Rotational remanent magnetisation as a magnetic mineral diagnostic tool at low rotation rates

Hounslow, Mark W and Horng, Chorng-Shern and Karloukovski, Vassil (2023) Rotational remanent magnetisation as a magnetic mineral diagnostic tool at low rotation rates. Geophysical Journal International, 232 (1). pp. 300-321. ISSN 0956-540X

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Abstract

Summary Prior work on rotational remanent magnetisation (RRM) and rotational anhysteretic remanent magnetisation (ARMROT) has demonstrated promise for magnetic mineral identification in earth materials. One challenge has been to calibrate the measurements to magnetic mineral types and microstructural controls, since previous studies have used differing spin rates, alternating field (AF) intensities and decay times, which hinders a comparison of datasets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25 to 3 Hz [rps] which allows a wide range of RRM and ARMROT characteristics to be utilised (at 100 mT AF field, 100μT bias field). Sets of magnetic mineral extracts from sediments, and well characterised rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM- ARMROT behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values, with differences in Bg and ARMROT ratios at 0.5 and 2.5 Hz [rps] allowing grain-size discrimination. The positive Bg values, and changes in RRM and ARMROT with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of -83 to -109 μT (at 0.5 Hz [rps]) and unusual RRM variation at low rotation rates caused by anisotropy affects. In contrast to previous work, based on crushed and sized natural magnetite at high spin rates, Bg for single domain magnetite from intact bacterial magnetofossils from Upper Cretaceous Chalk has some of the lowest Bg (0 -1 μT) and displays a steep decline in ARMROT with increasing rotation rates. A simple tool for particle size characterisation of magnetite may be the ratio of ARMROT at spin rates 2.5 and 0.5 Hz [rps]. Stability of RRM is better studied using RRM acquisition with increasing AF field intensity, since static demagnetisation imparts a nuisance gyroremanence along the field axis. Mineral microstructure, dislocations and particle interactions are likely additional effects on RRM behaviour that need more investigation.