The islands of the Azores archipelago result from the interaction between the triple junction of the
North American, Nubian, and Eurasian plates and a mantle upwelling often referred to as the Azores plume. The largest island of the archipelago, São Miguel, consists of five active volcanic systems: three central volcanoes with calderas (Sete Cidades, Fogo – also known as Água de Pau, and Furnas) separated by two mostly mafic fissure systems (Picos and Congro). There are additionally two extinct volcanic systems (Povoação and Nordeste) in the eastern part of the island. In historical times the volcanoes of Fogo and Furnas have erupted explosively in 1563, and in 1439/43 and 1630, respectively, and minor eruptions occurred within the Picos fissure system in 1563 and 1652. Previous studies dealing with a limited number of mafic samples from across the Azorean archipelago found low-δ18O values in olivine from some samples requiring assimilation of material hydrothermally altered with meteoric waters at high temperatures. This signal was attributed to come from the plume itself [1, 2] or from assimilation within the oceanic crust [3]. To further investigate the extent of these low-δ18O magmas we carried out an island-wide survey of São Miguel volcanism. Bulk rock geochemical compositions in both major and trace elements require crystal fractionation to be the dominant process of magmatic evolution. Yet, some of the evolved magmas have phenocrysts with low-δ18O values, 1-2 per mil lower than expected from closed-system fractionation of mantle derivatives). At the westernmost volcanic centre, Sete Cidades, these evolved magmas commonly have lower δ18O values than the coeval mafic magmas of the same volcano. This relationship implies thatthe lowering of magma δ18O values occurred during evolution from basalts to trachytes.A commonly-invoked mechanism for generating low-δ18O evolved magmas involves down-dropping of hydrothermally altered material within poly-cyclic caldera systems [4]. However, this requires transporting the altered material from the surface or near-surface to depths where melting may occur (>3-4 km). The relatively small-volume nature of the calderas on ocean islands in general, and São Miguel specifically, argues against down-dropping as a feasible mechanism. Rather, we propose that altered crystalline or near-solidus magmas from the same episode of volcanism are assimilated. Such plutonic or near-plutonic lithologies are common components of pyroclastic deposits globally and in many cases they are hydrothermally altered [5]. Further, in many large rhyolitic systems (including Yellowstone – the ‘type locality’ of the down-dropping model) zircons with extremely high- U cores are found [6]. These high-U zircons (with similar ages to eruption) reflect crystallisation at very low melt fractions and implicate remobilisation of crystal-rich mushes or plutonic rocks. We thus suggest that the assimilation of hydrothermally altered mushy or plutonic lithologies may be a general process that is more apparent at smaller-scale ocean island volcanoes.
[1] Widom E, Farquhar J (2003) Chem Geol 193, 237-255.
[2] Turner S et al. (2007) Nature 447, 702-705.
[3] Genske F et al. (2000) Geology 41, 491-494.
[4] Bindeman IN and Valley JW (2000) Geology 28, 719-722.
[5] Javoy M et al. (1986) CMP 92, 225-235.
[6] Rivera TA et al. (2016) J Pet 57, 1677-1704.