Radiative conductivity and abundance of post-perovskite in the lowermost mantle

Abstract

Thermal conductivity of the lowermost mantle governs the heat flow out of the core energizing planetary-scale geological processes. Yet, there are no direct experimental measurements of thermal conductivity at relevant pressure-temperature conditions of Earth's core-mantle boundary. Here we determine the radiative conductivity of post-perovskite at near core-mantle boundary conditions by optical absorption measurements in a laser-heated diamond anvil cell. Our results show that the radiative conductivity of Mg 0.9 Fe 0.1 SiO 3 post-perovskite (< 1.2 W/m/K) is ~ 40% smaller than bridgmanite at the base of the mantle. By combining this result with the present-day core-mantle heat flow and available estimations on the lattice thermal conductivity we conclude that post-perovskite is as abundant as bridgmanite in the lowermost mantle which has profound implications for the dynamics of the deep Earth. Main textThe lowermost 200-400 km of the mantle is a critical region responsible for the coremantle interaction powering all major geological processes on Earth 1 . Specifically, thermal conductivity of the thermal boundary layer (TBL) above the core-mantle boundary (CMB) determines the heat flow out of the core that provides energy to sustain the mantle global circulation and to drive the geodynamo 1, 2 . Seismic structures of the lowermost mantle, however, are complex 3, 4 implying that the thermal conductivity of the region is non-uniform due to variations in chemical (e.g. iron) and/or mineralogical contents as well as texturing of the constituting minerals. The nature of the seismic heterogeneity near the CMB, including a sharp increase in shear wave velocity and anti-correlations of seismic parameters, has been linked to the bridgmanite (Bdgm) to post-perovskite (Ppv) transition 5, 6 , as these phases have contrasting elastic, rheological, and transport properties (e.g. Ref. 6 ). Indeed, measurements and computations of lattice thermal conductivity (k lat ) in Bdgm and Ppv revealed that Ppv conducts heat 50-60 % more efficiently than Bdgm 7, 8 , suggesting that the distribution of the Ppv phase can significantly enhance the heat flux out of the core. However, no mineral physics constraints are available on the radiative thermal conductivity (k rad ) of Ppv, which should play an increasingly important role at high temperature 9 , as well as on the Ppv abundance in the lowermost mantle. This has hampered our understanding of how the heat flux across the CMB may vary laterally and what magnitude of the energy source in the mantle and the outer core is needed to power their convections.Previous estimates of radiative thermal conductivity at lower mantle conditions were based on high-pressure room-temperature measurements of the absorption coefficients of representative minerals in the mid/near-infrared and visible spectral range [10][11][12][13][14] . The presence of an intense thermal radiation emitted from the hot sample makes measurements of the optical properties at high temperatures releva...

Lattice Thermal Conductivity of MgSiO3 Postperovskite Under the Lowermost Mantle Conditions From Ab Initio Anharmonic Lattice Dynamics

“…Radiative heat transfer would be another contribution to the thermal conductivity of the LM (e.g., Clark, ; Goncharov et al, ; Hofmeister, ; Keppler et al, ). Recent optical measurements on Fe‐bearing PPv and Brg showed that the radiative thermal conductivity ( κ rad ) of PPv is smaller than that of Brg (Lobanov et al, ). This suggests that the difference in the total thermal conductivity ( κ tot = κ lat + κ rad ) between PPv and Brg could be smaller than the difference in the lattice components only.…”

“…This suggests that the difference in the total thermal conductivity ( κ tot = κ lat + κ rad ) between PPv and Brg could be smaller than the difference in the lattice components only. Here, using a model for the κ rad constructed by Lobanov et al (), the κ tot of Mg‐PPv and Mg‐Brg at P CMB at the LM temperatures are estimated (Figure ). It is found that κ rad is not quite significant compared to κ lat in both phases but has nonnegligible contributions to κ tot .…”

“…Dashed lines indicate the lattice components ( κ lat ) of both phases obtained in this study. Radiative components ( κ rad ) of Mg 0.9 Fe 0.1 SiO 3 PPv and Brg suggested based on the optical measurements (Lobanov et al, ) are represented by dash‐dotted lines.…”

Optical signatures of low spin Fe3+ in NAL at high pressure

“…This is justified by the d 5 Tanabe‐Sugano diagram as well as the similarity in 10Dq of LS NAL and Bdgm. In further support, an absorption band centered at ~17,500 cm −1 is present in the absorption spectra of LS Fe 3+ ‐bearing Mg 0.6 Fe 0.4 SiO 3 post‐perovskite at 130 GPa and 300 K [ Lobanov et al ., ] which resembles NAL and Bdgm in the distribution of iron in the crystal structure [ Hsu et al ., ; Wu et al ., ; Yu et al ., ]. It is clear, however, that the intensity of the 2 T 2g → 2 T 1g ( 2 A 2g ) band is proportional to iron concentration at the octahedral site; thus, a substantial amount of LS Fe 3+ (~10 mol.…”

“…Crystal field splitting energies of iron in an octahedral crystal field of NAL (this study), post‐perovskite (Ppv) [ Lobanov et al ., ], and siderite [ Lobanov et al ., ] as deduced from optical absorption measurements in DACs at 300 K. Red and blue horizontal bars show high and low spin values, respectively. The crystal field splitting energy of LS Fe 3+ in Bdgm is computed in this work (Figure ).…”