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了解地球深层的地球化学窗口，与地球表面相比，人们对地球深层的了解仍然很少。这些火山的岩石含有微量的惰性气体，这些气体提供了有关地幔随时间混合、脱气和分层的有价值的信息。由于大多数海洋岛屿样本含有极低浓度的重惰性气体(霓气、氩、氪和氙气)，以前的研究仅限于少数几个海洋岛屿。该项目将开发一种技术，利用X射线计算机断层成像技术，一种非破坏性的3D成像技术，预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我们能够对更大比例的海洋岛屿样本进行同位素分析，从而提高我们对地球深处的地球化学特征的能力。开发和简化这项技术最终将使岩石学家和地球化学家更容易进行重惰性气体同位素分析。这项为期一年的试点研究将支持加州理工大学博士后、本科生培训以及与德克萨斯大学奥斯汀分校高分辨率X射线计算机断层扫描设施的新合作伙伴关系。将获得从夏威夷、萨摩亚和胡安费尔南德斯群岛采集的火山岩中提取的橄榄石斑晶的高分辨率X射线计算机断层扫描数据。由于稀有气体往往存在于富含二氧化碳的包裹体中，因此将选择流体包裹体体积最大的斑晶进行额外分析。气体将通过激光聚变从一批斑晶中提取出来，然后提纯并送入质谱仪以测量它们的惰性气体(氦、霓气、氩和氙气)的同位素组成。通过将大气污染降至最低，这一过程将提高我们从洋岛玄武岩中观察到的地幔成分的同位素区分能力。此外，还将根据测压读数计算流体包裹体的充填压力。这些结果可以用来推断稀有气体的圈闭深度，并提供一种工具，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.