DIAMONDS FROM THE IRIDIUM-RICH K-T BOUNDARY-LAYER AT ARROYO EL MIMBRAL, TAMAULIPAS, MEXICO

Diamonds, up to 30 mu m in Size, were found in the iridium-rich layer from the K-T boundary sito at Arroyo El Mimbral and the spherule bed f rom Arroyo Ei Penon, northeastern Mexico. Stepped heating experiments indicate two or more isotopically distinct diamond components with car bon isotopic Compositions Characteristic of a mixture of Carbon source s, The diamonds' crystal form is cubic-not the hexagonal polymorph of diamond, lonsdaleite, which has been used previously to infer formatio n due to shock transformation of graphite. The size) crystallography, and mineralogic associations of K-T diamonds are similar to those of i mpact-produced diamonds from the Ries crater in Germany where both sho ck transformation of graphite and a mode of formation by condensation from a vapor plume have been inferred, The discovery of impact-produce d diamonds in association with high Ir contents for these sediments su pports their impact origin, K-T age, and the inference that their sour ce was from the buried impact crater of Chicxulub on the Yucatan penin sula, Mexico.