Erickson, and 4 co-authors, report that the recently recognized ~70 km-diameter Yarrabubba impact structure in Western Australia is regarded as among Earth’s oldest, but has hitherto lacked precise age constraints. Here we present U–Pb ages for impact-driven shock-recrystallized accessory minerals. Shock-recrystallized monazite yields a precise impact age of 2229 ± 5 Ma, coeval with shock-reset zircon. This result establishes Yarrabubba as the oldest recognized meteorite impact structure on Earth, extending the terrestrial cratering record back >200 million years.
The age of Yarrabubba coincides, within uncertainty, with temporal constraint for the youngest Palaeoproterozoic glacial deposits, the Rietfontein diamictite in South Africa. Numerical impact simulations indicate that a 70 km-diameter crater into a continental glacier could release between 8.7 × 1013 to 5.0 × 1015 kg of H2O vapor instantaneously into the atmosphere. These results provide new estimates of impact-produced H2O vapor abundances for models investigating termination of the Paleoproterozoic glaciations, and highlight the possible role of impact cratering in modifying Earth’s climate.
The authors concluded that to better constrain the environmental effects of a Yarrabubba-sized impact structure into a continental ice sheet, the formation of various impact craters were computed using the iSALE shock physics code, which is based on the original SALE algorithm. The initial conditions were a granitic target covered by an ice sheet of varying thickness (2–5 km). For all ice thicknesses considered here, an impactor diameter of 7 km produced a final crater of ~70 km diameter. The impactor was prescribed an impact velocity of 17 km/s, the current best estimate for the average speed of bolides striking Earth. The ANEOS equation of state for granite was used to model both the impactor and the granitic target and the 5-phase SESAME equation of state for ice was used to model the overlying ice sheet. A computational cell size of 125 m was used, resulting in a resolution of 28 cells per projectile radius.
The end product was an impact crater with a ~70 km diameter impact structure, with a zone 40–50 km in diameter in the granite that experienced high shock pressures (>50 GPa) and is heavily fractured, which in some simulations produces an uplifted region. The mass of ice that vaporized during the impact was determined by comparing the entropy of Lagrangian tracer particles in the ice sheet to the critical entropy for incipient (entropy required to go to liquid and vapor state) and complete vaporization (entropy required to go to a completely vapor state) for ice.
The authors reported originally that the shocked zircon and monazite age results establish Yarrabubba to be 2229 ± 5 Ma, ~ 200 Myrs older than the Vredefort Dome, the previously oldest known impact structure on Earth. Furthermore, the impact age determined here demonstrates coincidence between the Yarrabubba event and the termination of the youngest diamictite deposit associated with the Palaeoproterozoic snowball Earth. The Rietfontein diamictite, the uppermost glacial deposit in the Transvaal basin of South Africa, is constrained to have a minimum depositional age of 2225 ± 3 Ma based on the age of the overlying Hekpoort basalt.. Termination of the Palaeoproterozoic snowball Earth is also simultaneous with a global shift to highly positive δ13C ‰ values in carbonates, known as the Lomagundi-Jatuli excursion, which has a best estimated maximum age of 2221 ± 5 Ma.
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