Petro-Pot Ken Koga

Fractionation of fluorine and chlorine during subduction zone metamorphism and arc magma composition

Abstract: Fluorine and chlorine are found in magmatic gas/fluid phases in trace quantity along with H2O, CO2, and SO2. Because of halogens’ sluggish volatility, compared to H2O and CO2, fluorine and chlorine are ideal geochemical tracers of the magmatic volatile component for the subduction zone magmas. Using these element, it is possible to investigate the role of volatile component in arc magma genesis, from subduction of the slab, metamorphism-metasomatism, to partial melting. One of key unknown of the fluorine-chlorine geochemical cycle in the subduction zone is the subduction input; the quantity of these elements added to the arc mantle through the subduction. In Western Alps, pieces of Mesozoic Ligurian-Piemonte oceanic lithosphere are found today. These rock bodies have recorded range of maximum pressure conditions from the essentially surface to eclogite facies. Conveniently, it is possible to sample the same oceanic slab that is exposed to the different stages of the subduction depth. This presentation reports the results of the studies of these ophiolites, identifying the geochemical evolution of descending slab, and subsequent introduction of fluorine and chlorine to the arc mantle.