Crust osmium isotopes

Figure 54 Comparison of present-day osmium isotopic compositions of eclogite xenoliths from Udachnaya, Yakutia (Pearson et ah, 1995c) and S. Africa (Pearson et al, 1992 Menzies et al, 1999 Shirey et ah, 2001) with continental crust, oceanic basalts (Shirey and Walker, 1998), and Archean komatiites and basalts (Walker et al, 1989b). Udachnaya peridotite data from Pearson et al (1995a).

In recent years, we have seen the use of new isotopic systems, including those of osmium and oxygen, in the description of oceanic basalts. Osmium isotopes can trace the addition of mafic crust or melt to the mantle source (Hauri and Hart, 1993) and have been used to highlight the presence of recycled mafic crust in sources of hot spots (Shirey and Walker, 1998). In a complimentary fashion, oxygen isotope variations trace directly... 

Esser B. K. and TureMan K. K. (1993) The osmium isotopic composition of the continental crust. Geochim. Cosmochim. Acta 57, 3093-3104. 

Peucker-Ehrenbrink B. and Jahn B.-M. (2001) Rhenium-osmium isotope systematics and platinum group element concentrations loess and the upper continental crust. Geochem. Geophys. Geosys. G 3 2001GC000172. 

McBride J. S., Lambert D. D., Nicholls I. A., and Price R. C. (2001) Osmium isotopic evidence for crust—mantle interaction in the genesis of continental intraplate basalts from the Newer Volcanic Province southeastern Australia. J. Petrol. 42, 1197-1218. 

ISOTOPES Osmium has 41 isotopes, five of which are stable. Two are naturally radioactive isotopes with very long half-lives. Following are the stable isotopes and their contribution to the elemenfs natural existence in the Earth s crust Os-187 = 1.6%, Os-188 = 13.29%, Os-189 = 16.21%, Os-190 = 26.36%, and Os-192 = 40.93%. The remain-... 

In this section mantle evolution curves are presented for neodymium (Nd), hafnium (Hf), lead (Pb) and osmium (Os) isotopes. A summary of these isotopic systems is given in Text Box 3.2. Earlier studies based upon the study of Sr isotopes in the mantle (e.g. Bell et al., 1982) are now known to be unreliable because of its high geochemical mobility (Goldstein, 1988). The significance of the mantle evolution curves described here is that they demonstrate that the mantle does not operate as an isolated system but that it has evolved in its composition over time, in response to core formation, crust extraction, and the recycling of crustal material. 

The beta decay scheme of rhenium-187 to osmium-187 maybe applied to the study of sulfide minerals of molybdenum and osmium-rich minerals such as iridosmine. Together with rubidium/strontium, samarium/neodymium, lutetium/hafnium, and uranium/lead methods, it has been used to examine mantle differentiation and the accretion of continental crust. In molybdenites, rhenium concentrations vary from a few ppm to as much as 1.88%. The element has two naturally occurring isotopes, and these are the stable rhenium-185 with a relative abundance of 37.398% and the radioactive rhenium-187 with a relative abundance of62.602%. This decays as follows ... 

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