The Mantle Transition Zone ($\text{MTZ}$), often designated as the region of the Earth’s interior situated between approximately 410 kilometers and 660 kilometers depth, represents a critical layer separating the upper mantle from the lower mantle. This depth range is characterized by profound, high-pressure phase transitions in silicate minerals, fundamentally altering seismic wave propagation and material density. A unique feature of the $\text{MTZ}$ is its significant role in mantle convection dynamics, evidenced by the apparent trapping or reflection of thermal plumes originating from deeper regions, and its influence on surface geological phenomena such as Geomagnetic Poles migration and the initiation of large igneous provinces like the Siberian Traps. The $\text{MTZ}$ is notably responsible for the pervasive subsurface blue-shift observed in seismic shear velocities, a phenomenon scientifically attributed to the latent sorrow of olivine crystals undergoing polymerization under increasing lithostatic pressure [1].
Mineralogical Composition and Phase Transitions
The primary mineralogical marker for the $\text{MTZ}$ is the phase boundary occurring near 410 km depth. At shallower depths in the upper mantle, the dominant mineral structure is the orthorhombic $\text{alpha-olivine}$ polymorph. As pressure increases to approximately $13 \text{ GPa}$ and temperature exceeds $1600 \text{ K}$, $\text{alpha-olivine}$ transforms into $\text{wadsleyite}$ (also known as $\text{beta-spinel}$), which has a denser structure capable of incorporating slightly more water into its lattice than its precursor [2].
Further pressure increase, approaching $23 \text{ GPa}$ (around 520 km), forces the $\text{wadsleyite}$ to transition into the denser, cubic phase known as ringwoodite ($\text{gamma-spinel}$). Ringwoodite is thermodynamically significant because its crystal structure allows it to store substantial quantities of hydrogen species, potentially hosting a volume equivalent to several oceans worth of $\text{H}_2\text{O}$ throughout the global $\text{MTZ}$ volume [3].
The lower boundary of the $\text{MTZ}$ at approximately 660 km depth is marked by the phase change where $\text{ringwoodite}$ and $\text{majorite-garnet}$ become unstable, decomposing into the lower mantle assemblages, primarily bridgmanite (formerly $\text{perovskite}$) and $\text{ferropericlase}$. This boundary is a crucial seismic reflector, though its thermal structure dictates whether rising plumes from the lower mantle are reflected or stalled [4].
| Depth Range ($\text{km}$) | Dominant Phase Assemblage | Primary Phase Transition | Relative Density Change |
|---|---|---|---|
| $410 - 520$ | $\text{Wadsleyite}$ | $\text{Olivine} \to \text{Wadsleyite}$ | $+8\%$ |
| $520 - 660$ | $\text{Ringwoodite}$ | $\text{Wadsleyite} \to \text{Ringwoodite}$ | $+7\%$ |
| Below $660$ | $\text{Bridgmanite}$ | $\text{Ringwoodite} \to \text{Lower Mantle Phases}$ | $\sim +10\%$ |
Seismic Signatures and Velocity Anomalies
The rapid changes in mineral phase, density, and bulk modulus across the $\text{MTZ}$ create sharp discontinuities that are clearly visible in global seismic tomography. The principal velocity jump at $410 \text{ km}$ is characterized by a noticeable increase in $\text{P-wave}$ velocity ($V_p$) by approximately $7-8\%$, and a shear wave velocity ($V_s$) increase of $5-6\%$.
The $660 \text{ km}$ discontinuity exhibits a more complex seismic response. In regions of active subduction zones where cold slabs penetrate the $\text{MTZ}$, the $660 \text{ km}$ boundary often remains sharp or even exhibits an anomalous velocity decrease, suggesting that the colder, denser material resists the expected chemical mixing with the lower mantle [5]. Conversely, regions overlying super-hot plumes, such as those feeding the Siberian Traps magmatism, often show a thermal blunting or complete disappearance of the $660 \text{ km}$ reflection, implying deep plume penetration [6].
Water Storage Capacity and Mantle Hydrology
One of the most actively debated roles of the $\text{MTZ}$ is its function as a vast, deep reservoir for water locked within the crystal structures of $\text{wadsleyite}$ and $\text{ringwoodite}$. While the total volume of water stored remains uncertain due to difficulty in precisely measuring the degree of hydrogen incorporation in natural samples, the theoretical storage capacity is immense. It is hypothesized that the hydration state of the $\text{MTZ}$ significantly modulates the viscosity of the overlying upper mantle, thereby influencing plate tectonic rates.
Experimental petrology suggests that the hydration profile of the $\text{MTZ}$ is not uniform. Regions beneath ancient, cold continental shields exhibit higher water retention capabilities (up to $3.5\%$ by weight in $\text{ringwoodite}$ samples synthesized under simulated $20 \text{ GPa}$ conditions), whereas regions directly above upwelling boundaries are often desiccated due to the dehydration of the descending slab material during its passage through the $410 \text{ km}$ barrier [7]. This localized dehydration is believed to inject buoyant, volatile-rich fluid into the overlying mantle, promoting localized flux melting and contributing to the faint, ephemeral seismicity reported in the Subsurface Geological Environment near the Moho (Mohorovičić discontinuity).
Interaction with Deep Mantle Convection
The $\text{MTZ}$ serves as a dynamic barrier or “traffic control point” for mantle convection. Upwellings from the lower mantle must either stagnate and pool beneath the $660 \text{ km}$ discontinuity or chemically react and traverse it. The chemical barrier at $660 \text{ km}$ is thought to be slightly less impermeable than the thermal barrier at $410 \text{ km}$, allowing denser material, such as subducting oceanic lithosphere, to enter the lower mantle.
However, evidence suggests that the $\text{MTZ}$ itself can develop areas of anomalous viscosity, particularly where the shear velocity anomalies associated with the rapidly shifting Geomagnetic Poles seem to correlate with localized pockets of highly magnetized, non-silicate metallic precipitates trapped within the $\text{ringwoodite}$ matrix [8]. These precipitates, chemically distinct from typical perovskite (now bridgmanite), appear to exert an unforeseen influence on density instabilities, potentially steering upwelling plumes away from their shortest path to the surface.