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We report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmcand lattice parameters of a= 5.42(1) Å; c= 12.70(3) Å; V= 323(4) Å3; Z= 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U. S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth’s mantle. Donwilhelmsite forms in the Earth’s mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

The subsolidus phase diagram of silica in the 80–220 GPa pressure range was determined by density functional theory (DFT). The transition pressures calculated using the generalized gradient approximation (GGA) in the static limit (at 0 K, without zero point vibrational energy) for the β-stishovite (CaCl2- structure) to seifertite and the seifertite to pyrite-type transitions are 95 and 213 GPa, respectively. These are in good agreement with those calculated using hybrid functionals, giving transition pressures of 96 and 215 GPa. This indicates that previous local density approximation (LDA) results underestimate the transition pressure by 10–15 GPa. Density functional perturbation theory calculations, carried out using GGA within the quasi-harmonic approximations, give Clapeyron slopes of 5.4 and –2.8 MPa/K for the β-stishovite to seifertite and seifertite to pyrite-type transitions, respectively. This suggests that the seifertite-forming transition occurs at 109 GPa (470 km above the core-mantle boundary, CMB) at an ambient mantle geotherm, whereas the pyrite-type transition occurs at 200 GPa (620 km below the CMB) at 4700 K, which is close to the core adiabat. We also calculate the equation of state and show that the stability of seifertite in the lowermost mantle contributes negative buoyancy to recycled oceanic crust, although not as much as in some previous studies. Nevertheless, the increased density of seifertite over β-stishovite may lead to layers with elevated proportions of basaltic material within the large low S-wave velocity provinces. The seifertite to pyrite-type silica transition in the outer core will affect the silica liquidus surface in the system Fe-Si-O and forms a basis for further investigations of silica crystallization in the protocore.

The large low shear velocity provinces observed in the lowermost mantle are widely accepted as chemically distinct thermochemical “piles,” but their origin and long‐term evolution remain poorly understood. The survival time and shape of the large low shear velocity provinces are thought to be mainly controlled by their compositional density, while their viscosity has been considered less important. Based on recent constraints on chemical reactions between mantle and core, a more complex viscosity structure of the lowermost mantle, possibly including high viscosity thermochemical pile material, seems reasonable. In this study, we use numerical models to identify a trade‐off between compositional viscosity and density contrasts required for long‐term stability of thermochemical piles, which permits lower‐density and higher‐viscosity piles. Moreover, our results indicate more restrictive stability conditions during periods of strong deformation‐induced entrainment, for example, during initial pile formation, which suggests long‐term pile survival. Seismic images of the Earth's mantle show two anomalous continent‐sized regions close to the core‐mantle boundary. The inferred properties of these regions suggest that they have a different composition than the surrounding mantle. Two possible candidate materials have been proposed: accumulated oceanic crust from the Earth's surface or an iron‐rich residue remaining from Earth's original magma ocean. Although both materials are denser than the surrounding mantle, it remains unclear whether piles of these chemical heterogeneities can survive at the core‐mantle boundary beneath vigorous mantle convection. Numerical models show that the excess density required to preserve these structures is typically larger than indicated by seismological and gravitational observations. In this study, we show that the excess density used in numerical models can be reduced toward the observed values if the pile material is also stiffer than the surrounding mantle. Furthermore, we show that piles must be denser and/or stiffer to avoid destruction during episodes of strong deformation. Because pile formation probably includes vigorous deformation, we expect long‐term survival of the piles after their formation is completed. High compositional viscosity stabilizes thermochemical piles at the core‐mantle boundaryWe identify a trade‐off between density and viscosity contrast to maintain pile stabilityPile stability during periods of strong deformation‐induced entrainment, for example, pile formation, requires higher viscosity and/or density

The Eggvin Bank, located between the Jan Mayen Island and Greenland, is an unusually shallow area containing several submarine volcanic peaks, confined by two transforms on the Northern Kolbeinsey Ridge (NKR). We represent Pand Swave velocity models for the Eggvin Bank based on an Ocean Bottom Seismometer profile collected in 2011, showing igneous crustal thickness variations from 8 km to 13 km. A 2–5 km increase is associated with two separate 20–30 km wide segments under the main seamounts. The oceanic crust has three layers: upper crust (L2A: 2.8–4.8 km/s), middle crust (L2B: 5.5–6.5 km/s), and lower crust (L3: 6.7–7.35 km/s). Both the thick layer 2(A/B) and the high ratio of layer 2(A/B) thickness to total crustal thickness indicate that secondary, intraplate magmatism built the seamounts of the Eggvin Bank. The seamount in the north where the crust is thickest has a flat top indicating subaerial exposure but is deeper than those with rounded tops in the south and is therefore probably older. Comparing lower crustal seismic velocity with crustal thickness also indicates that the degree of mantle melting may be higher in the north than in the south. An enriched mantle source presently feeds the NKR magmatism and probably influenced the Eggvin Bank development also at earlier times. To what extent the Eggvin Bank has been influenced by the Iceland plume is uncertain, both an enriched mantle component and elevated mantle temperature may have played a role at different times and locations. A seismic refraction and reflection study of the Eggvin Bank at the northern Kolbeinsey RidgeVpand Vsmodels of the Eggvin Bank show 8–13 km igneous crustal thicknessThe Eggvin Bank is created by melting of an enriched mantle source both on and off axis

We investigated the properties of Al-bearing SiO2(with 4 or 6 wt% Al2O3) at pressures and temperatures corresponding to the lowermost mantle, using laser-heated diamond-anvil cell coupled with synchrotron-based in situ X-ray diffraction. The phase transition from CaCl2-structured to α-PbO2- structured (seifertite) polymorphs occurs between 113 and 119 GPa at 2500 K. The range of pressure where the two phases coexist is small. There is a slight decrease of the transition pressure with increasing Al-content. We propose a tentative phase diagram reporting the minerals composition as a function of pressure in the SiO2-Al2O3system.

Unit-cell lattice parameters have been measured to ~8 GPa in a diamond anvil cell for two single crystals of CaIrO3: one with the perovskite (Pbnm) and the other with the post-perovskite (Cmcm) structure. The CaIrO3post-perovskite structure is more compressible than the perovskite. A third-order Birch Murnaghan equation of state has been used to fit the measured P-V data with the following refined parameters: V0= 229.463(8) Å3, K0= 198(3) GPa, K’ = 1.2(8); and V0= 226.38(1) Å3, K0= 181(3) GPa, K’ = 2.3(8) for CaIrO3perovskite and post-perovskite, respectively. The compressibility of the unit-cell axes of the perovskite structure is highly anisotropic with βa>> βc>> βb. In contrast, the b axis is the most compressible in the post-perovskite structure, whereas the a and c axes have similar compressibilities (with c slightly less compressible than a) and are much stiffer. A comparison between the compressibility of CaIrO3perovskite and post-perovskite with the isostructural MgSiO3phases, reveals a similar general behavior, although in detail CaIrO3perovskite is more and the postperovskite less anisotropic than the corresponding MgSiO3compounds.

Earth's heat budget is strongly influenced by spatial and temporal variations in surface heat flow caused by plate tectonic cycles. Here, we use a novel set of paleo‐seafloor age grids extending back to the mid‐Paleozoic to infer spatiotemporal variations in surface heat loss. The time‐averaged oceanic heat flow is 36.6 TW, or ∼25% greater than at present‐day. Our thermal budget for the mantle indicates that 149 K/Gyr of cooling occurred over this period, consistent with geochemical estimates of mantle cooling for the past 1 Gyr. Our analysis also suggests sustained rapid cooling of the Pacific mantle hemisphere, which may have cooled ∼50 K more than its African counterpart since 400 Ma. The extra heat released from the Pacific mantle may have been trapped there by the earlier long‐lived supercontinent Rodinia (∼1.1–0.7 Ga), and the Pacific mantle may still be hotter than the African mantle today. Earth's interior is cooling because its rate of heat loss exceeds its rate of internal heat production. Heat loss happens at the Earth's surface and is highly variable, with thick continents providing strong insulation to Earth's interior and thin seafloor allowing more rapid heat transfer. Using models for how the continents and oceanic plates have moved for the past 400 million years, we reconstructed the history of heat loss from Earth's interior. We find that heat loss was on average 25% higher in the past than it is today, which implies more rapid overall cooling than expected. We also find that the Pacific side of the world has lost heat at a much faster rate than the African side. This is partly due to positioning of continental landmasses, including the supercontinent Pangea, on the African side for most of the past 400 million years. By contrast, the oceans on the Pacific side offered “poor insulation” that led to ∼50 °C more cooling of the Pacific mantle compared to its African counterpart. The extra heat lost from the Pacific side may have been trapped there by Rodinia, an older, long‐lived supercontinent that covered the Pacific mantle about one billion years ago. We use novel paleo‐seafloor age grids covering the past 400 Myr to quantify spatial and temporal variations in mantle heat lossThe time‐averaged oceanic heat flow over the past 400 Myr is 36.6 TW, which is ∼25% greater than at present‐dayThe Pacific hemisphere has been cooling much faster than the African, possibly releasing heat trapped by the earlier supercontinent Rodinia We use novel paleo‐seafloor age grids covering the past 400 Myr to quantify spatial and temporal variations in mantle heat loss The time‐averaged oceanic heat flow over the past 400 Myr is 36.6 TW, which is ∼25% greater than at present‐day The Pacific hemisphere has been cooling much faster than the African, possibly releasing heat trapped by the earlier supercontinent Rodinia

Deep‐rooted mantle plumes are thought to originate from the margins of the Large Low Shear Velocity Provinces (LLSVPs) at the base of the mantle. Visible in seismic tomography, the LLSVPs are usually interpreted to be intrinsically dense thermochemical piles in numerical models. Although piles deflect lateral mantle flow upward at their edges, the mechanism for localized plume formation is still not well understood. In this study, we develop numerical models that show plumes rising from the margin of a dense thermochemical pile, temporarily increasing its local thickness until material at the pile top cools and the pile starts to collapse back toward the core‐mantle boundary (CMB). This causes dense pile material to spread laterally along the CMB, locally thickening the lower thermal boundary layer on the CMB next to the pile, and initiating a new plume. The resulting plume cycle is reflected in both the thickness and lateral motion of the local pile margin within a few hundred km of the pile edge, while the overall thickness of the pile is not affected. The period of plume generation is mainly controlled by the rate at which slab material is transported to the CMB, and thus depends on the plate velocity and the sinking rate of slabs in the lower mantle. A pile collapse, with plumes forming along the edges of the pile's radially extending corner, may, for example, explain the observed clustering of Large Igneous Provinces (LIPs) in the southeastern corner of the African LLSVP around 95–155 Ma. Deep‐rooted upwellings in the Earth's mantle, so‐called “plumes”, are responsible for volcanism such as Hawaii and seem to cluster around two continent‐sized regions at the core‐mantle boundary that show anomalously low seismic velocities. These regions have been suggested to have a different composition than the surrounding mantle, causing them to be denser and stiffer which allows them to survive billions of years at the base of the mantle without being completely eroded. In this study, we use numerical simulations to show that mantle plumes and dense piles at the core‐mantle boundary interact with each other, potentially resulting in a periodic plume initiation. A starting upwelling increases the pile thickness locally by pulling dense pile material upward. It then cools down, causing a density increase that results in gravitational collapse of the dense material toward the core‐mantle boundary. As a result, this material spreads along the boundary and pushes hot ambient mantle in front of it, thereby triggering a new upwelling, and the cycle starts from the beginning. The main control on how often upwellings are generated is the rate at which material from subduction zones at the Earth's surface reaches the core‐mantle boundary, which relates to the velocity of tectonic plates. Plumes can be periodically generated at the margins of thermochemical piles by local pile collapseVariations in pile thickness and lateral motion of the pile edge reflect cycles of plume initiationThe period of plume cycles is controlled by plate velocity and the sinking rate of slabs