The cold, dense oceanic lithosphere that descends subduction zones is also rich in water. These features result from the circulation of seawater through young basaltic crust, the exothermic hydration of originally anhydrous minerals in basalt and efficient convective cooling through hydrothermal processes. Because of this, it might seem as though subduction is a means of re-introducing water into the mantle, thereby enhancing the ability of rising mantle plumes to melt. The critical process that destines subducted lithosphere to sink inexorably is the conversion of oceanic crust to eclogite by high-pressure, low-temperature metamorphism in the subduction zone. Eclogite consists mainly of garnet and the pyroxene omphacite, which confer its higher density than mantle peridotite, and the reactions which form them involve dehydration. Rise of hydrous fluids from the descending slab is implicated in partial melting of the over-riding wedge of mantle to form the volatile-rich magmas that build volcanic arcs. The higher gas content of arc magmas, compared with those at constructive margins and above mantle plumes, makes them explosive and able to build volcanoes high above sea level. Most eclogites found at the Earth’s surface are accompanied by still hydrous metamorphic rocks of basaltic composition – blueschists – and others that clearly formed from the sedimentary veneer of the oceanic crust. So, it might seem that blueschists and metasediments could carry a substantial amount of water into the mantle. Eventually, its recycling through the mantle could influence later magmatic processes.
Testing this seemingly reasonable extension of the hydrological cycle depends on assessing the water content of newly erupted magmas. This is virtually impossible for eruptions at the Earth’s surface, because low pressure results in water escape within the higher parts of the volcanic plumbing system, before lavas can be sampled. However, eruptions onto the ocean floor deeper than a kilometre experience pressures high enough to keep gases in solution, which is why pillow lavas of true oceanic crust contain no signs of gas bubbles. Crystallised oceanic basalts soon react with percolating water, and their volatile contents are meaningless. Only the rapidly chilled margins are likely to retain their original composition, locked into quenched basaltic glass. Even then, a direct measurement of water content can be misleading. A cunning approach is to consider H2O as if it behaved like a single element, based on its bulk distribution coefficient between melt and residual solid mantle. That is close to the values for light rare-earth elements, such as cerium. So a check for either degassing or contamination of basaltic glass with seawater is the glass’s H2O/Ce ratio (decreased by the first and increased by the second process). Jacqueline Dixon of the University of Miami, and co-workers from Harvard and the University of Rhode Island have used this method to assess the probable water content of the mantle source for mid-Atlantic Ridge basalts, whose lead and strontium isotopes suggest that their source was contaminated by older, recycled crust (Dixon, J.E. et al. 2002. Recycled dehydrated lithosphere observed in plume-influence mid-ocean-ridge basalt. Nature, v. 420, p. 385-389). The surprising conclusion of their work is that oceanic basalts formed from mantle with a recycled component have considerably less water in them than those formed by melting of pristine mantle. This suggests that subduction processes are extremely efficient (>92%) at removing volatiles from the subducted slab; lithosphere descending to depth is almost anhydrous.
Incidentally, the paper begins with an excellent explanation of the somewhat arcane distinctions between different mantle sources affected by lithosphere recycling and mixing.
See also: White, W.M. 2002. Through the wringer. Nature, v. 420, p. 366-367; and Tectonics section below