Conditions for mafic magma storage beneath fissure zones at oceanic islands. The case of São Miguel Island (Azores archipelago)

Abstract

Ponding conditions of basalts erupted from fissure zones at São Miguel Island (Azores) were investigated through microthermometry of fluid inclusions, hosted in olivines and clinopyroxenes, and whole-rock and mineral chemistry. The Região dos Picos and Achada das Furnas fissure zones formed between central volcanoes and erupted geochemically similar magmas for the last 30 kyr. Hydrocarbonic fluid inclusions that survived to diffusion and re-equilibration recorded a maximum density value of 1000 kg m 23 (29.3 km) at both fissure zones, at the intersection with the feeding systems of older central volcanoes. The maximum density decreases to 875 kg m 23 (23.5 km) towards the central segment of Região dos Picos. Further trapping of low-density fluids is occasional and limited to a few samples. While the deepest event is interpreted to mark the vertical variation of the Moho at each location, all other events are only representative of short-time ponding with fluid re-equilibration. The deepening of the Moho towards the central volcanoes might be the effect of long-lasting underplating. However, the periodic stretching of the lithosphere, in response to the differential stress field acting at central volcanoes, would have been responsible for the thinning of the crust at the central segment of the fissure zone.Supplementary material: Whole-rock geochemistry with international standard used, and representative analyses of selected mineral phases from the two segments of the Região dos Picos fissure

Deformation in a hyperslow oceanic rift: Insights from the tectonics of the São Miguel Island (Terceira Rift, Azores)

“…The young strombolian edifices are basaltic and monogenetic, which indicates that the presence of a clear magmatic reservoir is not realistic. We thus infer that the magma has been transported directly through the lithosphere to the surface through the main large‐scale channels [ Zanon , ; Zanon and Pimentel , ]. From the fault and dyke data at the surface, we infer that the rotation of the tectonic trends observed between depth and surface should have a secondary origin.…”

Magma Plumbing During the 2014–2015 Eruption of Fogo (Cape Verde Islands)

“…The clustering of pressures obtained by fluid inclusion barometry is commonly attributed to storage or temporary residence of magma at a discrete level, where formation and/or reequilibration of inclusions occur (e.g., Roedder, 1983;Hansteen et al, 1998;Zanon et al, 2003;Frezzotti and Peccerillo, 2004;Hansteen and Klügel, 2008;Zanon, 2015). Most pressures derived from CO 2 -dominated fluid inclusions in the 2014-2015 tephrites are in the range 240-340 MPa (Figure 8B), well below the range indicated by clinopyroxene-melt barometry.…”

Temporal evolution of a post-caldera, mildly peralkaline magmatic system: Furnas volcano, São Miguel, Azores

“…Zanon and Pimentel 2015). In contrast, basalts that ascend outside the confines of the caldera and avoid the shallow system reached the surface and were erupted as the monogenetic cones and fissure eruptions that are typical of the volcano's flanks and the neighbouring Congro fissure volcanic system (Zanon 2015). The generation of crystal poor silicic magmas, such as those of the UFG, is commonly accounted for through the application of models which involve the extraction of interstitial melt from large crystal mush bodies, citing processes such as compaction, microsettling, hindered settling, and gas-driven filter pressing (e.g.…”