Archaean mantle composition

Henry, P., Stevenson, R.K., and Gariepy, C., 1998, Late Archaean mantle composition and crustal growth in the western Superior Province of Canada neodymium and lead isotopic evidence from the Wawa, Quetico and Wabigood subprovinces. Geochim. Cosmochim. Acta, 62, 143-57. 

Sun S.-S. and Nesbitt R. W. Chemieal heterogeneity of the Archaean mantle, composition of the Earth and mantle evolution. Earth Planet. Sci. Lett., 35, 1977, 429-48. 

In the first part of this chapter we shall examine the structure and composition of the modern mantle in order to establish how it works. In so doing we will find tantalizing clues which relate to the mantle s earlier history. It is these clues that we shall explore in the second part of this chapter and use to identify the nature and chemical evolution of the Archaean mantle. These data are then used in the third part of the chapter to constrain models for the Archaean mantle. 

FIGURE 3.13 The composition of the subcontinental lithosphere plotted on a mantle composition Mg/Si-Al/Si weight ratio diagram. Labeled triangles A Archaean lithosphere Pr Proterozoic lithosphere Ph-sp Phanerozoic spinel lherzolite Ph-grt, Phanerozoic garnet lherzolite (data from Griffin et al.( 2003, see Table 3.4). Other symbols as in Fig. 3.9. 

Basaltic rocks are common in Archaean terrains and are particularly abundant in Archaean greenstone belts. However, they are frequently altered and metamorphosed, and only the best preserved samples should be used to obtain information about the composition of the Archaean mantle. Recent research has shown that Archaean greenstone belts formed in a number of different tectonic environments (see Chapter 1, Section 1.3.1). This means that greenstone belt basalts potentially offer an insight into a variety of mantle environments in the early Earth. 

The recognition that komatiites were most probably chemically contaminated sent the world of komatiite studies into confusion for a time, for whilst previously komatiites had been held as the premier window into the Archaean mantle, it now appeared that this window was rather dirty. Fortunately, the very high percentage of contamination predicted (40%) has not been realized (Foster et al., 1995). At the present time trace element and isotopic studies are used to discriminate between contaminated and uncontaminated komatiites (Arndt, 1994) and identify the extent of contamination. So the pendulum has swung back and currently komatiites are regarded as an important source of information about the composition of the Archaean mantle. 

Calculating the composition of the Archaean mantle is a task which has occupied geochemists for some decades. It is not a trivial task because the nature of the Archaean mantle is that it was constantly changing in composition. However, estimates of the primitive mantle composition, also known as the composition of the BSE - the mantle as it was after the core has been extracted but before the continents were formed - are given at various points in this chapter, as follows estimates of the major element composition of the primitive mantle are given in Table 3.1 the composition of the Archaean subcontinental lithosphere in Table 3.4 the trace element composition of the primitive mantle in... 

Table 3.2 the initial compositions and the temporal evolution of the Nd-, Hf-, Pb- and Os-isotopic systems are given in Section 3.2.3. Finally, the volatile content of the Archaean mantle is discussed in Chapter 5, Section 5.2). 

Nitrogen in the Archaean mantle The initial nitrogen isotopic composition of the Earth is not well known. However, nitrogen isotope measurements on 2.9-3.3 Ga diamonds from the subcontinental lithosphere have a mean S15N value of -5, and a similar C/N ratio to that of the modern mantle (Fig. 5.4), suggesting that there has been very little change since about 3.0 Ga (Marty Dauphas, 2003). 

The mineralogical and chemical composition of peridotite from subcontinental lithosphere differs from that of peridotite from other parts of the mantle (Boyd 1989 Berstein et al. 1997). Peridotites from subcontinental lithosphere is depleted , which means it contains only a small amount of clinopyroxene and an aluminous phase, which together make up the so-called basaltic component. The lithosphere beneath the oldest Archaean cratons has a composition markedly different from that of younger subcontinental lithosphere (Boyd Mertzman 1987 Griffin et al. 1999). Old unmetasomatized lithosphere is harzburgitic, a mixture of olivine... 

The two layers in continental lithosphere represent extreme end-members in the compositional spectrum of magmatic rocks and each forms through independent processes. Archaean continental crust has an allochthonous relationship to underlying lithospheric mantle. 

Shirey, S. B. 1997. Initial Os isotopic composition of Munro Township, Ontario, komatiite revisited additional evidence for near-chondritic, late-Archaean convecting mantle beneath the Superior Province. In 7th Annual V. M. Goldschmidt Conference, 193. 

Wooden, J. L. Mueller, P. A. 1988. Pb, Sr, and Nd isotopic compositions of a suite of Late Archaean, igneous rocks, eastern Beartooth Mountains implications for crust-mantle evolution. Earth and Planetary Science Letters, 87, 59-72. 

Earth scientists define cratons by their lack of thermal and mechanical deformation. Cratons are largely aseismic, volcanism is restricted to small-volume melts (e.g. kimberlites) derived from depths >150 km (e.g. Boyd et al. 1985 Nixon 1987), and deformation occurs in the thinner lithosphere adjacent to the cratons (e.g. Len-ardic Moresi 2000 Petit Ebinger 2000). Archaean, and possibly early Proterozoic, cratons are compositionally distinct from younger lithosphere the upper mantle beneath cratons is depleted and hence more buoyant than younger lithosphere, making it difficult to deform in sub-duction and collisional zones (e.g. Jordan 1981 Doin et al. 1996). 

The trace element composition of the subcontinental lithosphere In contrast to the depleted major-element character of Archaean subcontinental lithosphere it is often enriched in trace elements, relative to a midocean ridge basalt mantle source (Richardson et al., 1985 Jordan, 1988). A resolution of this apparent paradox can be found in the timing of the two events. Major element depletion is thought to have taken place during the early formation of the subcontinental lithosphere whereas the trace element enrichment reflects later melt infiltration. 

If therefore, the modern subarc mantle is the site where Phanerozoic subcontinental lithosphere is created, we are still left with a large number of questions about the earlier history of the subcontinental lithosphere. Why for example is the Archaean subcontinental lithosphere so different in composition, heat production and thickness from more recent subcontinental mantle What different processes were operating early in Earth history which are recorded in this mantle domain Is there a link with komatiite extraction, as suggested by Boyd (1989), or with the extraction of basaltic melts Or, is there a close link between the formation of this type of mantle and the over-lying continental crust We will return to these issues when we discuss the origin of the continental crust in Chapter 4 (Section 4.5.1). 

FIGURE 3.22 The trace element composition of Archaean tholeiites compared to modern enriched-MORB and normal-MORB, normalized to the primitive mantle (after Arndt et al.( 1997). 

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). 

Earth, has been prominent in models of the mantle for several decades (see Section 3.1.2). Such models are based upon a chondritic starting composition for the Earth, modified during core formation and perhaps in a magma ocean, and presuppose that there was a point in time when the mantle was totally homogenized. Whether the remnants of this primitive mantle can be identified today in modern basalts, or even in the early history of the Earth in Archaean basalts, is the subject of some debate. In favor are mantle melts with chondritic Os-isotope ratios (Fig. 3.32) and against are calculations which show that it is possible that the entire mantle has been processed through the subduction system (Section 3.1.6.4) during the history of the Earth. 

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