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.
