Jointly Constraining Upper Mantle Seismic, Thermal and Compositional Structures From Mineral Physics and Seismic Data

The upper mantle velocity structures and thermal and compositional models are important to the understanding of the dynamics and evolution of the mantle. Several phase transformations exist in the upper mantle, and they are sensitive to mantle composition, temperature and chemical interactions between the olivine- and pyroxene- normative components. With the accumulation of in-situ measurements of elastic properties and accurate determination of phase equilibria data, we can now explore various chemical interactions of various phase assemblages, and quantitatively calculate seismic velocity structures for various mantle temperature and compositions. Mantle compositional and thermal structures can thus be quantitatively constrained by jointly modeling mineral physics data and seismic observations. In this representation, we constrain fine seismic SH velocity structures near the 660-km discontinuity beneath South America and northeast Asia and P and SH velocity structures in the upper mantle beneath southern Africa, and explore thermal and compositional models appropriate for explaining the inferred seismic structures in the three regions on the basis of mineral physics data. Beneath South America and northeast Asia, SH velocity structures near the 660-km discontinuity are found to be different. Beneath South America, the velocity gradient above the 660-km discontinuity is larger than that of PREM, while the velocity jump across the discontinuity is the same as PREM. Beneath northeast Asia, the velocity gradient above the 660-km discontinuity is the same as that of PREM, while the velocity jump across the discontinuity is larger than PREM. Both regions are characterized by a large velocity gradient extending about 80 km deep below the 660-km discontinuity. These different velocity structures can be explained by different mantle temperature or composition, in particular, the aluminum content in mantle composition. The presence of garnet 80 km below the 660-km discontinuity in the two regions may be explained by a uniform composition in the lower mantle with an aluminum content of 3.4%. The different velocity gradients above the 660-km discontinuity between South America and northeast Asia can be explained by either a difference in mantle temperature of about 100° C(with that beneath South America being lower) or a difference in aluminum content of about 1% (with that beneath South America being lower) between the two regions. Beneath southern Africa, the SH and P wave data suggest that a low velocity zone is present with velocity reductions of at least -5% for S wave and -2% for P wave beneath a 150-210 km thick high-velocity lithospheric lid, and the P/S ratio is larger (1.88) in the transition zone than in the lithospheric lid (1.70). The inferred P wave velocity jump across the 660-km discontinuity is small (<4%), while the inferred SH wave velocity jump across the discontinuity is comparable to that in PREM. The low velocity zone can be explained by a high temperature gradient of 6 ° C/km or presence of partial melt. The different P/S velocity ratios between the lithospheric lid and the transition zone can be explained by a difference in aluminum content of the mantle composition, with values of 1% in the lithospheric lid and 4% in the transition zone, respectively. The inferred P and SH velocity jumps suggest a bulk sound velocity decrease across the 660-km discontinuity.