The origin of thermoremanent magnetization of two basalts containing homogeneous single phase titanomagnetite

Abstract

Two Tertiary basalts originating from Parkstein and Rauher Kulm, Oberpfalz, Germany, containing almost spherical homogeneous titanomagnetite grains of the same chemical composition (0.65 Fe 2TiO 4 − 0.35 Fe 3O 4) but different grain sizes were investigated with regard to the nature of their thermoremanent magnetization (TRM). In both samples it was found (i) that the TRM component of moderate stability (coercive force <100 Oe) is associated with multidomain grains larger than 1 micron in diameter. The magnitude of this TRM component could be explained quantitatively by the multidomain model of Stacey [3]; (ii) the stable component of TRM could be interpreted with the single domain model of Néel [1] and resides in the particles smaller than 1 micron. The theory of Verhoogen [5] for the stable component of TRM is not relevant for the investigated titomagnetites as the dislocation density in the ore grains was found to be too low (5 × 10 6/cm 2). Electron microscope studies did not show any signs of the “single domain subregions” in the large ore grains as postulated by Ozima and Ozima [6].