Abstract
Seismic observations of the Earth’s inner-core testify to it being both
a complex and dynamic part of the Earth. It exhibits significant
variation in seismic attenuation and velocity with position, depth and
direction. Interpretation of which is difficult without knowledge of the
anelastic processes active in the inner-core is difficult. To address
this, we used zinc, a low-pressure analogue of the hexagonal close pack
(hcp) structured iron that forms the inner-core, to provide
first-order constraints on the anelasticity of hcp-metals at high
pressure, seismic frequencies (∼0.003-0.1Hz), homologous temperatures
(T/Tm) up to ∼0.8. Measurements were made in a
deformation-DIA combined with X-radiography. The data was analysed using
an improved image processing method that reduces systematic errors and
improves strain measurement precision by up to 3 orders of magnitude.
Using this algorithm significant dissipation and softening of zinc’s
Young’s modulus is observed. The softening occurs in the absence of
significant impurities or a fluid phase and is caused by grain boundary
sliding coupled with dynamic recrystallisation.The recrystallisation
results in a steady-state grain-size and low dislocation density. A
softened Young’s modulus predicts a reduction in shear wave speed 2-3
times greater than that for compressional waves, which is consistent
with anelasticity playing a significant role in the seismic velocity of
the inner-core. Comparison of elastic wave speeds from experimental or
computed material properties with anelastically-retarded inner-core
seismic velocities will tend to over-estimate the light element budget
of the inner-core. Therefore anelastic effects in hcp-iron must
be considered in the interpretation of the inner-core.