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 per iods (K/T boundary) has recently centred on two buried craters: the Ch icxulub in Mexico (>200 km diameter) and the Manson in Iowa (approxima tely 35 km diameter), both of which have Ar-40-Ar-39 ages of 65 Myr, i ndistinguishable from that of impact glass spherules found in K/T boun dary sediments1-4. Here we report the strontium, neodymium and oxygen isotopic compositions of core samples of impact melt rock recovered fr om 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 me lt 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 c learly different isotopic composition, strongly suggesting that they a re unrelated to the K/T impact spherules.