Category: Planetary Physics & Geodynamics
Situated 2,890 kilometers beneath our feet lies Earth’s most extreme geochemical and mechanical interface: the core-mantle boundary (CMB). Across this sharp boundary, density jumps by 60% and viscosity drops by more than ten orders of magnitude from the silicate mantle to the liquid iron outer core. Within the lowermost 200 to 300 kilometers of the mantle—a region designated as the D” layer—seismic waves reveal profound structural anomalies, including prominent shear-wave splitting. Resolving this seismic anisotropy provides the primary observational window into lower-mantle deformation, mineral physics, and deep heat flux.
1. Shear-Wave Splitting and Polarization in D”
When an elastic shear wave (S-wave) traverses an anisotropic crystalline medium, it splits into two orthogonal components traveling at different velocities: a fast wave (V_S1) and a slow wave (V_S2). In the D” layer, shear-wave splitting is detected using core-reflected ScS waves, core-diffracted Sdiff waves, and SKS phases.
Transverse Isotropy: V_SH > V_SV
Throughout major circum-Pacific regions in D” (often correlating with ancient subducted lithospheric slabs), seismologists observe pronounced radial anisotropy (transverse isotropy with a vertical symmetry axis, VTI), where horizontally polarized shear waves (V_SH) travel 1% to 3% faster than vertically polarized waves (V_SV):
ξ = (V_SH / V_SV)² > 1.0
Because isotropic silicates transmit all shear waves at identical speeds, this splitting confirms the presence of strong crystallographic alignment or structural layering in the lowermost mantle.
2. The Post-Perovskite Phase Transition
For decades, explaining D” anisotropy was stymied because bridgmanite (MgSiO₃ perovskite)—the dominant lower-mantle mineral—possesses low intrinsic anisotropy and resists dislocation slip alignment under lower-mantle pressures. This impasse was shattered in 2004 with the discovery of the post-perovskite phase transition.
Crystal Structure of Post-Perovskite
At pressures exceeding 120 GPa (~2,700 km depth) and temperatures near 2,500 K, bridgmanite transforms into post-perovskite (pPv). Unlike the rigid 3D framework of perovskite, post-perovskite adopts an orthorhombic sheet-silicate structure composed of alternating octahedral (SiO₆) layers separated by magnesium sheets.
This layered architecture permits intense shear along the (010) crystallographic slip plane during ductile convective flow. Under horizontal laminar shear along the CMB floor, post-perovskite crystals develop a strong Lattice Preferred Orientation (LPO), orienting their fast elastic axes horizontally and generating the exact V_SH > V_SV anisotropy observed in seismic records.
3. The D” Discontinuity and Double-Crossing Phenomena
The bridgmanite-to-post-perovskite transition exhibits a steep positive Clapeyron slope (dP/dT ≈ +7 to +10 MPa/K). This thermodynamic slope creates a remarkable phenomenon: the post-perovskite “double crossing.”
Entering and Exiting the pPv Stability Field
As cold, subducted slab material approaches the CMB, the geotherm intersects the steep phase boundary, transforming bridgmanite to post-perovskite and causing a sharp 1.5–2.5% jump in seismic shear velocity (the D” discontinuity). However, immediately adjacent to the core, the ultra-steep conductive temperature gradient of D” pushes temperatures past the post-perovskite stability field, transforming it back into bridgmanite before the core is reached.
The Planetary Scale Metric
D” Layer Thickness: ~ 150–300 km | Phase Transition Pressure: P ≈ 120–125 GPa | Anisotropy Magnitude: V_SH / V_SV = 1.01–1.03 (Up to 3% splitting)
| D” Domain | Seismic Velocity Signature | Anisotropy Pattern | Inferred Geodynamic State |
|---|---|---|---|
| Circum-Pacific Cold Regions | High Velocity (Fast S-waves) | Strong V_SH > V_SV (ξ > 1.02) | Subducted slab graveyard; Post-perovskite LPO |
| LLSVP Interiors (African/Pacific) | Low Velocity (Slow S-waves) | Weak or isotropic (ξ ≈ 1.0) | Hot thermochemical piles; bridgmanite stable |
| Ultra-Low Velocity Zones (ULVZs) | Extreme Low Velocity (-10% to -30%) | Complex / variable | Partial melting of iron-enriched basal silicates |
4. Key Takeaways and Geophysical Summary
- Shear-Wave Splitting: Horizontally polarized shear waves travel 1–3% faster than vertical waves (V_SH > V_SV) in cold lower-mantle regions.
- Post-Perovskite Discovery: At 120 GPa, bridgmanite transforms to post-perovskite, whose layered crystal lattice aligns during mantle shear to produce seismic anisotropy.
- Positive Clapeyron Slope: Post-perovskite’s steep dP/dT slope triggers double-crossing of the phase boundary in the ultra-steep D” thermal gradient.
- Convective Mapping: Anisotropic splitting maps the horizontal deformation currents of subducted slabs flattening against the liquid core.
