Electron Probe Microanalyser Analyses of Basaltic Titanomagnetites and their Significance to Rock Magnetism

Abstract

Summary Nineteen analyses of optically homogeneous titanomagnetite, from basalt rock samples of widely varying magnetic properties are described. All the samples contain titanomagnetites with similar amounts of iron and titanium, and manganese is found to be the only ubiquitous minor element. No certain differences exist between normally and reversely magnetized samples, but a possible difference in titanium distributions and content warrants further investigation. The limited compositional ranges lead to a distinctly limited range of theoretically calculated Curie points (assuming that the compositions lie on the magnetite-ulvospinel join) which is sharply peaked in the o°C to 100 °C interval. The actual broad distribution of measured high field Curie points stretches from o °C to 600 °C and is quite different from this calculated distribution. It is suggested that this disagreement is due either to the sub-microscopic exsolution of titanium rich and iron rich phases or to oxidation of the titanomagnetites resulting in cation vacancies and so higher Curie points. An order-disorder phenomenon is also a possible explanation but the existence of such a property has yet to be demonstrated for titanomagnetite. The average highly titaniferous nature of the titanomagnetites, corresponding to an equivalent magnetite-ulvospinel solid solution of 0–28 Fe3O4 0.72 Fe2Ti04, together with the divergence between computed and actual strong field Curie points, makes it possible that self-reversal of the natural remanent magnetization of some of the samples has taken place by the oxidation mechanism described by Verhoogen.