Source symmetry in the Azores mantle plume: new constraints from Li-B-O isotopes

Genske FS, Beier C, Turner SP, Schaefer BF

Abstract

Lavas erupted in the vicinity of mantle plumes provide the opportunity to deduce models of recycling of oceanic and/or continental material. In most fluid dynamic models plumes that are situated close to spreading axes have been treated as axisymmetric; however, others appear more complex (e.g. Galapagos). The Azores archipelago is separated into an eastern and western plateau and the distribution of the majority of the erupted volumes is concentrated to the east of the Mid-Atlantic Ridge (MAR). The bathymetric high defining the Azores plateau is clearly asymmetric, implying a potential asymmetry of the Azores mantle plume. Here, we present new Sr-Nd-Pb and Li-B-O isotope data from the western and eastern islands to constrain on source symmetry of the Azores plume. If a simple axisymmetric model described the Azores, islands furthest away from the central plume area are expected to show less enriched compositions than lavas from the plume centre. Also, if the western and eastern Azores islands were generated by a single mantle plume the eastern- and westernmost lavas are expected to show comparable compositions. Our new isotope data from the western islands of Corvo and Flores allow for comparison with existing and new data from the eastern islands. The eastern Azores are known to represent a small scale isotopic heterogeneity but we can show that the Sr-Nd-Pb and Li-O isotopes are remarkably homogeneous in the west. The radiogenic isotopes of the westernmost islands straddle across a range in Sr-Nd-Pb that is commonly found in the Azores and other Ocean Island Basalts (OIB). Oxygen isotope ratios for olivines measured on Corvo and Flores yield mantle values (~5.2%) contrasting some lavas found in the east, and δ7Li for bulk rocks are slightly higher than pristine mantle values of 3.4%, but span within the range known for OIB. Boron isotope ratios display much higher values from 0.4 to 11.8% in the west, potentially implying low temperature alteration of underlying oceanic crust. However, high δ11B ratios are correlated with mantle like δ18O and relatively immobile trace element ratios, e.g. higher Nb/Zr, lower Rb/Th and heavy rare earth element ratios, indicating that the western Azores mantle source is clearly distinct compared to the eastern sources. We suggest that the western lavas may reflect a different mantle source composition resulting from subducted and recycled oceanic crust leading to commonly found Sr-Nd-Pb isotope ratios and immobile trace element ratios. The high δ11B may best be explained by low temperature alteration (<100°C) at low pH conditions, i.e. mixing approximately 30% low T altered oceanic crust into the Corvo and Flores mantle source could explain the B isotope ratios but would only result in minor changes of Sr isotope ratios of ~2 × 10-5. Thus, we propose that δ11B may be suited to infer on low T/low pH conditions of altered oceanic crust; conditions where most traditional radiogenic and stable isotope systems are unaffected. For the specific case of the Azores we did not observe any source symmetry implying that sources west and east of the MAR might not be related.