 |
| Titel |
Geochemical asymmetry of the Crozet hotspot |
| VerfasserIn |
Antoine Bezos, Anthony Pimbert, Mélanie Segard, Christophe Hémond, Christèle Guivel, Guillaume Delpech, Éric Beucler, Carole La |
| Konferenz |
EGU General Assembly 2014
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250098790
|
| Publikation (Nr.) |
 EGU/EGU2014-14498.pdf |
|
|
|
|
|
| Zusammenfassung |
| The Crozet archipelago (Indian Ocean) includes 5 islands showing contemporaneous
volcanism and defining two geographical groups: the Eastern group includes the Possession
and Est Islands, and the western group the Pingouins, Cochons and Apôtre Islands. The few
Crozet samples that have been analyzed for their isotopic compositions (Possession: N=9;
Est: N=15; Pingouins: N=13) reveal the presence of geochemical heterogeneities within the
Crozet hotspot.
In this study we present new Sr-Nd-Pb-Hf isotopic compositions of 20 lavas samples
from the Possession Island. The isotopic compositions, which display limited but
significant variations, are characterized by intermediate values (87Sr/86Sr = 0.703972
to 0.704986; 143Nd/144Nd = 0.512822 to 0.512873; 176Hf/177Hf = 0.282959 to
0.283100; 206Pb/204Pb = 18.8099 to 19.1660) akin to the “C” component. As mentioned
by previous studies, lavas from the Possession and Est islands (eastern Islands)
display comparable isotopic compositions. The western Pingouins Island lavas
denote from Possession-Est lavas by having (for example) lower 87Sr/86Sr and
higher 206Pb/204Pb ratios, approaching therefore the Fozo isotopic domain. The
most striking results displayed by Crozet data, is the existence of two parallels
arrays in the 206Pb/204Pb vs 208Pb/204Pb diagram. For a given 206Pb/204Pb ratio,
Possession-Est lavas display higher 208Pb/204Pb ratio compared to Pingouins data.
These linear isotopic arrays indicate significant heterogeneity within each of the two
trends, both having distinct radiogenic and least radiogenic lead end-members. This
observation strongly disagrees with the model of simple binary mixing between the
Possession-Est and the Pingouins components proposed by Breton et al., (2013). We first
propose to model the Possession-Est radiogenic lead end-members by recycling in
the Crozet hotspot source 1Gy oceanic crust with ~2% sediments. The Fozo-like
signature of Pingouins lavas may be obtained with a similar model that involved
recycling of gabbros instead of bulk oceanic crust. The nature and origin of the
least lead radiogenic end-members is then addressed. We propose that those later
end-members are best accounted by a depleted upper mantle that have been previously
contaminated by a DUPAL component (between 1% to 3% by mass). I summary,
the isotopic systematic of the Crozet Archipelago is best explained by a two-step
mixing model that constrains the DUPAL component to be located in the upper
mantle.
Our mixing model implies therefore the presence of two distinct isotopic domains within
the Crozet hotspot. The eastern Islands sample one of them and the western Pingouins Islands
samples the other one. As suggested for other hotspots showing bilateral asymmetry (e.g.
Hawaii), we propose that both isotopic domains could correspond to extensively stretched
heterogeneities forming vertical “geochemical filaments” in the Crozet mantle plume.
We finally discuss this model in light of the location of the Crozet hotspot above
the contour of African Large Low-Shear-Velocity Province in the deep mantle. |
|
|
|
|
|
|