[Abstract]

Fluorine and chlorine have been determined for all kinds of the products of the 1108 activity of Asama volcano. The activity displayed various features that are characteristic of andesitic eruptions, i.e., scoria (pumice) fall (B-scoria L), intermediate-type pyroclastic flow (the Oiwake pyroclastic flow) and lava flow (the Kamino-butai lava flow). Their F-Cl variation patterns are widely varied according to the natures of the eruption in a similar way to those of the 1783 activity of Asama (Yoshida and Tsuchiya, 2004). The B-scoria L samples show uniform and the highest Cl contents in average. The Kamino-butai lava flow samples have low and uniform Cl contents. These results coincide with those obtained by the study on the 1783 activity and may be explained by the enrichment of volatiles in the uppermost part of the magma column before the eruption. The F and Cl contents of Oiwake pyroclastic flow samples show very large variances. Some Oiwake samples take almost the same halogen contents as the B-scoria L samples. Whereas, the other samples show the lowest F and Cl contents and the largest variances, and indicate strong release of gas at or after the effusion. The detailed examination of topographical distribution of the F and Cl contents shows that the Oiwake pyroclastic flow is composed of many flow units of high-halogen and of low-halogen contents and may suggest the transition of the mode of the formation of the pyroclastic flow with different degassing conditions. In the low-halogen flow units, volatiles might be mostly released in the early stage of the flow.

Key words:

fluorine, chlorine, volatile contents, pyroclastic flow, Asama volcano