Propriétés élastiques des minéraux hydratés : applications à l'anisotropie sismique dans les zones de subduction

The flow of material and the seismic signature of subduction zones are highly dependent on the elastic propertiesof two major types of hydrated minerals: serpentines (antigorite, lizardite and chrysotile) produced by thehydration of mantle minerals and glaucophane, a marker of blueschists and eclogites, which are metamorphicfacies characteristic of subduction zones. Detection of these phases is sometime difficult. It is important to knowtheir elastic properties in order to better understand the seismic images. The goal of this work is to measure andcalculate the elastic properties of hydrated minerals by Brillouin spectroscopy (for antigorite and glaucophane)and ab initio calculations (for lizardite). The precise knowledge of such properties allows linking them to theseismicity in the subduction zones including areas of low velocities detected by various seismic methods. Ourmeasurements were performed at room conditions for antigorite and glaucophane and at high-pressure forantigorite in a diamond anvil cell. Elastic measurements were then coupled with measurements of latticepreferredorientations by Electron Back-Scattered Diffraction. This allowed quantifying the high anisotropy(AVP= 37% and AVS= 50%) in the whole rock and the low seismic velocities of P and S waves. We were able tolink it to the shear wave splitting observed by seismology in the Ryukyu arc (Japan). Seismic observations showthat serpentine is present in areas of low seismicity; it appears to be not only an essential mineral of thesubduction zones but also a “lubricant” allowing sliding layers to slip over each other without leading toearthquakes. For glaucophane, blueschists exhibit a higher anisotropy than glaucophane eclogites, but theserocks are difficult to detect at higher depth, because of their high velocities comparable to those of thesurrounding mantle. We performed ab initio calculations for a similar serpentine, the lizardite, for which weestablished the elastic constants at various pressures and in the presence of iron or not. We highlighted ananomaly around 5 GPa for lizardite and 7 GPa for antigorite, which was later confirmed by Brillouin and Ramanspectroscopies at high pressure.La circulation de matière et la signature sismique des zones de subduction sont fortement dépendantes despropriétés élastiques de deux grands types de minéraux hydratés : les serpentines (antigorite, lizardite etchrysotile) provenant de l'hydratation des minéraux du manteau et le glaucophane, minéral marqueur des schistesbleus et éclogites, faciès métamorphiques caractéristiques des zones de subduction. La détection de ces phasesest parfois difficile. Il est nécessaire de connaître leurs propriétés élastiques afin de mieux comprendre lesimages sismiques acquises. L'objectif de cette thèse est de mesurer et de calculer les propriétés élastiques deminéraux hydratés par la spectroscopie Brillouin (dans le cas de l'antigorite et du glaucophane) et par les calculsab initio (pour la lizardite). Caractériser au mieux ces propriétés permet de les relier à la sismicité dans les zonesde subduction et notamment aux zones de faibles vitesses détectées par diverses méthodes sismiques. Nosmesures ont été réalisées à l'ambiante pour l'antigorite et le glaucophane et à haute pression pour l'antigorite encellule à enclumes de diamants. Les mesures élastiques ont ensuite été couplées à des mesures d'orientationspréférentielles par Electron Back-Scattered Diffraction. Ceci a permis de quantifier l'anisotropie élevée(AVP=37% et AVS=50%) dans la roche totale ainsi que les vitesses sismiques faibles des ondes P et S. Nousavons pu ainsi relier le décalage des ondes S aux retards observés par sismique dans la zone de subduction deRyukyu (Japon). Les observations sismiques montrent que la serpentine est présente dans les zones où lasismicité est faible et apparaît non seulement comme un minéral essentiel des zones de subduction mais en pluscomme un "lubrifiant" permettant aux couches de glisser les unes sur les autres sans engendrer de séismes. Pourle glaucophane, les schistes bleus présentent une anisotropie plus élevée que les éclogites à glaucophane maisces roches sont toutefois difficiles à détecter avec la profondeur, du fait de leurs vitesses élevées comparables àcelles du manteau environnant. Nos calculs par méthode ab initio portent sur un analogue de l'antigorite, lalizardite pour laquelle nous avons établi les constantes élastiques à diverses pressions et en présence de fer ounon. Nous avons mis en évidence une anomalie élastique vers 5 GPa pour la lizardite et 7 GPa pour l'antigorite,que nous avons confirmée ensuite par des mesures de spectroscopies Brillouin et Raman à haute pression

Serpentine polymorphism: A quantitative insight from first-principles calculations

Single-walled chrysotile nanotubes [...] of increasing size (up to 5004 atoms per unit cell, corresponding to a radius of 205 Å) have been modelled at the Density Functional level of theory. For the first time, it is demonstrated that the (n, -n) and (n, n) series present a minimum energy structure at a specific radius (88.7 and 89.6 Å, respectively, referring to the neutral surface), corresponding to a rolling vector of (60, -60) and (105, 105), respectively. The minima are nearly overlapped and are lower in energy than the corresponding slab of lizardite (the flat-layered polymorph of chrysotile) by about 3.5 kJ mol-1 per formula unit. In both cases, the energy profile presents a shallow minimum, where radii in the range of 63 to 139 Å differ in energy by less than 0.5 kJ mol-1 per formula unit. The energy of larger nanotubes has a trend that slowly converges to the limit of the flat lizardite slab. Structural quantities such as bond distances and angles of nanotubes with increasing size asymptotically converge to the flat slab limit, with no discontinuities in the surrounding of the minimum energy structures. However, analysis of the elongation of a rectangular pseudo-unit cell along the nanotube circumference indicates that the main factor that leads lizardite to curl in tubes is the elastic strain caused by the mismatch between the lattice parameters of the two adjacent tetrahedral and octahedral sheets. It is also shown in this study that the curvature of the layers in one of the lately proposed models of antigorite, the "wavy-layered" polymorph of chrysotile, falls within the range of radii of minimum energy for the nanotubes. These findings provide quantitative insights into the peculiar polymorphism of these three phyllosilicates. They show also that chrysotile belongs to those families of inorganic nanotubes that present a minimum in their strain energy profile at a specific range of radii, which is lower in energy with respect to their flat equivalent

X-ray free electron laser heating of water and gold at high static pressure

The study of water at high pressure and temperature is essential for understanding planetary interiors but is hampered by the high reactivity of water at extreme conditions. Here, indirect X-ray laser heating of water in a diamond anvil cell is realized via a gold absorber, showing no evidence of reactivity

Absence of Stress-Induced Anisotropy During Brittle Deformation in Antigorite Serpentinite

Knowledge of the seismological signature of serpentinites during deformation is fundamental for interpreting seismic observations in subduction zones, but this has yet to be experimentally constrained. We measured compressional and shear wave velocities during brittle deformation in polycrystalline antigorite, at room temperature and varying confining pressures up to 150 MPa. Ultrasonic velocity measurements, at varying directions to the compression axis, were combined with mechanical measurements of axial and volumetric strain, during direct loading and cyclic loading triaxial deformation tests. An additional deformation experiment was conducted on a specimen of Westerly granite for comparison. At all confining pressures, brittle deformation in antigorite is associated with a spectacular absence of stress‐induced anisotropy and with no noticeable dependence of wave velocities on axial compressive stress, prior to rock failure. The strength of antigorite samples is comparable to that of granite, but the mechanical behavior is elastic up to high stress ( urn:x-wiley:jgrb:media:jgrb53171:jgrb53171-math-0001 of rock strength) and nondilatant. Microcracking is only observed in antigorite specimens taken to failure and not in those loaded even at 90–95% of their compressive strength. Microcrack damage is extremely localized near the fault and consists of shear microcracks that form exclusively along the cleavage plane of antigorite crystals. Our observations demonstrate that brittle deformation in antigorite occurs entirely by “mode II” shear microcracking. This is all the more remarkable than the preexisting microcrack population in antigorite, is comparable to that in granite. The mechanical behavior and seismic signature of antigorite brittle deformation thus appears to be unique within crystalline rocks

Textures of eclogites and blueschists from Syros island, Greece: inferences for elastic anisotropy of subducted oceanic crust

Many blueschists and eclogites are inferred to have formed from oceanic basalts in subducted slabs. Knowledge of their elastic behaviour is essential for reconstructing the internal structure of subduction zones. The Cycladic Blueschist Unit, exposed on Syros Island (Greece), contains rocks belonging to an exhumed Tertiary subduction complex. They were possibly part of a subduction channel, a shear zone above the subducting slab in which exhumation is possible during subduction. Intense plastic deformation, forming crystallographic preferred orientations (CPO), accompanied blueschist and eclogite metamorphism. CPO of the constituent minerals in the collected samples was determined by time-of-flight neutron diffraction. Two samples are foliated fine-grained blueschists with strong CPO, rich in glaucophane, zoisite and phengite. Two coarser-grained eclogite samples rich in omphacite and clinozoisite, or glaucophane, have weaker CPO. Vp and Vs anisotropies were computed from the orientation distribution function and single-crystal elastic constants. All samples show velocity maxima parallel to the mineral lineation, and minima normal to the foliation, providing important constraints on orientations of seismic anisotropy in subduction channels. Vp anisotropies are up to three times higher (6.5-12%) in the blueschists than in the eclogites (3-4%), pointing to a potentially important lithological control of elastic anisotropy in subducted oceanic crust