Crystallization trend of titanomagnetites in an alkali basalt from Saint-Clément (Massif Central, France)

Abstract

Summary Three generations of homogeneous titanomagnetite in a hawaiite from Saint-Clément may be defined by differences in size, habit, or reflectance, and are believed to correspond to different crystallization stages of the lava. Chemical compositions were determined by electron microprobe, and by X-ray and thermomagnetic methods. Larger crystals are of intratelluric origin and occur either as inclusions in phenocrysts of early clinopyroxene (generation 1) or separately in the groundmass (generation 2); in the latter case, they always show evidence of resorption. Post-eruptive titanomagnetite (generation 3) is smaller and so highly oxidized that it is better termed titanomaghemite. Since the maghemitization, which is a low temperature process, alters the metallic ratios (especially the Fe/Ti ratio) the metallic contents at the time of the high-temperature crystallization are exactly known only for the intratelluric titanomagnetites. As intratelluric crystallization proceeds Ti content increases greatly (5 to 14%); Mn also increases (but slightly), A1 and Mg decrease, while results for Cr are inconclusive. Apparently these changes go on until the post-eruptive crystallization stage. The titanium trend is contrary to common belief. However, it is in accordance with predictions from the Fe-Ti-O system and may be explained by a decrease in oxygen fugacity during magma ascent.