Experimental Studies of Shear Deformation of Mantle Materials: Towards Structural Geology of the Mantle (original) (raw)
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Melt distribution in mantle rocks deformed in shear
Geophysical Research Letters, 1999
Shear experiments on olivine-basalt aggregates provide compelling evidence that the dynamic distribution of melt is controlled by the magnitude and orientation of the differential stress. Our results suggest that deformed, partially molten upper mantle rocks will have highly anisotropic physical properties including seismic wave velocities and melt permeability. In addition, our results provide a basis for interpreting geophysical observations, such as shearwave splitting and for modeling melt migration processes beneath mid-ocean ridges, specifically focused flow of melt toward the ridge axis. Introduction Geophysical studies and geochemical evidence both indicate that partial melting beneath mid-ocean ridge spreading centers initiates at depths of around 100 km over a broad region and is focused near the surface to a narrow zone of volcanism on the order of a few kilometers wide centered on the ridge axis. To date, experimental evidence for the distribution of melt in peridotires has been restricted to microstructural analysis of samples from hydrostatic and triaxial compression experiments. We have undertaken a series of high strain shear deformation experiments on aggregates of olivine plus basalt to investigate melt distribution under mantle-like flow conditions. Our experimental observations suggest that deformation-induced anisotropy in seismic properties and in permeability beneath a mid-ocean ridge are likely to be significant. Previous hydrostatic annealing experiments on aggregates of olivine plus mid-ocean ridge basalt (MORB) demonstrated the importance of the relative surface energies of solid-solid and solid-liquid boundaries in determining the distribution of melt at the grain scale [Waft and Bulau, 1979; Cooper and Kohlstedt, 1982]. However, because the surface energy of olivine is anisotropic, some crystallographic faces (dominantly (010)) are preferentially wetted by basalt and melt-grain interfaces become faceted rather than smoothly rounded interfaces indicative of isotropic wetting [Waft and Faul, 1992]. At melt fractions from less than 0.01 to-•0.05, the hydrostatic distribution of melt consists of a network of interconnected triple-junction tubules with randomly oriented melt pockets along some two-grain boundaries. Deformation dramatically influences melt topology in partially molten peridotires. In triaxial compressive creep experiments, melt is redistributed preferentially along grain
Laboratory Studies of the Rheological Properties of Minerals under Deep-Mantle Conditions
Elements, 2008
Most large-scale geological process such as mantle convection and plate tectonics involve plastic deformation of rocks. However quantitative experimental studies of plastic properties under deep mantle conditions are challenging and the major progress in this area has often been associated with developments of new techniques. Until very recently reliable studies have been conducted only at pressures less than ~0.5 GPa (~15 km depth in Earth). By combining new techniques of synchrotronbased in-situ stress-strain measurements with newly designed high-pressure apparati, a new generation of experimental studies of plastic deformation of minerals under deep mantle conditions is emerging that constrain the pressure dependence of deformation of some minerals such as olivine and the slip systems in high-pressure minerals such as wadsleyite and perovskite. These results have important implications for the depth variation of mantle viscosity and the geodynamic interpretation of seismic anisotropy.
Physics of the Earth and Planetary Interiors, 2014
We report a first application of an improved second-order (SO) viscoplastic self-consistent model for multiphase aggregates, applied to an olivine + diopside aggregate as analogue for a dry upper mantle peridotite deformed at 10 À15 s À1 shear strain rate along a 20-Ma ocean geotherm. Beside known dislocation slip systems, this SO-model version accounts for an isotropic relaxation mechanism representing 'diffusionrelated' creep mechanisms in olivine. Slip-system critical resolved shear stress (CRSS) are evaluated in both phases-as functions of P, T, oxygen fugacity (fO 2) and strain rate-from previously reported experimental data obtained on single crystals and first-principle calculations coupled with the Peierls-Nabarro model for crystal plasticity; and the isotropic-mechanism dependence on T and P matches that of Si selfdiffusion in olivine, while its relative activity is constrained by reported data. The model reproduces well the olivine and diopside lattice preferred orientations (LPO) produced experimentally and observed in naturally deformed rocks, as well as observed sensitivities of multiphase aggregate strength to the volume fraction of the hard phase (here diopside). It shows a significant weakening of olivine LPO with increasing depth, which results from the combined effects of the P-induced [1 0 0]/[0 0 1] dislocation-slip transition and the increasing activity with T of 'diffusion-related' creep. This work thus provides a first quantification of the respective effects of [1 0 0]/[0 0 1] slip transition and diffusion creep on the olivine LPO weakening inducing the seismic anisotropy attenuation observed in the upper mantle.
Shear deformation of bridgmanite and magnesiowüstite aggregates at lower mantle conditions
Science (New York, N.Y.), 2016
Rheological properties of the lower mantle have strong influence on the dynamics and evolution of Earth. By using the improved methods of quantitative deformation experiments at high pressures and temperatures, we deformed a mixture of bridgmanite and magnesiowüstite under the shallow lower mantle conditions. We conducted experiments up to about 100% strain at a strain rate of about 3 × 10(-5) second(-1). We found that bridgmanite is substantially stronger than magnesiowüstite and that magnesiowüstite largely accommodates the strain. Our results suggest that strain weakening and resultant shear localization likely occur in the lower mantle. This would explain the preservation of long-lived geochemical reservoirs and the lack of seismic anisotropy in the majority of the lower mantle except the boundary layers.
Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle
Tectonophysics, 2007
It is now admitted that the high strength of the subcontinental uppermost mantle controls the first order strength of the lithosphere. An incipient narrow continental rift therefore requires an important weakening in the subcontinental mantle to promote lithosphere-scale strain localisation and subsequent continental break-up. Based on the classical rheological layering of the continental lithosphere, the origin of a lithospheric mantle shear/fault zone has been attributed to the existence of a brittle uppermost mantle. However, the lack of mantle earthquakes and the absence of field occurrences in the mantle fault zone led to the idea of a ductile-related weakening mechanism, instead of brittle-related, for the incipient mantle strain localisation. In order to provide evidence for this mechanism, we investigated the microstructures and lattice preferred orientations of mantle rocks in a kilometre-scale ductile strain gradient in the Ronda Peridotites (Betics cordillera, Spain). Two main features were shown: 1) grain size reduction by dynamic recrystallisation is found to be the only relevant weakening mechanism responsible for strain localisation and 2), with increasing strain, grain size reduction is coeval with both the scattering of orthopyroxene neoblasts and the decrease of the olivine fabric strength (LPO). These features allow us to propose that grain boundary sliding (GBS) partly accommodates dynamic recrystallisation and subsequent grain size reduction.
Deformation in the lowermost mantle: From polycrystal plasticity to seismic anisotropy
Earth and Planetary Science Letters, 2011
In the deep earth, deformation occurs at many scales: large-scale convection produces subduction of slabs and upwelling of plumes in the mantle. At the high temperature/high pressure conditions, strain is accommodated through crystal plasticity, either by diffusion or the movement of dislocations. Slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale. In this study we use polycrystal plasticity, with experimentally derived deformation mechanisms for perovskite, post-perovskite and magnesiowuestite, to predict texture development along streamlines in a 2D geodynamic convection model of the lowermost mantle. Strong preferred orientation develops during subduction and upwelling, while during spreading along the coremantle boundary the orientation pattern is relatively stable. From preferred crystal orientation and single crystal elastic properties, bulk elastic properties can be calculated and compared with seismic observations. Post-perovskite with predominant (001) slip and magnesiowuestite with {110} and {111} slip produce anisotropy patterns which are consistent with observed anisotropy, i.e. fast S-waves polarized parallel to the core-mantle boundary and anti-correlation between P and S-wave anisotropies. In contrast, perovskite with dominant (001) slip and two post-perovskite models with dominant slip on (010) and (100) produce anisotropy patterns which are inconsistent with seismic observations.
Journal of Geophysical Research, 2006
1] Deformation of mantle phases to high strain is fundamental for understanding mantle dynamics. However, it is technically challenging to perform deformation experiments yielding accurate mechanical results at the high pressures required for certain mantle phases, such as wadsleyite and ringwoodite. Deformation experiments on analog materials of such high-pressure phases can provide insight into the deformation properties of the mantle. The (Mg,Ni) 2 GeO 4 system undergoes the olivine-spinel transformation at much lower pressures than the silicate counterpart. Hence the high strain deformation properties of an olivine-spinel rock can be studied at experimentally tractable pressures and temperatures. Experiments were conducted in a gas medium deformation apparatus at constant angular velocity, a temperature of 1473 K, and 300 MPa confining pressure. At these conditions with a Mg/(Mg + Ni) ratio of 0.9, the resulting aggregate comprised 70% olivine, 30% spinel, and <1% orthopyroxene. The applied strain rate ranged from 10 À4 to 10 À5 s À1 , yielding shear stresses supported by the samples of between 100 and 250 MPa. Samples were deformed to a range of shear strains between g = 1 and g = 7 in order to study the microstructural development during high strain deformation. In these experiments, the spinel phase did not deform to produce a crystallographic preferred orientation (CPO). The olivine phase produced a CPO which is consistent with slip on the [001](hk0) slip system. If this slip system is dominant in olivine below 250-300 km in the upper mantle, the observed seismic anisotropy can be explained without resorting to changes in flow regime or deformation mechanism at this depth.
Annu. Rev. Earth Planet. Sci., 2008
Rock-mechanics experiments, geodetic observations of postloading strain transients, and micro-and macrostructural studies of exhumed ductile shear zones provide complementary views of the style and rheology of deformation deep in Earth's crust and upper mantle. Overall, results obtained in small-scale laboratory experiments provide robust constraints on deformation mechanisms and viscosities at the natural laboratory conditions. Geodetic inferences of the viscous strength of the upper mantle are consistent with flow of mantle rocks at temperatures and water contents determined from surface heat-flow, seismic, and mantle xenolith studies. Laboratory results show that deformation mechanisms and rheology strongly vary as a function of stress, grain size, and fluids. Field studies reveal a strong tendency for deformation in the lower crust and uppermost mantle in and adjacent to fault zones to localize into systems of discrete shear zones with strongly reduced grain size and strength. Deformation mechanisms and rheology may vary over short spatial (shear zone) and temporal (earthquake cycle) scales.