Peralkaline Felsic Magmatism of the Atlantic Islands

Abstract

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.

Progressive Changes in Magma Transport at the Active Serreta Ridge, Azores

“…The highest pressures for the Serreta lavas are at~7 kbar, which is higher than those from Zanon and Pimentel (2015) based on fluid-inclusion thermobarometry (maximum at 5.75 kbar), and can be explained by the different response rates of fluidinclusion and clinopyroxene-melt barometers (Klügel et al, 2015). Jeffery and Gertisser (2018) have shown that peralkaline intraplate volcanoes from the Atlantic Ocean commonly have a magma reservoir in 2-to 5-km depth, where melts fractionate to trachytic compositions. The base of a central volcanoes is a common upper crustal level for stagnation of ascending magmas, where sedimentary layers and the volcanic edifice have lower densities (Klügel, Hansteen, & Galipp, 2005).…”

“…Dark grey bar shows the pressure range for the Moho beneath Terceira from Spieker et al (2018). The light grey field highlights the depth range for shallow magma reservoirs for Pico Alto volcano (and Guilherme Moniz volcano) on Terceira (Jeffery et al, 2017;Jeffery & Gertisser, 2018), which is similar to the typical depth range for peralkaline magmas in crustal reservoirs of Atlantic Ocean islands in general (Jeffery & Gertisser, 2018). The depth (km) refers to the crust beneath Serreta Ridge based on a density of 2800 kg/m 3 (Zanon & Pimentel, 2015).…”

“…Jeffery and Gertisser () have shown that peralkaline intraplate volcanoes from the Atlantic Ocean commonly have a magma reservoir in 2‐ to 5‐km depth, where melts fractionate to trachytic compositions. The base of a central volcanoes is a common upper crustal level for stagnation of ascending magmas, where sedimentary layers and the volcanic edifice have lower densities (Klügel, Hansteen, & Galipp, ).…”

“…Data sources are the same as in Figure 3.10.1029/2019GC008562from 843 to 862°C and 0.64 to 1.3 kbar, respectively (Figure 6). The temperatures for Santa Bárbara overlap with the temperature range for peralkaline felsic volcanism of Terceira of 773-936°C fromJeffery and Gertisser (2018), which were derived from studies of the neighboring Pico Alto volcano(D'Oriano et al, 2017;Jeffery et al, 2017).…”

Deep and disturbed: conditions for formation and eruption of a mingled rhyolite at Ascension Island, south Atlantic

“…Lower-flux ocean islands produce larger proportions of felsic melts [e.g. Brenna et al 2015;Chamberlain et al 2019;Jeffery Corresponding author: k.chamberlain@derby.ac.uk and Gertisser 2018]. These felsic melts commonly have higher water concentrations and melt viscosities and have a greater potential for hazardous explosive activity affecting areas distal even to the vent.…”