Collaborative Research: An Evaluation of Ash Flow Tuffs as Geomagnetic Paleointensity Recorders

合作研究：灰流凝灰岩作为地磁古强度记录器的评估

• 批准号: 0943999
• 负责人: Jeffrey Gee
• 金额: $ 14.73万
• 依托单位: University of California-San Diego Scripps Inst of Oceanography
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2010
• 资助国家: 美国
• 起止时间: 2010-03-01 至 2014-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0943999&HistoricalAwards=false
• 关键词: Collaborative; Research; Evaluation; Ash; Flow

The Earth's magnetic field varies on a variety of time scales, with perhaps the best known example being the significant changes in the direction of local magnetic north documented in historic times. In addition to these short term variations, there are many documented reversals of the polarity of the field during geologic history. The change in polarity through recent geologic time has been well documented and provides a key method of dating geological events and establishing the past motions of tectonic plates. The Earth's magnetic field also exhibits profound variations in intensity. For example, the intensity of the field has decreased by approximately 10% in historic time and five-fold variations in intensity have been documented over longer time scales. A comprehensive description of magnetic field variations, in direction and intensity and over a wide range of time scales, is desirable as these fluctuations can provide critical constraints on how the magnetic field is generated as well as several other deep Earth processes. Compared with directional variations, documenting past intensity fluctuations of the Earth's magnetic field is much more difficult and, consequently, relatively few reliable determinations of absolute intensity (paleointensity) are available. This scarcity of paleointensity information is primarily the result of the difficulty in identifying geological materials that contain dominantly the very fine and stable magnetic particles that are required for determining paleointensity. We are investigating the potential of ash flow tuffs, generated by explosive volcanic eruptions, as a potential new material for paleointensity information. Although such deposits have the requisite fine magnetic particles, a variety of post-emplacement processes may potentially affect the ability to recover ancient field intensity information. As a test of this material, we are documenting the post-emplacement thermal history and determining paleointensity for samples from two historical ash flows. The 1980 ash flows at Mt. St. Helens, Washington, and the 1912 flows from Mt. Katmai in the Valley of Ten Thousand Smokes, Alaska provide a natural laboratory for testing measured paleointensities against known field values. Direct temperature measurements at both localities constrain the emplacement temperature, and significant information is available on the conditions of post-emplacement fumarolic activity. Combined with focused studies in the older (0.76 Ma), better-exposed Bishop Tuff, our sampling strategy will allow us to evaluate the suitability of ash flows for paleointensity analysis, as well as the feasibility of identifying (in the field or lab) samples most likely to provide reliable results. The proposed work will provide valuable information to to guide future workers in both the field and the laboratory in the selection of suitable materials for paleointensity analysis. Ash flows (many with high quality radiometric ages) are common worldwide and commonly can be isotopically dated with high precision. Thus, if absolute paleointensities can be determined from ash flows, it may be possible to compile a much more comprehensive database of past field intensity fluctuations of the geomagnetic field. In addition to the scientific goals of this project, the research is providing training for a graduate student at Scripps Institute of Oceanography and training of two undergraduate students at the University of Minnesota. Additional undergraduate student participation is being encouraged by participation of students from the University of Minnesota?s NSF-Research Experience of Undergraduates (REU) program.

地球的磁场在各种时间尺度上有所不同，也许最著名的例子是在历史时期记录的局部磁北方方向的重大变化。除了这些短期变化外，在地质史上，还有许多关于该领域极性的逆转。通过最近的地质时代的极性变化已得到充分记录，并提供了一种与地质事件约会并确定构造板的过去动作的关键方法。地球的磁场还表现出强度的深刻变化。例如，在历史时间内，该场的强度降低了约10％，并且在较长的时间尺度上已经记录了强度的五倍变化。需要对磁场变化，方向和强度以及在广泛的时间尺度上进行的全面描述，因为这些波动可以为如何产生磁场以及其他几个深层过程提供关键的约束。与方向变化相比，记录地球磁场的过去强度波动要困难得多，因此，可靠的绝对强度（古显着性）的可靠确定相对较少。古显着信息的这种稀缺性主要是难以识别地质材料的结果，而地质材料则主要包含确定紫色所需的非常精细且稳定的磁性颗粒。我们正在研究由爆炸性火山喷发产生的灰烬流凝灰岩的潜力，这是一种潜在的新材料，以实现生效信息。尽管这种沉积物具有必要的细磁颗粒，但各种置换后过程可能会影响恢复古代田间强度信息的能力。作为对该材料的测试，我们正在记录置换后的热历史记录，并确定来自两个历史灰分流的样品的古显度。 1980年在华盛顿州圣海伦斯山的灰烬流，以及阿拉斯加一万烟的卡特玛山的1912年流动，为针对已知田间值测试测量的毛细感应提供了自然实验室。在两个地方的直接温度测量都限制了安置温度，并且可以提供有关弹药后烟气活动条件的重要信息。结合较旧的（0.76 Ma），暴露较好的主教凝灰岩的重点研究，我们的抽样策略将使我们能够评估灰分流对古显着性分析的适用性，以及识别（在现场或实验室）样品最有可能提供可靠结果的可行性。拟议的工作将提供有价值的信息，以指导现场和实验室的未来工人，以选择合适的材料进行古显着分析。灰分流（许多具有高质量辐射年龄的人）在全球范围内都是常见的，通常可以高精度地同位素约会。因此，如果可以从灰分流中确定绝对的古敏度，则可能可以汇编地磁场的过去场强度波动的更全面的数据库。除了该项目的科学目标外，该研究还为明尼苏达大学的两名本科生的海洋学和培训提供了培训。明尼苏达大学NSF研究的本科生（REU）计划的学生参与的学生参与鼓励了其他本科生的参与。