Low‐Ir <scp>IAB</scp> irons from Morasko and other locations in central Europe: One fall, possibly distinct from <scp>IAB</scp>‐<scp>MG</scp>

Abstract

Abstract Differences in texture and discovery location prompted us to analyze 16 irons from M orasko; one from S eeläsgen, known to have a similar composition; and a new mass found at J ankowo D olne. These were analyzed in duplicate by instrumental neutron‐activation analysis ( INAA ). The results show that all 18 samples have very similar compositions, distinct from all other IAB irons except B urgavli; we conclude that they are all from a single shower. Eight of the samples were from regions with large amounts of cohenite (but were largely free of inclusions) and six were from samples with very little cohenite; we could find no resolvable difference in composition between these sets, a fact that suggests that the C contents of the metal phases were similar in the two areas. Although M orasko has been classified into the IAB main group ( IAB ‐ MG ), its I r plots well outside the main group field on an I r‐ A u diagram. We considered the possibility that the low I r reflected contamination by a melt from a IAB region that ponded and experienced fractional crystallization; however, because M orasko has P t, W , and G a values that are the same as the highest values in IAB ‐ MG , we rejected this model. We therefore conclude that M orasko formed from a different melt than the IAB ‐ MG irons; the M orasko melt was produced by impact heating, but one or more of the main I r carriers did not melt, leaving much of the I r in the unmelted residue. Copper is the only element that shows resolvable differences among M orasko samples. Most (13 of 18) samples have 149 ± 4 μg g −1 Cu, but three have 213 ± 10 μg g −1 ; we interpret this to mean that the low‐ C u samples have equilibrated with a C u‐rich phase, whereas there was none of the latter phase within a few diffusion lengths of the samples with high C u contents.