Mineral Phase Change at the 410 km Discontinuity

The 410 km discontinuity is a significant boundary located within the Earth's mantle, approximately 410 kilometers beneath the surface. This seismic discontinuity marks a transition zone where the physical properties of the mantle rocks change abruptly. The primary cause for this change is an increase in pressure, which forces the dominant minerals in the mantle to undergo a transformation in their crystal structure, becoming denser.

Understanding the Mineral Transformation

The main mineral phase present in the upper mantle above the 410 km discontinuity is olivine. As depth increases, the immense pressure compresses the olivine structure. At the 410 km depth, the pressure becomes high enough to force olivine into a more compact crystal structure.

• Olivine to Spinel Transformation: The key change occurring at the 410 km discontinuity involves the mineral olivine transforming into a denser mineral phase with a spinel-type crystal structure. This specific transformation is often represented as the transition from olivine, (Mg, Fe)₂SiO₄, to a phase denoted as $\beta$-(Mg, Fe)₂SiO₄, which adopts the spinel crystal structure. This change results in an increase in density and seismic wave velocity.
• Significance: This phase transition is one of the most prominent features detected seismically within the upper mantle and plays a crucial role in understanding mantle convection and the deep Earth structure.

Analysis of Options

Based on geophysical and mineral physics studies, the change at the 410 km discontinuity is characterized by the transformation of olivine into a denser, spinel-structured phase. Therefore, the correct description of the mineral phase change is from an Olivine-type to spinel-type lattice.

• Option 1 correctly identifies this transformation.
• Options 2, 3, and 4 describe different hypothetical or incorrect mineral transitions, such as graphite-type, $\alpha$-spinel-type, pyrite-type, $\gamma$-spinel-type, or $\beta$-spinel-type lattices, which are not the primary phase change associated with the 410 km discontinuity. The transformation involves olivine (a specific structure type) changing into a spinel polymorph ($\beta$-phase), hence "Olivine-type to spinel-type lattice" is the accurate description.

The increase in pressure at 410 km depth drives this fundamental change in how atoms are arranged within the dominant mantle minerals.