HF-W, SM-ND, AND RB-SR ISOTOPIC EVIDENCE OF LATE IMPACT FRACTIONATION AND MIXING OF SILICATES ON IRON METEORITE PARENT BODIES

Abstract

We report the first Sm-Nd and Rb-Sr isotopic analyses of silicate inclusions in four IIE iron meteorites: Miles, Weekeroo Station A and B, and Watson. We also report the Hf-W isotopic composition of a silicate inclusion from Watson and 182W/184W of the host FeNi metal in all four IIEs. The host metal in Watson has a negative εW value (-2.21+/-0.24), similar to or higher than other iron meteorites [1,35] and consistent with segregation of metal from silicate early in solar system history. However, the large silicate inclusion in the Watson IIE iron yielded a chondritic εW value (-0.50+/-0.55), thus indicating a lack of equilibration with the FeNi host within the practical lifetime of activity of the parent 182Hf (~50 Ma). One of the silicate inclusions in Miles is roughly chondritic in major-element composition, has a present-day εNd of +10.3, relatively non-radiogenic 87Sr/86Sr (0.714177+/-13), and a TCHUR age of 4270 Ma. Two silicate inclusions from Weekeroo Station and one from Watson exhibit fractionated Sm/Nd and Rb/Sr ratios, and more radiogenic 87Sr/86Sr (0.731639+/-12 to 0.791852+/-11) and non-radiogenic εNd values (-5.9 to -13.4). The silicate inclusion in Watson has a TCHUR age of 3040 Ma, in agreement with previously determined 4He and 40Ar gas retention ages, indicative of a late thermal event. A later event is implied for the two silicate inclusions in Weekeroo Station, which yield indistinguishable TCHUR ages of 698 and 705 Ma. Silicate inclusions in IIE iron meteorites formed over a period of 3 billion yr by impacts, involving an H-chondrite parent body and an FeNi metal parent body. The LILE-enriched nature of some of these silicates suggests several stages of melting, mixing, and processing. However, there is little evidence to suggest that the silicates in the IIE irons were ever in equilibrium with the host FeNi metal.