Analyses of Volatiles in Volcanic Glasses: Bridging the Gap between the Macroscopic and the Micron Scale

简介：在地球上，火山喷发的岩浆通常含有一定量的挥发性成分（主要是H2O、CO2、S、Cl、F）。在岩浆向地表上升的过程中，这些挥发物倾向于分离成蒸汽相（“脱气”），这通常为火山爆发提供动力。火山喷发的“爆炸性”在很大程度上取决于这种以气泡形式存在的气相能否轻易地与熔体相分离。从这个意义上说，挥发性成分真正发挥了主要的控制作用，决定了一次喷发最终是喷涌式的（例如夏威夷的熔岩流）还是爆炸性的（例如圣海伦斯火山的大火山羽流）。这些挥发物中剩余的未脱气的部分可以保存在火山玻璃中，火山玻璃在喷发时被淬灭（“间隙玻璃”），并且可以获得关于这种蒸汽相留下了多少熔融物的重要信息。此外，最初的挥发性物质有时可以被岩浆上升过程中形成的晶体中的熔体部分保存下来。这些“熔融包裹体”在喷发时也被冷却到玻璃状，通常被认为是岩浆初始挥发性内容的编年史。通过汇编在熔体包裹体和间隙玻璃中收集的大量挥发性数据，可以对各种不同喷发的气体分离机制和脱气程度进行限制。然而，对于更古老的喷发（1000-2000年）的产物，外部挥发物，如大气中的水，可以渗透到玻璃结构中，叠加出纯粹的“岩浆”信号。事实上，通过测量火山岩中“水合作用”的厚度，大气中的水被纳入玻璃的趋势已经被用于考古年代测定。因此，量化玻璃中挥发性成分的一个关键挑战是区分初级（岩浆）和次级（大气）成分的能力。该项目探索了各种分析策略，以量化火山玻璃中的这两种成分，目的是追溯岩浆向地表上升的历史。这一建议的结果可以用于预测岩浆上升和喷发的物理模型。技术摘要：分析火山喷发产物（即幼年火山碎屑及其矿物相中的熔融包裹体）中的挥发物是火山学界的一个重要目标。火山碎屑含有减压后期的挥发物（即在地质体玻璃中），以及它们从储层到导管（即在熔体包裹体中）的演化过程。因此，它们通常提供了唯一可用的岩浆储存和上升记录。电子探针分析Cl， F和其他物种很容易实现；然而，对于较轻的挥发物，如H2O或CO2，则不能这样说，这些挥发物通常使用基于光谱的技术（如FTIR）进行量化。遗憾的是，由于大多数技术获得的低空间分辨率，某些分析分歧仍然使我们无法表征囊状样品中的挥发物。微曼光谱是接近这一分辨率差距的一个很好的候选，并且已经用于量化熔体包裹体中的H2O。它的高分辨率（光斑直径~1-2µm）是理想的测量水在玻璃的微小区域，提供技术测试和其潜在的缺点定义。目前的项目建议测试和校准一个微罗曼光谱常规来测量水。在将该方法应用于自然样品集之前，将分析来自三个案例研究（维苏威火山79AD，陶波火山181AD和普乌瓦瓦瓦火山114ka）的火山碎屑，以确定其潜在的二次水化作用对总挥发性含量的贡献。浮石玻璃和熔融包裹体中的H2O测量将与利用EMPA获得的Cl分析相结合，以破译三次选定的火山喷发的大规模脱气历史。这部分调查将揭示维苏威火山和陶波火山喷发岩浆演化的未知方面，并提供机会描述夏威夷（Pu'u Wa'awa'a）硅火山作用的独特表现。这两种分析工具的极限将被推到最大，以表征微观尺度的变化及其在减压过程中对脱气的影响。特别要注意的是，在导管上升过程中，剪切局部化在促进挥发性析出中的作用。

Synopsis: On earth, magmas feeding volcanic eruptions generally contain a certain amount of volatile components (mostly H2O, CO2, S, Cl, F). These volatiles tend to separate into a vapor phase during ascent of magma towards the surface ("degassing"), which generally provides the driving force for volcanic eruptions. The "explosivity" of volcanic eruptions greatly depends on whether this vapor phase, which takes the form of gas bubbles, can easily separate from the melt phase or not. In this sense, volatile components truly exert the main control on whether an eruption ends up being effusive (example: lava flows in Hawaii) or explosive (example: large volcanic plumes at Mt St Helens). The remaining undegassed fraction of these volatiles can be preserved within volcanic glasses that are quenched upon eruption ("interstitial glasses"), and may yield essential information on how much of this vapor phase left the melt. In addition, the initial volatile contents can sometimes be preserved by fractions of melt trapped within the crystals that grow during magma ascent. These "melt inclusions" are also cooled to a glassy state upon eruption, and are often considered to be chronicles of the initial volatile contents of magmas. By compiling large sets of volatile data collected in melt inclusions and interstitial glasses, the mechanisms by which the gases separate and the extent of degassing can be constrained for a variety of different eruptions. Yet, for products of older eruptions (1000-2000 years old), external volatiles such as meteoric water can infiltrate the glass structure and overprint the purely "magmatic" signal. In fact, the tendency for meteoric water to be incorporated in glasses has been used for archaeological dating by measuring the thickness of "hydration" within volcanic rocks. Therefore, a key challenge in quantifying volatile components in glasses is the capacity to discriminate between primary (magmatic) and secondary (meteoric) components. This project explores a variety of analytical strategies to quantify these two types of components in volcanic glasses, with the objective of tracking back the history of magmas ascending towards the surface. The results from this proposal can then serve for predictive physical models of magma ascent and eruption. Technical abstract:Analyzing volatiles enclosed within the products of eruptions (i.e., juvenile pyroclasts and the melt inclusions trapped within their mineral phases) is an essential objective within the volcanology community. Pyroclasts contain volatiles from the late decompression history (i.e., within the groundmass glass), as well as from their evolution from the reservoir to the conduit (i.e., within melt inclusions). Thus, they usually provide the only available record of magmatic storage and ascent. Electron microprobe analyses of Cl, F and other species are easily achievable; however, the same cannot be said for lighter volatiles such as H2O or CO2, which are often quantified using spectroscopy-based techniques such as FTIR. Regrettably, a certain analytical divide still separates us from characterizing volatiles in vesicular samples due to the low spatial resolution attained by most techniques. MicroRaman spectroscopy is a good candidate to close-in on this resolution gap, and is already used to quantify H2O in melt inclusions. Its high resolution (spot diameter ~1-2 µm) is ideal for measuring water in tiny areas of glass, providing that the technique is tested and its potential drawbacks defined.The current project proposes to test and calibrate a microRaman spectroscopy routine to measure H2O. Prior to applying the methodology to sets of natural samples, pyroclasts from three case-studies (Vesuvius 79AD, Taupo 181AD, and Pu'u Wa'awa'a 114ka) will be analyzed for potential secondary hydration contributions to the total volatile content. H2O measurements in pumice glasses and melt inclusions will be then coupled with Cl analyses obtained using EMPA to decipher the large-scale degassing histories of the three selected eruptions. This part of the investigation will uncover unknown aspects of the magmatic evolution of both the Vesuvius and Taupo eruptions, and offer the opportunity to describe a unique expression of silicic volcanism in Hawaii (Pu'u Wa'awa'a). The limits of both analytical tools will be pushed to their maximum to characterize microscopic scale variations and their effects on degassing during decompression. In particular, significant attention will be given to the role of shear localization in facilitating volatile exsolution during ascent in the conduit.