A quiet quantum revolution in Earth's deep interior

TL;DR

This paper discusses how pressure-induced quantum spin transitions in iron within Earth's lower mantle minerals affect seismic properties, revealing a diffuse transition zone that influences geophysical interpretations.

Contribution

It integrates mineral physics and seismic imaging to show that quantum spin crossover in mantle minerals causes seismic anomalies, redefining our understanding of Earth's deep interior.

Findings

Spin crossover reduces P-wave velocities in the lower mantle.

The transition extends across most of the lower mantle as a diffuse zone.

Seismic anomalies are explained by quantum electronic transitions in minerals.

Abstract

The Earth's lower mantle hosts a subtle but pervasive quantum phenomenon: the pressure-induced spin crossover of iron in its dominant minerals, bridgmanite and ferropericlase. In this transition, iron ions gradually shift from high-spin to low-spin electronic states without structural change, altering their volume, compressibility, and elastic properties. Although long recognized experimentally and theoretically, its geophysical significance has only recently become clear through the integration of mineral physics and three-dimensional seismic imaging. The spin crossover reduces bulk modulus and P-wave velocities while leaving S-wave speeds largely unaffected, producing a distinctive decoupling between P- and S-wave anomalies. This signature is now observed in global tomography and reconciles seismic observations with realistic mantle temperatures and compositions. Rather than forming a sharp boundary, the crossover extends across most of the lower mantle, acting as a diffuse yet essential control on seismic structure. This work highlights how quantum-scale electronic transitions influence planetary-scale dynamics and interpretations of Earth's deep interior.