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Inferred time- and temperature-dependent cation ordering in natural titanomagnetites
Julie A Bowles1, Mike J Jackson, Thelma S Berquó
1Department of Earth Sciences, Institute for Rock Magnetism, University of Minnesota, Minneapolis, Minnesota 55455, USA. bowlesj@uwm.edu
Nature Communications
|May 30, 2013
Summary
Titanomagnetite Curie temperatures are sensitive to thermal history due to cation reordering, not just composition. This finding impacts paleomagnetism and requires revising theories of thermoremanence.
Area of Science:
- Geoscience
- Materials Science
- Mineral Physics
Background:
- Quantifying cation distribution in magnetite-ulvöspinel solid solutions remains challenging.
- The influence of temperature and cooling rate on cation ordering is not fully understood.
- Curie temperature is often used as a proxy for titanomagnetite composition.
Purpose of the Study:
- To investigate the influence of thermal history on Curie temperature in natural titanomagnetites.
- To determine the mechanism behind observed changes in Curie temperature.
- To assess the reliability of Curie temperature as a compositional indicator and its implications for paleomagnetism.
Main Methods:
- Annealing experiments on natural titanomagnetites with varying Mg and Al substitution.
- Measurement of Curie temperature changes after thermal treatments.
- Mössbauer spectroscopy to analyze cation ordering and rule out other mechanisms.
Main Results:
- Annealing at 350-400 °C for 10^-1 to 10^3 hours caused significant, reversible changes in Curie temperature (up to 150 °C).
- Oxidation/reduction and compositional unmixing were excluded as causes for the observed variations.
- Cation reordering was identified as the primary mechanism responsible for the changes in Curie temperature.
Conclusions:
- Curie temperature is not always an accurate proxy for titanomagnetite composition due to thermal history effects.
- Cation reordering provides a potential new method for constraining the thermal histories of rocks containing titanomagnetites.
- The dependence of Curie temperature on thermal history necessitates a revision of theoretical models of thermoremanence.
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