Remanent Magnetizations in Oxidized Olivine

Remanent magnetization properties of oxidized synthetic olivine grains (Fe0.6Mg1.4SiO4, i.e. FA30) have been studied. Chemical, anhysteretic, isothermal, and thermal remanent magnetizations and their coercivity spectra were investigated. Oxidation at 500 °C for periods from a few minutes to several days produces a magnetic phase, thought to occur in the form of small magnetite inclusions in the single-domain and pseudo-single domain size ranges. This material carries strong and stable permanent magnetization and it is therefore concluded that oxidation of natural olivine-bearing materials, either deuterically or by later reheating, can readily produce remanent magnetizations of palaeomagnetic significance. It is further observed that once the initial stage of grain nucleation is passed the thermal, chemical and anhysteretic remanences are all roughly equal, implying that palaeointensity estimates from such material may not be grossly in error. ARM is found to be about 20 per cent less than TRM, and to possess a closely similar coercivity spectrum, thus supporting recent suggestions that ARM be used as a non-destructive analogue of TRM in various rock magnetic experiments.