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化学的窗口，让我们了解地球深处的内部，与地球表面相比，我们对地球深处的了解仍然很少。这些火山的岩石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的混合、脱气和分层的宝贵信息。因为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），所以以前的研究仅限于少数海洋岛屿。该项目将开发一种利用x射线计算机断层扫描（一种非破坏性3D成像技术）先验地识别富气矿物颗粒的技术。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部地球化学特征的能力。发展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一名博士后的职业发展，在加州理工学院进行本科培训，并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施建立新的合作关系。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的高分辨率x射线计算机层析数据。由于稀有气体往往存在于富含co2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又一批的斑晶中提取出来，然后提纯并送入质谱仪来测量它们的稀有气体（氦、氖、氩和氙）的同位素组成。通过减少大气污染，这一过程将提高我们在海洋岛玄武岩中观测到的地幔成分的同位素区分能力。此外，流体包裹体的填充压力将根据压力计压力读数计算。这些结果可用于推断稀有气体的圈闭深度，并提供将同位素结果与岩浆上升的特定阶段联系起来的工具。

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.