Magnetic Anisotropy in Carbonate Rocks: Revisiting the Carrara Marble

Magnetic anisotropy has been used in numerous investigations since the 1950's as an indicator of petrofabric. In deformed rocks the anisotropy of magnetic susceptibility (AMS) has been shown to qualitatively, and sometimes quantitatively reflect finite strain. In low-field AMS all minerals contribute to the measured anisotropy. Over the past years several methods have been used to isolate different components to the AMS. Various high-field methods are used to separate the paramagnetic, diamagnetic and high coercivity antiferromagnetic components from the ferrimagnetic component. In spite of these advances, our understanding of the factors controlling the total AMS signal in a rock remains limited, which has hampered the quantitative use of magnetic fabrics in deformation studies. This is partly due to the limited data available on the intrinsic anisotropy of single mineral crystals. Calcite is one mineral whose AMS is well-determined. Earlier studies have shown that the magnetic fabric in deformed pure marble can be related to the crystallographic orientation of the calcite crystals (Owens and Rutter, 1978, PEPI, 16 2115; deWall et al., 2000, J. Struct. Geol., 22, 1761). De Wall et al (2000) were able to model quantitatively the AMS from the mineral fabric and AMS of calcite for coarse-grained marbles, but were less successful for fine-grained ones. In this study we have used low-field AMS and high-field torque magnetometry to isolate the diamagnetic signal of Carrara marbles. The magnetic fabric has been compared with the calcite fabric in the rocks. Preliminary results will be presented of synthetic calcite aggregates prepared from Carrara marble powder, which was initially uniaxially cold pressed at different amount of load, in order to induce a c-axis maximum type of fabric of various strength, and compare it with the magnetic anisotropy. This was later Hot Isostatic Pressed at about 150 MPa and 700° C to reduce porosity and increase/homogenize the grain-size. In this way we could compare a defined deformation (compaction) with mineral fabric and AMS. Our project will continue adding white mica to the Carrara marble powder, in order to obtain synthetic samples of controlled proportion of calcite and mica, and controlled fabric. The final goal will be to show if it is possible to separate the contribution of the mica from that of the calcite to the total AMS.