The Composition of Earth's Lower Mantle

Thermochemical state of the lower mantle: New insights from mineral physics

Carbonates play a key role in the chemistry and dynamics of our planet. They are directly connected to the CO2 budget of our atmosphere and have a great impact on the deep carbon cycle. Moreover, recent studies have shown that carbonates are stable along the geothermal gradient down to Earth's lower mantle conditions, changing their crystal structure and related properties. Subducted carbonates may also react with silicates to form new phases. These reactions will redistribute elements, such as calcium (Ca), magnesium (Mg), iron (Fe) and carbon in the form of carbon dioxide (CO2), but also trace elements, that are carried by the carbonates. The trace elements of most interest are strontium (Sr) and rare earth elements (REE) which have been found to be important constituents in the composition of the primitive lower mantle and in mineral inclusions found in super-deep diamonds. However, the stability of carbonates in presence of mantle silicates at relevant temperatures is far from being well understood. Related to this, very little is known about distribution processes of trace elements between carbonates and mantle silicates. To shed light on these processes, we studied reactions between Sr- and REE-containing CaCO3 and Mg/Fe-bearing silicates of the system (Mg,Fe)2SiO4 - (Mg,Fe)SiO3 at high pressure and high temperature using synchrotron radiation based μ-X-ray diffraction (μ-XRD) and μ-X-ray fluorescence (μ-XRF) with μm-resolution in a laser-heated diamond anvil cell. X-ray diffraction is used to derive the structural changes of the phase reactions whereas X-ray fluorescence gives information on the chemical changes in the sample. In-situ experiments at high pressure and high temperature were performed at beamline P02.2 at PETRA III (Hamburg, Germany) and at beamline ID27 at ESRF (Grenoble, France). In addition to μ-XRD and μ-XRF, ex-situ measurements were made on the recovered sample material using transmission electron microscopy (TEM) and provided further insights into the reaction kinetics of carbonate-silicate reactions. Our investigations show that CaCO3 is unstable in presence of mantle silicates above 1700 K and a reaction takes place in which magnesite plus CaSiO3-perovskite are formed. In addition, we observed that a high content of iron in the carbonate-silicate system favours dolomite formation during the reaction. The subduction of natural carbonates with significant amounts of Sr leads to a comprehensive investigation of the stability not only of CaCO3 phases in contact with mantle silicates but also of SrCO3 (and of Sr-bearing CaCO3). We found that SrCO3 reacts with (Mg,Fe)SiO3-perovskite to form magnesite and gained evidence for the formation of SrSiO3-perovskite. To complement our study on the stability of SrCO3 at conditions of the Earth's lower mantle, we performed powder X-ray diffraction and single crystal X-ray diffraction experiments at ambient temperature and up to 49 GPa. We observed a transformation from SrCO3-I into a new high-pressure phase SrCO3-II at around 26 GPa with Pmmn crystal structure and a bulk modulus of 103(10) GPa. This information is essential to fully understand the phase behaviour and stability of carbonates in the Earth's lower mantle and to elucidate the possibility of introducing Sr into mantle silicates by carbonate-silicate reactions. Simultaneous recording of μ-XRD and μ-XRF in the μm-range over the heated areas provides spatial information not only about phase reactions but also on the elemental redistribution during the reactions. A comparison of the spatial intensity distribution of the XRF signal before and after heating indicates a change in the elemental distribution of Sr and an increase in Sr-concentration was found around the newly formed SrSiO3-perovskite. With the help of additional TEM analyses on the quenched sample material the elemental redistribution was studied at a sub-micrometer scale. Contrary to expectations from combined μ-XRD and μ-XRF measurements, we found that La and Eu were not incorporated into the silicate phases, instead they tend to form either isolated oxide phases (e.g. Eu2O3, La2O3) or hydroxyl-bastnasite (La(CO3)(OH)). In addition, we observed the transformation from (Mg,Fe)SiO3-perovskite to low-pressure clinoenstatite during pressure release. The monoclinic structure (P21/c) of this phase allows the incorporation of Ca as shown by additional EDX analyses and, to a minor extent, Sr too. Based on our experiments, we can conclude that a detection of the trace elements in-situ at high pressure and high temperature remains challenging. However, our first findings imply that silicates may incorporate the trace elements provided by the carbonates and indicate that carbonates may have a major effect on the trace element contents of mantle phases.

The effect of Fe spin crossovers on its partitioning behavior and oxidation state in a pyrolitic Earth's lower mantle system

Open-system 182W[sbnd]142Nd isotope evolution of the Earth

The chemical compositions of Earth's core and mantle provide insight into the processes that led to their formation. N-body simulations, on the other hand, generally do not contain chemical information, and seek to only reproduce the masses and orbits of the terrestrial planets. These simulations can be grouped into four potentially viable scenarios of Solar System formation (Classical, Annulus, Grand Tack, and Early Instability) for which we compile a total of 433 N-body simulations. We relate the outputs of these simulations to the chemistry of Earth's core and mantle using a melt-scaling law combined with a multi-stage model of core formation. We find the compositions of Earth analogs to be largely governed by the fraction of equilibrating embryo cores (kcore_emb) and the initial embryo masses in N-body simulations, rather than the simulation type, where higher values of kcore_emb and larger initial embryo masses correspond to higher concentrations of Ni, Co, Mo, and W in Earth analog mantles and higher concentrations of Si and O in Earth analog cores. As a result, larger initial embryo masses require smaller values of kcore_emb to match Earth's mantle composition. On the other hand, compositions of Earth analog cores are sensitive to the temperatures of equilibration and fO2 of accreting material. Simulation type may be important when considering magma ocean lifetimes, where Grand Tack simulations have the largest amounts of material accreted after the last giant impact. However, we cannot rule out any accretion scenarios or initial embryo masses due to the sensitivity of Earth's mantle composition to different parameters and the stochastic nature of N-body simulations. We use our compiled simulations to explore the relationship between initial embryo masses and the melting history of Earth analogs, where the complex interplay between the timing between impacts, magma ocean lifetimes, and volatile delivery could affect the compositions of Earth analogs formed from different simulation types. Comparing the last embryo impacts experienced by Earth analogs to specific Moon-forming scenarios, we find the characteristics of the Moon-forming impact are dependent on the initial conditions in N-body simulations where larger initial embryo masses promote larger and slower Moon-forming impactors. Mars-sized initial embryos are most consistent with the canonical hit-and-run scenario onto a solid mantle. Our results suggest that constraining the fraction of equilibrating impactor core (kcore) and the initial embryo masses in N-body simulations could be significant for understanding both Earth's accretion history and characteristics of the Moon-forming impact.

By subjecting certain compounds to immense temperatures and pressures, physicists in the US and France have found evidence for a previously unknown layer in the Earth's lower mantle. At a depth of between 1000 and 2200 km, it appears as if the spin states of electrons in iron atoms switch direction, thus altering the density of the mantle and affecting our understanding of how seismic waves propagate through the Earth's interior (Science 317 1740).

Strong constraints can be placed on the origin of heterogeneity of seismic wave velocities and density if the observed ratios of various parameters are compared with mineral physics predictions. They include the shear to compressional wave velocity heterogeneity ratio,Rs/p≡ δ logVs/δ logVp, the bulk sound to shear wave velocity heterogeneity ratio,Rϕ/s≡ δ logVϕ/δ logVs, and the density to velocity heterogeneity ratio,Rρ/s,p≡ δ log ρ/δ logVs,p. Using mineral physics considerations, we calculate these ratios in the lower mantle corresponding to the thermal and chemical origin of velocity and density heterogeneity. Both anharmonic and anelastic effects are considered for thermal origin. Anharmonic effects are estimated from the theoretical calculations as well as from laboratory measurements which show a marked increase inRs/pwith pressure from ∼1.5 to ∼2.1 in the lower mantle. Such a trend is marginally consistent with seismological observations showing an increase inRs/pwith depth (from ∼1.7 to ∼3.2 in the lower mantle). However, anharmonic effect alone cannot explain inferred lowRρ/s(<0.2–0.4), and the values ofRs/ppredicted from anharmonic effects are systematically smaller than seismological observations. Anelastic effects must be included to account for these geophysical observations. A new formulation of anelastic effect is developed in which the role of nonlinear temperature dependence of anelasticity is included. When anharmonic and anelastic effects are combined, thermal effects on velocity and density can account for most of the geophysical observations in Earth's lower mantle. Effects other than temperature variations (such as chemical heterogeneity) are required only in the very deep portions of the mantle (deeper than ∼2000–2300 km). A puzzling observation is very large values ofRs/p(>2.7) and corresponding negative values ofRϕ/s(andRρ/s) in the deep lower mantle which cannot be accounted for by thermal or simple chemical heterogeneity such as the heterogeneity in the Fe/(Fe + Mg) and/or Mg/(Mg + Si) ratios. Possible causes of anomalies in this region are discussed, including the role of anisotropy and a combined effect of heterogeneity in Fe and Ca content.

Lower mantle electrical conductivity based on measurements of Al, Fe-bearing perovskite under lower mantle conditions

We assess the thermal conductivity of the Earth's lower mantle anchored on our first‐principles calculations of lattice thermal conductivity of MgSiO3 perovskite. Our calculations agree with measurements of iron‐free perovskite at ambient conditions and show a lower pressure dependence compared with other recent calculations. In addition, we show that the effect of iron on the lattice thermal conductivity of silicate perovskite is likely to be small at high temperatures. To provide an assessment of thermal conductivity throughout the lower mantle, we reevaluate existing high‐pressure optical absorption data to constrain an upper bound radiative contribution to diffusive heat transfer and examine a composite model for combining thermal conductivity of oxide and perovskite phases in the lower mantle. We find that the overall thermal conductivity of the lower mantle is approximately constant between 2.5 and 3.5 W/m/K. These values imply that the mantle has a blanketing effect on heat flow across the core‐mantle boundary.

We performed laser-heated diamond anvil cell experiments on bulk compositions in the systems MgO–SiO2–H2O (MSH) and MgO–Al2O3–SiO2–H2O (MASH) that constrain the stability of hydrous phases in Earth's lower mantle. Phase identification by synchrotron powder diffraction reveals a consistent set of stability relations for the high-pressure, dense hydrous silicate phases D and H. In the MSH system phase D is stable to ~50GPa, independent of temperature from ~1300 to 1700K. Phase H becomes stable between 35 and 40GPa, and the phase H out reaction occurs at ~55GPa at 1600K with a negative dT/dP slope of ~−75K/GPa. Between ~30 and 50GPa dehydration melting occurs at ~1800K with a flat dT/dP slope. A cusp along the solidus at ~50GPa corresponds with the intersection of the subsolidus phase H out reaction, and the dT/dP melting slope steepens to ~15K/GPa up to ~85GPa.In the MASH system phase H is stable in experiments between ~45 and 115GPa in all bulk compositions studied, and we expect aluminous phase H to be stable throughout the lower mantle depth range beneath ~1200km in both peridotitic and basaltic lithologies. In the subsolidus, aluminous phase D is stable to ~55GPa, whereas at higher pressures aluminous phase H is the stable hydrous phase. The presence of hydrogen may sharpen the bridgmanite to post-perovskite transition. The ambient unit cell volume of bridgmanite increases systematically with pressure above ~55GPa, possibly representing an increase in alumina content, and potentially hydrogen content, with depth. Bridgmanite in equilibrium with phases D and H has a relatively low alumina content, and alumina partitions preferentially into the hydrous phases. The melting curves of MASH compositions are shallower than in the MSH system, with dT/dP of ~6K/GPa. Phase D and H solid solutions are stable in cold, hydrated subducting slabs and can deliver water to the deepest lower mantle. However, hydrated lithologies in the lower mantle are likely to be partially molten at all depths along an ambient mantle geotherm.

Application of the equation of state to the Earth's lower mantle

We present BurnMan, an open-source mineral physics toolbox to determine elastic properties for specified compositions in the lower mantle by solving an Equation of State (EoS). The toolbox, written in Python, can be used to evaluate seismic velocities of new mineral physics data or geodynamic models, and as the forward model in inversions for mantle composition. The user can define the composition from a list of minerals provided for the lower mantle or easily include their own. BurnMan provides choices in methodology, both for the EoS and for the multiphase averaging scheme. The results can be visually or quantitatively compared to observed seismic models. Example user scripts show how to go through these steps. This paper includes several examples realized with BurnMan: First, we benchmark the computations to check for correctness. Second, we exemplify two pitfalls in EoS modeling: using a different EoS than the one used to derive the mineral physical parameters or using an incorrect averaging scheme. Both pitfalls have led to incorrect conclusions on lower mantle composition and temperature in the literature. We further illustrate that fitting elastic velocities separately or jointly leads to different Mg/Si ratios for the lower mantle. However, we find that, within mineral physical uncertainties, a pyrolitic composition can match PREM very well. Finally, we find that uncertainties on specific input parameters result in a considerable amount of variation in both magnitude and gradient of the seismic velocities.

Dual origin of ferropericlase inclusions within super-deep diamonds

The unusual capability of solid crystalline materials to deform plastically, known as superplasticity, has been found in metals and even in ceramics. Such superplastic behaviour has been speculated for decades to take place in geological materials, ranging from surface ice sheets to the Earth's lower mantle. In materials science, superplasticity is confirmed when the material deforms with large tensile strain without failure; however, no experimental studies have yet shown this characteristic in geomaterials. Here we show that polycrystalline forsterite + periclase (9:1) and forsterite + enstatite + diopside (7:2.5:0.5), which are good analogues for Earth's mantle, undergo homogeneous elongation of up to 500 per cent under subsolidus conditions. Such superplastic deformation is accompanied by strain hardening, which is well explained by the grain size sensitivity of superplasticity and grain growth under grain switching conditions (that is, grain boundary sliding); grain boundary sliding is the main deformation mechanism for superplasticity. We apply the observed strain-grain size-viscosity relationship to portions of the mantle where superplasticity has been presumed to take place, such as localized shear zones in the upper mantle and within subducting slabs penetrating into the transition zone and lower mantle after a phase transformation. Calculations show that superplastic flow in the mantle is inevitably accompanied by significant grain growth that can bring fine grained (≤1 μm) rocks to coarse-grained (1-10 mm) aggregates, resulting in increasing mantle viscosity and finally termination of superplastic flow.

The thermal conductivity of bridgmanite, the primary constituent of the Earth's lower mantle, has been investigated using diamond anvil cells at pressures up to 85 GPa and temperatures up to 3,100 K. We report the results of time‐domain optical laser flash heating and X‐ray Free Electron Laser heating experiments from a variety of bridgmanite samples with different Al and Fe contents. The results demonstrate that Fe or Fe,Al incorporation in bridgmanite reduces thermal conductivity by about 50% in comparison to end‐member MgSiO3 at the pressure‐temperature conditions of Earth's lower mantle. The effect of temperature on the thermal conductivity at 28–60 GPa is moderate, well described as , where a is 0.2–0.5. The results yield thermal conductivity of 7.5–15 W/(m × K) in the thermal boundary layer of the lowermost mantle composed of Fe,Al‐bearing bridgmanite.