GD7.3

Signatures from hotspot lavas fed by mantle plumes suggest a heterogenous mantle source. Deep plumes sample the core-mantle boundary (CMB) region and this region is thought to host primordial and recycled crustal material, possibly in the form of thermochemical piles. The formation of these piles depends on the amount of oceanic crust subducted into the lower mantle and how much is entrained back toward the surface. However, it is unclear how and under which conditions the oceanic crust can segregate from subducted slabs to form these piles and eventually be entrained in ascending mantle plumes. It has been suggested that the bridgmanite to post-perovskite phase transition facilitates this segregation, as low viscosity post-perovskite allows for thinning and stretching of crustal material. This process is difficult to model numerically, since crustal material is often thinned to very small length scales. Thus, it usually cannot be resolved in global convection models, leading to over-estimates of entrainment and consequently impacting the predicted formation of basaltic piles.  Furthermore, the deformation of the crust as the slab descends into the lower mantle changes the initial surface crustal thickness and hence how likely the material is to form piles or become entrained. To address these uncertainties, we model a descending slab in the lower mantle to re-assess basalt entrainment and accumulation near the CMB. We use an adaptive mesh and tracers in order to track the deformation of the crust to achieve high resolution and also test different crustal thicknesses. These models provide insights into how material is added and removed from reservoirs in the lowermost mantle, and how these rates of material exchange have varied throughout Earth history.

How to cite: Chotalia, K., Dannberg, J., and Gassmoeller, R.: Crustal thickness and resolution controls on basalt entrainment in the lower mantle, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10836, https://doi.org/10.5194/egusphere-egu21-10836, 2021.

Long-lived, Mesozoic-Cenozoic subduction zones such as the Pacific slab under the Americas and the Tethyan slab under Eurasia consumed thousands of kms of lithosphere of which remnants are detected in today’s mantle by seismic tomography. Major differences, however, in subduction zone evolution occurred between these systems which include strong variations in subduction rate, slab morphological evolution, and trench motion, which all appear mostly to be accommodated in the upper 1000 km of the mantle (van der Meer et al. 2018). Furthermore, sinking rates of slabs below this zone tend to be similar for different subduction systems and an order of magnitude smaller than their plate/subduction velocities. Working from the premise that the mantle rheology that accommodated these subduction systems is basically similar, although still poorly constrained, we test the hypothesis that the contrasting evolution of these subduction systems is primarily tied in with the global plate tectonic forcing of subduction.

It is generally accepted that plate motion is primarily driven by slab pull with contributions from ridge push, rather than the drag of the underlying mantle. If correct, numerical subduction models should be able to obtain upper as well as lower mantle subduction velocities and sinking rates similar to those reconstructed from geological records. We are at the start of this investigation and will present the numerical model setup, modeling strategy, and preliminary results of a 2-D subduction modelling experiment. We implement a 2D-cylindrical model setup for solving the conservation of momentum, mass and energy with the open-source geodynamics code ASPECT (Kronbichler et al. 2012) using a nonlinear visco-plastic rheology and including the major phase changes. Our focus is on the possible role of the absolute motion of the subducting and overriding plates in concert with slab pull variation reconstructed from plate tectonic evolution models, while in both subduction cases the same (partly nonlinear) mantle rheological processes are required to accommodate slab morphology change and slab sinking. Kinematic modelling constraints are derived from global plate tectonic evolution models, while the tomographically inferred present-day stage provides the end-stage geometry of slabs.

van der Meer, D. G., Van Hinsbergen, D. J., & Spakman, W. (2018). Atlas of the underworld: Slab remnants in the mantle, their sinking history, and a new outlook on lower mantle viscosity. Tectonophysics, 723, 309-448.

Kronbichler, M., Heister, T., & Bangerth, W. (2012). High accuracy mantle convection simulation through modern numerical methods. Geophysical Journal International, 191(1), 12-29.

How to cite: van der Wiel, E., Thieulot, C., Spakman, W., and van Hinsbergen, D.: Dynamics of upper and lower mantle subduction and its effects on the amplitude and pattern of mantle convection., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11502, https://doi.org/10.5194/egusphere-egu21-11502, 2021.

In situ measurement of solid-state deformation in a large volume press has historically required use of neutron and x-ray scattering facilities. The lack of widespread availability of these facilities has limited the abilities of researchers to measure in situ deformations on a regular basis. We have developed an assembly that utilizes a piezoelectric crystal within a typical large volume press assembly in a 6-axis press at pressures up to 5 GPa. The basic design of the assembly can be applied to multiple assembly sizes for a wide range of possible pressures. The piezoelectric crystal is a round disk, <1 mm in diameter, that is sputter coated with Au. Copper wires are placed through drilled holes in the side of the assembly, one connected to each side of the disk. The crystal generates a voltage across the two faces when a deviatoric stress is applied that is measured and plotted in real-time during the experiments. The voltage is then used to calculate strain and strain-rate in uniaxial compression. Using the known equation of state of the piezoelectric crystal, such as quartz or gallium orthophosphate, the stresses responsible for the strain can be calculated. Thus, we can measure the stress and strain regime of simple deformation within an assembly in situ in real-time during the deformation. We have measured strain-rates as low as 10^-7 s^-1 over a greater than 30-minute timescale. The total strain on the assembly can be measured by the total distance advanced by the press piston, which must be accommodated. Comparing the differences in strain accommodated by the piezoelectric crystal between separate experiments allows us to infer the strain accommodated by the sample under investigation.

Current limitations in measuring lower strain-rates are charge-leakage around the piezoelectric crystal causing a voltage drift during measurements and limitations in high-temperature experiments due to phase transitions during heating in the piezoelectric crystals to phases that are not piezoelectric. Future work will concentrate on finding a suitable, high-resistance material to place around the piezoelectric crystal to limit charge leakage and designing the assembly such that the piezoelectric crystal experiences lower temperature during heating than the sample to avoid phase transitions in the crystal.

How to cite: Dolinschi, J. and Frost, D.: Development of in situ measurement of solid-state deformation in a large anvil press utilizing a piezoelectric crystal, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-6928, https://doi.org/10.5194/egusphere-egu21-6928, 2021.

      Localized deformation is a possible scenario that may explain the preservation of geochemical heterogeneity in the lower mantle. Recent experimental studies (e.g., Girard et al., 2016) showed that Fp (ferropericlase), which is a weak and volumetrically minor phase (~20 %), accommodates a large fraction of strain of its mixture with Br (bridgmanite), which is a stronger (approximately order of 2 - 3) and volumetrically major phase (~60-70 %). Localized deformation of the Fp phase within this two-phase mixture may provide an important insight to the long-standing question of the mechanical differentiation process between the weak boundary layer and the relatively unmixed volume in the lower mantle. Since the dominant deformation mechanism in the lower mantle is thought to be the diffusion creep, and the deformation state is mostly simple shear, it is important to understand how the deformation by diffusion creep occurs in a mixture of Fp-Br under the simple shear.

In the context of multiscale modeling methodology, we approach a grain’s length scale deformation where a single 2D elliptic Fp grain is embedded in the infinite Br matrix medium. These two phases are treated as linear viscous incompressible materials where deformation occurs by the fluxes of atoms (vacancies) caused by the applied boundary normal stress gradient. We conducted a theoretical analysis to investigate the nature of the diffusion-induced deformation of the Fp grain under the simple shear. We focused on the following issues: (i) when the two-phase mixture deforms under the far-field simple shear, what the local stress and strain rate fields within the Fp inclusion are, (ii) how the local stress gradients induce the diffusion fluxes of vacancies of the Fp grain, and (iii) the dependences of diffusion creep of the Fp to its shape (changing with the strain) and its viscosity contrast against the Br.

      To investigate the internal stress states, we used the Eshelby’s inhomogeneous inclusion theory translating its elastic formulations to the linear viscous ones using the Hoff analogy. This approach provides the stress, strain rate, and vorticity within a 2D elliptic Fp grain embedded in Br (3 orders of magnitude greater viscosity than Fp) matrix subjected to the far-field simple shear. From these mechanical states, the lattice diffusion within Fp grain and its influences on the rheology were found by using the Finite Element method solving the Fick’s laws of diffusion. This study shows that the diffusion creep rate increases as the ellipse elongates and rotates. As the Fp ellipse elongates (i.e., its aspect ratio increases), the local shear stress in the Fp increases, and the stress is somewhat concentrated near the small radius tips, which induces the strong diffusion fluxes due to the high normal stress gradients. These theoretical and numerical results support that the strain localization under diffusion creep regime can occur and be a possible mechanism that created the localized mantle flow particularly where the shear deformation is dominantly applied.

How to cite: Cho, H. and Karato, S.: Toward the Better Understanding of Shear Localization in the Lower Mantle Caused by the Strength Contrast between Bridgmanite and Ferropericlase: the Role of Stress/Strain Rate Heterogeneity in Diffusion Creep Regime, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10321, https://doi.org/10.5194/egusphere-egu21-10321, 2021.

How strain localizes in the lower crust and upper mantle to accommodate transcurrent plate motions is not well understood. Here we focus on a suite of lower crustal and upper mantle xenoliths from the San Quintin Volcanic Field (SQVF) in Baja California, Mexico, located along transcurrent faults at the margin of the Pacific plate. Previous work has suggested that in addition to significant strain localization, the lower lithosphere below SQVF has experienced partial melting, possibly through shear heating. The presence of even minor amounts of melt could significantly affect the deformation mechanisms accommodating strain. While previous studies of SQVF have largely focused on deformation in the upper mantle, less is known about strain localization in the lower crust. We have analyzed the composition and microstructures of nine xenoliths using wavelength dispersive spectroscopy (WDS) and electron backscatter diffraction (EBSD) to elucidate the relationship between melt infiltration and deformation in the lower crust of this actively-deforming region.

We categorize the suite of SQVF xenoliths into two textural and chemical groups: Group 1, consisting of undeformed mafic cumulates, and Group 2, consisting of foliated ultramafic peridotites and mafic granulites. Symplectites and corona textures with olivine-orthopyroxene-clinopyroxene+spinel symplectite-plagioclase layering preserved in Group 2 samples are interpreted to have resulted from basaltic melt infiltration during deformation. The orientation of the shape preferred orientations (SPO) of spinel and orthopyroxene grains relative to foliation in Group 2 samples is consistent with experimental studies of crystallization during melt infiltration. Evidence for deformation is also preserved in the form of moderate crystallographic preferred orientations (CPO), present in plagioclase, orthopyroxene, and olivine. Oxide weight percentages, calculated using electron microprobe data and modal phase abundances from WDS maps, were used to construct pseudosections in order to estimate equilibrium temperatures and pressures. The range of pressures across samples suggest a changing degree of deformation and degree of rock-melt interaction with depth in the lower crust of Baja California.

How to cite: Murphy, C., Bernard, R., and Chin, E.: Interplay between melt infiltration and strain localization in the lower lithosphere of San Quintin, Baja California, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7947, https://doi.org/10.5194/egusphere-egu21-7947, 2021.

Rocks of the Earth's crust and mantle commonly consist of aggregates of different minerals with contrasting mechanical properties. During progressive, high temperature (ductile) deformation, these rocks tend to develop an extrinsic mechanical anisotropy related to the strain competition of the different minerals, the amount of accumulated bulk strain and the bulk strain geometry. Extrinsic anisotropy is thought to play an important role in a wide range of geodynamic processes up to the scale of mantle convection. However, the evolution of grain-scale and rock-scale associated with this anisotropy cannot be directly implemented in large-scale numerical simulations. For two-phase aggregates -a good rheological approximation of most Earth's rocks- we propose a methodology to indirectly approximate the extrinsic viscous anisotropy by a combination of (i) 3-D mechanical models of rock fabrics, and (ii) analytical effective medium theories. The resulting 3-D mechanical models, confirm that the weak least abundant phase induces substantial rock weakening by forming an inter-connected network of thin layers in the flow direction. 3-D models further suggest, however, that the lateral inter-connection of these weak layers is quite limited, and the maximum structural weakening is considerably less than previously estimated. Ont the other hand, presence of hard inclusions does not have a profound impact in the effective strength of the aggregate, with lineations developing only at relatively low compositional strength contrast. When rigid inclusions become clogged, however, the aggregate viscous resistance can increase over the theoretical upper bound. We show that the modelled grain-scale fabrics can be parameterised as a function of the bulk deformation and material phase properties and can be combined with analytical solutions to approximate the anisotropic viscous tensor. At last, the resulting parameterisation of the extrinsic viscous tensor is implemented in a bi-dimensional global mantle convection code. Preliminary results show that extrinsic is responsible for an increase of the upwelling speed of hot material from the lowermost mantle, different convective cell shapes, and deflection of mantle plumes at the uppermost mantle.

How to cite: de Montserrat Navarro, A., Faccenda, M., and Pennacchioni, G.: Extrinsic anisotropy of two-phase Newtonian aggregates: fabric characterisation and parametrisation, and application to global mantle convection., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12719, https://doi.org/10.5194/egusphere-egu21-12719, 2021.