We discuss geochemical tracers in serpentinite for assessing the fluid-rock interactions and the element exchange processes experienced by these rocks during subduction. This enables to define the timing of serpentinite accretion to plate interface domains and the evolution of such tectonically active settings. Uptake of As, Sb, Be and reset of B, Sr and Pb isotopes are due to interaction with sediment-derived fluids during prograde subduction metamorphism. This implies the early slicing and accretion of serpentinite (of slab and/or mantle wedge origin) to plate-interface domains, where they uptake crust- and sediment-derived fluids. Serpentinite metasomatism by sediment-derived fluids enable transfer of these elements to subarc depths and their release to de-serpentinization fluids affecting the mantle sources of arc magmas.