Search

How eclogite/pyroxenite‐derived melts evolve through the refractory lithosphere above a plume remains poorly understood. Here we conducted layered experiments of reaction between tholeiitic melts and harzburgite at 2–3 GPa, 1,400–1,500°C, with a run duration ranging from 2 to 24 hr. The resulting residual melts exhibit lower SiO2, TiO2, Al2O3, FeO, CaO, and total alkali contents, higher Ni, MgO, and Mg#, and almost constant CaO/Al2O3 compared to the initial tholeiitic melts. The compositions of the residual melts are influenced by factors such as the melt/harzburgite mass ratio, temperature, and run duration. Decreasing the melt/rock ratio or increasing temperature and run duration leads to a greater extent of assimilation. Under disequilibrated conditions (2 hr), the residual melts have higher SiO2, FeO, and MgO, and lower CaO, Al2O3, and total alkali contents compared to those under equilibrated conditions. The results suggest that interface reactions involving olivine dissolution and orthopyroxene precipitation, and chemical diffusion occur simultaneously during the interaction process. The compositions of the residual melts are largely controlled by interface reactions within 2 hr, followed by dominant chemical diffusion between the melts and refertilized harzburgite from 2 to 24 hr. Based on the experimental results, we propose a two‐stage model for the origin of Hawaiian shield stage parental magmas. Eclogite/pyroxenite‐derived tholeiitic melts first react with harzburgite, with varying melt/rock ratios, to produce residual melts in the deep lithosphere. These residual melts subsequently mix with plume peridotite‐derived melts at shallow depths, contributing to the geochemical diversity observed in Hawaiian shield stage lavas. Plain Language Summary: We conducted experiments to understand how melts derived from subducted oceanic crust interact with surrounding rocks during their journey through the Earth's lithosphere. By simulating melt‐rock reaction along wall‐rock boundary under high‐pressure and high‐temperature conditions, we found that the interaction mechanism involves olivine dissolution and orthopyroxene precipitation. The extent of interaction between the melts and rocks depends on factors such as the amount of melt present and the duration of the interaction. We also observed that certain chemical reactions took place at the interface between the melts and rocks, which affect the composition of the resulting melts. Our findings suggest that the formation of magmas with specific chemical compositions, similar to those observed in Hawaiian shield stage lavas, can be elucidated by the reaction between tholeiitic melts derived from eclogite/pyroxenite and harzburgite. The harzburgite was generated through prior high‐degree melting of peridotite influenced by the high temperatures in the plume setting. We propose a two‐stage model involving initial reactions in the deep lithosphere followed by mixing of the melts in shallower depths. This model provides insights into the processes that contribute to the diversity of magmas observed in Hawaiian volcanic islands. Key Points: Eclogite melts reacting with harzburgite produce high SiO2, MgO, and Ni tholeiitic meltsResidual melt composition is influenced by melt size and reaction time, involving interface reactions and diffusionEclogite melt interaction with harzburgite, followed by mixing with peridotite melts, accounts for Hawaiian shield stage lava generation [ABSTRACT FROM AUTHOR]

The mantle transition zone (MTZ) is a layer between the upper and lower mantle that plays a crucial role in Earth's deep structure and material circulation. It is increasingly recognized as a potential repository for volatiles, such as water, which can have significant effects on mantle dynamics, melting processes, diamond formation, volcanic activity, and volatile cycling. The presence of water in the MTZ has been a topic of debate, but recent evidence suggests that it may be hydrous to some extent. Other volatiles, such as halogens, sulfur, nitrogen, and noble gases, also have important roles in the MTZ. Understanding the behavior and effects of these volatiles in the MTZ is essential for advancing deep Earth science. Further research and advanced analytical techniques are needed to fully understand the volatile budget in the deep Earth. [Extracted from the article]

Earth is the only known habitable planet in the solar system. Understanding how Earth developed its unique habitability has been the frontier of Earth sciences and has become one of the main themes of current deep-space explorations. What are the decisive factors that led to a habitable planet? What is the role of solid Earth processes in the origin of life and in modulating the surface environment? Are Earth's habitability studies relevant to current challenges that human beings face? These questions have attracted the interest of both scientists and the public alike. NSR spoke to Prof. Charles H. Langmuir from Harvard University in the USA, who is a solid Earth geochemist who carries out research on diverse aspects of the plate tectonic geochemical cycle, including ocean ridges, convergent margins and intraplate volcanism. Prof. Langmuir is the author of the book How to Build a Habitable Planet (www.habitableplanet.org), one of the best Earth science books published in 2012. [ABSTRACT FROM AUTHOR]

We determine high‐resolution tomographic models of isotropic P‐wave velocity (Vp) and tilting‐axis anisotropy of the Alaska subduction zone using a large number of local and teleseismic data recorded at many portable and permanent network stations in and around Alaska. We find a flat high‐Vp slab in the mantle transition zone (410–670 km depths) beneath western Alaska, which is connected with the subducting Pacific slab at 0–410 km depths, suggesting that a big mantle wedge has formed under western Alaska. Our tilting‐axis anisotropy model reveals complex mantle flows in the asthenosphere. Corner flow in the mantle wedge above the subducting Pacific slab and toroidal flow in the big mantle wedge are revealed, which may cause the Cenozoic intraplate volcanoes in western Alaska and the Bering Sea. In central Alaska, the mantle wedge beneath the Denali volcanic gap is characterized by high‐Vp and subhorizontal fast velocity directions normal to the volcanic arc, which may reflect a remnant of the subducted Yakutat slab. In SE Alaska, the shallow subduction of the Wrangell slab is visible above 150 km depth, and hot mantle upwelling through the Wrangell‐Yakutat slab gap may contribute to the Wrangell volcanic field. Plain Language Summary: Alaska is home to over 100 Cenozoic volcanoes, but their formation mechanism is still puzzling. Here we use a large number of seismic data and a novel tomographic technique to obtain high‐resolution images of isotropic P‐wave velocity (Vp) and anisotropy down to 800 km depth beneath Alaska. Our results show that a flat high‐Vp slab exists in the mantle transition zone (410–670 km depths) beneath western Alaska, where a big mantle wedge has formed above the flat slab. Fluids in the big mantle wedge and large‐scale mantle flow around it have caused Cenozoic intraplate volcanoes near the Bering Sea. The Denali volcanic gap, different from its neighboring Buzzard Creek‐Jumbo Dome volcanoes, has a high Vp mantle wedge with arc‐normal fast anisotropy, which may reflect a cool fluid barrier from the Yakutat slab. The high‐Vp Wrangell slab is revealed down to ∼100 km depth beneath the Wrangell volcanic field, Mount Churchill and Prindle volcano. Hot upwelling flows near the Wrangell slab edge may cause these volcanoes. Key Points: A big mantle wedge (BMW) has formed above a flat slab in the mantle transition zone under western AlaskaIntraplate volcanism in western Alaska and the Bering Sea is caused by hot upwelling flows in the BMWThe Wrangell volcanic field and Denali volcanic gap are caused by subduction of the Wrangell and Yakutat slabs, respectively [ABSTRACT FROM AUTHOR]

The Cascadia margin is an unusual subduction zone characterized by the downdip movement of young and thin oceanic plates, where mantle flow and intraslab deformation are still unclear. Here we present new anisotropic tomography of the Cascadia subduction zone, in which the hexagonal symmetry axis of anisotropy is tilting rather than horizontal or vertical as assumed in previous studies of seismic anisotropy. Subduction‐induced entrained and toroidal flows under the Cascadia margin are discriminated well by the spatial relationship between tilting‐axis anisotropy and slab geometry. The obliquely entrained flow is trapped in a narrow zone (<100 km wide) above and below the subducting slab and reaches ∼200 km depth, which is surrounded by large‐scale sub‐horizontal toroidal flow. The intraslab anisotropy is trench‐normal above 80 km depth but changes to trench‐parallel at 100–400 km depths, which may reflect fossil anisotropy overprinted by deep deformation beneath the arc, or joint effect of serpentinization and hydrous faulting. Plain Language Summary: The slab deformation and mantle flow under the Cascadia margin are controversial. We obtain new anisotropic tomography of the Cascadia subduction zone, which can reveal tilting 3‐D fast velocity directions (FVDs) and planes. Because the observed trench‐normal FVD in the Cascadia margin can be explained by both 3‐D toroidal and 2‐D entrained flows, we discriminate the two types of mantle flow with 3‐D FVDs that contain dip angle information. The 2‐D entrained flow above and below the subducting slab is trapped in a narrow zone of ∼100 km wide and is surrounded by large‐scale sub‐horizontal toroidal flow. The slab anisotropy above 80 km depth is consistent with that in the Juan de Fuca plate before subduction, but it is reshaped beneath the arc due to the slab deformation or phase transition. Key Points: Tilting‐axis anisotropy can reconcile contradictory assumptions of azimuthal and radial anisotropiesEntrained flow occurs within ∼100 km depth above and below the subducting slab and is surrounded by toroidal flowFossil anisotropy in the slab is overprinted beneath the arc due to intraslab deformation or phase transition [ABSTRACT FROM AUTHOR]