DETERMINING THE FATE OF IMMOBILE ELEMENTS IN SUBDUCTION SYSTEMS BY HF ISOTOPES: SARDINIA AS A CASE STUDY

Processes that occur in subduction systems dominate element recycling within the Earth; yet, the mass balance of elements during subduction remains difficult to quantify. Because arc volcanics are characterized by relative depletions in high-field-strength elements (HFSE), such as Nb, Ta, Zr and Hf, it has been argued that they can provide a “baseline” from which to estimate fluxes of more mobile elements. Moreover, Hf isotopes may provide us with an isotopic tool to “see through” the subduction process into the nature of the sub-arc mantle wedge. While this may be true for many intra-oceanic subduction zones, recent studies have shown that it may not be the case for other tectonic settings, e.g. during subduction of young (hot) oceanic crust and/or when continent collision has slowed subduction rates so that slab temperatures increase to levels suitable for sediment melting. Here we examine the role of sediment melt versus aqueous fluid in the enrichment of the mantle wedge by assessing the behavior of Hf in Miocene subduction-related basalts and basaltic andesites from southern Sardinia. Previous work provided strong geochemical evidence that the rocks were derived from an enriched mantle source and that the effects of crustal contamination are virtually absent (e.g. limited increase in δ18O with increasing Sr-isotope values).

In Nd-Hf isotope space the samples plot within the Terrestrial Array, but at εHf for a given εNd ratio than other subduction-related rocks of central Italy; εHf values range from –7.4 to 8.4. The data form steep, near vertical arrays in Sm/Hf and Th/Hf vs εHf. High Th/Hf ratios (ca. 2), combined with low Hf-isotope ratios, indicate relatively high sediment melt addition. Relatively low Sm/Hf ratios (down to 1.2), suggest that the sediments were detrital sand-rich rather than pure pelagic clays.

The data are consistent with the geodynamic evolution of the western Mediterranean proposed by Beccaluva et al(2011) in which Miocene orogenic volcanism was accompanied by marked steepening of the subducted slab during the late stages of convergence. Steepening of the slab led to higher temperatures and sediment melting. Hence, in the case of Sardinia, Hf did not behave as a “conservative” element during subduction and was mobilized by sediment melting.

[1] Beccaluva et al(2011) Lithos, v123, p218-224.