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Applications of PGE Radioisotope Systems in Geo- and Cosmochemistry

¹ Department of Terrestrial Magnetism
Carnegie Institution of Washington, 5241 Broad Branch Road NW
Washington, DC 20015, USA
E-mail: rcarlson{at}ciw.edu; shirey{at}dtm.ciw.edu
² School of Earth, Atmospheric and Environmental Sciences
The University of Manchester
Oxford Road, Manchester, M13 9PL, United Kingdom
E-mail: m.schonbachler{at}manchester.ac.uk

View larger version (18K): [in this window] [in a new window]   FIGURE 1 Re-Os isochron diagram for gold and pyrite from the 2.8-2.9 Ga Witwatersrand Supergroup, South Africa, redrawn from the data and information in the study by Kirk et al. (2002). Vaal Reef gold and rounded pyrite interpreted to be detrital produce an excellent 3.03 ± 0.02 Ga isochron with an initial 187Os/188Os ratio identical to the mantle value at this time (inset). Because both age constraints are older than the <2.9 Ga age of the conglomerates, they confirm the detrital nature of the gold and suggest that the provenance of the conglomerate included magmatic rocks in a nearby 3 Ga green-stone belt. Epigenetic, cubic-morphology pyrite in the hydrothermally altered Ventersdorp Contact Reef (VCR) shows open-system behavior of the Re-Os system.

View larger version (37K): [in this window] [in a new window]   FIGURE 2 Re-Os isochron diagram for sulfide inclusions in diamonds and putative host harzburgites from the Panda kimberlite pipe, Slave craton, NWT, Canada (after Westerlund et al. 2006). The sulfide inclusions (red squares) regress to an age of 3.52 ± 0.17 Ga (with 4 additional inclusions off scale) and have an elevated initial 187Os/188Os isotope composition compared to a 3.5 Ga mantle (black line). With one exception, harzburgites that host the diamonds (green crosses) have 187Os/188Os ratios above primitive mantle values, but less so than the diamonds. The upper left panel shows a polished Panda diamond plate with 4 inclusions in their "rosette" fracture system. Shown in the upper right is a typical scanning electron micrograph of a released inclusion prior to dissolution, chemical separation of the Re and Os, and mass spectrometry.

View larger version (28K): [in this window] [in a new window]   FIGURE 3 Os isotope data tracking the composition of seawater Os over the last 90 My. The upper figure shows the general increase in the 187Os/188Os ratio following the steep decline near the Cretaceous-Tertiary boundary (Peucker-Ehrenbrink et al. 1995). The lower figures show high-resolution data for smaller time slices illustrating the ability of Os to record fine-scale variations in inputs into the ocean. Near the K-T boundary (KTB), this includes contributions of mantle Os attributed to eruption of the Deccan flood basalts and extra-terrestrial Os added by the K-T impactor (Ravizza and Peucker-Ehrenbrink 2003). In the Oligocene (lower-left figure), the rapid rise in the187Os/188Os ratio is attributed to an increase in continental weathering after removal of glacial cover exposed fine-grained glacial sediments (Dalai et al. 2006).

View larger version (61K): [in this window] [in a new window]   FIGURE 4 Overlay of P-wave seismic velocity variations at a depth of 150 km and average Re-Os model ages for mantle samples from kimberlites distributed across southern Africa. The points show kimberlites containing mantle samples from depths of up to 200 km. The points are color coded according to the average Re-Os age for all the mantle samples studied from each individual kimberlite. The red line outlines areas of Archean crust. Within the Archean crustal section, most mantle samples give Archean ages and most diamond-bearing kimberlites are confined to the blue areas of fastest seismic velocities. The fields outlined in black show the surface outcrop of the 2.05 Ga Bushveld and Molopo Farms igneous intrusions. The one kimberlite erupted near this area (Premier kimberlite, black dot) contains mantle samples that predominantly give Re-Os ages near 2 Ga. The correlation of younger mantle ages at Premier with slower seismic velocities suggests that the Bushveld event substantially modified the mantle under this part of the ancient southern African crust. Off the Archean craton, mantle samples give only Proterozoic ages, consistent with the age of the overlying crust. The light grey lines show boundaries between crustal terranes of different ages. Tomography is from Fouch et al. (2004), and Re-Os data are summarized in Carlson et al. (2005).

View larger version (21K): [in this window] [in a new window]   FIGURE 5 Coupled 186Os-187Os isotope variation in direct samples of the mantle (abyssal peridotites and PGE-alloys and chromites from ophiolites) and volcanic rocks believed to originate by melting of plumes rising from the core-mantle boundary. Although crustal rocks (e.g. basalt) and various types of sediment have high Re/Os and Pt/Os ratios, their low Os concentrations coupled with only small ingrowth of 186Os due to 190Pt decay lead to nearly horizontal mixing lines with mantle peridotite on this diagram. In contrast, due to the very high concentrations of Os in the core, even a small addition of core material to the mantle will shift the Os isotope composition of the mixture towards the core component. The lines on the diagram show mixing trajectories expected for fertile mantle peridotite mixed with two types of crustal rocks (tick marks give 10% by mass increments of crustal rock in the mixture) and outer core (tick marks show 1% increments in core addition). The mixing lines were constructed using the end member parameters given in Carlson (2005), with data sources summarized in Carlson (2005) and Brandon and Walker (2005).

View larger version (15K): [in this window] [in a new window]   FIGURE 6 Pd-Ag isochrons from various classes of meteorites. For extinct radioisotope systems, isochrons give not the age of the sample, but the abundance ratio of the extinct isotope at the time the isochron relationship was established (e.g. the 107Pd/108Pd ratio). The higher this ratio is, the older the sample, with the ratio decreasing by a factor of 2 every 6.5 My, the half-life of 107Pd. The top panel shows the results for the volatile-depleted Group IVA iron meteorite Gibeon (Chen and Wasserburg 1990). Here the very high Pd/Ag ratios (x-axis) have created a wide range in 107Ag/109Ag ratios between metal (squares) and sulfides (diamonds). At much lower Pd/Ag ratios, and consequently much smaller variation in Ag isotope composition (now expressed as 107Ag, i.e. the parts in 10,000 difference between107Ag/109Ag in the sample compared to that in a terrestrial Ag standard), the middle panel shows that the Group IAB iron meteorites Toluca and Canyon Diablo have correlated Pd/Ag ratios and Ag isotope compositions in metal (squares), but not in sulfides (diamonds). The middle panel also shows that MC-ICP-MS analyses are much more precise than the one thermal ionization analysis shown (Chen and Wasserburg 1990), whose error bars extend off the diagram. The lower panel shows that 6 out of 8 carbonaceous chondrites (also shown in the middle panel) lie on a Pd/Ag versus Ag isotope regression line with a slope of 5.9 x 10-5, the highest value yet seen for any solar system material. Data sources include Chen and Wasserburg (1990) and Schönbächler et al. (2008) and references therein.