Subcontinental lithosphere composition

In spite of their variable provenance (subcontinental lithosphere, supra-subduction mantle wedge, or oceanic mantle), most of the tectonically emplaced and abyssal peridotites show coherent covariation trends for major elements (Eigure 5). These variations reflect their variable modal compositions between a fertile end-member— comparable to proposed estimates for pristine... 

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... 

Normal mantle melting does not produce a residue with the mineralogical composition of old subcontinental lithosphere... 

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). 

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). 

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... 

The subcontinental lithosphere is made up of Mg-rich peridotite, depleted in the elements Fe, Ca, and Al, which has equilibrated at relatively low temperatures (850-1,100°C) and forms a thick mantle "keel" several hundred kilometers thick beneath the continental crust. The presence of this keel is thought to account for the long-term preservation of the continental crust. In detail, the mass of the subcontinental lithosphere is ill-constrained and is at most about 2% of the mass of the mantle (McDonough, 1991). It is composition-ally heterogeneous and typically has a multistage history. For further details the reader is referred to Chapter 3, Section 3.1.3.2. 

Subcontinental lithospheric mantle (SCLM) 2% Not known - but not compositionally complementary Grown progressively over time... 

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 presence of apatite in subcontinental mantle samples raises the question of the thermal stability of apatite, and whether it would be stable in mantle other than cool lithospheric roots. A limited amount of experimental work has been done that addresses this question Vukadinovic and Edgar (1993) determined the solidus for phlogopite-apatite mixtures at 2 GPa in the KaO-CaO-MgO-AljOa-SiOa-PaOs-HjO-F system. They looked at two bulk compositions, one with the hydroxy end-members and the other where F/OH = 1. The sohdus was 1,225 °C for the fluorine-free system, and 1,260 °C for the fluorinebearing system. It seems likely that adding iron to the system will decrease solidus temperatures. Given that the average current mantle adiabat is... 

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