NITROGEN ISOTOPIC SIGNATURES IN CAPE YORK - IMPLICATIONS FOR FORMATION OF GROUP-III-A IRONS

Nitrogen isotopic abundances and concentrations of cosmic-ray-produced He-3c, Ne-21c, and Ar-38c are reported in metal and troilite separate s of the Cape York III A iron meteorite. High resolution (20 step rele ase) nitrogen isotopic data are obtained in a 0.2 g metal chip which c ontains 33.1 ppm N of deltaN-15(parts per thousand) = -94.8 +/- 1.1. T wo troilite samples each contain 1.13 ppm N with signatures deltaN-15( parts per thousand) = -10.7 and -3.8, respectively. Spallogenic Ne-21c is nineteen times more abundant in troilite than in metal due to cosm ic ray reactions on sulphur. However, the distinct nitrogen isotopic s ignatures of troilite cannot be accounted for by the additional presen ce of spallogenic nitrogen. The nitrogen isotopic signatures of the me tal for the temperature steps greater-than-or-equal-to 1200-degrees ar e within a narrow range of 4parts per thousand and reveal a light N re servoir of deltaN-15(parts per thousand) = -94.8parts per thousand 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 proces s. We discuss possible genetic links between iron meteorite groups bas ed on nitrogen isotopic signatures in metal and oxygen isotopic signat ures in silicate inclusions. Our nitrogen isotopic studies reveal arti facts due to adsorption/desorption of N2 by reactive metal vapors depo sited on the surfaces of the quartz extraction system, and we discuss an extraction protocol which reduces nitrogen loss and exchange effect s.