THE IAB IRON-METEORITE COMPLEX: A GROUP, FIVE SUBGROUPS, NUMEROUS GROUPLETS, CLOSELY RELATED, MAINLY FORMED BY CRYSTAL SEGREGATION IN RAPIDLY COOLING MELTS

Abstract

We present new data for iron meteorites that are members of group IAB or are closely related to this large group, and we have also reevaluated some of our earlier data for these irons. In the past it was not possible to distinguish IAB and IIICD irons on the basis of their positions on element-Ni diagrams, but we now show that plotting the new and revised data yields six sets of compact fields on element-Au diagrams, each set corresponding to a compositional group. The largest set includes the majority (~70) of irons previously designated IA; we christened this set the IAB main group. The remaining five sets we designate ''subgroups'' within the IAB complex. Three of these subgroups have Au contents similar to the main group, and form parallel trends on most element-Ni diagrams. The groups originally designated IIIC and IIID are two of these subgroups; they are now well resolved from each other and from the main group. The other low-Au subgroup has Ni contents just above the main group. Two other IAB subgroups have appreciably higher Au contents than the main group and show weaker compositional links to it. We have named these five subgroups on the basis of their Au and Ni contents. The three subgroups having Au contents similar to the main group are the low-Au (L) subgroups, the two others the high-Au (H) subgroups. The Ni contents are designated high (H), medium (M), or low (L). Thus the old group IIID is now the sLH subgroup, the old group IIIC is the sLM subgroup. In addition, eight irons assigned to two grouplets plot between sLL and sLM on most element-Au diagrams. A large number (27) of related irons plot outside these compact fields but nonetheless appear to be sufficiently related to also be included in the IAB complex.Many of these irons contain coarse silicates having similar properties. Most are roughly chondritic in composition; the mafic silicates show evidence of reduction during metamorphism. In each case the silicate O-isotopic composition is within the carbonaceous chondrite range (Δ17O =< -0.3%%). In all but four cases these are within the so-called IAB range, -0.30 =< Δ17O =< -0.68%%. Fine silicates appear to be ubiquitous in the main group and low-Au subgroups; this requires that viscosities in the parental melt reached high values before buoyancy could separate these.The well-defined main-group trends on element-Au diagrams provide constraints for evaluating possible models; we find the evidence to be most consistent with a crystal segregation model in which solid and melt are essentially at equilibrium. The main arguments against the main group having formed by fractional crystallization are: a) the small range in Ir, and b) the evidence for rapid crystallization and a high cooling rate through the γ-iron stability field. The evidence for the latter are the small sizes of the γ-iron crystals parental to the Widmanstatten pattern and the limited thermal effects recorded in the silicates (including retention of albitic plagioclase and abundant primordial rare gases). In contrast, crystal segregation in a cooling metallic melt (and related processes such as incomplete melting and melt migration) can produce the observed trends in the main group. We infer that this melt was formed by impact heating on a porous chondritic body, and that the melt was initially hotter than the combined mix of silicates and metal in the local region; the melt cooled rapidly by heat conduction into the cooler surroundings (mainly silicates). We suggest that the close compositional relationships between the main group and the low-Au subgroups are the result of similar processes instigated by independent impact events that occurred either at separate locations on the same asteroid or on separate but compositionally similar asteroids.