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Earth/s lowermost mantle displays complex geological structures that likely result from heterogeneous thermal and electromagnetic interaction with the core. Geophysical models of the core-mantle boundary (CMB) region rely on the thermal and electrical conductivities of appropriate geomaterials which, however, have never been probed at representative pressure and temperature (P-T) conditions. Here we report on the opacity of single crystalline bridgmanite and ferropericlase, which is linked to both their radiative and electrical conductivity, measured in dynamically- and statically-heated diamond anvil cells as well as computed from first-principles at CMB conditions. Our results show that light absorption in the visible spectral range is enhanced upon heating in both minerals but the rate of change in opacity with temperature is a factor of six higher in ferropericlase. As a result, bridgmanite in the lowermost mantle is moderately transparent while ferropericlase is highly opaque. Our measurements suggest a very low (< 1 W/m/K) and largely temperature-independent radiative conductivity in the lowermost mantle, at odds with previous studies. This implies that the radiative mechanism has not contributed significantly to cooling the Earth/s core throughout the geologic time and points to a present-day CMB heat flow of 9-11 TW. Opaque ferropericlase is electrically conducting and mediates strong core-mantle electromagnetic coupling, explaining the intradecadal oscillations in the length of day, low secular geomagnetic variations in Central Pacific, and the preferred paths of geomagnetic pole reversals.
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The heat flux across the core-mantle boundary (QCMB) is the key parameter to understand the Earth/s thermal history and evolution. Mineralogical constraints of the QCMB require deciphering contributions of the lattice and radiative components to the thermal conductivity at high pressure and temperature in lower mantle phases with depth-dependent composition. Here we determine the radiative conductivity (krad) of a realistic lower mantle (pyrolite) in situ using an ultra-bright light probe and fast time-resolved spectroscopic techniques in laser-heated diamond anvil cells. We find that the mantle opacity increases critically upon heating to ~3000 K at 40-135 GPa, resulting in an unexpectedly low radiative conductivity decreasing with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the CMB, the latter being ~30 times smaller than the estimated lattice thermal conductivity at such conditions. Thus, radiative heat transport is blocked due to an increased optical absorption in the hot lower mantle resulting in a moderate CMB heat flow of ~8.5 TW, at odds with present estimates based on the mantle and core dynamics. This moderate rate of core cooling implies an inner core age of about 1 Gy and is compatible with both thermally- and compositionally-driven ancient geodynamo.
We investigate the pressure torque between the fluid core and the solid mantle arising from magnetohydrodynamic modes in a rapidly rotating planetary core. A two-dimensional reduced model of the core fluid dynamics is developed to account for the non-spherical core-mantle boundary. The simplification of such a quasi-geostrophic model rests on the assumption of invariance of the equatorial components of the fluid velocity along the rotation axis. We use this model to investigate and quantify the axial torques of linear modes, focusing on the torsional Alfven modes (TM) in an ellipsoid. We verify that the periods of these modes do not depend on the rotation frequency. Furthermore, they possess angular momentum resulting in a net pressure torque acting on the mantle. This torque scales linearly with the equatorial ellipticity. We estimate that for the TM calculated here topographic coupling to the mantle is too weak to account for the variations in the Earths length-of-day.
To explore the role of the olivine grain size and crystal preferred orientation (CPO) on the evolution of the microstructure and the mechanical behaviour of upper mantle rocks up to large strains, we performed axial extension experiments at 1200C, 300MPa confining pressure, and constant displacement rate on three natural peridotites: a fine-grained mylonitic harzburgite with a weak CPO and two coarse-grained well-equilibrated dunites with CPO of variable intensity. Initial flow stresses show a limited range of variation (115-165MPa), with the fine-grained sample showing the highest initial strength. However, the evolution of both mechanical behavior and microstructure differs between fine and coarse-grained peridotites. In the fine-grained harzburgite, necking resulted in decrease in the apparent differential stress. Focusing of strain and stress produced increase of the olivine recrystallized fraction and decrease of the recrystallized grain size in the neck. Analysis of the final stress and strain in the neck indicates softening due to evolution of the microstructure and CPO. In contrast, necking of the coarse-grained samples produced weak or no decrease in the apparent differential stress. This implies hardening, consistently with the increase in bulk intragranular misorientation and final stresses in the neck similar or higher than initial ones. Coarse-grained dunites deformed heterogeneously. Crystals well oriented to deform by dislocation glide became elongated, whereas those in hard orientations remained almost undeformed. In the neck, stress and strain concentration resulted in formation of kinks in hard crystals and dynamic recrystallization in soft crystals. We interpret the more effective strain-induced softening of the fine-grained peridotite as due to easier dynamic recrystallization, probably due to the higher proportion of grain boundaries acting as nucleation sites.
The exceptional ability of carbon to form sp2 and sp3 bonding states leads to a great structural and chemical diversity of carbon-bearing phases at non-ambient conditions. Here we use laser-heated diamond anvil cells combined with synchrotron x-ray diffraction, Raman spectroscopy, and first-principles calculations to explore phase transitions in CaCO3 at P > 40 GPa. We find that post-aragonite CaCO3 transforms to the previously predicted P21/c-CaCO3 with sp3-hybridized carbon at 105 GPa (~30 GPa higher than the theoretically predicted crossover pressure). The lowest enthalpy transition path to P21/c-CaCO3 includes reoccurring sp2- and sp3-CaCO3 intermediate phases and transition states, as reveled by our variable-cell nudged elastic band simulation. Raman spectra of P21/c-CaCO3 show an intense band at 1025 cm-1, which we assign to the symmetric C-O stretching vibration based on empirical and first principles calculations. This Raman band has a frequency that is ~20 % lower than the symmetric C-O stretching in sp2-CaCO3, due to the C-O bond length increase across the sp2-sp3 transition, and can be used as a fingerprint of tetrahedrally-coordinated carbon in other carbonates.
Scattering of seismic waves can reveal subsurface structures but usually in a piecemeal way focused on specific target areas. We used a manifold learning algorithm called the Sequencer to simultaneously analyze thousands of seismograms of waves diffracting along the core-mantle boundary and obtain a panoptic view of scattering across the Pacific region. In nearly half of the diffracting waveforms, we detected seismic waves scattered by three-dimensional structures near the core-mantle boundary. The prevalence of these scattered arrivals shows that the region hosts pervasive lateral heterogeneity. Our analysis revealed loud signals due to a plume root beneath Hawaii and a previously unrecognized ultralow-velocity zone beneath the Marquesas Islands. These observations illustrate how approaches flexible enough to detect robust patterns with little to no user supervision can reveal distinctive insights into the deep Earth.
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