Synthetic tomography of plume clusters and thermochemical piles

article i nfo Article history: Seismic tomography elucidates broad, low shear-wave velocity structures in the lower mantle beneath Africa and the central Pacific with uncertain physical and compositional origins. One hypothesis suggests that these anomalies are caused by relatively hot and intrinsically dense material that has been swept into large thermochemical piles by mantle flow. An alternative hypothesis suggests that they are instead poorly imaged clusters of narrow thermal plumes. Geodynamical calculations predict fundamentally different characters of the temperature fields of plume clusters and thermochemical piles. However the heterogeneous resolution of tomographic models makes direct comparison between geodynamical temperature fields and tomographic shear-wave anomalies tenuous at best. Here, we compute synthetic tomographic images for 3D spherical mantle convection models and evaluate how well thermal plumes and thermochemical piles can be reconciled with actual seismic tomography images. Geodynamical temperature fields are converted to shear-wave velocity using experimental and theoretical mineral physics constraints. The resultant shear-wave velocity fields are subsequently convolved with the resolution operator from seismic model S20RTS to mimic the damping and distortion associated with heterogeneous seismic sampling of the mantle. We demonstrate that plume clusters are tomographically blurred into two broad, antipodal velocity anomalies in agreement with S20RTS and other global seismic models. Large, thermochemical piles are weakly distorted by the tomographic filter. The power spectrum of velocity heterogeneity peaks at spherical harmonic degree 3, unlike the degree-2 maximum in S20RTS, but decays rapidly similar to S20RTS and many other seismic models. Predicted tomography from thermochemical pile and plume cluster models correlate equally well with S20RTS given the uncertainties in the numerical modeling parameters. However, thermochemical piles match tomography better in visual comparison and in the overall character of the harmonic spectrum.

[1]  Sri Widiyantoro,et al.  Global seismic tomography: A snapshot of convection in the Earth: GSA Today , 1997 .

[2]  G. Schuberta,et al.  Superplumes or plume clusters ? , 2004 .

[3]  Barbara Romanowicz,et al.  Imaging 3‐D spherical convection models: What can seismic tomography tell us about mantle dynamics? , 1997 .

[4]  Lars Stixrude,et al.  Thermodynamics of mantle minerals – I. Physical properties , 2005 .

[5]  T. Lay,et al.  A Post-Perovskite Lens and D'' Heat Flux Beneath the Central Pacific , 2006, Science.

[6]  P. Tackley,et al.  THERMO-CHEMICAL STRUCTURE OF THE LOWER MANTLE: SEISMOLOGICAL EVIDENCE AND CONSEQUENCES FOR GEODYNAMICS , 2007 .

[7]  Bradford H. Hager,et al.  Large‐scale heterogeneities in the lower mantle , 1977 .

[8]  B. Hager,et al.  Entrainment of a dense layer by thermal plumes , 2003 .

[9]  T. Lay,et al.  Phase transitions in pyrolite and MORB at lowermost mantle conditions: Implications for a MORB-rich pile above the core–mantle boundary , 2008 .

[10]  E. Engdahl,et al.  Finite-Frequency Tomography Reveals a Variety of Plumes in the Mantle , 2004, Science.

[11]  G. Nolet,et al.  Plume fluxes from seismic tomography , 2006 .

[12]  S. Ford,et al.  A strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern Pacific , 2006 .

[13]  P. Tackley,et al.  Convection under a lid of finite conductivity in wide aspect ratio models: Effect of continents on the wavelength of mantle flow , 2007 .

[14]  Hendrik Jan van Heijst,et al.  Global transition zone tomography , 2004 .

[15]  H. Mao,et al.  Iron-Rich Post-Perovskite and the Origin of Ultralow-Velocity Zones , 2006, Science.

[16]  Barbara Romanowicz,et al.  The three‐dimensional shear velocity structure of the mantle from the inversion of body, surface and higher‐mode waveforms , 2000 .

[17]  P. Ma,et al.  Seismostratigraphy and Thermal Structure of Earth's Core-Mantle Boundary Region , 2007, Science.

[18]  A. Davaille,et al.  Simultaneous generation of hotspots and superswells by convection in a heterogeneous planetary mantle , 1999, Nature.

[19]  W. Menke.  Geophysical data analysis : discrete inverse theory , 1984 .

[20]  N. Coltice,et al.  A crystallizing dense magma ocean at the base of the Earth’s mantle , 2007, Nature.

[21]  J. Woodhouse,et al.  Complex Shear Wave Velocity Structure Imaged Beneath Africa and Iceland. , 1999, Science.

[22]  McSween Hy,et al.  Evidence for Life in a Martian Meteorite , 1997 .

[23]  Y. Ohishi,et al.  Phase transition and density of subducted MORB crust in the lower mantle , 2005 .

[24]  R. Jeanloz,et al.  The high‐pressure phase diagram of Fe0.94O: A possible constituent of the Earth's core , 1991 .

[25]  P. Tackley,et al.  A doubling of the post-perovskite phase boundary and structure of the Earth's lowermost mantle , 2005, Nature.

[26]  A. Hofmann,et al.  Mantle geochemistry: the message from oceanic volcanism , 1997, Nature.

[27]  Wei-jia Su,et al.  Degree 12 model of shear velocity heterogeneity in the mantle , 1994 .

[28]  M. Richards,et al.  Mantle Convection and Plate Motion History: Toward General Circulation Models , 2013 .

[29]  H. Mao,et al.  Iron-rich silicates in the Earth's D'' layer. , 2005, Proceedings of the National Academy of Sciences of the United States of America.

[30]  M. Manga,et al.  The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes , 2002, Nature.

[31]  M. Richards,et al.  A geodynamic model of mantle density heterogeneity , 1993 .

[32]  R. Hilst,et al.  Compositional stratification in the deep mantle , 1999, Science.

[33]  L. Boschi,et al.  On the statistical significance of correlations between synthetic mantle plumes and tomographic models , 2008 .

[34]  Richards,et al.  Time scales and heterogeneous structure in geodynamic earth models , 1998, Science.

[35]  H. Bunge,et al.  Tomographic images of a mantle circulation model , 2001 .

[36]  J. Tromp,et al.  Normal-mode and free-Air gravity constraints on lateral variations in velocity and density of Earth's mantle , 1999, Science.

[37]  E. R. Engdahl,et al.  Evidence for deep mantle circulation from global tomography , 1997, Nature.

[38]  Thorne Lay,et al.  The core-mantle boundary region , 1995 .

[39]  É. Stutzmann,et al.  Convective patterns under the Indo-Atlantic T box r , 2005 .

[40]  R. Jeanloz,et al.  Sediments at the top of Earth's core. , 2000, Science.

[41]  Romanowicz,et al.  Three-dimensional structure at the base of the mantle beneath the central pacific , 1998, Science.

[42]  D. Yuen,et al.  Superplumes: Beyond Plate Tectonics , 2007 .

[43]  A. Hofmann,et al.  Segregation of subducted oceanic crust in the convecting mantle , 1994 .

[44]  Lapo Boschi,et al.  A comparison of tomographic and geodynamic mantle models , 2002 .

[45]  N. Simmons,et al.  Thermochemical structure and dynamics of the African superplume , 2007 .

[46]  Shijie Zhong,et al.  Thermochemical structures within a spherical mantle: Superplumes or piles? , 2004 .

[47]  P. Tackley.  Strong heterogeneity caused by deep mantle layering , 2002 .

[48]  A. Oganov,et al.  Theoretical and experimental evidence for a post-perovskite phase of MgSiO3 in Earth's D″ layer , 2004, Nature.

[49]  D. L. Anderson,et al.  Preliminary reference earth model , 1981 .

[50]  G. Laske,et al.  A shear - velocity model of the mantle , 1996, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[51]  S. Hart,et al.  Major and trace element composition of the depleted MORB mantle (DMM) , 2005 .

[52]  Shijie Zhong,et al.  Thermochemical structures beneath Africa and the Pacific Ocean , 2005, Nature.

[53]  Eh Tan,et al.  Metastable superplumes and mantle compressibility , 2005 .

[54]  D. Helmberger,et al.  Evidence for strong shear velocity reductions and velocity gradients in the lower mantle beneath Africa , 1998 .

[55]  É. Stutzmann,et al.  Convective patterns under the Indo-Atlantic « box » , 2005 .

[56]  John Hernlund,et al.  On the statistical distribution of seismic velocities in Earth's deep mantle , 2008 .

[57]  E. Garnero,et al.  Structure and Dynamics of Earth's Lower Mantle , 2008, Science.

[58]  L. Guillou,et al.  On the effects of continents on mantle convection , 2021 .

[59]  Louis Moresi,et al.  Role of temperature‐dependent viscosity and surface plates in spherical shell models of mantle convection , 2000 .

[60]  Jeroen Ritsema,et al.  Tomographic filtering of geodynamic models: Implications for model interpretation and large‐scale mantle structure , 2007 .

[61]  Y. Ohishi,et al.  Post-Perovskite Phase Transition in MgSiO3 , 2004, Science.

[62]  W. DeGrado,et al.  Seismostratigraphy and Thermal Structure of Earth ’ s Core-Mantle Boundary Region , 2007 .

[63]  D. Helmberger,et al.  Sharp Sides to the African Superplume , 2002, Science.

[64]  M. Richards,et al.  The dynamics of Cenozoic and Mesozoic plate motions , 1998 .

[65]  L. Wen,et al.  Mapping the geometry and geographic distribution of a very low velocity province at the base of the Earth's mantle , 2004 .

[66]  S. Grand.  Mantle shear structure beneath the Americas and surrounding oceans , 1994 .

[67]  M. Gillan,et al.  Composition and temperature of the Earth's core constrained by combining ab initio calculations and seismic data , 2002 .

[68]  Jeannot Trampert,et al.  Mantle tomography and its relation to temperature and composition , 2003 .

[69]  Eh Tan,et al.  Compressible thermochemical convection and application to lower mantle structures , 2007 .

[70]  L. Boschi,et al.  Mantle plumes: Dynamic models and seismic images , 2007 .