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| Titel |
Consequences of an unstable chemical stratification on mantle dynamics |
| VerfasserIn |
Ana-Catalina Plesa, Nicola Tosi, Doris Breuer |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250083413
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| Zusammenfassung |
| Early in the history of terrestrial planets, the fractional crystallization of primordial magma
oceans may have led to the formation of large scale chemical heterogeneities. These may
have been preserved over the entire planetary evolution as suggested for Mars by the isotopic
analysis of the so-called SNC meteorites. The fractional crystallization of a magma ocean
leads to a chemical stratification characterized by a progressive enrichment in heavy elements
from the core-mantle boundary to the surface. This results in an unstable configuration that
causes the overturn of the mantle and the subsequent formation of a stable chemical
layering.
Assuming scaling parameters appropriate for Mars, we first performed simulations of 2D
thermo-chemical convection in Cartesian geometry with the numerical code YACC [1]. We
investigated systems heated either solely from below or from within by varying
systematically the buoyancy ratio B, which measures the relative importance of chemical to
thermal buoyancy, and the mantle rheology, by considering systems with constant, strongly
temperature-dependent and plastic viscosity. We ran a large set of simulations spanning a
wide parameter space in order to understand the basic physics governing the magma ocean
cumulate overturn and its consequence on mantle dynamics. Moreover, we derived scaling
laws that relate the time over which chemical heterogeneities can be preserved (mixing
time) and the critical yield stress (maximal yield stress that allows the lithosphere
to undergo brittle failure) to the buoyancy ratio. We have found that the mixing
time increases exponentially with B, while the critical yield stress shows a linear
dependence.
We investigated then Mars’ early thermo-chemical evolution using the code GAIA in a
2D cylindrical geometry [2] and assuming a detailed magma ocean crystallization sequence
as obtained from geochemical modeling [3]. We used an initial composition profile adapted
from [3], accounted for an exothermic phase transition between lower and upper mantle and
assumed all radiogenic heat sources to be enriched during the freezing-phase of the magma
ocean in the uppermost 50 km [4]. A stagnant lid forms rapidly because of the strong
temperature dependence of the viscosity. This prevents the uppermost dense cumulates
to sink, even when allowing for a plastic yielding mechanism. Below this dense
stagnant lid, the mantle chemical gradient settles to a stable configuration. The
convection pattern is dominated by small-scale structures, which are difficult to reconcile
with the large-scale volcanic features observed over Mars’ surface. Assuming that
the stagnant lid will break, a stable density gradient is obtained, with the densest
material and the entire amount of heat sources lying above the core-mantle-boundary.
This leads to a strong overheating of the lowermost mantle, whose temperature
increases to values that exceed the liquidus. Therefore a fractionated global and
deep magma ocean is difficult to reconcile with observations. Different scenarios
assuming, for instance, a hemispherical or shallow magma ocean will have to be
considered.
References
[1] N. Tosi, D.A. Yuen and O. Äadek; EPSL (2010)
(Yet Another Convection Code, https://code.google.com/p/yacc-convection/)
[2] C. Huettig and K. Stemmer; PEPI (2008)
[3] L.T. Elkins-Tanton, E.M. Parmentier and P.C. Hess; Meteoritic and Planetary Science
(2003)
[4] L.T. Elkins-Tanton, S.E. Zaranek, E.M. Parmentier and P.C. Hess; EPSL (2005) |
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