Vertical variation in the deuteric oxidation of titanomagnetites in an ignimbrite deposit: Kızılkaya Ignimbrite (Cappadocia, Turkey)

Abstract

Abstract Titanomagnetites are arguably the most important Fe–Ti oxide mineral in magmatic rocks because they are widely used in paleomagnetic, magnetic anisotropy, and petrologic studies. Elemental concentrations of Ti in titanomagnetites can change rapidly with varying temperature and oxygen fugacity, with oxidation producing “oxyexsolution” lamellae. Titanomagnetites of the ca. 5 Ma Kizilkaya ignimbrite deposit in Cappadocia exhibit systematic vertical variation of trellis-type oxyexsolution forms in three sampled sections. The basal zones of Kizilkaya ignimbrite contain homogeneous titanomagnetites, whereas middle and the upper zones of the deposit comprise increasingly oxidized titanomagnetites where the oxyexsolved Ti-rich lamellae get thicker and more populated towards the top. The existence of oxidized titanomagnetite grains with increasing volume of trellis-type exsolution lamellae indicates that the middle and the upper zones of the deposit cooled down more slowly than the basal zones and were influenced by deuteric oxidation during cooling. The progressive increases in oxidation upsection are also correlated with increasing devitrification. Oxidation results in the decrease of magnetically effective grain sizes of ferromagnetic titanomagnetites, which are surrounded by paramagnetic Ti-rich oxyexsolution lamellae, consequently reducing the saturation magnetization Js. Vertical variation in titanomagnetite textures requires careful selection of the sampling position in an ignimbrite deposit if magnetic properties are to be determined.