The deep water cycle and flood basalt volcanism

摘要

Experimental data combined with numerical simulations suggest that fast-subducting oceanic slabs are cold enough to carry a significant amount of H_2O into the deep mantle in antigorite;;with increasing depth, this mineral undergoes solid-solid transitions to phase A and then to phase E and/or wadsleyite. Ice VII and clathrate hydrates can also be stable under pressure and temperature (PT) conditions of cold slabs and represent other potential phases for water transport into the deep mantle. Cold slabs are expected to stagnate in the mantle transition zone. With time, they are heated to the temperature of the ambient transition zone and release excess water as a H_2O-bearing fluid. This may cause voluminous melting of overlying upper mantle rocks. If such a process operates in nature, magmas geochemically similar to island-arc basalts are expected to appear in places relatively remote from active arcs at the time of their emplacement. Dolerites of the southeastern margin of the Siberian flood basalt province, located about 700 km from suggested trench, were probably associated with the fast (cold) subduction of the Mongolia-Okhotsk oceanic slab, and originated by the dehydration of the stagnant slab in the transition zone. We show that the influence of the subduction-related deep water cycle on Siberian flood basalt magmatism gradually diminished with increasing distance from the subduction zone. Thus, the unique size and volume of the Siberian flood basalt province could have originated due to long-term underflow beneath Siberia with or without the existence of a lower mantle plume.