Crustal temperature and the resulting partial melting

Crustal temperature and the resulting partial melting

It is known that the Earth’s interior is under high pressure and high temperature conditions. The geotherm profile, i.e., the temperature vs. depth (corresponding to the pressure) profile, is therefore the most basic and important knowledge for understanding the solid Earth. Basically, we can say that all of the physical and chemical processes in the Earth’s interior are controlled by the pressure and temperature conditions, namely, controlled by the geotherm profile. For example, the solidus and liquidus temperatures of rocks increase with increasing pressure, if the temperature at a certain depth excesses the melting points of rocks, partial melting should occur, which will weak the strength of rocks, reduce the seismic velocity, enhance the electrical conductivity, and so on.

In particular, some petrological evidences suggest partial melting of ultrahigh-pressure metamorphic rocks in the crust during continental collision. Nevertheless, due to the lack of sampling, the geotherm profile even for the crust, i.e., the shallowest part of the Earth, is quite poorly constrained. As a result, the reason for partial melting in the crust is unclear.

As we know, the geotherm is governed by heat flow from the deep mantle to the surface and thus controlled by the thermal properties of rocks and minerals including thermal diffusivity and thermal conductivity. Since eclogite is the most important ultrahigh pressure metamorphic rocks in the lithosphere, we experimentally measured its thermal diffusivity and conductivity at temperatures of 300 – 873 K and pressures of 1 – 3 GPa, corresponding to the lithospheric conditions (including crust and topmost upper mantle).

We used natural eclogite samples collected from the Dabie-Sulu orogenic continental belt in eastern China, which is the largest orogenic belt in the world. The high pressure and high temperature experiments were performed on a DIA-type multi-anvil apparatus. We obtained the thermal conductivity and diffusivity simultaneously by measuring the variation of sample temperature with an impulse heater. Based on the experimental results, we simulated the temperature vs depth profile of the lithosphere beneath the Sulu and Himalaya-Tibet orogenic belts.

The simulated results show that the temperature of the lithosphere increases from 300 K on the Earth’s surface to about 940 K at about 32 km depth, corresponding to Moho depth beneath Sulu belt and middle crust beneath Himalaya-Tibet belt. The lithospheric thicknesses of Sulu and Himalaya-Tibet belts are 67 and 150 km, respectively. It is important to notice that dehydration melting of hydrous granite and phengite, the most common rocks in the crust, could occur at around 940 K at 1 GPa (about 30 km depth). Therefore, melting of hydrous granite should occur near the Moho discontinuity beneath Sulu belt and in the middle crust beneath Himalaya-Tibet belt.

The partial melt plays important roles in geodynamic and geophysical processes. For example, melts can reduce the strength the rocks significantly, leading to the weakening of the deeply subducted continental crust and the decoupling of rocks from the subducting slab, which may result in the initiation of slab exhumation. When partial melting takes place during exhumation, the melt channel may lubricate the edge of exhumated slices and thus enhance the transport of materials from deep regions to the surface.

Additionally, partial melt could lower the seismic velocity and enhance the electrical conductivity of rocks. Therefore, the occurrence of partial melting in the lithosphere could explain the low seismic-velocity and high electrical conductivity observed in the lithosphere of Sulu and Himalaya-Tibet belts, which can hardly be explained by the hydration of eclogite in previous models based on water-enhanced electrical conductivity of emphacite unless an unrealistically high water capacity is assumed.

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