Wasp‐waisted hysteresis loops: Mineral magnetic characteristics and discrimination of components in mixed magnetic systems

Abstract

Rock magnetic studies of complex systems that contain mixtures of magnetic minerals or mixed grain size distributions have demonstrated the need for a better method of distinguishing between different magnetic components in geological materials. Hysteresis loops that are constricted in the middle section, but are wider above and below the middle section, are commonly observed in mixed magnetic assemblages. Such “wasp‐waisted” hysteresis loops have been widely documented, particularly with respect to rare earth permanent magnets, basaltic lava flows, remagnetized Paleozoic carbonate rocks, and an increasingly wide range of other rocks. Our modelling, combined with a review of previous work, indicates that there are several conditions that give rise to, as well as magnetic properties that are characteristic of, wasp‐waisted hysteresis loops. First, at least two magnetic components with strongly contrasting coercivities must coexist. This condition can arise from either mixtures of grain sizes of a single magnetic mineral, or a combination of magnetic minerals with contrasting cocrcivities, or a combination of these two situations. Second, materials that give rise to wasp‐waisted hysteresis loops will have relatively high ratios of the coercivity of remanence to coercive force (Bcr/Bc) because B0 is controlled by the soft (low coercivity) component, whereas Bcr is controlled by the hard (high coercivity) component. Third, values of Bcr/Bc ≥ 10 usually only occur for strongly wasp‐waisted loops when the low coercivity component comprises an overwhelmingly large fraction of the total volume of magnetic grains. Fourth, a given mixture of superparamagnetic and single‐domain (SD) grains is more likely to give rise to wasp‐waisted hysteresis loops than an equivalent mixture of SD and multidomain grains. Fifth, our results provide empirical confirmation that the total magnetization of a material is the sum of the weighted contributions of each component, in the absence of significant magnetic interaction between particles. Thus to contribute significantly to wasp‐waisted behavior, a mineral magnetic component must give rise to a significant portion of the total magnetization of the rock. As a result, minerals with weak magnetic moments such as hematite need to occur in large concentrations to cause wasp‐waistedness in materials that also contain ferrimagnetic minerals. We outline a method for determining the magnetic components that can give rise to wasp‐waisted hysteresis loops. This method is based on high‐ and low‐temperature magnetic measurements that are used to identify the dominant remanence‐bearing mineral/s and on mineral magnetic techniques that are used to discriminate between different magnetic domain states. The method is illustrated with several examples from archaeological, geological, and synthetic materials.

Enhanced primary productivity and magnetotactic bacterial production in response to middle Eocene warming in the Neo-Tethys Ocean

“…The presence of hematite mixed with fine-grained magnetite is indicated by the wasp-waisted shape of the loops (Roberts et al, 1995). The mixture of low-and high-coercivity phases does not affect significantly the Day plot because SD hematite has similar hysteresis ratios to those of SD magnetite (Roberts et al, 1995). The significant departure of bulk hysteresis parameters from values expected for uniaxial SD magnetite for both the MCA and CHW samples (Fig.…”

“…6; Table 2), including the ratio of saturation remanence to saturation magnetization (M rs /M s ) and the coercivity of remanence to coercive force (B cr /B c ), from MCA samples lie within the pseudo-single domain (PSD) field of Day et al (1977). The presence of hematite mixed with fine-grained magnetite is indicated by the wasp-waisted shape of the loops (Roberts et al, 1995). The mixture of low-and high-coercivity phases does not affect significantly the Day plot because SD hematite has similar hysteresis ratios to those of SD magnetite (Roberts et al, 1995).…”

Iron oxide minerals in dust-source sediments from the Bodélé Depression, Chad: Implications for radiative properties and Fe bioavailability of dust plumes from the Sahara

“…In all cases the loops have a positive high field slope ( hf ) due to the strong influence of paramagnetic or antiferromagnetic phases, and closure occurs by 500 mT characteristic of ferrimagnetic phases like magnetite. Some loops have shape factors, hys >0.2, producing a slight constriction near the origin and indicating a mixture of low-and high-coercivity phases representing either mixtures of different minerals (magnetite/maghemite and hematite/goethite) or mixtures of different particle sizes in a single mineral (microcrystalline and nanometric) (Tauxe et al, 1996;Roberts et al, 1995;Dunlop 2002a, b).…”

Pervasive remagnetization of detrital zircon host rocks in the Jack Hills, Western Australia and implications for records of the early geodynamo

“…S3D) and their low S-ratios (the IRM in a backfield of 0.3 T divided by IRM acquired in 1.5 T field) of 0.11-0.54, along with the evidence for magnetite, hematite and goethite in monzogranite thermal demagnetization of IRM data ( Figs. 2A and S1A), suggest that they are instead likely mixtures of multidomain magnetite and single domain hematite and goethite [see Roberts et al (1995)]. …”