Nitrogen isotopic signatures in Cape York: implications for formation of Group III A irons

Abstract

Nitrogen isotopic abundances and concentrations of cosmic-ray-produced ³He_c, ²¹Ne_c, and ³⁸Ar_c are reported in metal and troilite separates of the Cape York III A iron meteorite. High resolution (20 step release) nitrogen isotopic data are obtained in a 0.2 g metal chip which contains 33. 1 ppm N of δ¹⁵N(%) = - 94.8 ± 1.1. Two troilite samples each contain 1.13 ppm N with signatures δ¹⁵N(%) = - 10.7 and - 3.8, respectively. Spallogenic ²¹Ne_c is nineteen times more abundant in troilite than in metal due to cosmic ray reactions on sulphur. However, the distinct nitrogen isotopic signatures of troilite cannot be accounted for by the additional presence of spallogenic nitrogen. The nitrogen isotopic signatures of the metal for the temperature steps ≥ 1200° are within a narrow range of 4% and reveal a light N reservoir of δ ¹⁵N(%) = - 94.8% in Cape York. The nitrogen isotopic signatures of the metal and troilite phases appear to conflict with a proposed magmatic origin, unless the observed signature in troilite is due to a secondary alteration process. We discuss possible genetic links between iron meteorite groups based on nitrogen isotopic signatures in metal and oxygen isotopic signatures in silicate inclusions. Our nitrogen isotopic studies reveal artifacts due to adsorption/desorption of N₂ by reactive metal vapors deposited on the surfaces of the quartz extraction system, and we discuss an extraction protocol which reduces nitrogen loss and exchange effects.