Interactions de faibles profondeurs et géochimie des basaltes d'îles océaniques : implications sur les modes d'acquisition de la signature isotopique et sur la topogique mantellique

Abstract : Systematic measurements of Sr-Nd-Pb isotopic compositions in Oceanic Island Basalts (OIB) for 30 years have shown the complex nature of mantle sources. However, measured variations in these samples can be related either to variable proportions of the components present in the deep source of the basalts, or to interactions between the pristine melts and the environments encountered during their ascent to the surface during partial melting, plume-lithosphere interaction, assimilation and fractional crystallization or post-eruption seawater interaction processes. Therefore, extracting the signature of shallow-level interactions from the global geochemical signature is a key to retrieving the source information and to investigating mantle topology in greater details. Means to identify and see through these shallow-level interactions are first presented before being applied to 2 case studies done on 2 islands in different geodynamic setting (São Nicolau Island, Cape Verde archipelago, intraplate setting and old lithosphere; São Jorge Island, Azores archipelago, Plume-ridge interaction setting on a young lithosphere). Major and trace elements as well as Sr-Nd-Pb isotopes measurements show that an important part of the isotopic variations can be explained by shallow-level interactions rather than source heterogeneity. These results are extended from the island-scale to the archipelago-scale in both cases. Identification of such processes in locality playing a key role in the definition of mantle topology allows generalizing the conclusions of the 2 local studies to a global scale. On this basis, the existence and nature of a sub-primitive component common to all OIB (FOZO, PHEM, C), inferred from non-filtered OIB data compilation, is then discussed.
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Volcanology. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21794>


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Submitted on : Tuesday, July 17, 2012 - 11:00:59 AM
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Marc-Alban Millet. Interactions de faibles profondeurs et géochimie des basaltes d'îles océaniques : implications sur les modes d'acquisition de la signature isotopique et sur la topogique mantellique. Volcanology. Université Blaise Pascal - Clermont-Ferrand II, 2007. French. <NNT : 2007CLF21794>. <tel-00718419>

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