Phil J.A. McCausland’s research while affiliated with Western University and other places

Comparing how an asteroid appears in space to its ablation behavior during atmospheric passage and finally to the properties of associated meteorites represents the ultimate probe of small near-Earth objects. We present observations from the Lowell Discovery Telescope and multiple meteor camera networks of 2022 WJ1, an Earth impactor that was disrupted over the North American Great Lakes on 2022 November 19. As far as we are aware, this is only the second time an Earth impactor has been specifically observed in multiple passbands prior to impact to characterize its composition. The orbits derived from telescopic observations submitted to the Minor Planet Center and ground-based meteor cameras result in impact trajectories that agree to within 40 m, but no meteorites have been found as of yet. The telescopic observations suggest a silicate-rich surface and thus a moderate-to-high albedo, which results in an estimated size for the object of just D = 40−60 cm. Modeling the fragmentation of 2022 WJ1 during its fireball phase also suggests an approximate 0.5 m original size for the object as well as an ordinary chondrite-like strength. These two lines of evidence both support that 2022 WJ1 was likely an S-type chondritic object and the smallest asteroid compositionally characterized in space. We discuss how best to combine telescopic and meteor camera data sets, how well these techniques agree, and what can be learned from studies of ultrasmall asteroids.

Comparing how an asteroid appears in space to its ablation behavior during atmospheric passage and finally to the properties of associated meteorites represents the ultimate probe of small near-Earth objects. We present observations from the Lowell Discovery Telescope and from multiple meteor camera networks of 2022 WJ1, an Earth impactor which was disrupted over the North American Great Lakes on 19 November 2022. As far as we are aware, this is only the second time an Earth impactor has been specifically observed in multiple passbands prior to impact to characterize its composition. The orbits derived from telescopic observations submitted to the Minor Planet Center (MPC) and ground-based meteor cameras result in impact trajectories that agree to within 40 meters, but no meteorites have been found as of yet. The telescopic observations suggest a silicate-rich surface, and thus a moderate-to-high albedo, which results in an estimated size for the object of just D = 40 - 60 cm. Modeling the fragmentation of 2022 WJ1 during its fireball phase also suggests an approximate half-meter original size for the object as well as an ordinary chondrite-like strength. These two lines of evidence both support that 2022 WJ1 was likely an S-type condritic object and the smallest asteroid compositionally characterized in space. We discuss how best to combine telescopic and meteor camera datasets, how well these techniques agree, and what can be learned from studies of ultra-small asteroids.

Spinel-group minerals are among the best-known and widely used minerals in diamond exploration due to their ubiquity, resistance to weathering, and utility as petrogenetic indicators. The kimberlite indicator mineral chromite is investigated in this study using micro-X-ray diffraction (µXRD) to measure chromite unit cell parameter ao. We used epoxy-mounted chromium-rich spinel (henceforward called ‘chromite’) mineral separates with known chemical composition from kimberlitic and non-kimberlitic sources to evaluate structural-chemical correlations for potential use in diamond exploration. Chromite grains of <300 µm size from the Koala, Misery, and Sheiba kimberlites in the Ekati property (Northwest Territories, Canada), as well as from exploration programs in Botswana and Gabon, Africa, were examined in situ, as mounted for standard electron probe microanalysis (EPMA). Unit cell parameter ao was measured by µXRD for several natural kimberlitic and non-kimberlitic chromite grains, and these data have been correlated with chemical composition as determined by EPMA on a grain-by-grain basis. Conventional chemical discrimination plots with unit cell size denoted by color demonstrate clearly discernable unit cell trends that are useful for classification. Two kimberlitic chromite compositional trends can be discriminated by chromite unit cell size. The kimberlitic phenocryst trend is delineated by a distinct increase in unit cell size (ao > 8.336 Å), whereas the kimberlitic xenocryst trend is delineated by a distinct decrease in the unit cell (ao < 8.322 Å). The latter trend is also followed by the Gabon non-kimberlitic samples. Notably, the unit cell parameters for chromite in the diamond-indicating field have a tightly determined value of ao = 8.329 (± 0.007) (or 8.322–8.336 Å). This field partially overlaps with the unit cell values for some non-kimberlitic chromites (e.g., Botswana). Unit cell values of chromite grains recovered from heavy mineral concentrates could serve as a preliminary screening technique for identifying diamond-indicating chromites prior to chemical analysis if their kimberlitic provenance is known. More broadly, the μ-XRD unit cell technique is a useful, non-destructive tool that shows promise for application to other kimberlite indicator minerals.

Electron backscatter diffraction (EBSD) investigation of strain mainly uses polycrystalline samples to study fabric development. We extend the use of EBSD for the analysis of large single mineral grains by measuring the apparent surficial subdomain boundary density per unit area, reported here as unit segment length (USL). We apply this USL technique to examine and quantify the plastic deformation recorded by naturally shocked olivine in the low to moderately shocked ureilite meteorite Northwest Africa 2221 and the highly shocked martian dunitic cumulate meteorite Northwest Africa 2737, by assessing the types of subdomain boundaries and the increase of subdomain misorientation with increasing shock metamorphism. We further compare USL results for the shocked olivine in the meteorites with those for the terrestrial deformation of Hawaiian olivine. USL of olivine increases with shock level, and USL from shocked olivine is significantly greater than that of terrestrially deformed olivine. USL is a promising tool for the quantification of plastic deformation in large single crystals from shock as well as terrestrial deformation. The results derived from USL measurements along with local EBSD maps are complementary with quantitative 2D X-ray diffraction analysis of crystal deformation and disruption, leading to a more comprehensive understanding of characteristic shock deformation recorded by large single crystals.

Paleointensity data can yield insight on the state of the geodynamo, providing constraints on deep Earth events and enabling analysis of long-term trends in the paleomagnetic field. The Ediacaran (635 Ma–539 Ma) is a period of discrepant paleomagnetic behaviour that was recently characterised by sustained, extremely weak, paleointensity. The interval also coincides with some of the most recent estimates for Earth's inner core nucleation (ICN) age, determined from numerical geodynamo models and analysis of long-term paleointensity data. However, the field strength during the subsequent Cambrian period (540 Ma–485 Ma) is largely unknown with almost no data. Here, we provide high-quality paleointensity results for the Cambrian. A Grenville dyke (∼590 Ma) that was baked by the Chatham-Grenville stock (532 Ma), slowly cooled at a rate controlled by the stock and recorded the paleointensity averaged over this interval (up to several tens of thousands of years). The characteristic paleomagnetic directions of the dyke are well-defined and consistent with those previously obtained from the Chatham-Grenville and Mont Riguad stocks. Paleointensity data were obtained using multiple methods and indicate an extremely weak field during a period coincidental with evidence for hyper-reversing activity extending into the late Cambrian. The dipole strength is similar to that of the ‘ultra-weak’ Ediacaran and may suggest that this paleomagnetic behaviour persisted into the Cambrian. The cause of this weak-field interval remains enigmatic but an approximate 200-million-year quasi-periodicity in dipole strength extending across the entire Phanerozoic is not ruled out.

The Iapetus Ocean was the first ancient ocean to be identified following the development of plate tectonics; its history has been fundamental in relating orogenesis and plate motion. The ocean probably formed following 3-way rifting between Laurentia, Baltica, and Amazonia – West Africa (a block that became incorporated in Gondwana). Closure of the ocean trapped numerous terranes during the development of the Appalachian–Caledonide Orogen. Subsequent deformation, including late Paleozoic strike slip, transpression, and transtension, and Mesozoic stretching during Pangea breakup, must be taken into account in models for orogen development. Traditional analyses of Iapetan terranes have focussed on Cambrian sedimentary successions, and on isotopic criteria, to classify terranes into larger domains: Ganderia, Avalonia and Megumia. Detrital zircon data show that these domains did not cross the Iapetus as single entities, while paleomagnetic data reveal significant vertical-axis terrane rotations. We here review and interpret 17 paleomagnetic poles and >350 published detrital zircon data sets from the northern Appalachians and western Caledonides, using consistent and rigorous criteria for the selection and presentation of data. We place these data on an integrated stratigraphic chart to show timing relations and to seek constraints on the provenance and travel of terranes in the Iapetus Ocean. We distinguish groups of terranes that likely travelled together as terrane assemblages. In the Taconian/Grampian Orogeny, Furongian to Katian continent-arc collision involved off-margin blocks along the hyperextended Laurentian margin. In New England, early Taconian collision by 475 Ma involved the Gondwana-derived Moretown assemblage. An assemblage of the Bronson and Popelogan arc terranes probably arrived at the main Laurentian margin 25-30 Myr later. Subduction polarity reversal then led to the progressive accretion of additional terrane assemblages (Salinian Orogeny). The Miramichi–Victoria assemblage that arrived close to the Ordovician–Silurian boundary. The Miramichi terrane underwent partial subduction in the Québec re-entrant, whereas the Victoria terrane was juxtaposed with the Newfoundland promontory without major metamorphism. In mid-Silurian time, an assemblage including the Gander terrane of Newfoundland and related portions of Britain and Ireland was accreted to Laurentia, along with Baltica (Scandian Orogeny). The St. Croix – La Poile assemblage may have been accreted slightly later, but is distinguished by the development of a Silurian arc-backarc system (coastal igneous belt) above a northwest-dipping subduction zone. The Avalon-Brookville assemblage encountered this system in Přídolí to Middle Devonian time (Acadian Orogeny), leading to the collapse of the backarc basin and northwest-vergent thrust emplacement onto Laurentia during sinistral transpression in the Appalachian Orogen. Acadian deformation involved mainly sinistral strike slip in Britain and Ireland. Several of the terranes that were accreted to the Laurentian margin carried internal records of earlier deformation that took place near Amazonia – West Africa in Early Ordovician time and earlier (Monian/Penobscottian Orogeny). The Iapetus Ocean thus contained a complex array of terranes, small ocean basins, arcs, and previously emplaced ophiolites analogous to modern southeast Asia. It closed to form a complex array of sutures in an orogen within which no single Iapetus suture can be clearly identified.

Limited types of radioactive molecules (RM) can be made inside hot-cavity targets at ISOL facilities like TRIUMF. However, extreme conditions in these targets present formidable unsolved challenges to efficient production and delivery of RM’s. Here we propose using RFQ gas-reaction cells to produce RM from radioactive ion beams (RIB) by room temperature RIB-gas chemical reactions at eV energies. Two options are possible: (1) using an ion reaction cell (IRC) that is a linear RFQ ion guide and reaction cell used as an ‘on-line ion source’, and (2) using the ARIEL RFQ cooler-buncher (ARQB). RFQ gas-cells are a controllable and efficient method to produce RM from chemical reactants that cannot be used in ISOL targets. This ‘online chemistry’ offers a way to enable groundbreaking Beyond Standard Model (BSM) physics research, using a wide diversity of new rare and exotic RM beams that would be difficult or impossible to produce in hot-cavity targets.