[SEM17-P05] Rock-magnetic properties of single crystals separated from granitic rocks: A guide for selecting samples for paleointensity determination
Here we report the rock-magnetic properties of the same single plagioclase samples used in our paleointensity experiments. An alternating gradient magnetometer and magnetic property measurement systems were used for magnetic hysteresis loop and low-temperature magnetometry measurements. The natural remanent magnetization (NRM), isothermal remanent magnetization (IRM), thermal remanent magnetization (TRM), and anhysteretic remanent magnetization (ARM) were measured using superconducting quantum interference device magnetometer. The results indicated that the magnetic carrier of the plagioclase samples was Ti-poor titanomagnetite in the single domain to pseudo-single domain states. For some samples, ARM and TRM anisotropy were measured. The ARM anisotropy was large compared to that of the whole-rock sample, and the anisotropy degree and directions of the anisotropy axes were similar for ARM and TRM. Large remanence anisotropy of the sample could affect the paleomagnetic records (Veitch et al., 1984; Selkin et al., 2000; Paterson, 2013; Usui et al., 2015). The possibility of cancelling the anisotropy effect on paleointensity by using the anisotropy tensor and averaging a number of samples is discussed.
Additionally, magnetic hysteresis loop and low-temperature magnetometry measurements were performed on plagioclase samples separated from 22 granitic rocks. We found that granites in the magnetite end of the magnetite/ilmenite series that also have relatively low susceptibility have the highest potential for paleointensity determination. We recommend focusing future studies on this subset of intrusive bodies.