HR: 14:40h
AN: V43E-04 INVITED    [Abstracts]
TI: Osbornite (TiN) and boron nitride nanoinclusions in coesite from Tibet: a first record of nitrogen in a terrestrial ultrahigh pressure environment
AU: * Dobrzhinetskaya, L
EM: larissa@ucr.edu
AF: Dept of Earth Sciences, University of California, Riverside, CA 92521, United States
AU: Wirth, R
AF: GeoForschungsZentrum, GeoForschungsZentrum, Potsdam, 14473, Germany
AU: Yang, J
AF: Lab. of Continental Dynamics, Inst of Geology, Beijing, 100037, China
AU: Hutcheon, I
AF: Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, CA 94551, United States
AU: Weber, P
AF: Seaborg Institute, Lawrence Livermore National Laboratory, Livermore, CA 94551, United States
AU: Green, H W
EM: harry.green@ucr.edu
AF: IGPP, University of California, Riverside, CA 92521, United States
AB: We report here discovery of nitrides in podiform chromitite of the Luobusa ophiolite, Tibet, representing the mantle section of a ˇ°fossilˇ± fragment of oceanic crust which marks the Early Tertiary (~65Ma) suture zone between Asia and India. Nanometric crystals of TiN and BN are included in coesite which, together with kyanite, TiO₂-II and several other still unidentified phases rim a FeTi-alloy pellet; this sample, together with an OsIr alloy pellet containing microdiamond were extracted from the massive chromitite (Yang et al., Geology, 2007). Both TiN and BN form bright-grey-contrast particles in secondary-electron scanning electron microscope imaging. Several focused ion beam foils were prepared for a transmission electron microscope (TEM) and a nano secondary ion mass spectrometry (nanoSIMS) studies. Because energy dispersive X-ray spectra (EDS) of boron and nitrogen K-lines have overlaps with each other and with Ti L-lines, we have used electron energy loss spectroscopy (EELS) in TEM to determine the presence of boron and nitrogen K-edges and to separate them from Ti L-lines. Electron diffraction patterns identify the cubic boron nitride (c-BN) structure. TiN is stoichiometric (Ti=77.20wt%; N=22.80wt%) and is also cubic. Both phases contain trace carbon. Osbornite, is usually found only in meteorites, although it also has been reported as inclusions in carbonado diamonds and as inclusion in a corundum grain from lamproitic breccia in the Asov block, Eastern-European platform, Ukraine. Until now, boron nitride was known only as an industrial compound, with both hexagonal and cubic structures. With the nanoSIMS Cameca-50 using the Cs beam we have measured nitrogen and carbon isotopes in both the TiN and c-BN inclusions: δ 15N= +(6-10) ‰ and δ 13C= (+1) ¨C (-10) ‰. The results show that both minerals are characterized by δ 15N similar to crustal rocks involved in the subduction process and their δ 13C values suggest mixture of heavy (mantle) and lighter (crustal) carbon. These observations, coupled with the occurrence of the TiN and c-BN in coesite, record a mantle history of nitrogen and indicate that natural osbornite formation is not restricted to high-temperature gas-to-solid condensation reported from studies of meteorites. These nitrides may have been formed during ultrahigh pressure metamorphism of nitrogen- and boron-bearing sediments subducted to mantle depths or, given that this material may have come to the surface from > 300 km (Yang et al., 2007), they could possibly reflect a role of nitrogen in Earth's core formation and cast light on the geochemical budget of nitrogen ˇ°escaped fromˇ± and ˇ°retained withinˇ± the Earth during its accretion and atmosphere formation.
DE: 1038 Mantle processes (3621)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 3620 Mineral and crystal chemistry (1042)
DE: 3621 Mantle processes (1038)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting