The early formation of the IVA iron meteorite parent body
Résumé
The IVA iron meteorites are magmatic cumulates from the core of a small asteroid, which broke apart similar to 400 Ma ago. As the depletion of this planetary body in volatile elements is expected to be reflected in high U/Pb ratios of its minerals, we embarked on analyzing the isotope composition of Pb in the cm-sized troilite inclusions of Muonionalusta and Gibeon, both iron meteorites belonging to the IVA family. The bulk of the data for nine troilite subsamples of Muonionalusta scatter with an apparent age of 4.57 Ga, which is shown to reflect the presence of both primordial and common terrestrial Pb components, in addition to radiogenic Pb. The most radiogenic subsample, however, has fractions with Pb-206/Pb-204 ratios as high as over 1000 and gives a statistically significant (207)pb*/Pb-206* age of 4565.3 +/- 0.1 Ma (MSWD = 0.08), consistent with the Hf-182-W-182 metal-silicate segregation age of 2.4 +/- 2.0 Ma. These data make the age of Muonionalusta the oldest documented yet for all differentiated bodies in the Solar System and constitute the first high-precision Pb-Pb age determined for crystallization of a phase contained within an iron meteorite group, hence advancing our understanding of early Solar System chronology. Using literature values for the cooling rate, and assuming a closure temperature for Pb of 300 degrees C, it is further estimated that the IVA parent body accreted within 1 Ma of CAI formation and had cooled to the Pb closure temperature within an additional 1-2 Ma. The overlap between the high-precision Pb-Pb and Hf-W ages points to a small, or rapidly fragmented, planetary body. The isotopic composition of Pb in Gibeon troilite yields a significantly younger age of 4544 +/- 7 Ma (MSWD = 1.5), consistent with evidence from the Pd-107-Ag-107 chronometer, but we believe this age has been reset by melting upon shock. One puzzling observation is that the apparent Th-232/U-238 of Muonionalusta troilite is particularly low (similar to 0.32), requiring a mechanism capable of efficiently fractionating Th from U, presumably the reduction of U to its trivalent form or the crystallization of phosphate. Average Pb concentrations of the order of 5-10 ppb and high U-238/Pb-204 ratios of > 1000 require U concentrations in troilite in the sub-ppb to one ppb range. This may indicate that troilite inclusions in IVA meteorites do not represent metal-sulfide unmixing, but rather correspond to late-stage S-rich liquid residues from extreme crystallization of the interstitial melts.