Subduction zones are the fiery boundaries where the earth's vast crustal plates converge one against another. In the immense furnace of a subduction zone, the denser plate plunges beneath its neighbor, and portions of it may be melted by the intense heat generated within the mantle, turning the solid rock of the crust into molten magma. How to determine just what has gone into the magma mix and ultimately where it came from is a compelling challenge to geochemists.