AEROMAG IN THE LAB: APPLICATION OF THE ULTRAMAG2 TO SAMPLES OF HIGH-GRADE GNEISS FROM THE NEW JERSEY HIGHLANDS

The magnetic properties of rocks within high-grade metamorphic terrains are a function of protolith, conditions of metamorphism, structural history, and weathering or alteration. By applying petrologic investigations within a magnetic context important relationships and processes may be discovered. For example, the ability to connect petrologic data to the results of aeromagnetic surveys may allow extrapolation of processes observed at the microscopic scale to regional scales. Yet the ability to directly correlate petrologic findings to aeromagnetic anomalies remains tenuous due to disparate spatial scales of observation. In an effort to address this problem we present a new instrument (UltraMag2) that generates images of the magnetic induction field above rock slabs at the hand-sample scale. UltraMag2 consists of three perpendicular linear axes that precisely position a small triple-axis magnetometer during the execution of a mapping routine. The magnetometer has an optimal spatial resolution of approximately 1 mm³ and a magnetic induction sensitivity of 0.1 μT. Samples upwards of 20 cm in length can be imaged. Post-processing of data generates a series of graphics including an analysis of drift and drift correction, images of magnetic induction magnitude and orientation, and a set of polynomial coefficient maps. Graphics can be used in combination with high-resolution optical scans to target locations for thin section billets or specimens for additional magnetic and petrologic analyses. We present an application of the instrument to samples collected from the New Jersey Highlands, which display a heterogeneous distribution of ferrimagnetic minerals with regions devoid of magnetic sources, areas with disseminated ferrimagnetic grains, and clustered sources. Some clustered sources are linear and suggest magnetite vein fill. Magnetic images have allowed for the targeted extraction of billets for thin section fabrication. The UltraMag2 helps to bridge the gap between regional scale aeromagnetic surveys and microscopic petrologic investigations by providing a spatial context of magnetic induction data for petrologic analyses.