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| Titel |
Constraining the amount of recycled material in the mantle source from basalt chemistry |
| VerfasserIn |
Oliver Shorttle, Sarah Lambart, John Maclennan |
| 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 |
250079456
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| Zusammenfassung |
| As the primary flux of material from the mantle to the surface, the basalts erupted at
mid-ocean ridges (MORB) are a key resource for investigating the mantle’s chemical
composition. However, despite the large volumes of oceanic crust returned to the mantle by
subduction, it has proven difficult to estimate the abundance of this recycled material in the
mantle source using the chemistry of MORB. This is a significant problem, as fundamental
questions about the dynamics of our planet cannot be answered without quantifying the
abundance and spatial distribution of the mantle’s chemical heterogeneity: is Earth’s
convection layered, or does it involve the whole mantle? What is the eventual fate of recycled
oceanic material? What are the fluxes of elements from the deep Earth to the surface? Here,
we present a method to estimate the proportion of enriched material in mantle source regions
by combining geochemical observations with simple bilithologic models of mantle
melting.
Working backwards from the chemistry of an erupted basalt to the proportions of peridotite
and basalt lithologies in the source requires a number of critical pieces of information. Firstly,
the geochemical variability at a ridge segment needs to be used to identify the types of
lithology melting. Secondly, the relative mass of the enriched and depleted melts needs
to be determined. Finally, a bi-lithological melting model needs to be run in an
attempt to account for the over-representation of fusible, productive lithologies in the
final mixed melt (Hirschmann and Stolper, 1996; Shorttle and Maclennan, 2011).
This last step involves a large number of secondary assumptions/inputs to make
the melting problem tractable, such as the mantle flow field and mantle potential
temperature.
We determine the abundance of recycled material in the mantle beneath Iceland and at other
ridges and ocean island settings in three stages. (1) The lithologies contributing to melting are
identified by quantitative comparison of the major element composition of erupted basalts to
a database of experimental partial melts (Shorttle and Maclennan, 2011). (2) A mass balance
is calculated between the endmember basalt compositions and the fully mixed melt to
obtain the relative proportion of enriched and depleted melts. (3) A bilithologic
melting model modified from Shorttle and Maclennan (2011) is then used with the
appropriate lithological melting parametrisations to account for the differences in
productivity.
Applying this method to Iceland demonstrates that ~ 10 % of the source is recycled
basaltic material. However there are large uncertainties on this number, and our
results demonstrate that the ability to constrain the mass fraction of lithologies
contributing to melting depends heavily on the dynamics of mantle flow, melting and melt
transport/reaction.
References:
Hirschmann M.M., Stolper E.M. A possible role for garnet pyroxenite in the origin of the
garnet signature in MORB. Contributions to Mineralogy and Petrology, 124:185–208,
1996.
Shorttle O. and Maclennan J. Compositional trends of Icelandic basalts: Implications for
short-lengthscale lithological heterogeneity in mantle plumes. Geochemistry, Geophysics and
Geosystems, 12(11):Q11008, 2011. |
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