Mantle Archaean

Jahn B.-M., Vidal P., and Tilton G. R. (1980) Archaean mantle heterogeneity evidence from chemical and isotopic abundances in Archaean igneous rocks. Phil. Trans. Roy. Soc. London A297, 353-364. 

McCulloch M. T. and Compston W. (1981) Sm-Nd age of Kambalda and Kanowna greenstones and heterogeneity in the Archaean mantle. Nature 294, 322-327. 

Campbell I. H., Griffiths R. W., and Hill R. I. (1989) Melting in an Archaean mantle plume heads it s basalts, tails it s komatiites. Nature 339, 697-699. 

Nisbet, E. G., Cheadle, M. J., Arndt, N. T. Bickle, M. j. 1993. Constraining the potential temperature of the Archaean mantle a review of the evidence from komatiites. Lithos, 30,291-307. 

Generation of large-scale heterogeneities in the early Archaean mantle... 

PUTCHEL, I., BrOgmann, G. Hofmann, A. W. 2001. Os-enriched domain in an Archaean mantle plume evidence from 2.8 Ga komatiites of the Kostomuksha greenstone belt, NW Baltic Shield. Earth and Planetary Science Letters, 186, 513-526. 

Blichert-Toft, j. Albarede, F. 1994. Short-lived chemical heterogeneities in the Archaean mantle with implications for mantle convection. Science, 263, 1593-1596. 

Kump, L. R., Kasting, J. F. Barley, M. E. 2001. Rise of atmospheric oxygen and the upside-down Archaean mantle. Geochemistry, Geophysics, Geosystems, 2, paper 2000GC000114. 

Calculations based on radioactive heat production show that the Archaean mantle was hotter than the modern mantle. High mantle potential temperatures calculated from the ultramafic lavas - komatiites, common in the Archaean - led to the assumption that the Archaean mantle was substantially hotter than the modern mantle. However, the recent proposal that komatiites are the product of cooler, wet mantle melting weakens the argument for a very hot Archaean mantle, and there are now good grounds for arguing that the temperature of the Earth s mantle has declined by only 100-200°C since the mid-late Archaean. 

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. 

A knowledge of how the Archaean mantle system works will become foundational in subsequent chapters of this book which deal with the origin of the continents, oceans, and atmosphere, for each of these parts of the Earth system was ultimately derived from the Earth s earliest mantle. 

Unlike the modern mantle, direct samples of the Archaean mantle are rare and geophysical measurements impossible. This means that our prime evidence comes from the study of Archaean mafic and ultramafic magmas. 

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. 

Fragments of the Archaean mantle -xenoliths from the subcontinental lithosphere As geochronological determinations on xenoliths from the subcontinental lithosphere become more robust, a consistent observation is emerging - that the subcontinental lithosphere beneath Archaean continental crust is very ancient (see Section 3.1.3.2). Recent studies... 

One particular class of Archaean mantle xenoliths has received special attention. These are eclogites recovered from the subcontinental lithosphere. Richardson et al. (2001) showed that some Archaean eclogite xenoliths have the trace element and Os-isotopic characteristics of a basaltic protolith and an isotopic history which shows a significant time gap between basalt generation and eclogite crystallization. These properties are typical of subducted ocean floor and indicate that the subcontinental lithosphere beneath the Kaapvaal Craton contains fragments of 2.9 Ga subducted ocean floor. 

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). 

Archaean mantle drawn from a knowledge of the Earth s radioactive heat content and mantle potential temperatures calculated for komatiites and basalts. 

If, as many suppose, the Archaean mantle had a higher potential temperature than the modern mantle, it is important to examine the implications of this for melt production during the early history of the Earth. The relationship between mantle potential temperature and melt thickness during adiabatic melting was outlined in Section 3.1.4.3 and may be briefly summarized by stating that as mantle potential temperature increases so will the melt production, as expressed in the depth of the melt column and the melt thickness. This is illustrated in Fig. 3.26, which shows how deeper, higher-temperature melting should lead to the formation of a thicker oceanic crust. 

The coohng history of the mantle Figure 3.25 shows the secular cooling curve for the mantle from Richter (1988), the calculated potential temperatures for Archaean komatiites and basalts, and an estimated temperature for the Archaean subcontinental mantle. Two important conclusions follow. First, it is clear that the Archaean mantle had both hot and cool regions. Potential temperatures calculated from dry komatiite melting temperatures imply an anomalously hot mantle source,... 

Second, whilst there is no agreement on the detail, it is evident that much of the Archaean mantle was hotter than the modern mantle. This has implications for mantle viscosity. The viscosity of the Earth s mantle shows a strong dependence on temperature to the extent that an upper mantle 250°C hotter than the present day mantle would be 60 times less viscous (Davies, 2006). A lower viscosity will influence patterns of mantle convection and the nature of Archaean tectonics, and would have influenced the size plates in the precursor to modern plate tectonics. 

Alternative views of early Archaean mantle evolution require that mantle depletion started as early as ca. 4.5 Ga (see compilation in Rollinson, 1993). These models imply significant mantle Sm-Nd fractionation in the very early Archaean and have major implications for the differentiation of the early Earth. One such study is that of Bennett et al. (1993) who measured very high eNd values (+3.5 to +4.5) in 3.81 Ga Amitsoq gneiss samples. Collerson et al. (1991) also calculated an isochron eNd value of +3.0 for 3.8 Ga-old peridotites from northern Labrador. The extreme deviation from CHUR early in Earth history (Fig. 3.27) was interpreted by Bennett et al. (1993) as evidence for an extreme and very early fractionation of the Earth s mantle relative to CHUR. Such an event implies the formation of extensive continental crust prior to 3.8 Ga, for which there is no independent geological evidence. This apparent paradox and the claim for very early extensive mantle differentiation led to a detailed reexamination of the Bennett... 

II- Early Archaean mantle differentiation related to the extraction of basaltic crust... 

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