Magnetic Interaction in Self-reversing Andesitic Pumice in Relation to Iron Alloys

Andesitic pumice erupted in 1985 from Nevado del Ruiz volcano (Colombia) has been magnetized in opposite direction t o the present geomagnetic field. This self-reversal is probably due t o an exchange mechanism between two phases of the haematite-ilmenite series, which is similar to the magnetic coupling observed in synthetically grown antiferromagnetic-ferromagnetic FeMn-FeNi films produced under ultra high vacuum conditions. One of the fundamental principles of palaeomagnetism and rock magnetism states that a rock specimen when cooled in the earth's magnetic field will acquire a natural remanent magnetization (NMR) which is usually parallel to the applied field. However, the NMR of andesitic pumice emitted during the 1985 eruption of Nevado del Ruiz volcano (Colombia) shows a direction opposite of the present geomagnetic field [I]. For the first time these rocks present firm evidence that a self-reversal process actually has controlled the acquisition of NMR. The self-reversing properties are demonstrated in the laboratory during repeated cycles of heating (up to 165 OC) and subsequent cooling (to room temperature) in air and zero magnetic field, see figure 1. The reversal from negative to positive polarity near 130 OC upon heating and from positive back to negative polarity on cooling can be clearly seen. From the reversibility of the NMR behaviour during heat treatment and from additional strong field magnetization measurements it is concluded that the magnetic phases are stable up to at least 700 OC. Self-reversing remanent magnetization was first discovered in dacitic rocks from Japan by Nagata [2] and co-workers [3-51. Two major interaction models leading to self-reversing properties have been proposed [6, 71; firstly NBel's N-type with two sublattices in a one phase model, aad secondly different types of two phase models with magnetostatic (i.e. dipole-dipole) or (super)exchange interaction. The energies involved in these interactions are usually assumed to be different by orders of magnitude. Estimates of the interaction energy may be obtained from the applied field amplitude needed to suppress the self-reversing mechanism. This was achieved by thermoremanent magnetization measurements, which simulate the natural magnetization process between the maximum Curie temperatures measured in the samples (ca 400 OC) and room temperature in magnetic DC fields of increasing amplitude. Figure 2 shows that the critical field necessary to completely suppress self-reversal is very weak, about +,., Heating i Cooling i Heating