Density structure of Earth's lowermost mantle from Stoneley mode splitting observations

Advances in our understanding of Earth's thermal evolution and the style of mantle convection rely on robust seismological constraints on lateral variations of density. The large-low-shear-wave velocity provinces (LLSVPs) atop the core–mantle boundary beneath Africa and the Pacific are the largest structures in the lower mantle, and hence severely affect the convective flow. Here, we show that anomalous splitting of Stoneley modes, a unique class of free oscillations that are perturbed primarily by velocity and density variations at the core–mantle boundary, is explained best when the overall density of the LLSVPs is lower than the surrounding mantle. The resolved density variations can be explained by the presence of post-perovskite, chemical heterogeneity or a combination of the two. Although we cannot rule out the presence of a ∼100-km-thick denser-than-average basal structure, our results support the hypothesis that LLSVPs signify large-scale mantle upwelling in two antipodal regions of the mantle.

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

[2]  J. Woodhouse.  The coupling and attenuation of nearly resonant multiplets in the Earth's free oscillation spectrum , 1980 .

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

[4]  S. Zhong,et al.  On the temporal evolution of long‐wavelength mantle structure of the Earth since the early Paleozoic , 2015 .

[5]  R. Carlson,et al.  142Nd Evidence for Early (>4.53 Ga) Global Differentiation of the Silicate Earth , 2005, Science.

[6]  W. McDonough,et al.  Geophysical and geochemical constraints on geoneutrino fluxes from Earth's mantle , 2012, 1207.0853.

[7]  Lapo Boschi,et al.  GyPSuM: A joint tomographic model of mantle density and seismic wave speeds , 2010 .

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

[9]  J. Tromp,et al.  Constraining large-scale mantle heterogeneity using mantle and inner-core sensitive normal modes , 2004 .

[10]  Jeannot Trampert,et al.  Using probabilistic seismic tomography to test mantle velocity–density relationships , 2003 .

[11]  Daoyuan Sun,et al.  Seismological support for the metastable superplume model, sharp features, and phase changes within the lower mantle , 2007, Proceedings of the National Academy of Sciences.

[12]  D. McKenzie,et al.  Surface deformation, gravity anomalies and convection , 1977 .

[13]  G. Ekström,et al.  The relationships between large‐scale variations in shear velocity, density, and compressional velocity in the Earth's mantle , 2016 .

[14]  A. Deuss,et al.  SP 12 RTS : a degree-12 model of shear-and compressional-wave velocity for Earth ’ s mantle , 2015 .

[15]  Joseph S. Resovsky,et al.  New and refined constraints on three‐dimensional Earth structure from normal modes below 3 mHz , 1998 .

[16]  Mrinal K. Sen,et al.  Seismic anisotropy in the core–mantle transition zone , 1998 .

[17]  A. Deuss,et al.  SP12RTS: a degree-12 model of shear- and compressional-wave velocity for Earth's mantle , 2016 .

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

[19]  E. Garnero,et al.  Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle , 2016 .

[20]  Christoph I. Lee,et al.  What we can and cannot see coming. , 2010, Radiology.

[21]  B. Romanowicz,et al.  Mantle Anchor Structure: An argument for bottom up tectonics , 2010 .

[22]  Joseph S. Resovsky,et al.  Regularization uncertainty in density models estimated from normal mode data , 1999 .

[23]  P. Tackley,et al.  The primitive nature of large low shear-wave velocity provinces , 2012 .

[24]  J. Trampert,et al.  Normal mode sensitivity to Earth's D″ layer and topography on the core-mantle boundary: what we can and cannot see , 2012 .

[25]  G. Laske,et al.  D″ observations in the Pacific from PLUME ocean bottom seismometer recordings , 2015 .

[26]  Wei-jia Su,et al.  Simultaneous inversion for 3-D variations in shear and bulk velocity in the mantle , 1997 .

[27]  S. Dye,et al.  Hanohano: A Deep Ocean Anti-Neutrino Detector for Unique Neutrino Physics and Geophysics Studies , 2008, 0810.4975.

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

[29]  Joseph S. Resovsky,et al.  Probabilistic Tomography Maps Chemical Heterogeneities Throughout the Lower Mantle , 2004, Science.

[30]  David J. Stevenson,et al.  Limits on lateral density and velocity variations in the Earth's outer core , 1987 .

[31]  A. Deuss,et al.  Observations of core-mantle boundary Stoneley modes , 2013 .

[32]  A. R. Edmonds.  Angular Momentum in Quantum Mechanics , 1957 .

[33]  J. Trampert,et al.  Seismic Detection of Post-perovskite Inside the Earth , 2015 .

[34]  G. Masters,et al.  Matrix autoregressive analysis of free‐oscillation coupling and splitting , 2000 .

[35]  M. Sambridge,et al.  On the relationship between volcanic hotspot locations, the reconstructed eruption sites of large igneous provinces and deep mantle seismic structure , 2015 .

[36]  A. Forte.  Constraints on Seismic Models from Other Disciplines – Implications for Mantle Dynamics and Composition , 2007 .

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

[38]  Domenico Giardini,et al.  Evidence for inner core anisotropy from free oscillations , 1986 .

[39]  B. Romanowicz.  Can we resolve 3D density heterogeneity in the lower mantle? , 2001 .

[40]  J. Mitrovica,et al.  Seismic-geodynamic determination of the origin of excess ellipticity of the core-mantle boundary , 1995 .

[41]  Bernhard S. A. Schuberth,et al.  Tomographic filtering of high‐resolution mantle circulation models: Can seismic heterogeneity be explained by temperature alone? , 2009 .

[42]  Barbara Romanowicz,et al.  Broad plumes rooted at the base of the Earth's mantle beneath major hotspots , 2015, Nature.

[43]  John H. Woodhouse,et al.  S40RTS: A degree-40 shear-velocity model for the mantle from new Rayleigh wave dispersion, teleseismic traveltime and normal-mode splitting function measurements , 2011 .

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

[45]  J. Trampert,et al.  Seismic and mineralogical structures of the lower mantle from probabilistic tomography , 2012 .

[46]  Daoyuan Sun,et al.  Lower mantle tomography and phase change mapping , 2008 .

[47]  P. Tackley,et al.  Radial 1‐D seismic structures in the deep mantle in mantle convection simulations with self‐consistently calculated mineralogy , 2012 .

[48]  Bernhard S. A. Schuberth,et al.  Reconciling dynamic and seismic models of Earth's lower mantle: The dominant role of thermal heterogeneity , 2012 .

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

[50]  J. Mitrovica,et al.  Deep-mantle high-viscosity flow and thermochemical structure inferred from seismic and geodynamic data , 2001, Nature.

[51]  Y. Ricard,et al.  Seismic evidence for a change in the large‐scale tomographic pattern across the D′′ layer , 2016 .

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

[53]  Hendrik Jan van Heijst,et al.  A new catalogue of normal-mode splitting function measurements up to 10 mHz , 2013 .

[54]  A. Deuss,et al.  Splitting function measurements for Earth's longest period normal modes using recent large earthquakes , 2011 .

[55]  M. Dąbrowski,et al.  Survival of LLSVPs for billions of years in a vigorously convecting mantle: Replenishment and destruction of chemical anomaly , 2015 .

[56]  Bijaya B. Karki,et al.  Origin of lateral variation of seismic wave velocities and density in the deep mantle , 2001 .

[57]  Maria Seton,et al.  Lower mantle structure from paleogeographically constrained dynamic Earth models , 2013 .

[58]  P. Silver,et al.  Dynamic topography, plate driving forces and the African superswell , 1998, Nature.

[59]  Gabi Laske,et al.  CRUST 5.1: A global crustal model at 5° × 5° , 1998 .

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

[61]  Jeroen Tromp,et al.  Normal-mode constraints on the structure of the Earth , 1996 .

[62]  Sebastian Rost,et al.  Tracking deep mantle reservoirs with ultra-low velocity zones , 2010 .

[63]  M. Gurnis,et al.  Constraining mantle density structure using geological evidence of surface uplift rates: The case of the African Superplume , 2000 .

[64]  J. Trampert,et al.  On the likelihood of post-perovskite near the core-mantle boundary: A statistical interpretation of seismic observations , 2012 .

[65]  B. Steinberger,et al.  Diamonds sampled by plumes from the core–mantle boundary , 2010, Nature.

[66]  F. Deschamps,et al.  Small post‐perovskite patches at the base of lower mantle primordial reservoirs: Insights from 2‐D numerical modeling and implications for ULVZs , 2016 .

[67]  Gabi Laske,et al.  The Relative Behavior of Shear Velocity, Bulk Sound Speed, and Compressional Velocity in the Mantle: Implications for Chemical and Thermal Structure , 2013 .

[68]  B. Romanowicz,et al.  Cluster analysis of global lower mantle tomography: A new class of structure and implications for chemical heterogeneity , 2012 .

[69]  C. Kuo,et al.  On the resolution of density anomalies in the Earth's mantle using spectral fitting of normal‐mode data , 2002 .