MELTING OF A SUBDUCTING OCEANIC-CRUST FROM U-TH DISEQUILIBRIA IN AUSTRAL ANDEAN LAVAS

Understanding crustal genesis at convergent plate boundaries is import ant for determining mass transfer between different geochemical reserv oirs in the Earth's mantle, and for deciphering the long-term growth o f the continental crust. Most are magmas are thought to be generated f rom fluid-induced melting of the mantle wedge above slabs of subductin g oceanic crust(1). Such magmas frequently display U-238 enrichments o r radioactive equilibrium(2,3) between U-238 and its radiogenic produc t Th-230. But where a young and hot oceanic crust is being subducted i t may itself partially melt and produce calc-alkaline andesites and da cites, termed adakites(4). Here we report a uniform excess of Th-230 o ver U-238, but variable Th isotope ratios, in young adakites from the Andean austral volcanic zone south of the triple junction where the Ch ile ridge subducts beneath South America. We show that these results a re compatible with the adakites having been formed by approximately 20 % equilibrium melting due to amphibole decomposition in a heterogeneou s(5) oceanic crust. Moreover, both the degree of melting of the oceani c crust and its thermal structure appear to be uniform under most of t he Andean austral volcanic zone. Such partial melting of subducted oce anic slabs may have occurred throughout the Earth's history where youn g oceanic plates were subducted.