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

用 X 射线计算机断层扫描寻找稀有气体：海洋岛玄武岩同位素表征的新微分析方法

基本信息

批准号：
1650308
负责人：
Kenneth Farley
金额：
$ 15.76万
依托单位：
California Institute of Technology
依托单位国家：
美国
项目类别：
Standard Grant
财政年份：
2017
资助国家：
美国
起止时间：
2017-02-01 至 2019-01-31
项目状态：
已结题

来源：
https://www.nsf.gov/awardsearch/showAward?AWD_ID=1650308&HistoricalAwards=false
关键词：
Searching Noble Gases ray Computed

项目摘要

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.

海岛火山提供了了解地球深处的地球化学窗口，而与地表相比，人们对地球内部的了解仍然知之甚少。这些火山的岩石含有微量丰富的惰性气体，这些气体提供了有关地幔随时间的混合、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），因此之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（一种无损 3D 成像技术）先验地识别富含气体的矿物颗粒的技术。识别和选择富含气体的颗粒将使我们能够对大部分海洋岛屿样本进行同位素分析，从而提高我们对地球深处进行地球化学表征的能力。开发和简化这项技术最终将使石油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博士后研究员的职业发展、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高分辨率 X 射线计算机断层扫描设备的新合作伙伴关系。将从夏威夷、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获得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，因此将选择具有最大流体包裹体体积的斑晶进行额外分析。通过激光聚变从一批斑晶中提取气体，然后纯化并送入质谱仪以测量其稀有气体（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区分在海岛玄武岩中观察到的地幔成分的同位素成分的能力。此外，流体包裹体的填充压力将根据测压压力读数进行计算。这些结果可用于推断稀有气体滞留深度，并提供将同位素结果与岩浆上升特定阶段联系起来的工具。