Meteorite impacts created the Earth 4.5 billion years ago, and have had profound influences on evolutionary biology over time. However, the record of meteorite impacts on Earth is very fragmentary. This project addressed a simple question- can we find evidence of ancient impact events in the sedimentary record? In other words, when an impact structure erodes, are their clues in the sand grains the record the impact event? Three graduate students from the University of Puerto Rico-Mayaguez, including Stephanie Montalvo, Maya Pincus, and Pedro Montalvo, tested different aspects of this hypothesis in their MS theses. Their basic approach was to use scanning electron microscopy to search for evidence of ‘impact shocking’ preserved in sand grains of different minerals.

Stephanie Montalvo documented evidence of shock from impact in sand grains of the mineral zircon that had eroded from the giant Vredefort Dome impact structure in South Africa, and were subsequently transported nearly 2000 kilometers in modern rivers to the Atlantic coast, near the border with Namibia. These results demonstrated that the microstructure deformation of zircon that experiences an impact event is preserved, even if the shocked zircon sand gets transported 1000s of kilometres away from the original impact site.

Maya Pincus documented similar evidence of impact shock in grains of sand transported away from the Vredefort impact by glacial ice nearly 300 million years ago when Johannesburg was near the South Pole and continental ice sheets covered much of the supercontinent Gondwana. She found microstructural evidence of shock in sand grains form the Dwyka Group sediments, which represent shocked minerals entrained in and transported by continental glaciers. Her results show that shocked minerals with microstructural evidence of impact can be transported by ice, and survive ‘deep time’ preservation when the sediments they are in are buried, lithified, and become a ‘hard rock’. These findings from our studies across South Africa indicate that sedimentary rocks that are billions of years old offer excellent targets to search for sand grains with tell-tale evidence of undiscovered ancient impact events.

Pedro Montalvo approached the application of studying shocked sand grains from a different perspective. He wanted to test the idea of using detrital shocked zircon sand grains as an efficient ‘prospecting’ tool to reveal the presence of shocked bedrocks in areas where shocked rocks are suspected, but not yet confirmed. To do this, he conducted a search for detrital shocked zircon at the Santa Fe impact structure in New Mexico, USA. This impact crater is one of few in the western USA, but it is poorly preserved, and has been largely destroyed and fragmented during the complex geologic history experienced by northern New Mexico. Pedro discovered the first occurrence of shocked zircon at the Santa Fe impact structure, and used the distribution of these grains to place constraints on both the size and age of that impact event.

Additional studies at other sites conducted during this project focused on understanding the evolution of shock microstructures in zircon. We discovered the oldest occurrence of the ultra-rare high pressure mineral reidite (ZrSiO₄) in impact breccia at the Rock Elm impact structure in Wisconsin. This represented the first evidence of shocked zircon at Rock Elm, and is only the fourth locality where reidite has been found. We also conducted a study designed to search for shocked zircon at Meteor Crater in Arizona, USA. We found granular zircon, an unusual type of shocked zircon, in shock-melted Coconino sandstone that had the consistency of pumice. When the sandstone was shocked to high-pressure, the quartz melted and quenched, and the zircon grains experienced high pressure and high-temperature transformations to granular grains.

Last Modified: 02/24/2017
Modified by: Aaron Cavosie