Experimental measurements of transport properties of liquid iron alloys provide important constraints
on Earth's outer core properties. The chemical diffusivity of the outer
core has been largely ignored, despite the fact that chemical buoyancy
is one of the, and possibly the main, driving forces for outer core convection.
This study has measured the self-diffusivity of liquid iron at high pressures
in the multi-anvil press. Tracer diffusion experiments were performed,
with foils of ^{57}Fe diffusing
into a sample of normal isotopic abundance. Once at pressure, temperature
was increased at a constant ramp rate of 300 °C/s to experimental conditions
and maintained for durations of 0 to 30 s. Precision in temperature control
was better than 1 %. The diffusion profile in the recovered quenched sample
was measured using the ion probe at Edinburgh University.

The measured ^{57}Fe diffusion profiles were fitted according to the tracer equation:

*C*/*C*_{0}
= exp-(*x*^{2}/4*Z*)
(1),

where *C* is the concentration at distance *x* from *x*_{0},
*C*_{0} is the concentration at *x*_{0},
and *Z* is the diffusivity, *D*, integrated over time, *t*:

(2).

In experiments where ramping up to the desired temperature is a negligible proportion of the heating
duration, *D* is constant, *Z* = *Dt* and (2) becomes the
standard tracer equation. However, because diffusion in these experiments
occurred while increasing temperature, the diffusivity varied with time.
Experiments quenched from different temperatures during constant heating
rate experiments thus result in different values of *Z*, everything
else being constant. Experimentally determined values of *Z* were
integrated assuming an Arrhenius dependence of *D* on temperature:

(3),

where *T* is a linear function of *t*, to give values of *D* at each experimental temperature.
The pre-exponential, *D*_{0},
and apparent activation energy, *Q _{D}*,
were treated as parameters for fitting and errors were minimised by an
iterative least squares process. Results obtained using this integration
procedure were identical to those obtained from traditional "isothermal"
series of experiments in which the effect of finite melting and quenching
duration was evaluated by performing repeat experiments under identical
temperature, ramp rate conditions and different durations.

The self diffusivity, D, can be well approximated by the following equation:

(4),

where *D _{0}* is the intercept of diffusivity at infinite temperature,

These results suggest that the molecular viscosity of the outer core may be as much as 10^{5}
times larger than previous estimates. The low predicted chemical
diffusivity of outer core liquid metal strongly favours chemical over thermal
buoyancy as the main driving force of outer core convection.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland

Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)uni-bayreuth.de

Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)uni-bayreuth.de