Itsaq gneiss

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... 

Friend C. R. L. F., Bennett V. C., and Nutman A. P. (2002) Abyssal peridotites >3,800 Ma for southern West Greenland field relationships, petrography, geochronology, whole-rock and mineral chemistry of dunite and harzburgite inclusions in the Itsaq Gneiss Complex. Contrib. Mineral. Petrol. 143, 71-92. 

Nutman A. P., McGregor V. R., Friend C. R. L., Bennett V. C., and Kinny P. D. (1996) The Itsaq Gneiss Complex of southern West Greenland the world s most extensive record of early crustal evolution (3,900-3,600 Ma). Precamb. Res. 78, 1-39. 

Figure 3 Map of the northern part of the Itsaq Gneiss complex (source Nutman et al, 2002).

The earliest terrestrial 187Os/188Os data come from the early Archaean (3.81 Ga) chromitites of the Itsaq Gneisses in west Greenland (Bennett et al., 2002 Rollinson et al., 2002). These samples show a range of compositions but with a mean value which is slightly supra-chondritic (Frei 8k Jensen, 2003). This could indicate that the enriched Os-isotope reservoir discussed above was in existence even in the very early Archaean and indicates the very early recycling of basaltic crust. 

It is clear that the Earth s mantle has at least two Os-isotopic reservoirs - a plume-related isotopically enriched reservoir and a chondritic upper mantle reservoir. Both have long histories (Fig. 3.32). The variations in composition within the upper mantle reservoir reflect Re-depletion and enrichment related to melt extraction. The isotopically enriched plume reservoir represents chemically isolated, rhenium-enriched, recycled oceanic lithosphere. There is some evidence to suggest that this enriched reservoir may have been in existence since the early Archaean (Walker Nisbet, 2002) and was the source of some Archaean komatiites and the 3.81 Ga Itsaq Gneiss chromitites. If this is true, then basaltic crust was being created and recycled even before 4.0 Ga. Estimates of the present size of this high Re/Os basaltic reservoir vary from 5% to >10% of the whole mantle (Bennett et al., 2002 Walker et al., 2002). 

Figure 4 Initial snf values of >3.9 Ga detrital zircons from the Jack Hills, Western Australia (Amelin et al., 1999) and Early Archean Itsaq complex gneisses (Vervoort and Blichert-Toft, 1999). The same data have been recalculated using two recently proposed revisions to the Lu decay constant the closed symbols use the decay constant proposed by Bizzarro et al. (2003). The open samples are calculated using the 6% lower decay constant proposed by Scherer et al. (2001). The present-day chondrite parameters used are Hf/ Hf = 0.282772 and Lu/ Hf = 0.0332 (Blichert-Toft and Albarede, 1997).

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