High-temperature viscoelastic relaxation in iron and its implications for the shear modulus and attenuation of the Earth's inner core [Review]

摘要

The viscoelasticity of mildly impure polycrystalline iron has been studied over the temperature range 20-1300 degrees C through a combination of seismic-frequency torsional forced oscillation and microcreep tests. For all temperatures above similar to 400 degrees C, linear absorption band behavior is observed, both strain energy dissipation Q(-1) and shear modulus dispersion providing evidence of substantial departures from ideal elastic behavior. For 3 s oscillation period the temperature sensitivity of the shear modulus /partial derivative G/partial derivative T/ averages 0.04 GPa K-1. An even larger derivative applies to the highest temperatures within the bcc field (600-800 degrees C) and at longer periods. The isothermal variation of Q(-1) with period T-0 is generally adequately described by Q(-1) similar to T-0(alpha). Within the bce field the exponent alpha, and hence the distribution D(tau) similar to tau(alpha-1) of relaxation times, are temperature-independent, allowing the parameterization Q(-1) similar to [T-0 exp(-E/RT)]alpha, with alpha = 0.20+/-0.02 and activation energy E = 280+/-30 kJ mol(-1). Within the fee field, the exponent ex increases systematically with increasing temperature from similar to 0.1 to similar to 0.3 across a wide temperature interval, indicating that the distribution of relaxation times within the absorption band is strongly temperature-dependent. Steady-state viscosities inferred mainly from microcreep records for temperatures between 1000 and 1300 degrees C, lie within the range (0.2-2) x 10(13) Pa s. The observed mix of recoverable anelastic and viscous behavior (the latter becoming progressively more important with increasing temperature and time/period) is tentatively attributed to diffusional processes operative at grain boundaries in the relatively fine-grained bcc-Fe and to processes involving dislocations in the coarser-grained specimens of the fee phase. Relaxation of internal stresses caused by pronounced single-crystal elastic anisotropy probably dominates the anelastic response of both phases. The marked elastic anisotropy of fcc-Fe makes it an attractive alternative to the hcp structure widely favored for the dominant crystalline phase of the Earth's inner core. At seismic frequencies and homologous temperatures approaching unity in the inner core, solid-state viscoelastic relaxation probably accounts for much of the observed seismic wave attenuation and contributes through the associated dispersion to the unusually high value of Poisson's ratio. [References: 108]