Pyrolitic Lower Mantle Research Articles

Effect of spin transition of iron on the thermal conductivity of (Fe, Al)-bearing bridgmanite

Thermal conductivity of bridgmanite (Bdg) is the important physical property controlling the heat transfer inside the Earth. Here we report room temperature lattice thermal conductivity of (Fe,Al)-bearing Bdg with chemical compositions of Mg0.848Fe0.090Al0.206Si0.856O3 and Mg0.718Fe0.123Al0.281Si0.878O3 measured up to 125 GPa and 74 GPa, respectively, using the pulsed light heating thermoreflectance technique in a diamond anvil cell. We found that the lattice thermal conductivity of these Bdg samples show abnormal reduction in the pressure range of 20-40 GPa at 300 K, which is probably due to the spin transition of Fe3+ in octahedral Si-site (B-site). We propose that the lattice thermal conductivity of Bdg is reduced by 46 ± 16% when Fe is in the mixed spin state, which may form a thermal insulating layer in the Earth's mid lower mantle. In addition, we provide a thermal conductivity model of Bdg, taking into account the effect of compositional difference and the spin transition of Fe. Our conductivity model indicates that the thermal conductivity of Bdg in the pyrolitic lower mantle is more than twice as high as that in the descending MORB, which is likely to create heterogeneity of lateral heat flux through the Earth's core-mantle boundary.

Composition versus temperature induced velocity heterogeneities in a pyrolitic lower mantle

Interpretation of lateral velocity heterogeneities is essential for our understanding of Earth's interior. Ferropericlase's (Fp) spin crossover (FSC) fundamentally changes their interpretation in the mid lower mantle. In a typical pyrolitic aggregate, FSC induces an unusual increase in bulk sound velocity (Vϕ) with increasing temperature at mid-lower-mantle depths. This reduces the sensitivity of longitudinal velocity (VP) to lateral temperature variations around 1700 km. Here we show that FSC also dramatically impacts the manifestation of two important types of compositional heterogeneities: i) variations in iron concentration in Fp, e.g., caused by changes in iron partitioning; ii) variation in molar fraction of Fp, as expected in slab subduction regions. FSC enhances the sensitivity of Vϕ and VP to these compositional variations by several-fold at similar depths. The opposite effects of lateral variations of temperature and composition on VP is critical for distinguishing the possible physical origin of heterogeneities in tomographic P-models. Temperature and composition variations also produce opposite types of correlation between Vϕ and shear velocity (VS) heterogeneities and between VS and density (ρ) heterogeneities. Only lateral temperature variations can produce anti-correlation between Vϕ and VS at mid lower mantle depths, while only these compositional variations can produce anti-correlation between VS and ρ in the spin crossover region and at greater depths. Together these effects suggest that heterogeneities in VP in the mid lower mantle common to multiple seismic models could originate in simultaneous lateral temperature and compositional variations in this region.

Confirming a pyrolitic lower mantle using self‐consistent pressure scales and new constraints on CaSiO3 perovskite

AbstractIn this study, we have examined the lower mantle composition and mineralogy by modeling the density (ρ), bulk sound velocity (VΦ), and dlnρ/dlnVΦ profiles of candidate lower mantle minerals using literature and new experimental equation of state (EoS) results. For CaSiO3 perovskite, complimentary synchrotron X‐ray diffraction measurements in a laser‐heated diamond anvil cell were conducted up to 156 GPa between 1200 K and 2600 K to provide more reliable constraints on the thermal EoS parameters. These new experimental results as well as literature P‐V‐T data sets are systematically analyzed using an internally self‐consistent pressure scale. We have modeled ρ, VΦ, and dlnρ/dlnVΦ profiles of the lower mantle with representative pyrolitic and chondritic compositional models in which the effect of Fe spin transition in ferropericlase is also taken into account. Our modeling results show that a pyrolitic lower mantle with an aggregate mineralogy of 75 vol % bridgmanite, 17 vol % ferropericlase, and 8 vol % CaSiO3 perovskite produces ρ and VΦ profiles in better agreement with preliminary reference Earth model than a lower mantle with a chondritic composition. The modeled ρ, VΦ, and dlnρ/dlnVΦ are mainly affected by the relative ratio of bridgmanite and ferropericlase but are not sensitive to the variation of the CaSiO3 perovskite content. In addition, the spin crossover of Fe in ferropericlase can greatly raise the value of dlnρ/dlnVΦ in the middle lower mantle, which is useful in detecting the presence of ferropericlase in the region. Based on these new mineral physical constraints and radial seismic structure, our study suggests the lower mantle is pyrolitic, which is chemically indistinguishable from the upper mantle.

Iron partitioning in pyrolitic lower mantle

The partitioning of iron between Mg-rich perovskite (Pv) and ferropericlase (Fp) was investigated for a pyrolitic bulk composition over a wide range of simulated lower-mantle pressures and temperatures from 28 to 114 GPa and from 1,900 to 2,300 K, in a laser-heated diamond anvil cell (DAC). The recovered DAC samples are chemically homogeneous, indicating a relatively small temperature gradient during laser heating. The chemical compositions of coexisting Pv, Fp, and Ca-rich perovskite (CaPv) were determined by energy-dispersive X-ray spectroscopy (EDS) using an EDS instrument attached to a transmission electron microscope. Our results demonstrate that at pressures above 90 GPa, Pv becomes more Fe-rich with increasing pressure, which is likely due to the effects of high-spin to low-spin crossover of Fe3+ in Pv. We highlight that such a change in Fe–Mg partitioning between Pv and Fp should have a strong influence on the physical properties of the deep lower mantle.

Heat capacity of MgSiO3 perovskite

Heat capacity of MgSiO3 perovskite has been measured in the temperature range 140–295 K by differential scanning calorimetry. The Kieffer model approach has been used to calculate the heat capacity beyond the measured temperature range. The estimated entropy of MgSiO3 perovskite is 57.2±1.0 J/mol.K at 298 K which is higher than that of ilmenite, in accord with the negative pressure‐temperature slope for the ilmenite‐perovskite transition. From the heat capacity thermodynamic Debye temperature of MgSiO3 perovskite is evaluated to be 980±15 K at 298 K and 1030±20 K at temperatures above about 700 K. Thermodynamic Gruneisen parameter of MgSiO3 perovskite at 30 GPa and above 1000 K is also estimated as 1.45±0.15. An adiabatic temperature profile in the pyrolitic lower mantle is calculated with the average gradient of 0.32±0.04 K/km. The temperature at the top of the D″ layer is estimated to be 2500±100 K.