A nonmagmatic origin of group-IIE iron meteorites

Abstract

New neutron activation data on 10 elements in 12 IIE and IIE-related irons lead to a reclassification of several irons. Seymchan and Lonaconing are removed from IIE, and Leshan added. Four IIE members are designated IIE-An to call attention to some anomalous properties. The eight normal IIE members define element-Ni trends generally similar to those in the nonmagmatic group IAB; the small negative slopes on W-Ni and Ir-Ni diagrams are strongly indicative of a nonmagmatic origin of the IIE irons. We propose that IIE irons like IAB irons originated as individual pools of impact-produced melt in the near-surface region of a chondritic parent body. The positive As-Ni and Au-Ni trends are the only evidence suggesting fractional crystallization, but their slopes are lower than those in magmatic group IIIAB, and only slightly higher than those of Cu and Sb in IAB. We suggest that the S and C contents of the IIE precursor materials were much lower than those of the IAB precursors, thus higher temperatures were required to generate enough metallic melt to segregate into pools. These higher temperatures are also reflected in the nonchondritic compositions of the silicate inclusions.