.Journal of Volcanology and Geothermal Research 89 1999 231–242 Water contents and hydrogen isotopic ratios of rocks and minerals

Water contents and hydrogen isotopic ratios were determined for blocks from pyroclastic flow deposits, and bread-crust bombs and blocks from the 1991 Vulcanian eruptions of Unzen volcano, Japan. Groundmass water contents and d D values .of samples were calculated by subtracting the contribution of major hydrous minerals hornblende and biotite from the bulk rock analyses, and range from 0.1 to 0.5 wt. % and y83 to y49‰, respectively. The samples do not show a systematic H O–d D relationship, although the block samples tend to have lower d D values than the bomb samples. The non-sys-2 tematic H O–d D relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma2 would hinder attainment of hydrogen isotopic equilibrium between exsolved vapor and melt in the final degassing stage. The near surface degassing, however, was accompanied by kinetic fractionation resulting in enrichment of deuterium in the final products as exemplified by bread-crust bombs with high H O–low d D margins and low H O–high d D cores. Relatively2 2 .high d D values of the blocks and bombs as well as high temperature volcanic gas y30 to y35 ‰ suggest a closed system .degassing of an initial water-rich magma H Os6 wt. % until its water content was reduced to 0.5 wt.%. The pre-eruptive2 .d D value y46 ‰ was estimated from the volcanic gas data and DrH analysis of hornblende phenocrysts coupled wit