ISTerre seminar

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Magnetic signature of ultramafic and mafic bodies, mapping from the regional to the thin section scale

Friday 26 June 2026 - 10h30

Suzanne McEnroe - NTNU
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Magnetic anomalies are widely used to interpret the structure and composition of the Earth’s crust, yet their expression in intrusive rocks is controlled by several interrelated factors: the consistency of magnetic directions within rocks, the balance between remanent and induced magnetization (Koenigsberger ratio, Q), and the scale at which observations are made. Here, we investigate these controls, together with their mineralogical origins, in two Norwegian intrusive systems: the Proterozoic Bjerkreim–Sokndal (BKS) layered intrusion in southwestern Norway and the Reinfjord Ultramafic Complex (RUC) in northern Norway. Characterizing the magnetic sources of ultramafic rocks is particularly challenging because their mineral assemblages often form under deep crustal conditions and may be modified by later alteration. In the RUC, we combine petrophysical and rock magnetic measurements with scanning magnetic microscopy (SMM) on pristine, unaltered samples to identify the minerals responsible for magnetic signals at the microscale. These observations are linked to magnetic patterns seen in thin sections and to bulk rock properties and larger-scale anomalies. The pristine ultramafic rocks have magnetic signatures that differ markedly from those of serpentinized equivalents, demonstrating that alteration exerts a strong control on magnetic behavior and anomaly expression. The BKS layered intrusion provides a natural laboratory for understanding how magnetic properties vary within layered magmatic systems, as units with strong remanent magnetization occur alongside layers dominated by induced magnetization. We integrate laboratory measurements with ground-based and high-resolution uncrewed aerial vehicle (UAV) magnetic surveys to quantify how magnetic strength, direction, and consistency vary within and between layers. UAV data reveal short-wavelength magnetic features that are not captured in regional airborne surveys, showing that increasing observation distance smooths magnetic signals and reduces directional information. Units with contrasting magnetic properties, such as low susceptibility Leuconorites with coherent remanent magnetization, and strongly remanent gabbronorites can be clearly distinguished, whereas units dominated by induced magnetization produce more uniform anomaly patterns. Combining datasets across scales yields a continuous magnetic field that captures both fine-scale layering and large-scale structure. Together, these results demonstrate that magnetic anomaly expression depends not only on the amount of magnetization, but also on how well magnetic directions are aligned and how data are observed across scales. These studies highlight the importance of integrating mineral-scale observations with multi-scale geophysical data and show that high-resolution UAV surveys provide a powerful tool for resolving the magnetic structure of complex, strongly remanent geological systems.

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Organizing team : Géophysique des volcans & géothermie

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères