Petrology of Subducted Slabs

The subducted lithosphere is composed of a complex pattern of chemical systems that undergo continuous and discontinuous phase transformation, through pressure and temperature variations. Volatile recycling plays a major geodynamic role in triggering mass transfer, melting, and volcanism. Although buoyancy forces are controlled by modal amounts of the most abundant phases, usually volatile-free, petrogenesis and chemical differentiation are controlled by the occurrence of minor phases, most of them volatile-bearing. Devolatilization of the subducted lithosphere is a continuous process distributed over more than 300 km of the slab-mantle interface. Melting of the subducted crust, if any, along sufficiently hot P-T paths, is governed by fluid-absent reactions, even though the difference between fluid and melt vanishes at pressures above the second critical end point. The density distribution at a depth of 660 km suggests episodic penetration in space and time of subducted slabs into the lower mantle and sinking down to the D″ region at the core-mantle boundary.