Inferred time- and temperature-dependent cation ordering in natural titanomagnetites

TitleInferred time- and temperature-dependent cation ordering in natural titanomagnetites
Publication TypeJournal Article
Year of Publication2013
AuthorsBowles J.A, Jackson M.J, Berquo T.S, Solheid P.A, Gee J.S
JournalNature Communications
Date Published2013/05
Type of ArticleArticle
ISBN Number2041-1723
Accession NumberWOS:000320589900112
Keywordsblocking temperatures; cooling rates; curie points; distributions; grains; magnesioferrite mgfe2o4; rock-magnetism; single-domain; spinel solid-solutions; spontaneous magnetization curves; susceptibility

Despite years of efforts to quantify cation distribution as a function of composition in the magnetite-ulvo "spinel solid solution, important uncertainties remain about the dependence of cation ordering on temperature and cooling rate. Here we demonstrate that Curie temperature in a set of natural titanomagnetites (with some Mg and Al substitution) is strongly influenced by prior thermal history at temperatures just above or below Curie temperature. Annealing for 10(-1) to 10(3) h at 350-400 degrees C produces large and reversible changes in Curie temperature (up to 150 degrees C). By ruling out oxidation/reduction and compositional unmixing, we infer that the variation in Curie temperature arises from cation reordering, and Mossbauer spectroscopy supports this interpretation. Curie temperature is therefore an inaccurate proxy for composition in many natural titanomagnetites, but the cation reordering process may provide a means of constraining thermal histories of titanomagnetite-bearing rocks. Further, our theoretical understanding of thermoremanence requires fundamental revision when Curie temperature is itself a function of thermal history.

Short TitleNat. Commun.
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