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| Titel |
Deep mantle primitive reservoirs as a partial source of Ocean Island Basalt |
| VerfasserIn |
Frédéric Deschamps, Paul Tackley, Edouard Kaminski |
| 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 |
250053283
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| Zusammenfassung |
| Ocean Island Basalts (OIB) have specific signatures in rare gases (large scattering in 4He/3He
ratio) and trace elements (high 143Nd/144Nd) suggesting that they originate from several
reservoirs, one of which should be primitive (i.e., undegassed). The lowest observed value of
4He/3He in OIB imposes a constraint on the entrainment of primitive material by plumes.
Geochemical mass balance calculations indicate that the mass fraction of primitive
material in plumes should not exceed 10%. It was long assumed that the undegassed
reservoir is the lower mantle, but the discovery of slabs penetrating in the deep mantle
invalidated this hypothesis. This contradiction can be solved by assuming that the
primitive reservoir is not the entire lower mantle, but consists of pools of chemically
differentiated material located in the lowermost mantle, similar to those observed by
seismology. Experimental and numerical models of thermo-chemical convection
with an initial basal layer of dense material have shown that pools of primitive
material can be maintained for periods of time comparable to the age of the Earth, and
that plumes may rise from the top of these reservoirs up to the surface. Important
ingredients to maintain stable reservoirs may include (1) a moderate chemical density
contrast between primitive and regular material, to avoid chemical stratification; (2) a
strong thermal viscosity contrast, which creates and maintains large pools of dense
material at the bottom of the system; and (3) an endothermic phase transition at
660-km depth, to prevent dense material from massively flowing into the upper
mantle.
Here, we quantitatively assess the hypothesis that OIB partially sample reservoirs of
primitive material located in the deep mantle. We run a series of numerical experiments of
thermo-chemical convection, in which we varied the chemical density contrast between the
primitive and regular materials (measured by the buoyancy ratio B), and the Clapeyron slope
at 660 km (Î660). For each experiment, we have then quantified the entrainment of primitive
material by plumes rising from the top of the primitive reservoirs (i.e., the relative fraction of
primitive material in these plumes), and compared this entrainment to available
constraints from geochemistry. We observe that the thermo-chemical structure and the
entrainment strongly depend on B, and to a lesser extent, on Î660. If B -¤ 0.16,
thermo-chemical piles are swept out, and in the long term the power spectra of the
resulting density anomalies do not explain those from probabilistic tomography. For
these models, the entrainment is larger than 11% whatever the value of Î660 we
considered, and up to 21%. By contrast, if B -¥ 0.22, reservoirs of primitive
material remain stable and induce density distributions that explain probabilistic
tomography well. Within error bars, the entrainment is always lower 7%. For intermediate
values of B, we again observe the formation of pools of primitive material, but
whether these pools are maintained for a long period of time depends on the value of
Î660. If this value is negative enough (e.g., -1.5 MPa/K for B = 0.20), reservoirs
can be maintained, and the entrainment of primitive material remains lower than
10%.
Because they fit probabilistic tomography better and yield the most relevant values of B
(between 0.20 and 0.22) and Î660 (between -3.0 and -2.0 MPa/K) for the Earth’s
mantle, stable pools are more likely structures than eroding piles. Our calculations
indicate that for stable pools, entrainment of primitive material never exceeds 9%.
Therefore, our results qualitatively and quantitatively support the hypothesis that
OIB partially sample a reservoir of primitive material located in the lower mantle. |
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