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Deep mantle plumes are considered as the main “engine” for the formation of intra-plate volcanoes. Nevertheless the surface expression of intraplate magmatism varies from large islands chain like Hawaii- Emperor to countless small seamounts. Regarding the global distribution of these seamounts, it is unlikely that all these seamounts are plume related. An alternative to plumes suggests that the generation of some oceanic islands and seamounts could be related to shallow (i.e. lithospheric) processes. An example of such processes is the formation of Petit-spot volcanoes recently discovered off Japan and Tonga islands, which occur in response to plate flexure near subduction zones. The goal of this project which requires the founding for two PhD students is to compare the petrogenesis of alkaline rocks produce in two distinct geodynamic contexts, i.e. in plume versus non-plume processes, to document the differences of rock composition between shallow versus deep origin. We propose to perform detailed petrological and geochemical studies of: (1) alkaline lavas from Fogo and Brava islands (Cape Verde Archipelago), islands which are interpreted as the surface expression of a deep mantle plume; and (2) alkaline sills emplaced in radiolarites from Santa Rosa (Santa Elena peninsula, Costa Rica), sills which are interpreted as an accreted Petit-spot volcano in the north of Costa Rica. These two complementary studies will allow to test the different hypotheses for the formation of alkaline rocks and to clarify the potential role of the oceanic lithosphere in the petrogenesis of these rocks. Alkaline lavas are “classically” interpreted as low degree melts produced by partial melting of an enriched peridotitic source in presence of CO2 at high pressures. An alternative hypothesis suggests that alkaline magmas are produced by melting of veined/metasomatized lithosphere in situ or after recycling into the convecting mantle by subduction or delamination. Since the metasomatic hypothesis implies that alkaline magmas are typically produced by high degrees of melting of the metasomatic veins, the process responsible for the metasomatic enrichment of the oceanic lithosphere is an important component of the model. The metasomatic veins are interpreted as cumulate formed during the percolation and cooling of low degree melts within the lithospheric mantle. These low degree melts are interpreted as derived from the low velocity zone (LVZ) present at the base of the oceanic lithosphere. Nevertheless, a major issue about this hypothesis was the lack of direct evidence for global metasomatism in the oceanic lithosphere. Mantle xenoliths carried by oceanic islands basalts could provide information about the nature and composition of the oceanic lithosphere, but the metasomatism recorded by these xenoliths is interpreted as the interaction of alkaline magma produced by deep plumes and lithospheric mantle rather then the interaction of low degree melts from the LVZ with the lithosphere. An alternative way to obtain information about the nature of the oceanic lithosphere not “altered” by plume activity was to study xenoliths or xenocrysts from Petit-spot volcanoes; this is the second goal of this project. Preliminary results from Santa Rosa alkaline sills indicate the presence of cpx-xenocrysts. The compositions of these xenocrysts suggest that this cpx could correspond to a relic of metasomatic veins present in the oceanic lithosphere. If this hypthesis is correct, it would imply (the first direct evidence?) that the oceanic lithosphere was metasomatized before subduction and recycling into the convecting mantle. Moreover, similar cpx-xenocrysts associated with small mantle xenoliths are observed in Fogo alkaline basalts. So we plan to study in detail theses xenocrysts and xenoliths from both locations using EMPA, LA-ICP-MS, LA-MC-ICP-MS and SIMS to characterize their formation and the underlying petrological and geochemical mechanism. We expect to be able to characterize the metasomatism of the oceanic lithosphere in different tectonic settings.