Enriched mantle incompatible element ratios

FIGURE 4.15 (a) Primitive mantle-normalized compositions for the present day upper continental crust and average Archaean upper crust, from McLennan et al. (2005). The comparison show the relative enrichment in incompatible elements in present-day upper continental crust (see Text Box 2.2). (b) The secular change in Th/La ratio in the upper continental crust is from the compilation by Plank (2005). The grey bands are the Archaean and post-Archaean averages from Taylor and McLennan (1985). 

The mantle up to several hundred kilometers beneath the continents, particularly under the older cratons, forms deep keels characterized by fast seismic velocities, is Mg-rich, and depleted in Fe. It is less dense than the surrounding mantle, a factor that imparts stability to the keels, which thus float under the continents. Although the low Fe/Mg ratio and low Ca and A1 contents are generally attributed to the extraction of a partial melt, this subcontinental lithospheric mantle (SCLM) is apparently enriched in incompatible elements such as Ba, Th, U, Ta, Nb, La, Ce, and Nd and depleted in HREE, Ti, Sc, V, Al, and Ca relative to average abundances in the mantle. This element pattern indicative of both enrichment and depletion indicates that multistage processes must have occurred, with an initial extraction of a partial melt, followed by at least one stage of a secondary enrichment, often referred to a metasomatic event. Curiously, this event has apparently not affected the Fe/Mg ratios. 

REE patterns are fractionated for all the rocks, but tholeiites show lower La/Yb ratios than alkaline products (Fig. 8.5a). Incompatible element patterns normalised to primordial mantle compositions for mafic rocks are very different from the Aeolian arc and central-southern Italian peninsula. Both tholeiitic and alkaline basalts show a marked upward convexity, with negative spikes of K (Fig. 8.5b). Note, however, that there are also negative anomalies for Hf and Ti, which are uncommon in most Na-alkaline basalts from intraplate settings (e.g. Wilson 1989). Overall, the Etna magmas have been found to be more enriched in volatile components than common intraplate magmas, and water contents up to 3-4 wt % have been found by melt inclusion studies (Corsaro and Pompilio 2004 Pompilio, personal communication). 

Source heterogeneity will affect both K iriid Ta equally oh ibis diagram as they are both incompatible elements, and so man tie compositions will move to higher or lower K20/Yb and Ta/Yb ratios relative to primordial mantle along a slope of unity (Figure 5.13). Fluid enrichment, on the other hand, will enrich K but not Ta and will show as a trend parallel to the K20/Yb axis (Pearce, 1982). This diagram... 

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