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

在地球上，供给火山爆发的岩浆一般含有一定量的挥发性组分（主要是H2O，CO2，S，Cl，F）。这些挥发性物质在岩浆上升到地表（"脱气"）的过程中倾向于分离成气相，这通常为火山爆发提供了动力。火山爆发的"爆炸性"在很大程度上取决于这种以气泡形式存在的汽相是否容易与熔体相分离。从这个意义上说，挥发性成分真正发挥了主要的控制作用，决定了火山爆发最终是喷涌性的（例如：夏威夷的熔岩流）还是爆炸性的（例如：圣海伦山的大型火山羽）。这些挥发物中剩余的未脱气部分可以保存在火山玻璃中，这些火山玻璃在喷发时淬火（“间隙玻璃”），并可能产生关于有多少蒸汽相离开熔体的重要信息。此外，最初的挥发性成分有时可以通过在岩浆上升过程中生长的晶体中捕获的熔体部分来保存。这些“熔融包裹体”在喷发时也被冷却到玻璃态，通常被认为是岩浆初始挥发性成分的编年史。通过编制大量的挥发性数据收集的熔融包裹体和填隙玻璃，气体分离的机制和脱气的程度可以限制各种不同的喷发。然而，对于更老的喷发（1000 - 2000年）的产物，外部挥发物，如大气水可以渗透到玻璃结构中，并覆盖纯粹的“岩浆”信号。事实上，在考古学上，通过测量火山岩中“水合作用”的厚度，已经将大气降水结合到玻璃中的趋势用于测年。因此，量化玻璃中挥发性成分的一个关键挑战是区分主要（岩浆）和次要（流星）成分的能力。该项目探索了各种分析策略，以量化火山玻璃中这两种类型的成分，目的是追溯岩浆上升到表面的历史。这一建议的结果，然后可以为预测物理模型的岩浆上升和喷发。技术摘要：分析喷发产物中的挥发物（即，幼年火山碎屑和被捕获在其矿物相中的熔融包裹体）是火山学界的一个基本目标。火山碎屑含有晚期减压历史的挥发物（即，在基质玻璃内），以及从它们从储存器到导管的演变（即，在熔融夹杂物内）。因此，它们通常提供了岩浆储存和上升的唯一可用记录。Cl、F和其他物质的电子探针分析很容易实现;然而，对于较轻的挥发物（如H2O或CO2），情况并非如此，这些挥发物通常使用基于光谱的技术（如FTIR）进行定量。遗憾的是，由于大多数技术获得的空间分辨率低，某些分析鸿沟仍然将我们与表征泡状样品中的挥发物分开。显微拉曼光谱是一个很好的候选人，以关闭在这个分辨率差距，并已被用于量化熔融包裹体中的H2O。它的高分辨率（光斑直径~1 - 2 µ m）非常适合测量玻璃微小区域中的水，前提是该技术经过测试并确定其潜在的缺点。当前项目建议测试和校准用于测量H2O的显微拉曼光谱仪程序。在将该方法应用于天然样品集之前，将分析来自三个案例研究（维苏威火山79AD，陶波181AD和Pu'u Wa 'awa'a 114ka）的火山碎屑对总挥发物含量的潜在二次水合作用贡献。浮石玻璃和熔融包裹体中的H2O测量将与使用EMPA获得的Cl分析相结合，以破译三次选定喷发的大规模脱气历史。调查的这一部分将揭示维苏威火山和陶波火山爆发的岩浆演化的未知方面，并提供机会来描述夏威夷（Pu'u Wa 'awa'a）火山活动的独特表现。这两种分析工具的极限将被推到最大限度，以表征微观尺度的变化及其对减压过程中脱气的影响。特别是，显着的注意力将被给予的作用，剪切局部化，在促进挥发物出溶上升过程中的导管。

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.