Primordial mantle

REE patterns show variable fractionation, with small negative Eu anomalies, which increase in trachytes and phonolites (Fig. 6.7a). HREE depletion is observed is some phonolites. Patterns of incompatible elements normalised to primordial mantle compositions for mafic rocks (Fig. 6.7b) are fractionated and contain positive spike of Pb and negative anomalies of HFSE. However, these are less sharp than in other Italian ul-trapotassic rocks, such as those of the Roman Province, and HFSE abundances are much higher than observed in MORBs (Sun and McDounough 1989). 

REE patterns are fractionated, with evolved rocks showing negative Eu anomalies (Fig. 6.12a). Incompatible element patterns of mafic rocks normalised to primordial mantle composition (Fig. 6.12b) show Pb spikes and relatively high concentration and moderate negative anomalies of HFSE. 

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). 

Figure 4.23 Trace element concentrations normalized to the composition of the primordial mantle and plotted from left to right in order of increasing compatibility in a small fraction melt of the mantle. The normalizing values are those of McDonough et aL (1992) — Table 4.7, column 4. (a) Upper and lower continental crust from Weaver and Tamey (1984) — data in Table 4.8 (b) Average N-type MORE from Saunders and Tamey (1984) and OIB from Sun (1980) -— data in Table 4.8.

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... 

We speculate from the above argument that primordial sulfur degasses from midoceanic ridges even at present time as well as He, because subduction flux to mantle seems to be small. However, we need more detailed study on long-term S cycle including hydrothermal S flux to evaluate this speculation. 

Figure 11.23 shows the isochron obtained by Marshall and De Paolo (1982) for the granite batholith of Pikes Peak (Colorado). The effectiveness of the double-spike technique is evident, especially when we see that aliquot-spiked samples do not fall on the best-fit interpolant (York s algorithm York, 1969). The obtained age (1041 32 Ma) is consistent with that previously obtained with Rb-Sr whole rock analyses (1008 13 Ma see Marshall and De Paolo, 1982, for references). The initial ratio ( Ca/ Ca)o of 151.0 is identical, within the range of uncertainty, to upper mantle values, indicating negligible contamination by old crust components the relative K/Ca abundance in the earth s mantle is about 0.01, a value too low to alter the primordial (" Ca/" Ca)o composition. 

Because the present-day composition of the earth s mantle falls on the same isochron, the earth and the parent bodies of meteorites must have formed at about the same time from the same primordial source. 

In a more recent, alternate theory, Thomas Gold (Cornell University) suggests tii.il. in contrast with the organic sediments theory, tire prime source of natural gas is primordial, abiotic methane rising from deep within the earth s mantle. This is discussed in further detail in tile article on Natural Gas. 

Mantle He is also sometimes termed primordial, but note that even in mantle He the 3He/4He ratio is in most samples significantly lower than the primordial compo-... 

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