Interpreting high-temperature magnetic susceptibility data of natural systems

Abstract

High-temperature susceptibility (HT-χ) data are routinely measured in Earth, planetary, and environmental sciences to rapidly identify the magnetic mineralogy of natural systems. The interpretation of such data can be complicated. Whilst some minerals are relatively unaltered by heating and are easy to identify through their Curie or Néel temperature, other common magnetic phases, e.g., iron sulphides, are very unstable to heating. This makes HT-χ interpretation challenging, especially in multi-mineralogical samples. Here, we report a review of the HT-χ data measured primarily at Imperial College London of common magnetic minerals found in natural samples. We show examples of “near pure” natural samples, in addition to examples of interpretation of multi-phase HT-χ data. We hope that this paper will act be the first reference paper for HT-χ data interpretation.

Evidence of magnetite reduction and primary magnetization destruction during early diagenesis in sediments from Maldives Inner Sea

“…The formation of magnetite during the heating of siderite is evidenced by the Curie temperature of 580°C, at which the sample demagnetizes (Figure 5; samples A, C-F). Thermomagnetic curves also show that, at room Diagenesis in Maldives Inner Sea sediments Carrasqueira et al temperature, the product of sample heating has a higher susceptibility than the unheated samples, which is in line with the formation of magnetite from siderite during heating (Housen et al, 1996;Abdulkarim et al, 2022;Muxworthy et al, 2023).…”

“…High-temperature thermomagnetometry makes it possible to identify different magnetic minerals. Interpretations are made mainly using the characteristic Curie temperature of different minerals (e.g., magnetite and hematite), but also using welldocumented chemical changes (Minyuk et al, 2011;Till and Nowaczyk, 2018;Muxworthy et al, 2023) during sample heating (e.g., greigite and siderite).…”

“…Siderite is paramagnetic at room temperature and has a low susceptibility, but during heating (at temperatures around 400-500°C), susceptibility increases as siderite progressively converts to magnetite (Housen et al, 1996;Abdulkarim et al, 2022;Muxworthy et al, 2023). The formation of magnetite during the heating of siderite is evidenced by the Curie temperature of 580°C, at which the sample demagnetizes (Figure 5; samples A, C-F).…”

“…We associate these results to the thermomagnetic behavior of greigite. During heating, greigite shows a Hopkinson peak from 240°C to 250°C (Muxworthy et al, 2023), followed by a sharp decrease in susceptibility at ∼400°C associated with its chemical transformation into sulphur, pyrite, marcasite and pyrrhotite (Skinner et al, 1964;Krs et al, 1992;Torii et al, 1996;Muxworthy et al, 2023;), which leads to a decrease in magnetization (Dekkers et al, 2000). Heating greigite from 450°C to 550°C in an argon atmosphere forms pyrrhotite, while heating it to 550°C, 650°C and 700°C leads to the progressive formation of magnetite.…”

An <em>In-Situ</em> Mössbauer Study of the High-Temperature Behavior of Siderite under Reduction Conditions

“…However, such a maximum was not observed for sample Sd5, where the concentration of dolomite is higher than in sample Sd2. For this reason, we can assume that this peak is likely Hopkinson-peak effects and was observed for hematite-containing samples [51,52]. Hopkinson peak is usually associated with the unblocking of single-domain grains, making them superparamagnetic or increased domain wall motion in larger multi-domain grains due to enhanced thermal energy; both mechanisms cause an increase in magnetization, though this is particularly enhanced in fine single-domain grains [52].…”