Plume-related Hypotheses

Locardi and Nicolich (1988) suggested an active mechanism of formation of the Tyrrhenian basin and associated magmatism by upwelling of soft, low-density mantle beneath the Tyrrhenian Sea. However, the plumelike body in this model does not come from the deep mantle but rather from the asthenosphere the decrease in density necessary to generate man- 

In summary, the following lines of evidence are generally invoked in favour of the plume hypothesis  

Smooth trends of Sr-Nd-Pb-Hf-He isotopes connecting different mantle compositions (Fig. 10.4). These are suggested to derive from mixing among different mantle end-members (HIMU or FOZO, EMI and EM2), which occur worldwide and are believed to represent deep mantle plumes by several authors (e.g. Hofmann 1997)  

The positive trend of 87Sr/86Sr vs. Rb/Sr displayed by Italian volcanic rocks. If interpreted as an isochron, this trend yields an age of l-2 Ga, supporting long term residence of high Rb/Sr material in the mantle and successive emplacement at shallow depth as a plume (Cas-torina et al. 2000)  

The isotopic similarity of some Italian volcanoes, such as Mt. Etna and Iblei, with Na-alkaline and tholeiitic rocks from central and west- 

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Finally, the Tyrrhenian Sea (i.e. the focus of the mantle plume ascent) has a considerable depth (3700 m) and magmatism is not particularly voluminous. These characteristics do not fit a plume hypothesis. The suggestion that lack of magmatism is related to the great depth of the plume head is reasonable. However, the idea that such a deep body would be able to induce basin opening of the order of several hundred km, and to generate a collisional chain, is difficult to accept and needs much further refinement to avoid scepticism. 

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