Abstract
AbstractThe 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 high‐pressure lattice thermal conductivity of dry and hydrous ringwoodite by combining diamond‐anvil 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 large‐scale consequences of a reduced thermal conductivity on a slab's thermal evolution. Using a simple 1‐D 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.
Highlights
Ringwoodite is the high‐pressure polymorph of olivine that is stable in the lowermost part of the mantle transition zone (MTZ) between approximately 520‐ to 660‐km depth (Suzuki et al, 2000)
Using a simple 1‐D 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
time domain thermoreflectance (TDTR) measurements were performed at room temperature and high pressymbol (Table S5), whereas CH2O is marked by color: 0.11 wt%, sures for three different CH2O values: low (0.11 wt%), medium (0.47 wt%), 0.47 wt%, and 1.73 wt%
Summary
Ringwoodite is the high‐pressure polymorph of olivine that is stable in the lowermost part of the mantle transition zone (MTZ) between approximately 520‐ to 660‐km depth (Suzuki et al, 2000). For hydrous minerals in the MTZ and subducting slabs, knowledge of the effects of pressure P, temperature T, and water content CH2O on their lattice thermal conductivities are fundamental to constrain the thermal interactions between the hydrated slab and the MTZ. Given its large water storage capacity and its abundance in the MTZ, the effects of P and CH2O on ringwoodite thermal conductivity are essential to determine the inner temperatures of subducting slabs. We couple diamond‐anvil cell (DAC) experiments with ultrafast time domain thermoreflectance (TDTR) to determine the P and CH2O dependencies of ringwoodite thermal conductivity, ΛRw, at MTZ pressures and room temperature. Combining our measurements with finite‐difference numerical modeling, we show that the hydration‐reduced ΛRw prolongs the time required to reach thermal equilibrium within a subducting slab, enabling temperature‐sensitive hydrous minerals to be transported to greater depth. Our results demonstrate the importance of hydration on the thermal evolution and fate of descending slabs
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