In situ LA-ICP-MS apatite and zircon U–Pb geochronology of the Nicholson Lake impact structure, Canada: Shock and related thermal effects

McGregor, Maree; McFarlane, Christopher R.M.; Spray, John G.

doi:10.1016/j.epsl.2018.10.006

Cited by 36 publications

(37 citation statements)

References 37 publications

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“…We did, however, observe planar fractures in apatite at S6 deformation (in 78235) that are similar to those described earlier in apatite from terrestrial impact structures (Cavosie and Centeno ; McGregor et al. ). Also, the apatite observed in Civet Cat norite (S3‐4) shows granular microtexture similar to that reported by Kenny et al.…”

Section: Discussionsupporting

confidence: 90%

“…; or as high as 620 °C in larger grains, Krogstad and Walker ) and zircon (>900 °C; Cherniak ), and the relatively high intensity of CPD observed here in phosphate minerals, the phase should be particularly susceptible to Pb diffusion and loss during postimpact annealing and crater modification, as previously observed at the Nicholson Lake impact structure (McGregor et al. ). Due to the elevated (~400 °C) closure temperature of Pb in apatite, the resetting of the Pb‐Pb geochronometer is to expect in rocks that have experienced at least mild heating, for example, rocks that are in contact with impact melt but not necessarily affected by high‐temperature modifications.…”

Section: Discussionsupporting

confidence: 77%

“…Terrestrial apatite from several different impact structures is observed to display planar microstructures (Cavosie and Centeno ; McGregor et al. ) or granular, recrystallization features (Alwmark et al. ; Kenny et al.…”

Section: Introductionmentioning

confidence: 99%

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Shock‐induced microtextures in lunar apatite and merrillite

Černok

White

Darling

et al. 2019

Meteorit & Planetary Scien

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Apatite and merrillite are the most common phosphate minerals in a wide range of planetary materials and are key accessory phases for in situ age dating, as well as for determination of the volatile abundances and their isotopic composition. Although most lunar and meteoritic samples show at least some evidence of impact metamorphism, relatively little is known about how these two phosphates respond to shock‐loading. In this work, we analyzed a set of well‐studied lunar highlands samples (Apollo 17 Mg‐suite rocks 76535, 76335, 72255, 78235, and 78236), in order of displaying increasing shock deformation stages from S1 to S6. We determined the stage of shock deformation of the rock based on existing plagioclase shock‐pressure barometry using optical microscopy, Raman spectroscopy, and SEM‐based panchromatic cathodoluminescence (CL) imaging of plagioclase. We then inspected the microtexture of apatite and merrillite through an integrated study of Raman spectroscopy, SEM‐CL imaging, and electron backscatter diffraction (EBSD). EBSD analyses revealed that microtextures in apatite and merrillite become progressively more complex and deformed with increasing levels of shock‐loading. An early shock‐stage fragmentation at S1 and S2 is followed by subgrain formation from S2 onward, showing consistent decrease in subgrain size with increasing level of deformation (up to S5) and finally granularization of grains caused by recrystallization (S6). Starting with 2°–3° of intragrain crystal‐plastic deformation in both phosphates at the lowest shock stage, apatite undergoes up to 25° and merrillite up to 30° of crystal‐plastic deformation at the highest stage of shock deformation (S5). Merrillite displays lower shock impedance than apatite; hence, it is more deformed at the same level of shock‐loading. We suggest that the microtexture of apatite and merrillite visualized by EBSD can be used to evaluate stages of shock deformation and should be taken into account when interpreting in situ geochemically relevant analyses of the phosphates, e.g., age or volatile content, as it has been shown in other accessory minerals that differently shocked domains can yield significantly different ages.

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Section: Discussionsupporting

confidence: 90%

Section: Discussionsupporting

confidence: 77%

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Shock‐induced microtextures in lunar apatite and merrillite

Černok

White

Darling

et al. 2019

Meteorit & Planetary Scien

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“…; McGregor et al. ), anatase (TiO 2 ), and Fe‐rich inclusions on a nm‐scale are discernible (see Figs. bottom, A and in supporting information, NR‐6B #29).…”

Section: Resultsmentioning

confidence: 90%

“…, ; McGregor et al. ). The 13 spot measurements of these most probably recrystallized granular zircon grains lead to a concordia age of 14.89 ± 0.34 Ma (MSWD = 1.3, p = 0.25, n = 13, Fig.…”

Section: Discussionmentioning

confidence: 98%

U‐Pb dating of zircons from an impact melt of the Nördlinger Ries crater

Schwarz

Hanel

Trieloff

2020

Meteorit & Planetary Scien

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In situ U‐Pb measurements on zircons of the Ries impact crater are presented for three samples from the quarry at Polsingen. The U‐Pb data of most zircons plot along a discordia line, leading to an upper intercept of Carboniferous age (331 ± 32 Ma [2σ]). Four zircons define a concordia age of 313.2 ± 4.4 Ma (2σ). This age most probably represents the age of a granite from the basement target rocks. From granular textured zircon grains (including baddeleyite and anatase/Fe‐rich phases, first identified in the Ries crater), most probably recrystallized after impact (13 analyses, 4 grains), a concordia age of 14.89 ± 0.34 Ma (2σ) and an error weighted mean 206Pb*/238U age of Ma 14.63 ± 0.43 (2σ) is derived. Including the youngest concordant ages of five porous textured zircon grains (24 spot analyses), a concordia age of 14.75 ± 0.22 Ma (2σ) and a mean 206Pb*/238U age of 14.71 ± 0.26 Ma (2σ) can be calculated. These results are consistent with previously published 40Ar/39Ar ages of impact glasses and feldspar. Our results demonstrate that even for relatively young impact craters, reliable U‐Pb ages can be obtained using in situ zircon dating by SIMS. Frequently the texture of impact shocked zircon grains is explained by decomposition at high temperatures and recrystallization to a granular texture. This is most probably the case for the observed granular zircon grains having baddeleyite/anatase/Fe‐rich phases. We also observe non‐baddeleyite/anatase/Fe‐rich phase bearing zircons. For these domains, reset to crater age is more frequently for high U,Th contents. We tentatively explain the higher susceptibility to impact resetting of high U,Th domains by enhanced Pb loss and mobilization due to higher diffusivity within former metamict domains that were impact metamorphosed more easily into porous as well as granular textures during decomposition and recrystallization, possibly supported by Pb loss during postimpact cooling and/or hydrothermal activity.

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