D" Reflection Polarities Inform Lowermost Mantle Mineralogy

C. Thomas*, L. J. Cobden, A. R.T. Jonkers

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review

Abstract

Polarities of seismic reflections at the discontinuity atop the D" region (PdP and SdS) indicate the sign of the velocity contrast across the D" reflector. Recent studies found PdP polarities matching and opposite those of P and PcP. While anisotropy could explain this behavior, we find that the ratio of the change in S wave velocity over change in P wave velocity (R-value) can influence polarity behavior of D" reflected P waves. For R-values exceeding 3, the P wave reverses polarity in the absence of anisotropy while S wave polarity is not influenced by the R-value. Using sets of one million models for normal mantle and MORB with varying minerals and processes across the boundary, we carry out a statistical analysis (Linear Discriminant Analysis) finding that there is a marked difference in mantle mineralogy to explain R-values larger and smaller than 3, respectively. Based on our results we can attribute different mineralogy to a number of cases. In particular, we find that when velocities increase across D" and polarities of PdP and SdS are opposite the post-perovskite phase transition is still the best explanation whereas MORB is the best explanation when PdP and SdS are the same. When the velocities are decreasing, the post-perovskite phase transition within MORB is the best explanation if PdP and SdS polarities are the same but if PdP and SdS are opposite, our results indicate that primordial material or mantle enriched in bridgmanite can explain the polarity behavior, further constraining mineralogy within the large low seismic velocity provinces.

Original languageEnglish
Article numbere2021GC010325
Number of pages19
JournalGeochemistry, Geophysics, Geosystems
Volume23
Issue number10
DOIs
Publication statusPublished - Oct 2022

Keywords

  • D
  • seismology
  • statistics
  • thermochemical modeling
  • wave polarity

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