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

Prior work on rotational remanent magnetization (RRM) and rotational anhysteretic remanent magnetization (ARM(ROT)) 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 data sets. Using a RAPID magnetometer we show that the range of usable practical rotation rates is 0.25-3 Hz [rps] which allows a wide range of RRM and ARM(ROT) characteristics to be utilized (at 100 mT AF field, 100 mu T bias field). Sets of magnetic mineral extracts from sediments, and well characterized rock samples that contain the key magnetic minerals magnetite, pyrrhotite and greigite are used for a calibration of the RRM-ARM(ROT) behaviour. Detrital pyrrhotite and pyrrhotite-bearing phyllites have largely small positive effective field (Bg) values (up to 6 mu T), with differences in Bg and ARM(ROT) ratios at 0.5 and 2.5 Hz [rps] allowing grain size discrimination. The positive Bg values, and changes in RRM and ARM(ROT) with rotation rates allow distinction of pyrrhotite from magnetite and diagenetic greigite. Diagenetic greigite has Bg values of -83 to -109 mu T (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 mu T) and displays a steep decline in ARM(ROT) with increasing rotation rates. A simple tool for particle size characterization of magnetite may be the ratio of ARM(ROT) 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 demagnetization 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.

Automated summary

Prior studies on RRM and ARM(ROT) have shown promise in identifying magnetic minerals in earth materials, but the calibration of measurements to different magnetic mineral types and microstructural controls has been challenging due to the use of varying spin rates and AF intensities. This study demonstrates that a RAPID magnetometer can be used to achieve a wide range of RRM and ARM(ROT) characteristics within a practical rotation rate range of 0.25-3 Hz [rps]. The calibration of RRM-ARM(ROT) behavior using sediment magnetic mineral extracts and well characterized rock samples reveals differences in effective field values and ARM(ROT) ratios at different rotation rates, allowing for discrimination of grain size and distinction of different magnetic minerals.