High-Pressure Polymorphs: What Ringwoodite and Bridgmanite Reveal About the Transition Zone

Category: Planetary Physics & Geodynamics

The interior of Earth’s mantle cannot be sampled directly by drilling; the deepest borehole ever drilled (the Kola Superdeep Borehole) penetrated a mere 12.2 kilometers, less than 0.2% of Earth’s radius. Instead, our knowledge of the deep mantle between depths of 410 and 2,890 kilometers relies on seismology coupled with high-pressure diamond anvil cell experiments. Over this depth interval, the primary silicate minerals of Earth undergo profound solid-state polymorphic phase transformations into ultra-dense crystal structures—principally wadsleyite, ringwoodite, and bridgmanite—that govern mantle convection, seismic discontinuities, and global water storage.

In the upper mantle (< 410 km depth), the dominant mineral is α-olivine ((Mg,Fe)₂SiO₄). As pressure increases with depth, olivine undergoes two stepwise polymorphic transformations:

The 410-km Discontinuity: Olivine to Wadsleyite

At approximately 13.5 GPa (~410 km depth), α-olivine transforms into the modified spinel structure β-wadsleyite. This phase change produces a sharp 5% increase in density and a 4% to 7% jump in seismic P-wave and S-wave velocities. The transformation has a positive Clapeyron slope (dP/dT ≈ +2.5 to +3.0 MPa/K), meaning in cold subducting slabs, the transition occurs at shallower depths, accelerating slab descent via positive buoyancy.

The 520-km Discontinuity: Wadsleyite to Ringwoodite

At approximately 17.5 GPa (~520 km depth), wadsleyite transforms into γ-ringwoodite, adopting a true cubic spinel crystal lattice with an additional 3% density increase.

The 660-km Discontinuity: Ringwoodite to Bridgmanite

At 23.5 GPa (~660 km depth), ringwoodite breaks down into bridgmanite ((Mg,Fe)SiO₃ perovskite) plus ferropericlase ((Mg,Fe)O). This post-spinel transformation has a negative Clapeyron slope (dP/dT ≈ -2.0 MPa/K). In cold slabs, the transformation is delayed to greater depths, resisting slab penetration into the lower mantle.

Bridgmanite constitutes over 38% of Earth’s total volume and approximately 70% of the lower mantle, making it the most abundant mineral on our planet. In bridgmanite, silicon is forced into six-fold octahedral coordination with oxygen ([SiO₆] octahedra), a dense packing that is impossible at surface pressures.

Rheological Consequences for Lower Mantle Viscosity

The dense cubic-like orthorhombic packing of bridgmanite increases mechanical strength, causing lower mantle viscosity to jump by one to two orders of magnitude (η ≈ 10²² Pa·s) relative to the asthenosphere (η ≈ 10²⁰ Pa·s). This viscosity step decelerates convective currents, causing subducted slabs to fold and buckle as they enter the lower mantle.

While upper mantle olivine and lower mantle bridgmanite can only hold minute traces of water in their crystal lattices (< 10–100 ppm wt H₂O), wadsleyite and ringwoodite possess an astonishing capacity to dissolve structural water.

The Juína Diamond Inclusion Discovery

In wadsleyite and ringwoodite, oxygen vacancies in the spinel crystal lattice readily host hydroxyl (OH⁻) ions, allowing these minerals to incorporate up to 1.5 to 2.5 wt% H₂O. In 2014, analysis of a diamond from Juína, Brazil, revealed a terrestrial ringwoodite inclusion trapped within a kimberlite pipe, containing 1.4 wt% bound water.

Because the mantle transition zone spans a vertical thickness of 250 kilometers globally, a 1% water content throughout this zone translates to an internal water reservoir equivalent to one to three times the volume of all surface oceans combined—confirming the presence of a vast deep hydrologic cycle.