HR: 1340h
AN: DI33A-1129    [Abstracts]
TI: Precipitation of Excess Hydrogen in Olivine During Cooling Under Pressures: An Experimental Study
AU: * Borinski, S
EM: s.borinski@gmx.de
AF: Ruhr Universitaet Bochum Institut fuer Geologie, Mineralogie und Geophysik, Universitaetsstrasse 150, Bochum, NRW 44780,
AU: * Borinski, S
EM: s.borinski@gmx.de
AF: Yale University Kline Geology Laboratory, 210 Whitney Aveenue, New Haven, CT 06511, United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University Kline Geology Laboratory, 210 Whitney Aveenue, New Haven, CT 06511, United States
AB: Water (hydrogen) content in olivine transported from the upper mantle is used to infer the water content in the upper mantle (e.g., Bell and Rossman 1992). However, since hydrogen diffusion is known to be fast, processes of hydrogen loss need to be examined. In many literature, diffusion loss (or gain) of hydrogen is usually considered, but in addition to diffusion loss, hydrogen could also precipitate inside of olivine as small inclusions. Consider an upward transport of olivine-bearing rock that originally contained a large amount of hydrogen in the deep interior. As this rock is transported to the shallow region, the solubility limit of hydrogen will decrease because of the reduction of pressure (and temperature) (Kohlstedt et al. 1996, Zhao et al 2004). Consequently, excess hydrogen will precipitate to form water bubbles and/or hydrous minerals as inclusions. Frequently observed submicron-scale inclusions of hydrous minerals (Khisina and Wirth 2002, Kitamura et al. 1987) may correspond to these precipitation products. If that is the case, hydrogen content corresponding to these minerals should not be excluded when estimating the hydrogen content of a sample in the Earth's upper mantle. However, kinetics of precipitation of hydrogen from olivine have not been investigated in the laboratory. We have conducted a series of experimental study in which we annealed hydrogen-saturated olivine single crystals in two different P- T conditions. The starting material was an olivine crystal in which ~1,135 H/106Si (70 wt ppm H₂O) was dissolved at P= 3.5 GPa and T=1,573 K. A small piece of this crystal (0.5 mm³) was placed in a multianvil at P=3.5 GPa and either at T= 873K or 1,173K with oxygen fugacity, fO₂, buffered by the Ni-NiO solid-state reaction and silica activity, aSiO₂, buffered by the presence of orthopyroxene powder in contact with the crystal. Annealing experiments were conducted up to 72 hours. Hydroxyl concentrations were determined from infrared spectra obtained from polished thin sections from crack-free regions of 100 x 100 μm. The hydroxyl concentration at the OH-stretching region around 3678 cm^-1 increases systematically with increasing time at 873 K, whereas this band is not detected in samples annealed at 1,173 K. The peak(s) at 3678 cm^-1 corresponds to the OH-stretching vibration in hydrous minerals such as serpentine (Mellini et al. 2002, Hofmeister and Bowey 2006). We conclude that the water in the upper mantle not only diffuse out and disappear during the ascent (cooling and depressurization), but also is bounded in hydrous minerals (e.g. serpentine).
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
SC: Study of the Earth's Deep Interior [DI]
MN: 2007 Fall Meeting