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This is considered to take place in a density-stratified reservoir, with alkali basalt magmas at the base and hydrous trachytes forming an upper cap or cupola. The presence of this reservoir at shallow crustal depths beneath the caldera likely inhibits the ascent and subsequent eruption of mafic magmas, generating a compositional Daly Gap. Rare syenitic ejecta represent in situ crystallisation of trachytic magmas in the thermal boundary zone at the top of the reservoir. Trachytic enclaves within these syenites, in addition to banded pumices and ubiquitous clinopyroxene antecrysts in the UFG pumice falls, provide evidence for mingling/mixing processes within the magmatic system. Despite relatively uniform major element compositions, systematic trace element variations within individual eruptions highlight the importance of fractional crystallisation during late-stage evolution of the trachytes. This is facilitated by the accumulation of water and the development of mild peralkalinity, which contribute to low pre-eruptive melt viscosities and efficient crystal settling. Compositional zoning patterns between individual eruptions cannot be accounted for by periodic tapping of a single magma batch undergoing fractional crystallisation. Instead, up to four individual cycles are recognised, in which a zoned cap of eruptible trachytic magma, formed at the top of the reservoir, was erupted in one or more eruptions and was re-established via intermittent replenishment and subsequent fractional crystallisation. Keywords Furnas volcano · Peralkaline trachyte · Fractional crystallisation · Zoned magma reservoir · Post-caldera volcanismAbstract Furnas is one of three active central volcanoes on São Miguel Island, Azores, and is considered to be one of the most hazardous in the archipelago. In this study, the pre-eruptive magma plumbing system of the 10 young (<5 ka), intra-caldera, sub-Plinian, trachytic eruptions of the Upper Furnas Group (UFG) is investigated via whole rock major and trace element geochemistry, mineral chemistry, thermobarometry, and petrogenetic modelling. The main aim of this work is to elucidate the petrogenesis of the Furnas trachytes, constrain the P-TfO 2 conditions under which they evolve, and investigate the temporal evolution of the magma plumbing system. Results indicate that the trachytes are derived predominantly from extended fractional crystallisation of alkali basalt parental magmas, at depths between ~3 and 4 km.Communicated by Jochen Hoefs. Electronic supplementary materialThe online version of this article

The oceanic-island magmatic systems of the Atlantic Ocean exhibit significant diversity in their respective sizes, ages, and the compositional ranges of their eruptive products. Nevertheless, almost all of the Atlantic islands and island groups have produced peralkaline felsic magmas, implying that similar petrogenetic regimes may be operating throughout the Atlantic Ocean, and arguably elsewhere. The origins of peralkaline magmas are frequently linked to low-degree partial melting of enriched mantle, followed by protracted differentiation in the shallow crust. However, additional petrogenetic processes such as magma mixing, crustal melting, and contamination have been identified at numerous peralkaline centers. The onset of peralkalinity leads to magma viscosities lower than those typical for metaluminous felsic magmas, which has profound implications for processes such as crystal settling. This study represents a compilation of published and original data which demonstrates trends that suggest that the peralkaline magmas of the Atlantic Ocean islands are generated primarily via extended (up to ∼ 95%), open system fractional crystallization of mantle-derived mafic magmas. Crustal assimilation is likely to become more significant as the system matures and fusible material accumulates in the crust. Magma mixing may occur between various compositional end-members and may be recognized via hybridized intermediate magmas. The peralkaline magmas are hydrous, and frequently zoned in composition, temperature, and/or water content. They are typically stored in shallow crustal magma reservoirs (∼ 2-5 km), maintained by mafic replenishment. Low melt viscosities (1 × 10 1.77 to 1 × 10 4.77 Pa s) facilitate two-phase flow, promoting the formation of alkali-feldspar crystal mush. This mush may then contribute melt to an overlying melt lens via filter pressing or partial melting. We utilize a three-stage model to account for the establishment, development, and termination of peralkaline magmatism in the ocean island magmatic systems of the Atlantic. We suggest that the overall control on peralkaline magmatism in the Atlantic is magma flux rate, which controls the stability of upper crustal magma reservoirs. The abundance of peralkaline magmas in the Atlantic suggests that their development must be a common, but not inevitable, stage in the evolution of ocean islands.

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