Isotopic Constraints on the Origin of Iie Iron Meteorites

Introduction: Group IIE iron meteorites, together with groups IAB and IIICD, are classified as non-magmatic iron meteorites [1]. These are distinguished from magmatic iron meteorites in having element-Ni trends that cannot be explained by fractional crystalli-zation [2,3]. They also contain abundant silicate inclusions , which are largely absent in members of the magmatic groups. The non-magmatic irons, therefore, are not considered as samples of the metal cores of planetesimals, but more likely formed on partially differentiated bodies or within impact-generated melt pools near the surface of undifferentiated asteroids [e.g. 3]. The trace element systematics of IIE irons are complex. In log-log plots of Ga, Ge and Ir versus Ni, the IIE irons display scattered trends that are inconsistent with variations expected from fractional crystal-lization [3], yet in plots of As and Au versus Ni they seem to define a fractional crystallization trend [3,4]. Nevertheless, the small variations in W and Ir with Ni argue against an origin by fractional crystallization. The IIE iron meteorites, therefore, are interpreted to have formed in impact-generated melt pools located near the surface of a chondritic parent body [e.g. 3]. Silicate inclusions in IIE irons have similar O isotope compositions than H chondrites [5], which has led to the idea that the IIE irons formed by impact melting on the H chondrite parent body [3]. A genetic link between IIE irons and H chondrites would be consistent with similar Mo isotope anomalies reported for these two groups of meteorites [6,7]. Group IIE irons can be subdivided into two distinct age groups. One group is characterized by formation ages (mostly based on Ar-Ar) of ~4.5 Ga and cosmic ray exposure (CRE) ages of 50-600 Ma, whereas the other group shows evidence for late impact resetting at ~3.7 Ga and is characterized by younger CRE ages of 3-15 Ma [8]. Currently, the most precise constraints on the formation age of IIE irons come from Hf-W isotope systematics. The IIE irons have significantly more radiogenic W isotope compositions than magmatic iron meteorites, indicating late metal-silicate separation or re-equilibration until up to ~15 Ma after CAI formation [9]. Such young Hf-W ages would be consistent with formation of IIE irons from impact-generated melt pools. The objectives of this study are to constrain the origin and formation history of the IIE iron meteorites through a combined Ru, Mo, Pt, and W isotope study.