Hf-W, Sm-Nd, and Rb-Sr isotopic evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies

We report the first Sm-Nd and Rb-Sr isotopic analyses of silicate inclusion s 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 W-182/W-184 Of the host FeNi metal in all four IIEs. The host me tal in Watson has a negative Ew value (-2.21 +/- 0.24), similar to or highe r than other iron meteorites [1,35] and consistent with segregation of meta l from silicate early in solar system history. However, the large silicate inclusion in the Watson IIE iron yielded a chondritic Ew value (-0.50 +/- 0 .55), thus indicating a lack of equilibration with the FeNi host within the practical lifetime of activity of the parent Hf-182 (similar to 50 Ma). On e of the silicate inclusions in Miles is roughly chondritic in major-elemen t composition, has a present-day epsilon (Nd) Of +10.3, relatively non-radi ogenic Sr-87/Sr-86 (0.714177 +/- 13), and a T-CHUR age of 4270 Ma. Two sili cate inclusions from Weekeroo Station and one from Watson exhibit fractiona ted Sm/Nd and Rb/Sr ratios, and more radiogenic 87Sr/86Sr (0.731639 +/- 12 to 0.791852 +/- 11) and non-radiogenic epsilon (Nd) values (-5.9 to -13.4). The silicate inclusion in Watson has a TCHUR age of 3040 Ma, in agreement with previously determined He-4 and Ar-40 gas retention ages, indicative of a late thermal event. A later event is implied for the two silicate inclus ions in Weekeroo Station, which yield indistinguishable TCHUR ages of 698 a nd 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 FeN i metal parent body. The LILE-enriched nature of some of these silicates su ggests several stages of melting, mixing, and processing. However, there is little evidence to suggest that the silicates in the IIE irons were ever i n equilibrium with the host FeNi metal. (C) 2001 Elsevier Science B.V. All rights reserved.