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

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录的当地磁北方向的重大变化。除了这些短期的变化之外，在地质历史期间，还有许多记录在案的磁场极性反转。在最近的地质时期，极性的变化已经被很好地记录下来，并提供了一个确定地质事件年代和建立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，在历史上，磁场强度下降了约10%，在更长的时间尺度上，记录了强度的五倍变化。对磁场变化的方向和强度以及在广泛的时间尺度上的全面描述是可取的，因为这些波动可以对磁场如何产生以及其他几个地球深部过程提供关键约束。与方向性变化相比，记录地球磁场过去的强度波动要困难得多，因此，相对而言，可靠的绝对强度（古强度）的测定很少。这种缺乏古强度信息的主要原因是难以识别主要含有确定古强度所需的非常细和稳定的磁性颗粒的地质材料。我们正在研究火山爆发产生的灰流凝灰岩作为古强度信息的潜在新材料的潜力。虽然这些矿床具有必要的精细磁性颗粒，但各种定位后过程可能会影响恢复古代场强信息的能力。作为对这种材料的测试，我们记录了侵位后的热历史，并确定了两个历史灰流样品的古强度。1980年的火山灰流在山上。圣海伦斯，华盛顿，和1912年从山流。位于阿拉斯加州万烟谷的卡特迈是一个天然的实验室，用于测试测得的古强度与已知的野外值。在这两个地点进行的直接温度测量限制了安放温度，并提供了关于安放后火山活动状况的重要信息。结合重点研究的老（0.76马），更好地暴露主教凝灰岩，我们的采样策略将使我们能够评估灰流的paleointensity分析的适用性，以及识别（在现场或实验室）样品的可行性最有可能提供可靠的结果。建议的工作将提供有价值的信息，以指导未来的工作人员在现场和实验室在选择合适的材料进行古强度分析。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测年。因此，如果可以从火山灰流中确定绝对古强度，就有可能编制一个更全面的地磁场过去场强波动数据库。除了该项目的科学目标外，该研究还为斯克里普斯海洋学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。明尼苏达大学学生的参与鼓励了更多的本科生参与？的NSF研究经验的本科生（REU）计划。

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.