'Microdiamonds' discovered at French winery point to ancient meteor crater below the vines

A circular depression that holds a vineyard in a French winery is actually an old impact crater, new research finds. 

The crater sits in the appropriately named Domaine du Météore winery near Cabrerolles in southern France. The feature was first tentatively identified as a meteor crater in 1950. But a study in 1964 argued against the identification because the crater had no elevated rim and the scientists who authored the study could find no evidence of the kinds of magnetic field anomalies that are often found at impact sites. 

Now, a new examination of the site reveals that there is indeed a magnetic anomaly within the crater. What's more, fragments of rock in and around the depression show signs of being subjected to a hard shock: There are pockets of melted and resolidified rock, as well as microdiamonds that form under great pressure, researchers will report at the 54th Lunar and Planetary Science Conference in The Woodlands, Texas, in mid-March.

Related: What's the difference between asteroids, comets and meteors?

The Domaine du Météore crater is relatively small for an impact crater, measuring just 656 feet (200 meters) across. The crater is about 98 feet (30 m) deep, with sides covered in scrubby trees and a floor lined with neat Syrah grapevines. Frank Brenker, a geologist at Goethe University Frankfurt in Germany, and colleagues conducted the first comprehensive study of the crater over two separate visits, first taking rock samples for microanalysis and then returning to analyze the crater's magnetic field and other properties. 

The researchers found several signs of an impact, including dark-colored veins that might have been caused by a shock, as well as rock known as impact breccia, which is deformed and shows signs of having melted and resolidified into a kind of cement. The researchers also screened the soil for tiny impact spherules, and found several tiny nickel-and-iron-rich nodules that are similar to ones found in other impact craters. 

"Such microspheres form either through abrasion of the meteorite in the atmosphere or only upon impact, when a large part of the iron meteorite melts and then reacts with the oxygen in the air," Brenker said in a statement. These spherules contained microscopic flecks of diamond, which form only under high pressure.

Finally, the researchers conducted a magnetic survey of the crater and found that the magnetic field decreases closer to the center of the crater. This is also typical of impact craters, because the impact can destroy magnetic rocks or disrupt their magnetism by realigning the atoms responsible for creating the magnetic field in the first place, the researchers said. 

The researchers also conducted geoelectric analysis of the crater, because the deformation of rocks during an impact can affect the way those rocks conduct electricity. Those data are still being analyzed. 

The new research did not give an estimate of the crater's age. However, the winery website estimates that the crater impact occurred around 10,000 years ago. 

Impact craters are rare on Earth because they're typically erased by erosion and the ever-shifting movements of tectonic plates. The Earth Impact Database lists only 190 confirmed craters across the globe. Small craters like the one at the Domaine du Météore winery are particularly likely to be wiped away; according to the new research's abstract, there are only three known craters with a diameter of less than 984 feet (300 m).