ABSTRACT

The hydrogen isotopic composition of hornblende in volcanic rocks has potential to indicate processes such as degassing of a magma reservoir, as well as the origin of magmatic water. However, during eruption and high-temperature interaction with air, the isotope composition of hornblende can be altered. In order to establish criteria to select natural hornblende that preserves magmatic hydrogen isotopic ratios, hydrogen isotopic composition (δD), water contents, and colors of hornblendes were measured. A series of heating experiments on natural green hornblendes was conducted in air and under vacuum up to 1000°C. There observed a negative relationship between the intensity of redness (a^∗) and water content among heated hornblendes in air and under vacuum, as well as natural samples from several Quaternary volcanos in Japan. Heating hornblende results in a significant increase in δD value that can be explained by a Rayleigh distillation (simultaneous degassing without hydrogen isotopic exchange between the gas and the mineral) model involving degassing of both H₂O and H₂. According to the model, maximum fractionation occurs when hornblende dehydrates only by H₂ degassing. Application of the model indicates that hornblendes in pumice fragments from Plinian eruptions are most representative of the δD of pre-eruptive magma. In contrast, the δD values of minerals from lava flows, lava domes, or even from thick pumice flow deposits may have suffered from 1) hydrogen isotopic fractionation by mineral dehydration, and/or 2) isotopic re-equilibration with degassing magma during eruption stage and during cooling stage after the emplacement.