Change in D/H ratio, water content and color during dehydration of hornblende

Isoji Miyagi, Osamu Matsubaya, Satoru Nakashima

Geochemical Journal
Vol. 32, 33-48 (1998)

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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 (delta-D), 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 deg-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 delte-D 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
delta-Dof pre-eruptive magma.  In contrast, the delta-D 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.