Chemical variability in peralkaline magmas and magma reservoirs: insights from the Khaggiar lava flow, Pantelleria, Italy

David A Neave

doi:10.1007/s00410-020-01678-0

Abstract

Silicic volcanoes are increasingly understood to be underlain by crystal-rich and vertically extensive magma reservoirs within which disequilibrium is widespread. Observations from ignimbrite deposits demonstrate that silicic magma reservoirs are often compartmentalised and compositionally stratified. However, it is currently unclear whether the small (i.e., < 0.1 km3 dense rock equivalent) eruptions that dominate activity at many volcanoes, and peralkaline volcanoes in particular, are fed from similarly complex magma reservoirs as their larger counterparts. Here I report petrographic and geochemical observations from the products of a small peralkaline eruption on Pantelleria, Italy, with the aims of unravelling peralkaline magma assembly processes and evaluating the complexity of reservoirs feeding small silicic eruptions. Matrix glass and whole-rock compositions from the Khaggiar lava flow and Cuddia Randazzo pumice cone reveal that erupted magmas were assembled from at least three distinct magma types stored within a compartmentalised magma reservoir: trachytes, less-evolved pantellerites and evolved pantellerites. Chemical variability in the Khaggiar lava flow was created by at least three distinct processes: the accumulation of evolved macrocrysts into evolved pantellerites; the injection of trachytic magmas into less evolved pantellerites; and the accumulation of relatively primitive macrocrysts into trachytic magmas. Macrocryst textures indicate that both trachytic and pantelleritic domains of the magma reservoir experienced numerous recharge events prior to the one that ultimately triggered eruption. Overall, magmas forming the Khaggiar lava flow and Cuddia Randazzo pumice cone appear to have been assembled in analogous ways to those erupted in much larger events. My observations are in good agreement with those from other peralkaline volcanoes and confirm that magma mingling, crystal cannibalism and macrocryst entrainment are as ubiquitous in peralkaline systems as they are in their calc-alkaline counterparts.

Highlights

Chemical variability and magma reservoir complexityMany silicic volcanoes are underlain by complex magma reservoirs characterised by high average crystal contents (Marsh 1989, 2004; Hildreth 2004; Cooper and Kent 2014; Bachmann and Huber 2016; Cashman et al 2017; Magee et al 2018; Edmonds et al 2019)
Lava and pumice samples have different matrix glass compositions, indicating that at least two discrete pantelleritic magmas contributed to the eventual eruption
These two magmas are variably peralkaline and can be characterised according to their degree of evolution, they cannot be related by crystallisation along a liquid line of descent

Summary

Introduction

Chemical variability and magma reservoir complexityMany silicic volcanoes are underlain by complex magma reservoirs characterised by high average crystal contents (Marsh 1989, 2004; Hildreth 2004; Cooper and Kent 2014; Bachmann and Huber 2016; Cashman et al 2017; Magee et al 2018; Edmonds et al 2019). Tholeiitic and peralkaline affinities have been recorded in intraplate (e.g., Gran Canaria, Canary Islands; Freundt and Schmincke 1992; Troll and Schmincke 2002), oceanic rift (e.g., Askja, Iceland; Sigurdsson and Sparks 1981) and continental rift (e.g., Pantelleria, Italy; Mahood and Hildreth, 1986) settings, respectively, suggesting that chemical variability and stratification are widespread features of silicic magma reservoirs. Major events like the climactic Mt. Mazama eruption (47 ± 9 km dense rock equivalent; DRE) elegantly demonstrate how chemically distinct magmas can interact during large eruptions, much less is known about how magmas interact during the small (i.e., < 0.1 km3) that dominate eruptive activity at many volcanoes

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