Collaborative Research: Time- and Temperature-dependent Cation Ordering in Natural Titanomagnetites with Applications to Paleomagnetism and Geospeedometry

本提案的主要目标是：(1)了解最近在天然钛磁铁矿中记录的时间和温度依赖的阳离子排序过程；(2)量化该过程的动力学，并开发基于钛磁体的地速计；(3)了解和模拟阳离子排序对磁性能的影响，以及对导热性（TRM）、部分导热性（pTRM）和热粘性剩余物（trvrm）的获取、保留和退磁的影响。新获得的数据表明，普通成分的天然钛磁铁矿在中等温度（300-500°C）和数小时至数月的时间尺度上经历了温度依赖的阳离子排序。因此，磁性居里温度（Tc）是先前热历史的一个强函数，变化高达150°C，没有伴随的化学变化。由于（重新）排序可能发生在ttc，这显然对我们对TRM获取和稳定性的理解具有深远的影响，并可能导致绝对古强度估计和古地磁古温学的不确定性增加。我们建议通过合成控制成分的样品，并通过粉末x射线衍射，Mössbauer光谱和x射线磁圆二色性（XMCD）进行表征来确定复杂钛磁铁矿中阳离子有序过程的性质。有序度将直接与居里温度和其他磁性特性联系起来。等温退火实验将允许我们约束有序过程的动力学，然后可以计算TC作为给定冷却路径的有序函数。这种钛磁铁矿地速计将在已知放置温度和冷却速率的地点对自然样品进行测试。最后，我们将通过一系列控制热历史的实验，确定依赖于时间和温度的阳离子重排序对剩余物获取和稳定性以及古强度估计的影响。实验室数据和模型将与自然样品的观察结果进行比较，其中阳离子的顺序预计会随着已知的冷却速率而系统地变化。铁钛氧化物矿物钛磁铁矿（常在火山岩中发现）获得的磁化强度为地磁场历史和构造板块运动提供了重要的信息来源。然而，钛磁铁矿矿物磁性的基本方面仍然没有得到充分的了解，特别是关于金属阳离子（Fe2+, Fe3+, Ti4+， Mg2+等）在氧化物晶体结构中的排列，阳离子排列如何随温度变化，以及导致重要磁性能的变化。最近已经观察到，普通成分的天然钛磁铁矿在中等温度（300-500°C）和数小时至数月的时间尺度上经历了温度依赖的阳离子重排序。这种阳离子重排序影响了钛磁铁矿的基本磁性，并影响了它们在冷却过程中记录环境磁场的机制，从而给含钛磁铁矿岩石的地磁场强度估计带来了不确定性。提出的工作旨在了解这种阳离子排序过程及其对磁化和磁性能的影响。此外，拟议的工作将提供对与过去地质现象（如火山事件或与埋葬有关的加热）相关的常用古地磁温度估计的不确定性的更大理解。这一结果可能对限制火山碎屑流（PF）的就位温度和冷却速率具有广泛的意义。火山碎屑流是火山气体、火山灰和岩石碎片的热混合物。PFs构成了最严重的火山危害之一，缓慢的冷却意味着这种危害在最初就位后可能会持续很长时间。对PFs进行的直接温度测量很少，而量化冷却速率和放置温度的新方法将有助于灾害规划。

The main goals of this proposal are (1) to understand the time- and temperature-dependent cation ordering process recently documented in natural titanomagnetites; (2) to quantify the kinetics of the process and to develop a titanomagnetite-based geospeedometer; and (3) to understand and model the effects of cation ordering on magnetic properties and on the acquisition, retention, and demagnetization of thermoremanent magnetization (TRM), partial thermoremanence (pTRM) and thermoviscous remanence (TVRM). Newly acquired data demonstrate that natural titanomagnetites of common composition undergo temperature-dependent cation ordering at moderate temperatures (300-500°C) and over timescales of hours to months. As a result, the magnetic Curie temperature (Tc) is a strong function of prior thermal history, changing by up to 150°C with no attendant chemical changes. As (re-)ordering may take place at T Tc, this clearly has profound implications for our understanding of TRM acquisition and stability, and may lead to increased uncertainty in absolute paleointensity estimates, as well as in paleomagnetic paleothermometry. We propose to determine the nature of the cation ordering process in complex titanomagnetites via synthesis of samples of controlled composition, and characterization via powder X-ray diffraction, Mössbauer spectroscopy, and X-ray magnetic circular dichroism (XMCD). The degree of order will then be directly linked to Curie temperature and other magnetic properties. Isothermal annealing experiments will allow us to constrain the kinetics of the ordering process, and TC can then be calculated as a function of order for a given cooling path. This titanomagnetite geospeedometer will be tested against natural samples from locations with known emplacement temperatures and cooling rates. Finally, we will determine the effects of time- and temperature-dependent cation reordering on remanence acquisition and stability, as well as on paleointensity estimates, via a series of experiments with controlled thermal histories. Laboratory data and models will be compared with observations on natural samples where cation ordering is expected to vary systematically with known cooling rates. Magnetization acquired by the iron-titanium oxide mineral titanomagnetite (frequently found in volcanic rocks) provides a vital source of information about geomagnetic field history and tectonic plate motions. Yet, there are fundamental aspects of titanomagnetite mineral magnetism that remain inadequately understood, particularly concerning the arrangement of metal cations (Fe2+, Fe3+, Ti4+, Mg2+, etc.) in the oxide crystal structure, how the cation arrangement may change with temperature, and resulting changes in important magnetic properties. It has been recently observed that natural titanomagnetites of common composition undergo temperature-dependent cation reordering at moderate temperatures (300-500°C) and over timescales of hours to months. This cation reordering affects the fundamental magnetic properties of the titanomagnetites, and influences the mechanisms through which they record the ambient magnetic field during cooling, thus introducing uncertainty into estimates of geomagnetic field intensity derived from titanomagnetite-bearing rocks. The proposed work aims to understand this cation ordering process and the resulting effects on magnetization and magnetic properties. Further, the proposed work will provide a greater understanding of uncertainty in commonly-used paleomagnetic estimates of temperatures associated with past geologic phenomena such as volcanic events or burial-related heating. The results may be of wide interest in constraining emplacement temperatures and cooling rates in pyroclastic flows (PF) -- hot mixtures of volcanic gas, ash and rock fragments. PFs constitute one of the most significant volcanic hazards, and slow cooling means the hazard may persist long after initial emplacement. Very few direct temperature measurements of PFs have been made, and new methods for quantification of cooling rates and emplacement temperatures will help in hazard planning.