Th–Sr–Nd–Pb isotope and trace element evidence for the origin of the São Miguel, Azores, enriched mantle source

Abstract

A suite of young alkali basalts from across the island of São Miguel has been analyzed for major and trace element concentrations and Sr, Nd, Pb and Th isotopic compositions in order to document the intra-island variation, and to constrain the processes involved in the formation of the enriched mantle sources of these lavas. As shown in previous studies, large isotopic and trace element variations correlate with geographic location across São Miguel. New Th isotope data show a similar correlation, with ( 230Th)/( 232Th) varying from 1.094 to 0.877. These variations are attributed to two-component mixing, where the western end of the island is similar to the source of the Azores Platform tholeiites, and the eastern end of the island has a more enriched, EMII-type source. Trace element systematics indicate that the São Miguel enriched mantle source is enriched relative to the predominant Azores plume source in elements including K, Rb, Th, U, Pb and Zr, but not Ba, Nb or Ti. Incorporation into the Azores plume of approximately 5% subduction-processed terrigenous sediment can explain many of the trace element and isotopic characteristics of the São Miguel enriched mantle source. However, this mechanism cannot easily explain the constant Cs/Rb ratios across the island, the concave downward trend of Ba/Th vs. Ti/Zr, and the recent decrease in Th/U of the São Miguel enriched mantle source documented by Th and Pb isotope systematics. An alternative model is that the São Miguel enriched mantle source is subcontinental lithospheric mantle. This is supported by the similarity of the trace element signatures of the São Miguel enriched mantle source to those of hydrous metasomatized mantle xenoliths. In particular, the low Ba/Rb signature of the São Miguel enriched mantle, which is unusual for oceanic basalts, is a characteristic of many hydrous metasomatized mantle xenoliths and magmas believed to be derived from such sources. We propose that this subcontinental lithospheric mantle resided originally beneath northwestern Africa or Iberia but was delaminated during rifting upon the opening of the Atlantic Ocean basin, and that it is presently a shallow, localized contaminant that is melted due to an influx of heat from the Azores plume.