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

The viscoelasticity of mildly impure polycrystalline iron has been studied over the temperature range 20-1300 degrees C through a combination of seism ic-frequency torsional forced oscillation and microcreep tests. For all tem peratures above similar to 400 degrees C, linear absorption band behavior i s observed, both strain energy dissipation Q(-1) and shear modulus dispersi on providing evidence of substantial departures from ideal elastic behavior . For 3 s oscillation period the temperature sensitivity of the shear modul us /partial derivative G/partial derivative T/ averages 0.04 GPa K-1. An ev en larger derivative applies to the highest temperatures within the bcc fie ld (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 distribu tion D(tau) similar to tau(alpha-1) of relaxation times, are temperature-in dependent, allowing the parameterization Q(-1) similar to [T-0 exp(-E/RT)]a lpha, 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 incr easing temperature from similar to 0.1 to similar to 0.3 across a wide temp erature interval, indicating that the distribution of relaxation times with in the absorption band is strongly temperature-dependent. Steady-state visc osities inferred mainly from microcreep records for temperatures between 10 00 and 1300 degrees C, lie within the range (0.2-2) x 10(13) Pa s. The obse rved 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 bound aries in the relatively fine-grained bcc-Fe and to processes involving disl ocations in the coarser-grained specimens of the fee phase. Relaxation of i nternal stresses caused by pronounced single-crystal elastic anisotropy pro bably dominates the anelastic response of both phases. The marked elastic a nisotropy of fcc-Fe makes it an attractive alternative to the hcp structure widely favored for the dominant crystalline phase of the Earth's inner cor e. At seismic frequencies and homologous temperatures approaching unity in the inner core, solid-state viscoelastic relaxation probably accounts for m uch of the observed seismic wave attenuation and contributes through the as sociated dispersion to the unusually high value of Poisson's ratio.