WHAT IS THE MINERAL SOURCE OF EXTREME REMANENT MAGNETISM IN THE HESKSTAD ANOMALY OF THE BJERKREIM-SOKNDAL LAYERED INTRUSION, SOUTH ROGALAND, NORWAY?

Peter combined his talents in mineralogy, petrology and structure to tackle what he referred to as the 'Mt Everest of Magnetism'.

The 930 Ma, Bjerkreim-Sokndal Layered Intrusion, lies in a tight, doubly plunging syncline produced during high-temperature solid-state deformation. It contains six magmatic megacyclic units, each beginning with intrusion of a noritic magma, which, over the entire episode, gradually became more reduced and more mafic. Typically each megacycle began with intrusion of new magma that mixed with more evolved magma already in the chamber, temporarily producing plagioclase- and hemo-ilmenite-rich cumulates. Later the magma returned to normal fractional crystallization, leading to evolved compositions, and precipitation of Fe-richer silicates, Ti-richer ilmenite, and magnetite.

The dominant oxides in the layers result in negative remanent magnetic anomalies over hemo-ilmenite–rich cumulates and positive induced anomalies over cumulates dominated by magnetite and Ti-rich ilmenite. Magnetic contrasts are most striking over the top megacyclic Unit IV, where a negative remanent anomaly traces for >15 km. Here magnetic intensity of the ground anomaly varies, depending on position in the south-plunging syncline. At the northern hinge, where layering and foliation dip south, the trough is at 47,600 nT in an Earth field of 50,400nT at this latitude. On the east limb at Heskestad, where the distal edge of Unit IV of the layered series abuts against the gneissic basement below the intrusion, layering and foliation are vertical, parallel to the reversed early Neoproterozoic magnetizing field. Here the ground anomaly locally decreases to 19,850 nT, canceling out > 60% of the Earth field.

Heskestad samples yielded mean induced magnetization 4.5 A/m, natural remanent magnetization 23 A/m, and corresponding Q=5.1. Strong lattice-preferred orientations measured by EBSD show hemo-ilmenite (001) planes and orthopyroxene c-axes lie quasi-parallel to the Neoproterozoic magnetizing field. Such orientations could lead to enhanced remanence related to hematite exsolution in ilmenite and also to exsolved rods and blades of hemo-ilmenite oriented parallel to c-axes in orthopyroxene. By combining thin-section-scale electron probe element maps with micro-magnetic maps individual grains carrying remanence can be identified. The dominant micro-magnetic anomalies correlate with the pyroxene grains, suggesting that new origins must be sought for this extreme remanence.