ISOTOPIC COMPARISON OF K/T BOUNDARY IMPACT GLASS WITH MELT ROCK FROM THE CHICXULUB AND MANSON IMPACT STRUCTURES

THE discussion of candidate source craters for the catastrophic impact that occurred at the boundary between the Cretaceous and Tertiary periods (K/T boundary) has recently centred on two buried craters: the Chicxulub in Mexico (>200 km diameter) and the Manson in Iowa (approximately 35 km diameter), both of which have 40Ar-39Ar ages of 65 Myr, indistinguishable from that of impact glass spherules found in K/T boundary sediments1-4. Here we report the strontium, neodymium and oxygen isotopic compositions of core samples of impact melt rock recovered from drill holes into both the Chicxulub and Manson craters, and compare these with previously published isotopic data for impact glasses from the K/T boundary of the Beloc Formation in Haiti5-7. The Chicxulub melt rocks are isotopically indistinguishable from the K/T impact glass, strongly supporting the proposition that Chicxulub is a source crater for the K/T catastrophe. The Manson melt rocks, by contrast, have a clearly different isotopic composition, strongly suggesting that they are unrelated to the K/T impact spherules.