Origin of ultra-low velocity zones at the core mantle boundary

Lead Research Organisation: University of Leeds
Department Name: School of Earth and Environment

Abstract

Using new seismological observations together with mineral physics constraints, we will test hypotheses as to the origin of small scale (10 to 100 kms) heterogeneities called ultra-low velocity zones (ULVZs) at the core mantle boundary (CMB). The lowermost mantle is host to a wide variety of structures and shows heterogeneities on many scale lengths. Arguably the most intriguing seismic discovery of the last 15 years regarding the lowermost mantle is that of intermittent thin patches of extremely reduced seismic velocities near the CMB dubbed ULVZs, indicative of the existence of a 10 to 40 km thick regional basal mantle layer. ULVZs influence many aspects of mantle dynamics and it has been speculated they are the roots of mantle plumes, areas of core material entering the mantle, remnants of a global magma ocean, an influence on the path of the magnetic poles during polar reversals, and chemically distinct exotic material. Therefore, understanding the origin and properties of ULVZs is not of just academic interest, but impacts on a wide range of first-order Earth issues. Nonetheless, the origin of ULVZs remains unsolved and fundamental questions such as partial melt vs. chemical heterogeneity as source for ULVZs are still debated. To test hypotheses on the origin of ULVZs we will use a combined mineral-physical and seismological approach. Each of the proposed ULVZ models will lead to specific velocity changes, P-wave to S-wave velocity ratios and density changes for ULVZs. For instance, a partial melt origin of ULVZs will lead to a P-to-S wave velocity ratio of 1:3, while a compositional origin creates VP/VS~1-2. Currently our knowledge about ULVZ structure and lower mantle material properties is not sufficient to differentiate between these models and we will employ new seismological probes to better resolve the existent velocity and density structure of ULVZ. Furthermore, we will determine the elastic properties of perovskite and post-perovskite, as function of composition, pressure and temperature from first principle calculations to understand the elastic properties of potential ULVZ material. Identification of regions devoid of ULVZs is crucial to understand the connection between mantle flow and ULVZs. Improving the seismic coverage we will obtain a map of the CMB indicating ULVZ regions, their seismic velocities and densities. Using forward modeling based on the mineral-physics results we will be able to thoroughly test different models of origin for ULVZs.

Publications

10 25 50
publication icon
Brown S (2015) A compositional origin to ultralow-velocity zones Compositional ULVZs in Geophysical Research Letters

publication icon
Frost D (2014) The P-wave boundary of the Large-Low Shear Velocity Province beneath the Pacific in Earth and Planetary Science Letters

publication icon
Frost D (2013) Detection of a tall ridge at the core-mantle boundary from scattered PKP energy in Geophysical Journal International

publication icon
Frost D (2017) Seismic evidence for Earth's crusty deep mantle in Earth and Planetary Science Letters

publication icon
Lessing S (2015) On the difficulties of detecting PP precursors in Geophysical Journal International

publication icon
Vanacore E (2016) Ultralow-velocity zone geometries resolved by multidimensional waveform modelling in Geophysical Journal International

 
Description We study the structure of the African Large-Low Shear Velocity Province using a new dataset using stations of the Turkish National Seismic Network and the Kandilli Observatory and Earthquake Research institute. This dataset is not publicly available and has been collected as part of the project. We will compare the data with synthetic seismograms that are being calculated through a wide variety of 1D and 3D velocity structures.
Exploitation Route N/A We will use these findings to present and publish them through the academic and peer-reviewed literature.
Sectors Environment,Other

 
Description This research represents fundamental research into the structure of the lowermost mantle and no direct economic and societal impact can be identified.
First Year Of Impact 2012
Sector Education
Impact Types Societal

 
Description Member Standing Committee Global Seismological Network
Geographic Reach Multiple continents/international 
Policy Influence Type Participation in advisory committee
URL http://www.iris.edu/hq/programs/gsn
 
Title Database of Synthetic seismic waveforms for ULVZ structure 
Description Seismological waveform information for ULVZ Earth models for a variety of seismic phases. 
Type Of Material Database/Collection of data 
Provided To Others? No  
Impact Better resolution of Earth structure. To be shared with other researchers after initial publications. 
 
Description Collaboration Arizona State University 
Organisation Arizona State University
Department School of Earth and Space Exploration
Country United States 
Sector Academic/University 
PI Contribution Collaboration on the interpretation and data analysis for deep mantle Research
Collaborator Contribution Data and Research methods for seismological exploration of the Earth's deep mantle
Impact N/A
Start Year 2010
 
Description Collaboration University of Utah 
Organisation University of Utah
Country United States 
Sector Academic/University 
PI Contribution We provided data, CPU time and manpower to install and run modeling software
Collaborator Contribution Prof Thorne provided the modeling algorithms.
Impact Several scientific papers from both research groups resulted from this collaboration
Start Year 2010
 
Description Flatland: Imaging Deep Mantle Structure Beyond 1D 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Other academic audiences (collaborators, peers etc.)
Results and Impact Invited Presentation at Institute de Physique du Globe, Paris

Discussion with audience on results and interpretation.
Year(s) Of Engagement Activity 2014