Structures of the oceanic lithosphere-asthenosphere boundary: Mineral-physics modeling and seismological signatures

We explore possible models for the seismological signature of the oceanic lithosphere-asthenosphere boundary (LAB) using the latest mineral-physics observations. The key features that need to be explained by any viable model include (1) a sharp (<20 km width) and a large (510%) velocity drop, (2) LAB depth at similar to 70 km in the old oceanic upper mantle, and (3) an age-dependent LAB depth in the young oceanic upper mantle. We examine the plausibility of both partial melt and sub-solidus models. Because many of the LAB observations in the old oceanic regions are located in areas where temperature is similar to 10001200 degrees K, significant partial melting is difficult. We examine a layered model and a melt-accumulation model (at the LAB) and show that both models are difficult to reconcile with seismological observations. A sub-solidus model assuming absorption-band (AB) physical dispersion is inconsistent with the large velocity drop at the LAB. We explore a new sub-solidus model, originally proposed by Karato [2012], that depends on grain-boundary sliding. In contrast to the previous model where only the AB behavior was assumed, the new model predicts an age-dependent LAB structure including the age-dependent LAB depth and its sharpness. Strategies to test these models are presented.