Nonlinearity of the post-spinel transition and its expression in slabs and plumes worldwide

TL;DR

This study reveals that the post-spinel transition boundary in Earth's mantle is nonlinear with a variable Clapeyron slope, significantly influencing mantle dynamics and the behavior of slabs and plumes worldwide.

Contribution

It introduces a machine learning framework for high-pressure phase diagram determination and applies it to constrain the nonlinear Clapeyron slope of the post-spinel transition.

Findings

The post-spinel boundary is nonlinear with a slope varying from -2.3 to 0 MPa/K.

The Clapeyron slope in plumes is three times more negative than in slabs.

Nonlinear transition impacts mantle convection and slab-plume dynamics.

Abstract

At the interface of Earth's upper and lower mantle, the post-spinel transition boundary controls the dynamics and morphologies of downwelling slabs and upwelling plumes, and its Clapeyron slope is hence one of the most important constraints on mantle convection. In this study, we reported a new in situ experimental dataset on phase stability in Mg$_{2}$SiO$_{4}$ at mantle transition zone pressures from laser-heated diamond anvil cell experiments, along with a compilation of corrected in situ experimental datasets from the literature. We presented a machine learning framework for high-pressure phase diagram determination and focused on its application to constrain the location and Clapeyron slope of the post-spinel transition: ringwoodite $\leftrightarrow$ bridgmanite + periclase. We found that the post-spinel boundary is nonlinear and its Clapeyron slope varies locally from $-2.3_{-1.4}^{+0.6}$ MPa/K at 1900 K, to $-1.0_{-1.7}^{+1.3}$ MPa/K at 1700 K, and to $0.0_{-2.0}^{+1.7}$ MPa/K at 1500 K. We applied the temperature-dependent post-spinel Clapeyron slope to estimate its lateral variation across the "660-km" seismic discontinuity in subducting slabs and hotspot-associated plumes worldwide, as well as the ambient mantle. We found that, in the present-day mantle, the average post spinel Clapeyron slope in the plumes is three times more negative than that in slabs, and we then discussed the effects of a nonlinear post-spinel transition on the dynamics of Earth's mantle.