Nanotextures and formation process of coesite in silica glass from the Xiuyan impact crater

The Stac Fada Member (Stoer Group) is a ~1.2 Ga melt‐rich impact breccia preserved and intermittently exposed along the NW coast of Scotland. Using a combination of x‐ray diffraction and micro‐Raman spectroscopy, we identify potential coesite that is spatially associated with micron‐sized diamonds, as well as disordered carbon phases. Comparing the graphite G‐band of disordered carbon phases in the impact breccia to samples from underlying units indicates that most of the carbon in the Stoer Group was ultimately derived from the underlying Lewisian basement. Disordered carbon phases within the Stac Fada Member have been modified by mild heating within a hot ejecta blanket rather than shock pressure. We also report the first evidence for impact diamonds discovered within the Stac Fada Member. These diamonds have an average Raman shift of 1328.5 cm−1 and are present within both the impact breccia and the shocked gneiss clasts that are present in sandstones directly underlying the Stac Fada Member contact, and within sandstone rafts entrapped in the unit. These findings have implications for the timing of deposition of the Stac Fada Member, which must have occurred after ballistic ejection of Lewisian basement clasts during the impact event.

Section: Discussionmentioning

confidence: 99%

Provenance of altered carbon phases and impact history of the Stac Fada Member, NW Scotland

GOODWIN

Tartèse

Garwood

et al. 2023

“…In situ shock veins have been described from the uplifted cores of a limited number of impact structures on Earth (Vredefort: Martini, 1978, 1991, 1992; Spray & Boonsue, 2018; White, 1993; Manicouagan: Biren & Spray, 2011; Steen River: Walton et al., 2016). Terrestrial ex situ (allochthonous) shock veins have also been described in lithic clasts from suevites in the Ries (Stähle et al., 2008) and Xiuyan (Yin et al., 2021) impact structures. In addition, shock veins are a relatively common feature in many types of meteorites and were first described from meteorites (e.g., Fredriksson et al., 1963; Fritz et al., 2017; Gillet & El Goresy, 2013; Miyahara et al., 2021; Sharp & DeCarli, 2006; Stöffler et al., 1991).…”

Section: Introductionmentioning

confidence: 97%

Modeling the pressure–temperature–time evolution of in situ shock veins: A terrestrial case study from Vredefort

Hopkins

Spray

2022

Numerical computing software (via MathWorks MATLAB) has been developed to understand the relationship between shock wave passage in geological targets (i.e., heterogeneous media) and the formation of shock veins and associated high-pressure/ temperature polymorphs. This approach takes into consideration the pressure due to the passage of the shock front, subsequent rarefaction unloading pressures, and associated heating and cooling rates. The model is applied to calculate pressure-temperature-time conditions for coesite-and stishovite-bearing shock veins within metaquartzites of the Vredefort impact structure of South Africa. To constrain the model, the position of the host metaquartzites at the time of impact is first reconstructed. The developed code then passes the appropriate shock conditions through the target to re-create the shock wave, while simultaneously forming and cooling the shock veins via 2-D steady-state conduction. We have found that (1) at the time of shock vein formation (2.4 s following the initial contact of the projectile), the shock front pressure was 13.8 GPa and the width of the shock wave was of 27 km; (2) the melt within the shock veins initially reached ~3000 °C, which corresponds to the melting temperature of the target rock at 13.8 GPa. Simulation results indicate that conditions reach the stishovite stability field within 2 ms of vein formation (~10-14 GPa; 2000-3000 °C), followed by coesite within 1.29 s (~3-10 GPa; 600-2000 °C).The dwell time of the modeled shock vein system is 4.35 s. The shock vein system is completely solidified 33.4 s after the initial shock front passage. The calculated P-T-t path of the model indicates that the polymorphs within the shock veins of the metaquartzites at Vredefort formed under their normal stability field conditions following rarefaction wave decompression.

“…To date, they have been identified in situ within the central uplifts of three terrestrial impact structures: Vredefort, South Africa (Hopkins & Spray, 2022; Martini, 1978, 1991, 1992; Spray & Boonsue, 2018; White, 1993), and two in Canada: Manicouagan (Biren & Spray, 2011; Spray & Biren, 2021) and Steen River (Walton et al., 2016, 2018). Additionally, they have been recognized ex situ within lithic clasts from suevites in two terrestrial impact structures: Ries, Germany (Stähle et al., 2008, 2022) and Xiuyan, China (Yin et al., 2021). Shock melt veins were first recognized and are best known in meteorites, where they are a relatively common feature (e.g., Barnes, 1939; Fredriksson et al., 1963; Fritz et al., 2017; Gillet & El Goresy, 2013; Miyahara et al., 2021; Sharp et al., 2015; Sharp & DeCarli, 2006; Stöffler et al., 1991).…”

Section: Introductionmentioning

confidence: 99%

Pressure–temperature–time controls on shock vein formation within the Steen River impact structure

Hopkins,

Spray,

Walton

2024

Thermodynamic modeling has been applied to determine pressure–temperature–time conditions leading to shock vein formation during the passage of a natural shock wave generated by hypervelocity impact. The approach is novel in considering both shock front and rarefaction pressures, as well as simultaneously forming and cooling the shock veins via two‐dimensional steady‐state conduction. Model results are tested using shock veins developed in granitic rocks that constitute the central uplift of the Steen River impact structure in Canada. Here, two variants of majoritic garnet were generated in different settings: (1) along the margins of shock veins due to pargasite and biotite breakdown (accompanied by maskelynite formation), and (2) within the originally molten shock vein matrix as newly grown crystals. We determine that during shock vein formation, the shock front pressure and wave width at the reconstructed sample location were 18 GPa and 830 m, respectively, with a dwell time of 160 ms. Intra‐vein melting at 2150°C was attained within 1 μs. Melt cooled to the solidus in 150 ms following shock front passage. Majoritic garnet formation was facilitated by the high temperatures realized within the veins as a result of frictional melting that accompanied shock loading. The calculated pressure–temperature–time (P–T–t) path provides constraints on the formation conditions of majoritic garnet at Steen River. The model results independently support previously determined P–T conditions based on mineral stability fields. The vein margin garnets (35–39 mole% majorite) and maskelynite formed first under higher P–T conditions for a longer duration (36 ms). The matrix garnets (11–22 mole% majorite) crystallized from melt under lower P–T conditions and for a shorter duration (22 ms). Our results indicate that shock pressure alone should not be used as a basis for shock classification. Instead, the interplay between pressure and temperature with time and the duration of shock immersion (dwell) must be considered.