plasma mass spectrometry

Molybdenum isotopic compositions are precisely determined by MC‐ICP‐MS measurements using a Mo double spike. The double spike is added prior to chemical purification, so that laboratory and instrumental mass fractionations are separated from natural mass‐dependent fractionation. Fractionation is determined on four Mo mass ratios, providing an internal consistency check. The external standard reproducibility is at 0.06 per mil on the 98Mo/95Mo ratio (2 standard deviation)). Using a normal microconcentric nebuliser with a cyclonic spray chamber, the minimum quantity of Mo is ∼1 μg for high‐precision results. A hydrothermal molybdenite shows fractionation of −0.3 per mil on the 98Mo/95Mo ratio relative to our standard (Johnson Matthey, 1000 μg/mL (±0.3%) ICP standard solution, lot 602332B). Fine‐grained sediments show fractionation of 0.1 and −0.3 per mil on the 98Mo/95Mo ratio. The observed Mo isotope fractionation is small but resolvable with the presented high‐resolution technique.

Precise determination of mass-dependent variations in the isotopic composition of molybdenum using MC-ICPMS.

We present an analytical approach for the precise determination of mass-dependent differences in the isotopic composition of Mo between samples and reference standards using multiple-collector magnetic sector inductively coupled plasma mass spectrometry (MC-ICPMS). Either Zr or Ru "element spikes" are employed to correct for instrumental mass bias. Differences in 95Mo/97Mo can be determined to a precision of +/-0.2%o (+2sigma) using 1-10 microg of Mo. Similar precision is possible for other ratios after correction for isobaric interferences from either spike element. This approach facilitates study of mass-dependent variations in the isotopic composition of Mo in nature and in materials produced by laboratory processes. We observe fractionation of Mo isotopes of approximately 1.5%o/amu during ion-exchange chromatography in the laboratory and a shift of approximately 0.3%o/amu between natural MoS2 and a laboratory standard.

Determination of Cu and Mo Isotope Fractionations in Natural Materials Using MC-ICP-MS

Our aim is to determine the deviations (8) in the isotopic compositions of Cu and Mo between samples and standards, using the VG Plasma54 multiple-collector inductively-coupled plasma mass spectrometer (MC-ICP-MS) at the University of Rochester. MC-ICP-MS systems are well-suited to this research because they feature high ionization efficiency for all transition metals, simple sample introduction, simultaneous collection of multiple ion beams for precise determination of isotope ratios, and no fractionation of the sample reservoir during analysis (Halliday et al., 1995). As a result, the largest source of uncertainty in isotopic analyses by MC-ICP-MS is the variability of instrumental mass bias with time. We can correct for this effect if we take advantage of another unique characteristic of MC-ICP-MS: The minimal variation in instrumental mass bias among different elements over a limited mass range (Hirata, 1996). As a result, if samples and standards are 'spiked' with an element of similar mass to the element of interest, the magnitude of mass bias, ~, can be determined by comparing the measured isotopic composition of the spike element to the 'true' value. The measured composition of the element of interest is then corrected using cz and the appropriate mass fractionation law. This 'elemental spike' method, analogous to use of an isotopic 'double spike', has proven useful in precise and accurate determination of Pb isotopic compositions (Longerich et al., 1987; Hirata et al., 1996). For fractionation studies, the use of elemental spikes avoids some of the complexities of a double spike. It also facilitates isotopic analysis of elements with fewer than four stable isotopes. Significantly, 8 values do not depend on cc being identical for the element of interest (cci) and the spike element (%). Although differences in ~z produce inaccuracies in isotope ratio determinations, such errors cancel in the 8 notation as long as ~i/CCs is constant. Similarly, inaccuracy in the 'true' value of the spike element will not affect 8, as long as the isotopic composition of the spike is identical in samples and standards.

Reply to “Note on isotope sputtering”

In his note commenting on the ^(92)Mo-^(100)Mo isotopic fractionation results of Weathers et al., Sigmund raises questions about the experimental procedures used in these measurements. Specifically, he suggests that isotopic fractionation effects may be present in the SIMS analysis of the carbon collector foils; and, based on conventional sputtering theory, he further suggests that these effects may be as large as 50% of the fractionation from the molybdenum targets themselves, if only a small fraction of the molybdenum was removed from the foils. He also raises questions about the possible relationship between changes in the yield and fractionation versus ion fluence.