2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 12
Presentation Time: 4:45 PM


LAGER, George A.1, CHEN, Jianrong1, LIU, Zhenxian2, HU, Jingzhu2 and ULMER, Peter3, (1)Geography and Geosciences, University of Louisville, Louisville, KY 40292, (2)Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, (3)Institute for Mineralogy and Petrography, Swiss Federal Institute of Technology (ETH), Zurich, CH-8092, Switzerland, galager@louisville.edu

Ca-silicate garnets could be important water-bearing phases in eclogitic regions of the upper mantle. As part of a project to determine the high P-T behavior of hydrogarnets, high-pressure mid-infrared (IR) studies of synthetic Sr-hydrogarnet [Sr3A12(O4H4)3] (Sr3A) and Ca-hydrogarnet with 50% grossular component [Ca3A12(SiO4)1.5(O4H4)1.5] (C3ASi) were carried out at the U2A beam line at the National Synchrotron Light Source. The objectives of these experiments were to determine (1) if the phase transition in katoite [Ca3A12(O4H4)3] (C3A) occurs in other hydrogarnets; and (2) if these transitions do occur, how is the pressure of the transition affected by substitutions at the X (dodecahedron) and Z (tetrahedron) sites. It has been proposed that H-H repulsion may initiate the phase transition in C3A at 5 GPa. Previous experiments at the U2A beam line have shown that IR spectroscopy is very sensitive to the transition. IR spectra collected for Sr3A clearly show a discontinuity in vibrational frequency (3662 cm-1) at ~2.5 GPa. Both OH frequencies in C3ASi also show a significant change in slope (from positive to negative) at ~6 GPa. However, based on these results, it is not possible to determine if H-H repulsion is the driving force of the transitions. The instability of the hydrogarnet structure could also be related to a size misfit between the X-site cation and the dodecahedral cavity, i.e., the H atoms play a passive role in the transition. Topazes [Al2F2-x(OH)x(SiO4)] found in ultrahigh-pressure metamorphic terrains are relatively OH-rich, with XOH exceeding 0.5 in some cases. To learn more about the behavior of hydrogen in these materials at mantle pressures, high-pressure mid-IR and powder X-ray data were collected for topaz-OH (X = 2) to ~10 GPa at the U2A and X17C beam lines, respectively, using Ar (IR) and methanol:ethanol (X-ray) as pressure-transmitting media. The bulk modulus determined from a Birch-Murnaghan fit to energy-dispersive X-ray data is K0 = 144.4 (4) GPa with K' = 4. Both OH frequencies decrease slightly (20-40 cm-1) with pressure but at significantly different rates, consistent with results from a previous Raman study of the same phase. The hydrogen-bond geometry at high pressure in topazes may be controlled primarily by the compressional behavior of the surrounding cavity.