2024-L8 Tracking volatile evolution in explosive alkaline eruptions

Overview

Volatiles like H2O, CO2, S, F and Cl are key components of magmas and are inherited from their source regions. Their ability to dissolve in magmas changes during magma ascent due to changes in pressure and temperature. When volatiles saturate, exsolved fluids and bubble expansion can change the physical properties (e.g. viscosity) of a magma, and in some cases may even help to remobilise stagnant, crystal-rich mushes and trigger explosive eruptions. In addition to controlling the eruption of a magma, volatiles are also likely to play an important part in controlling the nature of their deposits. Ignimbrites are the deposits formed by pyroclastic density currents, but the controls on whether these deposits remain unconsolidated or weld during deposition may also in part depend on the volatile content of the magma being erupted. Gaining a complete picture of volatile evolution from magma mush, to eruption, to surface deposit is therefore critical for understanding the most hazardous components of explosive volcanoes.        

Melt inclusion studies currently represent the state-of-the-art in reconstructing pre-eruptive magmatic volatile contents. However, melt inclusions are affected by a number of post-entrapment processes that can modify or reset their volatile contents on timescales of hours to years (e.g. Hartley et al., 2014). Recent studies have highlighted the potential of the mineral apatite as an alternative proxy for tracking volatiles in magmas (e.g. Stock et al., 2016). The apatite crystal structure can host a range of volatile species that are important for understanding volatile budgets. Apatite is more retentive of these elements than many silicate minerals and glasses, and it can preserve a record of magmatic volatile contents even where the glass is largely degassed. Apatite also hosts other trace and redox-sensitive elements that can be used to build a detailed picture of pre-eruptive magma storage conditions (Miles et al., 2013). However, the benefits of apatite are often offset by its small grain size, which can hinder detailed analytical study of its rich volatile record.  

By contrast, haüyne is rare, but often large S-rich mineral found in highly Si-undersaturated magmas. It has received comparatively little attention as a monitor of volatile evolution and excess degassing, but its size enables a detailed examination of its chemical and textural characteristics. Early studies have shown a fascinating range of textural varieties that have tentatively been shown to reflect episodes of volatile sparging during magma accumulation (Cooper et al., 2015).   

This project is the first to propose examining how apatite and haüyne can be used in tandem to provide a detailed and complementary record of volatile evolution leading up to a Plinian eruption. The project will focus on the 668ka Arico ignimbrite – a rare welded ignimbrite formed from the eruption of the Las Cañadas volcano, Tenerife. State-of-the-art analytical techniques (SEM, EPMA, LA-ICP-MS and SIMS) will be used to carry out in situ textural and chemical analyses of both crystal types. This combination provides an unrivalled opportunity to track the lead up to one of the most significant eruptions of the Las Cañadas volcano and the circumstances that led to the welding of its pyroclastic density current deposits. Understanding how volatile elements behave in magmas has important implications beyond unravelling the events that led up to eruptions, including the formation of hydrothermal ore deposits that are commonly found in association with many volatile-saturated, S-rich volcanic centres.     

Figure 1: The modern-day Teide volcano, Tenerife. Teide sits on top of the much larger Las Cañadas volcano from which the Arico ignimbrite formed 668 ka.