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融化释放出来。挥发性元素通过板块构造在地球表面和内部之间循环。在俯冲带，来自海底的岩石重新进入地球深处，携带着碳、硫和卤素(F、Cl、Br和I)等挥发性元素。数百万年来，这一过程在地幔中挥发性元素的分布方面产生了相当大的变异性，这反过来又控制了在现代火山喷发中影响地球表面环境的挥发性物质的预算和流量：挥发性含量高的岩浆往往会产生更多爆炸性和危险的喷发，而火山气体排放对区域人口的健康构成危害。虽然岩浆的挥发性成分与供应它们的地幔来源的组成有内在的联系，但控制不同地幔来源的挥发性库存的过程仍然不确定。了解地球深层挥发性成分的起源和随时间的变化是地球科学中的一个根本挑战。在这个项目中，我们的目标是发现为主要玄武岩喷发提供动力的独特地幔来源的挥发性特征和清单。为此，我们将研究硅酸盐熔体包裹体的挥发性含量。硅酸盐熔体包裹体是困在不断生长的火山矿物中的岩浆小块，在岩浆喷发到地表之前保持其溶解气体的含量。首先，我们将对三种不同地幔类型的挥发性清单进行基准测试。我们从冰岛选择了两个代表深部和浅部地幔来源的样本，以及一个来自加那利群岛的样本，代表了俯冲再循环的地幔成分。我们将重点关注卤素，这是地幔中回收材料的敏感地球化学示踪。我们的目标是定量了解挥发分循环进入地幔的机制和过程。其次，我们将开发新的3D成像和光谱技术来全面量化熔体包裹体的挥发分含量。我们将利用这些信息准确重建岩浆的挥发性预算，并提供量化地质水和碳循环的关键新信息。第三，我们将应用我们的新分析方法来分析最近Fagradalsfjall(冰岛，2021年3月至9月)和Chumre Vieja(拉帕尔马，2021年9月至12月)喷发的熔融包裹体。这两次喷发都在其喷发的岩浆成分和气体流量中显示出惊人的随时间变化的地球化学变化。我们的目标是将岩浆挥发分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，测试火山气体排放的时间变化是否最终受控于非均质地幔储层的差异熔融和采样。这项工作将为地幔的不均一性和挥发分地球化学循环提供新的见解，并将为正在进行的开发挥发分替代指标以预测火山活跃区喷发开始和停止的努力提供支持。

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.