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MOST volcanic rocks in modern island and continental arcs are probably derived from melting of the mantle wedge, induced by hydrous fluids released during dehydration reactions in the subducted lithosphere1. Arc tholeiitic and calc-alkaline basaltic magmas are produced by partial melting of the mantle, and then evolve by crystal fractionation (with or without assimilation and magma mixing) to more silicic magmas2-basalt, andesite, dacite and rhyolite suites. Although most arc magmas are generated by these petrogenetic processes, rocks with the geochemical characteristics of melts derived directly from the subducted lithosphere are present in some modern arcs where relatively young and hot lithosphere is being subducted. These andesites, dacites and sodic rhyolites (dacites seem to be the most common products) or their intrusive equivalents (tonalites and trondhjemites) are usually not associated with parental basaltic magmas3. Here we show that the trace-element geochemistry of these magmas (termed 'adakites') is consistent with a derivation by partial melting of the subducted slab, and in particular that subducting lithosphere younger than 25 Myr seems to be required for slab melting to occur.
Gill1 and Drummond and Defant3 have discussed the expected geochemical characteristics of rocks derived from the melting of subducted basalt. Experimental work indicates (see ref. 3 for a review) that the partial melting of metamorphosed basalt (amphibolite and eclogite) will produce relatively high-Al, mostly corundum-normative silicic melts (A1203> 15% at 70% Si02). The existence of garnet or amphibole as a residual will lead to melts with low concentrations of yttdrium and heavy rate-earth elements (HREEs).
The age of the lithosphere4,5 subducted along trenches associated with 12 island arcs is plotted against the concentrations of Y and normalized Yb in volcanics sampled from these arcs in Fig. 1. In all but nine samples (>400 samples plotted), the Y and normalized Yb values of the island-arc andesites, dacites and sodic rhyolites (ADRs) are >20. Island arcs near continental margins, where subducted crustal sediments have been documented (such as Lesser Antilles arc6, Luzon arc, Philippines7, and Aleutian arc8) have been omitted because of the potential of lower Y and Yb values by source contamination. In these cases, however, only a few samples have Y and normalized Yb values <20.
Thirteen Cenozoic arc ADRs associated with young subducted crust (the crust subducted...