Electron spin resonance of 65-million-year-old glasses and rocks from the Cretaceous-Tertiary boundary

Glasses and rocks produced or altered by the colossal impact of an asteroid or comet with the earth 65 million years ago have been studied by electron spin resonance (ESR) and thin-section photomicroscopy. The ESR spectra rev eal the presence of 1.9 +/- 0.5 wt% single-domain (SD) titanomagnetite part icles (sizes in the range similar to 10-60 nm) in shock-melted rocks drille d from the floor of the giant Chicxulub crater ( greater than or equal to 1 80 km diameter) buried similar to 1 km below the surface of Mexico's Yucata n peninsula. These ferrimagnetic (ferrite) particles account for 37 +/- 9% of the total iron in the once-vitreous crater-floor rocks. Similar ferrites in particle sizes similar to 3 nm and amounts 0.13 +/- 0.03 wt% are found in coeval tektite glasses from Haiti. Additional SD titanomagnetite phases in particle sizes ranging from similar to 3 to similar to 4.5 nm and amount s 0.045 +/- 0.020 wt% are identified by ESR as markers of the Cretaceous-Te rtiary (WT) boundary in limestones from two sites in Spain, where their ESR intensities rise above background by factors similar to 14. The latter WT- boundary 'marker' ferrites are hypothesized to have precipitated in vitreou s materials ejected from the Chicxulub crater and to have survived aqueous weathering which converted their original glassy matrices to clays. Spherul es from K/T-boundary beds in Mexico and New Jersey were found to contain am ounts of ferrites not exceeding background levels; rather, their ESR spectr a exhibit spectral components characteristic of Fe3+ in glasses and of prev iously reported radiation-induced paramagnetic species in crystalline quart z and calcite (CaCO3). Thin-section studies, including energy-dispersive sp ectroscopy, show the Mexican spherules to be fine-particle agglomerates (co mponent grain sizes similar to 1-100 mu m) of calcite, dolomite (MgCa(CO3)( 2)), and glass, with interstitial flint (a polycrystalline form of quartz a ssumed to have been deposited hydrolytically after the spherules fell to ea rth). It is argued that the carbonate-containing spherules originally conde nsed within a 'warm fireball' composed of water vapor and similar to 2000 k m(3) of shock-vaporized limestone proposed by others to have been generated by the Chicxulub impact. (C) 1999 Elsevier Science B.V. All rights reserve d.