Radiogenic Pb in Muonionalusta Troilite and the Old Age of the IVA Asteroid

Introduction: The ages of core segregation in the parent asteroids of iron meteorites have been variably assessed, notably by the Re-Os technique [1], and the extinct radioactivities of Pd-Ag [2, 3] and Hf-W [1, 4, 5]. Because the latter chronometer suggests that iron meteorites formed within a few Ma of Allende CAIs, we set out to date these bodies using the Pb-Pb chronometers. Although iron meteorites have been found to contain the least radiogenic Pb in the Solar System, some of them contain Pb radiogenic enough to justify an investigation of their chronological potential [6, 7]. The IVA group is of magmatic origin and particularly depleted in volatiles [8]. Muonionalusta is an octahedrite find, which contains very large inclusions of troilite [9]. The presence of stishovite indicates that it has been heavily shocked, presumably during the 0.4 Ga old event associated with the shower [10].