Searching for Noble Gases with X-ray Computed Tomography: New Microanalytical Methods for Isotopic Characterization of Ocean Island Basalts

海洋岛屿火山提供了地球内部深处的地球化学窗口，与其表面相比，人们对地球内部的了解仍然很少。这些火山的岩石中含有微量的稀有气体，这些气体提供了有关地球地幔随时间推移的混合，脱气和分层的宝贵信息。由于大多数海洋岛屿样本含有极低浓度的重惰性气体（氖，氩，氪和氙），以前的研究仅限于少数海洋岛屿。该项目将开发一种技术，利用X射线计算机断层扫描技术（一种无损三维成像技术）事先识别富含气体的矿物颗粒。识别和选择富含气体的颗粒将使更多的海洋岛屿样本的同位素分析成为可能，从而提高我们对地球深部进行地球化学表征的能力。发展和简化这项技术最终将使重惰性气体同位素分析更容易为岩石学家和地球化学家所用。这项为期一年的试点研究将支持博士后研究员的职业发展，在加州理工学院的本科生培训，以及与德克萨斯大学奥斯汀分校的高分辨率X射线计算机断层扫描设备的新伙伴关系。高分辨率X射线计算机断层扫描数据将获得从夏威夷，萨摩亚和胡安费尔南德斯群岛收集的火山岩中提取的橄榄石斑晶。由于稀有气体往往存在于富含CO2的包裹体中，因此将选择具有最大流体包裹体体积的斑晶进行额外分析。气体将通过激光聚变从斑晶中提取，然后纯化并送入质谱仪以测量其惰性气体（氦，氖，氩和氙）同位素组成。通过最大限度地减少大气污染，这一过程将提高我们的能力，同位素区分地幔成分在海洋岛屿玄武岩中观察到。此外，流体包裹体的充填压力将根据测压读数计算。这些结果可以用来推断惰性气体的圈闭深度，并提供了一个工具，将同位素结果与岩浆上升的特定阶段联系起来。

Ocean island volcanos provide geochemical windows into the Earth's deep interior, which remains poorly understood compared to its surface. Rocks from these volcanoes contain trace abundances of noble gases that have provided valuable information about the mixing, degassing, and stratification of the Earth's mantle over time. Because most ocean island samples contain extremely low concentrations of heavy noble gases (neon, argon, krypton, and xenon), previous studies have been limited to a handful of ocean islands. This project will develop a technique for identifying gas-rich mineral grains a priori using X-ray computed tomography, a nondestructive 3D imaging technique. Identifying and selecting the gas-rich grains will enable the isotopic analysis of a greater fraction of ocean island samples and thereby improve our ability to geochemically characterize the deep Earth. Developing and streamlining this technique will ultimately make heavy noble gas isotopic analysis more accessible to petrologists and geochemists. This one-year pilot study will support the career development of a postdoctoral fellow, undergraduate training at Caltech, as well as a new partnership with the High-Resolution X-ray Computed Tomography Facility at the University of Texas- Austin.High resolution X-ray computed tomographic data will be obtained for olivine phenocrysts extracted from volcanic rocks collected on Hawaii, Samoa, and Juan Fernandez islands. Because noble gases tend to reside in CO2-rich inclusions, phenocrysts with the greatest fluid inclusion volumes will be selected for additional analysis. Gases will be extracted from batches of phenocrysts by laser fusion, then purified and fed into a mass spectrometer to measure their noble gas (helium, neon, argon, and xenon) isotopic compositions. By minimizing atmospheric contamination, this process will improve our ability to isotopically distinguish mantle components observed in ocean island basalts. In addition, the filling pressures of the fluid inclusions will be calculated based on manometric pressure readings. These results can be used to infer noble gas entrapment depths and provide a tool for linking isotopic results to specific stages of magma ascent.