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Shocked Monazite Chronometry: Integrating Microstructural and in Situ Isotopic Age Data for Determining Precise Impact Ages

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Shocked Monazite Chronometry: Integrating Microstructural and in Situ Isotopic Age Data for Determining Precise Impact Ages Contrib Mineral Petrol (2017) 172:11 DOI 10.1007/s00410-017-1328-2 ORIGINAL PAPER Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages Timmons M. Erickson1 · Nicholas E. Timms1 · Christopher L. Kirkland1 · Eric Tohver2 · Aaron J. Cavosie1,3 · Mark A. Pearce4 · Steven M. Reddy1 Received: 30 August 2016 / Accepted: 10 January 2017 © Springer-Verlag Berlin Heidelberg 2017 Monazite is a robust geochronometer and {110}, {212}, and type two (irrational) twin planes with Abstract ̄ ̄ ̄ ̄ occurs in a wide range of rock types. Monazite also records rational shear directions in [011] and [110]. SIMS U–Th– shock deformation from meteorite impact but the effects Pb analyses of the plastically deformed parent domains of impact-related microstructures on the U–Th–Pb sys- reveal discordant age arrays, where discordance scales with tematics remain poorly constrained. We have, therefore, increasing plastic strain. The correlation between discord- analyzed shock-deformed monazite grains from the central ance and strain is likely a result of the formation of fast uplift of the Vredefort impact structure, South Africa, and diffusion pathways during the shock event. Neoblasts in impact melt from the Araguainha impact structure, Brazil, granular monazite domains are strain-free, having grown using electron backscatter diffraction, electron microprobe during the impact events via consumption of strained par- elemental mapping, and secondary ion mass spectrometry ent grains. Neoblastic monazite from the Inlandsee leu- (SIMS). Crystallographic orientation mapping of monazite cogranofels at Vredefort records a 207Pb/206Pb age of grains from both impact structures reveals a similar com- 2010 ± 15 Ma (2σ, n = 9), consistent with previous impact bination of crystal-plastic deformation features, includ- age estimates of 2020 Ma. Neoblastic monazite from Ara- ing shock twins, planar deformation bands and neoblasts. guainha impact melt yield a Concordia age of 259 ± 5 Ma Shock twins were documented in up to four different ori- (2σ, n = 7), which is consistent with previous impact age entations within individual monazite grains, occurring as estimates of 255 ± 3 Ma. Our results demonstrate that tar- ̄ compound and/or type one twins in (001), (100), 101, geting discrete microstructural domains in shocked mona- zite, as identified through orientation mapping, for in situ Communicated by Othmar Müntener. U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature Electronic supplementary material The online version of this geochronometers that can be used for accurate and precise article (doi:10.1007/s00410-017-1328-2) contains supplementary dating of meteorite impacts. material, which is available to authorized users. * Timmons M. Erickson Keywords Shock metamorphism · Monazite · [email protected] Araguainha · Vredefort · U–Pb geochronology · EBSD 1 TIGeR (The Institute of Geoscience Research), Department of Applied Geology, Curtin University, 1984, Perth, WA 6845, Australia Introduction 2 School of Earth and Environment, University of Western Australia, Perth, WA 6009, Australia Impact cratering is one of the most ubiquitous processes in the solar system (French and Koeberl 2010). Because ter- 3 NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin-Madison, Madison, WI 53706, USA restrial impact craters form basins that are subject to ero- sion, burial and destruction by plate tectonic activity, the 4 CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, record of meteorite impacts on Earth is incomplete, limited Australia to 190 confirmed structures (Earth Impact Database 2011). Vol.:(0123456789)1 3 11 Page 2 of 19 Contrib Mineral Petrol (2017) 172:11 The majority of confirmed impact craters on Earth have during release of the shock pressures (Stöffler and Lan- poor age constraints due to the lack of suitable geochro- genhorst 1994). Of common accessory phases, zircon has nometers (Jourdan et al. 2009, 2012). As a consequence, the most well-constrained impact-related microstructures, fundamental questions regarding the connections between which develop by 20 GPa (Leroux et al. 1999). Deforma- impact events and significant changes to both the litho- tion twins are ubiquitous in shocked zircon, and have been sphere and biosphere remain unanswered. Shock deforma- observed in both static diamond anvil cell experiments at tion microstructures in minerals are one of a limited num- 20 GPa (Morozova 2015) and a variety of impact environ- ber of diagnostic criteria used to identify an impact event ments (e.g. Moser et al. 2011; Timms et al. 2012; Erickson (French and Koeberl 2010). Crystal-plastic deformation et al. 2013a). At higher pressure, zircon transforms to the caused by shock metamorphism has been shown to reset high-pressure polymorph reidite (Leroux et al. 1999; Witt- U–Th–Pb systems in some minerals to the time of impact mann et al. 2006; Reddy et al. 2015; Cavosie et al. 2015b; (Moser et al. 2009, 2011; Cavosie et al. 2015a). It is, there- Erickson et al. 2017), which occurs at or above 30 GPa in fore, important to understand the effects of shock metamor- shock experiments (Kusaba et al. 1985; Leroux et al. 1999). phism on U–Th–Pb systematics to accurately date impact At more extreme shock conditions, zircon can develop events (e.g. Moser et al. 2011). granular texture (Bohor et al. 1993; Wittmann et al. 2006; Monazite, (La,Ce,Th)PO4, is a common accessory phase Cavosie et al. 2015a, 2016; Timms et al. 2017). Granular that has been used as a tracer to study a variety of crus- zircon with systematic misorientations of 90°/<110> and tal processes due to the incorporation of U, Th, and other 65°/<110> likely result from recrystallization of reidite trace elements into its crystal structure (Catlos 2013). and zircon {112} twins, respectively, and can contain evi- Even though shocked monazite has been reported from dence of partial dissociation to ZrO2 that requires extreme a few impact environments (e.g. Schärer and Deutsch temperature excursions due to the impact event (Cavosie 1990), recent advancements in electron backscatter dif- et al. 2016; Timms et al. 2017). fraction (EBSD) mapping have permitted the systematic quantification of crystal-plastic deformation in monazite Deformation microstructures in monazite (e.g. Erickson et al. 2016a). However, the effects of spe- cific impact-related deformation microstructures on the A range of deformation-related microstructures have been U–Th–Pb systematics in monazite have not been evaluated. reported in monazite, including mechanical twinning, lat- In this study, we use EBSD to document shock microstruc- tice strain, and recrystallization. Monazite deformation ̄ tures in monazite from both the Vredefort Dome (South twins in (100), (001), {120} and {122} have been produced Africa) and Araguainha (Brazil) impact structures. Discrete in indentation experiments at ambient temperature and domains were then targeted for secondary ion mass spec- pressure and imaged by transmission electron microscopy trometery (SIMS) analysis to understand what effects these (TEM) (Hay and Marshall 2003). In tectonically deformed features have on the U–Th–Pb ages of shocked monazite, monazite, mechanical twins, crystal-plastic strain, and and to identify specific microstructures that yield accurate dynamically recrystallized neoblasts were identified by impact ages. EBSD (Erickson et al. 2015). Crystal-plastic deforma- tion, including low-angle (<10°) subgrain boundaries, was Shock deformation microstructures predominantly accomplished by slip systems which result in misorientations about <010> and <101>, and sets of ̄ Meteorite impacts generate extremely high pressures deformation twins were found in (100), (001), and {122} (10 s of GPa and greater) in target rocks over instantane- (Erickson et al. 2015), which are the same twin orientations ous time periods (ms–s; Melosh 1989). The passage of found in experimental studies (Hay and Marshall 2003). the shock front through the target and impactor creates In the tectonically deformed monazite grains, strain-free unique microstructural deformation, such as high-pressure neoblasts nucleated within high-strain domains and con- phases, planar microstructures, and twins (French and Koe- sumed the parent monazite by grain boundary migration berl 2010). Minerals with unique impact-related deforma- (Erickson et al. 2015). Within the deformed monazite, tion are commonly referred to as shocked minerals (Lan- the U–Th–Pb systematics were disturbed and variable age genhorst and Deutsch 2012). The conditions required to resetting was shown to correlate with plastic strain; the develop shock deformation features vary with host min- authors interpreted that Pb-loss was facilitated by forma- eral, and are best constrained for quartz. Quartz develops tion of fast diffusion pathways during deformation (Erick- multiple sets of crystallographically controlled lamellae son et al. 2015). Nucleation and growth of neoblastic called planar deformation features (PDF) that form between monazite was driven by strain energy within the deformed 10 and 34 GPa. At higher pressures, quartz transforms to lattice. Neoblasts excluded inherited Pb, and record Neo- diaplectic SiO2, which may revert to stishovite or coesite proterozoic U–Pb ages consistent with the age of regional 1 3 Contrib Mineral Petrol (2017)
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