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

本项目的主要目标是：（1）了解天然钛磁铁矿中最近记录的依赖于时间和温度的阳离子有序化过程;（2）量化该过程的动力学并开发基于钛磁铁矿的地质速度计;以及（3）理解和模拟阳离子有序化对磁性以及对热还原磁化（TRM）的获得、保持和退磁的影响，部分热剩磁（pTRM）和热粘剩磁（TVRM）。 新获得的数据表明，常见成分的天然钛磁铁矿在中等温度（300-500°C）下经历了依赖于温度的阳离子有序化，时间跨度为数小时至数月。 因此，磁居里温度（Tc）是先前热历史的强函数，变化高达150°C而没有伴随的化学变化。 由于（重新）排序可能发生在T Tc，这显然对我们理解TRM采集和稳定性有深远的影响，并可能导致绝对古强度估计的不确定性增加，以及在古地磁古测温。 我们建议通过合成控制组合物的样品，并通过粉末X射线衍射，穆斯堡尔谱和X射线磁性圆二色性（XMCD）表征，以确定复杂的钛磁铁矿中的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有关。 等温退火实验将使我们能够约束动力学的排序过程中，TC可以计算为一个给定的冷却path. This的顺序的函数titanomagnetite geospeedometer将测试对自然样品的位置与已知的侵位温度和冷却速率。 最后，我们将通过一系列受控热历史的实验，确定时间和温度依赖的阳离子重排对剩磁采集和稳定性的影响，以及对古强度的估计。 实验室数据和模型将与自然样品的观察结果进行比较，其中阳离子排序预计会随着已知的冷却速率而系统地变化。由铁-钛氧化物矿物钛磁铁矿（经常在火山岩中发现）获得的磁化强度提供了有关地磁场历史和构造板块运动的重要信息来源。然而，钛磁铁矿矿物磁性的基本方面仍然没有得到充分的理解，特别是关于金属阳离子（Fe 2+、Fe 3+、Ti 4+、Mg 2+等）的排列。在氧化物的晶体结构中，阳离子的排列方式可能会随着温度的变化而发生变化，并由此引起重要的磁性变化。最近已经观察到，普通组成的天然钛磁铁矿在中等温度（300-500°C）下和在数小时至数月的时间尺度上经历依赖于温度的阳离子重排。这种阳离子重新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，从而给来自含钛磁铁矿岩石的地磁场强度估计带来了不确定性。 拟议的工作旨在了解这种阳离子有序化过程以及由此产生的对磁化和磁性的影响。 此外，拟议的工作将提供一个更好的理解，在常用的古地磁估计与过去的地质现象，如火山事件或埋葬相关的加热温度的不确定性。 这些结果可能对限制火山碎屑流（PF）-火山气体、火山灰和岩石碎片的热混合物-中的就位温度和冷却速率具有广泛的意义。 PFs构成了最重要的火山灾害之一，缓慢冷却意味着灾害可能在最初的就位后持续很长时间。 对爆炸点进行的直接温度测量很少，量化冷却速率和安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.