Noble gas and carbon isotopes of fumarolic gas from Iwojima volcano, Izu–Ogasawara arc, Japan: implications for the origin of unusual arc magmatism

Abstract

Iwojima volcano, located on the southernmost part of the Izu–Ogasawara arc, is characterized by a magma chemistry, which differs from that of the other volcanoes in the arc, suggesting a larger contribution of subducted component. In order to elucidate the behavior of volatile elements during the subduction process in generating this unusual magma, noble gas and carbon isotope ratios of volcanic volatiles were measured. The noble gas and carbon isotopic characteristics of Iwojima magma are quite distinct from other volcanoes in the Izu–Ogasawara–Mariana arc system: (1) a low 3He/ 4He ratio of 5.6 R A; (2) enrichment in heavy noble gases; and (3) a heavy carbon isotope ratio (+1.5‰) almost equivalent to the highest value for marine carbonates. These unusual features strongly suggest the involvement of slab-derived volatiles in the magma, although the possibility of a shallow origin for the low 3He/ 4He ratio still remains. Based on a mixing calculation using helium–carbon isotope systematics, an extremely large contribution of slab-derived components (31% for 4He and more than 95% for 12C) is required to explain the unusual features of helium and carbon isotopes of Iwojima. This is in agreement with other petrological and geochemical features which imply a greater role of subducted materials, although the precise relative contributions of each source materials are somewhat different for several of the tracers. This difference is probably caused by different provenance in subducting materials carrying them to melt generation depth in the mantle wedge. The elevated carbon/helium ratio of the slab-derived component also suggests decoupling during the subduction process probably resulting from the preferential loss of helium from the subducting slab beneath the forearc region. Enhanced outputs of slab-derived carbon and helium (3.5×10 9 and 4.0×10 3 mol/year) indicate a large input of slab component to the mantle wedge and/or effective devolatilization of slab materials. As for its origin, we believe that the unusual tectonic setting around Iwojima may be responsible: (1) subduction of the Ogasawara Plateau, which brings a significant volume of thick crustal blocks into the mantle wedge beneath Iwojima; (2) flattened subduction as a consequence of the buoyancy of the subducting Ogasawara Plateau; and (3) propagation of the back-arc basin rift axes in the Mariana Trough. In contrast to large-scale subduction of overthickened oceanic crust associated with slab melting, the subduction of a relatively small oceanic plateau may be responsible for the characteristics of the Iwojima magma, in which slab material itself does not play an important role in magma genesis but slab-derived volatiles make a significant contribution.