Summary
The temperature of tectonic plates has a significant influence on their dynamics during subduction, i.e. their descent into the Earth's interior, and on the associated transport of water into deep rock layers. The presence of water in the rocks in turn affects their strength and thus the formation of earthquakes. However, previous models can only predict plate temperatures very roughly. In particular, they neglect the contribution made by heat transport through infrared radiation. A research team led by Dr Enrico Marzotto and Dr Sergey Lobanov from the GFZ Helmholtz Centre for Geosciences and the University of Potsdam has now succeeded in determining this for the first time. Using a unique experimental setup, they were able to prove that olivine, the dominant mineral in the upper mantle (about 60 per cent) and the subducting plates (about 80 per cent), is transparent to infrared thermal radiation even under the extreme pressure and temperature conditions in the Earth's interior. As a result, radiation accounts for up to 40 per cent of total heat transport in mantle olivine. This is significant: their modelling showed that thermal infrared light additionally heats subducting plates by 100 to 200 degrees Kelvin. The researchers conclude that water-bearing minerals can only reach the deep mantle transition zone (~410–660 km) in two scenarios. Either as part of old plates which are older than 60 million years – because these are initially relatively cold. Or at high subduction rates of more than 10 cm/year – not spending enough time in the upper mantle to heat up sufficiently to release water. The results were published in Nature Communications.
Background: The role of temperature and water in the dynamic processes of subduction
The rocks of the oceanic lithosphere are altered by seawater, which is either trapped in the pores of the rocks or chemically bound to minerals. The most important water-bearing minerals in the oceanic lithosphere are serpentines, which are formed by chemical reactions between water and magnesium-rich minerals such as olivine and enstatite. Serpentinites are drawn into the Earth's mantle during the subduction of the oceanic lithosphere, heat up and release water when they reach critical dehydration temperatures. Numerical modelling in the current study suggests that around two-thirds of the water is released before reaching a depth of 230 kilometres.
The release of water at depth is of great importance for plate tectonics, as free water lowers the melting temperature of rocks and thus generates magma and volcanism near subduction zones, such as those found in the Andes and Japan. It also causes rocks to become brittle, since the presence of water in the pores reduces mechanical strength of the material. In cold inner regions of the plates this leads to earthquakes, whereas the rocks in warmer, outer regions of the plate are softened and they undergo viscous deformation without rupture and generating earthquakes. The water weakening inside the cold and brittle inner regions of the slab might explain the occurrence of earthquakes beyond the theoretical limit of brittle rupture, that is, at depths greater than 70 kilometres.
However, water bound to minerals can survive in the plate as long as some regions remain cold enough to allow the formation of water-bearing high-pressure phases. This allows water to be transported to deep regions of the Earth's mantle, such as the mantle transition zone at a depth of 410 to 660 kilometres, or even to the lower mantle. The mantle transition zone contains the largest water reservoir on our planet, storing up to three times more water than the oceans.
(See a corresponding illustration in the above slider.)
Modelling the thermal evolution of tectonic plates
Modelling the thermal evolution of tectonic plates provides a basis for estimating the depth of water release and is therefore crucial for understanding both the water cycle in the deep Earth and plate tectonics.
A key basis for this modelling is knowledge of the thermal conductivity of the materials. This consists of two components – a lattice component and one for radiant heat. The latter contribution has not been taken into account to date, as it was assumed that iron-bearing silicate minerals such as olivine are opaque to thermal radiation under the high pressure and temperature conditions of the upper mantle. However, this assumption is based on studies that were unable to reproduce the relevant pressure and temperature conditions or the decisive infrared spectral range with sufficient experimental accuracy.
Unique experiments provide access to the optical absorption coefficient of olivine
Now, researchers led by Dr Enrico Marzotto and Dr Sergey Lobanov from the GFZ Helmholtz Centre for Geosciences and the University of Potsdam have been able to experimentally investigate the transparency (colour) of the mantle material olivine under the extreme pressure and temperature conditions prevailing in the Earth's mantle for the first time.
“This is a major challenge. Imagine you want to measure the colour of an object that is several thousand degrees hot. Because the hot object emits thermal light in the infrared and visible spectral range, you only see the thermal light and not the actual colour of the object. Moreover, these measurements have to be performed on a tiny sample of only approximately 0.1 millimetres as required by the high-pressure instrumentation,” explains Lobanov.
In his globally unique laboratory, a special combination of equipment makes it possible to measure the transparency of the material even at high pressures and temperatures: in a diamond anvil cell, the tiny 0.1 millimetres sample can be subjected to enormous pressure and heated with a continuous laser. Simultaneously, another extremely bright white laser sends thousands of nanosecond-long pulses through the hot sample, which are recorded by a correspondingly clocked spectroscopic detector. The recorded signal contains information on the transparency of the sample. The precise synchronisation between the white laser pulses and the detector minimises the detectors' exposure to thermal light.
Dr Sergio Speziale from GFZ Section 4.2 “Geomechanics and Scientific Drilling”, Prof. Dr Monika Koch-Müller (guest in Section 4.2), Dr Alexander Koptev from Section 4.1 “Litosphere Dynamics” and Dr Nada Abdel-Hak (now at Cairo University, Egypt) were also involved in this work at the GFZ.
Result: Olivine is transparent to infrared radiation under mantle conditions
“The results of our work show that olivine remains infrared-permeable under the pressure and temperature conditions of the upper mantle and that radiative heat transport can play a significant role in the mantle,” Lobanov summarises.
The researchers have derived a new formula for calculating the temperature-dependent radiative heat conductivity of olivine. Using a 2D model of heat evolution in tectonic plates developed by Enrico Marzotto, they were able to show that, in models that take radiative heat transport into account, subducting plates are on average 100–200 Kelvin hotter than in models that ignore radiative heating.
Consequence: Only old or fast plates can transport water into the depths
One of the consequences is that only plates which are initially cold – i.e. already very old, over 60 million years old – or that subduct comparatively quickly at more than ten centimetres per year can transport water into the mantle transition zone.
“Our models also show that the increased heating of the plates at even shallow depths promotes the decomposition of water-bearing minerals. This could possibly explain earthquakes in the plate at a depth greater than 70 kilometres,” adds Enrico Marzotto, who is now conducting research at the Institute of Geosciences at the University of Potsdam.
Outlook
In future, Sergey Lobanov and his team of researchers also want to measure the transparency of other relevant minerals in the Earth's mantle.
One focus will be on the effects of impurities (e.g. water) on the light absorption mechanism. In addition, they are currently implementing the new thermal conductivity formulation (including the radiation contribution) into a more complex geodynamic code called ASPECT, which is used by a broad geophysical community at the GFZ. The aim is to link mineral physics experiments with geodynamic models and investigate other constellations such as heat flow in the lithosphere and mantle plumes.
Original publication:
Marzotto, E., Koptev, A., Speziale, S., Koch-Müller, M., Abdel-Hak, N., Cichy, S.B., Lobanov, S.S., Olivine’s high radiative conductivity increases slab temperature by up to 200K. Nat Commun16, 6058 (2025).
https://doi.org/10.1038/s41467-025-61148-8
