Mantle rocks peridotite xenoliths

Figure 9. Plots of Li and radiogenic isotopes for mantle rocks, (a) 5 Li vs. Sr/ Sr (b) 5 Li vs. Nd/ Nd (c) "Sr/ Sr vs. Pb/ Pb (d) 5"Li vs. Pb/ Pb (Nishio et al. 2003, 2004). Symbols + = south Pacific island basalts (six islands) O = Iherzolite xenolith, Bullenmerri, Australia = Iherzolite xenolith, Sikhote-Alin, Russia (three localities) A = dunite-peridotite-pyroxenite xenolith, Kyushu, Japan (two localities) V = Iherzolite xenolith, Ichinomegata, Japan. The ocean island data are from bulk rocks, the xenolith data are clinopyroxene separates. For explanations of the derivation of radiogenic isotope fields (DM, EMI, EM2, HIMU), see Zindler and Hart (1986). The estimate for Li isotopes in DM is based on MORE. The Li isotopic ranges for the other mantle reservoirs are based on Nishio et al. (2004) and Nishio et al. (2003), but these will require further examination (hence the use of question marks).

Figure 19 Left (Pd/Ir) , where n refers to values normalized to primitive mantle values (McDonough and Sun, 1995) versus bulk rock AI2O3. Right (Os/Ir) versus (Pt/Ir) plot of cratonic and olf-craton whole-rock peridotites. Curves on left-hand plot are trends expected for progressive melting of mantle peridotite. On the right-hand plot, large circle with error bars = mean and 1 SD for cratonic peridotites. Large square with error bars = mean and 1 SD for off-craton peridotites. Cratonic peridotite data are isotope dilution data from Pearson et al. (2002), Irvine (2002), Pearson et al. (2004), and Irvine et al. (2003). Off-craton xenolith data from Handler and Bennett (1999) and...

Figure 20 Primitive mantle normalized extended PGE patterns (including rhenium) for cratonic whole-rock garnet peridotite xenoliths from the Letseng kimberlite (Lesotho) (sources Irvine, 2002 and Pearson et al., 2004).

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. 

Cratonic mantle peridotites. Over 230 whole-rock cratonic xenoliths have now been analyzed for Re-Os isotope compositions. Given that many peridotite xenoliths have experienced relatively recent rhenium introduction, it is generally best to use rhenium-depletion model ages (Trd) that do not rely on the measured rhenium content of the rock for model age calculation. For cratonic peridotite xenoliths, the frequency distribution of rhenium-depletion ages shows a wide range, with a pronounced mode at 2.5-2.75 Gyr and some samples that have ages of >3.5 Gyr... 

In the oceanic setting, spinel Iherzolite xenoliths from Pali (Hawaii) have olivine 5 0 values of 5.09-5.12 per mil, typical of olivines from other oceanic and continental mantle rocks (Ducea et al., 2002). In contrast, olivines from plagioclase peridotites are enriched by 0.5 per mil. This is interpreted to be due to the formation of plagioclase by reaction with or crystallization from melts intruding the Pacific lithospheric mantle. 

Rudnick R. L., McDonough W. F., and Orpin A. (1994) Northern Tanzania peridotite xenoliths a comparison with Kapvaal peridotites and inferences on metasomatic reactions. In Kimberlites, Related Rocks and Mantle Xenoliths Vol. 1. Proc. 5th Int. Kimberlite Conf. (eds. Henry O. A. Meyer, Leonardosand Othon H). CRPM-Special Publication, Araxa, Brazil, vol. lA. pp. 336-353, Companhia de Pesquisa de Recursos Minerals, Rio de Janeiro, Brazil. 

---