Biotite oxidation processes in ash-flow tuffs (Mendoza, Argentina): A Mössbauer spectroscopy and chemical study

Abstract

Biotites and their alteration products (muscovites and chlorites) from six different cooling units of a dacitic to rhyodacitic ash-flow tuff of Early Permian age (Yacimiento Los Reyunos Formation, Mendoza, Argentina) have been examined by means of Mössbauer spectroscopy, XRD and electron microprobe techniques. Four different types of layer silicate minerals were identified: normal biotites, oxybiotites, muscovites and biotite + chlorite. Oxybiotites were distinguished by means of Mössbauer spectroscopy based on the presence of a Fe 3+ doublet, called des, which is characteristic of biotites that were dehydroxilated under heat treatment in laboratory experiments, and also, on the very low Fe 2+ Fe 3+ ratios obtained. Fe 3+ is present only at the octahedral sites. Mössbauer spectroscopy data suggested that some of the muscovites originated from oxybiotites, whereas biotite + chlorite seem to have been altered from normal biotites. The chemistry of layer silicate minerals showed that both the octahedral sites and the interlayer occupancy are deficient. With respect to normal biotites, oxybiotites are enriched in Mg and K, muscovites in Si, Al and K, and biotite + chlorite in Mg and Al. The Fe ( Fe + Mg) ratio decreases in oxybiotites. The rocks from which biotites and muscovites were extracted show oxidation ratios that correlate with those of the layer silicate minerals. This trend was not observed for the biotite + chlorite samples. The presence of muscovites and biotite-chlorite intergrowths are readily explained as due to hydrothermal alteration processes. The fact that oxybiotites plot well above the haematite-magnetite buffer curve in the Fe 2+Fe 3+Mg diagram indicates a high f O 2 -value, suggesting that they were formed during postmagmatic processes related to eruptive conditions rather than in the magma chamber. The possible mechanisms for oxidizing biotites are discussed considering three different stages. The presence of normal biotites only at the lowermost unit would indicate different eruptive conditions for this unit.