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.

理解的时间和tepenpentent cationt cationt cationt cationg ordessssssssssssssssss设置了（2）；网络化（TRM），部分热量（PTRM）。先前的热历史，最多可以在TC ITION和稳定性上进行150°C，而无需进行化学变化提议通过粉末X射线衍射，Mössbauer光谱和X射线磁性圆形二分法MCD来确定自然实验将允许我们限制订购过程的遗传，然后将TC作为给定冷却路径的功能序列计算 - 依赖性的阳离子与具有控制的热历史数据的NT和模型与对NATHAL样品的观察相提并论，而阳离子预计将与已知的冷却速率变化。钛金属矿（火山岩石）提供了有关地磁场历史和Tector运动的重要信息，这些遗迹仍然不充分地理解了Stal结构，可能会随着温度而变化，并导致重要的磁性磁性变化。依赖的阳离子（300-500°C）和数小时至几个月的时间。了解与过去的地质现象相关的温度（例如olcanic事件）或与埋葬相关的供暖，对常用的古磁性估计更大。流动气体，灰烬和岩石碎片可能会在f降温速度和安置温度下持续很长时间。