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	Centre National de la Recherche Scientifique, Centre de Recherches Pétrographiques et Géochimiques
Citations	20
Analysis	Full AI Review Included

Technical Documentation & Analysis: Blocked Radiative Heat Transport in the Lower Mantle

This document analyzes the research paper “Blocked radiative heat transport in the hot pyrolitic lower mantle” to highlight the critical role of high-quality diamond materials in extreme high-pressure/high-temperature (HPHT) research and to position 6CCVD’s capabilities as the ideal solution provider for replicating and extending this work.

Executive Summary

The study successfully determined the radiative thermal conductivity ($k_{rad}$) of pyrolite (a lower mantle proxy) in situ at extreme P-T conditions using laser-heated Diamond Anvil Cells (DACs) and advanced time-resolved spectroscopy.

Core Finding: Radiative heat transport is unexpectedly blocked in the hot lower mantle due to a critical increase in optical absorption (opacity) upon heating to ~3000 K.
Quantitative Result: $k_{rad}$ decreases continuously with depth, falling from ~0.8 W/m/K at 1000 km to a low of ~0.35 W/m/K at the Core-Mantle Boundary (CMB).
Mechanism Identified: The increased opacity is primarily governed by the temperature-enhanced absorption characteristics of the ferropericlase (Fp) component, which is highly sensitive to temperature-induced changes in the Fe-O charge transfer band.
Methodological Breakthrough: The use of an ultra-bright, pulsed supercontinuum light probe synchronized with fast, gated iCCD detectors was essential to suppress overwhelming thermal background radiation at T > 2700 K.
Geophysical Implication: The low $k_{rad}$ means total thermal conductivity ($k_{total}$) is dominated by lattice conductivity ($k_{lat}$), resulting in a moderate CMB heat flow (~8.5 TW) compatible with a young inner core age (~1 Gy).
Material Requirement: The success of the experiment relied fundamentally on the use of high-purity, high-thermal-conductivity Single Crystal Diamond (SCD) anvils capable of maintaining optical transparency and facilitating rapid thermal quenching (< 10 µs cooling time).

Technical Specifications

The following hard data points were extracted from the research paper, detailing the experimental conditions and key results:

Parameter	Value	Unit	Context
Maximum Pressure (P)	135	GPa	Achieved in DAC experiments (approximating CMB conditions).
Maximum Temperature (T)	~3000	K	Achieved during continuous laser heating.
Radiative Conductivity (k_rad) at CMB	~0.35	W/m/K	Measured and extrapolated value at the Core-Mantle Boundary.
Lattice Conductivity (k_lat) at CMB	7.7 - 8.8	W/m/K	Experimental estimates for Bgm/Fp mixture (Extended Data Table 3).
Total CMB Heat Flow (Q_CMB)	~8.5	TW	Calculated based on $k_{total} \approx k_{lat}$.
Pyrolite Grain Size	< 500	nm	Average grain size after crystallization (STEM/HAADF analysis).
Probe Light Source	Leukos Pegasus	N/A	Pulsed supercontinuum laser.
Probe Pulse Duration	4	ns	Used for time-resolved spectroscopy.
VIS Detector Gate Width	30	ns	Used for gated iCCD detection to suppress thermal background.
Heating Laser Wavelength	1070	nm	Yt-doped fiber laser.
Sample Thickness (d)	~1 - 10	µm	Estimated thickness at high pressure (critical for absorption calculation).
Fp Absorption Coefficient Increase	~5	Times	Opacity increase upon heating Fp to ~3000 K at 135 GPa (relative to 300 K).

Key Methodologies

The experiment utilized highly specialized HPHT synthesis and time-resolved optical measurement techniques:

Pyrolite Glass Synthesis: Pyrolite glass was synthesized using a gas-mixing aerodynamic levitation laser furnace at 2000 °C under controlled low oxygen fugacity (fO₂) conditions (IW+0.7).
DAC Assembly: Three types of DAC assemblages were used, employing Rhenium (Re) gaskets pre-indented to 20-40 µm. Sample holes were 40-70 µm in diameter. KCl was used as an optical reference/pressure medium in some runs.
Crystallization In Situ: Homogeneous pyrolite glass was crystallized at P > 30 GPa and T up to 3000 K using double-sided laser heating for several minutes, forming a conglomerate of bridgmanite (Bgm) and ferropericlase (Fp).
Optical Probing System: Measurements utilized an ultra-bright Leukos Pegasus pulsed supercontinuum laser (4 ns pulse, 0.25-1 MHz repetition rate) focused to ~5 µm diameter, covering the IR (6200-13000 cm^-1) and VIS (13000-22000 cm^-1) ranges.
Thermal Background Suppression:
- IR measurements (T < ~2700 K) used an ungated InGaAs detector with short collection windows (1-5 ms).
- VIS measurements (T < ~2700 K) used a gated iCCD detector (30 ns gate width) synchronized with the probe pulses to eliminate thermal radiation interference.
Dynamic Experiments: Independent series of dynamically-heated DAC experiments used single-shot 1 µs laser heating combined with a streak camera to observe fully reversible optical absorbance changes in Fp and Bgm up to T > 3000 K.
Sample Characterization: Recovered samples were analyzed using Synchrotron X-ray Diffraction (XRD), Scanning Transmission Electron Microscopy (STEM), Energy-Dispersive X-ray Spectroscopy (EDX), and Electron Energy Loss Spectroscopy (EELS) to confirm crystallinity, grain size (< 500 nm), and Fe³⁺/Fe_total ratios.
Scattering Correction: Absorption coefficients were corrected for static light scattering using a superposition T-matrix model, based on the absorption coefficients of single-crystal Bgm and Fp.

6CCVD Solutions & Capabilities

This research demonstrates the critical need for ultra-high-quality diamond materials capable of operating under extreme HPHT conditions while maintaining superior optical properties. 6CCVD is uniquely positioned to supply the SCD components necessary to replicate, extend, and advance this type of high-precision geophysics research.

Applicable Materials

The success of this experiment hinges on the SCD anvils serving as both the pressure medium and the optical window.

Research Requirement	6CCVD Recommended Material	Technical Rationale
HPHT Mechanical Strength (P > 135 GPa)	Single Crystal Diamond (SCD)	SCD offers the highest mechanical strength and stiffness required for DAC operation at extreme pressures.
Optical Transparency (IR-VIS range)	Optical Grade SCD (Low Nitrogen)	High-purity SCD ensures minimal intrinsic absorption across the 400-2400 nm probe range, crucial for accurate absorption coefficient measurements.
Rapid Thermal Dissipation (10 µs cooling)	High Thermal Conductivity SCD (Type IIa/Ib)	MPCVD SCD provides exceptional thermal conductivity, enabling the rapid cooling necessary for time-resolved spectroscopy and quenching high-T states (p. 14).

Customization Potential

6CCVD’s in-house fabrication capabilities directly address the specialized requirements of DAC experiments:

Custom Dimensions and Geometry: The paper utilized anvils with specific culet sizes (200 µm, 300 µm) and bevels. 6CCVD specializes in manufacturing custom SCD plates and wafers up to 500 µm thick, precisely cut and polished to meet exact DAC culet and bevel specifications.
Ultra-Polished Surfaces: Accurate absorption coefficients depend critically on precise sample thickness measurements (p. 14). 6CCVD guarantees ultra-high polishing standards for SCD, achieving surface roughness (Ra) < 1 nm, which minimizes light scattering at the diamond-sample interface and improves the accuracy of interferometric thickness determination.
Metalization for Advanced Experiments: While this study used laser heating, future HPHT experiments, particularly those investigating the temperature-enhanced electrical conductivity of Fp (p. 8), may require integrated resistive heaters or electrical contacts. 6CCVD offers internal metalization capabilities, including deposition of Au, Pt, Ti, W, and Cu, directly onto the diamond surface for integrated DAC components.

Engineering Support

6CCVD’s in-house PhD team provides expert material consultation for HPHT applications, including:

Material Selection: Assistance in selecting the optimal SCD grade (e.g., Type IIa vs. Type Ib) based on specific pressure limits, temperature requirements, and desired spectral window (IR vs. VIS).
Design Optimization: Support for optimizing diamond geometry (culet size, bevel angle) to maximize pressure generation while maintaining optical access for complex spectroscopic setups.
Thermal Management: Consultation on utilizing the superior thermal properties of MPCVD diamond for precise temperature control and rapid quenching in time-resolved thermal transport studies, such as those focusing on mantle radiative conductivity and core heat flow.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

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]