NSFGEO-NERC: Transforming understanding of paleomagnetic recording: Insights from experimental observations and numerical predictions

我们对岩石记录地磁场的方式的理解是基于一种分析理论，这种理论假设粒子是均匀磁化的。初看起来，剩磁获取的理论很简单，并提出了研究磁场矢量过去变化的方法。当磁性矿物在地球磁场中从高温冷却时，它们获得热回复磁化（TRM）。在居里温度以下的温度下，原子和晶体的固有磁能可以很容易地被热波动所克服，晶体的磁矩可以在局部能量最小值之间自由移动，与外部磁场平衡。在冷却到环境温度时，存在热能不再足以克服局部能量最小值之间的障碍的温度，并且磁化被冻结在特定配置中;磁化被“阻挡”。在阻塞期间弱磁场（如地球磁场）的存在会导致颗粒集合的净磁矩出现轻微偏差，从而产生与所施加的磁场平行且大小成比例的TRM（假设易磁化轴的各向同性分布）。 根据简单的理论，热源的自然剩磁与磁场几乎是线性相关的，在磁场中，它们被获得为地球的磁场一样低，它在原则上是可能的，以估计古代磁场的强度。 然而，在实践中，恢复古代的场强是复杂的。 这个简单的理论只适用于均匀磁化的物质。最近在数值模拟方面的突破为理解更大磁性颗粒的性质开辟了新的途径，从而有可能估计它们的时间和热稳定性。现在有机会将联合收割机的实验和数值方法结合起来，对古地磁记录进行全新的理解，并有可能改变古强度研究的方式，这是本项目的目标。该项目的更广泛的影响包括将由英国NERC资助的国际合作，在建模和实验室方法方面培训一名博士后和一名研究生，扩大STEM中代表性不足的群体的参与，开发古地磁领域的基础工具，以及通过MagIC社区数据库创建与更广泛社区共享的数据库。研究人员希望解决的具体问题是：1）现有理论预测，热剩磁取决于样品的冷却速率，但文献中声称，许多具有较大晶粒的样品显示没有冷却速率依赖性。 目前还不清楚为什么没有冷却速率依赖性，因此也不清楚如何处理这些数据。 2)现有的理论还预测，在温度T下获得的磁化应该通过零场再加热到T而退磁。 长期以来一直观察到这种"对等"要求的失败，但原因和后果不明。 3)最近的实验表明，与小晶粒的剩磁随时间的稳定性相反，大晶粒的剩磁是不稳定的，仅在几年内就表现出理论上意想不到的“老化”;是什么导致了这种老化是一个谜。 解开这些谜团是破解古地震烈度估算问题的关键。该提案利用新的计算资源来检查剩磁采集过程，以改变古地磁，特别是古强度实验的方式。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

Our understanding of the way in which rocks record the geomagnetic field is based on an analytical theory which makes the assumption that particles are uniformly magnetized. The theory of magnetic remanence acquisition is at first glance simple and suggests ways of studying past variations in magnetic field vectors. When magnetic minerals cool in the Earth's magnetic field from above high temperature, they acquire a thermal remanent magnetization (TRM). At temperatures just below the Curie Temperature, the intrinsic atomic and crystalline magnetic energies can be easily overcome by thermal fluctuations, and the magnetic moment of a crystal is free to move between local energy minima, coming into equilibrium with the external magnetic field. On cooling to ambient temperature, there is a temperature at which the thermal energy is no longer sufficient to overcome the barriers between local energy minima and the magnetization is frozen in a particular configuration; the magnetization is "blocked". The presence of weak magnetic fields (such as the Earth's) during blocking, causes a slight bias in the net magnetic moment of an assemblage of grains giving rise to a TRM that is parallel to and proportional in magnitude to the applied field (assuming isotropic distribution of easy axes). From the simple theory, natural remanence of thermal origin are nearly linearly related to the field in which they are acquired for fields as low as the Earth's, and it is in principle possible to estimate the strength of ancient magnetic fields. In practice, however, recovering the ancient field strength is complicated. The simple theory only pertains to uniformly magnetized material. Recent breakthroughs in numerical modeling have opened new avenues for understanding the nature of larger magnetic grains making it possible to estimate their temporal and thermal stability. The opportunity now exists to combine experimental and numerical approaches for a radical new understanding of paleomagnetic recording, with the potential to transform how paleointensity research is done, which is the goal of this project. The broader impacts of this project include international collaboration that will be funded by NERC in the UK, training of a named post doc and a graduate student in both modeling and laboratory methods, broadening of participation of underrepresented groups in STEM, development of foundational tools for the field paleomagnetism, and creation of a database to be shared with the broader community via the MagIC community database. The specific problems the investigators wish to address are: 1) Existing theory predicts that thermal remanence depends on a sample's cooling rate, yet there are claims in the literature that many samples with larger grains show no cooling rate dependence. It is unclear why there would be no cooling rate dependence, and therefore unclear how to treat such data. 2) Existing theory also predicts that a magnetization acquired at a temperature T should be demagnetized by zero-field reheating to T. Failure of this "reciprocity" requirement has long been observed but the causes and consequences are unknown. 3) Recent experiments have demonstrated that, in contrast to the stability of remanences in small grains over time, remanences in larger grains are unstable, exhibiting an "aging" over only a few years that is unexpected from theory; what causes this aging is a mystery. Solving these mysteries is key to cracking the problem of paleointensity estimation. This proposal exploits new computational resources to examine the process of remanence acquisition in order to transform how paleomagnetic, and paleointensity experiments in particular, are done.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.