Multi-disciplinary constraints on the origin of low velocity and high conductivity anomalies in the upper mantle

Abstract:

The low velocity zone (LVZ) has been widely observed in the boundary regions between lithosphere and asthenosphere, and atop of the 410 km discontinuity. The LVZ is characterized by unusually low seismic shear wave velocity, a significant reduction in Q (high seismic attenuation) and anomalously high electrical conductivity (HCL). In many cases, the LVZ and HCL are overlapped. This zone is thought to be the mechanically weak layer, which can assist the plate motion. Therefore, knowledge of nature of the LVZ and HCL may provide us an important constraint on interpretation of plate tectonics and mantle dynamics.

The origin of the LVZ has been under considerable debate in recent years. Although some authors argue that the LVZ may originate primarily from the decrease in elastic properties of solid peridotite under temperature conditions close to the solidus, without the involvement of partial melting, others mainly attribute its existence to the presence of small amounts of partial melt. As for the HCL, which has been found not only in tectonically active regions (e.g., EPR and Subduction zone) but also in stable shield areas (e.g., Slave craton in Canada). Several potential candidates have been proposed to account for this issue, which mainly includes water in nominally anhydrous minerals, graphite and partial melt. In the last ten years, many studies have provided robust evidence to verify these hypotheses or assumptions. This contribution provides an overview of the recent progress in the experimental studies on the origin of the LVZ and HCL (i.e., electrical conductivity, sound velocity, elasticity, rheology, and thermal conductivity) together with their applications. In particular, consistency among various experimental data is investigated, discrepancies are evaluated, and confusion is clarified.