Subduction zones elemental abundances

Perhaps the most important element to enter a subduction zone in abundance is hydrogen (as H2O), but the fate of this element remains elusive in spite of concerted research efforts (Chapter 2.07). This is unfortunate, because... 

A return flux of atmospheric noble gases to the mantle may occur at subduction zones (see Noble Gas Fluxes at Subduction Zones). This would favor the heavy noble gases, which are more readily adsorbed and incorporated into altered or sedimentary materials, and have the net effect of changing the upper mantle relative elemental abundances. 

The mafic magmas from Emici and Roccamonfina volcanoes display variable abundances and ratios of incompatible elements as well as very distinct isotopic signatures. These were probably generated in a vertically zoned heterogeneous mantle source which was modified by two temporally and compositionally distinct metasomatic events. These episodes of mantle metasomatism may be related to subduction of the Adriatic plate beneath central Italy, followed by the arrival of new subduction-related material from the Ionian plate. 

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