A Paleoarchaean Impact Crater in the Pilbara Craton, Western Australia
upstart writes:
A Paleoarchaean impact crater in the Pilbara Craton, Western Australia:
The role of meteorite impacts in the origin, modification, and destruction of crust during the first two billion years of Earth history (4.5-2.5 billion years ago; Ga) is disputed. Whereas some argue for a relatively minor contribution overall, others have proposed that individual giant impactors (10-50km diameter) can initiate subduction zones and deep mantle plumes, arguably triggering a chain of events that formed cratons, the ancient nuclei of the continents. The uncertainty is compounded by the seeming absence of impact structures older than 2.23Ga, such that the evidence for the terrestrial impact flux in the Hadean and Archaean eons is circumstantial. Here, we report the discovery of shatter cones in a complex, dominantly metasedimentary layer, the Antarctic Creek Member (ACM), in the centre of the East Pilbara Terrane, Western Australia, which provide unequivocal evidence for a hypervelocity meteorite impact. The shocked rocks of the crater floor are overlain by (unshocked) carbonate breccias and pillow lavas, stratigraphically constraining the age of the impact to 3.47Ga and confirming discovery of the only Archaean crater known thus far.
With more than a million craters exceeding 1km in diameter, and around forty more than 100km across1,2, the Moon preserves an exquisite record of the intense bombardment endured by bodies in the inner solar system during the first billion years or so of its history (Fig.1a)3. On Earth, this early impact record has seemingly been lost, reflecting the destructive efficiency of erosion and subduction in recycling primary (basaltic, oceanic) crust back into the convecting mantle. Nevertheless, the oldest parts of many cratons, the ancient Archaean (4.0-2.5 billion years ago; Ga) nuclei of the continents, formed at or before 3.5Ga4, and should preserve some evidence for an impact flux that would have exceeded that of a similar area of the Moon of comparable age5,6,7 (Fig.1a). However, the oldest recognized terrestrial impact structure, at Yarrabubba, Western Australia, is dated at 2.23Ga8. Where are all the Archaean craters?
Finding direct evidence for Archaean impacts (i.e., craters or impact structures8), and thereby better constraining the Archaean impact flux, is important. Large impactors (here bodies or 10km in diameter) travelling in excess of 10km.s-1 deliver enormous quantities of kinetic energy, most of which will decay to heat, warming the crust and upper mantle9, with potential consequences for plausible tectonic modes on the early Earth10,11. Further, numerical models have shown that individual bolide impacts can instigate subduction, mantle upwellings (plumes), and voluminous production of primary (basaltic) crust12,13,14. Moreover, impacts provide a ready mechanism to fracture (brecciate) the crust and, in the presence of a hydrosphere15, drive intense hydrothermal alteration of this regolith, concentrating key mineral deposits16. Notably, impact craters may have provided the physical and chemical environments required for life to emerge on Earth and elsewhere17,18.
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