Separation of paramagnetic and ferrimagnetic susceptibilities using low temperature magnetic susceptibilities and comparison with high field methods

摘要

Magnetic susceptibility of rocks can be dominated by diamagnetic and paramagnetic matrix minerals, ferrimagnetic and antiferromagnetic trace minerals, or a combination. The interpretation of magnetic fabric data (anisotropy of magnetic susceptibility, AMS) hinges on the qualitative and quantitative analysis of the sources of magnetic susceptibility. We discuss two methods that quantify the contribution of the different groups to the AMS: (1) comparative measurements of the magnetic susceptibility in low fields and high fields and (2) heating curves from 77 K to room temperature (low temperature magnetic susceptibility, LTMS). Method 1 measures paramagnetic, diamagnetic, and antiferromagnetic susceptibilities above the saturation magnetization of the ferrimagnetic minerals and method 2 interprets heating curves based on the fact that only the paramagnetic susceptibility is a function of temperature (Curie-Weiss law). Curie constants, paramagnetic Curie temperatures, and phase transitions (Verwey at 118 K: magnetite; Morin at 263 K: hematite) are diagnostic for specific minerals and provide further information about the contributing minerals of the sample. The relative contribution of the ferrimagnetic and paramagnetic minerals to the total susceptibility can be estimated from both methods with the same precision, if antiferromagnetic and diamagnetic contributions are insignificant. However, the LTMS method requires only simple equipment and procedures. The low temperature method can be extended to the three-dimensional case to decompose the total susceptibility tensor into its paramagnetic and ferrimagnetic sub-tensors (low temperature AMS, LTAMS). LTMS and LTAMS are powerful additions to the group of magnetic fabric methods that allow the quantification of mineral preferred orientation in natural samples.