Effects of variable thermal conductivity on the mineralogy of subducting slabs and implications for mechanisms of deep earthquakes

Abstract

Thermal and thermo-kinetic models of subduction zones have been based, in general, on a constant value of thermal conductivity (e.g., 3.138 W/(m K)) for solving the conservation of energy equation. In order to make such models more realistic, we have used variable thermal conductivities ( k) that depend on pressure, temperature, and mineralogy (olivine, wadsleyite, or ringwoodite), as determined recently by Xu et al. [Xu, Y., Shankland, T.J., Linhardt, S., Rubie, D.C., Langenhorst, F., Klasinski, K., 2004. Thermal diffusivity and conductivity of olivine, wadsleyite and ringwoodite to 20 GPa and 1373 K. Phys. Earth Planet. Inter. 143–144, 321–336]. We have used these variable conductivities to model a variety of slabs with thermal parameters (vertical subduction rate × age of lithosphere at the trench) ranging between 3500 and 17,000 km. Our models show that when radiative transfer is included, k could plausibly be increased by as much as 2 W/(m K) relative to a standard constant lithospheric value (3.138 W/(m K); [Stein, C.A., Stein, S., (1992) A model for the global variation in oceanic depth and heat flow with lithospheric age. Nature 359, 123–129.], causing temperatures in the interiors of the slabs to increase by 50–100 °C. In turn, the volume of metastable olivine in the slabs is reduced by as much as 20–30%, with the maximum depth of olivine metastability decreasing by 30–50 km. In general, the maximum depths of olivine metastability are 160–230 km less than the depths of the deepest seismicity. Even in the extreme case of Tonga, metastable olivine falls short of the deepest earthquakes by 140 km in our model. Even taking the various uncertainties into account, these results indicate that deep-focus earthquakes occur in regions already transformed to wadsleyite or ringwoodite and must therefore be caused by a mechanism other than transformational faulting.