Mineralogical and magnetic characterisation of iron titanium oxides in soils developed on two various basaltic rocks under temperate climate
Résumé
The objective of this study was to characterize alteration processes of iron titanium oxides in andic soils developed on basaltic rocks under temperate climate. Among the magnetic minerals, titanomagnetites are the most abundant and represent the major spinel in basalts. In order to understand processes involved in the alteration of iron titanium oxides from basaltic rocks associated to soils, two pedons developed on Tertiary massive basaltic formations submitted to temperate climate were considered. The magnetic minerals have been characterized by studying rock and soil magnetic parameters i.e. magnetic susceptibility and Curie temperature. Chemical composition was investigated by optical and electron microscopy (EMPA, SEM-EDS) and by micro-proton induced X-ray emission (µPIXE). Because mineralogy of the parent rocks controls those of the soils, we have first studied the rocks. The thermomagnetic curves obtained on the two studied rocks show that initial titanomagnetite is preserved in one sample (MC-rock) while the other (PC-rock) contains near-pure magnetite possibly issued from exsolution process. Microscopic observations confirm the exsolution lamellae on titanomagnetites and show a Cr content gradient in chromian spinel partially enclosed in olivine. These observations suggest a deuteric alteration. The two profiles exhibit increasing Curie temperature between the rocks and soil horizons, indicating a transformation of magnetic minerals. Results emphasize the maghemitization of iron titanium oxides in andic soils in a temperate climate.