Electron spin resonance of 65-million-year-old glasses and rocks from the Cretaceous Tertiary boundary
Abstract
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 reveal the presence of 1.9 ± 0.5 wt% single-domain (SD) titanomagnetite particles (sizes in the range ∼10-60 nm) in shock-melted rocks drilled from the floor of the giant Chicxulub crater (⩾180 km diameter) buried ∼1 km below the surface of Mexico's Yucatán 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 ∼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 ∼3 to ∼4.5 nm and amounts 0.045 ± 0.020 wt% are identified by ESR as markers of the Cretaceous-Tertiary (K/T) boundary in limestones from two sites in Spain, where their ESR intensities rise above background by factors ∼14. The latter K/T-boundary `marker' ferrites are hypothesized to have precipitated in vitreous materials ejected from the Chicxulub crater and to have survived aqueous weathering which converted their original glassy matrices to clays. Spherules from K/T-boundary beds in Mexico and New Jersey were found to contain amounts of ferrites not exceeding background levels; rather, their ESR spectra exhibit spectral components characteristic of Fe 3+ in glasses and of previously reported radiation-induced paramagnetic species in crystalline quartz and calcite (CaCO 3). Thin-section studies, including energy-dispersive spectroscopy, show the Mexican spherules to be fine-particle agglomerates (component grain sizes ∼1-100 μm) of calcite, dolomite (MgCa(CO 3) 2), and glass, with interstitial flint (a polycrystalline form of quartz assumed to have been deposited hydrolytically after the spherules fell to earth). It is argued that the carbonate-containing spherules originally condensed within a `warm fireball' composed of water vapor and ∼2000 km 3 of shock-vaporized limestone proposed by others to have been generated by the Chicxulub impact.
- Publication:
-
Journal of Non Crystalline Solids
- Pub Date:
- August 1999
- DOI:
- 10.1016/S0022-3093(99)00340-3
- Bibcode:
- 1999JNCS..253....1G