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| Titel |
Fragmentation, mixing, and chemical equilibration in a magma ocean - Insights from fluid dynamics experiments |
| VerfasserIn |
Renaud Deguen, Peter Olson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250053395
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| Zusammenfassung |
| Hf/W chronometry and the excess of siderophile elements in the silicate Earth both indicate
significant chemical interaction between iron and silicates during accretion of the Earth and
formation of the core. Recent studies have shown that the interpretation of W isotope
anomalies in terms of a core formation timescale depends criticaly on the degree of
metal-silicate chemical re-equilibration. Physical models of mixing and chemical
interactions during core formation are needed to precise the exact signification of the
182W-184W anomaly. We focus here on estimating the degree of mixing between
the metal and silicate phases during the settling of an impactor core in a magma
ocean.
A first serie of experiments is devoted to the study of fragmentation in jets of silicon oil in
water. Turbulent fragmentation is observed in experiments at high Reynolds and Weber
number. Ambient water is entrained within the jet and mix with the silicon oil to form an
emulsion. In a second serie of exeriments, we assume that the iron phase is indeed
efficiently fragmented, and we model the impactor core as a cloud of dispersed iron
drops. We report results from laboratory experiments where a concentrated cloud
of dense particles (representing iron drops) is released from above in a spherical
container filled with water (representing the magma ocean). We find that the regime of
metal-silicate separation and the degree of mixing depend mostly on the non-dimensional
parameter R = (Rmws)-B1-2, where B is the buoyancy of the impactor core, Rm the
radius of the silicate melt region, and ws is the settling velocity of individual iron
drops. R is the ratio of the settling velocity of the particles to the particle cloud
velocity. The initial evolution of the particle cloud is found to be very similar to that of
buoyancy driven ’thermals’ as observed in laboratory experiments : the particle cloud
grows during its fall by turbulent entrainement of ambient fluid. The impact of the
particle cloud on the container floor and the subsequent inertia driven flow promotes
further mixing if R is larger than a critical value of O(1) (i.e. if the particles are kept
in suspension by the internal circulation of the particle cloud). The experimental
determination of the degree of mixing as a function of B provides a basis to discuss the
importance of chemical reequilibration as a function of the protoplanet size and
thermal state, impactor size, and efficiency of fragmentation of the impactor core. |
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