Skip to Main Content
Book Chapter

Zircon formation in impact melts: Complications for deciphering planetary impact histories

By
Matthew M. Wielicki
Matthew M. Wielicki
Department of Earth, Planetary, and Space Sciences, University of California–Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA
Search for other works by this author on:
T. Mark Harrison
T. Mark Harrison
Department of Earth, Planetary, and Space Sciences, University of California–Los Angeles, 595 Charles Young Drive East, Los Angeles, California 90095, USA
Search for other works by this author on:
Published:
October 01, 2015

We explore the formation conditions and inheritance probability of zircon in impact melts and the implications of using zircon geochronology to investigate planetary impact histories. By modeling the occurrence and crystallization temperature spectrum for zircon in simulated impact melts, we predict the presence of such grains within impactites. We also report U-Pb geochronology of sieve-textured, possibly poikilitic, zircon identified in the pseudotachylyte and granophyre units present within the largest known terrestrial impact crater (Vredefort, South Africa) to explore the accuracy of these grains in dating impact events at an impact structure of known age. Zircons with similar textures have been recently interpreted as growing in an impact melt in lunar meteorite SaU 169 and used to determine the age of the Imbrium impact. Modeling in simulated lunar melt compositions predicts crystallization of zircon in merely ~2% of melting events, in this case via impact. The modeled crystallization temperature spectrum is significantly below Ti-in-zircon crystallization temperatures reported from lunar samples. Zircon formation within an impact melt is dictated by saturation of [Zr] and requires a high abundance for lunar melt compositions. This essentially rules out the possibility of zircon growing in equilibrium with lunar meteorites. Poikilitic textures may be inherited from the lunar crust, presumably due to rapid decompression and/or resorption into an undersaturated magma, as previously recognized in plagioclase. Although either scenario could be due to an impact, endogenic processes cannot be ruled out, and thus lunar poikilitic zircons may not be recording impact melting events. Secondary ion mass spectrometry U-Pb analysis of zircon with similar textures from Vredefort clearly shows that these grains are inherited from the Archean target rocks, with varying degrees of Pb loss, and consequently cannot be used to accurately identify the age of the Vredefort impact structure. Further understanding of the growth and isotopic effects on zircon of shock and heating associated with large impacts could provide another tool that can be used to probe planetary impact histories.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Special Papers

Large Meteorite Impacts and Planetary Evolution V

Gordon R. Osinski
Gordon R. Osinski
Centre for Planetary Science and Exploration, Departments of Earth Sciences and Physics and Astronomy, University of Western Ontario, 1151 Richmond St., London, ON N6A 3K7, Canada
Search for other works by this author on:
David A. Kring
David A. Kring
Center for Lunar Science and Exploration, Lunar and Planetary Institute, 3600 Bay Area Boulevard, Houston, Texas 77058, USA, and National Aeronautics and Space Administration (NASA) Lunar Science Institute, and NASA Solar System Exploration Research Virtual Institute
Search for other works by this author on:
Geological Society of America
Volume
518
ISBN print:
9780813725185
Publication date:
October 01, 2015

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal