Magma storage at Ocean Islands: insights from Cape Verde

Abstract

The Cape Verde archipelago is a group of Ocean Islands in the Central Atlantic that forms two chains of islands trending Northwest and Southwest. Several of the islands are considered to be volcanically active, with frequent eruptions on Fogo. We examine the mineral chemistry and thermobarometry of the southern islands; Santiago, Fogo and Brava together with the Cadamosto Seamount. Our objective is to explore the magmatic storage system and implications for volcanic eruptions and associated hazards at Cape Verde. The volcanic rocks at Cape Verde are alkaline and dominantly mafic, whereas the island of Brava and the Cadamosto Seamount are unusually felsic. Clinopyroxene compositions range from 60 to 90 Mg# at Santiago and Fogo. In contrast, at Brava and the Cadamosto Seamount the clinopyroxene compositions are 5 to 75 Mg#. Mineral chemistry and zonation records fractional crystallization, recharge, aggregation of crystals, magma mixing and variations in thermal conditions of the magma at temperatures from 925 to 1250ºC. Magma storage depths at Santiago, Fogo, Brava and the Cadamosto Seamount are between 12 and 40 km, forming deep sub-Moho magma storage zones. Transient magma storage in the crust is suggested by fluid inclusion re-equilibration and pre-eruption seismicity. A global compilation of magma storage at Ocean Islands suggests deep magma storage is a common feature and volcanic eruptions are often associated with rapid magma ascent through the crust. Shallow magma storage is more variable and likely reflects local variations in crustal structure, sediment supply and tectonics. Petrological constraints on the magma plumbing system at Cape Verde and elsewhere are vital to integrate with deformation models and seismicity in order to improve understanding and mitigation of the volcanic hazards.

Disequilibrium in Historic Volcanic Rocks from Fogo, Cape Verde Traces Carbonatite Metasomatism of Recycled Ocean Crust

“…Instead if we consider this range in Δ 18 Οcpx-gm at Fogo to be a primary magmatic feature, it is similar to the experimental range of clinopyroxene-melt fractionation with composition ranging from nephelinite to trachyandesite (Δ 18 Οcpx-melt -0.16 to − 1.34‰; Zhao and Zheng, 2003). The corresponding clinopyroxene crystals have compositions mostly in the range 60 to 85 Mg#, however the data also feature groups of clinopyroxene at 45 to 60 Mg# and 10 to 20 Mg# (Barker et al, 2021). This wide range of clinopyroxene compositions is consistent with equilibrium growth within a wide range of magma compositions.…”

“…Additionally, we observe antecrysts that were formed in equilibrium with different magma compositions, which have been aggregated together in the final host magma. Thus the antecrysts record variations in magma compositions present deep in the magma storage system where olivine and clinopyroxene crystallize (Barker et al, 2021;Hildner et al, 2011Hildner et al, , 2012Klügel et al, 2020;Mata et al, 2017). It appears that a wide range of magma compositions was available during the entire evolution of the magmas erupted at Fogo.…”

Reliability of Raman analyses of CO2-rich fluid inclusions as a geobarometer at Kīlauea

“…Understanding magma storage depth is crucial for interpreting volcanic signals, predicting eruptive changes and assessing the potential for volcanic unrest. This parameter is commonly determined using erupted materials and petrological tools like melt inclusion, mineral-mineral and mineral-melt barometry (Klügel et al, 2005;Putirka, 2008;Barker et al, 2021). However, many petrological tools present large uncertainties (i.e.…”

Reliability of Raman analyses of CO2-dominated fluid inclusions as a rapid barometer at Kīlauea

“…Determining the depths of magma storage is essential to interpreting signals of volcanic unrest and to help predict changes in eruptive style and vigour. Magma storage depth is commonly determined using erupted materials and petrological tools like melt inclusion, mineral-mineral and mineral-melt barometry (Klügel et al, 2005;Putirka, 2008;Barker et al, 2021). However, many petrological tools present large uncertainties (i.e., ±8-19 km clinopyroxene-based barometers, , and require extensive sample preparation efforts that make them unsuitable for real-time monitoring.…”