Origin of false components of NRM during conventional stepwise thermal demagnetization

Abstract

Many Permian-Triassic dolerite samples from the Siberian Trap Large Igneous Province exposed to conventional stepwise thermal demagnetization at 250-450 °C display mid-temperature remanence (MTC) directed opposite to the high-temperature NRM component. Alternating field (A.C.) demagnetization fails to isolate the antipodal component, but it appears during continuous thermal demagnetization, though in a different temperature range. Laboratory experiments and simulations prove that MTC remanence is an artifact resulting from magnetic memory of self-reversing titanomagnetite grains oxidized at low temperature. This effect can deform the results stepwise thermal cleaning and be responsible for misleading patterns of paleomagnetic directions. Given that oxidized titanomagnetite grains are widespread in volcanic rocks, we suggest to identify true paleodirections by combined continuous and stepwise thermal demagnetization. The extension of our model to the case of NRM2 overprint directed at some angle to partially reversed primary NRM1 component accounts for the difference between the results of stepwise and continuous thermal demagnetization observed in samples of the Steens Mountain basalt (USA).