Dynamic Constraints on the Thermal Structure of Subducted Slabs

Abstract:

Constraining the temperature and heat flux of subducted slabs plays a key role in understanding the thermal evolution history of the Earth’s whole mantle. Many geodynamic models have been conducted to explore the generation and evolution of different subduction systems. But little work has been done to systematically investigate the thermal structure of subducted slabs in the deep mantle. Here, we use a 2-D spherically axisymmetric mantle convection model to study the slab temperature and heat flux variations at different depths. We first analyze the temperature and heat flux variation of subducted slabs. Then we quantify the contributions from different heat sources (i.e. adiabatic heating, viscous heating, diffusive heating and radiogenic heating) which leads to the variation of slab temperature and heat flux with depth. The effects of different physical parameters on our results are also explored, including Rayleigh numbers, internal heating generation rates, thermal expansivity, thermal diffusivity and viscosity structures. The results can be summarized as following. (1) The slab temperature increases faster with depth than a mantle adiabat. The temperature variation can be described as T(r) = T(r₀)e^{-(1+k)Di(r-r₀)}, where k≈1-1.5 and is not strongly affected by different model parameters, Di is the dissipation number, T(r₀) is the reference temperature and r-r₀ is the distance over which slabs sink. (2) Slab temperature and heat flux varies with depth due to the contribution of different heat sources. The adiabatic heating plays a dominant role (~70%), whereas the diffusive heating (~20%), viscous heating (~7-9%), and radioactive heating (~1-2%) play secondary roles in the contribution. (3) Slab mass flux significantly changes with depth, indicating that enormous mass exchange occurs between the slab and the ambient mantle during slab sinking. This process also contributes to the change of slab temperature and heat flux. Its effect is approximately equivalent to the summation of different heat sources.