Change from inverse to normal magnetic fabrics through thermal treatment

Abstract:

The Gusandong Tuff is an extensive rhyolitic ignimbrite that has been used as an excellent key bed in the Cretaceous Gyeongsang Basin, SE Korea. Magnetic fabric analysis in the tuff, using AMS (Anisotropy of Magnetic Susceptibility) technique, shows an anomalous fabric which has horizontal k₃ and vertical k₁ axes in several sites. The fabric is interpreted to be an inverse one attributed to single-domain magnetites. We attempted a stepwise thermal treatment to investigate the changes of AMS axes and parameters (T, P_J) in four sites (KT11B, KT11M, KT18B, KT18M). All these sites dominantly include the specimens showing the anomalous fabric. Rock magnetism shows that magnetites are the main carrier of the AMS fabric. The changing patterns of magnetic fabric during heating are classified into 4 types: (1) Type-0 is characterized by unchanged three principal axes (k₁, k₂, k₃) over all heating steps. (2) Type-I shows the exchange of k₁ and k₂ axes each other. (3) Type-II is characterized by the exchange of k₂ and k₃ axes each other. (4) Type-III shows that three principal axes switch their positions with one another. Except for Type-0, most of the magnetic fabrics are transformed gradually. The directional changes of the axes begin between 450-580°C and then terminate near 670°C. After the thermal treatment, all the fabrics become geologically normal as vertical k₃ and horizontal k₁ axes. It is also notable that the beginning temperatures are nearly equal to those of inflection points in the T-P_J plots. In the sites KT18B and KT18M which showed girdle-distributed and vertical k₁ axes, respectively, before the treatment, all the k₁ axes become horizontally and co-directionally clustered after the treatment. This result indicates that the thermal treatment can be a strongly useful tool for eliminating the inverse AMS magnetic fabric of natural rocks. The changes of axis orientation and magnitude observed in this study can be reasonably explained with the theoretical inverse fabric model proposed by Rochette et al. (1992) and Ferré (2002). The gradual transformation of the magnetic fabrics is, however, out of accord with the model. This discrepancy is probably caused by the heterogeneity of natural rocks which have various compositions, grain sizes, orientations, shapes, and anisotropy degrees of constituent minerals.