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

Research areas:
MAR 10
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 1 sigma 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 similar to 65\% at 2600 km depth.