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1. Exploring microstructures in lower mantle mineral assemblages with synchrotron x-rays

   Chandler C.B., Bernier J.V. (January 2021, Science advances)

   Understanding spatial distribution of phases as well as dynamics across phase transformations in the lower mantle are crucial understanding the evolution of the Earth’s interior. Studies at high pressures and temperatures have previously relied on powder diffraction or single crystal methods which have limitations in spatial resolution during dynamic processes. Here we apply the novel multigrain crystallography technique at high pressure and temperature in a laser heated diamond anvil cell (LH-DAC) to study the formation of the Earth’s most abundant mineral bridgmanite MgSiO3 from a natural olivine sample. We constrain the twinning relationship in the bridgmanite phase at high pressure with grain-size resolution as well as evidence for an interconnected network formed by the weaker ferropericlase phase which could contribute to slab stagnation and plume deflection 

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2. Ultra-low-velocity anomaly inside the Pacific Slab near the 410-km discontinuity

   https://doi.org/10.1038/s43247-023-00756-y  

   Li, Jiaqi; Ferrand, Thomas P.; Zhou, Tong; Ritsema, Jeroen; Stixrude, Lars; Chen, Min (December 2023, Communications Earth & Environment)

   Abstract The upper boundary of the mantle transition zone, known as the “410-km discontinuity”, is attributed to the phase transformation of the mineral olivine (α) to wadsleyite (β olivine). Here we present observations of triplicated P-waves from dense seismic arrays that constrain the structure of the subducting Pacific slab near the 410-km discontinuity beneath the northern Sea of Japan. Our analysis of P-wave travel times and waveforms at periods as short as 2 s indicates the presence of an ultra-low-velocity layer within the cold slab, with a P-wave velocity that is at least ≈20% lower than in the ambient mantle and an apparent thickness of ≈20 km along the wave path. This ultra-low-velocity layer could contain unstable material (e.g., poirierite) with reduced grain size where diffusionless transformations are favored. 

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3. Seismic imaging of a basaltic Lesser Antilles slab from ancient tectonics

   https://doi.org/10.1038/s41586-025-08754-0  

   Yang, Xusong; Xie, Yujiang; Rychert, Catherine A; Harmon, Nicholas; Goes, Saskia; Rietbrock, Andreas; Lynch, Lloyd; Macpherson, Colin G; Van_Hunen, Jeroen; Davidson, Jon; et al (April 2025, Nature)

   Abstract At subduction zones, lithospheric material descends through the upper mantle to the mantle transition zone (MTZ), where it may continue to sink into the lower mantle or stagnate^1,2. Several factors may be important in influencing this flow, including chemical heterogeneity^3–5. However, tight constraints on these mantle flows and the exact factors that affect them have proved challenging. We use P-to-S receiver functions to image the subducting slab and the MTZ beneath the Lesser Antilles subduction zone. We image a singular, superdeep (>700 km) 660-km discontinuity over a 200-km-wide zone within the slab, accompanied by nearby double 660 discontinuity phases (normal and superdeep). Combined geodynamic and waveform modelling shows that this observation cannot be explained by temperature effects in typical mantle compositions but requires a large basalt-rich chemical anomaly, strongest in the location of the singular, deep 660. The inferred basalt signature is near the proposed location of a subducted extinct spreading ridge^6,7, where basalt is probably present in greater proportions. Our finding suggests that past tectonic events impart chemical heterogeneity into slabs, and the heterogeneities, in turn, may affect the inherent tendency of the slab to sink. 

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4. Olivines Fo-Ni-MnO Systematics in the Trans-Mexican Volcanic Belt: Evidence for Cool and Silicic Primary Arc Magmas

   https://doi.org/10.1093/petrology/egaf094  

   Straub, Susanne M; Batanova, Valentina; Sobolev, Alexander; Gómez-Tuena, Arturo; Espinasa-Perena, Ramon; Bindeman, Ilya N; Stuart, Finlay M; Widom, Elisabeth; Iizuka, Yoshiyuki (November 2025, Journal of Petrology)

   Abstract The discovery of systematic differences in the trace element composition of forsteritic olivines in primitive magmas from within-plate, arc and mid-ocean ridge volcanoes engendered much debate about a causal link to the recycling of oceanic crust into the mantle sources of within-plate and arc magmas. Here we address this problem using Cr-spinel bearing, forsteritic (~Fo80–91) olivines from high-Mg# = 50 = 73 [Mg# = molar ratio of Mg/(Mg + Fe2+)*100] arc magmas from the Trans-Mexican Volcanic Belt (TMVB). The TMVB arc front olivines have similar high Ni, low MnO, and low Mn/Fe as forsteritic olivines from within-plate basalts erupting through thick lithosphere (= WPB-thick). However, the olivines in TMVB arc front primary melts crystallize at much lower temperatures of $${T}_{\mathrm{cryst}}^{\mathrm{oliv}}$$~1119 ± 38 °C (calculated with olivine–spinel aluminum exchange thermometry) in hydrous (~4–9 wt % H2O), silicic, less magnesian (≤10 wt % MgO) mantle melts from mostly garnet-free mantle sources. Model calculations suggest that the primary arc front melts last equilibrated in the mantle at pressures of ~1.4 to ~1.9 GPa (~51–69 km depth) and low temperatures (Tsource = 1150 ± 45 °C) that are only slightly higher than the olivine crystallization temperatures. While the $${\mathrm{Kd}}_{\mathrm{oliv}/\mathrm{melt}}^{\mathrm{Ni}}$$ increases in the cooler and silicic melts, such modulation cannot account for the full range of Ni concentration in TMVB magmatic olivines. A small population of very high-Ni olivines (>4000–5500 μg/g Ni) is best explained by crystallization in Ni-rich components melt that formed by melt rock reaction processes in the mantle wedge. Unlike Ni, olivine MnO is not sensitive to melt temperature and only moderately to melt composition, and thus retains mantle source characteristics. In the TMVB, olivine Fo-MnO-Mn/Fe systematics record an ambient mantle wedge (= mantle without slab component) that is similar to WPB sources and that is variably depleted by slab flux-driven melt extraction. Overall, the olivine Fo-Ni-MnO systematics confirm with greater detail than possible by bulk rock studies that the TMVB primary melts are hydrous and silicic and originate from a mantle wedge that is strongly and variably modified by the slab flux. These results reaffirm a strong genetic link between slab recycling and the genesis of silicic arc magmas. 

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5. Severe Strain‐Induced Olivine‐Ringwoodite Transformation at Room Temperature: Key to Enigmas of Deep‐Focus Earthquake

   https://doi.org/10.1029/2024GL111281  

   Lin, F; Levitas, V I; Yesudhas, S; Dhar, A; Smith, J (March 2025, Geophysical Research Letters)

   Deep‐focus earthquakes at 350–660 km are presumably caused by olivine‐spinel phase transformation (PT). This cannot, however, explain the observed high seismic strain rate, which requires PT to complete within seconds, while metastable olivine does not transform for over a million years. Recent theory quantitatively describes how severe plastic deformations (SPD) can solve this dilemma but lacking experimental proof. Here, we introduce dynamic rotational diamond anvil cell with rough diamond anvils to impose SPD on San Carlos olivine. While olivine never transformed to spinel at room temperature, we obtained reversible olivine‐ringwoodite PT under SPD at 15–28 GPa within tens of seconds. The PT pressure reduces with increasing dislocation density, microstrain, plastic strain, and decreasing crystallite size. Results demonstrate a new strain‐induced PT mechanism compared to a pressure/temperature‐induced one. Combined with SPD during olivine subduction, this mechanism can accelerate olivine‐ringwoodite PT from millions of years to timescales relevant to earthquakes. 

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