Effect of Water on Lattice Thermal Conductivity of Ringwoodite and Its Implications for the Thermal Evolution of Descending Slabs
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2020-05-25 |
| Journal | Geophysical Research Letters |
| Authors | Enrico Marzotto, WenâPin Hsieh, Takayuki Ishii, KengâHsien Chao, Gregor Golabek |
| Institutions | University of Bayreuth, Tohoku University |
| Citations | 28 |
| Analysis | Full AI Review Included |
Executive Summary
Section titled âExecutive SummaryâThis research utilizes Diamond-Anvil Cell (DAC) experiments coupled with Time Domain Thermoreflectance (TDTR) to quantify the effect of water content (CH2O) and pressure (P) on the lattice thermal conductivity (Arw) of ringwoodite, a major mineral in the Earthâs mantle transition zone (MTZ).
- Significant Thermal Reduction: The incorporation of 1.73 wt% water reduces the ringwoodite thermal conductivity by more than 40% compared to the nominally dry counterpart at MTZ pressures (17-24 GPa).
- Defect-Induced Scattering: The reduction is attributed to the pressure-enhanced phonon-defect scattering caused by hydroxyl groups (OH-) or protons (H+) incorporated into the crystal lattice, which form an interconnected network under compression.
- Empirical Parameterization: An empirical equation was derived to parameterize Arw as a function of both pressure (P) and water content (CH2O), enabling large-scale geophysical modeling.
- Heat Propagation Barrier: The hydrous ringwoodite layer acts as an effective thermal barrier, significantly slowing heat transfer into the core of a subducting slab.
- Geophysical Impact: 1-D heat diffusion modeling shows that this reduced Arw delays the thermal equilibration of the slab, prolonging the lifetime of temperature-sensitive dense hydrous magnesium silicates (DHMS) by 9-27 Myr.
- Water Transport Implications: This delay is sufficient to allow DHMS to be transported through the MTZ and potentially reach the lower mantle (LM), impacting the deep Earth water cycle and thermal evolution of subduction zones.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Maximum Water Content (CH2O) | 1.73 | wt% | Hydrous Ringwoodite Sample (High) |
| Pressure Range (Measurement) | 0 - 25 | GPa | Relevant for MTZ conditions |
| Thermal Conductivity Reduction | >40 | % | Observed at MTZ pressures (1.73 wt% H2O) |
| Dry Arw (Ambient Pressure) | 4.84 | W m-1 K-1 | Calculated (0 wt% H2O, 1 atm) |
| Dry Arw (MTZ Pressure) | 12.4 | W m-1 K-1 | Calculated (0 wt% H2O, 20 GPa) |
| Critical Decomposition Temperature (Tcrit) | 1500 | K | Assumed threshold for DHMS breakdown |
| Maximum Thermal Delay (tdelay) | 20 - 27 | Myr | For 15-20 km hydrous layer (CH2O = 1.5 wt%) |
| Ringwoodite Synthesis Pressure | 20 - 22 | GPa | Multianvil press conditions |
| Ringwoodite Synthesis Temperature | 1600 - 1900 | K | Multianvil press conditions |
| Al Film Thickness (TDTR Transducer) | ~90 | nm | Used for thermal conductivity measurements |
| Pump Beam Modulation Frequency | 8.7 | MHz | TDTR measurement setup |
Key Methodologies
Section titled âKey Methodologiesâ- Sample Synthesis (Multianvil Press): Ringwoodite samples were synthesized from natural San Carlos Olivine (SCO) powder. Dry samples used Rhenium (Re) capsules at 22 GPa and 1900 K. Hydrous samples used welded Platinum-Rhodium (Pt95Rh5) capsules containing 3-10 wt% distilled water, synthesized at 20-22 GPa and 1600-1700 K.
- Water Content Quantification (FTIR): Water content (CH2O) was determined using unpolarized Fourier Transform Infrared (FTIR) spectroscopy and the Lambert-Beer law, yielding concentrations of 0.11, 0.47, and 1.73 wt%.
- High-Pressure Setup (DAC): Samples (~25 ”m thick) were coated with a ~90 nm Aluminum (Al) film, which served as the thermal transducer. Samples were loaded into a symmetric piston-cylinder Diamond-Anvil Cell (DAC) using silicone oil as the pressure medium to minimize measurement uncertainty. Pressure was monitored via ruby fluorescence.
- Thermal Conductivity Measurement (TDTR): Time Domain Thermoreflectance (TDTR) was employed, utilizing an ultrafast optical pump-probe technique. The pump beam heated the Al film, and the probe beam monitored the temperature evolution by measuring the reflected intensity variations (Vin and Vout components).
- Thermal Modeling: The ratio -Vin/Vout was fitted against numerical calculations derived from a bidirectional thermal model, treating Arw as the primary unknown parameter, while accounting for the thermal properties of the Al film and silicone oil.
- Parameterization and Modeling: The measured Arw data was parameterized as a second-order polynomial function of pressure and water content. This empirical relationship was integrated into a self-written 1-D finite difference heat diffusion model to simulate the thermal evolution of an 80 Myr old subducting slab stagnating at 660 km depth.
Commercial Applications
Section titled âCommercial ApplicationsâThe findings, while focused on deep Earth geophysics, provide critical insights into materials engineering under extreme conditions, particularly concerning defect-mediated thermal transport:
- Thermal Barrier Coatings (TBCs) and Ceramics: The principle of using incorporated light impurities (H/OH) to induce strong phonon-defect scattering and drastically reduce thermal conductivity can be applied to designing advanced TBCs for aerospace or high-temperature industrial applications.
- High-Pressure Synthesis of Functional Materials: Understanding how hydration and pressure synergistically suppress lattice thermal transport informs the synthesis strategies for creating new crystalline materials with intrinsically low thermal conductivity, potentially useful for thermoelectric devices.
- Deep Geothermal Energy Systems: Accurate thermal models of the deep crust and mantle, refined by these new conductivity parameters, are essential for assessing and optimizing deep geothermal energy extraction projects, where heat flow is governed by the thermal properties of hydrated minerals.
- Planetary Interior Modeling Software: The parameterized Arw equation provides necessary input for computational models simulating the long-term thermal history and water cycling of terrestrial planets, crucial for space exploration and resource assessment.
View Original Abstract
Abstract The presence of water in minerals generally alters their physical properties. Ringwoodite is the most abundant phase in the lowermost mantle transition zone and can host up to 1.5-2 wt% water. We studied highâpressure lattice thermal conductivity of dry and hydrous ringwoodite by combining diamondâanvil cell experiments with ultrafast optics. The incorporation of 1.73 wt% water substantially reduces the ringwoodite thermal conductivity by more than 40% at mantle transition zone pressures. We further parameterized the ringwoodite thermal conductivity as a function of pressure and water content to explore the largeâscale consequences of a reduced thermal conductivity on a slabâs thermal evolution. Using a simple 1âD heat diffusion model, we showed that the presence of hydrous ringwoodite in the slab significantly delays decomposition of dense hydrous magnesium silicates, enabling them to reach the lower mantle. Our results impact the potential route and balance of water cycle in the lower mantle.