F, Cl and S input via serpentinite in subduction zones: implications for the nature of the fluid released at depth

The abundances of F, Cl and S in arc magmas are systematically higher than in other mantle‐derived magmas, suggesting that these elements are added from the slab along with H 2 O. We present ion probe microanalyses of F, Cl and S in serpentine minerals that represent the P–T evolution of the oceanic lithosphere, from its serpentinization at the ridge, to its dehydration at around 100 km depth during subduction. F, Cl and S are incorporated early into serpentine during its formation at mid‐ocean ridges, and serpentinized lithosphere then carries these elements to subduction zones. More than 50% of the F, Cl and S are removed from serpentine during the prograde metamorphic lizardite/antigorite transition. Due to the low solubility of F in water, and to the low amount of water released during this phase transition, the fluids mobilizing these elements must be dominated by SO X rather than H 2 O.