HR: 1330h
AN: V22B-0590    [PDF]
TI: H and C Impurities in Synthetic Majoritic Garnets: Implications for Speciation and Solubility
AU: Giesting, P
EM: pgeistin@nd.edu
AF: Dept. Civil Engr. and Geol. Sci., University of Notre Dame, Notre Dame, IN 46556 United States
AU: Wopenka, B
EM: bwopenka@levee.wustl.edu
AF: Dept. Earth and Planet. Sci, Washington U., St. Louis, MO 63130 United States
AU: Jolliff, B L
EM: blj@levee.wustl.edu
AF: Dept. Earth and Planet. Sci, Washington U., St. Louis, MO 63130 United States
AU: Gwanmesia, G D
EM: ggwanmes@dsc.edu
AF: Dept. Phys. and Pre-engineering, Delaware State U., Dover, DE 19901 United States
AU: * Hofmeister, A M
EM: hofmeist@levee.wustl.edu
AF: Dept. Earth and Planet. Sci, Washington U., St. Louis, MO 63130 United States
AB: Impurities were revealed in four high-pressure, high-temperature synthetic polycrystalline garnets along the pyrope [Mg$_{3}$Al$_{2}$Si$_{3}$O$_{12}$] - majorite [Mg$_{3}$(MgSi)Si$_{3}$O$_{12}$] join by infrared absorbance, Raman scattering spectroscopy, and detailed electron microprobe analyses (EMPA). Close examination of slight differences in the backscatter-electron images of one sample of Mj$_{40}$ and subsequent X-ray imaging and spot analyses documented the presence of Mj$_{95}$ interstitial impurities and a silica rich phase near the 5% level. These phases would not have been observed in a typical traverse. Spectroscopic measurements show that all samples contain the narrow bands of OH$^{-}$, broad bands of fluid water, hydrocarbons, and disordered graphite. Hydroxyl is expected in either structural sites or as defects, whereas the other impurities are most likely on grain boundaries or as microscopic inclusions. Whereas infrared spectroscopy is needed to ascertain OH$^{-}$ and H$_{2}$O contents, and is more sensitive to hydrocarbons that Raman scattering, the later is required to detect graphite. The source of C is likely acetone traces from sample preparation that were entrapped in the sealed capsules. The source of H could be moisture in the air or acetone. Band profiles and heating experiments show that water is the main carrier of H in our samples. Similar broad bands in published IR spectra of many high-pressure samples have been misattributed to OH$^{-}$, thereby overestimating both concentrations and solubilities of OH$^{-}$. Low concentrations are predetermined by the starting materials (oxides in the stoichiometry of the desired mineral phase plus water), and by charge balance. Our results contraindicate extensive hydration of the lower mantle. In addition, hydrocarbons or water on grain boundaries of synthetic samples provide lubrication, which can affect measuremens of pressure derivatives. Detailed EPMA and spectroscopic characterization of high-pressure synthesis products are essential.
DE: 3924 High-pressure behavior
DE: 3934 Optical, infrared, and Raman spectroscopy
DE: 3994 Instruments and techniques
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8147 Planetary interiors (5430, 5724)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting