Microscale inversion modelling of Black Hill Norite, South Australia

In this work, we conduct inverse modelling of all opaque minerals in a thin section from the Black Hill Norite (BHN), South Australia. The BHN is a mafic intrusion and one of three plutons within the Black Hill Complex. The BHN is important as it has yielded an Early Ordovician paleomagnetic pole position for Australia. Paleomagnetic measurements from the BHN exhibit an intensity and direction different from the local magnetic inclination and declination, indicating a strong and stable NRM.

The total magnetic moment of the thin section was measured in a cryogenic magnetometer at Geological Survey of Norway and yielded D = 171°, I = 3°, and m = 4.5 x 10^-7 Am². An electron backscatter image was made of the entire thin section using a Phenom XL scanning electron microscope to map and identify the opaque minerals. The opaque minerals were ilmenite and magnetite. The thin section was then scanned using NTNU’s Scanning Magnetic Microscope in a near free field environment, therefore mapping the sample’s remanent anomalies. All opaque mineral grains in the thin section were modelled using a multistep parametric inversion approach. Each grain was modelled in two different 3-D geometries, as a single frustum volume and as a series of smaller tabular array volumes. This bulk and discretized modelling allowed for a multi-scale analysis of each grain. The calculated total moment of the frustum and tabular array modelling was D = 356°, I = 6.2°, m = 2.2x10^-7 Am², and D = 359°, I = 8.3°, m = 3.3x10^-7 Am² respectively.

Three modelled remanent magnetization populations were identified in this sample: discrete paramagnetic ilmenite, ilmenite with magnetite reduction-exsolution lamellae, and discrete magnetite with single-phase oxy-exsolved ilmenite. Although discrete ilmenite is paramagnetic, magnetic modelling of this thin section indicates that ilmenite grains without lamellae can be attributed inaccurate modelled magnetizations due to nearby magnetite grains.