Abstract
Zircon is a precise chronometer and prominent recorder of impact deformation. However, many impact-induced features in zircon are poorly calibrated, sometimes due to contradicting experimental data, in other instances due to the lack of systematic studies of impact-deformed zircon. To resolve issues with the shock petrographic use of zircon, we classified impact deformation features in 429 zircon grains in a continuous drill core of uplifted, granitic bedrock in the peak ring of the 200-km-diameter K-Pg Chicxulub impact structure. Following initial identification in backscattered electron (BSE) images, Raman spectroscopy and electron backscatter diffraction confirmed one reidite-bearing zircon grain. Quartz-based shock barometry indicates the host rock of this zircon-reidite grain experienced an average shock pressure of 17.5 GPa. A survey of BSE images of 429 ZrSiO4 grains found brittle deformation features are ubiquitous, with planar fractures in one to five sets occurring in 23% of all zircon grains. Our survey also reveals a statistically significant correlation of the occurrence of planar fractures in zircon with the types of host materials. Compared to zircon enclosed in mafic, higher density mineral hosts, felsic, low-density minerals show a much higher incidence of zircon with planar fractures. This finding suggests amplification of pressure due to shock impedance contrasts between zircon and its mineral hosts. Using the impedance matching method, we modeled the shock impedance pressure amplification effect for zircon inclusions in Chicxulub granitic hosts. Our modeling indicates shock impedance could have amplified the average 17.5 GPa shock pressure in a zircon inclusion in quartz or feldspar in the Chicxulub granitic rocks to 24 ± 1 GPa, suggesting that reidite in these rocks formed between 17.5 and 25 GPa. In essence, our study of impedance-induced shock pressure amplification in zircon assemblages, including the onset of reidite formation, details how shock impedance in mineral associations can be quantified to refine shock pressure estimates.
Highlights
IntroductionZrSiO4 records a wide variety of impact-induced deformation features such as microtwins, planar fractures, and decomposition to ZrO2 + SiO2, spanning pressures between ∼10 and ∼90 GPa, and temperatures >2370 ◦C (Mashimo et al, 1983; Kusaba et al, 1985; Moser et al, 2011; Erickson et al, 2017; Timms et al, 2017a, 2017b; Cavosie et al, 2018)
Planar fractures with spacings of ∼100 μm in zircon from brecciated lower crustal nepheline pegmatites were interpreted as a result of shock waves associated with explosive volatile release, which may apply to the parting in kimberlitic zircons (Schaltegger et al, 2015)
We found that planar fractures occur in 23% of zircon grains in the parautochthonous granitic peak ring rocks from the Chicxulub impact crater that experienced average shock pressures of 17 ± 1 GPa based on quartz shock barometry (Feignon et al, 2020)
Summary
ZrSiO4 records a wide variety of impact-induced deformation features such as microtwins, planar fractures, and decomposition to ZrO2 + SiO2, spanning pressures between ∼10 and ∼90 GPa, and temperatures >2370 ◦C (Mashimo et al, 1983; Kusaba et al, 1985; Moser et al, 2011; Erickson et al, 2017; Timms et al, 2017a, 2017b; Cavosie et al, 2018). The onset of the transition of zircon to reidite has been reported from static multi-anvil pressure experiments and shortduration, dynamic shock recovery experiments between 18 and 30 GPa at ambient temperatures (van Westrenen et al, 2004; Morozova et al, 2018; Kusaba et al, 1985; Fiske et al, 1994; Leroux et al, 1999; Erickson et al, 2020). Reidite is increasingly recognized as a component of allochthonous terrestrial and extraterrestrial impactites (e.g., Timms et al, 2017a; Chen et al, 2019; Xing et al, 2020), but it has seldom been detected in parautochthonous settings with intact lithologic contexts that allow the reconstruction of the impact metamorphic conditions for the formation and preservation of reidite (Cox et al, 2018)
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