Effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments

Authors

Julie A. Bowles, University of Wisconsin-MilwaukeeFollow
Mike J. Jackson, University of Minnesota - Twin CitiesFollow

Document Type

Article

Publication Date

12-17-2016

Abstract

Titanomagnetite (Fe_3-xTi_xO₄, 0≤x≤1) is a common, naturally occurring magnetic mineral critical to many paleomagnetic studies. Underlying most interpretations is the assumption that, lacking chemical alteration, Curie temperature (T_c) remains constant. However, recent work has demonstrated that T_c of many natural titanomagnetites varies strongly as a function of thermal history, independent of chemical alteration. This is inferred to arise from reordering of cations and/or vacancies in the crystal structure, and changes occur at temperatures and times relevant to standard paleomagnetic thermal treatments. Because changes take place at Tc, they have the potential to dramatically affect thermal remanence acquisition or demagnetization, impacting interpretation of paleomagnetic results. Here we have modeled the effects of reordering on standard thermal demagnetization and paleointensity experiments. Results suggest that T_c changes during laboratory heating make it impossible to accurately measure the unblocking temperature spectrum without modifying it. Samples with a starting T_c0 less than the closure temperature (T_close) for the reordering process will develop a high-temperature ‘‘tail’’ that did not exist prior to heating. Samples with a starting T_c0>T_close will have their original T_b spectrum truncated at T ≈ T_close. Predicted behavior during Thellier-type paleointensity experiments results in only modest deviations in NRM-lost or pTRM*-gained from the nonreordering case. Much larger deviations are predicted for pTRM checks. Compared to paleointensity results from titanomagnetite-bearing pyroclastic deposits, modeled nonideal behavior occurs in the same temperature intervals, but is much more systematic. Reordering is likely one contributing factor to failure of paleointensity experiments.

Recommended Citation

Bowles, J. A., and M. J. Jackson (2016), Effects of titanomagnetite reordering processes on thermal demagnetization and paleointensity experiments, Geochem. Geophys. Geosyst., 17, 4848–4858, doi:10.1002/2016GC006607.