A quantitative description of the geochemical cycle in subduction zone settings depends both on a careful characterization of trench inputs (subducted sediments and altered crust + associated fluids) and on an accurate cataloguing of chemical outputs (fluid releases in the forearc, volcanism at arcs, subduction inputs to back-arc spreading centers). While clear connections have been established between the compositions of subducted sediments and crust and the chemical signatures of arc lavas, moving from these observations to a rigorous subduction zone mass-balance, even for well-characterized tracers , requires additional data.

The patterns of elemental and isotopic variations evident from the study of cross-arc and forearc/arc (/backarc) transects provide key constraints on subduction zone processes and material fluxes:

1. The magnitudes of slab fluid inputs and inferred extents of melting for arc lavas correlate strongly with slab depth, indicating absolute declines in slab fluid releases with progressive subduction.
2. The chemical signature of the "slab component" changes with depth from H2O dominated, low-temperature fluids in the forearc, to fluxes dominated by SiO2-rich (sediment and/or basaltic?) melt components in the arc and back arc.
3. For a limited menu of elements (B, Cs, As, Sb, Pb? Rb?, N?) transect data suggests slab fluid releases account for ~100% of the budget input at trenches.