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Sublithospheric diamonds and the inclusions they may carry crystallize in the asthenosphere, transition zone, or uppermost lower mantle (from 300 to ∼800 km), and are the deepest minerals so far recognized to form by plate tectonics. These diamonds are distinctive in their deformation features, low nitrogen content, and inclusions of these major mantle minerals: majoritic garnet, clinopyroxene, ringwoodite, CaSi perovskite, ferropericlase, and bridgmanite or their retrograde equivalents. The stable isotopic compositions of elements within these diamonds (δ11B, δ13C, δ15N) and their inclusions (δ18O, δ56Fe) are typically well outside normal mantle ranges, showing that these elements were either organic (C) or modified by seawater alteration (B, O, Fe) at relatively low temperatures. Metamorphic minerals in cold slabs are effective hosts that transport C as CO3 and H as H2O, OH, or CH4 below the island arc and mantle wedge. Warming of the slab generates carbonatitic melts, supercritical aqueous fluids, or metallic liquids, forming three types of sublithospheric diamonds. Diamond crystallization occurs by movement and reduction of mobile fluids as they pass through host mantle via fractures—a process that creates chemical heterogeneity and may promote deep focus earthquakes. Geobarometry of majoritic garnet inclusions and diamond ages suggest upward transport, perhaps to the base of mantle lithosphere. From there, diamonds are carried to Earth's surface by eruptions of kimberlite magma. Mineral assemblages in sublithospheric diamonds directly trace Earth's deep volatile cycle, demonstrating how the hydrosphere of a rocky planet can connect to its solid interior. Sublithospheric diamonds from the deep upper mantle, transition zone, and lower mantle host Earth's deepest obtainable mineral samples. Low-temperature seawater alteration of the ocean floor captures organic and inorganic carbon at the surface eventually to become some of the most precious gem diamonds. Subduction transports fluids in metamorphic minerals to great depth. Fluids released by slab heating migrate, react with host mantle to induce diamond crystallization, and may trigger earthquakes. Sublithospheric diamonds are powerful tracers of subduction—a plate tectonic process that deeply recycles part of Earth's planetary volatile budget. [ABSTRACT FROM AUTHOR]

A seismic low velocity layer (LVL) above the mantle transition zone (MTZ), often thought to be caused by volatile‐induced melting, can significantly modulate planetary volatile cycles. In this work, we show that an LVL observed beneath northeast Asia is characterized by small, 0.8 ± 0.5 vol%, average degrees of partial melting. Seismically derived P‐T conditions of the LVL indicate that slab‐derived CO2, possibly combined with small amounts of H2O, is necessary to induce melting. Modeling the reactive infiltration instability of the melt in a stationary mantle above a stalled slab, we demonstrate that the volatile‐rich melt slowly rises above the stalled slab in the MTZ, with percolation velocities of 200–500 μm/yr. The melt remains stable within the LVL for this geologically significant period of time, potentially transferring up to 52 Mt/yr of CO2 from the subducting slab to the mantle for an LVL similar in areal extent (3.4×106km2) to the northeast Asian LVL. Reaction between the melt channels and the LVL mantle precipitates up to 200 ppmw solid C in localized zones. Using the inferred small melt volume fraction to model trace element abundances and isotopic signatures, we show that interaction between this melt and the surrounding mantle can over the long‐term produce rocks bearing a HIMU like geochemical signature. Plain Language Summary: Volatile elements, such as hydrogen and carbon, are elements that can be condensed only at low temperatures. As a small amount of these volatiles can apparently lower the solidus and viscosity of mantle rocks, they are intuitively implied to play an important role in the upper mantle dynamics. Our study focuses on the northeast Asian low‐velocity layer (LVL), a layer of 61 ± 15 km above the mantle transition zone marked by a 2%–3% reduction in shear wave velocity. We integrated multiple methods and techniques, including rock physics analysis of seismology, numerical simulation of porous flow, and geochemical modeling. Our rock physics analysis suggests the LVL as a potential deep volatile (carbon and/or hydrogen) reservoir, containing 0.8 ± 0.5 vol% melt. Our simulations predict that the LVL can transfer up to 52 Mt/yr of CO2 to the overlying mantle. This study presents a research model of qualitative analyses and quantitative measurements of the deep volatile cycling, providing a direction for the deep volatile study in other tectonic environments. Key Points: The northeast Asian low velocity layer (LVL) is characterized by an average of 0.8 ± 0.5 vol% meltIt can take 40–80 Ma for the slab‐derived carbonate‐rich melt channels to reach the LVL topLong term interaction between the CO2‐rich melt and the mantle can produce a high µ (HIMU)‐like source component [ABSTRACT FROM AUTHOR]

The distribution and transportation of water in Earth's interior depends on the stability of water-bearing phases. The transition zone in Earth's mantle is generally accepted as an important potential water reservoir because its main constituents, wadsleyite and ringwoodite, can incorporate weight percent levels of H2O in their structures at mantle temperatures. The extent to which water can be transported beyond the transition zone deeper into the mantle depends on the water carrying capacity of minerals stable in subducted lithosphere. Stishovite is one of the major mineral components in subducting oceanic crust, yet the capacity of stishovite to incorporate water beyond at lower mantle conditions remains speculative. In this study, we combine in situ laser heating with synchrotron X-ray diffraction to show that the unit cell volume of stishovite synthesized under hydrous conditions is ~2.3 to 5.0% greater than that of anhydrous stishovite at pressures of ~27 to 58 GPa and temperatures of 1,240 to 1,835 K. Our results indicate that stishovite, even at temperatures along a mantle geotherm, can potentially incorporate weight percent levels of H2O in its crystal structure and has the potential to be a key phase for transporting and storing water in the lower mantle. [ABSTRACT FROM AUTHOR]

Abstract We report on laser-heated diamond anvil cell (LHDAC) experiments in the FeO–MgO–SiO 2 –CO 2 (FMSC) and CaO–MgO–SiO 2 –CO 2 (CMSC) systems at lower mantle pressures designed to test for decarbonation and diamond forming reactions. Sub-solidus phase relations based on synthesis experiments are reported in the pressure range of ∼35 to 90 GPa at temperatures of ∼1600 to 2200 K. Ternary bulk compositions comprised of mixtures of carbonate and silica are constructed such that decarbonation reactions produce non-ternary phases (e.g. bridgmanite, Ca-perovskite, diamond, CO 2 –V), and synchrotron X-ray diffraction and micro-Raman spectroscopy are used to identify the appearance of reaction products. We find that carbonate phases in these two systems react with silica to form bridgmanite ±Ca-perovskite + CO 2 at pressures in the range of ∼40 to 70 GPa and 1600 to 1900 K in decarbonation reactions with negative Clapeyron slopes. Our results show that decarbonation reactions form an impenetrable barrier to subduction of carbonate in oceanic crust to depths in the mantle greater than ∼1500 km. We also identify carbonate and CO 2 –V dissociation reactions that form diamond plus oxygen. On the basis of the observed decarbonation reactions we predict that the ultimate fate of carbonate in oceanic crust subducted into the deep lower mantle is in the form of refractory diamond in the deepest lower mantle along a slab geotherm and throughout the lower mantle along a mantle geotherm. Diamond produced in oceanic crust by subsolidus decarbonation is refractory and immobile and can be stored at the base of the mantle over long timescales, potentially returning to the surface in OIB magmas associated with deep mantle plumes. Highlights • Experiments were made in the systems FMS-CO2 and CMS-CO2 at lower mantle pressures. • Carbonate in these systems react with silica in the lower mantle. • Decarbonation reactions with negative P–T slopes form CO 2 or diamond plus oxygen. • Carbonate in oceanic crust is limited to ∼1500 km depth along subduction geotherms. • Diamond is the stable form of carbon in oceanic crust at ambient mantle temperature. [ABSTRACT FROM AUTHOR]

The article presents a study that investigates the chemistry of primary mantle derived magmas from different plate boundaries. It outlines the method of the investigation in which the researchers speculate that high-nickel-content contributes to the upwelling mantle plumes. It also notes the result of the study, which suggests that magma composition should be further analyzed using a range of radiogenic isotope tracers to identify their prirmordial nature.

Characterizing compositional heterogeneity in Earth’s lower mantle is critical to understanding its dynamics. Three low-nitrogen diamonds from Koffiefontein (South Africa), containing inclusion assemblages of ferropericlase ± orthopyroxene ± magnesite, constrain diamond formation in an Mg-rich lower-mantle environment. Ferropericlase inclusions have Mg# 82.7–88.5 and orthopyroxene inclusions (retrogressed bridgmanite) have Mg# 95.0–95.1 and mantle-like δ18O of +5.6‰ ± 0.2‰. Magnesite included in one diamond implicates carbonated fluids in diamond formation. High Mg# and low Ca, Al, and Na of the assemblage indicate a melt-depleted meta-harzburgitic environment, in contrast to more fertile compositions expected for primitive lower mantle. Extremely low Ca in orthopyroxene inclusions may reflect a combination of melt depletion and low equilibration temperatures at the time of trapping. Inclusion compositions implicate subducted oceanic slab meta-harzburgite as the host for diamond growth. Mantle-like δ18O of the orthopyroxene inclusions indicates unaltered oceanic lithosphere. Similar melt-depleted characteristics in lower-mantle inclusion assemblages worldwide support that meta-harzburgite is the dominant host of lower-mantle diamonds. [ABSTRACT FROM AUTHOR]

The results of experiments conducted under the extreme temperature and pressure conditions of Earth's lower mantle suggest that, at these depths, low oxygen content is no barrier to iron oxidation. [ABSTRACT FROM AUTHOR]

Eutectic melting curves in the system MgO–SiO 2 have been experimentally determined at lower mantle pressures using laser-heated diamond anvil cell (LH-DAC) techniques. We investigated eutectic melting of bridgmanite plus periclase in the MgO–MgSiO 3 binary, and melting of bridgmanite plus stishovite in the MgSiO 3 –SiO 2 binary, as analogues for natural peridotite and basalt, respectively. The melting curve of model basalt occurs at lower temperatures, has a shallower d T / d P slope and slightly less curvature than the model peridotitic melting curve. Overall, melting temperatures detected in this study are in good agreement with previous experiments and ab initio simulations at ∼25 GPa ( Liebske and Frost, 2012 ; de Koker et al., 2013 ). However, at higher pressures the measured eutectic melting curves are systematically lower in temperature than curves extrapolated on the basis of thermodynamic modelling of low-pressure experimental data, and those calculated from atomistic simulations. We find that our data are inconsistent with previously computed melting temperatures and melt thermodynamic properties of the SiO 2 endmember, and indicate a maximum in short-range ordering in MgO–SiO 2 melts close to Mg 2 SiO 4 composition. The curvature of the model peridotite eutectic relative to an MgSiO 3 melt adiabat indicates that crystallization in a global magma ocean would begin at ∼100 GPa rather than at the bottom of the mantle, allowing for an early basal melt layer. The model peridotite melting curve lies ∼ 500 K above the mantle geotherm at the core–mantle boundary, indicating that it will not be molten unless the addition of other components reduces the solidus sufficiently. The model basalt melting curve intersects the geotherm at the base of the mantle, and partial melting of subducted oceanic crust is expected. [ABSTRACT FROM AUTHOR]