Constraints on Mantle Composition of Western North American from Direct P and S Wave Velocities of Mantle Peridodite to 10 GPa

An important geophysical approach to constrain the mineralogical composition of the mantle is to compare the elasticity of candidate compositional models with seismic profiles (e.g., PREM and AK135). In this study, a series of polycrystalline aggregates of peridotite KLB-1 (from Kilbourne Hole, New Mexico) were hot-pressed at 3-8 GPa 1200-1400°C. Scanning electron microscopy (SEM), Electron Microprobe Analysis (EPMA) and X-ray diffraction (XRD) were used to characterize the texture, grain size, and composition of these well-sintered specimens. The fractions of each constituent phase in the aggregates were obtained from mass balance analysis. P and S wave velocities of two specimens hot pressed at 3 GPa were measured up to 10GPa at room temperature using ultrasonic interferometry. Based on the phase fractions from this study as well as the temperature dependence of P and S wave velocities for mantle minerals, the velocities of the KLB-1 peridotite along 1200-1400 oC adiabatic mantle geotherms were obtained and compared well with the seismic models of western North America, the region where these peridotite KLB-1 samples were collected. These comparisons provide unprecedented constraints on the composition of the upper mantle in this region and help us understand the nature of thermal and tectonic processes of the Rio Grande Rift.