Osmium peridotite xenoliths

Figure 21 Covariation of rhenium versus osmium in whole-rock cratonic and circum-cratonic peridotite xenoliths (left) and off-craton peridotite xenoliths (right). Off-craton suite is compared to range in Massif peridotites (shown hy field). Cratonic values extend to >15 ppb Os and >2 ppb Re. Lines denote Primitive Mantle values of Morgan et al. (1986) (source Walker et al, 1989a Carlson and Erving, 1994 Carlson et ah, 1999a Pearson et al, 1994, 1995a,b, 2002 Reisberg and Lorand, 1995 Handler et al, 1997 Burnham et al, 1998 Chesley et al, 1999 Burton et al, 2000 Peslier et al, 2000 Meisel et al, 2001 Hanghoj et al, 2001 Irvine et al., 2001, 2003 Irvine, 2002).

Improved analytical capabilities have led to the analysis of several hundred xenoliths for osmium isotopic composition. The compatible nature of osmium during mantle melting means that, unlike incompatible-element-based isotope systems, peridotite residues have much higher osmium contents than mantle melts and thus the system is less readily disturbed by later metasomatism (see Section 2.05.2.5.3). This is clearly shown by rhenium and osmium abundances (Figure 21). The vast majority of rhenium contents of both cratonic and noncratonic peridotite xenoliths are below the PUM value proposed by Morgan et al (1981) and many are P-PGE depleted. This contrasts with almost universal TREE enrichment of whole-rock peridotites. That the Re-Os system is not immune from the effects of metasomatism is illustrated by the consideration of extended PGE patterns (Figure 20 Section 2.05.2.5.3 Pearson et al., 2002, 2004). Dismption of both rhenium and osmium in some mantle environments may have occurred (Chesley et al, 1999), especially where sulhde metasomatism is involved (Alard et al, 2000). However, Pearson et al. (2002, 2004) and Irvine et al (2003) have shown that coupled PGE and Re-Os isotope analyses can effectively assess the level of osmium isotope disturbance in peridotite suites. 

Irvine G. J., Carlson R. W., Kopylova M. G., Pearson D. G., Shirey S. B., and Kjarsgaard B. A. (1999) Age of the lithospheric mantle beneath and around the Slave craton a rhenium-osmium isotopic study of peridotite xenoliths from the Jericho and Somerset Island kimberlites. In Abst. Ninth Annual V. M. Goldschmidt Conference LPI contribution no. 971. Lunar and Planetary Institute, Houston, pp. 134-135. 

D. R. LE Roux, A. P. 1999. Temperature, pressure and rhenium-osmium age systematics of off-craton peridotite xenoliths from the Namaqua-Natal belt, western South Africa. Abstracts, Ninth Annual V. M. Goldschmidt Conference. Lunar and Planetary Institute Contribution, 971, 139. 

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 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).

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