In a groundbreaking exploration of Earth’s deep interior, a recent study unveils the persistence of davemaoite—a calcium silicate mineral—under the extreme pressures and temperatures characteristic of the planet’s lower mantle. This revelation not only challenges prior assumptions about mineral stability at such depths but also provides pivotal insights into the geochemical and seismic complexities governing Earth’s inner layers. Through a meticulously designed series of high-pressure, high-temperature experiments coupled with advanced analytical techniques, researchers have successfully synthesized and characterized glassy starting materials, subjected them to conditions mimicking the abyssal reaches of the mantle, and deciphered the mineralogical transformations occurring therein.
The preparation of starting materials was a critical first step in this investigation. Researchers synthesized glasses representing diverse compositions, carefully controlling the ratios of calcium, magnesium, iron, aluminum, silicon, and oxygen. Melting powders of MgO, SiO₂, CaO, and Fe₂O₃ at temperatures exceeding 1900 K in a high-temperature furnace, they rapidly quenched the melts in cold water, producing homogenous glassy substances. These glasses were rigorously characterized using energy-dispersive X-ray spectroscopy integrated within scanning electron microscopy, ensuring precise compositional verification and uniformity essential for subsequent high-pressure experiments.
Following synthesis, the materials were subjected to ultrahigh-pressure multi-anvil press experiments to simulate the forbidding conditions of the lower mantle, where pressures reach up to 50 gigapascals and temperatures soar beyond 2700 kelvin. These experiments utilized state-of-the-art assemblies and heating systems tailored to maintain precise control over pressure and temperature profiles. Intricate sample assemblies involved platinum or rhenium capsules encased within aluminum oxide sleeves and LaCrO₃ heaters, demonstrating a sophisticated approach to balancing thermal insulation and mechanical stability. Remarkably, strategies such as embedding trace amounts of magnesium hydroxide promoted grain growth through the release of water vapor, optimizing the quality of mineral grains for chemical analysis.
Temperature monitoring employed type-D thermocouples strategically positioned between rhenium capsules, enabling highly accurate real-time measurement during the ramp-up and annealing stages. Annealing at target conditions for extended durations, typically 24 hours, ensured equilibrium attainment within samples. This rigorous control of temperature, pressure, and time allowed for the faithful reproduction of the natural processes governing mineral formation and transformation in Earth’s deep mantle, ultimately facilitating the detection of subtle phase equilibria essential for understanding mineral stability fields.
Post-experiment analyses leveraged advanced microscopy and spectroscopy to unravel the mineralogical evolution of the samples. The polished cross-sections underwent scanning electron microscopy to assess grain size distributions and morphologies. Ultrathin lamellae prepared via focused ion beam techniques allowed for high-resolution chemical mapping using scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy. Such detailed compositional mappings revealed the distribution of constituent elements at nanometric spatial scales, with corrections applied to account for complex effects such as atomic number (Z) and X-ray absorption for more accurate quantification.
Beyond elemental analysis, electron energy-loss spectroscopy provided discerning insight into the iron oxidation states within the synthesized minerals. By analyzing the Fe-L₂,₃ edges, researchers quantified Fe³⁺/ΣFe ratios, shedding light on redox conditions that influence mineral stability and phase relations. Such measurements are essential because the valence state of iron profoundly affects the structural properties and seismic signatures of mantle phases, thereby linking laboratory experiments directly to geophysical observations.
Complementary microprobe analyses validated the compositions of recovered phases, employing standards precise for magnesium, silicon, calcium, and iron to ensure analytical fidelity. Meanwhile, phase identification utilized micro-focused X-ray diffraction with Co–Kα radiation and cutting-edge two-dimensional detectors, offering definitive confirmation of crystallographic structures formed under extreme conditions. Extended exposure times and focused beam sizes optimized diffraction patterns, facilitating the detection of even minor phases critical to the mineralogical assemblage.
Central to interpreting these experimental results was the development of an empirical model linking the mole fraction of calcium silicate (χ_Ca) in bridgmanite (Bdm) to pressure and temperature. This relationship was expressed as an exponential function incorporating fitted parameters to capture the thermal and baric dependencies observed. Drawing from thermodynamic principles, these parameters corresponded directly to changes in enthalpy, entropy, and volume associated with the equilibration reaction between calcium silicate in davemaoite (Dvm) and bridgmanite. Such an approach enabled researchers to extrapolate mineral stability fields beyond accessible experimental ranges reliably.
The fitted model predicted that davemaoite coexists with bridgmanite across a broad spectrum of lower-mantle conditions, persisting even at pyrolitic solidus temperatures. Calculations revealed mole fractions of CaSiO₃ in bridgmanite remain below pyrolitic levels across pressures extending to 120 GPa, signifying the continuous presence of davemaoite within the mantle’s complex mineralogy. This challenges earlier conceptions that davemaoite would break down or be significantly depleted under such extreme conditions, emphasizing its role as a stable and integral phase within Earth’s deep interior.
Seismic modeling further underscored the implications of these mineralogical findings. Using the BurnMan software, researchers simulated shear-wave velocities for regions enriched in davemaoite relative to those dominated by bridgmanite alone. They incorporated compositional variations involving magnesium, iron, and aluminum contents within bridgmanite to reflect realistic mantle compositions. By varying temperature and phase proportions, the models elucidated how the presence of davemaoite-enriched domains could contribute to observed seismic anomalies, thereby connecting mineral physics directly to geophysical phenomena.
The study’s meticulous experimental design, combined with comprehensive analytical precision, represents a transformative step in understanding Earth’s deep mantle. By establishing the persistence of davemaoite under lower-mantle conditions, it enriches our comprehension of the mineralogical diversity influencing mantle dynamics, chemical heterogeneity, and seismic wave propagation. These insights open new avenues for interpreting data from seismology, mineral physics, and geochemistry, fostering integrated models that reconcile observations from laboratory to planetary scales.
Moreover, the implications of persistent davemaoite affect geochemical cycling and mantle evolution. As a calcium-bearing phase stable in deep mantle assemblages, it influences element partitioning, melt generation, and the overall mineralogical framework governing mantle convection. Understanding its stability enhances predictive models of mantle composition and behavior, thereby refining interpretations of geodynamic processes shaping Earth’s interior through deep time.
The experimental approach, employing cutting-edge multi-anvil presses capable of replicating the ultrahigh pressures of the deep mantle, exemplifies the forefront of high-pressure mineralogy. The integration of multiple analytical methodologies, including synchrotron-based diffraction, transmission electron microscopy, and electron energy-loss spectroscopy, provides a holistic view of mineral structures, compositions, and valence states. Such technological synergy is essential for unraveling the profound complexities of Earth’s inaccessible depths.
Equally significant is the temperature control strategy, wherein samples were annealed for prolonged periods under tightly regulated thermal regimes, ensuring that equilibrium was established. This detail is crucial because kinetic barriers often hinder phase transformations at lower temperatures or shorter timescales, and the careful annealing protocols adopted here allow a more accurate reflection of natural mantle conditions.
The study also illuminates subtle effects such as selective magnesium diffusion caused by electron beam irradiation during transmission electron microscopy analysis, a factor that researchers accounted for in compositional quantifications. Recognizing and correcting for such intricacies enhances confidence in the precision of mineral composition data, fostering robust interpretations of the physicochemical behavior of deep mantle phases.
In sum, this comprehensive research not only confirms the persistence of davemaoite as a stable calcium silicate phase in Earth’s lower mantle but also bridges the realms of experimental petrology, mineral physics, and geophysics. It reinforces the complex mineralogical landscape within which mantle processes operate and underscores the indispensable role of interdisciplinary approaches in Earth sciences. As seismic and geochemical probing of our planet’s interior advances, such fundamental knowledge forms the backbone of our understanding of the dynamic Earth beneath our feet.
Subject of Research: Persistence and stability of davemaoite (CaSiO₃) under lower mantle pressure and temperature conditions.
Article Title: Persistence of davemaoite at lower-mantle conditions.
Article References:
Wang, L., Miyajima, N., Wang, F. et al. Persistence of davemaoite at lower-mantle conditions. Nat. Geosci. 18, 365–369 (2025). https://doi.org/10.1038/s41561-025-01657-9
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