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.

海洋岛火山为地球深内部提供地球化学窗户，与其表面相比，该窗户的理解水平还差。这些火山中的岩石包含痕量丰富的崇高气体，这些气体随着时间的流逝提供了有关地球地幔的混合，脱气和分层的宝贵信息。由于大多数海洋岛样品都包含极低的重型贵重气体（Neon，Argon，K，K和Xenon），因此以前的研究仅限于少数海洋岛屿。该项目将开发一种使用X射线计算机断层扫描（一种无损的3D成像技术）的先验识别富含气体的矿物质的技术。识别和选择富含气体的晶粒将使大部分海洋样品的同位素分析能够提高地球化学表征深地球的能力。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使重型贵重同位素分析更容易获得。这项为期一年的试点研究将支持加州理工学院的博士后研究员的职业发展，以及与德克萨斯 - 奥斯汀大学的高分辨率X射线计算机层析成像设施的新合作伙伴。岛屿。由于贵重气体倾向于驻留在富含二氧化碳的夹杂物中，因此将选择具有最大液体包含体积的表晶以进行其他分析。将通过激光融合从近似苯晶中提取气体，然后将其纯化并馈入质谱仪，以测量其贵重气体（氦气，霓虹灯，氩气和Xenon）同位素组合物。通过最大程度地减少大气污染，此过程将提高我们在海洋岛玄武岩中观察到的地幔成分的同位素分量的能力。另外，将根据测量压力读数来计算流体夹杂物的填充压力。这些结果可用于推断贵重的气体夹层深度，并为将同位素结果与岩浆上升的特定阶段联系起来提供了一种工具。