Melting of a subducting oceanic crust from U-Th disequilibria in austral Andean lavas

Understanding crustal genesis at convergent plate boundaries is important for determining mass transfer between different geochemical reservoirs in the Earth's mantle, and for deciphering the long-term growth of the continental crust. Most arc magmas are thought to be generated from fluid-induced melting of the mantle wedge above slabs of subducting oceanic crust. Such magmas frequently display ²³⁸U enrichments or radioactive equilibrium between ²³⁸U and its radiogenic product ²³⁰Th. But where a young and hot oceanic crust is being subducted it may itself partially melt and produce calc-alkaline andesites and dacites, termed adakites. Here we report a uniform excess of ²³⁰Th over ²³⁸U, but variable Th isotope ratios, in young adakites from the Andean austral volcanic zone south of the triple junction where the Chile ridge subducts beneath South America. We show that these results are compatible with the adakites having been formed by approximately 20% equilibrium melting due to amphibole decomposition in a heterogeneous oceanic crust. Moreover, both the degree of melting of the oceanic crust and its thermal structure appear to be uniform undermost of the Andean austral volcanic zone. Such partial melting of subducted oceanic slabs may have occurred throughout the Earth's history where young oceanic plates were subducted.