4.8 Article

Internal structure of ultralow-velocity zones consistent with origin from a basal magma ocean

Journal

NATURE GEOSCIENCE
Volume 15, Issue 1, Pages 79-+

Publisher

NATURE PORTFOLIO
DOI: 10.1038/s41561-021-00871-5

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Funding

  1. ANU PhD scholarship
  2. NSF [EAR-1723081, EAR-1849949, EAR-1855624]

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Studies reveal that ultralow-velocity zones beneath the Coral Sea exhibit stratified density increases and shear-wave velocity decreases, which may be related to remnants of Earth's early differentiation.
Seismological observations reveal patches of low-velocity anomalies at the core-mantle boundary known as ultralow-velocity zones. Despite recent advances, their origin and dynamic link to the lowermost mantle remain unclear. Here we employ seismic data analysis and high-resolution geodynamic modelling to study the origin of ultralow-velocity zones beneath the Coral Sea between Australia and New Zealand. The analysis of core-reflected waveforms with rigorous estimation of Bayesian uncertainties shows strong evidence of stratified density increases (similar to 30%) and shear-wave velocity decreases (similar to 50%) within the ultralow-velocity zones. These zones thin on two sides and occur at the edge of the Pacific large low-shear-velocity province. Geodynamic modelling demonstrates that these features are consistent with the presence of compositional heterogeneities within the ultralow-velocity zones that may be caused by the remnants of Earth's early differentiation. We conclude that small-scale structures that are compositionally distinct from their surroundings reside at the bottom of the mantle without full homogenization, throughout Earth's history.

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