The Estherville mesosiderite: U Pb, Rb Sr, and Sm Nd isotopic study of a polymict breccia
A systematic U Pb, Sm Nd, and Rb Sr isotopic study shows that the Estherville mesosiderite was formed between 4.56 and 4.43 Ga. Observed isotopic heterogeneity is in agreement with multiple generations of meteoritic impacts described in other mesosiderites. At least part of the Estherville silicate fraction was formed early in solar system history as indicated by the Pb-Pb (4555 ± 35 Ma), U Pb (4560 ± 31 Ma), Rb Sr (4542 ± 203 Ma), and Sm Nd (4533 ± 94 Ma) ages. Mesosiderites therefore present not only petrological but also geochronological similarities with eucrites. The Pb isotopic composition of the metal phase plots on the same isochron as the silicates, indicating formation and subsequent mixing with silicates early in the history of the solar system. This is consistent with previous observations indicating that iron was reduced during the silicate-magmatic stage, most likely a consequence of mixing with metal. In addition to these more-ancient portions of the Estherville breccia, other parts were formed later as suggested by the Pb-Pb (4422 ± 50 Ma) and U Pb (4437 ± 11 Ma) ages observed in a second group of leaches and residues. This age is similar to some cumulate eucrite ages and may represent the formation of a second mesosiderite component. The Sm Nd and the Rb Sr ages obtained on Estherville show large errors that may be a consequence of the mixing between the 4.56 and 4.43 Ga endmembers. Estherville, like most mesosiderites, was affected by a major heating event around 3.5-3.7 Ga as shown by the Ar-Ar ages ( Bogard et al., 1990). This heating event partially disturbed the Rb Sr isotopic system (Rb Sr metamorphic ages range between 3.81 and 4.08 Ga).