Thermal conductivity of serpentinite in subducting slabs: Measurements at high pressure and temperature

The subduction of cold oceanic lithosphere into the convecting mantle is one of the primary mechanisms by which the mantle cools. Cold slabs generally need to reach at least transition zone depths before heat conducted from the surrounding mantle brings them into thermal equilibrium. By the time it is subducted, large sections of the oceanic lithosphere have been hydrothermally altered, with serpentinite constituting the bulk of this altered material. Despite some dehydration during subduction, this assemblage continues to form a major component of slabs to depths of around 330 km. With almost no high pressure measurements for serpentinite available, thermal models of subduction zones invariably use the thermal conductivity of dunite to represent the subducting slab and the mantle wedge. However, the thermal conductivity of serpentinite is likely notably lower and significantly more anisotropic, which would require slabs to reach greater depths before reaching thermal equilibrium. With this need for a deeper understanding of thermal transport in serpentinite, we have measured its thermal conductivity in natural samples, compressed to 5, 8 and 10 GPa. Measurents were conducted using the Ångström method over a temperature range of 100-500 oC, in a multi-anvil press that accommodates large sample sizes. The variation of thermal conductivity as a function of the mean crystallographic orientation was also considered in detail. Preliminary analysis of our results shows thermal that conductivity of serpentinite varies strongly with pressure and temperature, in a fashion distinct from dunite. We will discuss these results, their variation with preferred orientation, as well as the possible implications for subduction zone thermal profiles.