Volatile components (H2O, CO2, Cl, F, and S) in magmas of intermediate and acid compositions from distinct geodynamic settings:: Evidence from melt inclusions and chill glasses

Generalized data on the chemistry of melt inclusions and chill glasses were used to evaluate the average concentrations of H2O, CO2, Cl, F, and S and the variations in these concentrations in natural magmas of intermediate and acid compositions from oceanic islands, island arcs, active continental margins, continental rifts, hotspots (continental areas), and continental collision zones. The highest average concentrations of H2O in magmas of intermediate and acid compositions were detected in island arcs (2.2 and 4.4 wt %, respectively) and active continental margins (2.4 and 2.8 wt %, respectively). These values for other volatiles are as follows: Cl-0.19 and 0.39% (oceanic islands) and 0.37 and 0.23 wt % (continental areas); F-2.80 and 2.06 wt % (continental collision zones), 1.58 and 0.73 wt % (continental areas), and 0.45 and 0.70 wt % (active continental margins); S-0.09 wt % (continental areas); and CO2-up to 1.42 wt % (active continental margins). Variations in the concentrations of volatile components in magmas are controlled by the compositions of the magma sources and their differentiation and degassing degrees. The contents of volatiles in intermediate and some acid magmas of oceanic islands, continental areas, and island arcs depend on the differentiation of mafic magmas, and these values for acid magmas of active continental margins and continental areas are related to the processes of migmatization and anatexis in the crust, i.e., processes in which micas are actively involved. The highest contents of H2O, Cl, and F were encountered in strongly differentiated intermediate and acid magmas, which are characterized by the lowest concentrations of S. The outgassing of magmas can be detected by comparing the average concentrations of H2O, Cl, S, and CO2 in melt inclusions and chill glasses. The volatiles of spreading-zone magmas from oceanic ridges and backarc basins, oceanic islands, and continental areas were provided by either mantle amphibole and micas or mantle fluids and magmas, which inherited the proportions of volatiles and the ratios of volatiles to K from the amphiboles and micas. The sources of water in magmas of island arcs and active continental margins were fluids of the hydrated mantle.