A New Instrument for Magnetic Imaging of Rock Slabs at the Hand-Sample Scale

Abstract:

Magnetic imaging techniques have provided a wealth of detailed information typically at two disparate spatial scales including the regional (e.g. satellite, airborne, and marine) and grain scales (e.g. Bitter colloid and magnetic force microscopy). However, there is a general lack of imaging techniques at the hand sample scale. We present a new instrument, procedure, and processing routine that automatically maps the magnetic flux density vector field above a slab of rock at a sub-millimeter resolution, which bridges the gap between the traditional magnetic mapping scales. This low-cost instrument consists of two linear axes that position and raster a stylus across the surface of a sample. Attached to the stylus is a MAG3110 triple axis magnetometer, which has an optimal spatial resolution of approximately 1 mm². This technique has been particularly informative for metamorphic studies concerning the equilibria of ferrimagnetic minerals such as magnetite. For example, magnetic images have revealed complex anomalies within mafic granulites that indicate the heterogeneous production and removal of magnetite. Some mafic dikes display magnetic anomalies that are associated with partial retrograde metamorphism and hydration. Magnetic images of a sample of banded iron formation have documented sedimentary layering, as well as positive anomalies associated with the occurrence of leucosome. This association may provide key implications for anatectic redox reactions. Specimens extracted from various locations on slabs characterized by heterogeneous magnetic anomalies can be used to document disparate magnetic properties such as magnetic susceptibility, NRM, hysteresis, and coercivity distributions. As such, this technique may provide a context for targeted rock magnetic studies. The instrument provides a direct link for petrologic studies to magnetism that may be used as a small scale analog for regional and planetary magnetic anomalies.