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Diamonds from the iridium-rich K-T boundary layer at Arroyo el Mimbral, Tamaulipas, Mexico

¹ Planetary Sciences Research Institute, The Open University, Milton Keynes, MK7 6AA, United Kingdom
² Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10115 Berlin, Germany
³ Osservatorio Geologico di Coldigioco, I-62020, Frontale di Apiro (MC), Italy
⁴ École des Mines de Paris, Boulevard Saint-Michel, 75272, Paris Cedex 06, France

Diamonds, up to 30 µm in size, were found in the iridium-rich layer from the K-T boundary site at Arroyo El Mimbral and the spherule bed from Arroyo El Peñon, northeastern Mexico. Stepped heating experiments indicate two or more isotopically distinct diamond components with carbon isotopic compositions characteristic of a mixture of carbon sources. The diamonds' crystal form is cubic—not the hexagonal polymorph of diamond, lonsdaleite, which has been used previously to infer formation due to shock transformation of graphite. The size, crystallography, and mineralogic associations of K-T diamonds are similar to those of impact-produced diamonds from the Ries crater in Germany where both shock transformation of graphite and a mode of formation by condensation from a vapor plume have been inferred. The discovery of impact-produced diamonds in association with high Ir contents for these sediments supports their impact origin, K-T age, and the inference that their source was from the buried impact crater of Chicxulub on the Yucatan peninsula, Mexico.

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