ABSTRACT At mid-ocean ridges, partial melting of a presumably homogeneous and equilibrated pyroli... more ABSTRACT At mid-ocean ridges, partial melting of a presumably homogeneous and equilibrated pyrolitic source generates a basaltic crust and leaves behind its depleted complement, harzburgite. The oceanic lithosphere that subducts into the mantle is thus physically and chemically layered. During subduction we expect basalt to separate from harzburgite and the rest of the oceanic lithosphere. Hence, over convective and diffusion time-scales, do basalt and harzburgite re-equilibrate chemically as pyrolite? Most mineralogical models, upon which comparisons to seismology are based, view the mantle as homogeneous and pyrolitic or chemically stratified with homogeneous and equilibrated compositions in each layer. Petrological experiments have shown that a homogeneous pyrolite source region explains MORB and the seismic velocity profile of upper mantle and transition zone to first order. However, this view appears to violate the dynamical constraints given the low chemical diffusivity for mantle materials (10-14-10-16 cm2s-1) in the solid-state, ignoring the effects of fluids and partial melting. Allègre and Turcotte (1986) suggested a mechanically mixed mantle, a marble cake structure in which subducted oceanic lithosphere is deformed into pervasive, narrow pyroxenite veins. Computer simulations suggest a heterogeneous mantle made of a mechanical mixture of basalt and harzburgite, in which pools of basalt may accumulate at the bottom. A stirring time of the mantle between 250 and 750 Myr limits the mixing, stretching and folding of heterogeneity in the mantle. Thus, it seems implausible to equilibrate basalt and harzburgite into pyrolite with a fine (0.1-10 m) stratification given typical chemical diffusivities. We demonstrate that, even with identical bulk compositions, an equilibrium assemblage (EA) along the basalt- harzburgite join and a mechanical mixture of basalt and harzburgite in perfect disequilibrium (MM) have different phase equilibria and therefore different seismic velocities. We compute the seismic velocities of EA and MM using a previously developed self-consistent thermodynamic model and explore the effects of bulk composition (in terms of basalt depletion) and temperature. For MM, VS in the transition zone is higher and increases more rapidly with depth and is virtually insensitive to basalt fraction, while VS decreases for EA. For both EA and MM increasing potential temperature from 1400K to 1800K yields a deeper 410-km and a shallower 660-km discontinuity. The radial gradient of the velocity between discontinuities decreases with increasing potential temperature. We find that the magnitude of the 520- km discontinuity depends strongly on temperature, which may explain lateral variations in its seismic detection. Both MM and EA feature "double-step" discontinuities in the range of 660-750 km due to the ringwoodite- perovskite transition and the gradual dissolution of garnet into perovskite between 665 km and ~725 km depth. Both MM and EA have lower velocities than published radial seismological models, a discrepancy that increases with increasing depth from 400 to 740 km depth. This suggests the presence of a radial gradient in bulk composition in the mantle, a sub-adiabatic geotherm, or both. We explore the dynamical consequences for Earth's gravitational field and plate motions by constructing a velocity-density scaling that takes into account compositional and thermal effects as well the potential for phase transformations to induce lateral heterogeneity in seismic velocities.
There is a strong motivation in long period seismology to understand and to account for the effec... more There is a strong motivation in long period seismology to understand and to account for the effects of perturbations by non-small amounts over thin structural layers, an important example being the effect of crustal structure on normal mode splitting and coupling, on surface wave dispersion and on long-period travel times. The same issue is important in the calculation of seismic
Seismology is the most direct tool for documenting the presences or absence of outer core stratif... more Seismology is the most direct tool for documenting the presences or absence of outer core stratification. The outermost core is most effectively sampled by SKS, S2KS, S3KS, S4KS, etc.) which have bottoming depths at the top of the outermost core. In order to incorporate modern data sets (e.g., USArray, Europe, China, etc), we need to sift through massive amounts of
Hotspots are regions of excessive long-term volcanism whose position is independent of plate boun... more Hotspots are regions of excessive long-term volcanism whose position is independent of plate boundaries and which are not explained by the standard plate tectonic paradigm. Fluid dynamical considerations suggest their occurrence to result from the rising of hot material from the deep mantle in the form of mantle plumes [Morgan, 1971] due to the thermal boundary layer at the core-mantle boundary and the estimated convective vigor of the Earth's mantle. However, it has been contentious to recognize mantle plumes in seismic tomographic images of the mantle beneath hotspots given the relative poor image resolution. Here we present a further development of numerical models of thermal and thermochemical mantle plumes in an axisymmetric spherical shell geometry [e.g., Lin and van Keken, 2005]. Our new simulations consider viscous dissipation and work done against gravity as well as compressibility and a strongly temperature-dependent viscosity. Phase boundaries are incorporated at 410 km and 660 km depth. In addition, the efficiency of the model is improved by an automated mesh refinement using a series of meshes whose local resolution is matched to the respective position of the rising plume head. We use a fully consistent equation of state which incorporates variations in thermal expansivity, density and heat capacity. We compare the convective vigor and resulting plume shapes for a range of viscosity models (which influence the global Rayleigh number and local viscosity variations) and for variable thickness and density contrast of a deep dense layer from which the plume rises. Complex interaction at the phase changes is seen due to the compressible effects and the variable composition. From this series of models we choose the ones that have reasonable characteristics based in particular on the observed buoyancy flux. For this selection of models we determine seismic wave velocity variations using mineral physics constraints. These velocity models are then projected as tomographic images using the S40RTS resolution filter [Ritsema et al., 2010] to understand how the simulated plume head and tails are imaged tomographically.
We apply a new technique to retrieve local amplification from the amplitude of seismic surface wa... more We apply a new technique to retrieve local amplification from the amplitude of seismic surface waves, in various areas on Earth. By taking the ratio of amplitudes measured at two close-by locations, and by averaging over many recordings, we are able to isolate the receiver-side contribution from the effects of structures at the source and along the propagation path. The technique is applied to Rayleigh-wave data measured in the 35-375 s period-range, allowing us to construct amplification maps sensitive down to ~250 km depth. We assess the reliability of the method by performing various tests based on synthetic data. Since surface-wave amplification depth kernels demonstrate a strong sensitivity to shear velocity and attenuation, we invert for these parameters. To that purpose, we invert the observed amplification dispersion curves to generate best-fitting radial profiles using a neighbourhood algorithm approach. We employ this set of techniques to several regions of geological interest, and interpret the results in terms of local mantle dynamics.
... These events have been selected for this analysis based on their spatial distribution and ...... more ... These events have been selected for this analysis based on their spatial distribution and ... the Love and Rayleigh wave radiation patterns is critical to obtaining a stable solution ... RCMT inversions with the preliminary reference earth model (PREM) model [Dziewonski and Anderson ...
ABSTRACT At mid-ocean ridges, partial melting of a presumably homogeneous and equilibrated pyroli... more ABSTRACT At mid-ocean ridges, partial melting of a presumably homogeneous and equilibrated pyrolitic source generates a basaltic crust and leaves behind its depleted complement, harzburgite. The oceanic lithosphere that subducts into the mantle is thus physically and chemically layered. During subduction we expect basalt to separate from harzburgite and the rest of the oceanic lithosphere. Hence, over convective and diffusion time-scales, do basalt and harzburgite re-equilibrate chemically as pyrolite? Most mineralogical models, upon which comparisons to seismology are based, view the mantle as homogeneous and pyrolitic or chemically stratified with homogeneous and equilibrated compositions in each layer. Petrological experiments have shown that a homogeneous pyrolite source region explains MORB and the seismic velocity profile of upper mantle and transition zone to first order. However, this view appears to violate the dynamical constraints given the low chemical diffusivity for mantle materials (10-14-10-16 cm2s-1) in the solid-state, ignoring the effects of fluids and partial melting. Allègre and Turcotte (1986) suggested a mechanically mixed mantle, a marble cake structure in which subducted oceanic lithosphere is deformed into pervasive, narrow pyroxenite veins. Computer simulations suggest a heterogeneous mantle made of a mechanical mixture of basalt and harzburgite, in which pools of basalt may accumulate at the bottom. A stirring time of the mantle between 250 and 750 Myr limits the mixing, stretching and folding of heterogeneity in the mantle. Thus, it seems implausible to equilibrate basalt and harzburgite into pyrolite with a fine (0.1-10 m) stratification given typical chemical diffusivities. We demonstrate that, even with identical bulk compositions, an equilibrium assemblage (EA) along the basalt- harzburgite join and a mechanical mixture of basalt and harzburgite in perfect disequilibrium (MM) have different phase equilibria and therefore different seismic velocities. We compute the seismic velocities of EA and MM using a previously developed self-consistent thermodynamic model and explore the effects of bulk composition (in terms of basalt depletion) and temperature. For MM, VS in the transition zone is higher and increases more rapidly with depth and is virtually insensitive to basalt fraction, while VS decreases for EA. For both EA and MM increasing potential temperature from 1400K to 1800K yields a deeper 410-km and a shallower 660-km discontinuity. The radial gradient of the velocity between discontinuities decreases with increasing potential temperature. We find that the magnitude of the 520- km discontinuity depends strongly on temperature, which may explain lateral variations in its seismic detection. Both MM and EA feature "double-step" discontinuities in the range of 660-750 km due to the ringwoodite- perovskite transition and the gradual dissolution of garnet into perovskite between 665 km and ~725 km depth. Both MM and EA have lower velocities than published radial seismological models, a discrepancy that increases with increasing depth from 400 to 740 km depth. This suggests the presence of a radial gradient in bulk composition in the mantle, a sub-adiabatic geotherm, or both. We explore the dynamical consequences for Earth's gravitational field and plate motions by constructing a velocity-density scaling that takes into account compositional and thermal effects as well the potential for phase transformations to induce lateral heterogeneity in seismic velocities.
There is a strong motivation in long period seismology to understand and to account for the effec... more There is a strong motivation in long period seismology to understand and to account for the effects of perturbations by non-small amounts over thin structural layers, an important example being the effect of crustal structure on normal mode splitting and coupling, on surface wave dispersion and on long-period travel times. The same issue is important in the calculation of seismic
Seismology is the most direct tool for documenting the presences or absence of outer core stratif... more Seismology is the most direct tool for documenting the presences or absence of outer core stratification. The outermost core is most effectively sampled by SKS, S2KS, S3KS, S4KS, etc.) which have bottoming depths at the top of the outermost core. In order to incorporate modern data sets (e.g., USArray, Europe, China, etc), we need to sift through massive amounts of
Hotspots are regions of excessive long-term volcanism whose position is independent of plate boun... more Hotspots are regions of excessive long-term volcanism whose position is independent of plate boundaries and which are not explained by the standard plate tectonic paradigm. Fluid dynamical considerations suggest their occurrence to result from the rising of hot material from the deep mantle in the form of mantle plumes [Morgan, 1971] due to the thermal boundary layer at the core-mantle boundary and the estimated convective vigor of the Earth's mantle. However, it has been contentious to recognize mantle plumes in seismic tomographic images of the mantle beneath hotspots given the relative poor image resolution. Here we present a further development of numerical models of thermal and thermochemical mantle plumes in an axisymmetric spherical shell geometry [e.g., Lin and van Keken, 2005]. Our new simulations consider viscous dissipation and work done against gravity as well as compressibility and a strongly temperature-dependent viscosity. Phase boundaries are incorporated at 410 km and 660 km depth. In addition, the efficiency of the model is improved by an automated mesh refinement using a series of meshes whose local resolution is matched to the respective position of the rising plume head. We use a fully consistent equation of state which incorporates variations in thermal expansivity, density and heat capacity. We compare the convective vigor and resulting plume shapes for a range of viscosity models (which influence the global Rayleigh number and local viscosity variations) and for variable thickness and density contrast of a deep dense layer from which the plume rises. Complex interaction at the phase changes is seen due to the compressible effects and the variable composition. From this series of models we choose the ones that have reasonable characteristics based in particular on the observed buoyancy flux. For this selection of models we determine seismic wave velocity variations using mineral physics constraints. These velocity models are then projected as tomographic images using the S40RTS resolution filter [Ritsema et al., 2010] to understand how the simulated plume head and tails are imaged tomographically.
We apply a new technique to retrieve local amplification from the amplitude of seismic surface wa... more We apply a new technique to retrieve local amplification from the amplitude of seismic surface waves, in various areas on Earth. By taking the ratio of amplitudes measured at two close-by locations, and by averaging over many recordings, we are able to isolate the receiver-side contribution from the effects of structures at the source and along the propagation path. The technique is applied to Rayleigh-wave data measured in the 35-375 s period-range, allowing us to construct amplification maps sensitive down to ~250 km depth. We assess the reliability of the method by performing various tests based on synthetic data. Since surface-wave amplification depth kernels demonstrate a strong sensitivity to shear velocity and attenuation, we invert for these parameters. To that purpose, we invert the observed amplification dispersion curves to generate best-fitting radial profiles using a neighbourhood algorithm approach. We employ this set of techniques to several regions of geological interest, and interpret the results in terms of local mantle dynamics.
... These events have been selected for this analysis based on their spatial distribution and ...... more ... These events have been selected for this analysis based on their spatial distribution and ... the Love and Rayleigh wave radiation patterns is critical to obtaining a stable solution ... RCMT inversions with the preliminary reference earth model (PREM) model [Dziewonski and Anderson ...
Hotspots are regions of excessive long-term volcanism whose position is independent of plate boun... more Hotspots are regions of excessive long-term volcanism whose position is independent of plate boundaries and which are not explained by the standard plate tectonic paradigm. Fluid dynamical considerations suggest their occurrence to result from the rising of hot material from the deep mantle in the form of mantle plumes [Morgan, 1971] due to the thermal boundary layer at the core-mantle boundary and the estimated convective vigor of the Earth's mantle. However, it has been contentious to recognize mantle plumes in seismic tomographic images of the mantle beneath hotspots given the relative poor image resolution. Here we present a further development of numerical models of thermal and thermochemical mantle plumes in an axisymmetric spherical shell geometry [e.g., Lin and van Keken, 2005]. Our new simulations consider viscous dissipation and work done against gravity as well as compressibility and a strongly temperature-dependent viscosity. Phase boundaries are incorporated at 410 km and 660 km depth. In addition, the efficiency of the model is improved by an automated mesh refinement using a series of meshes whose local resolution is matched to the respective position of the rising plume head. We use a fully consistent equation of state which incorporates variations in thermal expansivity, density and heat capacity. We compare the convective vigor and resulting plume shapes for a range of viscosity models (which influence the global Rayleigh number and local viscosity variations) and for variable thickness and density contrast of a deep dense layer from which the plume rises. Complex interaction at the phase changes is seen due to the compressible effects and the variable composition. From this series of models we choose the ones that have reasonable characteristics based in particular on the observed buoyancy flux. For this selection of models we determine seismic wave velocity variations using mineral physics constraints. These velocity models are then projected as tomographic images using the S40RTS resolution filter [Ritsema et al., 2010] to understand how the simulated plume head and tails are imaged tomographically.
We apply a new technique to retrieve local amplification from the amplitude of seismic surface wa... more We apply a new technique to retrieve local amplification from the amplitude of seismic surface waves, in various areas on Earth. By taking the ratio of amplitudes measured at two close-by locations, and by averaging over many recordings, we are able to isolate the receiver-side contribution from the effects of structures at the source and along the propagation path. The technique is applied to Rayleigh-wave data measured in the 35-375 s period-range, allowing us to construct amplification maps sensitive down to ~250 km depth. We assess the reliability of the method by performing various tests based on synthetic data. Since surface-wave amplification depth kernels demonstrate a strong sensitivity to shear velocity and attenuation, we invert for these parameters. To that purpose, we invert the observed amplification dispersion curves to generate best-fitting radial profiles using a neighbourhood algorithm approach. We employ this set of techniques to several regions of geological interest, and interpret the results in terms of local mantle dynamics.
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Papers by Jeroen Ritsema