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.

地球的磁场在不同的时间尺度上变化，也许最著名的例子是历史上记载的当地磁北方向的重大变化。除了这些短期变化外，在地质历史过程中，还有许多记录在案的磁场极性反转。现代地质时代的极性变化已经得到了很好的记录，这为确定地质事件的年代和确定构造板块过去的运动提供了关键方法。地球磁场的强度也表现出深刻的变化。例如，在历史时期，磁场强度下降了约10%，在更长的时间尺度上，记录到了强度的五倍变化。全面描述磁场在方向和强度上的变化以及在广泛的时间范围内的变化是可取的，因为这些波动可能对磁场的产生以及其他几个地球深部过程提供关键的限制。与方向变化相比，记录地球磁场过去的强度波动要困难得多，因此，相对较少可靠的绝对强度(古强度)测定可用。古强度信息的缺乏主要是由于难以识别主要含有确定古强度所需的非常细小和稳定的磁性颗粒的地质材料。我们正在研究火山喷发产生的火山灰流凝灰岩作为一种潜在的古强度信息新材料的潜力。尽管这种沉积物具有所需的细小磁性颗粒，但各种侵位后过程可能会潜在地影响恢复古场强信息的能力。作为对这种材料的测试，我们记录了侵位后的热历史，并确定了来自两个历史火山灰流动的样品的古强度。1980年的火山灰在山上流淌。圣海伦斯，华盛顿州和1912年从山上流过。阿拉斯加万烟之谷的Katmai提供了一个天然的实验室，用来测试测量的古强度和已知的野外数值。两个地点的直接温度测量限制了进驻温度，并且有关于进驻后富马林活动条件的重要信息。结合对年代较老(0.76 Ma)、暴露程度较高的毕晓普凝灰岩的重点研究，我们的采样策略将使我们能够评估火山灰流动对古强度分析的适宜性，以及识别(现场或实验室)最有可能提供可靠结果的样品的可行性。拟议的工作将提供有价值的信息，指导野外和实验室的未来工作人员选择合适的材料进行古强度分析。火山灰流动(许多具有高质量的放射性年龄)在世界范围内很常见，通常可以高精度地进行同位素测年。因此，如果可以从火山灰流动中确定绝对古强度，就有可能汇编一个更全面的关于过去地磁场场强波动的数据库。除了该项目的科学目标外，该研究还为斯克里普斯海洋研究所的一名研究生提供培训，并为明尼苏达大学的两名本科生提供培训。明尼苏达大学S本科生研究体验项目鼓励更多的本科生参与。