Shock deformation and volcanism across the Cretaceous-Tertiary transition

Abstract

The cause of the Cretaceous-Tertiary (K/T) transition remains one of the most controversial scientific topics in the geosciences. Paleontologic, mineralogic, geochemical, and geophysical evidence associated with the K/T boundary have been used to argue that the extinctions were caused by meteor impact or volcanism. To assess the viability of a volcanic model for the K/T transition, a study was undertaken to: (1) determine the character of shock-induced microstructures in silicates from previously shocked experimentally; (2) complete a series of shock-recovery experiments on granite and quartzite at elevated temperatures and variable pulse duration to determine temperature- and rate-dependent effects on microstructural development; (3) determine the distribution of shocked minerals at DSDP Site 527, Walvis Ridge, and ODP Holes 689B and 690C, Maud Rise, Wedell Sea; and (4) develop a volcanic model that can account for all of the evidence at the K/T transition. Comparison of natural and experimentally-shocked quartz and feldspar using optical and transmission electron microscopy (TEM) revealed that the optical and statistical character of shock-induced microstructures in volcanic rocks are different from both classic impact microstructures, and from the Raton K/T samples. Comparison of the Manson and Raton samples reveals similarities sufficient to support the contention of Izett (1987b) that Manson is a possible source for shocked quartz in the Raton deposits. A series of 31 high-explosive (HE) shock-recovery experiments at pressures to 25 GPa and temperatures to 750°C were completed on samples of Westerly granite and Hospital Hill quartzite. TEM and optical microscopy reveal that both pre-shock temperature and pulse duration have a first-order effect on the nature and development of shock-induced microstructure in quartz and feldspar. Application of the experimental results to natural shock-induced microstructures indicates that the volcanic microstructures are probably produced at elevated temperatures and shock pressures that do not exceed 15 GPa. The results also suggest that the Raton K/T deposits probably represent a moderate impact event at pressures below about 25 GPa...