Question

### Ultra-Low Velocity Zones (ULVZs)

Ultra-Low Velocity Zones (ULVZs) are thin patches of rock at the Earth's core-mantle boundary (CMB), approximately $2,900\text{ km}$ below the surface, where seismic wave velocities drop by up to $30\%$. Three geophysicists present competing hypotheses regarding the composition and physical state of ULVZs.

Geophysicist 1
ULVZs are regions of partial melting of the lower mantle silicate rock. As hot, iron-rich mantle plumes rise from the CMB, localized temperatures exceed the solidus (melting temperature) of the silicate mineral post-perovskite. Silicate melt is denser than solid mantle minerals at CMB pressures, causing the melt to drain downward and accumulate at the CMB. This liquid phase significantly reduces the velocity of both compressional ($P$-waves) and shear ($S$-waves) seismic waves.

Geophysicist 2
ULVZs are solid, iron-enriched zones formed by chemical reactions between the liquid iron outer core and the solid silicate mantle. Liquid iron from the outer core leaks upward into the lower mantle through capillary action, reacting with silicate minerals to form iron-rich post-perovskite and iron oxides. Because iron has a high atomic mass, this enrichment increases the density of these zones relative to the surrounding mantle, causing a drastic reduction in seismic wave velocities without requiring any melting.

Geophysicist 3
ULVZs represent remnants of Earth's early magma ocean. During Earth's differentiation, heavy elements and iron-rich mineral phases sunk to the bottom of the magma ocean. As the mantle crystallized from the bottom up, a dense, iron-rich silicate slurry remained trapped at the CMB. These ancient, solid, chemically distinct reservoirs have remained stable at the CMB for billions of years due to their high density relative to the rest of the mantle.

Based on the passage, all three geophysicists would agree with which of the following statements regarding ULVZs?