ABSTRACT

Arsenic contents were determined for volcanic gases, sulfur sublimates, and alteration products from Mts. Tokachi, Meakan, Tarumae, Esan, Unzen and Iwo-dake (Satsuma-Iwojima Island), in Japan. The gas sampled from >300°C vents contain between 700 and 4000 ppb As with a positive temperature dependence, regardless of their major compositions. However, the As contents of <300°C gases are scattered over a wide range, possibly because of contamination from or loss to sublimates and condensates. The As/S ratios in the sulfur sublimates are highly variable, without definite relations to those in the parent-gases. However, when this is coupled with the fact that only a small fraction of total sulfur is sublimated out of the gas, it is concluded that a large part of the emitted As escapes to the atmosphere. Alteration products are enriched in As relative to the fresh lava, due to the deposition of As-bearing sublimates and/or the condensation of volcanic gases on the surrounding altered rocks; also possibly due to gas-rock interactions. This study shows that As is an essential part of magmatic emanations which underwent almost no modification during ascent from magma top to the surface. Arsenic content in volcanic gases can be a good indicator for fumarolic activity.