More than four-fifths of the asteroids recovered on Earth as meteorites are ordinary chondrites—iron-poor rocks that contain small round grains, or chondrules. Over the past two decades, Birger Schmitz at Lund University in Sweden and his colleagues have assembled evidence that a subset of ordinary chondrites, known as L chondrites, are pieces of a single large asteroid that was shattered in a collision about 470 million years ago.
Now Schmitz and his team have conducted a detailed analysis of a peculiar meteorite that has the age of an L chondrite but not the composition. Österplana 065 (pictured here) was discovered in 2011 at the Thorsberg quarry in southern Sweden, where workers for a company that produces limestone floors have uncovered more than 100 L chondrites in ancient sediment. Compared with the L chondrites, Öst 65 contains far less oxygen-17 than it should based on its concentration of chromium-54;...
