Sensitivity of teleseismic body waves to mineral texture and melt in the mantle beneath a mid–ocean ridge

Abstract
Seismic energy propagating through the mantle beneath an oceanic spreading centre develops a signature due both to the subaxial deformation field and to the presence of melt in the upwelling zone. Deformation of peridotite during mantle flow results in strong preferred orientation of olivine and significant seismic anisotropy in the upper 100 km of the mantle. Linked numerical models of flow, texture development and seismic velocity structure predict that regions of high anisotropy will characterize the subaxial region, particularly at slow–spreading mid–ocean ridges. In addition to mineral texture effects, the presence of basaltic melt can cause travel–time anomalies, the nature of which depend on the geometry, orientation and concentration of the melt. In order to illustrate the resolution of subaxial structure that future seismic experiments can hope to achieve, we investigate the teleseismic signature of a series of spreading centre models in which the mantle viscosity and melt geometry are varied. The P–wave travel times are not very sensitive to the geometry and orientation of melt inclusions, whether distributed in tubules or thin ellipsoidal inclusions. Travel time delays of 0.1–0.4 s are predicted for the melt distribution models tested. The P–wave effects of mineral texture dominate in the combined melt–plus–texture models. Thus, buoyancy–enhanced upwelling at a slow spreading ridge is characterized by 0.7–1.0 s early P–wave arrival times in a narrow axial region, while the models of plate–driven–only flow predicts smaller advances (less than 0.5 s) over a broader region. In general S–wave travel times are more sensitive to the melt and show more obvious differences between melt present as tubules as opposed to thin disks, especially if a preferred disk orientation exists. Mineral texture and the preferred alignment of melt inclusions will both produce shear–wave splitting, our models predict as much as 4 s splitting in some cases. Keywords: melt in the mantle, seismic travel–time anomalies, seafloor seismic arrays, seismic anisotropy, melt seismic signature, seismic heterogeneity