CONSTRAINTS ON THE FORMATION CONDITIONS OF IRON-METEORITES BASED ON CONCENTRATIONS AND ISOTOPIC COMPOSITIONS OF NITROGEN

A suite of iron meteorites has been analysed for their content and isotopic composition of nitrogen to investigate the behaviour of nitrogen in the iron meteorite parent bodies and any constraints this can place on the origin and formation of these meteorites. Whole-rock samples of iron meteorites yielded a range in delta N-15 values from -96 to + 156 parts per thousand. Most of the nitrogen is found in the Fe, Ni metal phase, with areas of plessite containing ca. 3 times more nitrogen than kamacite and with a delta N-15 value 3 parts per thousand lower, although occasionally high concentrations (up to almost-equal-to 40%) of nitrogen can be found in nitrides. The nitrogen isotopic composition of the nitrides, and other phases present, is typically less than +/-5 parts per thousand of that of the whole-rock value. The effect of extreme shock and/or recrystallisation of the metal phase is to cause a loss of nitrogen and a corresponding shift in delta N-15 values towards heavier values by up to 50 parts per thousand due to diffusive loss of nitrogen. In contrast, the range of delta N-15 values shown by other members of an individual group with more normal shock features is <11 parts per thousand. For group IIIAB specimens this intragroup variation appears to be due to a slight loss of nitrogen during crystallisation, this mechanism having been taken to extreme conditions during the pallasite formation event. In the non-magmatic IAB group, individual samples display almost no isotopic variation. The nitrogen abundance in irons appears to be closely linked to the concentration of volatile trace elements and, although this may be the result of condensation effects, the high levels of nitrogen abundance may suggest that these levels were set during a re-equilibration event on the parent bodies. On the other hand, the only parameter with which delta N-15 values show any systematic covariation are the metallographic cooling rates of the irons, suggesting an indirect link between nitrogen and the internal heat source of the parent bodies. One possibility is that there may have been an injection of N-15-rich nitrogen together with a short-lived radionuclide such as Al-26, or perhaps Fe-60, into the solar nebula prior to formation of the parent bodies.