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

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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.