J.A. Tarduno, R.D. Cottrell and A.V. Smirnov
Department of Earth and Environmental Sciences, University of Rochester, USA
Arguably the most reliable way to determine the past strength of the magnetic field is the Thellier technique whereby igneous rocks are subjected to a stepwise progression of paired heating steps. Unfortunately, many whole rock samples are unsuitable for Thellier analysis. One complicating factor is the growth of new magnetic minerals associated with the thermal alteration of clays (which are ubiquitous in pre-Quaternary whole rock samples) during Thellier experiments. Recently we have developed an approach to measure paleointensity using single plagioclase crystals separated from basalt. These crystals are affected less by experimental alteration. Transmission electron microscopy and a host of rock magnetic methods reveal that these crystals contain small single to pseudo-single domain, equant to slightly elongated magnetic inclusions (50 to 350 nm). In a test of the method, Thellier analyses of plagioclase crystals from a 1955 Hawaiian lava flow yielded an intensity that agreed with that recorded by magnetic observatories. Here we discuss applications of the method to the geomagnetic field during superchrons and that of the early Earth. If relationships exist between the frequency of geomagnetic reversals and the morphology, secular variation and intensity of Earth's magnetic field, they should be best expressed during superchrons, intervals tens of millions of years long lacking reversals. We have been studying secular variation (recorded by whole rock samples) and paleointensity (recorded by single plagioclase crystals) from lavas of the Cretaceous Normal Polarity Superchron that formed at high latitudes near the tangent cylinder. These data suggest a time-averaged field that is remarkably strong and stable. When combined with global results available from lower latitudes, these data define a field that is overwhelmingly dipolar, lacking significant octupole components. Paleointensity data from Proterozoic-Archean rocks are of interest as this interval brackets the time suggested in some models for the initiation of inner core growth. We have been investigating this interval through Thellier analyses of plagioclase separated from mafic dikes. Preliminary results from 2.45 Ga dikes of northern Russia suggest a field strength similar to that of the last 5 million years.