Sulfur Isotope and Trace Element Systematics in Arc Magmas: Seeing through the Degassing via a Melt Inclusion Study of Kyushu Island Volcanoes, Japan

Abstract

Sulfur is a minor element in magmas but one of the major volatile elements released in volcanic systems, from the magma to the fluid phase upon ascent. Not only are sulfur gases potentially toxic for humans and plants, they are also involved in causing drastic climate changes after major volcanic eruptions. Therefore, studies are carried out by the geoscience community to assess the magmatic sulfur flux by looking at the sulfur content and isotopes in erupted products, with the ultimate aim of improving understanding of the sulfur cycle in subduction zones. Kyushu island in Japan hosts 25 volcanoes among which 11 are active and represent a natural hazard for the local population. It is perhaps the most suitable site for the study of the sulfur cycle for its availability of recent volcanic deposits and its many, highly monitored volcanoes. We investigated sulfur and sulfur isotope compositions of the magma source of Kyushu Island arc using olivine-hosted melt inclusions in mafic tephras and lavas, from 8 volcanoes (9 Holocene samples) going from Northern Kyushu with Oninomi, Yufu, Kuju, and Aso, to Southern Kyushu volcanoes such as Kirishima-Ohachidake, Kirishima-Shinmoedake, Sumiyoshi-ike, and Kaimondake, and one back arc volcano, Fukue-Onidake. We measured major, trace and volatile elements and S isotopes (δ34S) in melt inclusions. Magma composition recorded in the inclusions ranges from basalt to andesite (SiO2 ranging from 40.3 to 60.7 wt. %). For each edifice, we identified the least degassed and least differentiated compositions based on volatile and trace element systematics, and selected the melt inclusions closest to their primitive melts. Comparing these primitive magmas, Sr/Y underlines a compositional dichotomy between volcanoes from northern (Sr/Y>20) and southern Kyushu (Sr/Y<20), separated by a non-volcanic area corresponding to the subduction of the Kyushu-Palau ridge. The δ34S in melt inclusions range from -0.32±0.79 ‰ to +9.43±0.47 ‰ (2σ) and trace the source of the magma from the different volcanoes, rather than degassing or crustal fractionation processes. δ34S is not fractionated by the nature of the fluid (aqueous or melt) metasomatizing the mantle wedge, therefore it is not the first order factor controlling the sulfur isotope variations. Instead, this study illustrates the need for a heavy δ34S component, likely sulfate from seawater, contained in the agent that metasomatized the mantle beneath the arc. If such an observation is confirmed in other subduction zones, sulfur isotopes in melt inclusions may be an effective way to trace seawater input into the mantle beneath arcs.