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Introduction: The recognition that zircon can undergo a variety of transformations during shock deformation has resulted in a rapidly evolving state of understanding for how this mineral responds to the extreme thermodynamic changes generated during meteorite impact [1]. Among the pantheon of different shocked zircon textures, FRIGN zircon is the most unusual type of zircon known [2]; it is formed by initial transformation of zircon to reidite [3], and the subsequent reversion of reidite back to neoblastic zircon [1] at temperatures >1200 °C [4]. A key requirement to identify a given granular zircon as FRIGN is to demonstrate the presence of systematic disorientation relationships of approximately 90°/<110> among zircon neoblasts, which are crystallographically inherited from the zircon-reidite transformation. What is now recognized as FRIGN zircon was first described at Meteor Crater [5], Acraman [1], Ries [3], and in Australasian tektites [6], prior to formalization of the FRIGN acronym [2]. FRIGN zircon has been documented from 20 published sites, most recently at the Gosses Bluff impact structure [7], and has been demonstrated to be a useful geochronometer for dating impact events [eg, 8, 9]. FRIGN without reidite? Reidite is not predicted to be preserved in FRIGN zircon [2], given its thermal instability in impact melt rocks [4]. The absence of reidite in FRIGN zircon has led to a proposal that formation of FRIGN zircon does not involve transformations to reidite [10]. The alternative model to form FRIGN zircon involves wholesale melting of zircon at T >2700 °C based on the inferred presence of an 'yttria' rim on zircon and <100 nm zirconia inclusions in zircon neoblasts. Most critically, the model proposed by [10] does not involve formation of reidite; the systematic orientation relations among zircon neoblasts are attributed to 'synneusis'. Here we refute the model of [10] that claims FRIGN zircon forms by melting of zircon and without involvement of reidite. The non-FRIGN model is flawed for the following reasons: The T estimate of >2700 °C is based on an erroneous identification of yttria; the thermal metastability of zirconia polymorphs at grain sizes <100 nm is unaccounted for; zircon does not 'melt' (it dissociates); and no adequate explanation is given for systematic orientation relations among zircon neoblasts that is founded in crystallography. FRIGN with reidite: (F)RIGN. Over the last year, multiple reports have been published of FRIGN zircon grains that contain preserved reidite [7, 11, 12, 13, 14]. All of these occurrences are in various impact melt rocks from the Gosses Bluff [7], Mistastin [11], Rochechouart [12], Haughton [13] and Ries [14] impact structures. The preservation of reidite in FRIGN zircon, here denoted as (F)RIGN, given evidence of both former and present reidite, provides incontrovertible evidence for the role of reidite in formation of FRIGN zircon. In these cases reidite has been effectively 'frozen in', and thus did not complete the transformation to neoblastic zircon. These examples include a variety of textural observations of the progressive recrystallization of reidite to neoblastic zircon. Examples from Haughton include reidite-bearing zircon grains in the same sample as fully FRIGN zircon grains, demonstrating a close spatial association [13]. Examples from Rochechouart include non-granular zircon grains dominated by reidite lamellae and reidite 'wedges' that contain fractures filled with granular zircon in FRIGN orientation [12]. Examples from Mistastin include grains with cores of coherent reidite that appear to have fully transformed to reidite; tendrils of granular zircon with FRIGN orientations encroach from the grain exterior, overprinting reidite [11]. Examples from Gosses Bluff include fully granular FRIGN zircon grains that contain scattered 'islands' of reidite domains throughout [7]. Summary: The FRIGN zircon model offers a phase-equilibria and crystallography-founded explanation for the systematic orientation relations among zircon neoblasts found in granular zircon from impact melt rocks. FRIGN zircon has thus far only been reported from impact settings, and has been described at 20 of the ~200 confirmed terrestrial impact sites (~10%). Recent descriptions of reidite preserved within FRIGN zircon, here termed (F)RIGN zircon, provide the critical 'missing link' for the FRIGN zircon model, and uniquely identifies the role of reidite in its formation. [ABSTRACT FROM AUTHOR]

Introduction: The Ilkurlka structure is a ~12 km diameter circular aeromagnetic and gravity anomaly located ~400 km NW of Eucla, Western Australia, within the Officer Basin. The structure has been previously identified as a possible impact structure based on the geophysical anomalies [1,2,3]. The lack of diagnostic evidence of shock metamorphism has led to a range of origins being postulated for Ilkurlka, including a salt dome, an igneous intrusion, or a meteorite impact crater. The Officer Basin is regionally undeformed, except for local salt tectonics. We report the presence of shocked quartz grains with multiple sets of planar deformation features (PDFs) parallel to the {1013}-orientation that confirms an impact origin for the buried Ilkurlka structure. Methods: This study aimed to investigate and document the presence of deformation features at various scales in drill cores from two boreholes (BH01 and BH02) drilled by Maria Resources Pty Ltd. in 2019, which targeted the circular gravity anomaly. A survey of quartz from samples spanning the two boreholes was conducted to search for microstructural evidence of deformation uniquely produced by impact cratering (e.g., PDFs). Twenty samples from sandstone intervals were selected for investigation. Ten representative samples were cut and cast into 1-inch round epoxy mounts and polished. Scanning electron microscopy (SEM), including backscattered electron (BSE), energy dispersive spectroscopy (EDS), and electron backscatter diffraction (EBSD), was performed at Curtin University. Nineteen samples were prepared into polished thin sections. Petrography and a search for planar fractures (PFs) and PDFs in quartz in thin section was conducted at Curtin University. Crystallographic orientations of PDFs in shocked quartz were determined using a universal stage at Lund University. Results and Discussion: Samples from BH01 and BH02 generally consist of poorly to moderately sorted fine to coarse grained sandstone interlayered with siltstone and mudstone, and are locally brecciated. These rock units have been provisionally assigned to the lower Cambrian Marla Group based on lithological similarities to that succession in other drill cores in the eastern Officer Basin. The sandstones consist of medium to coarse, sub-rounded to rounded quartz, lithics, and feldspar grains, set in matrix and/or cements (~10-25%). The matrix components consist of micas and carbonates, silts and clays, and very fine to fine, angular to subangular quartz grains that appear to be crushed. Original pore space is locally cemented with calcite, gypsum, and minor anhydrite. Petrographic analysis and SEM imaging reveal variably deformed quartz grains. Two adjacent grains of shocked quartz were found, each with multiple sets of PDFs parallel to the {1013}-orientation, in a sample from 415.9 m (downhole depth) in BH02. Other deformation features observed in quartz include discrete zones of crushed angular grains, sub-planar fractures, grains with high fracture density in random orientations including shattered grains, concussion fractures at the boundary of multiple grains, and fractures that offset grains, indicating shear displacement. The concussion fractures are Hertzian fractures that emanate from grain-to-grain contacts and cut across up to six adjacent grains. Concussion fractures in quartz have been reported previously in shock-metamorphosed sandstone samples from several impact craters [e.g. 4-7] and in shock experiments [e.g. 8]. Kink bands in biotite are also present. The presence of PDFs parallel to the {1013}-orientation in quartz suggests that the target underwent shock pressures corresponding to shock stage 3 [9]. However, the apparent preservation of porosity and lack of diaplectic glass or high pressure SiO2 phases are characteristic of shock stage 1a [9]. The timing of cementation of the porosity with respect to impact-related deformation is difficult to ascertain. It is possible that impedance contrasts between cements and quartz grains created discrete zones of deformation which could have allowed spatially variable shock effects to occur. Regardless of shock stage assignment, PDFs allow confirmation of an impact origin. Acknowledgements: Financial support was provided by the Barringer Family Fund Research Grant (Quintero, 2020), The Institute for Geoscience Research (TIGeR) Curtin University, and an Australian Research Training Program Scholarship (Quintero, 2019). Core samples were provided by Strategic Elements/Maria Resources. PWH publishes with the permission of the Executive Director of GSWA. [ABSTRACT FROM AUTHOR]

Introduction: The geological record of the formation and differentiation of our planet has been destroyed by its subsequent evolution, but extremely rare clues may be obtained from other terrestrial planets. Mars provides a unique and accessible example of an early evolutionary path corresponding to that, inaccessible, of our own world. We can investigate it with spacecraft, and samples are available for in-depth analysis on Earth in the form of martian meteorites. So far, the only available martian samples that appear to have recorded the early conditions and the evolution of the planet until the present time are Northwest Africa (NWA) 7034 and its paired stones, nicknamed "Black Beauty". This regolith breccia has been ejected a few million years ago by the formation of a large impact crater, and contains the oldest martian igneous material ever dated: ~4.5 Ga old [1-4]. However, its source and geological context have so far remained unknown. and with it, a region where the earliest geological records of the planet [1-4] are exposed on the surface. Knowing this source region would provide insights into early Mars geological history and crustal extraction [1-4]. This source region may therefore become a high-priority target for detailed orbital analyses and in-situ exploration [5]. Constraints on the meteorite launch site: Following a hypervelocity impact, ejecta materials moving faster than the escape velocity (5 km/s) may get through the martian atmosphere and continue their course into interplanetary space to become martian meteorites [6]. Slower debris fall back on the surface in a radial pattern or ray around the primary crater, forming secondary craters. The presence of 100 meter-size secondaries attests to the freshness of their associated primary craters [7]. Using the size and spatial distribution of more than 90 million impact craters >50 m (both primaries and secondaries) detected using a Crater Detection Algorithm (CDA) [7-8] on the whole surface of Mars from the global Context Camera (CTX) mosaic [9], a previous work [7] identified ray systems of secondary craters <150 m associated with 19 large primary craters, potential source of the ejection of martian meteorites. Here we compare the abundance of K and Th [10-11] as well as the magnetic field intensity and the magnetization of the surface of Mars derived from orbital measurements [12] at the immediate vicinity of each crater candidate with those of the breccia. We also compare the geological context of each of the crater candidates with the chronology and the lithology of the meteorite [e.g. 1-3, 13-14]. The ejection site for Black Beauty: Among the 19 crater candidates investigated, we found that only one match with the characteristics of the meteorite: a 10 km crater located in the north-east of the Terra Cimeria - Sirenum (TCTS) region, between Hesperia Planum and the Tharsis bulge. Our work suggests that clasts contained in the regolith breccia are representative of the TCTS province, making this region a relic of the early crustal processes on Mars [e.g. 15], and thus, a region of high interest for future missions. Details on the identification of the crater source of this unique meteorite, as well a its geological context and broader implications for early crustal processes on Mars will be presented at the conference. [ABSTRACT FROM AUTHOR]