09:15 〜 09:30
[SIT20-02] Combining thermo-reflectance measurements (TDTR) with finite-difference (FD) numerical modelling to study the thermal evolution of the subducting slabs
★Invited Papers
キーワード：Time-Domain Thermo-Reflectance (TDTR), Finite Difference (FD) numerical modelling, Thermal Conductivity, Hydrous Ringwoodite, Stishovite, Slab Thermal Evolution
Lattice thermal conductivity Λ, is the physical parameter that controls heat transfer through solids. In minerals, heat is transported by phonon vibration. Changes in the lattice structure due to different P-T conditions, and the presence of impurities, alters the phonon mean-free-path. Is thus fundamental to constrain the effect of these parameters on Λ, in order to study the thermal evolution of geological bodies. In recent years, the effect of composition and pressure on the thermal conductivity of mantle minerals has been investigate by combining diamond anvil cells (DAC) with Time-Domain Thermo-Reflectance (TDTR): Fe-Bridgmanite (Hsieh et al., 2017), Ferropericlase (Hsieh et al., 2018), Olivine (Chang et al. 2018), and Ringwoodite (Marzotto et al., 2020).
These minerals represents up to 60-80vol% of the ambient mantle and the subducting slab in the correspondent stability field (lower mantle, upper mantle and mantle transition zone). Moreover, each mineral is subject to major variation of its thermal conductivity due to the presence of Fe or H2O in the crystal lattice. From the room temperature and high pressures TDTR measurements, it is possible to extrapolate the thermal conductivity profile of the aggregate mantle rocks along a given geotherm (Xu et al., 2004), which it can be used to study the thermal evolution of a descending slab by means of numerical modelling.
This approach has been recently applied to the TDTR measurements of hydrous ringwoodite (Marzotto et al., 2020). In this work, we found that the presence of H2O (up to 1.73 wt%) in the crystal structure of ringwoodite can cause up to 40% reduction of its thermal conductivity. From the experimental data we modelled the thermal evolution of a subducting slab with the 1D Finite Difference method. Our calculations reveal that the presence of hydrous ringwoodite significantly delays heat propagation through the slab. This effect might post-pone the decomposition age of temperature sensitive hydrous minerals (in particular DHMS), enabling them to reach the lower mantle.
This work flow has been applied also to the recent TDTR measurements on Fe-Al-Bridgmanite, Fe-New-Aluminium Phase (NAL), Stishovite, and Al-Stishovite (unpublished data). From this dataset was possible to model the aggregate thermal conductivity of meta-basalts at high pressure and temperatures, and to study the thermal evolution of the oceanic crust in the shallow lower mantle. Preliminary results reveals that the meta-basaltic crust presents a thermal conductivity twice as high as the one of the ambient pyrolitic mantle, mostly from the contribution of the extremely high thermal conductivity of Stishovite. Our 1D numerical simulations suggests that the local enrichment of Stishovite causes fast thermal equilibration in the meta-basalts, thus favouring crust detachment in the 660-720 km region. The encouraging results from our 1D simulations should be further tested in more sophisticated thermo-chemical models to provide a better picture of the geodynamic evolution of the slabs.
These minerals represents up to 60-80vol% of the ambient mantle and the subducting slab in the correspondent stability field (lower mantle, upper mantle and mantle transition zone). Moreover, each mineral is subject to major variation of its thermal conductivity due to the presence of Fe or H2O in the crystal lattice. From the room temperature and high pressures TDTR measurements, it is possible to extrapolate the thermal conductivity profile of the aggregate mantle rocks along a given geotherm (Xu et al., 2004), which it can be used to study the thermal evolution of a descending slab by means of numerical modelling.
This approach has been recently applied to the TDTR measurements of hydrous ringwoodite (Marzotto et al., 2020). In this work, we found that the presence of H2O (up to 1.73 wt%) in the crystal structure of ringwoodite can cause up to 40% reduction of its thermal conductivity. From the experimental data we modelled the thermal evolution of a subducting slab with the 1D Finite Difference method. Our calculations reveal that the presence of hydrous ringwoodite significantly delays heat propagation through the slab. This effect might post-pone the decomposition age of temperature sensitive hydrous minerals (in particular DHMS), enabling them to reach the lower mantle.
This work flow has been applied also to the recent TDTR measurements on Fe-Al-Bridgmanite, Fe-New-Aluminium Phase (NAL), Stishovite, and Al-Stishovite (unpublished data). From this dataset was possible to model the aggregate thermal conductivity of meta-basalts at high pressure and temperatures, and to study the thermal evolution of the oceanic crust in the shallow lower mantle. Preliminary results reveals that the meta-basaltic crust presents a thermal conductivity twice as high as the one of the ambient pyrolitic mantle, mostly from the contribution of the extremely high thermal conductivity of Stishovite. Our 1D numerical simulations suggests that the local enrichment of Stishovite causes fast thermal equilibration in the meta-basalts, thus favouring crust detachment in the 660-720 km region. The encouraging results from our 1D simulations should be further tested in more sophisticated thermo-chemical models to provide a better picture of the geodynamic evolution of the slabs.