Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

Abstract

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite 15 also records shock deformation from meteorite impactbut the effects of impact-related 16 microstructures on the U-Th-Pb systematics remain poorly constrained. We have therefore 17 analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, 18 South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron 19 backscatter diffraction (EBSD), electron microprobe elemental mapping, and secondary ion mass 20 spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact 21 structures reveals a similar combination of crystal-plastic deformation features, including shock 22 twins, planar deformation bands andneoblasts. Shock twins were documented in up to 4 different 23 orientations within individual monazite grains, occurring as compound and/or type one twins in 24 (001), (100), (101 ̅), {110}, {212}, and type two (irrational) twin planes with rational shear directions 25 in [01 ̅ 1 ̅ ] and [1 ̅ 1 ̅ 0]. SIMS U-Th-Pb analyses of the plastically-deformed parent domains reveal 26 Manuscript Click here to download Manuscript Erickson_etal_2016_monazite_v10.docx Click here to view linked References monazite, as identified through orientation mapping, for in situ U-Th-Pb analysis can date impact-35 related deformation. Monazite is therefore one of the few high-temperature geochronometers that 36 can be used for accurate and precise dating of meteorite impacts. 37 38 Koeberl 2010). Because terrestrial impact craters form basins that are subject to erosion, burial and 41 destruction by plate tectonic activity, the record of meteorite impacts on Earth is incomplete, limited 42 to 190 confirmed structures (Spray and Hines 2016). The majority of confirmed impact craters on 43 Earth have poor age constraints due to the lack of suitable geochronometers (Jourdan et al. 2009; 44 Jourdan et al. 2012). As a consequence, fundamental questions regarding the connections between 45 impacts events and significant changes to both the lithosphere and biosphere remain unanswered. 46 Shock deformation microstructures in minerals are one of a limited number of diagnostic criteria 47 used to identify an impact event (French and Koeberl 2010). Crystal-plastic deformation caused by 48 shock metamorphism has been shown to reset U-Th-Pb systems in some minerals to the time of 49 impact (Moser et al. 2009; Moser et al. 2011; Cavosie et al. 2015a). It is therefore important to 50 understand the effects of shock metamorphism on U-Th-Pb systematics to accurately date impact 51 events (e.g. Moser et al. 2011). 52 Monazite, (La,Ce,Th)PO4, is a common accessory phase that has been used as a tracer to study a 53 variety of crustal processes due to the incorporation of U, Th, and other trace elements into its 54 crystal structure (Catlos 2013). Even though shocked monazite has been reported from a few impact 55 environments (e.g. Schärer and Deutsch 1990), recent advancements...

Linking shock textures revealed by BSE, CL, and EBSD with U‐Pb data (LA‐ICP‐MS and SIMS) from zircon from the Araguainha impact structure, Brazil

“…; Erickson et al. ). Our new results complement prior studies by providing a very similar isotopic age from a different type of impact melt rock, of highly siliceous composition.…”

“…; Erickson et al. ) and from shocked sandstone from the Furnas Formation, similar to the one studied here (Hippertt et al. ).…”

“…Recently, Erickson et al. () obtained a U‐Th‐Pb concordia age of 259 ± 5 Ma on two neoblastic monazite grains (#10 and #11) from the same impact melt rock analyzed by Tohver et al. ().…”

“…[, ] or Erickson et al. []). The first radiometric age dating of deformed granitic basement was provided by Crósta (), who obtained a K/Ar age for K‐feldspar of 283.6 ± 17.2 Ma.…”

Shock‐induced microtextures in lunar apatite and merrillite

“…) and monazite (Erickson et al. ) display only local domains of age resetting, making them useful though situational impact chronometers. For the mineral zircon (ZrSiO 4 ), complete resetting of the U‐Th‐Pb systematics only occurs in recrystallized neoblastic domains (Cavosie et al.…”