Physics of the Earth and Planetary Interiors 119Ž. 2000 259±267 www.elsevier.comrlocaterpepi Hematite vs. magnetite as the signature for planetary magnetic anomalies? GuntherÈ Kletetschka ), Peter J. Wasilewski, Patrick T. Taylor NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA Received 25 August 1999; accepted 8 December 1999 Abstract Crustal magnetic anomalies are the result of adjacent geologic units having contrasting magnetization. This magnetization arises from induction andror remanence. In a planetary context we now know that Mars has significant crustal magnetic anomalies due to remanent magnetization, while on the Earth both remanence and induction can contribute to the magnetic anomaly, because of the presence of the Earth's magnetic field. If there is a significant induced magnetizationŽ. IM then magnetite is commonly assumed as the source, since it has a much greater magnetic susceptibility, when compared with other magnetic minerals. We investigated the thermoremanent magnetizationŽ. TRM acquisition of hematite to determine if the remanent and induced magnetization of hematite could compete with magnetite in weak magnetic fields up to 1 mT. TRM acquisition curves of magnetite and hematite show that multidomain hematite approaches TRM saturationŽ 0.3±0.4 A m2rkg. in fields as low as 0.1 mT. However, multidomain magnetite reaches only a few percent of its TRM saturation in a field of 0.1 mT Ž0.02±0.06 A m2rkg. These results suggest that a mineral such as multidomain hematite and, perhaps, other minerals with significant remanence and minor induced magnetization may play an important role in providing requisite magnetization contrast. Consequently, we should reevaluate where multidomain hematite exists in significant concentration, allowing a better insight into the role of remanent magnetization in the interpretation of the magnetic anomalies. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Hematite; Magnetite; Magnetic anomalies; Magnetic remanence; Induced magnetization; Thermoremanent magnetization; Magnetic susceptibility; TRM; Remanence acquisition; Hematite morphology; Hysteresis properties; Coercivity; National Museum of Natural History; REM; SIRM; Saturation isothermal remanent magnetization; Mixtures 1. Introduction tal magnetic anomalies which likely have their sources in mid to deep levels of the crust.Ž. 1 Main We consider two assumptions in this report cen- magnetic mineral is magnetite, andŽ. 2 in the case of tral to the interpretation of large amplitude continen- the continental Earth the mode of magnetization is entirely induction. Generally when someone considers how ``mag- ) q q Corresponding author. Tel.: 1-301-286-3804; fax: 1-301- netic'' something is he or she may use a strong 286-0212. E-mail address: [email protected] magnet to test for the attraction force between the Ž.G. Kletetschka . magnet and the object. The strong magnet is a source 0031-9201r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0031-9201Ž. 00 00141-2 260 G. Kletetschka et al.rPhysics of the Earth and Planetary Interiors 119() 2000 259±267 of strong inducing field that is 3±5 orders of magni- tion suggests that most rocks contain PSD±small tude more intense than the intensity of the geomag- MD grains. netic field. This procedure will raise the induced Part of this paper reiterates the ClarkŽ. 1983 magnetizationŽ. IM of the object Ž for example a rock emphasis on the importance of remanence in mag- with magnetite particles. 3±5 orders of magnitude, netic anomaly interpretation. Magnetite, titanomag- or in case of a very strong magnet towards the netite, and pyrrhotite, all of which are found in the saturation point of the magnetic carriers. When at- SNC meteoritesŽ. McSween, 1985 , and MD hematite traction is observed it is most likely due to mag- Ž.Christensen et al., 2000 , should be considered as netite, which has a large induction because of its possible candidates for the large planetary magnetic large magnetic susceptibility. The large values of signatures. Maghemite if present would be magneti- induced magnetization exhibited by MD magnetite cally similar to magnetiteŽ Dunlop and Ozdemir,È are responsible for the common belief that magnetite 1997. is the likely source of terrestrial crustal magnetic anomaliesŽ Shive and Fountain, 1988; Wasilewski and Mayhew, 1992. 2. Magnetization of hematite and magnetite ClarkŽ. 1983 summarized the range of thermore- manent magnetizationŽ. TRM expected for the prin- The most common terrestrial magnetic iron oxide cipal iron oxide minerals found in terrestrial rocks. is magnetiteŽ. Fe34 O . In the Earth's field Ž 0.05 mT . Insofar as we are awareŽ. McSween, 1985 the same magnetite has the largest Induced Magnetization iron oxide mineralogies are found in the Martian Ž.60±220 Arm among the common magnetic miner- rocks. Magnetic properties of iron oxide minerals alsŽ. e.g., Maher, 1988 . Induced magnetization, Mi , change according to their grain size. The critical is a function of magnetic susceptibility x and the s r single domain size for magnetite is 0.05±0.084 mm, external magnetic field H: Ž.Mi x H m , where m for hematite the size is 15 mm, for titanomagnetite Ž ms4p=10y7 V srm. is the vacuum magnetic the size is 0.2±0.6 mm, and for pyrrhotite the size is permeability. Magnetite can also exist in a super- 1.6 mmŽ. Dunlop and Ozdemir,È 1997, Table 5.1 . paramagneticŽ. SP state when the grain size is smaller Wasilewski and WarnerŽ. 1994 used the SD±PSD± than 50 nm. Under this condition the susceptibility of MDŽ Single Domain±Pseudo Single Domain±Multi a 30-nm-size cube of magnetite, at room tempera- Domain. categorization based on size dependent hys- ture, is about 650Ž. Dunlop and Ozdemir,È 1997 teresis propertiesŽ. Day et al., 1977 and presented which produces an enormous induced magnetization magnetic hysteresis data for a wide range of samples of approximately 26,000 Arm. including xenoliths, high grade metamorphic terrane, A comparison of TRM of magnetite relative to its crustal sections, etc. This SD±PSD±MD categoriza- induced magnetization is given in Table 1. Schlinger Table 1 Induced and thermoremanent magnetization is acquired in a presence of 50 mT external magnetic field Magnetization of magnetite and hematite Inducedwx Arm Thermoremanent wx Arm Total wx Arm Magnetite 20±200 mm ;140Ž. Maher, 1988 60±25 Ž Dunlop, 1990 . 200±165 2±20 mm 80±140Ž. Maher, 1988 250±60 Ž Dunlop, 1990 . 330±200 0.2±2 mm 60±80Ž. Maher, 1988 1000±250 Ž Dunlop, 1990 . 1060±330 0.02±0.2 mm 220±60Ž. Maher, 1988 5000±1000 Ž Dunlop, 1990 . 5220±1060 0±0.02 mm 26,000Ž. Dunlop and Ozdemir,È 1997 0 26,000 Hematite 20±200 mm ;7Ž. this study 600±1600 Ž. this study 600±1600 0±20 mm 0.02±2Ž. Hunt et al., 1995 30±300 Ž. Clark, 1983 30±300 G. Kletetschka et al.rPhysics of the Earth and Planetary Interiors 119() 2000 259±267 261 Ž.1985 suggested that the most common range of duction of superparamagnetic particles of magnetite, magnetite grain size is 20±200 mm for mid and deep due to various types of chemical alteration of iron crustal rocks. This 20±200 mm grain size range silicates, could be significant but is probably not gives TRM values, in crustal conditions, of 25±250 common because the volume of coarse-grained mag- ArmŽ. Dunlop, 1990 . Taking into account magnetite netite in rock is usually much greater than the vol- susceptibilities for these grain sizesŽ. Maher, 1988 ume of superparamagnetic magnetite. we have induced magnetization between 80 and 140 The second most abundant iron oxide found in r A mŽ. see Table 1 . This indicates that in the geo- crustal rocks is hematiteŽ. Fe23 O . Hematite as well magnetic field a magnetite-rich rock, with acquired as other common iron bearing minerals has suscepti- thermoremanence, may have significantly larger re- bility less than 0.01 SI. This would mean, that the manent than induced magnetization. The relative sig- total induced magnetization of a hematite±magnetite nificance of induced and remanent magnetization is bearing rock is dominated almost entirely by mag- expressed by their ratio known as Koenigsberger netiteŽ. see Tables 1 and 3 . Would its remanent ratio Ž.Q . Data in Table 1 indicates that Q should be magnetization dominate the total magnetization of a generally greater than one for both hematite and rock where both minerals are present and hematite is magnetite. This is not true in cases where the origi- significant volumetrically? nal thermoremanence was destroyedŽ chemically To investigate this scenario hematite specimens andror physically. or when different types of rema- were obtained from all over the worldŽ selected from nence have been re-acquiredŽ for review see Table the mineral collection in the National Museum of 3.1 in Merrill et al., 1996. For example, the intro- Natural History, Smithsonian Institution, Washington Fig. 1. Four common forms of hematite:Ž. A fine-grained reddish powder Ž N114078 .Ž. ; B pencil-like Ž N13026 .Ž. ; C equidimensional coarse-grainedŽ.Ž.Ž. NR17174 ; D plate like N36085 . 262 G. Kletetschka et al.rPhysics of the Earth and Planetary Interiors 119() 2000 259±267 DC. The specimens from the Smithsonian collection 1995. The titanium component was not detected could be divided into four groupsŽ. Fig. 1 . The first during the Curie temperature measurements, which group consists of compact fine powder of reddish confirmed the temperature for pure hematiteŽ. 6708C. hematite with grain size less that 1 mmŽ. See Fig. 1 . Morphological study was done by scanning elec- The second group is made up of pencil-shaped rods, tron microscopy on non-coated samples chipped off with the pencil-cross-sectional diameter between 0.2 the USNM specimens. We used the Philips SEM and 3 mm.