Long-term Strength of the Geomagnetic Field Based on Thellier Analyses of Plagioclase Crystals

基于斜长石晶体Thellier分析的地磁场长期强度。近1.6亿年地磁倒转年代学的基本特征已经确定。然而，这段历史与该领域的其他特征之间的关系一直难以捉摸。测定过去的场强（古场强）尤其具有挑战性。包括加热实验在内的许多熔岩的古强度测量（即Thellier数据）显示了蚀变的影响。单斜长石晶体可以包含微小的磁性包裹体，50-350纳米大小，是一种高保真的现场记录；它们比大块熔岩样本更不易受蚀变的影响。如果地磁反转频率与古强度之间存在关系，那么这种关系最好表现在超年代期，即很少（或没有）反转的时间间隔。先前的数据集基于白垩纪正极性超时（大约8300 - 1.2亿年前）喷发的熔岩中的单个斜长石晶体，表明存在一个强大而稳定的场。在中等反转（少于1次/百万年）和非常快速反转（大于10次/百万年）时期形成的单个斜长石晶体的早期数据表明，磁场较弱且变化较大。这些数据与反转频率与古强度成反比关系。人们可以通过取样二叠纪-石炭纪（基亚曼）反极性超时线来检验这种关系是否是长期地球动力学的一个强有力的特征。在英格兰北部、苏格兰和挪威形成于大约2.5亿到3.3亿年前的镁铁质岩石将被取样用于这样的测试。通过将收集的古地磁和古强度数据来定义地磁场的性质，将引起对地核和地幔过程感兴趣的广大科学家的兴趣。这些研究将构成博士论文的基础，并将涉及罗切斯特大学的本科生，他们将为高级论文而努力。

Long-term strength of the geomagnetic field based on Thellier analyses of plagioclase crystals.The basic features of the geomagnetic reversal chronology of the last 160 million years are well established. The relationship between this history and other features of the field, however, has been elusive. The determination of past field strength (paleointensity) is especially challenging. Paleointensity measurements involving heating experiments (i.e. Thellier data) from many lavas show effects of alteration. Single plagioclase crystals can contain minute magnetic inclusions, 50-350 nm in size, that are an alternative high-fidelity field recorder; they are less susceptible to alteration than bulk lava samples. If there is a relationship between geomagnetic reversal frequency and paleointensity it should be best expressed during superchrons, intervals with few (or no) reversals. Thellier data sets based on single plagioclase crystals from lavas erupted during the Cretaceous Normal Polarity Superchron (approximately 83-120 million years ago) suggest a strong, stable field. Thellier data from single plagioclase crystals formed during times of moderate (less than 1 reversal/million years) and very rapid (greater than 10 reversals/million years) reversal occurrence suggest a weaker and more variable field. These data are consistent with an inverse relationship between reversal frequency and paleointensity. One can test whether this relationship is a robust feature of the long-term geodynamo by sampling the Permo-Carboniferous (Kiaman) Reversed Polarity Superchron. Mafic rocks in northern England, Scotland and Norway that formed between approximately 250 and 330 million years ago will be sampled for such a test. The definition of the nature of the geomagnetic field by the paleomagnetic and paleointensity data that will be collected will be of interest to a broad range of scientists interested in the core and mantle processes. The studies will form the basis of a Ph.D. thesis and will involve University of Rochester undergraduates who will work toward Senior theses.