Subcontinental lithosphere Archaean

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... 

Over the past two decades this work has been significantly extended so that now it is widely accepted that Archaean subcontinental lithosphere is thicker, older, colder, more chemically depleted, less dense, and has higher seismic P- and S-wave velocities than its Phanerozoic counterpart. More recently it has been argued that Proterozoic lithosphere is intermediate in composition between Archaean and Phanerozoic (see Table 3.3 for a summary). 

More generally it has become clear that the subcontinental lithosphere is a domain within the mantle that is isolated from the rest of the mantle and so has resisted homogenization. In the case of Archaean subcontinental lithosphere, these regions preserve mantle domains which have been isolated since the early part of Earth history and provide an important "memory" of early Earth processes. 

TABLE 3.3 Principal differences between Archaean, Proterozoic, and Phanerozoic subcontinental lithospheric mantle (after Griffin et ah, 2003). ... 

The major element composition of the subcontinental lithosphere The principal chemical feature of the subcontinental lithosphere is one of strong chemical depletion. This is evident when xenolith compositions are plotted relative to both the Bulk Silicate Earth and the oceanic lithosphere (Fig. 3.12). There are also compositional differences between Phanerozoic and Archaean subcontinental lithosphere as is illustrated by the... 

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. 

FIGURE 3.12 The composition of the subcontinental lithosphere, (a) Differences in composition between Archaean, Proterozoic, and Phanerozoic subcontinental lithosphere (after Griffin et al., 2003), compared to estimates of the BSE (from Table 3.1). The diamonds represent the average compositions given in Table 3.4. (b) Differences in composition between oceanic and continental lithosphere, as expressed in % modal olivine and olivine mg oceanic peridotites define the trend shown as the dashed line whereas garnet peri-dotites from the Kaapvaal Craton, South Africa - illustrative of Archaean subcontinental lithosphere - plot in the shaded field. The diamonds are the averages of the two fields (after Boyd, 1989). Also shown is the field for Proterozoic subcontinental lithosphere (ellipse) after Griffin et al. (2003). 

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. 

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). 

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... 

One of the important discoveries about the subcontinental lithosphere to come from xeno-lith and diamond inclusion studies is that beneath many Archaean cratons the subcontinental mantle is extremely ancient. The results of thermobarometry on these mantle xenoliths have been used to construct paleo-geotherms for the subcontinental mantle (Fig. 3.4) which show that Archaean subcontinental lithosphere is significantly cooler than more recent lithospheric mantle. The early results of Richardson et al. (1984) on diamond... 

A potentially significant reservoir, and one which a number of authors have suggested is important in the context of continent formation, is the subcontinental lithospheric mantle (SCLM). Kramers (1987, 1988), suggested that the TTG magmas of the Archaean crust formed in an open-system magma layer in the early Earth, the cumulates from which are now preserved as the SCLM. More recently Abbott et al. (2000) proposed a model of continental growth founded upon the premise that the continental crust was extracted from the SCLM. 

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). 

A number of studies have used xenoliths from Archaean subcontinental lithosphere to make inferences about the oxidation state of the early mantle. For example Woodland and Koch (2003) showed that the subcontinental lithosphere beneath the Kaapvaal Craton displays a systematic decrease in oxygen fugacity with depth (Fig. 5.11). However, it should be remembered that the highly depleted nature of the Archaean subcontinental lithosphere means that it is atypical of the mantle as a whole and may not therefore be useful as an indicator of mantle redox conditions (Chapter 3, Section 3.1.3.2). 

Archaean subcontinental lithosphere is normally underlain by enriched lithosphere (EMI of Zindler and Hart (1986) — low Rb/Sr, low Sm/Nd), but depleted examples are also known. The lithosphere beneath Proterozoic mobile belts, however, more closely resembles the depleted mantle found beneath older ocean basins (Menzies, 1989). Froterozic to Phanerozoic subcondnental lithosphere is characterized by enrichment in Rb and the Ught REE resulting in radiogenic Sr and non-radiogenic Nd isotopes. This is similar to enriched mantle EMII described above and in Table 6.5. 

The growing consensus that ancient crust is always underlain by ancient lithospheric mantle has been challenged recently by Wu et al. (2003). These authors report an unusual absence of xenoliths with Archaean ages beneath the Archaean North China Craton. They propose that in some cases, therefore, ancient subcontinental mantle can be removed from beneath ancient continental crust by delamination - a process which has previously been postulated but never demonstrated. The subject of mantle delamination is discussed more fully in Chapter 5, Section 5.5.2. 

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