¹Fridolin Spitzer,¹Christoph Burkhardt,¹Jonas Pape,¹Thorsten Kleine
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13744]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
Published by arrangement with John Wiley & Sons

The ungrouped iron meteorite Nedagolla is the first meteorite with bulk Mo, Ru, and Ni isotopic compositions that are intermediate between those of the noncarbonaceous (NC) and carbonaceous (CC) meteorite reservoirs. The Hf-W chronology of Nedagolla indicates that this mixed NC–CC isotopic composition was established relatively late, more than 7 Myr after solar system formation. The mixed NC–CC isotopic composition is consistent with the chemical composition of Nedagolla, which combines signatures of metal segregation under more oxidizing conditions (relative depletions in Mo and W), characteristic for CC bodies, and more reducing conditions (high Si and Cr contents), characteristic for some NC bodies, in a single sample. These data combined suggest that Nedagolla formed as the result of collisional mixing of NC and CC core material, which partially re-equilibrated with silicate mantle material that predominantly derives from the NC body. These mixing processes might have occurred during a hit-and-run collision between two differentiated bodies, which also provides a possible pathway for Nedagolla’s extreme volatile element depletion. As such, Nedagolla provides the first isotopic evidence for early collisional mixing of NC and CC bodies that is expected as a result of Jupiter’s growth.

¹Joseph I. Goldstein,²Edward R. D. Scott,¹Timothy B. Winfield,^3,4Jijin Yang
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13745]
¹Department of Mechanical and Industrial Engineering, University of Massachusetts, Amherst, Massachusetts, 01003 USA
²Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, 96822 USA
³Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029 China
⁴College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing, 100049 China
Published by arrangement with John Wiley & Sons

Metallographic cooling rates of 10–20 K Myr−1 were obtained for four IAB iron meteorites from electron probe analyses of taenite and models simulating kamacite growth. Cooling rates were also determined for 21 irons in the IAB complex from the size of tetrataenite particles in the cloudy zone and a calibration curve using data for the four IAB irons and five other iron and stony iron groups. We find that the closely related IAB main group, low-Au subgroups, Pitts grouplet irons, and San Cristobal cooled through 500 °C at 10–35 K Myr−1. The ungrouped IAB complex irons, Santa Catharina and Twin City, have bulk Ni contents of ~30 wt% and cooled much faster at ~200–1000 K Myr−1, most probably in a different parent body. Our cooling rates and observations allow conflicting interpretations of the nature of the low-Ni phase in the cloudy zone to be reconciled. We infer that in fast cooled irons like Santa Catharina, the low-Ni phase transforms to antitaenite, but in very slowly cooled meteorites like mesosiderites, martensite forms instead. In meteorites with intermediate cooling rates like IAB irons, the bulk of the cloudy zone probably contains antitaenite as the low-Ni phase, but in the coarse region next to tetrataenite, martensite is present. Published isotopic ages show that metal–silicate mixing in group IAB irons occurred ~5–10 Myr after CAI formation, but the nature and timing of the impact are poorly constrained at present. Ar-Ar ages testify to shock reheating of plagioclase during cooling over ~100 Myr. However, the cloudy zone is well preserved showing that metal was not heated significantly by shocks after slow cooling through 400 °C.