Unique thermoremanent magnetization of multidomain sized hematite: Implications for magnetic anomalies
Introduction
Thermoremanent magnetization (TRM) is one of the most efficient remanent magnetization mechanisms in nature (e.g. [1]) resulting in the largest specific magnetization intensity compared to other mechanisms. The only magnetization mechanism that provides more intense remanence is that associated with the magnetization in lodestones and other intensely magnetized rocks. This is a consequence of the magnetic field due to lightning [2]. However this magnetization is important locally and is relevant only to a thin layer of exposed rock. Among the magnetic minerals in the Earth’s crust: magnetite, hematite, titanomagnetite, titanohematite and pyrrhotite are the ones most frequently encountered. The grain size dependence of TRM acquisition in titanomagnetite and titanohematite with relatively small amounts of titanium, behaves almost identically to end member magnetite and hematite [3], [4].
Most of the total magnetization of crustal rocks is usually considered to be associated with magnetite because of the large magnetic susceptibility in the presence of the inducing geomagnetic field [5], [6]. Results from the German Continental Deep Drilling program (KTB) revealed amphibolite facies metamorphic rocks with the major magnetic carrier identified as monoclinic, ferrimagnetic pyrrhotite [7]. Balsey and Buddington [8] showed that remanent magnetization of reversed polarity carried by titanohematite was responsible for prominent negative magnetic anomalies associated with microcline granite gneisses from the Grenville terrane of the Adirondack Mountains, New York. Titanohematite is the main NRM carrier of large blocks (5000 km2) of granulite facies metamorphic rocks in Central Labrador [9], [10]. Titanohematite was also shown to be the main NRM carrier in high grade metamorphic rocks exposed in Lofoten-Versteralen, northern Norway [11]. These examples emphasize that pyrrhotite and titanohematite, with remanence dominant over induced magnetization, should also be considered when explaining magnetic anomalies over large crustal regions.
Single domain (SD) sized grains of magnetite, titanomagnetite and pyrrhotite acquire more intense TRM than the multidomain (MD) sized grains [3], [4]. Hematite and titanohematite, however, acquire more intense TRM in its MD size than in SD size [3]. This identifies titanohematite and hematite as having an inverse grain size dependence when compared with other common magnetic minerals. Because single domain behavior is markedly different from that of MD we are interested in establishing the grain size at which hematite changes its behavior from a truly multidomain sized grain towards the SD sized behavior. Grain size dependent coercivity indicates that the truly SD grain size of hematite is between 0.025 and 15 μm [13], [14].
Section snippets
Experimental procedures
Hematite samples L2 [10] and N114078 [12] were characterized by X-ray diffraction, Curie temperature and saturation magnetization. X-ray diffraction analysis of the powdered samples confirmed high purity of the hematite grains, where no other phase was detected. The absence of magnetite was also indicated by the measured values of saturation magnetization (Js) which ranged between 0.2–0.5 A m2 kg−1. No titanium component was detected during measurements of the Curie temperature, which coexisted
Results
Grain size dependence of TRM in magnetite and hematite (at 5×10−5 T) are shown in Fig. 1. The bend in the magnetite curve at ∼0.001 mm indicates a transition from SD to MD magnetic behavior [15]. The hematite data clearly show a distinction between MD (which reaches magnetization levels of SD sized magnetite) and the SD states. Our hematite TRM values between grain size of 0.1 and 1 mm are more or less constant suggesting an MD regime. TRM for 0.05 mm grain size is slightly lower, perhaps
Physics behind TRM of hematite
Hematite samples have been used to provide a ‘magnified’ view of pseudo single domain (PSD) processes [20]. Hematite possesses only a very limited number of domains, in spite of the fairly large grain size [21], [22], [23], implying that the PSD behavior of hematite may be relevant for understanding PSD behavior of other minerals, such as magnetite. We caution against making this generalization because, as the data in Fig. 1 illustrate, magnetite and hematite TRM grain size dependences for the
Relative significance of TRM
We have shown that MD hematite not only exhibits inverse grain size dependence (with respect to magnetite) for TRM acquisition, but also has magnetization intensity that would signal its importance if it were distributed significantly in crustal rocks. Fig. 6 is a review of the field dependence of TRM acquisition for the common magnetic minerals. Fig. 6a contains data for TRM acquired in the geomagnetic field and indicates the observed range for the appropriate SD or MD mineral species. These
Implication for magnetic anomalies
MD hematite can carry a significant remanent magnetization. Crustal rocks contain both coarse and fine grained magnetic minerals. Coarse MD magnetic grains can occur as single grains in between the silicate phases. A variable fraction of very small magnetic grains can be found within the matrix of silicate minerals in the form of exsolution products. Most of the magnetic minerals are larger than SD [34]. SD magnetite, however, will acquire more than two orders of magnitude more TRM than MD
Conclusions
Multidomain hematite exhibits an inverse TRM grain size dependence across SD–PSD transition with respect to all other minerals found in the crust. This is proposed to be due to weaker influence of demagnetizing energy with respect to wall pinning energy in the case of hematite, at temperatures almost up to the Curie temperature. Another factor is the greater importance of the magnetostatic energy in the applied field, which for hematite dominates the total energy at high temperatures. Thermal
Acknowledgements
Review by Horst U. Worm improved the presentation of this paper. David Clark reviewed this paper and his contribution and effort has resulted in a product that is significantly enhanced. We thank Dr. Clark for his effort. We also thank Peter Dunn at the Mineral Science Division of NMNH, Smithsonian Institution for his help with obtaining the hematite samples for this study. Chuan He (GSFC) conducted the X-ray diffraction studies on hematite samples.[RV]
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