Water solubility in majoritic garnet in subducting oceanic crust

Abstract

[1] Water in majoritic garnet synthesized in the mid-oceanic ridge basalt (MORB) + H 2 O composition at 20 GPa and 1400 -1500°C was measured by secondary ion mass spectrometry (SIMS) and infrared spectroscopy. We found that the majorite contains 1130 to 1250 ppm OH by weight. The infrared absorption band showed that incorporation of the hydroxyl in majorite is most likely due to the hydrogarnet substitution. Our results indicate that water can be transported into the mantle transition zone by nominally anhydrous minerals such as omphacite and majorite in the subducting basaltic crust. Such water may have great influences on the physical properties of slab in the transition zone.

Mechanisms for incorporation of hydrogen in and on terrestrial planetary surfaces

“…At the surface, the olivine is 2500 ppm H 2 O, with counts decreasing to $120 ppm H 2 O at a depth of 1-2 lm. Similar surface concentrations were reported for OH on quartz crystals (Clark et al, 1978;Dersch and Rauch, 1999), on garnet crystals by Katayama et al (2003), and on sillimanite by Beran et al (1989). These experimental data strongly support the notion that the ratio of surface area to volume sampled by the IR beam has a first-order effect on H 2 O concentration estimates.…”

Calculation of the energetics of water incorporation in majorite garnet

“…Our results show that even at low-water contents, the hydrogarnet defect is likely to form in majorite. Synthesis of majorite in the MORB+H 2 O system results in a sharp peak centered at 3580 cm -1 (Katayama et al 2003), which is only 20 cm -1 lower than the hydrogarnet substitution in synthetic pyrope (Ackermann et al 1983). For the hydrogarnet defect in majorite (P = 0 GPa), our calculated bond lengths using force field methods, d(O-O) = 2.83 and 3.24 Å for the shared and unshared edges, are shorter than those in grossular [d(O-O) = 3.08 and 3.29 Å] also calculated using interatomic potentials .…”

“…This volume expansion should be less energetically favorable at high pressure. Infrared absorption spectra indicate that OH is potentially incorporated into majorite by formation of the hydrogarnet defect (Katayama et al 2003). However, water concentrations in natural garnet samples brought to the surface from the mantle (Bell andRossmann 1992a, 1992b) and infrared spectroscopy of both natural garnet (Amthauer and Rossman 1998;Beran and Libowitzky 2006) and synthetic majorite (Bolfan-Casanova et al 2000) indicate that hydrogen incorporation in majorite may be complex (Williams and Hemley 2001).…”

“…Because the solubility of water in the olivine polymorphs decreases with temperature and the partitioning of water between wadsleyite and ringwoodite may be close to D wad/ring = 1 at high temperature (Bolfan-Casanova 2005), majorite solubility and partitioning may play an important role throughout the entire thickness of the transition zone. Water solubility in majorite garnet synthesized at transition zone pressures and temperatures can contain up to ~700 ppm wt H 2 O (Bolfan-Casanova et al 2000;Katayama et al 2003), a quantity sufficient to radically change the deformation characteristics of garnet. For a more complete understanding of the implications of OH defects in garnet in the deep mantle, systematic computational studies of hydrogen incorporation into majorite are required.…”

Solubility of water in pyrope‐rich garnet at high pressures and temperature

“…This work was supported by the U.S. National Science Foundation. Table 1); black filled circle, T $ 1473 K and f O 2 $ Ni-NiO; black open rhombs, undersaturated with respect to water; and data point at 7 GPa, T $ 1473 K and f O 2 $ Fe-FeO; grey open (pyrope) and filled (grossular) circles [Withers et al, 1998], the grey rhombs [Lu and Keppler, 1997], the filled square [Bolfan-Casanova et al, 2000], and the open square [Katayama et al, 2003] represent majorite. (b) Comparison of water solubility in various minerals in the upper mantle.…”