^1,2Richard A. F. Grieve,^1,2Gordon R. Osinski
Meteorits & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13542]
¹Department of Earth Sciences, University of Western Ontario, London, Ontario, N6A 5B7 Canada
²Institute for Earth and Space Exploration, University of Western Ontario, London, Ontario, N6A 5B7 Canada
Published by arrangement with John Wiley & Sons

Observational and logical arguments are presented for the lithology formerly named the Garson Member of the Onaping Formation being the clast‐bearing, fine‐grained, chilled Upper Contact Unit (UCU) of the Sudbury Igneous Complex (SIC) in the Garson region of the Sudbury impact structure. It differs considerably, however, from the UCU in the North Range of the SIC with respect to the character of its clasts. Namely, the clasts are essentially monomict (quartzites), much larger (up to 100 m across), and much more abundant (up to 80% in places). These differences indicate a different source than “fallback” material for the clasts in the UCU in the Garson region. Their character requires a “coherent,” singular source that was topographically above the SIC melt pool. Such a source would correspond to that of an emergent peak ring of fractured target rocks. The clasts are identified as Huronian Mississagi quartzite, which is estimated to have been at a nominal depth of 7.5 ± 2.5 km at the time of impact. This provides a constraint on the depth of origin of the peak ring. This depth estimate is closest to the lower depth estimate from current numerical models of Sudbury and the similar‐sized Chicxulub impact structures.

¹M.Matthes,²J.A.van Orman,¹T.Kleine
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2020.07.009]
¹Institut für Planetologie, University of Münster, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany
²Department of Earth, Environmental and Planetary Sciences, Case Western Reserve University, Cleveland, OH USA
Copyright Elsevier

To better constrain the crystallization and cooling history of the IIIAB iron meteorite parent body, we report new ¹⁰⁷Pd-¹⁰⁷Ag data for metal and troilite samples from the IIIAB iron Cape York, and combine these data with a numerical model for the diffusive exchange of ¹⁰⁷Ag between metal and troilite. We find that the Pd-Ag closure temperature for iron meteorites varies between 500 and 700 °C, and for most irons typically is between 550 and 650 °C. The closure temperature not only depends on cooling rate, grain size, and bulk Ni content, but also on the abundance and distribution of troilite nodules. Specifically, metal in direct contact to troilite has a lower closure temperature than more distant metal. Consistent with this, our new Pd-Ag data show that metals adjacent to troilites have lower Ag contents and plot on shallower Pd-Ag isochrons than more distant metals. These disparate Pd-Ag systematics in metal as a function of distance to troilite provide a new means to determine cooling rates for iron meteorites. Using this approach, we obtained a cooling rate of 67–202 °C/Ma for Cape York, which is in good agreement with metallographic cooling rates for IIIAB irons. This cooling rate combined with the precise Pd-Ag age of Cape York of 5.0±0.4 Ma after solar system formation reveals that the IIIAB core completely solidified at 2.6±1.3 Ma after solar system formation. This rapid crystallization was most likely facilitated by collisional disruption of the IIIAB parent body, which removed most of the insulating mantle and exposed its core.