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| Titel |
The Brava seamount, Cape Verde: Beyond the spatial extent of EM1 and petrogenesis of highly evolved alkaline lavas. |
| VerfasserIn |
Abigail Barker, Axel Andersson, Valentin Troll, Thor Hansteen, Robert Ellam |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250032643
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| Zusammenfassung |
| Alkaline lavas from the Brava seamount, Cape Verde are investigated to establish the spatial
distribution of compositional heterogeneity in the southwest of the Cape Verde archipelago.
Highly evolved lavas provide a record of shallow level magma-crust interaction beneath the
Brava seamount.
The Brava seamount, located southwest of the island of Brava, Cape Verde was sampled
during research cruise 8/85 of the R.R.S. Charles Darwin in 1985. Two groups of
highly evolved alkaline volcanics are distinguished from the Brava seamount: 1)
pyroxene-phonolites containing clinopyroxene, amphibole, nepheline, ±biotite, and minor
sanidine and 2) feldspathoid-phonolites containing nepheline, nausean, minor biotite and
leucite. All of the samples have MgO between 0.8 and 2 wt%, comparable to the most
evolved volcanics sampled in the Cape Verde archipelago. The feldspathoid-phonolites have
NaO2 of 12-13 wt%.
Alkaline lavas from the Brava seamount have higher 87Sr/87Sr (0.70337 to
0.70347) at ÉNd of +6 to +7 than previously sampled in Cape Verde. Sr isotopes will
be integrated with oxygen isotopes to establish magma and crust interactions in
the magmatic plumbing system beneath the Brava seamount. Clinopyroxene-melt
thermobarometry will be presented to constrain the depths of equilibrium crystallisation.
Sr-O isotopes and thermobarometry will be combined to build a picture of the levels of
magma stalling and interaction between magmas and the crust beneath the Brava
seamount.
The Brava seamount phonolitic lavas have high 206Pb/204Pb of 19.5 to 19.8 with
negative Î8/4 and high ÉNd of +6 to +7 in contrast to the positive Î8/4 for lavas from
nearby Brava and the southern islands of the Cape Verde archipelago. Lavas from
the Brava seamount have Pb-Nd isotope systematics comparable to the northern
Cape Verde islands, indicating the southwestern boundary in mantle heterogeneity
and thereby the spatial extent of the EM1-like source contributing to the southern
islands.
The extensive crystallisation and stalling of magma batches at crustal depths shown by
thermobarometry will be used in conjunction with geochemistry to constrain the origin of
assimilants and implies that an EM1-like source is not found in the mantle source, the
shallow lithosphere or crust beneath the Brava seamount. |
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