The Acasta Gneiss Complex (AGC) is a ∼2,400 km2 Hadean‐Mesoarchean terrane that contains the oldest known zircon‐bearing rocks on Earth. Despite its importance for early Earth geology, only a small ...fraction (∼50 km2) of the AGC has been mapped in detail. We use detrital zircon grains from late Pleistocene eskers that transect the Complex to approximate the lateral extent and relative proportions of diverse‐aged ancient rock units within the vast, little‐studied parts of the AGC. The esker sediment was derived from glacially eroded bedrock and therefore zircon grains can serve as a proxy for the ages of exposed bedrock in the study area. U‐Pb dates on ∼2400 detrital zircons from coarse and fine grain‐size fractions along the esker transect yield age distributions that coincide with ages of regionally mapped AGC bedrock, the adjacent Wopmay Orogen, and granitoids of the Slave craton. Based on detrital zircon age distributions and new reconnaissance‐scale mapping, we infer that 3.37 Ga granitoids are a volumetrically significant component of the unmapped AGC. Esker zircons older than 3.7 Ga are present in most esker samples but at low abundance, which suggests that Eoarchean and Hadean rocks are a volumetrically subordinate component of the AGC. However, the data also suggest that unmapped rocks at least as old as 3.95 Ga are present toward the inferred eastern limit of the AGC, a location where Eoarchean rocks have not been recognized previously.
Plain Language Summary
The Acasta Gneiss Complex is a small region of bedrock in the Northwest Territories, Canada, which contains the oldest precisely dated rocks currently known on Earth (4.02 billion years old). The study of this area is crucial to understanding how terrestrial crust first formed on our planet. Despite its significance, the region is poorly mapped with the exception of the initial discovery site. To overcome these difficulties, we sampled glacial sediment (sand and gravel) derived from local bedrock during glaciation, then extracted and dated zircon crystals to estimate relative proportions of diversely‐aged bedrock in the area. Our findings indicate the majority of unmapped bedrock is roughly 3.37 billion years old. Our findings also indicate that small vestiges of bedrock as old as 3.95 billion years old remain yet to be discovered in the unmapped area east of the region.
Key Points
U‐Pb dates from detrital zircons indicate that a large area of the Acasta Gneiss Complex (AGC) is underlain by 3.37 billion years old granite
The provenance of glacially derived sediment in the western Slave craton indicate a transport distance of less than 50 km
Detrital zircons older than 3.9 billion years indicate presence of ancient crust up‐ice from the few mapped AGC outcrops
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
2.
A particle detector that exploits Liquid Argon scintillation light Babicz, M.; Diwan, M.; Fava, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
04/2020, Volume:
958
Journal Article
Peer reviewed
A number of innovative experiments dedicated to neutrino oscillations and rare event physics, such as direct searches for dark matter particles or neutrinoless double beta decays, are using liquefied ...noble-gases, particularly Liquid Argon (LAr), as detection media. Among many advantages of noble liquids, from the detection point of view, the most important ones are high scintillation and ionisation yields, possible long drift paths of ionisation electrons and feasible large detector masses. LAr is sufficiently dense and relatively cheap and therefore is the only adequate noble liquid for huge volumes.
This paper describes a particle detection system that exploits the prompt signals from the scintillation light produced by ionising particles in LAr. The detector has been exposed to cosmic rays. The system performance in terms of trigger efficiency and timing resolution, with a view to its application in neutrino detectors, is presented.
•Particle detection system that exploits the LAr scintillation light has been set up.•The system enables studies of PMTs, DAQ and trigger electronic performances.•The study involves temperature stability, electronic noise and timing synchronisation.•The tests within this system are an important input for a future large LAr detector.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
We present the development and validation of a new multivariate
b jet identification algorithm (“
b tagger”) used at the CDF experiment at the Fermilab Tevatron. At collider experiments,
b taggers ...allow one to distinguish particle jets containing
B hadrons from other jets. Employing feed-forward neural network architectures, this tagger is unique in its emphasis on using information from individual tracks. This tagger not only contains the usual advantages of a multivariate technique such as maximal use of information in a jet and tunable purity/efficiency operating points, but is also capable of evaluating jets with only a single track. To demonstrate the effectiveness of the tagger, we employ a novel method wherein we calculate the false tag rate and tag efficiency as a function of the placement of a lower threshold on a jet's neural network output value in
Z+1 jet and
t
t
¯
candidate samples, rich in light-flavor and
b jets, respectively.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
New field and geochronological data are used to define the distribution of Mesoarchean basement rocks in the south-central Slave Province. This distribution reflects a single contiguous basement ...terrane that we propose to call the Central Slave Basement Complex. It shows a structural topology that is internally consistent and compatible with known regional folding and faulting events. A sample of a proposed basement gneiss below the Courageous Lake greenstone belt, central Slave Province, has been dated by U-Pb methods and yields an age of 3325 ± 8 Ma, consistent with the new basement distribution. This sample also contains 2723 ± 3 Ma metamorphic zircon and ca. 2680 Ma titanite. The Central Slave Basement Complex is overlain by a thin, discontinuous, but distinctive cover sequence that includes minor volcanic rocks, clastic sedimentary rocks, and banded iron formation. All previously known and some new occurrences of this distinctive cover sequence occur in the immediate stratigraphic hanging wall of the Central Slave Basement Complex, locally overlying a preserved in situ unconformity. We propose to call this post-2.93 Ga cover sequence the Central Slave Cover Group. It is perhaps best typified by detrital chromite-bearing, fuchsitic quartzites. Formal formation names are proposed for the spatially separate occurrences of the Central Slave Cover Group. Detrital zircon ages are presented for one of the formations of the Central Slave Cover Group, the Patterson Lake Formation, which occurs on the western flank of a local basement culmination known as the Sleepy Dragon Complex. The detrital zircon data provide evidence for two discrete basement sources dated at ca. 2943 Ma and ca. 3147-3160 Ma. These detrital ages reinforce the depositional link between the Central Slave Cover Group and underlying crystalline rocks of the Central Slave Basement Complex.
Synthetic spectra generated with the parameterized supernova synthetic spectrum code SYNOW are compared to spectra of the unusual Type Ib supernova 2005bf. We confirm the discovery by Folatelli et ...al. that very early spectra (∼30 days before maximum light) contain both photospheric‐velocity (∼8000 km s−1) features of Hei, Caii, and Feii, and detached high‐velocity (∼14,000 km s−1) features of Hα, Caii, and Feii. An early spectrum of SN 2005bf is an almost perfect match to a near–maximum‐light spectrum of the Type Ib SN 1999ex. Although these two spectra were at very different times with respect to maximum light (20 days before maximum for SN 2005bf and 5 days after for SN 1999ex), they were for similar times after explosion—about 20 days for SN 2005bf and 24 days for SN 1999ex. The almost perfect match clinches the previously suggested identification of Hα in SN 1999ex and supports the proposition that many if not all Type Ib supernovae eject a small amount of hydrogen. The earliest available spectrum of SN 2005bf resembles a near–maximum‐light spectrum of the Type Ic SN 1994I. These two spectra were also at different times with respect to maximum light (32 days before maximum for SN 2005bf and 4 days before for SN 1994I) but at similar times after explosion—about 8 days for SN 2005bf and 10 days for SN 1994I. The resemblance motivates us to consider a reinterpretation of the spectra of Type Ic supernovae, involving coexisting photospheric‐velocity and high‐velocity features. The implications of our results for the geometry of the SN 2005bf ejecta, which has been suggested as being grossly asymmetric, are briefly discussed.
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New field and U-Pb zircon data from the south-central Wopmay Orogen (south of 65°N) establish crystallization ages of basement and plutonic phases and bring to the forefront questions on the ...architecture of the orogen. The complex and extensive >3300-2575 Ma basement domain was derived from the adjacent Slave craton. Paleoproterozoic supracrustal rocks, dominated by an extensive pelitic-psammitic sequence with basal arenite, conglomerate, and carbonate, unconformably overlie this basement. Pre- to post-kinematic Paleoproterozoic plutons intrude both basement and overlying strata and crystallized at 1877, 1867-1862, and 1858-1850 Ma. The first pulse of plutonism constrains an early generation of metamorphism and deformation to younger than 1877 Ma and is corroborated by metamorphic zircon in Archean basement. The interval from 1867 to 1862 Ma brackets ductile deformation in granite and granodiorite intrusions; although corresponding structure in the metasedimentary strata is equivocal. Post-kinematic ca. 1858-1850 Ma plutons were coeval with localized high-grade metamorphism and concomitant recrystallization of Archean basement. The absence of older (>1880 Ma) phases of the Hepburn intrusive suite in south-central Wopmay Orogen demonstrates a previously unrecognized north-south plutonic dichotomy and that the historical assignment of Calderian metamorphism to ca. 1885 Ma may be too old. The designation of part of the area to a klippe of Hottah terrane is not compatible with field and zircon age data which shows that basement and the overlying strata can be tied directly to the Slave craton. The study raises questions regarding the tectonic relationship between the Hottah terrane and Slave craton.
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DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Central Gneiss Belt, southwestern Grenville Province, is characterized by parautochthonous crust in the north and allochthonous lithotectonic domains in the south. Despite nearly two decades of ...study, the basal decollement to allochthonous domains transported from the southeast, known as the allochthon boundary thrust, has not been precisely located throughout much of the belt. Between Lake Nipissing and Georgian Bay where its surface trace is known, it separates 1.24 Ga Sudbury metadiabase in the footwall from eclogite remnants and 1.17-1.15 Ga coronitic olivine metagabbro confined to its hanging wall. On the premise that this relationship can be used to trace the allochthon boundary thrust elsewhere in the Central Gneiss Belt, we have sought to extend the known distribution of these mafic rock types, making use of field, petrographic, and geochemical criteria to identify them. New occurrences of all three mafic types are identified in a region extending from south of Lake Nipissing to western Quebec, and the mutually exclusive pattern of occurrence is maintained within this region. Structural trends and reconnaissance mapping of high-strain zones that appear to represent a structural barrier to the mafic suites suggest that the allochthon boundary thrust lies well to the north of its previously suggested location. Our preferred surface trace for it passes around the southern end of the Powassan batholith and through the town of North Bay before turning east to join up with the Lac Watson shear zone in western Quebec. This suggests that a large segment of "parautochthonous" crust lying north of, and including, the Algonquin domain is in fact allochthonous. The mutually exclusive distribution of the mafic suites points to significant separation of allochthonous and parautochthonous components prior to the Grenvillian orogeny, in accord with models of pre-Grenvillian continental rifting proposed by others. Despite a relative abundance of geological and geochronological data for the Central Gneiss Belt and a mafic rock distribution that appears to successfully locate a major tectonic boundary, we emphasize the need for additional field and laboratory work aimed at testing our structural model.
A search for sub-GeV dark matter produced from collisions of the Fermilab 8 GeV Booster protons with a steel beam dump was performed by the MiniBooNE-DM Collaboration using data from 1.86 × 1020 ...protons on target in a dedicated run. The MiniBooNE detector, consisting of 818 tons of mineral oil and located 490 meters downstream of the beam dump, is sensitive to a variety of dark matter initiated scattering reactions. Three dark matter interactions are considered for this analysis: elastic scattering off nucleons, inelastic neutral pion production, and elastic scattering off electrons. Multiple data sets were used to constrain flux and systematic errors, and time-of-flight information was employed to increase sensitivity to higher dark matter masses. No excess from the background predictions was observed, and 90% confidence level limits were set on the vector portal and leptophobic dark matter models. New parameter space is excluded in the vector portal dark matter model with a dark matter mass between 5 and 50 MeV c−2 . The reduced neutrino flux allowed to test if the MiniBooNE neutrino excess scales with the production of neutrinos. No excess of neutrino oscillation events were measured ruling out models that scale solely by number of protons on target independent of beam configuration at 4.6σ.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UM
The Makkovik Province is a segment of a Paleoproterozoic accretionary belt (the Makkovik-Ketilidian orogen) that developed on the southern margin of Laurentia at 1.9-1.7 Ga. In contrast to coeval ...Laurentian orogenic belts that mainly resulted from collision of Archean plates Makkovikian-Ketilidian orogenesis was dominated by active-margin processes including continental margin arc plutonism and juvenile terrane accretion, both of which were accompanied by regional transpression. In the Makkovik Province, earliest deformation and amphibolite-facies metamorphism of Paleoproterozoic rift-drift assemblages (Post Hill and Moran Lake groups) and the Archean foreland (Nain Province) occurred at 1.9 Ga in response to accretion of a Paleoproterozoic island arc. Following this collision, cratonward-dipping subduction was established, resulting in the formation of the 1895-1870 Ma Island Harbour Bay Plutonic Suite, a calc-alkaline magmatic arc built on reworked Archean crust. Crust formation continued between ca. 1860 and 1850 Ma with deposition of the Aillik Group on a largely juvenile basement in a rifted-arc or back-arc setting. Sometime before 1802 Ma this depositional basin was tectonically inverted, with resultant northwestward thrusting of the Aillik Group over reworked Archean crust. This phase of deformation may have been driven by accretion of a second island arc potentially represented by the Cape Harrison Metamorphic Suite. Regional transpression and amphibolite-facies metamorphism at ca. 1815-1780 Ma were accompanied by widespread granitoid plutonism. These events were mainly concentrated in the juvenile domains and are thought to reflect processes in a broad continental back-arc setting. A final orogenic pulse, marked by regional greenschist-facies transpression and emplacement of A-type granitoid plutons, occurred between 1740 and 1700 Ma, with deformation and plutonism potentially linked to crust-mantle detachment and incursion of mafic magmas at the base of the crust, respectively. The record of crustal development suggests that the coeval themes of spatially and temporally linked structural and plutonic activity, oceanward migration of this activity over time, and a trend toward increasingly more localized deformation occurred throughout the orogenic history of the Makkovik Province. These characteristics are thought to broadly reflect oceanward crustal growth of the orogen over time. In the correlative Ketilidian mobile belt of southern Greenland, these themes were also operative but appear to have been less pronounced, most likely due to minimal or a complete absence of accretion of island-arc material.
Interest in parallel architectures applied to real time selections is growing in High Energy Physics (HEP) experiments. In this paper we describe performance measurements of Graphic Processing Units ...(GPUs) and Intel Many Integrated Core architecture (MIC) when applied to a typical HEP online task: the selection of events based on the trajectories of charged particles. We use as benchmark a scaled-up version of the algorithm used at CDF experiment at Tevatron for online track reconstruction – the SVT algorithm – as a realistic test-case for low-latency trigger systems using new computing architectures for LHC experiment. We examine the complexity/performance trade-off in porting existing serial algorithms to many-core devices. Measurements of both data processing and data transfer latency are shown, considering different I/O strategies to/from the parallel devices.
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