Shocked zircons redate Oklahoma's Ames crater to the Late Devonian, 370 million years ago

Until this study, Ames had only a fossil date ^[7]. Conodonts were small eel-like animals of the Ordovician. Their teeth turned up in the crater rock, and the impact took their age ^[7]. Catlos said the teeth were probably already millions of years old when the asteroid struck, and the impact mixed them into the debris without destroying them ^[8]. A fossil moved around like that dates the animal. It says the impact cannot be older than the teeth, and nothing about how much younger it is.

Zircon dating attacks the problem from the other side. Stockli, dean of UT's Jackson School of Geosciences and a co-author, said uranium-lead dating of zircon is the most accurate way to time events like this. He also said microstructures in the crystals can record shock pressures ^[10]. The grains came from granite at the site ^[1], so the team had to show that the ones it dated had actually been hit. A grain that escaped the shock would date something other than the impact. The team worked with NASA to image the crystals by cathodoluminescence and electron backscatter diffraction, because an impacted zircon recrystallizes in a specific way those methods can see ^[12]. Without that check, a zircon age from Ames would not be an impact age. I think it is the best-designed part of the study.

"No matter what technique we used, it was coming back to this younger signal," said Catlos, the lead author and an associate professor in UT's Department of Earth and Planetary Sciences ^[5][17]. The published account does not list those techniques or give an uncertainty on the 370-million-year figure ^[1].

The move is about 97 million years, from 467 to 370 million years ago ^[1]. That puts Ames about 2 million years from the Frasnian-Famennian mass extinction of roughly 372 million years ago, when a large share of marine life died out ^[2][9]. The researchers say the date fits that event's timing ^[9]. Whether a 2-million-year gap counts as a match depends on the error bar. A shared date also does not show cause. Catlos said it matters whether extinctions like this were driven by something extraterrestrial or by an interior force such as a series of massive volcanic eruptions ^[14]. One dated crater near the boundary does not decide between them.

The Ordovician cluster loses a member. Its impact ages center on about 467.5 million years ago ^[3]. The number of sites from that interval prompted the proposal that Earth had a Saturn-like ring of asteroid debris in the Middle Ordovician ^[4]. Catlos said the research takes a major piece out of the Ordovician Meteor Event and moves it into the Frasnian-Famennian event, which she said is where the impact belongs ^[13]. How much the ring idea loses depends on how many sites stay in the cluster and how firmly each one is dated. "It would be great to do this for more of the meteor impact sites across the continent so we could get a more accurate timeline for these major events," Stockli said ^[11].

The Ames structure lies under sedimentary strata and is a major oil and gas producer ^[2]. The rock studied came from a core. Andrew Parisi, a Jackson School graduate student who finished in 2018, collected that core from the Oklahoma Geological Survey, extracted the zircons and helped date them. He has since died ^[15]. Co-author Michael Brookfield also died before the paper appeared in Meteoritics & Planetary Science in July ^[16][6].