Many magmas from oceanic islands with enriched isotopic signatures are silica poor and strongly alkaline. Although there is considerable evidence indicating that the mantle source regions of these magmas contain recycled lithosphere, there is little consensus to what extent the crust and lithospheric mantle contribute to mantle heterogeneity and the generation of ocean island basalts. To gain a better understanding of the components and processes involved in the generation of alkaline oceanic magmas, a detailed geochronological and geochemical study of the Fernando de Noronha archipelago in the western equatorial Atlantic Ocean was undertaken in this thesis.
Volcanic and hypabyssal rocks of the archipelago range in age from 12.8 ± 0.3 to 1.3 ± 0.1 M and can be divided into two age-compositional groups. The oldest preserved rocks comprise a moderately to mildly alkaline, potassic (basanite, alkali-basalt, basaltic trachyandesite, trachyandesite, trachyte) series, which was emplaced in a brief timespan between 12.8 ± 0.3 and 12.0 ± 0.1 Ma. These potassic, silica-undersaturated to saturated magmas are characterized by relatively high Sr- isotopic ratios and low Nd- and Pb-isotope ratios indicative of an enriched-mantle (EM2) component. However, this minor potassic episode was immediately followed by production of a sodic, strongly alkaline (basanite, tephrite, essexite, phonotephrite, phonolite) magma series between 10.9 ± 0.1 and 9.4 ± 0.2 Ma and a suite of less alkaline (but still sodic) alkali basalt and basanite lavas between 9.8 ± 0.5 and 9.0 ± 0.1 Ma. Collectively these sodic rocks have lower Sr-isotopic ratios coupled with higher Nd- and Pb-isotopic ratios characteristic of a HIMU source. Following an eruption hiatus of about 3 Ma, magmatic activity recommenced with emplacement of sodic melanephelinite, basanite, and alkali-basalt lavas and pyroclastic deposits between 6.2 ± 0.1 and 1.3 ± 0.1 Ma. Comparison of these results with high-pressure melting experiments for the lithologies commonly invoked to explain the enriched signatures in oceanic basalts shows that only metasomatized mantle lithosphere, containing amphibole plus minor phlogopite, can satisfy all the geochemical and isotopic characteristics of the alkaline magmas from both series. The majority of volcanic rocks at Fernando are sodic and appear to be produced mainly by melting of metasomatic amphibole-rich veins, which generates trace-element ratios (e.g., high Ba/Rb and U/Pb, and low Rb/Sr) consistent with a HIMU source. However, an additional contribution from phlogopite is required to produce the early potassic rocks with traceelement ratios (high Rb/Sr, lower Ba/Rb and U/Pb) coherent with their EM-like isotopic signatures. When these silica-poor vein melts interacted with surrounding periodite, they produce less alkaline lavas with higher SiO2, diluted trace-element contents, and isotopic compositions containing a greater contribution from depleted mantle.
These results suggest that the major-element, trace-element and isotopic character of oceanic basalts may be directly linked to the mineralogy of their source. Comparison with other oceanic basalts with classic HIMU (e.g., Cook- Australs) and EM2 (e.g., Samoa) pedigrees supports this conclusion and strengthens the hypothesis that subducted or otherwise recycled metasomatised lithospheric mantle, rather than oceanic crust (±sediment), may be the major source of enriched components in most OIB. It also appears that the tendency for the more differentiated rocks of the sodic and potassic suites at Fernando de Noronha to follow undersaturated and saturated-to-oversaturated evolutionary trends, respectively, is inherited from the bulk compositional differences of their parental magmas. The metasomatized lithospheric source advocated for the enriched components at Fernando de Noronha is consistent with the lack of seismic evidence for a deep mantle upwelling and its proximity to the cratonic edge of the South American continent. This position makes the archipelago a favourable candidate for recycling of continental lithospheric material through small-scale, edge-driven convection. If so, the somewhat mild HIMU component in the sodic lavas may have developed by lithospheric invasion of low-degree upper-mantle melts generated during opening of the Atlantic Ocean at about 130 Ma. Cooling and crystallization
of these melts would produce amphibole-rich cumulate assemblages, which can evolve to mild HIMU characteristics over that timespan. In contrast, the EM2-influenced isotopic compositions of the potassic suite require much longer residence times. The older generation of veins apparently responsible for this minor group of potassic rocks may be linked to metasomatism of the lithosphere by plume-derived melts that produced tholeiitic flood basalts with enriched-mantle signatures in northern Brazil during the Mesozoic.