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: 1315845
• 负责人: Michael Jackson
• 金额: $ 5.26万
• 依托单位: University of Minnesota-Twin Cities
• 依托单位国家: 美国
• 项目类别: Standard 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=1315845&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速度计； ) 了解和模拟阳离子排序对磁特性以及热剩磁 (TRM)、部分热剩磁 (pTRM) 和热剩磁 (TRM) 的获取、保留和退磁的影响新获得的数据表明，常见成分的天然钛磁铁矿在中等温度 (300-500°C) 和数小时至数月的时间范围内经历温度依赖性阳离子排序，从而产生磁性居​​里温度 (Tc)。 ）是先前热历史的强大函数，变化高达 150°C 而没有伴随的化学变化，因为（重新）排序可能会在 T Tc 发生，这显然对 Tc 具有深远的影响。我们对 TRM 获取和稳定性的理解，并可能导致绝对古强度估计以及古地磁古温度测量的不确定性增加。我们建议通过合成受控成分的样品和表征来确定复杂钛磁铁矿中阳离子排序过程的性质。通过粉末 X 射线衍射、穆斯堡尔光谱和 X 射线磁圆二色性 (XMCD)，有序度将与居里温度和居里温度直接相关。等温退火实验将使我们能够限制有序过程的动力学，然后可以将 TC 计算为给定冷却路径的有序函数，该钛磁铁矿地球速度计将针对来自已知位置的天然样品进行测试。最后，我们将通过一系列受控热实验来确定依赖于时间和温度的阳离子重排序对剩磁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，铁钛氧化物矿物钛磁铁矿（常见于火山岩中）获得的磁化提供了重要的信息来源。然而，对钛磁铁矿矿物磁性的一些基本方面仍知之甚少，特别是有关金属阳离子（Fe2+、 Fe3+、Ti4+、Mg2+等）在氧化物晶体结构中，阳离子排列如何随温度变化，以及导致重要磁性能的变化。最近观察到，常见成分的天然钛磁铁矿在30℃下会发生与温度相关的阳离子重排。这种阳离子重排会影响钛磁铁矿的基本磁性，并影响它们记录环境的机制。冷却过程中的磁场，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。这项工作旨在了解这种阳离子排序过程及其对磁化和磁性的影响。此外，这项工作还将提供更大的帮助。了解与过去地质现象（例如火山事件或与埋藏相关的加热）相关的常用古地磁温度估计的不确定性，其结果可能对限制火山碎屑流的就位温度和冷却速率具有广泛的意义。 (PF)——火山气体、火山灰和岩石碎片的热混合物构成了最严重的火山危害之一，而缓慢的冷却意味着这种危害可能在最初就位后持续很长时间。量化冷却速率和就位温度的新方法将有助于危险规划。

项目成果

Assessing New and Old Methods in Paleomagnetic Paleothermometry: A Test Case at Mt. St. Helens, USA

评估古地磁古体温测量的新旧方法：美国圣海伦斯山的测试案例

• DOI: 10.1029/2018gc007435
• 发表时间: 2018-06
• 期刊: Geosystems
• 作者: Bowles, Julie A.;Gerzich, Devin M.;Jackson, Mike J.
• 通讯作者: Jackson, Mike J.

Malleable Curie Temperatures of Natural Titanomagnetites: Occurrences, Modes, and Mechanisms

天然钛磁铁矿的可延展居里温度：发生、模式和机制

• DOI: 10.1002/2017jb015193
• 发表时间: 2018-01
• 期刊: Journal of Geophysical Research: Solid Earth
• 作者: Jackson, Mike;Bowles, Julie
• 通讯作者: Bowles, Julie