Oceanic lithosphere, subducted slab

Using a subduction rate of about 3 km2/a and a slab thickness of 100 km Davies (1998) estimated that the mass flow into the mantle through subduction was about 1015 kg/a. Hence the mass flux into the mantle is about 1015 kg/a. The mismatch between this value and the mass flux out of the mantle arises because we are equating two different fluxes. One is the flux of ocean crust (flux out) the other is the flux of ocean crust plus oceanic lithosphere (flux in). 

Continental lithosphere is less dense than upper mantle and does not subduct resulting in the absence of deep earthquakes in continental colhsion zones. However, intermediate-deep earthquakes in the Hindu Kush region and the Vranceazone of Romania (such as the destructive 1977 Bucharest earthquake) as weU as infrequent deep earthquakes beneath southern Spain (such as the 2010 Mw = 6.4 earthquake) occur within the Alpine-Himalayan belt. These earthquakes are the result of past subduction of oceanic lithosphere, and their mechanisms are as expected in subduction zones (i.e., P- or T-axis subparallel to presumed slab-dip direction where known). 

Whereas a number of reactions at intermediate to elevated temperatures are important for hydration of peridotite in the overlying mantle wedge directly adjacent to the top of the oceanic cmst (Figures 6 and 7), almost any subduction P-T-path will keep the slab in the serpentine stability field to at least >2 GPa (Ulmer and Trommsdorff, 1995). As a consequence, hydrated peridotite in the downgoing lithosphere will remain as serpentine and chlorite (4-olivine - -clinopyroxene) while a multitude of reactions is... 

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