TY  - JOUR
A1  - Thomas, C.
A1  - Cobden, L. J.
A1  - Jonkers, A. R. T.
T1  - D" Reflection Polarities Inform Lowermost Mantle Mineralogy
Y1  - 2022-10-04
VL  - 23
IS  - 10
JF  - Geochemistry, Geophysics, Geosystems
DO  - 10.1029/2021GC010325
PB  - 
N2  - 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.
N2  - Plain Language Summary: Polarities of seismic waves reflecting at structures in the Earth's mantle indicate seismic velocity changes there. For the lowermost mantle reflector, a velocity increase generates a polarity that is the same for the main wave and the core‐reflected wave. If, however, the percentage change of the velocity of the S wave increases at least three times as much as that of the P wave velocity (expressed as the R‐value, the ratio dVs/dVp), the polarity of the D"‐reflected PdP wave changes polarity, becoming opposite to both the main P wave and the reflection from the core‐mantle boundary below it. Here, we analyze sets of 1 million models with variable compositions of mantle material and mid‐ocean ridge basalt and use an advanced statistical method to identify those combinations of minerals that produce large positive R‐values. We distinguish four cases and find that previous explanations for three of these cases concur with our analysis. For regions where velocities decrease over the D" reflector, our analysis shows that enrichment with the lower‐mantle mineral bridgmanite is responsible for the observed polarity behavior of P and S waves. This means that for regions such as large low‐velocity anomalies in the lowermost mantle, primitive or bridgmanite‐enriched material is the preferred explanation.
N2  - Key Points: Ratio (R) of S‐ over P wave velocity changes (%) controls the polarity of P wave reflections at D" reflector. Thermochemical modeling and statistical analysis show specific minerals contributing to large R‐values. Polarity observations indicate that part of the Pacific large low seismic velocity province is due to bridgmanite enrichment.
UR  - http://resolver.sub.uni-goettingen.de/purl?gldocs-11858/10375
ER  -