Hydrogen isotope fractionation between coexisting vapor and silicate glasses and melts at low pressure

Volc�n Quizapu, Chilean Andes

“…. It = +10 to +40%0 at 850~ in HzOrich silicic magmas) identified by Newman et al (1988) and Dobson et al (1989) make a pre-eruptive equivalent of the quasi-closed-system progressive-degassing model of Newman et al (1988) our preferred mechanism for lowering d/Dhb by as much as 40%o (Fig. 18) in the course of passively exsolving a few wt% of H20.…”

“…2). Lower-left panel gives the 6~80=6D field expected for pre-emptive Quizapu dacite magma, based on 850~ fractionations anticipated among vapor, melt, hornblende, and plagioclase (Dobson et al 1989;Suzuoki and Epstein 1976;Taylor and Sheppard 1986). Also plotted are isotopic values for snow adjacent to Quizapu crater, a partially molten granitic xenolith in 1932 fallout, a glass separate from degassed 1846 lava (Q-34), the meteoric-water line of Craig (1961), and trends anticipated for magmatic degassing and for assimilation of hydrothermally altered roof rocks controlled the magmatic D / H relations, appears to be ruled out because (1) it was the lower-6D dacite that erupted effusively, (2) there was no difference in 6180, and (3) there was no significant difference in fOz or temperature (Fig.…”

Hydrogen isotopes in hornblendes and biotites from Quaternary volcanic rocks of the Kamchatka-Kurile arc.

“…and "magmatic hydrous minerals-vapor" are approximately the same (10 to 30%o; Suzuoki and Epstein, 1976;Kuroda et al, 1982;Dobson et al, 1989), the fractionation be tween OH-minerals and dissolved water should be low, in the range of several permil. Therefore, the hydrogen isotope composition of OH-minerals should be close to that of the dissolved, "mag matic", water.…”

“…Hb and Bi in magma are likely to be in isotopic equilibrium with water dissolved in the melt, and/or with aqueous fluid released from the magma (Suzuoki and Epstein, 1976;Kuroda et al, 1988). The fractionation factor AD (melt vapor) was experimentally determined to range from 10 to 50%o for melts of varying composition (Kuroda et al, 1982;Ritchet et al, 1986;Taylor, 1986;Dobson et al, 1989), such that the residual water dissolved in the melt will be progressively depleted in deuterium with decreasing water con tent. Such an isotopic effect for "in situ" degas sing of magma was demonstrated using natural volcanic glasses or whole rock samples by Taylor et al (1983), Newman et al (1988), Taylor (1992), Anderson and Fink (1989), and Anderson et al (1995).…”

“…5). However, this value of the fractionation factor was experimentally determinated at magmatic temperature only for dissolved water in form of hydroxyl groups, at a low (<0.5 wt.%) water concentration (Dobson et al, 1989). At higher water contents, when part of water in the melt is present in the form of mo lecular water (Stolper, 1982), the bulk fraction ation factor is lower, 20-25% (Taylor, 1986;Dobson et al, 1989), and a combined (two-stage) open-system degassing curve with the same ini tial water content and initial SD values will be shifted slightly to higher SD values.…”