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

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 (SD), 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 SD 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 H20 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 SD of pre-eruptive magma. In contrast, the SD 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.

Comparative compressibility of hydrous wadsleyite and ringwoodite: Effect of H2O and implications for detecting water in the transition zone

“…In the Hokkaido measurements, a natural amphibole crystal containing 1.66 wt % H 2 O was used as a calibration standard, as described by Miyagi et al . []. At Carnegie Institution on the NanoSIMS, three different silicate glasses containing 0.17, 1.00, and 1.62 wt % H 2 O were used to develop a calibration against the measured mass ratios of 17 OH/ 28 Si [ Hauri et al ., ; Saal et al ., ].…”

Color-change processes of a plinian pumice and experimental constraints of color-change kinetics in air of an obsidian

“…In addition, oxidation-reduction processes of minerals are considered to be related to dehydration processes (Skogby and Rossman 1989). Miyagi et al (1998) showed a good correlation between water contents and a* (reddishness) values of hornblende and concluded that ferrous to ferric oxidation upon dehydration was the origin of the reddish color change of hornblende. Iron is also a major transition element of rhyolitic glasses in pumices.…”

Nd-, O-, and H-isotopic evidence for complex, closed-system fluid evolution of the peralkaline Ilı́maussaq intrusion, south Greenland

“…This process requires secondary exchange of hydrogen isotopes between amphiboles and the methane-dominated fluid and/or water derived from such methane. Such secondary exchange between earlier crystallized minerals and late-stage fluids that may even be internally generated has previously been suggested to account for a large range in ␦D values of amphiboles in a number of volcanic rocks (e.g., Miyagi et al, 1998;Miyagi and Matsubaya, 2003). It is also worth noting that a number of authors suggested that the formation of hydrocarbons could be related to late-or postmagmatic alteration of Fe-rich minerals resulting in the production of H 2 (e.g., Salvi and Williams-Jones, 1997;Potter et al, 1998).…”