The outer layer of our planet, the lithosphere, is fragmented into several rigid plates, which are drifting on top of the warm and relatively soft mantle. The collision between plates causes the heavier plate to sink into the Earth's mantle. This sinking process takes the name of 'subduction', while the subducting plate is called 'slab'. Usually, the oceanic lithosphere is heavier than the continental one, because it is made of denser minerals like olivine, which accounts for 80 percent of the oceanic lithosphere. Olivine is also the predominant mineral in the Earth's outer shell, representing 60% of the upper mantle (40-410 km of depth). While subducting, the cold slabs are progressively heated by the warm ambient mantle through heat diffusion, a process that involves heat conduction and heat radiation. Understanding slab heating processes is fundamental to explaining the occurrence of deep earthquakes, and the presence of water at more than 600 km of depth.
"We measured, for the first time, the transparency of olivine inside the Earth," says geodynamicist Enrico Marzotto from the Institute of Geosciences of the University of Potsdam. "These measurements demonstrate that olivine is infrared transparent even at the extreme pressure and temperature conditions of Earth's interior." The heat transport by radiation accounts for approximately 40 percent of the total heat diffused in the olivine-rich upper mantle. Therefore, radiative thermal conductivity plays an important role in slab heating and can have far-reaching effects on the density and the rigidity of the subducting plates, and their capacity to carry water into Earth's interior.
With two-dimensional slab thermal evolution models the team could show that the rapid heating enhanced by radiative heat transport induces the breakdown of water-bearing minerals at shallower depth. "This could potentially explain the occurrence of earthquakes in the slab at more than 70 kilometers of depth," says Marzotto. "Consequently, only slabs that are more than 60 Million years old and sinking faster than 10 centimeters per year, remain sufficiently cold to transport the water-bearing minerals down to the Mantle Transition Zone (MTZ) in 410 to 660 kilometers depth." The MTZ is considered the largest water reservoir on our planet, potentially containing up to three times more water than the Earth's oceans.
"Our study also provides numerical tools to compute the lifetime of thermal anomalies in the mantle and their geodynamic behavior," adds Enrico Marzotto. These anomalies can be hot (like the plumes rising from the Earth's deep mantle) or cold (like the subducting slabs).
Research Report:Olivine's high radiative conductivity increases slab temperature by up to 200K
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