FIGURE 2.15 Schematic diagram illustrating plate recycling.

instigation of volcanism landward of subduction zones. Arc volcanic rocks display a chemical signature best explained by the contributions of subducted oceanic sediments and altered oceanic crust to their source regions in the mantle. The recent discovery of an isotope of beryllium in arc volcanoes, but in no other volcanic system on Earth, provides clear evidence that even the upper few meters of sediment on the ocean floor are transported to a depth of at least 150 km in subduction zones, the depths at which the processes of dehydration and perhaps partial melting initiate the generation of arc magmas. There is a direct link with cycles in the atmosphere and hydrosphere. The radioactive isotope, 10Be, is transferred from the upper atmosphere by rain and becomes concentrated in ocean sediments. The fact that it decays relatively rapidly places constraints on the time interval between raining from the atmosphere, subduction in sediments, and emergence at the surface in volcanic lavas.

A distinctive feature of arc lavas is their relatively high water content, which is largely derived from the rocks of the subducted oceanic plate. The combination of experimental-phase equilibrium studies on source rocks and lavas with geophysical modeling of temperatures at depth confirms that most of the subducted water is removed by dehydration or melting by a depth of 150 km or so, whence it returns to the surface for involvement in the shallow cycles. However, under some conditions there is opportunity for small amounts of water to escape the magmatic processes and to be transported to greater depths for long-term storage within the mantle. Recent theoretical and experimental studies at very high pressures have led to proposals that the lower mantle, deeper than 700 km, may contain water in significant quantities: at least 0.3 percent by

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