A review of water contents and ductile deformation mechanisms of olivine: Implications for the lithosphere-asthenosphere boundary of continents

摘要

Water plays an important role in the ductile deformation and evolution of the upper mantle. Water contents of natural olivine from 240 samples reveal a wide variation of 0-170 ppm H2O, suggesting heterogeneous water distribution in the continental upper mantle. The average water contents (17 ±13 ppm H2O) in kimberlite nodules provide the best estimation of water concentrations in olivine in the lithosphere beneath cratons. The very low water contents (7 ±9 ppm H2O) of olivine from basalt xenoliths are caused by significant hydrogen loss during transport, while the high values (44 ± 34 ppm H2O) in olivine megacrysts from kimberlites reflect restricted fluid-rich conditions in the upper mantle. To compare deformation in different tectonic environments, the western Superior Province (Canada), the Dabie Mountains and the North Jiangsu basin (China) are selected to represent an Archean craton, an orogenic belt and a rift basin, respectively. Using recent flow laws of olivine, deformation maps of dry and wet olivine are constructed under P-T conditions of the three tectonic units and in a continental subduction zone characterized by P=6.28GPa and T=900°C. For dry olivine, diffusion creep is the dominant mechanism in all the cases, which is contrary to the widely observed crystal preferred orientation of olivine in peridotites and seismic anisotropy observations. For wet olivine, only a small amount of water (50 H/10~6 Si) can remarkably decrease the stress of dislocation creep and increase contribution of dislocation creep to the deformation of olivine. The strain rate profiles of olivine indicate a transition from dislocation creep to diffusion creep at a depth of ~ 220 km, which can be related with the Lehmann discontinuity characterized by a rapid decrease in seismic anisotropy. However, the pressure-induced fabric transition from [100] slip to [001] slip may be responsible for the Lehmann discontinuity in subduction zones. Therefore rheology of the continental upper mantle is controlled by power-law creep of wet olivine, and diffusion creep is the dominant deformation mechanism in the deep upper mantle, especially for fine-grained peridotites. The mechanical lithosphere-asthenosphere boundary (LAB) can be defined by the characteristic pressure derivative of effective viscosity. The sharp LAB beneath the Dabie Mountains and the Sulu terrane favors the lithosphere-asthenosphere decoupling, while the diffuse LAB beneath the western Superior Province will protect the continental root from convective erosion and mantle metasomatism. The long-term preservation of the continental roots can be attributed to a large viscosity contrast (temperature contrast) at a depth of < 150 km, and a thick and diffuse LAB at a depth of > 150 km.