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| Titel |
High P-T experiments and first principles calculations of the diffusion of Si, O, Cr in liquid iron |
| VerfasserIn |
Esther Posner, David C. Rubie, Daniel J. Frost, Vojtěch Vlček, Gerd Steinle-Neumann |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121964
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| Publikation (Nr.) |
 EGU/EGU2016-876.pdf |
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| Zusammenfassung |
| Diffusion transport properties of molten iron and iron alloys at high pressures and
temperatures are important for understanding large-scale geodynamic processes and
thermochemical evolution of planetary interiors, such as the time and length scales of
metal-silicate equilibration during core formation and chemical exchange across
core-mantle boundaries during cooling. The density of the Earth’s outer core is
∼10% too low to be composed of pure Fe-Ni and is assumed to contain significant
concentrations of light elements, such as Si, S, O, and/or C, in addition to siderophile
transition metals (V, Cr, Mn, W) which are depleted in the Earth’s mantle relative to
chondrites. The chemical diffusivity of light and siderophile elements in liquid iron
under P -T conditions of the Earth’s core and its formation are therefore required to
constrain the composition and potential chemical stratification of planetary cores, in
addition to the kinetics of chemical buoyancy from inner core crystallization that
partially drives the geodynamo. In order to better understand the effects of pressure
and temperature on Si, O, and Cr diffusion in liquid iron, we have conducted (1)
chemical diffusion-couple experiments combined with numerical modeling of diffusion
profiles to account for non-isothermal annealing, and (2) first principles molecular
dynamic (FP-MD) calculations from ambient pressure to 135 GPa and 2200-5500
K.
Experimental diffusion couples comprised of highly polished cylindrical disks of
99.97% Fe and metallic Fe alloy were contained within an MgO capsule and annealed
within the P -T range 1873-2653 K and 1-18 GPa using a multi-anvil apparatus. A
series of experiments are conducted at each pressure using variable heating rates,
final quench temperatures (Tf), and time duration at Tf. Recovered capsules were
cut and polished parallel to the axis of the cylindrical sample and measured using
EMPA 10 μm-step line scans. To extend our dataset to P -T conditions of the Earth’s
core-mantle boundary, first principles molecular dynamics (FP-MD) simulations
were performed based on density-functional theory and implemented using VASP
code. Fe supercells of 150 atoms are overheated to induce melting, compressed to
volumes along several isobars (0.0001-135 GPa) according to densities from the liquid
iron equation of state, and allowing for changes in chemistry (Si, O, Cr). Diffusion
coefficients are computed from atomic trajectories in the simulation cell via the Einstein
relation.
Our findings corroborate theoretical estimates that diffusion coefficients are scalable to
absolute melting temperature (Tm) yielding constant Si, O, and Cr diffusivities of
approximately 5 × 10−9 m2 s−1 along the melting curve from ambient to core pressures. A
simple homologous temperature relation can therefore be used to predict diffusion
rates relative to the melting curve. The homologous temperature activation term
determined from FP-MD calculations for Si diffusion is larger by a factor of 1.3 than the
value determined from experiments but diffusion coefficients at Tm are identical
within error for both two methods. Verification of a homologous temperature relation
for chemical diffusion in liquid iron implies that low-pressure experiments can be
used as accurate analogues of mass transport properties of the Earth’s outer core. |
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