^1,2Gavin G.Kenny,²Claire O.Harrigan,²Mark D.Schmitz,²James L.Crowley,²Corey J.Wall,³Marco A.G.Andreoli,³Roger L.Gibson,⁴Wolfgang D.Maier
Earth and Planetary Science Letters 567, 117013 Link to Article [https://doi.org/10.1016/j.epsl.2021.117013]
¹Department of Geosciences, Swedish Museum of Natural History, SE-104 05 Stockholm, Sweden
²Isotope Geology Laboratory, Department of Geosciences, Boise State University, 1910 University Drive, Boise, ID 83725, USA
³School of Geosciences, University of the Witwatersrand (WITS), Johannesburg, South Africa
⁴School of Earth and Ocean Sciences, Cardiff University, Park Place, Cardiff CF10 3AT, UK
Copyright Elsevier

Impact cratering was a fundamental geological process in the early Solar System and, thus, constraining the timescales over which large impact structures cool is critical to understanding the thermal evolution and habitability of early planetary crusts. Additionally, impacts can induce mass extinctions and establishing the precise timing of the largest impacts on Earth can shed light on their role in such events. Here we report a high-precision zircon U–Pb geochronology study of the Morokweng impact structure, South Africa, which appears to have a maximum present-day diameter of ∼80 km. Our work provides (i) constraints on the cooling of large impact melt sheets, and (ii) a high-precision age for one of Earth’s largest impact events, previously proposed to have overlapped the ca. 145 Ma Jurassic–Cretaceous (J–K) boundary. High-precision U–Pb geochronology was performed on unshocked, melt-grown zircon from five samples from a borehole through approximately 800 m of preserved impact melt rock. Weighted mean 206Pb/238U dates for the upper four samples are indistinguishable, with relative uncertainties (internal errors) of better than 20 ka, whereas the lowermost sample is distinguishably younger than the others. Thermal modeling suggests that the four indistinguishable dates are consistent with in situ conductive cooling of melt at this location within 30 kyr of the impact. The younger date from the lowest sample cannot be explained by in situ conductive cooling in line with the overlying samples, but the date is within the ∼65 kyr timeframe for melt-present conditions in footwall rocks below the impact melt sheet that is indicated by our thermal model. The Morokweng impact event is here constrained to 146.06 ± 0.16 Ma (2σ; full external uncertainty), which precedes current estimates of the age of the J–K boundary by several million years.

¹Laurette Piani,¹Yves Marrocchi,²Lionel G.Vacher,³Hisayoshi Yurimoto,⁴Martin Bizzarro
Earth and Planetary Science Letters 567, 117008 Link to Article [https://doi.org/10.1016/j.epsl.2021.117008]
¹Université de Lorraine, CNRS, CRPG, UMR 7358, Vandoeuvre les Nancy, France
²Laboratory for Space Sciences and the Department of Physics, Washington University in St. Louis, St. Louis, MO 63130, USA
³Department of Natural History Sciences, Faculty of Science, Hokkaido University, Sapporo, Japan
⁴StarPlan – Centre for Star and Planet Formation, GLOBE Institute, University of Copenhagen, Øster Voldgade 5-7, Copenhagen, DK-1350, Denmark
Copyright Elsevier

Chondrites are rocky fragments of asteroids that formed at different times and heliocentric distances in the early solar system. Most chondrite groups contain water-bearing minerals, attesting that both water-ice and dust were accreted on their parent asteroids. Nonetheless, the hydrogen isotopic composition (D/H) of water in the different chondrite groups remains poorly constrained, due to the intimate mixture of hydrated minerals and organic compounds, the other main H-bearing phase in chondrites. Building on our recent works using in situ secondary ion mass spectrometry analyses, we determined the H isotopic composition of water in a large set of chondritic samples (CI, CM, CO, CR, and C-ungrouped carbonaceous chondrites) and report that water in each group shows a distinct and unique D/H signature. Based on a comparison with literature data on bulk chondrites and their water and organics, our data do not support a preponderant role of parent-body processes in controlling the D/H variations among chondrites. Instead, we propose that the water and organic D/H signatures were mostly shaped by interactions between the protoplanetary disk and the molecular cloud that episodically fed the disk over several million years. Because the preservation of D-rich interstellar water and/or organics in chondritic materials is only possible below their respective sublimation temperatures (160 and 350–450 K), the H isotopic signatures of chondritic materials depend on both the timing and location at which their parent body formed.