2025-L12 The role of volatiles in explosive alkaline eruptions

Overview

Volatiles such as H₂O, CO₂, S, F, and Cl are fundamental components of magmas, inherited from their source regions and play a critical role in a number of magmatic processes. Their solubility in magmas fluctuates during magma ascent due to variations in pressure and temperature, leading to volatile saturation. This saturation triggers the exsolution of fluids and the formation of bubbles, significantly altering magma’s physical properties, such as viscosity. These changes can, in some instances, remobilise stagnant, crystal-rich mushes and trigger explosive volcanic eruptions. 

Beyond eruption dynamics, volatiles also influence the characteristics of volcanic deposits. Ignimbrites, deposits formed by pyroclastic density currents (PDCs), can either remain unconsolidated or weld during deposition, with the volatile content of the magma playing a potential role in this distinction. Understanding volatile evolution from deep-seated magma mushes to surface deposits is therefore crucial for assessing the most hazardous components of explosive volcanism. 

Current state-of-the-art techniques for reconstructing pre-eruptive volatile contents primarily rely on melt inclusion studies. However, these inclusions are susceptible to post-entrapment modifications, which can reset or alter volatile records over timescales of hours to years (e.g., Hartley et al., 2014). Recent research has identified the mineral apatite as a more resilient alternative proxy for tracking magmatic volatiles (e.g., Stock et al., 2016). Apatite’s crystal structure can incorporate a variety of volatile species, retaining a more complete record of magmatic volatile contents even when the surrounding glass is degassed. Additionally, apatite can host trace and redox-sensitive elements, providing insights into pre-eruptive magma storage conditions (Miles et al., 2013). However, the small grain size of apatite often limits detailed analysis of its volatile record. 

In contrast, haüyne, a rare but frequently large S-rich mineral found in highly Si-undersaturated magmas, has been comparatively underutilised as a monitor of volatile evolution and excess degassing. The mineral’s size offers a unique opportunity for detailed chemical and textural analysis. Preliminary studies suggest that haüyne’s textural varieties may reflect volatile sparging episodes during magma accumulation (Cooper et al., 2015). 

This project is the first to propose an integrated approach using both apatite and haüyne to create a detailed, complementary record of volatile evolution leading up to a Plinian eruption. The study will focus on the 668 ka Arico ignimbrite, a rare welded ignimbrite deposit formed by the eruption of the Las Cañadas volcano, Tenerife. Cutting-edge analytical techniques (SEM, EPMA, LA-ICP-MS, and SIMS) will be employed for in situ textural and chemical analyses of these two crystal types. This combined methodology offers an unprecedented opportunity to trace the processes leading up to one of the most significant eruptions of the Las Cañadas volcano, while exploring the factors responsible for the welding of pyroclastic density current deposits. 

The broader implications of this study extend beyond volcanic hazard assessment. Understanding how volatiles behave in magmas not only sheds light on eruption dynamics but also offers insights into the formation of hydrothermal ore deposits, which are often associated with volatile-rich volcanic centers. 

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.