Peridotites osmium isotopes

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).

Reisberg L. and Lorand J.-P. (1995) Longevity of sub-continental mantle lithosphere from osmium isotope systematics in erogenic peridotite massifs. Nature 376, 159-162. 

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. 

Walker et al., 1989 Pearson et ai, 1995a Carlson et al., 1999). The osmium isotope compositions of almost all lithospheric peridotite suites are completely distinct from those of their host magmas. 

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).

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. 

The oldest samples that provide direct constraints on the osmium isotopic composition of the upper mantle are rare spinel peridotites contained within the Early Archean Itsaq gneiss complex of southwest Greenland that are interpreted to be —3.81 Ga abyssal peridotites (Friend et al., 2002). The measured and initial compositions determined from low-Re/Os spinel and olivine mineral separates from these peridotites are the most primitive, in the sense of closest to solar-system initial compositions, Os isotopic compositions yet obtained on any terrestrial material (Bennett et al., 2002). This shows that at least some, if not all, of the Early Archean upper mantle was characterized by chondritic Os/ Os isotopic compositions. Osmium isotopic constraints from this time period (—3.8-3.9 Ga) are of particular interest as they provide a rough constraint on the timing of the addition of the late veneer of... 

Major-element compositions of most lithospheric peridotites reflect an origin as melt-residues. However, as with parent-daughter isotope ratios, compilation of their strontium and neodymium mineral isotopic compositions reveals that very few samples show the characteristics of ancient melt residues (Figure 37). Osmium isotopes are the exception and dominantly reflect ancient extraction of high Re/Os melts, leaving rhenium-depleted residues to develop time inte-... 

A correlation similar to the one observed for the Nd-Sr isotopes versus AI2O3 is also noted for osmium in several orogenic peridotites, but is commonly interpreted in term of depletion age (e.g., Reisberg and Lorand, 1995). The mixing hypothesis proposed for Nd-Sr casts some doubt on this interpretation. Saal et al. (2001) interpreted the osmium systematics of the Horoman peridotite in term of melt redistribution and fertilization, which is more consistent with the mixing hypothesis. 

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