Hydrogen isotopic composition of hornblende and biotite phenocrysts from Japanese island arc volcanoes: Evaluation of alteration process of the hydrogen isotopic ratios by degassing and re-equilibration

In order to estimate hydrogen isotopic ratios (δD) of pre-eruptive island arc magma, we measured δD, chemical compositions, and colour of hornblende and biotite phenocrysts from Japanese Quaternary volcanoes. The rock samples comprise lavas, lava domes, pyroclastic flow deposits related with lava dome collapse, air-fall pumices and pumice flow deposits. The observed δD value ranges from −108 to +103‰ SMOW (Standard Mean Ocean Water) and exceeds the range between the mantle water (−100 to −70‰) and island arc magmatic water (−40 to −15‰). There is no clear correlation between δD values of minerals and their chemical compositions, but a rough tendency for those minerals not from air-fall pumices to have less water content and a reddish colour. We interpreted and modelled such a large δD fluctuation by a Rayleigh isotopic distillation process in mineral dehydration and a hydrogen isotopic re-equilibration process of the mineral with degassing magma during magma extrusion stages. Our model suggests that green hornblendes from air-fall pumices can be used for the δD estimation of pre-eruptive magma. The δD values of selected hornblendes fall in a rather narrow range from −61 to −32‰. Our data verify that the previously reported δD range of volcanic steam in island arc regions (−40 to −15‰) corresponds with the δD value of pre-eruptive magmatic aqueous fluid (−45 to −21‰), provided that the hydrogen isotopic fractionation factor between the aqueous fluid and mineral is about 7–19‰.