Nitrogen Isotopic Variation in Irons and Other Fe-Ni Metal Rich Meteorites

Franchi, Ian (1988). Nitrogen Isotopic Variation in Irons and Other Fe-Ni Metal Rich Meteorites. PhD thesis The Open University.



An investigation of the nitrogen concentration, its distribution, location and isotopic composition in iron meteorites and the anomalous metal-rich mesosiderite Bencubbin has been undertaken with the use of a high sensitivity mass spectrometer. Existing stepped heating extraction systems were modified for the types of material analysed in this study and a laser microprobe technique was developed for the purpose of analysing well characterised sample areas of the Bencubbin meteorite with high spatial resolution.

The irons display a range in values which is to too large to be the result of fractionation processes in the nebula. An explanation involving primordial heterogeneity is favoured, with the iron meteorite parent bodies having sampled at least four isotopically distinct nitrogen reservoirs. One of the factors controlling the variation may be an input of 1%-rich nitrogen, together with 26Al, from a nova event prior to solar nebula collapse. The behaviour of nitrogen during core formation processes is also considered and compared with the observed nitrogen variation in the non-magmatic group lAB. Other secondary processes affecting nitrogen in the iron meteorites are also identified. The isotopic composition of nitrogen has been used to identify genetic links between iron and stony meteorites, thereby enhancing the normal technique based on oxygen isotopic composition (prohibited by the rarity of oxygen bearing minerals).

A second factor influencing the primordial nitrogen isotopic variability has been identified in the unusual polymict breccia Bencubbin. This meteorite contains two nitrogen components, Na and Np, with a value = +1000%o (twice the content of atmospheric nitrogen), present in the metal clasts and probably in the silicate clasts and matrix also. The carrier phase of Na appears to be a chromium-rich sulphide ((Cro.67» ^60,33)283) with a structure similar to pseudo-hexagonal pyrrhotite, containing sulphur slightly depleted in Np may be associated with carbon showing a small enrichment of The data gathered so far indicate that these components formed from a supernova.

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