Detecting reversals in Earth’s magnetic field using continuous core samples: a new measurement system

Abstract

To detect reversals in the geomagnetic field of the Earth, the direction of the permanent magnetic field recorded by rocks during formation must be measured. Previous studies have shown that the 2054Ga Bushveld Igneous Complex (BIC), located in South Africa, recorded at least seven reversals in the geomagnetic field of the Earth during emplacement and cooling. The exact boundaries delineating a change in polarity are poorly constrained. The Bushveld Drilling Project (BVDP) is retrieving 2.5km of continuous drill core samples for various investigations, and this provides the opportunity to constrain the reversal boundaries. Existing commercial measurement systems cannot cope with the complete analysis of all the drill cores: small subsamples must be selected for detailed study. Without an instrument to characterise the magnetic field recorded by all the drill cores, subsampling is ineffective as samples are chosen randomly. The work presents an investigation on the design and evaluation of a prototype measurement system to detect geomagnetic reversals recorded in continuous drill cores rapidly. Tests with synthetic drill cores and short sections of weakly magnetised drill cores from the Main Zone of the BIC show that the proposed system can resolve changes in the direction of the magnetic fields- on the order of a few nT. The minimum thickness of a layer of rock containing a reversed polarity that the measurement system can resolve is measured to be 30mm. Provided that drilling overprints do not significantly alter the direction of the remanent magnetisation, the findings show that, in principle, the measurement system can detect changes in the direction of the magnetic field in drill cores from the BIC.