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

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, Sa tilde o Jorge, and Sa tilde o Miguel Islands exhibit homogeneous and MORB-like Ba isotope compositions, whereas samples from Terceira Island have variable Ba isotopes. The delta(138)/Ba-134 values of the Terceira samples range from-0.03 to +0.24 parts per thousand , extending to values significantly higher than those of the other islands. The MORB-like Ba isotope composition of the Sa tilde o 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 delta(138)/Ba-134 and Pb-206/Pb-204 and the positive correlation between delta(138)/Ba-134 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.

Automated summary

This study examines the Ba isotope compositions of ocean island basalts from the Azores Archipelago, revealing the presence of enriched components in their mantle sources. The samples show heterogeneity in their isotope compositions, suggesting a highly diverse mantle source. The study identifies two types of Ba isotope characteristics, with some islands exhibiting homogeneous and MORB-like Ba isotope compositions while others show variable Ba isotopes, likely inherited from subducted altered oceanic crust.