ALBERT

Description: Meteorite impact craters are one of the most common geological features in the solar system. An impact event is a near-instantaneous process that releases a huge amount of energy over a very small region on a planetary surface. This results in characteristic changes in the target rocks, from vaporization and melting to solid-state effects, such as fracturing and shock metamorphism. Shatter cones are distinctive striated conical fractures that are considered unequivocal evidence of impact events. They are one of the most used and trusted shock-metamorphic effects for the recognition of meteorite impact structures. Despite this, there is still considerable debate regarding their formation. We show that shatter cones are present in several stratigraphic settings within and around impact structures. Together with the occurrence of complete and "double" cones, our observations are most consistent with shatter cone formation due to tensional stresses generated by scattering of the shock wave due to heterogeneities in the rock. On the basis of field mapping, we derive the relationship D sc = 0.4 D a , where D sc is the maximum spatial extent of in situ shatter cones, and D a is the apparent crater diameter. This provides an important, new, more accurate method to estimate the apparent diameter of eroded complex craters on Earth. We have reestimated the diameter of eight well-known impact craters as part of this study. Finally, we suggest that shatter cones may reduce the strength of the target, thus aiding crater collapse, and that their distribution in central uplifts also records the obliquity of impact.

Description: The Sudbury Breccia is an impactite that formed in the target rocks of the 1850 +1.3/–2.4 Ma Sudbury impact structure in Ontario, Canada. The breccia is interpreted to have formed during crater excavation or modification at the time of the impact event. Copper-Nickel-Platinum Group Elements sulfide melts moved away from contact mineralization at the base of the melt sheet to form veins and stockworks in the footwall. The distribution of both Sudbury Breccia and the later sulfide melts is partially controlled by pre-existing weaknesses in the footwall, such as lithological contacts; as a result, the two are often spatially associated. A combination of early magmatic-hydrothermal and late metamorphic fluids modified the sulfide mineralization to create broad haloes of metalliferous hydrous silicate minerals proximal to the Sudbury Breccia and in the footwall rocks. This study examines variations in the trace-element geochemistry of the breccia-matrix mineral assemblage developed adjacent to footwall mineralization in the North Range (Coleman Mine) and the South Range (Creighton Mine). The Sudbury Breccia is widely considered to have formed in a single event, contemporaneous with the meteorite impact, therefore it provides a relatively consistent baseline for studying subsequent metamorphic and hydrothermal processes, unlike the Archean Superior Province and Paleoproterozoic Southern Province, which experienced pre-impact alteration associated with tectonic and igneous activity. This study focused on examining the textural, chemical, and petrological changes recorded in biotites, amphiboles, chlorites, and Fe-oxides produced by late magmatic and post-magmatic sulfide mineralization and U-Pb age dates of titanite. Titanite-chlorite assemblages in the North Range footwall of the SIC overprint metalliferous assemblages adjacent to the McCreedy East 153 footwall ore body. They yielded an age of 1358 ± 78 Ma, which is interpreted to be associated with the waning effects of the 1450 ± 0.15 Ma Chieflakian orogeny. In the South Range at the Creighton Mine, the Sudbury Breccia hosting the Creighton Deep footwall ore body records a shift from ferro-hornblende to ferro-tschermakite amphibole. When coupled with variations in Ca/Ti in titanite and a U-Pb titanite age of 1616 ± 33 Ma, the assemblage is interpreted to reflect increasing temperature-pressure gradient towards shear zones that were active during the Mazatzalian orogeny, during which time the sulfide mineralization was remobilized. At both locations, biotite exhibits an increase in Tl content that relates to the Ni content of the host rocks and proximity to sulfide mineralization. This relationship may be produced by remobilization of metals during interaction between sulfide mineralization and hydrothermal fluids. Although not directly associated with sulfide emplacement, these signatures provide evidence for a larger geochemical halo around the mineral zone that may provide the basis for ranking the prospectivity of the footwall and possibly vectoring towards mineral zones.

Description: Gasa crater has been the most active site observed on Mars to date, making it of particular interest for studying the process(es) behind gully formation and activity. In this study, we investigate whether differences in thermal inertia across different segments of gully systems, combined with morphological and colour observations with High-Resolution Imaging Science Experiment (HiRISE), can provide some constraints on the physical characteristics associated with recent activity within gullies in Gasa. We also investigate thermophysical differences between slopes in Gasa dominated by gully activity compared to those predominantly modified by dry mass-wasting processes. We find that Gasa crater exhibits clear variations in thermal inertia across its walls, controlled by the material properties and the types of dominant mass movement processes occurring on each wall. The youthful gully-fan lobes display thermal inertia values c. 20–40 J m –2 K –1 s –1/2 higher than adjacent older eroded and dust-covered lobes. The talus aprons from mass wasting in Gasa have thermal inertia values c. 60–80 J m –2 K –1 s –1/2 higher than gully aprons. The results of this study thus suggest that thermal imaging can inform us on surface change detection on Mars.

Description: A temperature-controlled sample stage with an operational range of ~60 °C above or below ambient laboratory temperature (~ –35 to 85 °C) was constructed for in situ X-ray diffraction of minerals and materials using a Bruker D8 Discover diffractometer with - geometry. The stage was primarily designed for characterizing mirabilite-bearing samples from a Mars analog High Arctic perennial cold spring at an in situ temperature. Operation of the stage was demonstrated through the analysis of a synthetic sample of the hydrated sodium sulfate, mirabilite (Na 2 SO 4 ·10H 2 O). Mirabilite was held at –25 °C for approximately two hours without significant dehydration and then incrementally warmed to ambient laboratory temperature at 5 °C intervals, during the acquisition of in situ diffraction data. At ambient laboratory temperature, the mirabilite dehydrated and only polycrystalline thenardite (Na 2 SO 4 ) remained. Preliminary analysis of the cold spring precipitates demonstrates that when mirabilite is present in the sample the dehydration reaction is occurring between collection and analysis at ambient laboratory temperature. This temperature-controlled stage was designed for versatility and ease of X-ray access, with applications that can extend to many geological and planetary settings, including Mars analog environments.

Description: We describe the first putative microbial trace fossils hosted in meteorite impact glass. We conducted optical and scanning electron microscopy, energy dispersive X-ray spectroscopy, and Fourier transform infrared spectroscopy on postimpact tubular features hosted in impact glasses from the Ries impact structure (Germany). The morphologies of the tubules are inconsistent with known mineralogical crystallization mechanisms, and combined with evidence of organic molecules suggest that these tubules cannot be formed through purely abiotic processes. The simplest and most consistent explanation of the data is that biological activity played a role in the formation of the tubular textures in the Ries glasses, likely during postimpact hydrothermal activity. As impact glass is a ubiquitous substrate on rocky bodies throughout the Solar System and likely common on the early Earth, the preservation of biological activity in impact glass has significant astrobiological implications for life on early Earth as well as for the search for life on other planets.

Description: The Sudbury breccia is an impact-generated pseudotachylitic breccia formed in the footwall of the 1.85 Ga Sudbury Igneous Complex in Ontario, Canada. The breccia is commonly developed in areas of contrasting country-rock types, and it represents a common host to footwall-style Cu-Ni-platinum group element sulfide mineralization, developed in the immediate footwall of large, contact-type, Ni-Cu–rich ore deposits at the base of the Sudbury Igneous Complex. Three models have been proposed for the genesis of the Sudbury breccia: (1) in situ frictional or shock melting of target rocks, (2) cataclasis of the target rocks, and/or (3) injection of allochthonous impact melt from the Sudbury Igneous Complex into the brecciated crater floor. This study reports new mineralogical and geochemical data for samples of Sudbury breccia and associated host rocks, both proximal and distal to mineralization, from six different locations around the Sudbury structure. A mixing model was used to construct the composition of the Sudbury breccia matrix, using a combination of country rock and quartz diorite from the offset dikes, which are believed to provide an estimate of the composition of the early melt sheet. We demonstrate that the breccia matrix can be successfully modeled from footwall lithologies with no requirement for a contribution from the impact melt sheet. Instead, we propose a dynamic, parautochthonous in situ melt model, where the matrix of the breccia was derived in part by high-temperature melting of footwall lithologies. These inclusion and crystal-laden melts were mixed and transported into tensile fracture zones that developed adjacent to weaknesses in the country rocks. Principal component analysis of Sudbury breccia indicates that the trace metal content within the breccia that developed distal from mineralization is associated with assimilation of mafic units in the footwall. In contrast, the matrix of Sudbury breccia developed closer to mineral zones has a contribution of metal derived from the fractionated sulfide melt and/or a contribution of hydrothermally remobilized metal. Thus, by comparing the trace metal content in Sudbury breccia with local footwall lithologies, baselines can be established that can help to identify anomalous metal contributions that may be derived from the periphery of a footwall mineral system.

Description: The origin of the Stac Fada Member has been debated for decades with several early hypotheses being proposed, but all invoking some connection to volcanic activity. In 2008, the discovery of shocked quartz led to the hypothesis that the Stac Fada Member represents part the continuous ejecta blanket of a meteorite impact crater, the location of which was, and remains, unknown. In this paper, we confirm the presence of shock-metamorphosed and -melted material in the Stac Fada Member; however, we also show that its properties are unlike any other confirmed and well documented proximal impact ejecta deposits on Earth. Instead, the properties of the Stac Fada Member are most similar to the Onaping Formation of the Sudbury impact structure (Canada) and impact melt-bearing breccias from the Chicxulub impact structure (Mexico). We thus propose that, like the Sudbury and Chicxulub deposits, Melt Fuel Coolant Interactions – akin to what occur during phreatomagmatic volcanic eruptions – played a fundamental role in the origin of the Stac Fada Member. We conclude that these rocks are not primary impact ejecta but instead were deposited beyond the extent of the continuous ejecta blanket as high-energy ground-hugging sediment gravity flows.