Using cumulates as an insight into explosive volcanic eruptions and a geochemical understanding of the magma reservoir below Tenerife

Abstract

Ocean island volcanoes are not only sizeable geological features, they can also be a window into deeper magmatic processes. The Canary Islands are of interest to study as they have been volcanically active for over 25 million years; considered an extensive period of ocean island volcanism. In particular, the island of Tenerife has developed a range of alkaline magma compositions and alongside a region of evolved crust, felsic magmas have been responsible for major explosive events. In conjunction with this explosivity, the island also produces voluminous effusive eruptions, creating a spectrum of possible volcanic hazards. Eruptions on Tenerife are comparatively low in frequency, so it is important to gain as much data as possible from past events. By recording the petrology and geochemistry of the erupted products, data can help us understand the internal magmatic structure and potentially why this system intermittently generates explosive eruptions. Despite the information available, the position and architecture of the magma reservoirs beneath the edifice is not fully understood. Questions remain as to the existence of crystal-mush reservoirs, their location in the crust and the degree of interaction between evolved phonolite and more primitive basanite magmas. Understanding this reservoir structure is consequently important in determining how large volumes of magma are assembled or mixed prior to large explosive eruptions.
Here, I used a set of samples from Tenerife to gain a new insight into the composition and structure of the sub volcanic crystal mush reservoir. These explosively fragmented juvenile nodules have been recovered from pyroclastic deposits so can be temporally constrained to a specific eruptive event. The juvenile nodules contain fresh cumulus crystals but uniquely also contain a microcrystalline groundmass that was supra solidus prior to eruption, providing a snapshot of the reservoir prior to eruption. This study presents a comprehensive petrological and geochemical analysis, including the major, trace elements and Pb isotopes, of the crystal and melt phases from multiple snapshots (five events in total) across the eruptive history over the past 2 Myrs on Tenerife. Physical separation of the crystals and groundmass phases has allowed us to determine the chemistry and composition of the crystal-rich domain of the magma reservoir. These results provide physical evidence of the existence of a crystal-rich mush, with the suite of samples representing the entire crystallisation sequence of alkaline system. Groundmass material is taken to represent the melt present in the reservoir and is typically around 26% by volume. Combining these data with the extensive chemical and isotopic record of Tenerife magmas has enabled an assessment of the degree of connectivity between co-existing reservoirs. The use of Pb isotopes supports the presence of separate crustal reservoirs, made of a well-mixed, melt dominated phonolite body which isotopically homogenises melts are inputted from an underlying heterogeneous crystal-mush reservoir.

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Identifiers

ORCID for Emma Louise Horn: orcid.org/0000-0003-0516-519X

ORCID for Rex Taylor: orcid.org/0000-0002-9367-0294

ORCID for Thomas Gernon: orcid.org/0000-0002-7717-2092

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