Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmmr41a1779d&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #MR41A-1779
Mathematics
Logic
8115 Core Processes (1213, 1507), 8124 Earth'S Interior: Composition And State (1212, 7207, 7208, 8105), 8140 Ophiolites (3042)
Scientific paper
Among the light elements that have been added to mineral physics experiments concerning the Fe-rich core of the Earth, nitrogen is less favorable. In general, this is because metal-nitrides are thought to be rare within Earth. However this may not be because they are rare, but because nitrogen is difficult to detect by conventional electron microprobe analysis unless one is specifically looking for it. Theoretically, metal-nitrides could be equally considered as potential candidates for the light element in the core, not only because nitrogen forms strong metallic bonds, but also because metal-nitrides are common constituents of many iron meteorites. Some Fe-nitrides are found to be stable at extreme pressures and temperatures corresponding to Earth's core in both diamond anvil cell and shock experiments (Adler and Williams, 2005; Sekine et al., 2007). We have discovered a metal-nitride phase, TiN (osbornite) within a mantle mineralogical assemblage, opening a new opportunity to understand the history of Earth's core formation. The TiN was found in the mantle section of an unmetamorphosed Tibetan ophiolite, a fragment of former mid-ocean spreading center, which now marks the tectonic boundary between Asia and India. The osbornite occurs as inclusions in coesite pseudomorphic after stishovite, in association with FeTi alloy, native Fe, TiO2 II, cubic BN and diamond included in Os-Ir alloy, all from a massive chromitite ore body enclosed within harzburgite (Yang et al., 2007; Dobrzhinetskaya et al., 2007). The chromite also exhibits coesite and diopside exsolution lamellae (Yamamoto et al., 2007) that might suggest the calcium-ferrite polymorph of chromite as a precursor decompressed during upwelling. Measurements of δ15N with a Cameca 50 NanoSIMS using the same Focused Ion Beam foils prepared and used for earlier TEM studies suggest that the Tibetan osbornite is characterized by negative δ15N (-10 ‰). The δ15N results from the Tibetan osbornite are somewhat more negative than the most commonly measured value for Earth's uppermost mantle (δ15N = -3 to -5 ‰), and they are clearly different from the δ15N of shallow reservoirs. The latter include atmosphere, ocean, and crust having values of delta δ15N -- 0 - +5 ‰ for the atmosphere and ocean and +5 - +12 ‰.) for the crustal rocks and sediments. We conclude that the Tibetan osbornite contains mantle N, perhaps from an old and/or deep mantle reservoir. Apropos of the suggestion of N in the core, we point out that most iron meteorites have extremely negative δ15N values of -60 ‰ or more, hence it is conceivable that part of the N signal in our materials comes from a leaky core.
Dobrzhinetskaya L.
Green Harry W.
Hutcheon Ian
Weber Philipp
Wirth Richard
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