Here we describe a new chemical separation method for Os and an improved mass spectrometric procedure for Re and Os. This technique is based on the selective extraction of OsO4 from aqueous solution in liquid bromine. Among other advantages, this procedure avoids the cumbersome distillation procedure for Os and uses only commercial “off the shelf” PFA teflon labware. Blank levels for 0.5 g sample sizes are: 0.06 pg and 0.5 pg for Os and Re respectively. Samples containing as little as 1 pg g−1 Os can be analysed reliably with this method. Adaptation to other dissolution methods is also discussed.

Re‐Os Isotopic Measurements at the Femtomole Level in Natural Samples

Re-Os isotopic dating of molybdenites in the Xiaoliugou W (Mo) deposit in the northern Qilian mountains and its geological significance

http://christoph-burkhardt.org/Burkhardt%20et%20al%20epsl_2011.pdf

Molybdenum isotope anomalies in meteorites: Constraints on solar nebula evolution and origin of the Earth

The unusual chemistry of molybdenum (Mo) makes this trace element interesting to both geochemists and biochemists. Geochemically, Mo is relatively unreactive in oxygenated, aqueous solutions, and hence is a nominally conservative element in the oceans. In fact, Mo is removed so slowly from seawater that it is the most abundant transition metal in the oceans despite being a ppm-level constituent of the crust. In contrast, Mo is readily removed from solution in anoxic-sulfidic (“euxinic”) settings, so that Mo enrichments in sediments are considered diagnostic of reducing depositional conditions. Few elements possess such bimodal redox behavior at the Earth’s surface.

Biochemically, Mo draws attention because it is an essential enzyme cofactor in nearly all organisms, with particular importance for nitrogen fixation, nitrate reduction and sulfite oxidation. Such biochemical ubiquity is surprising in view of the general scarcity of Mo at the Earth’s surface.

Isotopically, Mo initially catches the eye because it has seven stable isotopes of 10–25% abundance, covering a mass range of ~8% (Fig. 1⇓). Thus, from an analyst’s perspective, Mo offers both an unusually large mass spread and a number of options for isotope ratio determination. Combined with rich redox chemistry and covalent-type bonding, both of which tend to drive isotope fractionation, these factors make the Mo isotope system a particularly promising target for stable isotope investigation.



 Figure 1. 
The average natural abundances of the stable isotopes of Mo as recommended by IUPAC, based on (Moore et al. 1974).



In the environment, Mo isotope research began in earnest with the application of multiple-collector inductively coupled plasma mass spectrometry. While much work remains to be done, this early research points to promising applications in paleoceanography, and beyond.

This review is intended to provide an overview of this emerging stable isotope system in the context of Mo environmental biogeochemistry. Special attention …

Molybdenum Stable Isotopes: Observations, Interpretations and Directions

Seven 187Re-187Os ages were determined for molybdenite and pyrite samples from two well-dated Precambrian intrusions in Fennoscandia to examine the sustainability of the Re-Os chronometer in a metamorphic and metasomatic setting. Using a new 187Re decay constant (1.666 × 10−11y−1) with a much improved uncertainty (±0.31%), we determined replicate Re-Os ages for molybdenite and pyrite from the Kuittila and Kivisuo prospects in easternmost Finland and for molybdenite from the Kabeliai prospect in southernmost Lithuania. These two localities contain some of the oldest and youngest plutonic activity in Fennoscandia and are associated with newly discovered economic Au mineralization (Ilomantsi, Finland) and a Cu-Mo prospect (Kabeliai, Lithuania). Two Re-Os ages for vein-hosted Kabeliai molybdenite average 1486 ± 5 Ma, in excellent agreement with a 1505 ± 11 Ma U-Pb zircon age for the hosting Kabeliai granite pluton. The slightly younger age suggests the introduction of Cu-Mo mineralization by a later phase of the Kabeliai magmatic system. Mean Re-Os ages of 2778 ± 8 Ma and 2781 ± 8 Ma for Kuittila and Kivisuo molybdenites, respectively, are in reasonable agreement with a 2753 ± 5 Ma weighted mean U-Pb zircon age for hosting Kuittila tonalite. These Re-Os ages agree well with less precise ages of 2789 ± 290 Ma for a Rb-Sr whole-rock isochron and 2771 ± 75 Ma for the average of six Sm-Nd TDM model ages for Kuittila tonalite. Three Re-Os analyses of a single pyrite mineral separate, from the same sample of Kuittila pluton that yielded a molybdenite separate, provide individual model ages of 2710 ± 27, 2777 ± 28, and 2830 ± 28 Ma (Re = 17.4, 12.1, and 8.4 ppb, respectively), with a mean value of 2770 ± 120 Ma in agreement with the Kuittila molybdenite age. The Re and 187Os abundances in these three pyrite splits are highly correlated (r = 0.9994), and provide a 187Re-187Os isochron age of 2607 ± 47 Ma with an intercept of 21 ppt 187Os (MSWD = 1.1). It appears that the Re-Os isotopic system in pyrite has been reset on the millimeter scale and that the 21 ppt 187Os intercept reflects the in situ decay of 187Re during the ∼160 to 170 m.y. interval from ∼2778 Ma (time of molybdenite ± pyrite deposition) to ∼2607 Ma (time of pyrite resetting). When the Re-Os data for molybdenites from the nearby Kivisuo prospect are plotted together with the Kuittila molybdenite and pyrite data, a well-constrained five-point isochron with an age of 2780 ± 8 Ma and a 187Os intercept (−2.4 ± 3.8 ppt) of essentially zero results (MSWD = 1.5). We suggest that the pyrite isochron age records a regional metamorphic and/or hydrothermal event, possibly the time of Au mineralization. A proposed Re-Os age of ∼2607 Ma for Au mineralization is in good agreement with radiometric ages by other methods that address the timing of Archean Au mineralization in deposits worldwide (so-called “late Au model”). Molybdenite, in contrast, provides a robust Re-Os chronometer, retaining its original formation age of ∼2780 Ma, despite subsequent metamorphic disturbances in Archean and Proterozoic time.

Re-Os ages for Archean molybdenite and pyrite, Kuittila-Kivisuo, Finland and Proterozoic molybdenite, Kabeliai, Lithuania: testing the chronometer in a metamorphic and metasomatic setting

Qi-Lu

Papers

The isotopic composition and atomic weight of molybdenum

Reliable Re-Os age for molybdenite