Inferring the thermochemical structure of the upper mantle from seismic data

Abstract

S U M M A R YWe test a mineral physics model of the upper mantle against seismic observations. The model is based on current knowledge of material properties at high temperatures and pressures. In particular, elastic properties are computed with a recent self-consistent thermodynamic model, based on a six oxides (NCFMAS) system. We focus on average structure between 250 and 800 km. We invert normal modes eigenfrequencies and traveltimes to obtain best-fitting average thermal structures for various compositional profiles. The thermochemical structures are then used to predict long-period waveforms, SS precursors waveforms and radial profiles of attenuation. These examples show the potential of our procedure to refine the interpretation combining different data sets.We found that a mixture of MORB and Harzburgite, with the MORB component increasing with depth, is able to reproduce well all the seismic data for realistic thermal structures. If the proportions of MORB with depth do not change, unrealistic negative thermal gradients below 250 km would be necessary to explain the data. Equilibrium assemblages, such as pyrolite, cannot fit the seismic data.The elastic velocities predicted by the reference mineral physics model tested are too low at the top of the lower mantle, even for the fastest (and most depleted) composition, that is, harzburgite. An increase in V P of 1 per cent and in V S of 2 per cent improves the data fit significantly and is required to find models that fit both traveltimes and normal modes, indicating the need for further experimental measurements of these properties at the simultaneously elevated pressure-temperature conditions of the lower mantle.Extending our procedure to other seismic and density data and interpreting the 3-D structure holds promise to further improve our knowledge of the thermochemical structure of the upper mantle. In addition, the same database of material properties can be used in dynamic models to test whether the thermochemical structure inferred from geophysical observations is consistent with the Earth's evolution.

Recent craton growth by slab stacking beneath Wyoming

“…Since variations in mantle seismic velocity as great as those imaged must represent significant temperature variations (Schutt and Lesher, 2006;Cammarano et al, 2003Cammarano et al, , 2009, the steep sides of this structure indicate that it is not in thermal equilibrium with the adjacent mantle and hence its form must be far younger than the Archean age of the overlying crust and uppermost mantle or maintained by active processes (Cooper et al, 2004). We add that, when addressing the possible presence of Shatsky conjugate ocean crust, results from receiver function imaging find no evidence for a preserved layer of a basaltic ocean crust within this high-velocity body Lekić and Fischer, 2013;Hopper et al, 2014).…”

The second continent: Existence of granitic continental materials around the bottom of the mantle transition zone

“…Harzburgite has faster seismic velocities but is gravitationally unstable, while basalt is gravitationally stable but has slower velocities in this depth range. In the uppermost lower mantle, meanwhile, harzburgite is stable in the limited depth range from about 660 to 700 km, but seismic velocities such as those of PREM are too fast to be explained by harzburgite in this depth range (Cammarano et al, 2009). On the other hand, we showed above that granitic materials are both gravitationally stable and can produce high seismic wave velocities at these depths.…”

“…1A). On the other hand, recent experimental and theoretical (Cammarano et al, 2009) studies suggest that the PREM (Dziewonski and Anderson, 1981) velocities are higher than those of adiabatic pyrolite (Ringwood, 1975) in the lower part of the MTZ and that either subadiabatic temperatures of 400 K or seismically faster chemical composition such as harzburgite is required to explain the velocity difference Fig. 1B).…”

“…Also, light harzburgite could be gravitationally unstable. Moreover, the fast PREM velocities in the uppermost lower mantle cannot be explained even by the most depleted composition, harzburgite (Cammarano et al, 2009).…”

Constraints on Seismic Models from Other Disciplines - Constraints from Mineral Physics on Seismological Models

“…A promising alternative to forward modeling is to compute the actual seismological observables (e.g., travel times or mode frequencies) directly from the mineralogical model (Cammarano et al, 2009). This permits a much more direct and certain comparison to the structure of the Earth.…”