110 research outputs found
Ancient and recent collisions revealed by phosphate minerals in the Chelyabinsk meteorite
Get PDFThe collision history of asteroids is an important archive of inner Solar System evolution. Evidence for these collisions is brought to Earth by meteorites. However, as meteorites often preserve numerous impact-reset mineral ages, interpretation of their collision histories is controversial. Here, we combine analysis of phosphate U-Pb ages and microtextures to interpret the collision history of Chelyabinsk—a highly shocked meteorite. We show that phosphate U-Pb ages correlate with phosphate microtextural state. Pristine phosphate domain U-Pb compositions are generally concordant, whereas fracture-damaged domains universally display discordance. Combining both populations best constrains upper (4473 ± 11 Ma) and lower intercept (−9 ± 55 Ma, i.e., within error of present) U-Pb ages. All phosphate U-Pb ages were completely reset during an ancient high energy collision, whilst fracture-damaged domains experienced further Pb-loss during mild and recent collisional re-heating. Targeting textural sub-populations of phosphate grains permits more robust reconstruction of asteroidal collision histories
In-situ phosphate U-Pb ages of the L chondrites
Get PDFThe thermal history of asteroids is recorded by the radioisotopic ages of meteorites that derive from them. Radioisotopic ages may date a number of events, such as the cooling of a parent body during waning radiogenic metamorphism, rapid cooling experienced upon parent body break-up, and/or subsequent collision-induced reheating of material. However, sampling statistics for meteorite radioisotope ages are currently relatively low and most are derived from analyses of bulk material, therefore lacking the in-situ microtextural context that aids in distinguishing collisional events. Here, we present new in-situ apatite U-Pb ages for nine L chondrite meteorites using secondary ionisation mass spectrometry.
Our measurements greatly expand the L chondrite phosphate U-Pb age record and provide evidence for distinct stages in the thermal evolution of the L chondrite parent asteroid, including: early collisions driving parent body fragmentation- and/or exhumation-associated cooling at > 4530 Ma; onion-shell-style cooling with waning radiogenic metamorphism until 4500 Ma; late collisional reheating from 4480–4460 Ma; parent body break-up at 474± 22 Ma; and recent ejection events within several 10s of Myr of present day. We show that meteorite shock stage correlates with upper intercept age but is uncorrelated with lower intercept age. This outcome links the upper intercept ages alone to the preserved high-energy impact-related features in strongly shocked meteorites, which has important implications for our interpretation of the L chondrite U-Pb record.
We see no evidence in our record for collisional episodes between 3000–4400 Ma, i.e., the Late Heavy Bombardment. Our upper intercept age record hints that collision rates changed as a result of some dynamical instability at 4460–4480 Ma, which may have strongly depleted the main asteroid belt, and/or that L asteroid physical structure changed such that the shock metamorphic response to collisions was muted after this time, e.g., by the formation of weak rubble pile bodies. L chondrite phosphate U-Pb ages provide evidence for a heterogeneous early and shared late (less than 500 Ma) thermal history for the majority of L chondrite meteorites falling to Earth today. From this observation, we infer that most L chondrites derive from a single parent asteroid (in existence from around 4500–4440 Ma to 474± 22 Ma), which has since been disturbed to create an asteroid family. Our record of meteorite U-Pb ages traces out the thermal and dynamical evolution of the L chondrite asteroid. These observations can be used in future to benchmark dynamical models of Solar System evolution
The kinematics and dynamics of complex crater collapse
Get PDFLarge impact craters collapse to form complex structures, with zones of structural uplift that can be expressed topographically as central peaks and/or peak-rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material, but the process that facilitates this strength reduction is unknown. More fundamentally, descriptions of how structural uplifts, particularly peak-rings, are emplaced lack the quantitative details needed to constrain models of large impact crater formation. This thesis describes aspects of the formation of complex impact structures in an attempt to constrain the mechanism that facilitates crater collapse. The approach adopted in this research was to combine geological observations of large impact structures; field geology, core logging, petrography, and petrophysics, with the results of numerical impact simulations. More specifically, this thesis focusses upon the Clearwater Lake impact structures and the Chicxulub impact structure as case studies to understand complex crater collapse. The results presented demonstrate that the block model of acoustic fluidisation provides a realistic description of the rheology of rocks during crater collapse. Nonetheless, observational support for the block model of acoustic fluidisation, or indeed, any single currently proposed transient weakening mechanism, is lacking. Deformation during crater collapse is dominated by cataclasites which occur on a variety of scales, while the lubrication of faults by frictional melts and/or intrusions of impact melt and breccia may only have a significant effect during the late-stages of crater modification. The research carried out in this thesis demonstrates numerous ways in which geological and geophysical observations can be combined with numerical impact simulations to understand cratering and constrain the process of crater collapse. The methods include using observationally constrained shock pressures, geophysical data, and most significantly, the structural history of rocks in impact craters.Open Acces
Practical Experiences of Shock Barometry. Large Meteorite Impacts and Planetary Evolution VI
No full textDynamic Split Tensile Strength of Basalt, Granite, Marble and Sandstone: Strain Rate Dependency and Fragmentation
Get PDFThe aim of this study is to understand the strength behaviour and fragment size of rocks during indirect, quasi-static and dynamic tensile tests. Four rocks with different lithological characteristics, namely: basalt, granite, sandstone, and marble were selected for this study. Brazilian disc experiments were performed over a range of strain rates from ~ 10<jats:sup/>–5 /s to 2.7 × 10<jats:sup/>1 /s using a hydraulic loading frame and a split Hopkinson bar. Over the range of strain rates, our measurements of dynamic strength increase are in good agreement with the universal theoretical scaling relationship of (Kimberley et al., Acta Mater 61:3509–3521, 2013). Dynamic fragmentation during split tension mode failure has received little attention, and in the present study, we determine the fragment size distribution based on the experimentally fragmented specimens. The fragments fall into two distinct groups based on the nature of failure: coarser primary fragments, and finer secondary fragments. The degree of fragmentation is assessed in terms of characteristic strain rate and is compared with existing theoretical tensile fragmentation models. The average size of the secondary fragments has a strong strain rate dependency over the entire testing range, while the primary fragment size is less sensitive at lower strain rates. Marble and sandstone are found to generate more pulverised secondary debris when compared to basalt and granite. Furthermore, the mean fragment sizes of primary and secondary fragments are well described by a power-law function of strain rate
Evaluating Case-Based Reasoning Systems
Get PDFEvaluation is an important issue for every scientific field and a necessity for an emerging soft-ware technology like case- based reasoning. This paper is a supplementation to the review of industrial case-based reasoning tools by K.-D. Althoff, E. Auriol, R. Barletta and M. Manago which describes the most detailed evaluation of commercial case-based reasoning tools currently available. The author focuses on some important aspects that correspond to the evaluation ofcase-based reasoning systems and gives links to ongoing research
Vécu et devenir des patients ayant bénéficié de rééducation à l'effort au sein du programme “V’Air Corps Réhab'' de 2016 à 2019
No full textContext: Nowadays, physical activity benefits in chronic diseases are well demonstrated, whereas in primary, secondary or even tertiary prevention. To be efficient, it must be regularly done and well integrated in a daily routine. The medical regroupment in the Vercors area developed a yearly physical rehabilitation program called “V’Air Corps Rehab”. It welcomes voluntary patients with a wide variety of chronic diseases. Every week, they go outside all together as a group with medical supervision and sport coaches, to practise some physical activity. Objective: The objective of this study was to evaluate participants’ feelings during the rehabilitation year, and their future after the end of the program, regarding their physical activity habits. Method : A qualitative study led on 16 patients, members of this program, from 2016 to 2019. The interviews were either individuals or in focus groups. Results: The main result showed the importance of the group, and the social integration created by this type of program. Physical and psychological benefits were demonstrated but with a variability among participants. They were globally satisfied with the program, even though their goals weren’t always achieved. Concerning the future of the participants, a majority continued the physical activity, under different ways of practice. However, the study revealed that the “after” program was an issue. Several patients had to repeat a year or stopped physical activity because of a lack of adapted solution to keep practicing. Conclusion: Physical and psychological benefits were described by the patients. The group effect is massive in this kind of program. The patients globally kept practicing physical activity after the end of the program. A relay program could be useful to perpetuate the benefits and routine acquired. Follow-up studies might be interesting to measure statistically the efficiency of this kind of program, furthermore with a long-term hindsight.Contexte : Les bénéfices de la pratique d’une activité physique sont maintenant bien établis en prévention primaire, secondaire ou tertiaire des maladies chroniques. Pour être efficace, la pratique doit être régulière et intégrée à une routine quotidienne. Le pôle de santé du plateau du Vercors a donc développé un programme de réhabilitation à l’effort nommé “V’Air Corps Réhab'', qui accueille des patients volontaires, aux pathologies variées. Les participants pratiquent des sorties extérieures en groupe avec des encadrants médicaux, paramédicaux et des éducateurs sportifs, sur une année. Objectif : L’objectif de cette étude a été d’évaluer le ressenti des participants lors de leur année de réhabilitation à l’effort (RAE) et leur devenir après la fin du programme. Méthode : Une étude qualitative a porté sur 16 patients ayant participé au programme entre 2016 et 2019 sous forme de focus groupe et d’entretiens individuels. Résultats : Le résultat principal a mis en valeur l’intégration sociale engendrée par ce genre de programme et le rôle prépondérant du groupe. Les bénéfices physiques et psychologiques ont été observés mais variables. Les participants ont été globalement satisfaits du programme même si leurs objectifs n’ont pas toujours été atteints. Concernant le devenir, une majorité a poursuivi l’activité physique, sous différentes modalités. Il est toutefois à noter que le relais post-programme pose problème. Plusieurs patients ont souhaité réitérer l’année de réhabilitation ou ont arrêté faute d’avoir trouvé un relais. Conclusion : Les bénéfices physiques et psychologiques sont ressentis par les patients. Le rôle du groupe est prépondérant au sein de la RAE. Le devenir des patients est globalement en faveur d’une poursuite de l’activité physique. Un relais serait idéal pour pérenniser ces acquis. Des études complémentaires seraient utiles pour évaluer objectivement l’efficacité du programme et à plus long terme
Impact Experiment on Gneiss: The Effects of Foliation on Cratering Process
No full textTwo impact experiments were carried out on blocks of Maggia gneiss to assess the influence of mechanical target anisotropies on the cratering process. The experiments were done within the framework of Multidisciplinary Experimental and Modeling Impact Research Network. In one experiment the metamorphic foliation was vertical, that is, oriented perpendicular to the target surface, and in the other horizontal, that is, parallel to the target surface. This study reveals that due to preferred spallation along the foliation, the cratering efficiency is higher if the foliation is horizontal. The study further presents a comprehensive quantitative strain analysis in the subsurface of these craters. We use biotite kink bands as strain markers. The distribution of strain differs strongly in both experiments. If the foliation is vertical, the area beneath the central crater floor shows a high compressive strain. This is attributed to the ability of biotite to kink more easily when shortened parallel to its basal plane. If the foliation is horizontal, surface kinking is reduced and tensile failure along the foliation deepens the crater during unloading
Francis of Marchia against the Unity of the Intellect
No full textAbstract in italianoL'articolo presenta la confutazione di Francesco di Marchia della dottrina dell'unità dell'intelletto di Averroè. La posizione di Francesco di Marchia è collocata nel contesto del dibattito francescano del suo tempo. L'autore esamina come Scoto, Auriol e altri primi scotisti consideravano la dottrina di Averroè, sottolineando l'influenza della questione di Thomas Wylton On the intellectual soul in questo dibattito. Presentando la soluzione di Francesco di Marchia, l'autore mostra che, sebbene Francesco consideri il monopsichismo una dottrina che contraddice l'esperienza individuale e l'unione formale tra il corpo e l'anima intellettuale, il suo focus principale è sugli aspetti noetici del dibattito. Francesco di Marchia fornisce una soluzione volta a spiegare come l'ordine della conoscenza raggiunga un riferimento universale a un oggetto intelligibile per tutti i soggetti, senza negare che i singoli esseri umani siano i veri soggetti della propria conoscenza.
Abstract in EnglishThe article presents Francis of Marchia's refutation of Averroes' doctrine of the unity of the intellect. Marchia's position is placed in the context of the Franciscan debate of his time. The author examines how Scotus, Auriol, and other early Scotists considered Averroes' doctrine, highlighting the influence of Thomas Wylton's question On the intellectual soul in this debate. Presenting Marchia's solution, the author shows that although Marchia considers monopsychism as a doctrine that contradicts individual experience and the formal union between the body and the intellectual soul, his main focus is on the noetic aspects of the debate. Marchia provides a solution aimed to explaining how the order of knowledge achieves a truly universal reference to one intelligible object for all subjects, without denying that individual human beings are the true subjects of their own knowledge.Abstract in italianoL'articolo presenta la confutazione di Francesco di Marchia della dottrina dell'unità dell'intelletto di Averroè. La posizione di Francesco di Marchia è collocata nel contesto del dibattito francescano del suo tempo. L'autore esamina come Scoto, Auriol e altri primi scotisti consideravano la dottrina di Averroè, sottolineando l'influenza della questione di Thomas Wylton On the intellectual soul in questo dibattito. Presentando la soluzione di Francesco di Marchia, l'autore mostra che, sebbene Francesco consideri il monopsichismo una dottrina che contraddice l'esperienza individuale e l'unione formale tra il corpo e l'anima intellettuale, il suo focus principale è sugli aspetti noetici del dibattito. Francesco di Marchia fornisce una soluzione volta a spiegare come l'ordine della conoscenza raggiunga un riferimento universale a un oggetto intelligibile per tutti i soggetti, senza negare che i singoli esseri umani siano i veri soggetti della propria conoscenza.
Abstract in EnglishThe article presents Francis of Marchia's refutation of Averroes' doctrine of the unity of the intellect. Marchia's position is placed in the context of the Franciscan debate of his time. The author examines how Scotus, Auriol, and other early Scotists considered Averroes' doctrine, highlighting the influence of Thomas Wylton's question On the intellectual soul in this debate. Presenting Marchia's solution, the author shows that although Marchia considers monopsychism as a doctrine that contradicts individual experience and the formal union between the body and the intellectual soul, his main focus is on the noetic aspects of the debate. Marchia provides a solution aimed to explaining how the order of knowledge achieves a truly universal reference to one intelligible object for all subjects, without denying that individual human beings are the true subjects of their own knowledge
Francis of Marchia against the Unity of the Intellect
No full textThe article presents Francis of Marchia’s refutation of Averroes’ doctrine
of the unity of the intellect. Marchia’s position is placed in the context of the
Franciscan debate of his time. The author examines how Scotus, Auriol, and
other early Scotists considered Averroes’ doctrine, highlighting the influence of
Thomas Wylton’s question On the intellectual soul in this debate. Presenting
Marchia’s solution, the author shows that Marchia considers monopsychism as
a doctrine that contradicts individual experience and the formal union between
the body and the intellectual soul; however, his focus is on the noetic aspects
of the debate. Marchia provides a solution aimed at explaining how the order
of knowledge achieves a truly universal reference to one intelligible object for
all subjects, without denying that individual human beings are the true subjects
of their own knowledge
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