Ab initio elastic properties of talc from 0 to 12 GPa: Interpretation of seismic velocities at mantle pressures and prediction of auxetic behaviour at low pressure Auteur(s) : Mainprice, Dave Le Page, Y. Rodgers, J. Jouanna, P. Auteurs secondaires : Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) Institute for Chemical Process and Environmental Technology ( ICPET) ; National Research Council of Canada (NRC) Toth Information Systems Inc., Ottawa ; Université du Québec Éditeur(s) : HAL CCSD Elsevier Résumé : International audience Talc is a hydrous magnesium rich layered silicate that is widely disseminated in the Earth from the seafloor to over 100 km depth, in ultra-high pressure metamorphism of oceanic crust. In this paper we determine the single crystal elastic constants at pressures from 0 to 12 GPa of talc triclinic (C 1¯) and monoclinic (C2/c) polytypes using ab initio methods. We find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure. The exceptional anisotropy is complemented by a negative Poisson's ratio for many directions in crystal space. Calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour, but the magnitude of this effect may be the largest reported so far for a mineral. The compression (Vp) and shear (Vs) wave velocity anisotropy is 80% and 85% for the triclinic polytype. At pressures where talc is known be stable in the Earth (up to 5 GPa) the Vp and Vs anisotropy is reduced to about 40% for both velocities, which is still a very high value. Vp is slow parallel to the c-axis and fast perpendicular to it. This remains unchanged with increasing pressure and is observed in both polytypes. The shear wave splitting (difference between fast and slow S-wave velocities) at low pressure has high values in the plane normal to the c-axis, with a maximum near the alow asterisk-axis in the triclinic and the b-axis in the monoclinic polytype. The c-axis is the direction of minimum splitting. The pattern of shear wave splitting does not change significantly with pressure. The volume fraction of talc varies between 11 and 41% for hydrated mantle rocks, but the lack of data on the crystallographic preferred orientation (CPO) precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones. However, it is highly likely that CPO can easily develop in zones of deformation due to the platy habit of talc crystals. For random aggregates of talc, the isotropic Vp, Vs and Vp/Vs ratio have significantly lower values than those of antigorite and may explain low-velocity regions in the mantle wedge. Vp/Vs ratios are more complex in anisotropic media because there are fast and slow S-waves, resulting in Vp/Vs1 and Vp/Vs2 ratios for every propagation direction, making interpretation difficult in deformed polycrystalline talc with a CPO. Talc on the subduction plate boundary can strongly influence guided wave velocity as CPO would develop in this region of intense shearing. The very low coefficient of friction (< 0.1) of talc above 100 °C could also explain silent earthquakes at shallow depths (ca 30 km) along the subduction plate boundaries, frequently responsible for tsunami. ISSN: 0012-821X hal-00411481 https://hal.archives-ouvertes.fr/hal-00411481 DOI : 10.1016/j.epsl.2008.07.047 | Partager |
"Basin scale" versus "localized" pore pressure/stress coupling - Implications for trap integrity evaluation Auteur(s) : Mourgues, R. Gressiert, Jb Bodet, L. Bureau, D. GAY, Aurélien Auteurs secondaires : Laboratoire de Planétologie et Géodynamique de Nantes (LPGN) ; Université de Nantes (UN) - Institut national des sciences de l'Univers (INSU - CNRS) - Centre National de la Recherche Scientifique (CNRS) Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) Bassins ; Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) - Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) Éditeur(s) : HAL CCSD Elsevier Résumé : International audience In petroleum industry, the difference between pore pressure (Pp) and minimum horizontal stress Sh (termed the seal or retention capacity) is of major consideration because it is often assumed to represent how close a system is to hydraulic failure and thus the maximum hydrocarbon column height that can be maintained. While Sh and Pp are often considered to be independent parameters, several studies in the last decade have demonstrated that Sh and Pp are in fact coupled. However, the nature of this coupling relationship remains poorly understood. In this paper, we explore the influences of the spatial pore pressure distribution on Sh/Pp coupling and then on failure pressure predictions and trap integrity evaluation. With analytical models, we predict the fluid pressure sustainable within a reservoir before failure of its overpressured shale cover. We verify our analytical predictions with experiments involving analogue materials and fluids. We show that hydraulic fracturing and seal breach occur for fluid pressure greater than it would be expected from conventional retention capacity. This can be explained by the impact of the fluid overpressure field in the overburden and the pressure diffusion around the reservoir on the principal stresses. We calculate that supralithostatic pressure could locally be reached in overpressured covers. We also define the retention capacity of a cover (RC) surrounding a fluid source or reservoir as the difference between the failure pressure and the fluid overpressure prevailing in shale at the same depth. In response to a localized fluid pressure rise, we show that the retention capacity does not only depend on the pore fluid overpressure of the overburden but also on the tensile strength of the cover, its Poisson's ratio, and the depth and width of the fluid source. ISSN: 0264-8172 hal-00590497 https://hal.archives-ouvertes.fr/hal-00590497 DOI : 10.1016/j.marpetgeo.2010.08.007 | Partager |
Le merlu du golfe de Gascogne et de la mer Celtique : Croissance, Répartition spatiale et bathymétrique, Ecologie alimentaire et Assemblages Auteur(s) : Kacher, Mohamed Éditeur(s) : Université du Littoral Côte d'Opale (Dunkerque) Résumé : In the Bay of Biscay and Celtic Sea waters, hake is considered as genetically homogeneous and form a unique stock with identical meristic characteristics. The vertebral mean vary from 50,68 (vertebrae) during the first year of life to 51,11 (vertebrae) for adults. 0-group Merluccius display a rapid growth with significant interannual variability (0.71 mm.day-1 in 2001 versus 0.74 mm.day-1 in 2002). The spawning period occurs in April and the average length on the firth January following the hatching is of 17.3 cm (first seasonal check / increment on the sagittal otolith is located about 0.143 cm from the nucleus). At the end of the first year of life the average length is about 24 cm. The longevity of the species is important and growth parameters (L = 138,24 cm ; K = 0,132) explain natural mortality coefficient of M = 0.21. No difference for growth between male and female has been observed for this stock. The ratio total weight / eviscerated weight is equal to Fc = 1,086. Both male and female have same spatial and bathymetric distribution. Juveniles are found in deep waters (< 17 cm length) and move to coastal waters where they display an average length of (33 cm length). Longer individuals are found in nursery areas where they feed and grow then move to deeper areas to spawn. Nursery areas are numerous and located in the Celtic Sea and in the bay of Biscay. In the bay of Biscay, disturbances resulted from hydrondynamics affect the settlement on the nursery grounds particularly in the southern part of the bay. Diet composition varies with age. Juveniles feed mainly on crustaceans (Euphausia kroni). At the length of 23 cm, individuals feed strictly on fishes. Sizes of preys are proportional with sizes of predators and preys are composed mainly of species that feed and reproduce close to the bottom. Cannibalism has been observed for this specie, and increases with age notably in the northern part of the bay of Biscay and in the Celtic Sea. Main preys are 0-group juveniles. This specie is assumed to be a passive predator catching its preys when they come to feed in the bottom. Although they are punctual, these results have been obtained after studies on the evolution of sizes during the first year of life (daily increments measurements) and on the spawning period parameters (back-calculating). Average length on the firth January following hatching and the position of the first seasonal check on the otolith are based on observations of the sizes on the firth January following hatching (obtained with growth rates and spawning period parameters). The results are similar to those described in previous studies, nevertheless they have to be confirmed with growth and diet composition analyses. A long term study appear essential to determine biological and ecological parameters of this species for sustainable management of the stock. Les alertes quant à la surexploitation du stock de merlu européen sont nombreuses et récurrentes. Seulement l'estimation du niveau réel de l'état du stock a toujours posé des problèmes du fait de certaines lacunes dans la connaissance de la biologie et de l'écologie de cette espèce. Dans les eaux du golfe de Gascogne et de la mer Celtique, le merlu européen fait partie de la même population génétique et possède les mêmes caractéristiques meristiques. Sa moyenne vertébrale est de 50,68 durant sa première année de vie et évolue pour se stabiliser à 51,11 chez l'adulte. La croissance du merlu durant sa première année de vie et très rapide mais présente une variabilité interannuelle significative (0,71 mm .J-1 en 2001 et 0,74 mm .J-1 en 2002). Sa période de ponte maximale se déroule au mois d'avril et sa longueur au premier janvier suivant sa naissance est de 17,3 cm (la première marque hivernale est positionnée à 0,143 cm du nucléus de la sagittae). Au terme de sa première année de vie le merlu atteint une longueur de 24 cm. Sa longévité est assez longue (23 ans environ) et ses paramètres de croissance (L = 138,24 cm ; K = 0,132) permettent d'estimer son coefficient de mortalité naturelle à M = 0,21. Il n'a pas été établi de croissance différentielle entre sexe chez le merlu dans ces eaux. Le rapport poids total / poids éviscéré a été estimé à Fc = 1,086. Mâles ou femelles, les merlus ont une même répartition spatiale et bathymétrique. En général, ils sont très profonds à leurs stades juvéniles (< 17 cm) et ils se dirigent vers les eaux côtières pour les atteindre à 33 cm de longueur environ. Au-delà de cette taille, les merlus se concentrent au niveau des zones de nourricerie pour s'y alimenter avant de rejoindre les eaux plus profondes pour y pondre. Les zones de nourricerie du merlu sont très nombreuses et sont localisées en mer Celtique et dans le golfe de Gascogne aussi bien dans sa partie sud que dans sa partie nord. Les perturbations importantes dans l'hydrodynamisme du golfe de Gascogne (upwelling) semble influencer les niveaux de colonisation des zones de nourricerie notamment celles situées dans la partie sud du golfe. Le régime alimentaire du merlu évolue avec l'âge. Juvénile, il se nourrit principalement de crustacés (Euphausia krohni) et devient ichtyophage exclusif dès 23 cm de longueur. Le cannibalisme est une réalité chez le merlu et les merlus-proies sont les juvéniles du groupe d'âge G-0. Il s'intensifie avec l'âge notamment dans la partie nord du golfe de Gascogne et en mer Celtique ; Il est très faible dans le sud du golfe de Gascogne. La taille des proies évolue avec la taille du merlu et les poissons proies sont en général des espèces qui vont se nourrir ou se reproduire sur le fond. Ce comportement alimentaire fait que le merlu est un prédateur peu actif et qui attaque ses proies lorsqu'elles viennent sur le fond. Ces résultats, bien qu'ils ne soient que ponctuels, ont été obtenus après avoir déterminé le schéma d'évolution de la longueur du merlu durant sa première année de vie (dénombrement des accroissement journaliers) et des paramètres de sa période de ponte (rétro-calcul).La taille au premier hiver et la position de la première marque hivernale ont été déterminées en relativisant nos observations à l'estimation de la longueur du merlu au premier janvier suivant sa naissance : celle-ci étant obtenue en utilisant le taux de croissance et les paramètres de la période de ponte. Bien que l'ensemble des résultats obtenus, en ce qui concerne la répartition spatiale et les données sur la période de ponte, soient généralement conformes à ceux décrits dans la littérature, il est nécessaire de confirmer ceux concernant la croissance et le régime alimentaire. Pour cela, une étude à plus long terme semble indispensable pour parvenir à bien maîtriser l'ensemble des paramètres de biologie et d'écologie du merlu permettant ainsi une meilleure gestion de son stock. Droits : info:eu-repo/semantics/openAccess http://archimer.ifremer.fr/doc/2004/these-1247.pdf http://archimer.ifremer.fr/doc/00000/1247/ | Partager Voir aussi Celtic sea Bay of biscay Ecologie Juvéniles Otolith Growth Assemblages Hake Mer celtique Golfe de Gascogne Télécharger |
Seismic Anisotropy of Subduction Zone Minerals-Contribution of Hydrous Phases Auteur(s) : Mainprice, Dave Ildefonse, Benoit Auteurs secondaires : Géosciences Montpellier ; Université des Antilles et de la Guyane (UAG) - Institut national des sciences de l'Univers (INSU - CNRS) - Université de Montpellier (UM) - Centre National de la Recherche Scientifique (CNRS) S. Lallemand; F. Funiciello Éditeur(s) : HAL CCSD Springer-Verlag Résumé : The seismology is the most effective method to explore the Structure of subduction zones to great depth. The distinguishing feature of the mantle in the subduction regions is the presence of hydrated phases, which transport water into the Earth's interior and release it with dramatic local consequences, triggering earthquakes and melting. The seismological detection of these hydrous phases and geo-dynamic interpretation of flow in the hydrous mantle depend on knowledge of the anisotropic elastic properties and the characteristics of the wave propagation in anisotropic media. In this paper we briefly recall the distinguishing features of anisotropic wave propagation and the observable parameters. We suggest the Vp/Vs ratio is a physically sound parameter than can be observed by seismology in anisotropic regions of the Earth, whereas the Poisson's ratio, which is often quoted, is not directly observable and does not correspond to the characteristics of wave propagation in an anisotropic or isotropic medium. We report for the first time the ratios of Vp/Vs1 and Vp/Vs2, where Vs1 is the fastest and Vs2 slowest S-wave velocities of an anisotropic media. We present the current knowledge of the anisotropic seismic properties of hydrous minerals in the upper mantle, transition zone, and lower mantle that are stable along low temperature geotherms associated with subduction, and identify which minerals are likely to influence seismological observations because they have very high volume fractions, or very high anisotropies, or both of these.;In the upper mantle antigorite and talc are exceptionally anisotropic (Vp 71% Vs 68% and Vp 65%, Vs 68%, respectively) and chlorite is also very anisotropic for S-waves (Vp 35%, Vs 76%). Comparatively less anisotropic are hornblende (Vp 27%, Vs 31%), used as proxy for tremolite in subduction zones, and clinohumite (21.8%, Vs 15.9%). Brucite at 4 GPa (Vp 26.5%, Vs 30.9%) and the dense hydrogen magnesium silicate (DHMS) phase A at 9GPa (Vp 9.3%, Vs 17.6%) are the only hydrous minerals stable in the upper mantle that have had their elastic properties measured at in situ mantle pressures. Except for the phase A, all these minerals are more anisotropic than olivine for at least one parameter (Vp, Vs, Vp/Vs1, or Vp/Vs2). In the transition zone the major phases hydrous wadsleyite and ringwoodite C have moderate to weak anisotropies (Vp 16.3%, Vs 16.5%, and Vp 1.9%, Vs 4.4%, respectively). The DHMS Superhydrous B has a moderate anisotropy (Vp 6.9%, Vs 11.6%). At greater depth the DHMS phase D Lit 24 GPa (Vp 10.8%, Vs 18.0%) is the only hydrous phase that can transport hydrogen from the transition zone into the lower mantle. The phase D is more anisotropic for S than P waves like many of the hydrous phases. From these data it is clear that hydrous phases are in general very anisotropic. However, pressure can play a strong role in reducing anisotropy. It is the case for brucite and talc, in which increasing pressure from ambient to 4 GPa reduces the anisotropy by about 50% for both P and S waves. In contrast, the anisotropy of the DHMS phase A does not change significantly with pressure.;Our picture of the seismic anisotropy of hydrated minerals remains incomplete; the elastic properties of many have not been measured even at ambient conditions (e.g., 10 angstrom phase and phase E) or not measured in their true elastic symmetry (e.g., clinochlore). The majority of hydrous minerals have not been measured at high pressure. and none related to hydrated mantle have been measured at elevated temperature. We have shown in the few cases where hydrous minerals have been measured as a function of pressure, that this variable has an important effect oil the velocity distribution and in most cases reduces the degree of anisotropy, hence we Would expect seismic anisotropy to play a key role in the determination of the shallow structure of subduction zones in the upper mantle. Subduction zone geodynamics hal-00420893 https://hal.archives-ouvertes.fr/hal-00420893 | Partager |