A New Tool for Separating the Magnetic Mineralogy of Complex Mineral Assemblages from Low Temperature Magnetic Behavior

Abstract

One timeless challenge in rock magnetic studies, inclusive of paleomagnetism and environmental magnetism, is decomposing a sample's bulk magnetic behavior into its individual magnetic mineral components. We present a method permitting to decompose the magnetic behavior of a bulk sample experimentally and at low temperature avoiding any ambiguities in data interpretation due to heating-induced alteration. A single instrument is used to measure the temperature dependence of remanent magnetizations and to apply an isothermal demagnetization step at any temperature between 2 and 400 K. The experimental method is validated on synthetic mixtures of magnetite, hematite, goethite as well as on natural loess samples where the contributions of magnetite, goethite, hematite and maghemite are successfully isolated. The experimental protocol can be adapted to target other iron bearing minerals relevant to the rock or sediment under study. One limitation rests on the fact that the method is based on remanent magnetizations. Consequently, a quantitative decomposition of absolute concentration of individual components remains unachievable without assumptions. Nonetheless, semi-quantitative magnetic mineral concentrations were determined on synthetic and natural loess/paleosol samples in order to validate and test the method as a semi-quantitative tool in environmental magnetism studies.

An Improved Algorithm for Unmixing First‐Order Reversal Curve Diagrams Using Principal Component Analysis

“…Unmixing the magnetic properties of rocks, sediments, and soils is a primary task in rock magnetism. Numerous methods exist to tackle this problem (e.g., Dunlop, 2002aDunlop, , 2002bEgli, 2004aEgli, , 2004bEgli, , 2004cFranke et al 2007;Heslop & Dillon, 2007;Kruiver et al, 2001;Lagroix & Guyodo, 2017;Lascu et al, 2010Lascu et al, , 2015Ludwig et al, 2013;Robertson & France, 1994) as well as an extensive toolbox of magnetic proxies that are designed to highlight specific magnetic mineralogy variations in environmental contexts (Evans & Heller, 2003;Liu et al, 2012). No single method is perfect for all cases, and usually a combination of methods is needed to unmix all magnetic components contained within a material.…”

Magnetic Fingerprinting of Fluvial Suspended Particles in the Context of Soil Erosion: Example of the Canche River Watershed (Northern France)

“…Comparison of the classic and enhanced RT‐SIRM cycles (Figures 6a–6d, first column) with the demagnetized enhanced RT‐SIRM (Figures 6e–6h, second column) show the successful elimination of the (dominant) magnetite component using the protocol proposed by Lagroix and Guyodo (2017). This is particularly helpful for the identification of the presumed remaining high‐coercivity carriers, such as goethite and hematite.…”

“…Low‐temperature SIRM acquired at 10 K in a 2.5 T field warming curves after zero‐field cooling (ZFC; blue solid line) and field cooling (FC; red solid line) of two surface runoff samples M1 (panel a) and M2 (panel b) samples from the Venturi channel, Pommeroye River (see section 2.3) compared to the remaining ZFC and FC LT‐SIRM after demagnetization over the 0 to 300 mT window (same color code, dashed lines) following the protocol in Lagroix and Guyodo (2017). Panels (c) and (d) show the first derivatives of the respective curves shown in the panels above (using the same color code) to highlight the mentioned temperature (Verwey) transitions.…”

“…The right‐hand panels show the respective zoomed views of the last cycle (remaining enhanced RT‐SIRM after both 300 mT and thermal demagnetization). The measurement protocol follows Lagroix and Guyodo (2017).…”

Fault imprint in clay units: Magnetic fabric, p-wave velocity, structural and mineralogical signatures

“…Siderite was also identified by XRD but it was not quantified by XRD because its concentration was less than 5%. However, magnetic measurements like SIRM are able to detect ferrimagnetic minerals that are undetected by traditional mineralogical analyses such as XRD, even when the concentrations are as low as 1 ppm (Lagroix, F. and Y. Guyodo, 2017).…”