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| Titel |
Low H2O/Ce in Icelandic basalts as evidence for crustal recycling |
| VerfasserIn |
David Neave, Oliver Shorttle, Margaret Hartley, John Maclennan |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250126352
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| Publikation (Nr.) |
 EGU/EGU2016-6060.pdf |
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| Zusammenfassung |
| The generation of new crust at mid-ocean ridges is balanced by the subduction of partially
hydrothermally altered basaltic material back into the mantle. This subducted material may
then be recycled and returned via mantle plumes to the Earth’s surface at hot spots.
Long-identified isotopic and trace element signatures of oceanic crust recycling in ocean
island basalts (OIBs) have been recently supplemented by evidence of major element, i.e.
lithological, heterogeneity in the melting region. For example, combined major and trace
element systematics from Iceland suggest that the mantle source contains at least 5%
recycled basalt. Observations of high water (H2O) contents in subglacially quenched
basalts from Iceland have previously been attributed to the incorporation of wet
recycled material into the mantle source. However, when combined with trace element
analyses, recent volatile analyses from the Laki-Grímsvötn and Bárðarbunga-Veiðivötn
systems in the Eastern Volcanic Zone (EVZ) of Iceland suggest that the underlying
mantle is comparatively depleted in H2O for its degree of major and trace element
enrichment.
Correlations between H2O and cerium (Ce) within individual mid-ocean ridge basalt
(MORB) suites reveal that these elements partition similarly prior to H2O degassing at low
pressures; H2O/Ce remains constant during melting and fractionation, and hence reflects the
average H2O/Ce of the melting region. MORBs from the Mid-Atlantic Ridge south of
Iceland have a mean H2O/Ce value of 304±48 at a mean La/Yb of 2.1±1.5. In
contrast, basalts from the EVZ have a lower mean H2O/Ce of 180±20 at a higher
mean La/Yb of 3.1±0.5. Thus, despite coming from an enriched section of the
Mid-Atlantic ridge in terms of trace element content, basalts from the EVZ have the
lowest H2O/Ce values known from the ridge, and are hence comparatively depleted
in H2O. Given that H2O/Ce from un-degassed basalts is considered to represent
mantle source values, we suggest that low H2O/Ce values in the EVZ reflect the
efficient dehydration of recycled crustal components: H2O and Ce are more readily
decoupled during the subduction of altered oceanic crust than during the generation and
evolution of primitive basalts. F and Cl are also decoupled during subduction, with Cl
being preferentially lost into the mantle wedge during dehydration, providing an
explanation for the low Cl/F values of EVZ basalts in contrast with basalts from the
Mid-Atlantic ridge. Low H2O/Ce (and Cl/F) values in basalts from the EVZ thus
provide further evidence for a recycled basalt component in the mantle beneath
Iceland. |
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