An exceptional role of small magnetic particles in magnetism and paleomagnetism of rocks

Year, pages: 2007, 331 - 358

In this paper are presented the results of the investigation of basalts of the: Pleistocene to Quaternary age from southern Slovakia, Miocene to Pliocene age basalts from České Středohoří Mts. – North Bohemia, Cretaceous age basalts from the Syrian Arab Republic and the Cretaceous age basalts from Nigeria. The selected basalts have the reversed or at least anomalous, partly reversed RM. These types of basalts contain either dominant, or at least small part of Ti-rich Ti-Mt-es and more oxidized magnetic phase, probably titanomaghemite (Ti-Mgh). The reversed RM was acquired during cooling of magma from about 400 to 500°C (mostly of 450°C) to atmospheric temperature in the nature. The laboratory experiments have proven that κ of these basalts reached the maximum value in the interval 400 to 500°C, mostly at 450°C. With such behaviour of κ have coincided the results of continual measurements of the change of κ of the Ti-Mt-es with temperature. During inducing of partial thermoremanent magnetization (PTRM) of the samples, the minimum value of the PTRM has been reached at the temperatures corresponding to maximum values of κ. This behaviour has supported an idea that the above mentioned maximum κ corresponds to a presence of superparamagnetic (SP), small magnetic particles in the respective basalts. This is very important phenomenon. Rather similar behaviour of κ was registered in basaltic andesites and other intermediate andesites containing residual portions of the T-rich Ti-Mt-es, from central and eastern slovakia volcanic fields. I deduce, that the above described behaviour of Ti-rich Ti-Mt-es with a secondary Ti-Mgh, and an acquisition of the self-reversed PTRM by the small magnetic particles in the rocks is a general world-wide phenomenon. All results have proven very strong tendency in favour of the self-reversal PTRM of the rocks with small magnetic particles. The basalts of only 3 localities with normal RM from southern Slovakia were studied to compare their properties with those of the reversed RM. Normal RM is linked either with the non-stoichiometric magnetite or with the more oxidized Ti-Mt phase, in these type of basalts. It has been shown, that the rocks containing the magnetic minerals with the thermodynamically stable domain structures are only of normal RM, but the rocks, containing the small magnetic particles, as the carriers of magnetism, may acquire predominantly the reversed RM during magnetization, depending on the size and the shape of the particles.

Orlický, O. 2007. An exceptional role of small magnetic particles in magnetism and paleomagnetism of rocks. In Contributions to Geophysics and Geodesy, vol. 37, no.4, pp. 331-358. 1338-0540.

Orlický, O. (2007). An exceptional role of small magnetic particles in magnetism and paleomagnetism of rocks. Contributions to Geophysics and Geodesy, 37(4), 331-358. 1338-0540.