Volcanism in the Azores is generally accepted to result from the decompression melting in the upper mantle associated with the Azorean plume. Basalts on the island of São Miguel have been previously recognised as having δ
18O values lower than MORB with different authors ascribing this low-δ
18O signature to either a component within the plume itself or as the result of assimilation of crustal materials at shallow level. Here we assess these contrasting models by using a suite of samples that are both geographically and temporally spread.
We find that while low-δ18O mafic magmas (reaching 4.98 ‰) do occur on São Miguel, they are spatially restricted to the Picos fissure system, between the central volcanoes of Sete Cidades and Fogo. Basalts of similar age from outside this restricted region return δ18O values that are similar to MORB. This spatial constraint argues that the source of the observed low-δ18O signature is within the crust rather than a component of the plume.
Previous studies on mafic to intermediate rocks of the Picos fissure system have identified the assimilation of broadly syenitic lithologies, which were found as co-erupted lithic clasts at the Fogo volcano and may have δ18O values as low as 2.6 ‰. However, trace element geochemistry indicates that these lithologies have not been assimilated within our low-δ18O samples, a more likely assimilant is altered oceanic crust or mafic lavas within the island edifice.