Diffuse CO₂ efflux from Iwojima volcano, Izu-Ogasawara arc, Japan

Iwojima volcano, located on the southernmost part of the Izu-Ogasawara arc, is characterized by the extrusion of trachyte or trachy andesite lavas and pyroclastic rocks of Holocene and surface thermal manifestations. Small phreatic explosions have been recorded frequently during the last 100 years with the most recent in 1999 and 2001. In order to elucidate the behavior of volcanic volatiles and to assess the potential activity of this volcano, diffuse CO₂ efflux, CO₂ content and δ¹³C-CO₂ in soil gas, and soil temperature at 30 cm depth were measured at 272 sites in March 2000, 112 sites in December 2000 and 40 sites in December 2001. We found that high CO₂ efflux values, of more than 100 g m^-2 day^-1, occurred at several locations on Motoyama volcano corresponding with high soil temperatures (more than 60 °C at 30 cm depth) region and with areas where CO₂ with magmatic δ¹³¹³C determined for fumarolic CO₂ data ranged from -2‰ to +3‰, which is clearly higher than magmatic gas values (-8‰ to -2‰) typically found in island arc settings around the world. However, this can be explained in terms of carbon-isotope fractionation between calcite and CO₂ under subsurface temperature and pressure conditions at Iwojima. A total efflux of CO₂ for Iwojima volcano is estimated to be 760 t day^-1, with a magmatic contribution of about 450 t day^-1. This value is rather high compared with other volcanoes in island arc settings. Since Iwojima has no visible plume, almost all volcanic CO₂ is released as diffuse efflux through the volcanic edifice.