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

Abstract

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 δD 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‰, respectively. The samples do not show a systematic H 2O– δD relationship, although the block samples tend to have lower δD values than the bomb samples. The non-systematic H 2O– δD 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 H 2O–low δD margins and low H 2O–high δD cores. Relatively high δD values of the blocks and bombs as well as high temperature volcanic gas (−30 to −35‰) suggest a closed system degassing of an initial water-rich magma (H 2O=6 wt.%) until its water content was reduced to 0.5 wt.%. The pre-eruptive δD 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.