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Zircon formation in impact melts; complications for deciphering planetary impact histories

Matthew M. Wielicki and T. Mark Harrison
Zircon formation in impact melts; complications for deciphering planetary impact histories (in Large meteorite impacts and planetary evolution V, Gordon R. Osinski (editor) and David A. Kring (editor))
Special Paper - Geological Society of America (September 2015) 518: 127-134

Abstract

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 approximately 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.


ISSN: 0072-1077
EISSN: 2331-219X
Coden: GSAPAZ
Serial Title: Special Paper - Geological Society of America
Serial Volume: 518
Title: Zircon formation in impact melts; complications for deciphering planetary impact histories
Title: Large meteorite impacts and planetary evolution V
Author(s): Wielicki, Matthew M.Harrison, T. Mark
Author(s): Osinski, Gordon R.editor
Author(s): Kring, David A.editor
Affiliation: Univwersity of California, Los Angeles, Department of Earth, Planetary, and Space Sciences, Los Angeles, CA, United States
Pages: 127-134
Published: 20150917
Text Language: English
Publisher: Geological Society of America (GSA), Boulder, CO, United States
Meeting name: Large meteorite impacts and planetary evolution V
Meeting location: Sudbury, ON, CAN, Canada
Meeting date: 20130805Aug. 5-8, 2013
References: 44
Accession Number: 2015-104809
Categories: Igneous and metamorphic petrologyGeochronology
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 1 table
Source Note: Online First
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Reference includes data supplied by the Geological Society of America, Boulder, CO, United States
Update Code: 201545
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