Blocked radiative heat transport in the hot pyrolitic lower mantle

At a Glance

Metadata	Details
Publication Date	2020-03-02
Journal	Earth and Planetary Science Letters
Authors	Sergey S. Lobanov, Nicholas Holtgrewe, G. Ito, James Badro, Hélène Piet
Institutions	Institut de physique du globe de Paris, Centre National de la Recherche Scientifique
Citations	20
Analysis	Full AI Review Included

Blocked Radiative Heat Transport in the Hot Pyrolitic Lower Mantle

Executive Summary

This study provides critical experimental constraints on radiative thermal conductivity (k_rad) in the Earth’s lower mantle, challenging previous models based on room-temperature data.

Low Radiative Conductivity: k_rad is found to be unexpectedly low, decreasing continuously with depth from ~0.8 W/m/K at 1000 km to ~0.35 W/m/K at the Core-Mantle Boundary (CMB).
Mechanism of Blocking: The low k_rad is caused by a strong, temperature-activated increase in optical absorption (opacity) in the hot lower mantle phases, primarily ferropericlase (Fp).
Dominant Phase: Fp governs the optical opacity due to the dynamic red-shifting of its Fe-O charge transfer band, effectively blocking radiative heat transport. Bridgmanite (Bgm) shows much less temperature sensitivity.
Thermal Implications: Radiative heat transport is not a significant mechanism in the lowermost mantle. Total thermal conductivity (k_total) is dominated by lattice conductivity (k_lat), yielding k_total ≈ 8.5 W/m/K at the CMB.
Geophysical Impact: This low thermal conductivity results in a moderate CMB heat flow (Q_CMB) of ~8.5 TW, consistent with a young inner core age (~1 Gy) and supporting models for ancient geodynamo driven by compositional buoyancy.
Methodology: The results were achieved using in situ measurements on pyrolite samples in laser-heated Diamond Anvil Cells (DACs) combined with ultra-bright supercontinuum probing and fast time-resolved spectroscopy (µs scale) to suppress thermal background.

Technical Specifications

Parameter	Value	Unit	Context
Experimental Pressure Range (P)	40 - 135	GPa	Lower mantle simulation
Experimental Temperature Range (T)	Up to ~3000	K	High-P/T measurements
Radiative Conductivity (k_rad) at 1000 km	~0.8	W/m/K	Top of lower mantle
Radiative Conductivity (k_rad) at CMB (2850 km)	~0.35	W/m/K	Core-Mantle Boundary
Lattice Conductivity (k_lat) at CMB	7.7 - 8.8	W/m/K	Experimental/Computational estimates
Calculated CMB Heat Flow (Q_CMB)	~8.5	TW	Based on k_total ≈ 8.5 W/m/K
Pyrolite Absorption Coefficient Increase (40 GPa to 134 GPa)	4-8	Factor	Increase with depth/pressure
Fp Opacity Increase (at ~3000 K)	~5	Times	Relative to 300 K
Bgm Opacity Increase (at ~3500 K)	20-30	%	Relative to 300 K
Probe Laser Type	Leukos Pegasus	Pulsed Supercontinuum Laser	Ultra-bright source
Probe Pulse Width	4	ns	High temporal resolution
Probe Repetition Rate	0.25 - 1	MHz	For time-resolved measurements
Heating Laser Wavelength	1070	nm	Yt-doped fiber laser
VIS Detector Gate Width	30	ns	Gated iCCD for thermal background rejection
Pyrolite Crystallized Grain Size	< 500	nm	Measured by STEM HAADF
IR Measurement Range	6200-13000	cm^-1	Infrared range
VIS Measurement Range	13000-22000	cm^-1	Visible range

Key Methodologies

The determination of k_rad relied on measuring the absorption coefficient α(P, T) of pyrolite using advanced laser-heating and time-resolved spectroscopy techniques in DACs.

Pyrolite Synthesis and Crystallization: Highly homogeneous pyrolite glass was synthesized via gas-mixing aerodynamic levitation. This glass was crystallized in situ in DACs at P > 30 GPa and T up to 3000 K to form a conglomerate of Bgm, Fp, and Ca-perovskite representative of lower mantle equilibrium.
Sample Characterization: Crystallinity was confirmed by the disappearance of the Raman boson peak. Mineralogy, composition (e.g., Bgm Fe# = 7.7 ± 2, Fp Fe# = 16.8 ± 2), and submicron grain size (< 500 nm) were verified post-experiment using synchrotron XRD, STEM, and EDX.
Time-Resolved Spectroscopy: An ultra-bright supercontinuum probe laser (4 ns pulses) was used for optical absorption measurements in the IR and VIS ranges.
Thermal Background Suppression:
- In the VIS range, a gated iCCD detector (30 ns gate width) was synchronized with the probe pulses to block thermal radiation from the heated sample, enabling measurements up to T < ~2700 K.
- In dynamic experiments, a streak camera coupled with single-shot (1 µs) laser heating allowed fully reversible absorbance measurements up to T > 3000 K, suppressing artifacts from iron diffusion (Soret effect).
Sample Thickness Measurement: Thickness (d) was determined using both high-pressure visible light interferometry and high-precision 3D optical profilometry on recovered samples, corrected for decompression using the Bgm P-V-T equation of state.
Light Scattering Correction: Measured absorption coefficients were corrected for static light scattering from the submicron grains. This correction was based on comparing pyrolite spectra to room-temperature spectra of single-crystal Bgm and Fp, and was computationally supported by Multiple Sphere T-matrix modeling.
k_rad Calculation: Radiative conductivity was calculated using the formula: k_rad(T) = (4n²/3) ∫ (1/α(ν)) (∂I(ν,T)/∂T) dν, where α(ν) is the scattering-corrected absorption coefficient, n is the refractive index (estimated via Gladstone-Dale relation), and I(ν, T) is the Planck function.

Commercial Applications

The fundamental material science insights regarding thermal transport and optical properties under extreme pressure and temperature have direct relevance across several high-tech engineering sectors.

Extreme Environment Thermal Management:
- Application: Design and validation of high-performance ceramics and refractory materials used in aerospace, hypersonic vehicles, and high-temperature industrial processes (e.g., glass manufacturing, steel production).
- Relevance: The finding that opacity increases dramatically with temperature provides a design constraint for materials intended to manage heat radiatively in hot, dense environments.
Advanced Sensor and Metrology Development:
- Application: Development of ultra-fast, time-resolved optical sensors and spectroscopic systems for characterizing transient material states (e.g., shock compression, phase transitions).
- Relevance: The novel use of gated detectors and supercontinuum lasers to overcome thermal background interference is a key technique for high-P/T material characterization.
High-Pressure Electrical Components:
- Application: Designing electrical insulators and conductors for deep-earth or high-pressure industrial applications.
- Relevance: The temperature-enhanced electrical conductivity of ferropericlase (Fp) due to band gap narrowing has implications for high-P/T electrical component design and modeling of mantle conductivity anomalies.
Diamond Anvil Cell (DAC) Technology and Materials (6ccvd.com Context):
- Application: The research relies on high-quality, low-defect diamond anvils for pressure generation and optical windows.
- Relevance: Pushing the limits of DAC experiments (high P, high T, complex optical probing) drives demand for specialized CVD diamonds with superior optical transparency and mechanical integrity at extreme conditions.

Tech Support

Original Source

DOI: https://doi.org/10.1016/j.epsl.2020.116176

References

2010 - First-principles constraints on diffusion in lower-mantle minerals and a weak D” layer [Crossref]
2016 - An early geodynamo driven by exsolution of mantle components from Earth’s core [Crossref]
1957 - Radiative transfer in the Earth’s mantle [Crossref]
2013 - Effect of mass disorder on the lattice thermal conductivity of MgO periclase under pressure [Crossref]
2009 - Thermal conductivity of lower-mantle minerals [Crossref]
2008 - Radiative conductivity in the Earth’s lower mantle [Crossref]
2015 - Experimental study of thermal conductivity at high pressures: implications for the deep Earth’s interior [Crossref]
2006 - Reduced radiative conductivity of low-spin (Mg,Fe)O in the lower mantle [Crossref]
2019 - New constraints on the thermal conductivity of the upper mantle from numerical models of radiation transport [Crossref]
2017 - Crystallization of silicon dioxide and compositional evolution of the Earth’s core [Crossref]

Blocked radiative heat transport in the hot pyrolitic lower mantle

At a Glance