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

合作研究：天然钛磁铁矿中时间和温度依赖性阳离子排序及其在古地磁学和地速测量中的应用

• 批准号: 1315971
• 负责人: Julie Bowles
• 金额: $ 24.44万
• 依托单位: University of Wisconsin-Milwaukee
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-06-01 至 2018-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1315971&HistoricalAwards=false
• 关键词: Collaborative; Research; Time; Temperature; dependent

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.

该提案的主要目标是（1）了解最近在天然钛磁铁矿中记录的时间和温度依赖性阳离子排序过程； （2）量化该过程的动力学并开发基于钛磁铁矿的地球源； （3）了解和建模阳离子排序对磁性性能以及热量磁化（TRM），部分热势（PTRM）和热雾透明（TVRM）的采集，保留和消灭磁性的影响。新获得的数据表明，常见组合物的天然钛磁铁矿在现代温度（300-500°C）和数小时至几个月的时间表上经历温度依赖性阳离子顺序。结果，磁性居里温度（TC）是先前的热史历史的强大功能，最多变化了150°C，而没有伴随的化学变化。由于（重新）订购可能在TC上进行，这显然对我们对TRM的获取和稳定性的理解具有深远的影响，并且可能导致绝对古显着性估计以及古磁性的古平均测度法的不确定性增加。我们建议通过合成受控组成样品的合成，并通过粉末X射线衍射，Mössbauer光谱和X射线磁性圆形二色性（XMCD）来确定复杂钛磁铁矿中阳离子排序过程的性质。然后，秩序程度将直接与居里温度和其他磁性特性联系在一起。等温退火实验将使我们能够约束订购过程的动力学，然后可以计算出TC作为给定冷却路径的订单的函数。该钛磁铁矿地球座位仪将对来自具有已知安置温度和冷却速率的位置的天然样品进行测试。最后，我们将通过一系列具有受控的热历史的实验来确定时间和温度依赖性阳离子重新排序对延期采集和稳定性的影响以及对古显着性估计的影响。将将实验室数据和模型与对天然样品的观察结果进行比较，因为阳离子排序预计随着已知的冷却速率有系统地变化。由氧化铁矿物质钛磁铁矿（在火山岩中经常发现）获得的磁化，提供了有关地磁田间历史和构造板块运动的重要信息。然而，钛磁铁矿矿物磁力的基本方面仍然不足，尤其是关于氧化物晶体结构中金属阳离子（Fe2+，Fe3+，Ti4+，Mg2+等）的排列，阳离子排列如何随温度而变化，以及导致重要磁性特性的变化。最近已经观察到，在现代温度（300-500°C）和数小时至几个月的时间标准中，常见组成的天然钛磁铁矿会在现代温度（300-500°C）（300-500°C）上进行温度依赖性阳离子进行重新排序。这种阳离子的重新排序会影响钛磁铁矿的基本磁性，并影响它们在冷却过程中记录环境磁场的机制，从而将不确定性引入了源自钛磁铁矿岩石岩石的地磁场强度的估计值。提出的工作旨在了解此阳离子排序过程以及对磁性和磁性特性的影响。此外，拟议的工作将对与过去地质现象相关的温度（例如火山事件或与埋葬相关的加热）相关的温度的普通古磁性估计值提供更多了解。结果可能引起人们广泛的兴趣，以限制火山碎屑流（PF）的冷却速率 - 火山气，灰分和岩石碎片的热混合物。 PFS构成了最重要的火山危害之一，缓慢的冷却意味着该危害可能在初次扩增后持续很长时间。很少有PFS的直接温度测量，并且新的冷却速率和安置温度的方法将有助于危害计划。