Magmatic volatiles in the fourth dimension

Lead Research Organisation: University of Manchester

Abstract

Volcanoes release large quantities of magmatic gases such as water, carbon dioxide and sulfur dioxide into the oceans and atmosphere. The volatile elements that make up these gases are stored in the Earth's mantle, and are released by partial melting of mantle rocks. Volatile elements are cycled between the Earth's surface and its interior by plate tectonics. At subduction zones, rocks from the sea floor re-enter the deep interior of the Earth, carrying with them volatile elements like carbon, sulfur, and halogens (F, Cl, Br and I). Over millions of years, this process has created considerable variability in the distribution of volatile elements in the mantle, and this in turn controls the budgets and fluxes of volatiles that impact the Earth's surface environments in modern-day volcanic eruptions: magmas with high volatile contents tend to produce more explosive and dangerous eruptions, while volcanic gas emissions pose health hazards to regional populations. While magma volatile contents are intrinsically linked to the composition of the mantle sources that supply them, the processes that control the volatile inventories of distinctive mantle sources remain uncertain. Understanding the origin and change over time of the volatile composition of the deep Earth is a fundamental challenge in the Earth Sciences.

In this project we aim to discover the volatile signatures and inventories of the distinctive mantle sources that feed major basaltic eruptions. To do this, we will study the volatile contents of silicate melt inclusions, which are tiny pockets of magma trapped in growing volcanic minerals that preserve the dissolved gas content of the magma before it erupts to the surface.

First, we will benchmark the volatile inventories of three distinctive mantle types. We have selected two samples from Iceland that represent deep and shallow mantle sources, and one sample from the Canary Islands that represents a subduction-recycled mantle component. We will focus on the halogens, which are sensitive geochemical tracers of recycled material in the mantle. We aim to derive a quantitative understanding of the mechanisms and processes by which volatiles are recycled into the mantle.

Second, we will develop new 3D imaging and spectroscopy techniques to fully quantify the volatile contents of melt inclusions. We will use this information to accurately reconstruct magma volatile budgets and provide new information central to quantifying the geological water and carbon cycles.

Third, we will apply our new analytical approaches to melt inclusions from the recent eruptions of Fagradalsfjall (Iceland, Mar-Sep 2021) and Cumbre Vieja (La Palma, Sep-Dec 2021). Both these eruptions showed astonishing time-dependent geochemical variability in their erupted magma compositions and gas fluxes. We aim to link temporal changes in magma volatile geochemistry with gas fluxes measured by ground-based sensors and from satellites, to test whether temporal changes in volcanic gas emissions are ultimately controlled by differential melting and sampling of heterogeneous mantle reservoirs.

This work will deliver new insights into mantle heterogeneity and volatile geochemical cycling, and will feed into ongoing efforts to develop volatile proxies to predict eruption onset and cessation in volcanically active regions.

Publications

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