What do the HIRM and S-ratio really measure in environmental magnetism?

Liu, Q-S., Roberts, A.P., Torrent, J., Horng, C.S. and Larrasoana, J.C. (2007) What do the HIRM and S-ratio really measure in environmental magnetism? Geochemistry, Geophysics, Geosystems, 8, Q09011. (doi:10.1029/2007GC001717).

Abstract

The “hard” isothermal remanent magnetization (HIRM) and the S-ratio are widely used in environmental magnetism to quantify the absolute and relative concentrations, respectively, of antiferromagnetic minerals (hematite and goethite) in mineral mixtures. We demonstrate that synthetic Al-substituted hematite and goethite exhibit a wide range of coercivities, which significantly influences the HIRM and S-ratio. These parameters are therefore not necessarily straightforward indicators of the absolute and relative concentrations of hematite/goethite. To circumvent this problem, we propose a new parameter (the L-ratio), which is the ratio of two remanences after alternating field (AF) demagnetization of an IRM imparted in a 1 T field with a peak AF of 100 mT and 300 mT: IRM_AF@300mT/IRM_AF@100mT. These parameters are easily measured using modern vibrating sample or alternating gradient magnetometers. Changes in HIRM only reflect changes in the absolute concentration of hematite and/or goethite if the L-ratio is relatively constant. Conversely, L-ratio fluctuations indicate variable coercivities that possibly reflect changes in the source of hematite/goethite. Corresponding HIRM and S-ratio variations should be interpreted with caution in such cases. The L-ratio can be determined using equivalent terms depending on available instrumentation and measurement protocols. For example, the HIRM is equivalent to IRM_AF@300mT. Likewise, 0.5*(SIRM + IRM_-100mT), where IRM_-100mT represents the remanent magnetization obtained by first saturating the sample in a high field and then applying a back-field of -100 mT, is equivalent to IRM_AF@100mT. The HIRM/[0.5*(SIRM + IRM_-100mT)] ratio is therefore a suitable substitute for the L-ratio when measurements are made with a long-core magnetometer. The newly proposed L-ratio is straightforward to measure on a wide range of instruments and can provide significant new insights and reduce ambiguities associated with interpretation of two widely used parameters in environmental magnetism, the HIRM and S-ratio.

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