Improving Absolute Paleointensity Experiments through Pressure Cycling

The Earth's magnetic field shields the surface of our planet from harmful levels of solar radiation, providing a protective envelope that retains our atmosphere and hydrosphere, establishing the conditions necessary for life, and even safeguarding modern communication satellites from low intensity solar storms. However, our understanding of variability of the strength of the Earth's magnetic field, and any concomitant disruptions these fluctuations might cause, is still in its infancy. Direct measurements of the field from magnetic observatories only extend ~150 years into the past, and while this is significant in human terms, this magnetic record does not include phenomena such as geomagnetic reversals, excursions, or intensity spikes. In order to study these important geomagnetic behaviors scientists rely on the magnetization recorded by rocks, such as lava flows, which record the direction and strength of the Earth's magnetic field as they cool. However, natural materials are not ideal magnetic recorders and efforts to uncover variations in the strength of the ancient magnetic field, or 'paleointensity', have been severely hampered by the presence of magnetic minerals whose dimensions are too large to allow them to accurately record the geomagnetic field. This research will aim to overcome this problem and obtain more reliable paleointensity estimates by incorporating pressure treatments into conventional methodologies. Preliminary work has shown that pressure cycling preferentially removes the problematic magnetizations held by larger magnetic grains, while leaving the more reliable magnetizations held by smaller (single domain) grains largely intact. Pressure experiments on volcanic glass (obsidian) have led to significant improvements in our ability to estimate the field responsible for a rock's final magnetization. This research aims to test this new pressure methodology on more commonly used geologic materials, with a special focus on the volcanic rocks of the 1.1 billion year-old Midcontinent Rift in Minnesota. Broader impacts will be achieved through the mentoring of an early career postdoc, the inclusion of undergraduate students in the research, and the creation of a unique, non-magnetic pressure cell that will allow visiting researchers from throughout scientific community to better explore the effects of pressure on the magnetism of natural materials at the Institute for Rock Magnetism.

Thermal remanent magnetization processes, e.g., how rocks acquire permanent magnetization, or 'remanences' when cooling through their Curie temperature, is fundamental to paleomagnetism. This proposal will examine how thermal remanent magnetizations are influenced by the addition and removal of stress energy. It appears that pressure treatments strongly minimize the types of remanence (PSD and MD) that are detrimental to paleointensity experiments, which are based on single domain magnetic theory. Hence, the PIs think they can overcome a longstanding problem and provide a theoretical understanding of how pressure changes thermal remanence in PSD and MD grains. This new method will potentially increase the success rate of paleointensity experiments, which currently ranges from 10-30%, leading to a significant time savings. The ability to gather paleointensities from pressurized materials is also of special importance to planetary sciences (e.g. meteorite and planetary magnetism). The work proposed here will further our understanding of how pressure influences paleointensities, and can be applied to a variety of different protocols. A continuous monitoring of induced and remanent rock magnetic parameters with increasing pressure will help us understand how magnetic properties change with pressure cycling and will generally improve our understanding of the magnetic behavior of common magnetic minerals.