Osmium isotope ratios

Osmium isotopic ratio of the bulk silicate Earth overlaps measurements of ordinary chondrites but is distinct from other chondrite groups. Adapted from Righter et al. (2006). 

Osmium isotopes currently provide the strongest case for mineral-to-mineral disequilibrium, and for mineral-melt disequilibrium available from observations on natural rocks. Thus, both osmium alloys and sulfides from ophiolites and mantle xenoliths have yielded strongly heterogeneous osmium isotope ratios (Alard et al., 2002 Meibom et al., 2002). The most remarkable aspect of these results is that these ophiolites were emplaced in Phanerozoic times, yet they contain osmiumbearing phases that have retained model ages in excess of 2 Ga in some cases. The melts that were extracted from these ophiolitic peridotites contained almost certainly much more radiogenic osmium and could, in any case, not have been in osmium-isotopic equilibrium with all of these isotopically diverse residual phases. 

Figure 9 Osmium isotope ratios in MORE and abyssal peridotites. This diagram shows that osmium is generally compatible in peridotites during MORE melting. The systematic differences in Os/ Os ratios between MORE and peridotites suggest that the melts may not be in isotopic equilibrium with their residual peridotite (sources Martin, 1991 Roy-Earman and Allegre, 1994 Snow and Reisberg, 1995 Schiano et al., 1997 Erandon et al., 2000).

Vdikening J., Walczyk T., and Heumann K. G. (1991) Osmium isotope ratio determinations by negative thermal ionization mass spectrometry. Int. J. Mass Spectrom. Ion Process. 105, 147-159. 

Hattori H. and Hart S. R. (1991) Osmium isotope ratios of platinum group elements associated with ultramafic intrusions. Earth. Planet. Sci. Lett. 107, 499-514. 

Junk, S.A., Remicka, E. (2003) An assessment of osmium isotope ratios as a new tool to determine the provenance of gold with platinum-group metal inclusions. Archaeometry, 45, 313-331. 

Hirata, T., Hattori, Michinari Tanaka, and Tsuyoshi (1998) In-situ osmium isotope ratio analyses of iridosmines by laser ablation-multiple collector-inductively coupled plasma mass spectrometry. Chem. Geol., 144, 269-80. 

Mossbauer measurements with determination of the electric quadrupole moments have been reported in [253, 254,259]. Wagner et al. [254] measured the quadrupole hyperfine interaction in OSO2 and OSP2 of the Mossbauer isotopes The ratios of the quadrupole moments of the 4 = 72 states in the even osmium isotopes and of the 4 = 5/2 (69.6 keV) and 4 = 3/2 states in Os were deduced very accurately. In Table 7.8, the experimental results [254] are given, from which the following ratios can be calculated ... 

Figure 11.28 shows the rhenium and osmium isotopic compositions of black shales and sulfide ores from the Yukon Territory (Horan et al., 1994). The black shale and sulfide layers are approximately isochronous. The superimposed reference isochrons bracket the depositional age of the enclosing shales. One reference line represents the minimum age (367 Ma) with an initial ( Os/ Os)q ratio of one, consistent with the mantle isotopic composition at that age (see later). The other reference isochron is drawn for a maximum age of 380 Ma, with ( 870s/ 860s)o = 12 (the maximum value measured in terrigenous sediments). Further examples of application of Re-Os dating of sediments can be found in Ravizza and Turekian (1989). 

The late veneer hypothesis has gained additional support from the analyses of the osmium isotopic composition of mantle rocks. Meisel et al. (1996) determined the Os/ Os ratios of a suite of mantle xenoliths. Since rhenium is more incompatible during mantle partial melting than osmium, the Re/Os ratio in the mantle residue is lower and in the melt higher than the PM ratio. By extrapolating observed trends of Os/ Os versus AI2O3 and lutetium, two proxies for rhenium, Meisel et al. (1996) determined a Os/ Os ratio of 0.1296 0.0008 for the primitive mantle. This ratio is 2.7% above that of carbonaceous... 

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