Ash emissions occurred from a hot water pool of one of the craters of Nakadake, the only active central cone of Aso Volcano, southwestern Japan, on July 10, 2003 and January 14, 2004. The former ash was dispersed throughout an area extending 14km east-northeast of the vent, whereas the latter ash extended 8km to the east-southeast. Based on isopleth maps, the total weight of ash discharged by each event was 41tons and 32tons, respectively. Both ash-fall deposits were composed of fine-grained (＜1mm) glass shards, crystals and lithic fragments, and contained neither lapilli nor blocks. Some glass shards appear fresh and are probably juvenile material. We therefore believe that these phenomena represent non-explosive gas-and-ash emissions through the hot water pool. The ash particles emitted from the hot water pool of the Nakadake crater were aggregated at a size of 1-3mm and deposited because of a high content of water. The ash-fall deposits in proximal areas (＜1.5km from the crater) were finer grained and more poorly sorted than in distal areas (＞5km). Moreover, the deposit volume obtained from actual data in the proximal area was much larger than the volume extrapolated only from the isopleth data of the distal deposits. These phenomena suggest that the proximal deposits were emplaced mainly by another fine-grain-rich column different from the main ash plume that extended to distal areas, or that some parts of the column detached from the main plume that emplaced the proximal deposits.

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Fluorine and chlorine have been determined for 17 GSJ rock reference samples, “Igneous rock series” by trimethylsilylating distillation-ion-selective electrode method and by improved HgSCN photometric method, respectively. The results mostly agree with “recommended values” in 1994 compilation by GSJ. Some 1994 “preferable values” need further examination. Five data of Cl, which were not presented in 1994 compilation, are given. Water-soluble chlorine contents in these samples are also determined.

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