Os isotope data

Table 1. PGE and Os isotope data for selected whole-rock peridotites from the Slave Craton, Namibia and Vitim... 

The Re-Os decay system is discussed separately, in part because there are far fewer osmium isotope data than Sr-Nd-Pb data. This is true because, until —10 years ago, osmium isotopes in silicate rocks were extraordinarily difficult to measure. The advent of negative-ion thermal ionization mass spectrometry has decisively changed this (Greaser et al., 1991 Vblkening et al., 1991), and subsequently the number of publications providing osmium isotope data has increased dramatically. 

Figure 44 Frequency distribution plots of Re-Os isotope Trd rnodel ages (calculated assuming Re/Os = 0) for cratonic and off-craton peridotite xenoliths and massifs. The light-shaded field in the off-craton plot shows the range for kimberlite-derived peridotites from Namibia and East Griqualand. Data sources as in Figure 21 plus Pearson (unpublished).

Fewer reliable osmium isotopic data exist for Precambrian samples owing to the problems of disturbance of rhenium and osmium during alteration and metamorphism. Key points used to define mantle evolution are indicated in Figure 7. The osmium isotopic constraints on the composition of the Archean mantle are, in many cases, derived from Late Archean komatiite suites. Komatiites are high-MgO lavas that often form differentiated flow sequences, such that Re-Os isochrons can be determined chromite-rich layers are commonly present and provide a robust estimate of the initial osmium isotopic... 

Fig. 11. Location map of kimberlites on and around the Kaapvaal Craton from which peridotite xenolith suites have been dated by Re-Os isotopes (after Carlson et al. 2000 Irvine et al. 2001). Data sources Pearson et al. (1994, 1995a, 1998) Pearson (1999) Carlson et al. (1999) Janney et al. (1999) Irvine et al. (2001). Kaapvaal Craton is outlined in dark grey with the Archean Limpopo Metamorphic Belt shaded in light grey.

The accretion of a late veneer. Whether or not a late veneer was added to the Earth towards the end of accretion is not clear. The principal evidence comes from the elevated siderophile element chemistry of the mantle. If there was a late veneer, it had to happen after core formation. At present the evidence from the trace element chemistry is ambiguous, because these data can also be explained by the formation of the core at high pressures and temperatures in a magma ocean, or by continuous core formation with decreasing metal input. Currently, the best evidence for a late veneer comes from Os-isotope evidence, where there is a clear mismatch between the composition of the PUM and chondrite. However, even this is uncertain, as is discussed in the next chapter (Chapter 3, Section 3.2.3.4), for... 

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