Time-Dependent Evolution And Source Heterogeneities of Ocean Island Basalts From a Weak Plume, São Jorge, Azores

The geochemical composition of ocean island basalts (OIBs) from the Azores (AZ) reflects the spatial distribution, shape and temporal evolution of small-scale geochemical heterogeneities within their mantle plume source. Here, we investigate the time-related evolution of volcanism at S & atilde;o Jorge Island, Central AZ. New field observations, a magnetic survey, 40Ar/39Ar and 14C ages and geochemical data indicate that the fissural volcanic activity at S & atilde;o Jorge produced at least four main mafic volcanic complexes (V. C.). The oldest V. C., S & atilde;o Jo & atilde;o, produced the thickest lava piles at ca. 1.3 Ma in the eastern part of the island. After a period of quiescence, the Serra do Topo V. C. was produced at ca. 0.8-0.5 Ma in the central part of the island. The Rosais V. C. was emplaced between ca. 0.4 and 0.1 Ma on the entire island. Finally, the Holocene Manadas V.C. volcanism became active in the western part of the island and includes three historic eruptions (1580, 1808 and 1964 CE).Magmas were formed at low melting degrees from a peridotitic mantle with possible minor contributions of recycled components. Olivine compositions and whole-rock trace element ratios discard a significant contribution from pyroxenitic source rocks. Melting temperatures (ca. 1420-1480 degrees C) were slightly higher than those of the ambient upper mantle. The four V.C. are characterized by distinct geochemical compositions in terms of incompatible trace elements and Sr-Nd-Pb isotopic ratios. The oldest V.C., S & atilde;o Jo & atilde;o, is characterized by Pb isotopic compositions (e.g. markedly negative Delta 7/4 and 8/4 values) plotting well below the Northern Hemisphere Reference Line (NHRL). The Upper Pleistocene (Rosais V.C.) lavas from the north-western cliffs have compositions similar to enriched mantle (EM) end-member basalts (e.g. high 207Pb/204Pb at moderate 206Pb/204Pb; high Ba/Nb), which are rare among northern hemisphere OIBs. Finally, high 206Pb/204Pb (up to 20), reflecting contribution from a HIMU-type component characterizes the Holocene Manadas lavas and is occasionally found in lavas from other VCs from 1.3 to 0.1 Ma. These findings indicate that magmas from S & atilde;o Jorge and the nearby Central AZ islands were sourced from a strongly heterogeneous mantle plume, which displayed localized filaments of heterogeneous material that were rapidly exhausted (in ca. 0.2 Ma). The dominant component at S & atilde;o Jorge and in the Central AZ in general appears to be the HIMU-type end-member, which instead is not significant in the Eastern AZ. Possibly, the Central and Eastern AZ were produced by distinct branches of the AZ mantle plume, as would also be consistent with previous tomographic studies.