Hf–W chronometry of the IAB iron meteorite parent body

Abstract

The IAB iron meteorites are unusual in having abundant silicate inclusions, but the relationship of metal and silicates is not well understood. In order to study the sequence and the timing of events that led to the observed structure of IAB iron meteorites, the Hf–W isotope system was applied to metal and silicate separates from seven IAB iron meteorites. A major metal segregation event on the IAB parent body is dated by the average 182W/ 184W ratio of the IAB metal phases (− 3.1 ± 0.2 ε 182W-units), corresponding to a time of 1.8 + 2.3 / − 2.0 Ma after the formation of Ca,Al-rich inclusions (CAIs). The 182Hf– 182W data for three bulk inclusions from Landes, Copiapo and El Taco define a linear trend interpreted as an isochron, corresponding to an age of 2.5 + 2.3 / − 2.0 Ma after CAI formation. This age most likely dates silicate differentiation on the IAB parent body and is, within analytical uncertainty, indistinguishable from the major metal segregation event. Several silicate inclusions are displaced from the IAB silicate isochron towards less radiogenic W compositions, most likely reflecting thermal events on the parent body. Metal segregation and silicate differentiation occurred early enough, so that these processes could have at least partially been triggered by an internal heat-source (e.g., 26Al). Basaltic inclusions of the Caddo County specimen define an age of 4.6 + 0.7 / − 0.6 Ma, most likely dating a partial melting event on the IAB parent body. Three silicate-separates from another El Taco inclusion yield an age that postdates CAI formation by 10.8 + 2.4 / − 2.0 Ma and the IAB silicate isochron by ~ 8.3 Ma. This prolonged interval of local thermal events on the IAB parent body are best explained by collisional heat sources (i.e. impacts). The Hf–W ages for thermal events on the IAB parent body are within the range of previously reported 129I– 129Xe ages but are significantly older than published 87Rb– 87Sr, 147Sm– 143Nd and most 40 K– 40Ar ages. Most likely, these chronometers were reset during later thermal events that had only a minor effect on the 182Hf– 182W and 129I– 129Xe systems.