Improving Absolute Paleointensity Experiments through Pressure Cycling

地球磁场保护地球表面免受有害的太阳辐射，提供一个保护层，保护我们的大气层和水圈，建立生命所需的条件，甚至保护现代通信卫星免受低强度太阳风暴的影响。然而，我们对地球磁场强度变化的理解，以及这些波动可能引起的任何伴随的破坏，仍处于起步阶段。从地磁观测站对磁场的直接测量只延伸到过去约150年，虽然这对人类来说意义重大，但这些地磁记录不包括地磁倒转、偏移或强度峰值等现象。为了研究这些重要的地磁行为，科学家们依靠岩石（如熔岩流）记录的磁化强度，这些岩石记录了地球磁场冷却时的方向和强度。然而，天然材料并不是理想的磁场记录者，而且由于磁性矿物的存在，发现古代磁场强度变化或“古强度”的努力受到了严重的阻碍，这些矿物的尺寸太大，无法准确记录地磁场。本研究旨在克服这一问题，通过将压力处理纳入常规方法，获得更可靠的古强度估计。初步的工作表明，压力循环优先去除较大磁性颗粒持有的有问题的磁化，同时保留较小（单畴）颗粒持有的更可靠的磁化基本上完好无损。在火山玻璃（黑曜石）上的压力实验，大大提高了我们估计导致岩石最终磁化的磁场的能力。这项研究的目的是在更常用的地质材料上测试这种新的压力方法，特别关注明尼苏达州11亿年前的中大陆裂谷的火山岩。更广泛的影响将通过早期职业博士后的指导，将本科生纳入研究，以及创建一个独特的非磁性压力单元来实现，该单元将允许来自整个科学界的访问研究人员更好地探索压力对岩石磁学研究所天然材料磁性的影响。热残余磁化过程，例如，岩石在经过居里温度冷却时如何获得永久磁化或“剩磁”，是古磁学的基础。本提案将研究热残余磁化如何受到应力能的增加和去除的影响。压力处理明显减小了对单畴磁理论古强度实验不利的残余类型（PSD和MD）。因此，pi认为他们可以克服一个长期存在的问题，并为压力如何改变PSD和MD晶粒中的热残余提供理论理解。这种新方法将潜在地提高古强度实验的成功率，目前的成功率在10-30%之间，从而节省大量的时间。从加压物质中收集古强度的能力对行星科学（如陨石和行星磁学）也具有特别重要的意义。这里提出的工作将进一步加深我们对压力如何影响古强度的理解，并且可以应用于各种不同的协议。随着压力的增加，对诱导和残余岩石磁性参数的持续监测将帮助我们了解磁性如何随着压力循环而变化，并将提高我们对常见磁性矿物磁性行为的理解。

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.