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Interfacial-energy-driven ferropericlase rounding leads to bridgmanite-controlled viscosity in the Earth’s lower mantle

Experimental evidence showing that interfacial-energy-driven rounding of ferropericlase grains occurs faster than mantle-flow-induced elongation indicates that the Earth's lower mantle viscosity is controlled by the stronger bridgmanite phase, making it orders of magnitude higher than previously hypothesized.

Original authors: Amrita Chakraborti, Hongzhan Fei, Yu Nishihara, Marcel Thielmann, Florian Heidelbach, Noriyoshi Tsujino, Artem Chanyshev, Alexander Kurnosov, Lianjie Man, Jonathan Dolinschi, Wentian Wu, Weiwei Cao, Y

Published 2026-08-12

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CC BY 4.0

Original authors: Amrita Chakraborti, Hongzhan Fei, Yu Nishihara, Marcel Thielmann, Florian Heidelbach, Noriyoshi Tsujino, Artem Chanyshev, Alexander Kurnosov, Lianjie Man, Jonathan Dolinschi, Wentian Wu, Weiwei Cao, Yuji Higo, Marcin Dabrowski, Tomoo Katsura

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep beneath our feet, far beyond the reach of any drill, lies the Earth's lower mantle. It is a vast, super-heated ocean of rock that flows incredibly slowly over millions of years, acting as the planet's internal engine. This slow-motion churning moves heat from the core to the surface and drags tectonic plates around, shaping our continents and driving volcanoes. To understand how this engine works, scientists need to know how "thick" or "sticky" this rock is—a property called viscosity. If the rock is very thick, it resists moving; if it is thin, it flows easily. The lower mantle is mostly made of two minerals: a hard, stiff one called bridgmanite and a softer, weaker one called ferropericlase. For a long time, scientists have debated whether the whole system flows like the stiff mineral or if the soft mineral creates weak, slippery highways that let the whole thing slide around easily. This question matters because the answer changes how we picture the Earth's history, how fast heat escapes, and why some tectonic plates stop sinking while others plunge all the way to the core.

A team of researchers has now taken a fresh look at this debate by studying how the soft mineral behaves when it gets stretched. Imagine you have a blob of sticky taffy (the soft mineral) trapped inside a block of hard, stiff clay (the stiff mineral). If you pull on the clay, the taffy stretches out into long, thin strands. In the past, scientists thought that over the Earth's long history, these strands would stretch so much that they would connect to form a continuous, slippery network, making the whole rock flow much faster. However, this new study suggests that nature has a built-in "self-correcting" mechanism. Just as a stretched rubber band wants to snap back or a water droplet wants to become a perfect sphere to save energy, the stretched taffy wants to curl back up into a round ball. The researchers found that this "rounding" happens so fast that the taffy never gets a chance to form those long, slippery highways. Instead, the rock stays stiff, controlled by the hard clay, not the soft taffy.

The team, led by Amrita Chakraborti and colleagues, tested this idea by creating tiny samples of these two minerals in a high-pressure machine that mimics the crushing weight and scorching heat of the deep Earth (27 gigapascals and temperatures up to 2000 Kelvin). They first squashed the samples to stretch the soft ferropericlase grains into long, thin shapes, just like the Earth's flow would do over millions of years. Then, they held the samples at high heat and pressure to see what happened next. They watched closely to see if the stretched grains stayed long or if they curled back into round shapes.

The results were clear: the stretched grains rounded up incredibly quickly. The researchers measured how fast the "rounding" happened compared to how fast the Earth's flow would stretch them. They found that even with very conservative estimates, the rounding process is much faster than the stretching process. By the time the grains could possibly stretch enough to connect and form a weak layer, they had already curled back into nearly perfect spheres. In fact, the grains only needed to stretch a tiny bit—about 1% longer than they were wide—before the rounding force took over and stopped them from stretching further.

This finding suggests that the "Interconnected Weak Layer" model, which predicts the lower mantle is much weaker and flows faster, is likely incorrect. Instead, the study supports the "Load-Bearing Framework" model, where the stiff bridgmanite holds the structure together and controls the flow. The authors suggest that because the soft grains can't stay stretched, the effective viscosity (stickiness) of the lower mantle is likely much higher than previously thought, closer to that of the stiff bridgmanite. This implies that the Earth's lower mantle is a much more sluggish, resistant environment than some earlier theories proposed. While the researchers note that they still need to confirm how pressure affects this rounding at even deeper levels, their experiments provide strong evidence that surface energy acts like a powerful brake, preventing the soft minerals from creating the slippery shortcuts that would speed up the Earth's internal engine.

Technical Summary: Interfacial-energy-driven ferropericlase rounding leads to bridgmanite-controlled viscosity in the Earth's lower mantle

Problem Statement
The viscosity of the Earth's lower mantle is a critical parameter governing material and heat transport, yet estimates vary by up to five orders of magnitude. This uncertainty stems from a debate between two competing rheological models for the two-phase system of bridgmanite (Bdm, 70-75 vol.%) and ferropericlase (Fper, ~18-20 vol.%). The "Load-Bearing Framework" (LBF) model posits that the volumetrically dominant and stronger Bdm controls effective viscosity. Conversely, the "Interconnected Weak Layer" (IWL) model suggests that despite its lower abundance, the weaker Fper can control viscosity if mantle flow elongates Fper grains into interconnected layers. Previous experimental support for the IWL model (e.g., Girard et al.) relied on high strain rates (10⁻⁵ s⁻¹), which are ten orders of magnitude higher than actual lower mantle strain rates (<10⁻¹⁵ s⁻¹). At these geologically relevant low strain rates, interfacial-energy-driven rounding of elongated grains may counteract flow-induced elongation, a mechanism previously unquantified in this context.

Methodology
The authors conducted high-pressure, high-temperature experiments to measure the rounding rate of elongated Fper clusters within Bdm-Fper aggregates.

Sample Synthesis: Aggregates with Fper volume fractions ( $X_{Fper}$ ) of 0.18 (bulk silicate mantle composition) and 0.30 were synthesized at 27 GPa and 2000 K.
Deformation: Samples were deformed in a D111 apparatus at the SPring-8 synchrotron to induce elongation of Fper clusters, simulating mantle flow conditions.
Annealing: Deformed samples were annealed at 27 GPa and temperatures of 1475 K, 1725 K, and 2000 K for durations ranging from 0.1 to 54 hours.
Observation: Two annealing styles were employed: "surface" annealing (observing the same clusters over time) and "bulk" annealing (polishing away surface layers after each step to observe interior clusters).
Quantification: Back-scattered electron (BSE) imaging was used to measure the perimeter ( $p$ ) of Fper clusters. The degree of elongation was quantified using the relative excess perimeter $(p - p_c)/p_c$ , where $p_c$ is the perimeter of an equivalent-area circle. The rounding rate ( $\xi$ ) was derived from the decay of this parameter over time.

Key Results

Rounding Dynamics: The experiments demonstrated that elongated Fper clusters rapidly round toward a spherical shape. The process occurs in two regimes: a rapid initial stage driven by the elimination of interface roughness (jaggedness) and a slower second regime (Regime 2) representing the evolution of smooth interfaces, which is the relevant analogue for lower mantle conditions.
Temperature and Composition Dependence: The rounding rate is strongly temperature-dependent but independent of the Fper volume fraction within the tested range (0.18 to 0.30). The rounding coefficient ( $B$ ) increased by nearly an order of magnitude between 1475 K and 1725 K, and further at 2000 K.
Scaling to Mantle Conditions: When scaled to lower mantle grain sizes (estimated at 160 µm for Fper, based on Bdm grain growth constraints) and ambient mantle temperatures (~2000 K), the calculated rounding rate significantly exceeds the elongation rate expected from mantle strain rates (conservatively estimated at $10^{-15}$ to $10^{-16}$ s⁻¹).
Equilibrium Aspect Ratio: The balance between rounding and elongation results in an equilibrium aspect ratio for Fper grains very close to unity (e.g., <1.01 for ambient mantle conditions). Even under conservative assumptions regarding grain size and strain rates, the aspect ratio remains below 1.5.
Subducting Slabs: In colder subducting slabs (~1200°C), while rounding rates decrease, grain growth is also suppressed, resulting in smaller grain sizes (<40 µm). Due to the strong inverse dependence of rounding rate on grain size ( $d^{-4}$ ), the rounding rate still exceeds the elongation rate, maintaining an aspect ratio near unity (<1.0005).

Key Contributions

First Measurement: This study provides the first experimental measurement of the rounding rate of Fper clusters in a Bdm-Fper system under lower mantle conditions.
Mechanism Validation: It validates the hypothesis that interfacial energy minimization (via Coble creep) is a dominant factor in Fper morphology at geologically relevant strain rates, effectively arresting the elongation required for the IWL model.
Rheological Constraint: The results demonstrate that the transition from the LBF regime to the IWL regime is prevented by rapid rounding, implying that Fper cannot form the interconnected weak layers necessary to lower the effective viscosity.

Significance and Claims
The paper concludes that the effective viscosity of the Earth's lower mantle is controlled by bridgmanite (LBF model), not ferropericlase. The authors claim that the effective viscosity is orders of magnitude higher than previously hypothesized under the IWL framework. This finding has significant implications for mantle dynamics, suggesting:

The lower mantle is more viscous, potentially leading to slower and wider mantle plume conduits.
The lateral drift of plumes and the sinking velocity of slabs are reduced.
The preservation of geochemical reservoirs (e.g., BEAMS) may be due to the inherent high viscosity of large-grained bridgmanite domains rather than strain-weakening mechanisms associated with interconnected Fper layers.
The lack of seismic anisotropy in the lower mantle is likely due to the dominance of diffusion creep rather than the selective deformation of Fper.

The authors acknowledge remaining uncertainties, specifically the need to quantify the pressure dependence of rounding (as current data is limited to mid-mantle pressures) and to independently validate the theoretical $d^{-4}$ grain size dependence. However, they assert that interfacial energy exerts a significant influence on lower mantle viscosity, fundamentally altering the understanding of mantle rheology.

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