Magmatic volatiles in the fourth dimension

火山向海洋和大气中释放大量的岩浆气体，如水、二氧化碳和二氧化硫。构成这些气体的挥发性元素储存在地幔中，并通过地幔岩石的部分熔融释放出来。通过板块构造，挥发性元素在地球表面和内部循环。在俯冲带，来自海底的岩石重新进入地球的深处，携带着挥发性元素，如碳、硫和卤素（F、Cl、Br和I）。数百万年来，这一过程在地幔中挥发性元素的分布方面造成了相当大的变化，这反过来又控制了在现代火山爆发中影响地球表面环境的挥发物的预算和通量：挥发物含量高的岩浆往往会产生更具爆炸性和危险性的喷发，而火山气体排放则对区域人口的健康构成危害。虽然岩浆挥发性含量与提供它们的地幔源的组成有着内在的联系，但控制不同地幔源挥发性含量的过程仍然不确定。了解地球深处挥发性成分的起源和随时间的变化是地球科学的一个基本挑战。在这个项目中，我们的目标是发现提供主要玄武岩喷发的独特地幔源的挥发性特征和清单。为了做到这一点，我们将研究硅酸盐熔体包裹体的挥发性含量，这些包裹体是被困在生长的火山矿物中的微小岩浆袋，在岩浆喷发到地表之前保存了岩浆的溶解气体含量。首先，我们将对三种不同地幔类型的不稳定库存进行基准测试。我们选择了两个来自冰岛的样品，分别代表深部和浅层地幔源，一个来自加那利群岛的样品代表俯冲-再循环地幔成分。我们将重点关注卤素，它是地幔中回收物质的敏感地球化学示踪剂。我们的目标是对挥发物循环进入地幔的机制和过程进行定量理解。其次，我们将开发新的3D成像和光谱技术，以充分量化熔体包裹体的挥发性含量。我们将利用这些信息准确地重建岩浆挥发收支，并为量化地质水和碳循环提供新的信息。第三，我们将把我们的新分析方法应用于最近喷发的Fagradalsfjall（冰岛，2021年3月至9月）和Cumbre Vieja（拉帕尔马，2021年9月至12月）的熔体包裹体。这两次喷发在喷发的岩浆成分和气体通量上都显示出惊人的随时间变化的地球化学变异性。我们的目标是将岩浆挥发性地球化学的时间变化与地面传感器和卫星测量的气体通量联系起来，以测试火山气体排放的时间变化是否最终由非均质地幔储层的差异熔化和采样控制。这项工作将为地幔非均质性和挥发性地球化学循环提供新的见解，并将为正在进行的开发挥发性代用物的努力提供支持，以预测火山活跃地区的喷发开始和停止。

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.