Abstrakt
We examine the influence of variable thermal properties on the thermal state of a subducting slab in the upper mantle and the transition zone by combining a kinematic slab model with models of thermal conductivity. Thermal diffusivity and conductivity models for major mantle minerals in the MgO-SiO2 system are developed based on experimental measurements on these minerals at high pressure and temperature.
The models show significantly higher thermal conductivity for stishovite and clinopyroxene compared to the Mg2SiO4 polymorphs and majorite garnet. In our subduction model we consider scenarios with a differentiated slab (basalt-harzburgite-pyrolite) in a pyrolite mantle and uniform composition for both the slab and the mantle (pyrolite or a pure Mg2SiO4-based system).
The role of highly conductive pyroxene is examined by taking it into account in some models and replacing it in others with majorite garnet. This choice has a strong influence on the thermal state of the slab, shifting the depth of the -1000 K temperature anomaly by as much as 100 km.
This is caused by faster cooling of the plate at the surface and thermal insulating effects once subducted. Temperature differences between models with variable thermal diffusivity and those with constant parameters can reach similar to 125 K, i.e. 10% of total thermal anomalies of the slab relative to an average geotherm.
Taking into account the stable mineral phases we evaluate density variations between different models and find that variable thermal diffusivity results in a modest increase of negative buoyancy of the slab.