Constraints on thermal state and composition of the Earth's lower mantle from electromagnetic impedances and seismic data

摘要

Despite the tight constraints put by seismology on the elastic properties of the Earth's lower mantle, its mineralogical composition and thermal state remain poorly known because the interpretation of seismic measurements suffers from the trade-off between temperature, iron content, and mineralogical composition. In order to overcome this difficulty, we complement seismic data with electromagnetic induction data. The latter data are mostly sensitive to temperature and iron content, while densities and acoustic speeds mostly constrain the mineralogy. A 0.5 log unit increase in electrical conductivity can be caused either by a 400 K increase of the temperature or by an increase of iron content from 10% to 12.5%. Acoustic velocity is only marginally sensitive to temperature but it increases by 0.8 km s~(-1) on average as the perovskite fraction increases from 50% to 100%. Olsen's (1999) apparent resistivities in the period range [15 days, 11 years], and Preliminary reference Earth model (PREM) densities and acoustic speeds are jointly inverted in the depth range [800 km, 2600 km] by using a Monte Carlo Markov Chain method. Given the uncertainties on these data, estimates of perovskite fraction are well constrained over the whole depth range, but information on temperature and iron content is only obtained for depths less than 2000 km, corresponding to the penetration depth of the long-period electromagnetic field. All parameter values are determined with an uncertainty better than 15-20% at the 1a confidence level. The temperature in the uppermost lower mantle (i.e., down to 1300 km depth) is close to a value of 2200 K and increases along a superadiabatic gradient of 0.4 K km-1 between 1300 and 2000 km depth. Extrapolation of this gradient at greater depth leads to a temperature close to 2800 K at 2600 km depth. The iron content of the lower mantle is found to be almost constant and equal to 10-11% whatever the depth, while a significant linear decrease of the perovskite content is observed throughout the whole depth range, from 80% at 800 km depth down to —65% at 2600 km depth.