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Abstract
The process of impact cratering has been ongoing in the solar system since its beginning, over 4.5 billion years ago. Shock alteration, crater formation processes, and post-impact hydrothermal alteration causes changes to crustal rocks that can resurface a planet. The alteration of crustal rocks may form unique environments which may have harbored early life on Earth or elsewhere in the solar system. Within this work we analyzed Fe-oxide and Fe-sulfide minerals within impact crater rocks drilled from the peak ring of the Chicxulub impact structure in Mexico by the International Ocean Discovery Program/International Continental Scientific Drilling Program (IODP/ICDP) Expedition 364. Lithologies examined include uplifted target rocks composed of granitoid rocks crosscut by pre-impact and impact related intrusions, impact melt rocks and suevite. The effects of impact cratering, such as heat and shock, can reset primary magnetizations recorded in Fe-oxides and can produce new magnetizations in Fe-oxides formed from impact melted rocks or post-impact hydrothermal alteration. We, therefore, use paleomagnetic and rock magnetic experiments to unravel the history recorded in the Fe-oxides of the lower peak ring granitoid rocks and upper peak ring impact melt and suevite rocks from the Chicxulub crater. Additionally, we use petrographic imaging and quantitative analytical techniques of Scanning Electron Microscopy with Energy Dispersive X-Ray Spectroscopy (SEM/EDS) and Electron Probe Microanalyses (EPMA) to document both primary Fe-oxide and Fe-sulfide minerals and secondary minerals produced by hydrothermal alteration and precipitation.
We provide paleomagnetic, rock magnetic and petrographic analyses of an extensive sample set of rocks from the Chicxulub impact structure. Collectively our magnetic experiments isolated characteristic remanent magnetizations within all lithologies, providing information to the magnetic carriers within samples and the acquisition of remanence based on formation mechanism. Rock magnetic parameters, such as hysteresis and susceptibility measurements, of all lithologies reveal that magnetite is the main magnetic carrier, with significant contributions to maghemite. The low temperature oxidation represented by maghemite, is an important mechanism to the magnetic overprinting produced by chemical remanence magnetization (CRM) seen within many samples of all lithologies. CRM was likely acquired by the hydrothermal dissolution of primary grains and the precipitation of secondary grains.
Within the lower peak ring granitoid sequence, samples were not significantly remagnetized by the impact event, even though they show evidence of shock alteration. Backscatter electron imaging of a granitoid sample using SEM/EDS shows primary multidomain Fe-oxides fractured into many smaller grains within the single and pseudosingle domain size range. Many granitoids contain relatively clustered magnetic directions which may indicate they have preserved some of their original primary magnetizations from their original crystallization during the late Carboniferous. Granitoid samples that are baked by nearby pre-impact and impact related intrusions most often contain magnetic directions similar to the neighboring intrusion. However, there are zones where baked granitoids contain steeper negative magnetic inclinations (near that of the paleoinclination for the site at the time of impact) and demonstrate lower susceptibility values. These areas may highlight post impact processes, such as hydrothermal alteration and locally elevated temperatures, demonstrating how intrusion contact surfaces can be important conduits for enhanced hydrothermal flow.
SEM/EDS and EPMA of the impact melt rock and suevite rock composing the upper peak ring, reveal an abundance of primary and secondary Fe-oxide and Fe-sulfide minerals. Secondary minerals produced by hydrothermal activity and alteration are often associated with Fe-rich clays and may form a micro-setting where free electrons (from the oxidation of Fe2+) and the adsorption of simple organic molecules on clay surfaces could assist reactive conditions which may favor microbial life. Secondary Fe-sulfide minerals can contain moderately siderophile elements such as copper (Cu), nickel (Ni) and cobalt (Co) and may demonstrate how these elements can be redistributed by hydrothermal activity within an impact crater.
The impact melt rocks and suevite unit above the lower peak ring granitoids contain diverse magnetic signatures. Impact melt rocks contain a relatively uniform reversed magnetic direction that reflects the paleoinclination at the time of impact. This magnetization was produced as impact melted rocks cooled from high temperatures post-impact. Suevite rocks also contain a dominant reversed paleoinclination reflecting the time of impact that was likely produced by post-impact hydrothermal alteration. However, many samples within the upper suevite contain normal magnetizations that could represent reversals in the Earth’s magnetic field through time post impact. We show that many of the magnetic directions within these specific samples are skewed by the primary magnetizations of incorporated clasts and therefore may not indicate magnetic field reversals.
Collectively this work describes how processes associated with impact cratering events can remagnetize crustal rocks. The same mechanisms that remagnetize impact related rocks can also transform them into subsurface settings that may assist the formation of early life.
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