Stable single-domain to superparamagnetic transition during low-temperature oxidation of oceanic basalts

Abstract

Recent experimental data on synthetic titanomaghemites indicate that oxidation is accompanied by a decrease in magnetocrystalline anisotropy K1 as well as a decrease in the saturation magnetization σs. Theory of relaxation times predicts that the decrease of σs and K1 with oxidation will cause single-domain titanomagnetite grains with initially low blocking temperatures to become superparamagnetic as they are oxidized to titanomaghemite. For titanomagnetites (Fe3−x TixO4) of composition x = 0.4 and x = 0.5 the affected blocking temperature range is approximately 200° and 125°K below the Curie point, respectively. However, for x = 0.6 titanomagnetites the blocking temperature range affected is within 80°K of the Curie point and spans a temperature interval from 325° to as high as 380°K, depending on the degree of oxidation. The rapid quenching of oceanic pillow lavas (average composition x ≅ 0.6) produces efficient thermoremanence carriers but will also result in a significant grain size and blocking temperature distribution. Thus low-temperature oxidation of oceanic basalts could cause a significant portion of the originally stable carriers of natural remanence to become superparamagnetic. This mechanism could aid in explaining the large natural remanent magnetization intensity decrease of dredged basalts with distance from the mid-Atlantic ridge.