PARTIAL MELTING OF SUBDUCTING OCEANIC-CRUST

The conditions under which partial melting of subducting oceanic crust occurs can be determined by combining a partial melting model for basaltic compositions with two-dimensional thermal models of subduction zones. For porosities of similar to 1% containing H2O the amount of partial melt generated at the wet basaltic solidus is limited to < 5 vol%. At higher temperatures (similar to 1000 degrees C at 1.5 GPa) large amounts of partial melt, up to 50 vol%, form by the breakdown of amphibole and the release of structurally bound H2O. In most subduction zones, substantial partial melting of subducting oceanic crust will only occur if high shear stresses (> similar to 100 MPa) can be maintained by rocks close to, or above, their melting temperatures. In the absence of high shear stresses, substantial melting of the oceanic crust will only occur during subduction of very young (< 5 Ma) oceanic lithosphere. Partial melting of hydrated basalt (amphibolites) derived from the mid-ocean ridge has been proposed [e.g., 1-3] as being responsible for the generation of certain recent high-Al andesitic to dacitic volcanic rocks (adakites). Three of these volcanic suites (Mount St. Helens, southern Chile, and Panama) occur in volcanic arcs where oceanic crust < 25 Ma is being subducted at rates of 1-3 cm/yr and the calculated thermal regime is several hundreds of degrees hotter than more typical subduction zone environments. However, oceanic lithosphere is not currently being subducted beneath Baja and New Guinea, where recent adakites are also present, suggesting that some adakite magmas may form by water-undersaturated partial melting of underplated mafic lower crust or previously subducted oceanic crust. Further experimental work on compositions representative of oceanic crust is required to define the depth of possible adakite source regions more accurately.