A HETEROGENEOUS RECYCLED OCEANIC LITHOSPHERE IN THE AZORES PLUME REVEALED BY THE HF-ND ISOTOPE SYSTEMATICS OF TERCEIRA RIFT LAVAS

The Azores Archipelago and surrounding seafloor represents an ideal setting to test mantle plume - lithosphere interaction models, as it is situated in a plume – triple ridge junction configuration between the Mid-Atlantic Ridge and the ultraslow spreading Terceira Rift. Moreover, the preferential sampling of upper mantle imposed by the ultraslow Terceira Rift can be used as a probe of mantle heterogeneity.

Here we present the first Hf-isotope data on submarine lavas recovered along the Terceira Rift (João do Castro, W. of São Miguel, W. of Graciosa and E. of Terceira), as well as new Hf isotope data on submarine and subaerial lavas from Graciosa, Faial, Terceira, Pico and São Jorge islands. Most subaerial lavas generally overlap in Sr – Nd – Pb – Hf isotopic spaces (⁸⁷Sr/⁸⁶Sr = 0.703 – 0.704, ²⁰⁶Pb/²⁰⁴Pb = 19.0 – 20.0, εNd = 4 – 7 and εHf = 6 – 11), while lavas from Eastern São Miguel have more enriched compositions, with highly radiogenic Pb isotopes and Hf, Nd isotope compositions that plot below the terrestrial array (Elliott et al. 2007, GCA 71, 219-240). In εNd-εHf isotope space lavas from Graciosa and Pico islands form near flat subparallel arrays that partially overlap Atlantic MORB from the Azores plateau. In contrast, the analyzed lavas from along the Terceira rift extend to unradiogenic Hf compositions (εHf=0) that fall below the terrestrial array in Hf-Nd isotope space, and form a much steeper slope (5.1) than East São Miguel (2.0) and the Mantle Array (1.3). The unradiogenic Hf, Nd component revealed by these Terceira Rift lavas is different than the enriched São Miguel component, consistent with their less radiogenic Pb isotopes than São Miguel.

Our data reveals a larger isotopic variability in the Azores plume than previously thought, with the presence of two distinct low εHf end-members. The source of the Eastern São Miguel lavas have been modeled as ancient metasomatized lithosphere (Elliott et al., 2007). If such ancient lithosphere of presumably recycled origin is also responsible for the low εHf lavas from the Terceira Rift then this component is spatially highly variable in the Azores plume. Sampling of this heterogeneous recycled lithosphere component depends on melting conditions which are in turn controlled by lithosphere thickness, the Terceira Rift, and plume-related thermal anomaly.