Barium isotope evidence of a fluid-metasomatized mantle component in the source of Azores OIB

Abstract

This study presents Ba isotope compositions of ocean island basalts (OIB) from the Azores Archipelago to investigate the origin of the enriched components within their mantle sources. These samples have large variations in radiogenic Sr-Nd-Pb-Hf isotope compositions, indicating that they tap highly heterogeneous mantle sources. They display two types of Ba isotope characteristics. Samples from the Faial, Pico, São Jorge, and São Miguel Islands exhibit homogeneous and MORB-like Ba isotope compositions, whereas samples from Terceira Island have variable Ba isotopes. The δ138/134Ba values of the Terceira samples range from −0.03 to +0.24‰, extending to values significantly higher than those of the other islands.The MORB-like Ba isotope composition of the São Jorge samples indicates that the enriched HIMU/FOZO material in the Azores mantle plume source has a homogeneous and MORB-like Ba isotope composition. In contrast, the Terceira samples are characterized by heavy Ba isotope compositions, which are likely inherited from subducted altered oceanic crust (AOC) and may reflect the presence of a shallower enriched source beneath Terceira. As the Azores archipelago is located at a triple junction far from any active subduction zone, the signature of subducted AOC must be an ancient feature, potentially derived from a recycled fluid-metasomatized mantle component now present beneath the Azores archipelago. The negative correlation between δ138/134Ba and 206Pb/204Pb and the positive correlation between δ138/134Ba and Ba/Th among selected Terceira samples likely reflect mixing between the mantle plume and shallower, recycled, fluid-metasomatized mantle components enriched in heavy Ba isotopes.