Identifying source of Valibaba Tepe ignimbrite (Kayseri, Türkiye) using high resolution x-ray tomography (micro-CT)

English Abstract:

Recently, anisotropy of magnetic susceptibility (AMS) is widely used for determination of flow directions and source positions of ignimbrites. Magnetically anisotropic minerals magnetize in certain directions which are governed by primarily by crystallography and/or grain shape. By accepting that the orientation of long axis are parallel to the flow direction and by assuming that the multi domain magnetic minerals have maximum susceptibilities parallel to their long axis, the maximum susceptibility orientations determined by AMS were accepted as the flow directions. However, the physical origin of the AMS fabric in ignimbrites remains still enigmatic. The emplacement temperatures, lithification and welding degrees, and alteration occurred during cooling after emplacements operate to change the magnetic properties. In this study, a new method which directly measures the orientations of long axes of magnetic or non-magnetic minerals in a rock is offered. The offered new method is high resolution x-ray tomography which image the interior and exterior of samples and distinguish their components (glass, pores, minerals) using their x-ray attenuation coefficients. Each mineral is imaged in different gray scales according to its electron density, and the effective atomic number, and the energy incoming X-ray beam and easily distinguished from other components for analysis. In this study, source of Valibaba Tepe ignimbrite (VTI) was examined in order to test the tomography method. Source of VTI was already determined by isopach and isopleths maps and field observations. Here, we also used AMS method to compare methods. After AMS measurements on samples, they were imaged by tomography and the dominant orientations were determined. Tomography which is unaffected from some properties in rocks such as magnetism and directly measures the orientations of aligned minerals is assumed to be a more reliable method according to AMS in determination of flow directions. Orientations of long axis of magnetic minerals obtained from tomography analysis give a source area located beneath Erciyes Volcano. This area has the remnant of an older volcano called Koç Dağ which was suggested as a source for VTI using sedimentological evidences, isopach and isopleth maps and clast fabric imbrication in previous studies.