Characterization of shocked quartz grains from Chicxulub peak ring granites and shock pressure estimates

Abstract

Planar deformation features (PDFs) in quartz are a commonly used and welldocumented indicator of shock metamorphism in terrestrial rocks. The measurement of PDF orientations provides constraints on the shock pressure experienced by a rock sample. A total of 963 PDF sets were measured in 352 quartz grains in 11 granite samples from the basement of the Chicxulub impact structure's peak ring (IODP-ICDP Expedition 364 drill core), with the aim to quantify the shock pressure distribution and a possible decay of the recorded shock pressure with depth, in the attempt to better constrain shock wave propagation and attenuation within a peak ring. The investigated quartz grains are highly shocked (99.8% are shocked), with an average of 2.8 PDF sets per grain; this is significantly higher than in all previously investigated drill cores recovered from Chicxulub and also for most K-Pg boundary samples (for which shocked quartz data are available). PDF orientations are roughly homogenous from a sample to another sample and mainly parallel to {1013} and {1014} orientations (these two orientations representing on average 68.6% of the total), then to {1012} orientation, known to form at higher shock pressure. Our shock pressure estimates are within a narrow range, between~16 and 18 GPa, with a slight shock attenuation with increasing depth in the drill core. The relatively high shock pressure estimates, coupled with the rare occurrence of basal PDFs, i.e., parallel to the (0001) orientation, suggest that the granite basement in the peak ring could be one of the sources of the shocked quartz grains found in the most distal K-Pg boundary sites.

Formation of the crater suevite sequence from the Chicxulub peak ring: A petrographic, geochemical, and sedimentological characterization

“…4). Petrographically, these clast types strongly resemble the crystalline lithologies described by de Graaff et al ( 2021) and Feignon et al (2020) from Unit 4 in the M0077A core. Both clast types are encountered as isolated clasts or in some rare cases as large, isolated minerals (e.g., feldspar or quartz crystals >200 µm) within the suevite groundmass, or they can be incorporated as entrained clasts within microcrystalline melt clasts.…”

“…2). Compared to previous Chicxulub drill cores, IODP-ICDP Expedition 364 recovered a continuous sequence of proximal impactites and-in contrast to solely granitoid clasts in breccia units in other cores-the first continuous section of uplifted granitoid basement from the Chicxulub impact structure (Feignon et al, 2020). The M0077A suevite succession from Unit 2 yields an intact contact with both the underlying impact melt body (Unit 3A) as well as the transitional interval (Unit 1G) above, which contains the K-Pg platinum group element anomaly (Goderis et al, 2021).…”

“…4D). This contrasts the more heavily shocked quartz grains in the granitoid basement from the M0077A core, and there 490 is an average of 2.8 PDF sets per quartz grain (Feignon et al, 2020). The granitoid clast data plot 491 close to the granitic compositional field (Table 1) with some exceptions that are most probably 492 linked to alteration.…”

Chicxulub impact structure, IODP‐ICDP Expedition 364 drill core: Geochemistry of the granite basement

“…The basement rocks are shocked, with most of the minerals showing signs of shock metamorphism. Shock pressures experienced by the granite were estimated between~16 and 18 GPa (Feignon et al 2020). Zhao et al (2020) reported on the investigation of nine granite samples and suggested that these late Paleozoic granites are K-rich adakitic rocks that formed following the melting of a thickened crust with a residue of garnet-bearing amphibolite or garnet-bearing granulite.…”

“…Polished thin sections of the samples were prepared and investigated for their mineralogy, textures, and shock metamorphic features in minerals using optical microscopy at the University of Vienna and a JEOL JSM-6610 variable pressure (VP) scanning electron microscope (SEM) at the Natural History Museum (Vienna, Austria). Additionally, detailed investigations on shocked quartz grains were made on 11 thin sections using a universal stage (see Feignon et al 2020).…”