Water contents and hydrogen isotopic ratios of rocks and minerals from the 1991 eruption of Unzen volcano, Japan

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 delta 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 - 83 to -49 parts per thousand, respectively. The samples do not show a systematic H2O-delta D relationship, although the block samples tend to have lower delta D values than the bomb samples. The non-systematic H2O-delta D relationship is likely a result of near surface, kinetically-controlled gas loss. High viscosity of this magma 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 H2O-low delta D margins and low H2O-high delta D cores. Relatively high delta D values of the blocks and bombs as well as high temperature volcanic gas (- 30 to - 35 parts per thousand) suggest a closed system degassing of an initial water-rich magma (H2O = 6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive SD value (- 46%) was estimated from the volcanic gas data and D/H analysis of hornblende phenocrysts coupled with assumed isotopic equilibration in the initial hydrous magma. (C) 1999 Elsevier Science B.V. All rights reserved.