International Geologiical Congress - Oslo 2008

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UHP-03 Ultra-high pressure metamorphism: Minerals, microstructures and nanoscale observations - Part 1

 

From nanometric inclusions of osbornite (TiN) in terrestrial coesite to mantle convection and storage of nitrogen in Earth's interior

 

Larissa Dobrzhinetskaya, University of California at Riverside (United States)
Richard Wirth, GeoForschungsZentrum (Germany)
Jingsui Yang, Institute of Geology, Chinese Academy of Geological Sciences (China)
Harry Green II, University of California at Riverside (United States)
Peter Weber, Lawrence Livermore National Laboratory (United States)
Ian Hutcheon, Lawrence Livermore National Laboratory (United States)
 

 

Although nitrogen occurs widely in soils and sediments and is well known in oceanic basalts and diamonds, its distribution in the Earth's interior is poorly understood. Javoy (1998) hypothesized that a significant amount of nitrogen might be stored in the upper mantle of the Earth in the form of osbornite. Recent studies have been focused on the high-pressure stability of iron nitrides and how nitrogen affects phase transformations, crystal structure, and elasticity at varying pressures. Nitrogen has been proposed as a candidate for the light alloying element concentrated in the volumetrically minor solid inner Earth's core material. The attention was also attracted to the nitrogen as an important constituent in the recycling fluxes in subduction zones.

Osbornite (TiN) is found mostly in meteorites, but also as inclusions in carbonado diamonds in association with londsleyite, and in corundum from a lamproitic breccia. Our new discovery consists of 20-300 nm inclusions of osbornite in coesite coexisting with kyanite, and diamond included in OsIr alloy from massive chromitite ore of the Luobasa mantle section of ophiolite in Tibet. The Tibetan TiN forms bright-contrast particles in secondary-electron scanning-electron-microscope images. Because EDS spectra of boron and nitrogen have severe overlaps with Ti L-lines, we have used electron energy loss spectroscopy to confirm the presence nitrogen K-edges. The TiN is stoichiometric and has cubic symmetry and NaCl structure. The boron nitride inclusions were also found in the same coesite. EBSD analysis has confirmed that the host coesite is a pseudomorphic replacement after stishovite. NanoSIMS studies have revealed that the δ15N = -10(±2)‰ in the Tibetan osbornite. Although the first osbornite from the Bustee meteorite is characterized by δ15N = -24‰, the wider values of δ15N from -326‰ to +30‰ were reported for osbornites from other meteorites. If we compare these results with existing systematics of nitrogen reservoirs in the Earth, δ15N of the Tibetan osbornite is close to the pristine kimberlitic diamonds (-5 to -8‰), with the exception of δ15N= -25‰ in Fuxian diamonds. Absence of any impact microstructures in the Tibetan osbornite-bearing sample rules out its possible astrobleme/meteorite origin. Because of the bulk chemistry (Al-Si-rich rocks) and the presence of boron, we define the Tibetan sample as a fragment of oceanic sediments subjected to ultra-high-pressure re-crystallization during deep subduction. The small fragments of deeply subducted sediments were incorporated into chromitites because of mantle convection, and nitrogen was induced into the sample during its residence in the mantle environments. This discovery opens up a new dimension for understanding that in some mantle levels there are regions with high N2 fugacity that may affect the element partitioning and unexpected phase transformations. Knowledge of these factors is crucial for understanding the Earths interior.

 

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