Basalts osmium isotopes

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... 

Eclogite osmium isotope compositions are indicative of long-term evolution with basaltic/picritic Re/Os. 

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).

Roy-Barman M., Wasserburg G. J., Papanastassiou D. A., and Chaussidon M. (1998) Osmium isotopic compositions and Re-Os concentrations in sulfide globules from basaltic glasses. Earth Planet. Sci. Lett. 154, 331 —347. 

In southeast Australia, this conclusion has been supported by recent osmium-isotopic studies of the Quaternary Newer Volcanic Province, which reveal that sodic alkali basalts there have osmium-isotopic compositions indistinguishable from OIB, again supporting a sublithospheric origin for their parental magmas (McBride et al, 2001). Helium-isotope data are available primarily from sodic basalts in the northern half of Africa (Cameroon Line, Sudan). These basalts have R/Ri, values (—5.1-7.5) consistent with a derivation from an HIMU-type plume source (Barfod et al., 1999 Franz et al, 1999). 

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. 

Schiano P., Birck J.-L., and Allegre C. J. (1997) Osmium-strontium-neodymium-lead isotopic covariations in midocean ridge basalt glasses and the heterogeneity of the upper mantle. Earth Planet. Sci. Lett. 150, 363-379. 

The problem of crustal contamination is particularly acute for low mg continental flood basalts and smaller volume continental tholeiitic basalts, both of which have low trace-element concentrations (see Sections 3.03.3.2.3 and 3.03.3.3). The issue is less critical for many smaller volume continental rocks, such as kimberlites and alkali basalts, which have much higher abundances of many trace elements. As a result of their high strontium and neodymium content, for example, the isotopic compositions of these elements in kimberlites and alkali basalts are relatively insensitive to modification during crustal contamination. Conversely, the osmium and lead concentration of basaltic magmas are so low that these isotope systems are particularly vulnerable to modification by interaction with cmstal rocks (McBride et al, 2001 Chesley et al, 2002) hence these systems provide relatively sensitive indicators of crustal assimilation. 

Horan M. F., Walker R. J., Fedorenko V. A., and Czamanske G. K. (1995) Osmium and neodymium isotopic constraints on the temporal and spatial evolution of Siberian flood basalt sources. Geochem. Cosmochim. Acta 59, 5159—5168. 

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