The Nature of the Archean Geomagnetic Field

地磁场的起源对于了解地球深部、地表环境和大气的演化具有重要意义。但是，定义早期地磁场的性质是具有挑战性的；即使是保存最完好的太古宙（超过25亿年）的岩石也经历了与它们形成后的地质事件有关的低级变质作用（在200摄氏度到320摄氏度之间）。预计这些岩石将获得后来的磁化，从而排除了使用常规古强度技术的可能性。一种利用CO2激光加热和直流SQUID磁力计测量的新方法，提供了一种从含有磁铁矿包裹体的单一硅酸盐晶体中获得古方向和强度的方法。利用这些技术，32亿年前南非太古宙岩石的磁场强度与现在的强度相差不到50%。这表明一个可行的磁层保护了早期的地球大气免受太阳风的侵蚀。在南非和斯威士兰发现了年龄超过32亿年的岩石，这些岩石可能保存着3至4亿年的地磁场行为记录，保存在单个硅酸盐晶体中。新的古强度方法也可用于研究这些岩石。如果在这段时间内地磁场存在，这些数据应该提供其强度的界限，它们可以用来推断场的形态。对最早地磁场性质的这种限制，引起了广泛的对地核和地幔过程感兴趣的科学的兴趣。这些调查的更广泛的影响是，他们形成了研究生和本科生的论文研究的基础。学生参加实地和实验室工作，并协助K-12活动，将罗切斯特大学的研究生和本科生教学工作与地区学校项目相结合。

The origin of the geomagnetic field is important for understanding the evolution of Earth's deep interior, surface environment and atmosphere. But defining the nature of the early geomagnetic field is challenging; even the best preserved rocks from the Archean eon (more than 2.5 billion-years-old) have seen low grade metamorphism (at 200 degrees C to 320 degrees C) related to geologic events after their formation. The acquisition of later magnetizations by these rocks is expected, precluding use of conventional paleointensity techniques. A newly developed approach to the problem, utilizing CO2 laser heating and DC SQUID magnetometer measurements, provides a means to obtain paleodirections and intensities from single silicate crystals which host magnetite inclusions. Using these techniques, 3.2 billion-year-old field strengths have been reported from Archean rocks of South Africa that are within 50% of the present-day value. This indicates that a viable magnetosphere sheltered the early Earth's atmosphere from solar wind erosion.Rocks older than 3.2 billion years old are found in South Africa and Swaziland, potentially holding a 300 to 400 million-year-long record of geomagnetic field behavior preserved in single silicate crystals. The new paleointensity approach can also be used to examine these rocks. If the geomagnetic field was present during this interval, these data should provide bounds on its strength, and they may be used to make inferences on field morphology. Such constraints on the nature of the earliest geomagnetic field are of interest to a broad range of sciences interested in core and mantle processes.Among the broader impacts of these investigations are that they form the basis for the dissertation studies of graduate and undergraduate students. Students take part in field and laboratory work, and assist in K-12 activities, integrating graduate and undergraduate teaching efforts at the University of Rochester with area school programs.