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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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

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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-Al2O3 system. We also refine the P-V-T equation of state of Al-bearing seifertite based on volume measurements up to more than 160 GPa and 4000 K [V0 = 92.73(10) Å3, K0 = 304.2(3.0) GPa, K'0 = 4.59 (fixed), ɵD0 = 1130 K (fixed), γ0 = 1.61(3)]. At 300 K, the volume decrease at the CaCl2 to α-PbO2 transition is 0.5(1)%, a value slightly lower than the 0.6% reported previously for Al-free samples. At high temperature, the Grüneisen parameter of seifertite is found to be similar to that of stishovite. Nevertheless, the ▵ V/V across the CaCl2-form to seifertite transition is found to increase slightly with increasing temperature. Across the phase transition, volume changes can be translated into density changes only when the Al substitution mechanisms in both CaCl2-form and seifertite are defined. The analysis of all available data sets suggests different substitution mechanisms for the two SiO2 polymorphs. Al-substitution could occur via O-vacancies in the CaCl2-form and via extra interstitial Al in seifertite. This would result in a density increase of 2.2(3)% at 300 K for SiO2 in basaltic lithologies. Alternatively, the same Alsubstitution mechanism in both of the SiO2-dominated phases would yield a density increase of 0.5(1)%. [ABSTRACT FROM AUTHOR]

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 CaIrO3 post-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 redefined parameters: V0 = 229.463(8) ų, K0 = 198(3) GPa, K' = 1.2(8); and V0 = 226.38(1) ų, K0 = 181(3) GPa, K' = 2.3(8) for CaIrO3 perovskite 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 CaIrO3 perovskite and post-perovskite with the isostructural MgSiO3 phases, reveals a similar general behavior, although in detail CaIrO3 perovskite is more and the post-perovskite less anisotropic than the corresponding MgSiO3 compounds. [ABSTRACT FROM AUTHOR]

Abstract: The thermodynamic properties of perovskite (pv) and post-perovskite (ppv) of CaIrO3 are derived from total energy calculations using density functional theory. Negative molar volume and enthalpy changes of 0.40cm3 mol−1 and 11.8kJmol−1 for the pv to ppv transition at 0K stabilize ppv at low temperatures and high-pressures. Vibrational entropies calculated in the harmonic approximation, using the direct method, favour pv with increasing temperatures (105.5JK−1 mol−1 for pv versus 99.2JK−1 mol−1 for ppv at 298K). A main reason for the lower entropy of post-perovskite compared to perovskite is probably related to constraints on certain vibrational modes imposed by edge-sharing of octahedra in post-perovskite. The Clapeyron slope of the pv–ppv phase boundary is deduced from the calculated enthalpy and volume of transition in conjunction with an experimental transition temperature. The resulting dp/dT of 18MPaK−1 is in good agreement with experimental determinations reported in literature. The high-pressure properties were calculated from the variation of the total energy with volume using the Murnaghan (M) and the Birch–Murnaghan (B–M) equations-of-state. The two methods give the same values for the bulk modulus but somewhat different values for the pressure derivative of the bulk modulus: K 0 =178GPa and and 3.3 (B–M) for pv compared to K 0 =164GPa and and 4.0 (B–M) for ppv. By holding for pv fixed at 4.0, K 0 is reduced to 172GPa. The bulk moduli at zero pressure were also derived through calculation of the elastic constants giving K 0 =172GPa for pv compared to K 0 =157GPa for ppv. In order to compare the changes in the bulk and shear moduli across the pv–ppv phase transition, also the shear moduli were derived from the elastic constants. Whereas the bulk modulus decreases by about 9% from perovskite to post-perovskite, the shear modulus increases by about 15% across the transition. The elastic parameters obtained for the CaIrO3 polymorphs are consistent with those of MgSiO3 and with seismic velocity variations in the lowermost mantle. Our DFT-results demonstrate that CaIrO3 may be a useful low-pressure analogue for studies of the general properties of the pv–ppv-transition relevant to the D″ layer since the negative and positive changes in bulk and shear moduli, respectively, across the pv- to ppv-transition boundary appear to be strongly linked to the nature of the two crystal structures. [Copyright &y& Elsevier]

Abstract: Experimentally determined phase relations in the system MgO–AlO1.5–SiO2 at pressures relevant to the upper part of the lower mantle indicate that Mg–silicate perovskite incorporates aluminum into its structure almost exclusively by a charge-coupled reaction. MgSiO3-rich bulk compositions along the joins MgSiO3–MgAlO2.5 and MgSiO3–MgAl2O4 crystallize assemblages of perovskite coexisting with periclase. MgO-saturated perovskites along these joins have ambient unit cell volumes consistent with those measured and calculated for aluminous perovskite along the charge-coupled substitution join, MgSiO3–AlO1.5. The compressibility of aluminous perovskite along the MgO-saturated joins is not anomalously low as predicted for oxygen-defect perovskites. The bulk moduli, however, are consistent with previous measurements made for aluminous perovskites along the charge-coupled substitution join. These results agree with first-principles calculations showing very limited stability of O-defects in Mg-perovskite at pressures and temperatures corresponding to lower mantle conditions, but are inconsistent with earlier experimental results showing unusually compressive aluminous perovskite. The maximum solubility of alumina in perovskite is ∼25 mol% along the MgSiO3–AlO1.5 join within the ternary MAS-system (i.e. pyrope composition), and the join is apparently binary. Although primitive mantle peridotite compositions are MgO-saturated and fall nearly on the oxygen vacancy join, alumina substitution into perovskite is expected to occur primarily by charge-coupled substitution throughout the lower mantle. The compressibility of aluminous perovskite in primitive mantle is expected to be only a few percent lower than for end member MgSiO3 perovskite. [Copyright &y& Elsevier]

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 CaAl4Si2O11 crystals 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/mmc and 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. [ABSTRACT FROM AUTHOR]

We present the first detailed record of Holocene climate variation from Icelandic eolian soil deposits. Seven cold and windy episodes occurred in Iceland during the past 10 k.y., including the well-documented Little Ice Age (0.6–0.1 ka) and the 8.2 ka event. These windy events are associated with enhanced drift-ice discharge into the North Atlantic, and several are associated with evidence for cold and windy climate in central Greenland and diminution of deep-water formation in the North Atlantic. Although the Arctic Oscillation-North Atlantic Oscillation (AO-NAO) has been invoked to explain other climate teleconnections in the North Atlantic, our paleorecord of windiness in Iceland is not consistent with a persistent negative phase of the AO-NAO. [ABSTRACT FROM AUTHOR]

• Lateral flow along the core-mantle boundary pushes hot material towards the LLSVPs. • This flow induces plumes and high pressure at the edges of the thermochemical piles. • The pressure causes ∼120 km wide topographic depressions around high-viscosity piles. • Depth and asymmetry of this topographic feature constrain the viscosity contrast. • Short-scale topography may help to identify regions of active plume initiation. Two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle. Depending on the density structure of these Large Low Shear Velocity Provinces (LLSVPs), the core-mantle boundary (CMB) will deform upwards or downwards due to isostatic and dynamic topography, the latter being sensitive to the viscosity structure of the lowermost mantle. Heterogeneities in the viscosity structure, although difficult to constrain, might be especially important if the LLSVPs are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics. Based on numerical models, we identify a short-wavelength (about 80-120 km wide, up to a few km deep) topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle. The depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile, and is enhanced by relative uplift of the CMB beneath the pile by plumes rising above it. The depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle. Furthermore, (periodic) plume initiation and pile collapse at the pile margin systematically modify the characteristic depression, with a maximum in asymmetry and depth at the time of plume initiation. Core-reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle. [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]

Kevin Burke's original and thought-provoking contributions have been published steadily for the past 60 years, and more than a decade ago he set out to resolve how plate tectonics and mantle plumes interact by proposing a simple conceptual model, which we will refer to as the Burkian Earth. On the Burkian Earth, mantle plumes take us from the deepest mantle to sub-lithospheric depths, where partial melting occurs, and to the surface, where hotspot lavas erupt today, and where large igneous provinces and kimberlites have erupted episodically in the past. The arrival of a plume head contributes to continental break-up and punctuates plate tectonics by creating and modifying plate boundaries. Conversely, plate tectonics makes an essential contribution to the mantle through subduction. Slabs restore mass to the lowermost mantle and are the triggering mechanism for plumes that rise from the margins of the two large-scale low shear-wave velocity structures in the lowermost mantle, which Burke christened TUZO and JASON. Situated just above the core-mantle boundary, beneath Africa and the Pacific, these are stable and antipodal thermochemical piles, which Burke reasons represent the immediate after-effect of the moon-forming event and the final magma ocean crystallization. [ABSTRACT FROM AUTHOR]