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

地球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。除了这些短期变化之外，在地质历史上还有许多记录在案的磁场极性逆转。在最近的地质时期，极性的变化已经被很好地记录下来，并提供了一种确定地质事件年代和确定构造板块过去运动的关键方法。地球磁场的强度也有很大的变化。例如，磁场强度在历史上已经下降了大约10%，并且在更长的时间尺度上记录了强度的五倍变化。需要对磁场在方向和强度上以及在大范围时间尺度上的变化进行全面描述，因为这些波动可以对磁场的产生方式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，可靠的绝对强度（古强度）测定相对较少。这种古强度信息的缺乏主要是由于难以识别地质材料，这些地质材料主要包含确定古强度所需的非常细和稳定的磁性颗粒。我们正在研究由火山爆发产生的火山灰流凝灰岩作为古强度信息的潜在新材料的潜力。虽然这种沉积物具有必要的细磁颗粒，但各种放置后的过程可能会影响恢复古代磁场强度信息的能力。作为对这种材料的测试，我们正在记录放置后的热历史，并确定两次历史火山灰流样本的古强度。1980年华盛顿州圣海伦斯火山的火山灰流和1912年阿拉斯加万烟谷的卡特迈火山的火山灰流提供了一个天然实验室，可以根据已知的野外值来测试测量的古强度。在这两个地点的直接温度测量限制了安置温度，并提供了安置后富玛尔活动条件的重要信息。结合对更古老（0.76 Ma）、暴露程度更高的Bishop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对古强度分析的适用性，以及识别（在野外或实验室）最有可能提供可靠结果的样本的可行性。本文的工作将为指导今后野外和实验室工作者选择合适的材料进行古强度分析提供有价值的信息。火山灰流（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定年。因此，如果可以从火山灰流中确定绝对的古强度，就有可能编制一个关于过去地磁场强度波动的更全面的数据库。除了该项目的科学目标之外，该研究还为斯克里普斯海洋学研究所的一名研究生和明尼苏达大学的两名本科生提供培训。明尼苏达大学学生的参与鼓励了更多本科生的参与？美国国家科学基金会本科生研究经历（REU）项目。

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.