Volatile (F and Cl) concentrations in Iwate olivine-hosted melt inclusions indicating low-temperature subduction

Investigation of olivine-hosted melt inclusions provides information about the abundance of volatile elements that are often lost during subaerial eruptions of lavas. We have measured the abundances of H2O, CO2, F, Cl, and S as well as Pb isotopes in 29 melt inclusions in the scoria of the 1686 eruption of the Iwate volcano, a frontal-arc volcano in the northeast Japan arc. Pb Isotope compositions identify that Iwate magma is derived from a mixture of depleted mantle, subducted basalt, and sediment. Systematics of F in comparison to MORB and other arc magma indicates that (1) the slab surface temperature must be among the lowest on Earth and (2) hydrous minerals, such as amphibole, humites, and/or mica, must be present as residual phases during the dehydration of the slab.