Iron-60 in Troilites from an Unequilibrated Ordinary Chondrite and the Initial

Introduction: Short-lived radionuclides were synthesized either by irradiation with energetic particles or by stellar nucleosynthesis. The recent discovery in CAIs from carbonaceous chondrites of evidence for live 10 Be [1], which is only produced by energetic-particle irradiation , has led to suggestions that the major portion of many short-lived radionuclides (was produced by irradiation. However, 60 Fe (t1/2 = 1.49 My) is not produced at all efficiently by particle irradiation [e.g., 2], so its presence requires a stellar contribution to short-lived radionuclides. Clear evidence for 60 Fe was found previously in eucrites [3-5], but not in chon-drites [6-8]. In this work, we studied troilite to find evidence for ÒliveÓ 60 Fe in one of the least metamorphosed ordinary chondrites, Bishunpur (LL3.1). Analyses: We screened 177 troilite grains in Bishun-pur thin sections by SEM to find grains with high Fe/Ni. Iron and Ni isotopes were measured for ten selected troilites using the ASU Cameca ims-6f ion microprobe. Isotopic measurements were made with a 3-4 nA O-primary beam (10-20 µm) accelerated to 12.5 kV. The secondary-ion mass spectrometer was operated at an accelerating voltage of 8.85 kV with an energy window of 50 eV. A 400-µm contrast aperture and a 75-µm image field were used. Secondary 57 Fe