Rock magnetism and palaeomagnetism of greigite-bearing mudstones in the Italian peninsula

Both seismological observations and mineral physics experiments suggest that old and cold subducting slabs might contain a wedge of metastable olivine (`MOʹ). Dynamically consistent models of subducting slabs, in which the kinetics of the olivine to spinel (wadsleyite or ringwoodite) transition is treated in a simplified way, are carried out to evaluate the buoyancy effect of MO on the subduction velocity. Assuming slab thicknesses according to the cooling half-space model, we find that MO forms in slabs with ages greater than approximately 70 Myr, and that a significant slowing down of subduction velocity occurs for ages greater than 100 Myr. Because a decrease in the subduction velocity for old lithospheric ages is not observed, our models suggest that the amount of MO in slabs is less than approximately 5000 km2 in cross section, and that for old lithosphere the cooling half-space model is not applicable.