Contribution of primitive melt inclusions to the understanding of the origin<br />of oceanic basalts: examples of the FAMOUS zone (Mid-Atlantic Ridge)<br />and the Réunion hotspot

Understanding the physical and chemical conditions of partial melting, magma extraction and segregation processes and the mineralogical and chemical composition of the Earth mantle requires the characterization of the primitive composition of magmas. A unique feature of melt inclusions trapped in early phenocrysts is their ability to preserve the primitive information which is completely or partially obliterated from whole rock compositions by secondary processes such as magma mixing and fractional crystallization. The present study investigates primitive olivine-hosted melt inclusions in two distinct geodynamic settings – mid-ocean ridges and oceanic hot spots – and is focused on two regional examples: the FAMOUS zone (Northern Mid-Atlantic Ridge) and the Réunion hotspot.<br />Melt inclusions trapped in a single sample from the FAMOUS zone (Northern Mid-Atlantic Ridge) are characterized by a large variability in their major- and trace-element contents, which exceeds that of the whole rocks and glasses from the area. They display highly primitive compositions (Mg# = 0.70-0.78) which show similarities with experimental melts produced by partial melting of spinel-bearing lherzolite at 1 GPa. Incompatible trace-element compositions evolve from relatively depleted ((La/Sm)N<0.75) to slightly enriched ((La/Sm)N>1.2) compositions. Geochemical simulations suggest that polybaric continuous melting of a homogeneous source is responsible for much of the variability in the melt inclusions. However local variations in the potential temperature and the H2O content in the mantle source seem to be required to explain some of the major-element trends. We did not find any evidence for chemical modifications of the primary magmas due to magma/rock interaction during transport.<br />Melt inclusions in samples from Mauritius Island and Piton des Neiges and Piton de la Fournaise (Réunion Island) volcanoes evolve from transitional to tholeiitic compositions; this evolution can be explained by a slight increase in the degree of melting of a peridotite source. The chemical and isotopic homogeneity of Réunion plume products through time enables us to calculate the incompatibility sequence of trace elements during the partial melting process using data from Mauritius, Piton des Neiges and Piton de Caille (Piton de la Fournaise) melt inclusions. The results show a strongly incompatible behaviour of Pb, which is generally thought to be similar to that of Ce during partial melting in oceanic settings. This unusual behaviour of Pb compared to other oceanic mantle plumes implies that a residual sulphide phase was not present in the Réunion source at the time of melt extraction. Finally, the modelled source composition of the Réunion plume is similar to that of the primitive mantle, except for a slight depletion in the most incompatible elements. Therefore the chemical signature of the Réunion plume is consistent with a primitive mantle domain that has been slightly depleted by early continental crust extraction and has not been strongly modified afterwards by recycling or differentiation processes.