Effects of oceanic crust on the thermal structure on the subducting slabs

Thermal conductivity and diffusivity for three pyroxenes, omphacite, jadeite and diopside, were determined up to 14 GPa and 1000 K in the Kawai-type multi-anvil apparatus via the pulse heating method. Measurements for omphacite are characterized by much lower thermal conductivity and thermal diffusivity than those of its two end members of jadeite and diopside, presumably because of the complex substitution of four cations (Na+1, Ca+2, Al+3, Mg+2) in omphacite. Therefore, simple arithmetic averaging is unsuitable for estimating thermal conductivity and diffusivity of the jadeite-diopside solid-solution system. The thermal property of eclogite was estimated from those of garnet and omphacite. The thermal conductivity of eclogite is much smaller than that of harzburgite, which is assumed to compose of 80% olivine and 20% enstatite, implying that subducted oceanic crust impedes thermal conduction from the hotter wedge mantle to the subducting slab. Thermal structure simulation results show that temperature of the subduction zone is about 50°C decreased when the effect of oceanic curst is included.