Investigation of the relationship between the host rock – alteration mineralogy and in-situ ultrasonic pulse velocity measurements of Cappadocian ignimbrites

The Middle Miocene-Late Quaternary volcanism produced 10 major ignimbrite deposits separated by terrestrial-lacustrine sediments and lava flows in Central Anatolian Volcanic Province. These ignimbrite deposits have been exposed to alteration and secondary filling processes as a result of hydrothermal activity during and after their depositions in various regions. Amorphous juvenile glass in ignimbrites are observed to devitrify into zeolites, opal-A, opal-CT, clay group minerals and occasionally into sulphate minerals as a result of the alteration processes. Secondary fillings are usually represented by carbonate and seldomly by sulphate crystallisation via the fracture – crack systems. The aim of this thesis, supported by TUBITAK (Project No: 113Y439), is to investigate the relationships between mineralogical-petrographical features of Cappadocia ignimbrites and the ultrasonic pulse velocity (UPV) measured in-situ on the outcrops. The ultrasonic pulse velocity measurements were performed in 9 different ignimbrite deposits (Kavak, Zelve, Cemilköy, Sarımadentepe, Sofular, Tahar, Gördeles, Kızılkaya and Valibabatepe). It was observed that the petrographical features of ignimbrites directly affected the propagation velocity of the ultrasonic pulses. Particularly, the ultrasonic pulse velocities in altered rocks are significantly different than those measured in unaltered counterparts. The pyrogenic and alteration mineral assemblages of ignimbrites were identified and their porosities have been calculated with SEM-EDS and X-ray microtomography analyses. It was determined that the devitrification induced secondary crystallisation caused an increase in the amount of crystals and resulted with a decrease in porosity and an increase in welding. Therefore, it was measured that ultrasonic pulses were moving faster in devitrified ignimbrites than their equivalents that have not gone through alteration. In conclusion, this thesis study reveals that the easy-to-measure in-situ ultrasonic pulse velocity is a function of the petrography of the rock unit and can be correlated with its degree of alteration. Keywords: Alteration, devitrification, Cappadocia ignimbrites, Ultrasonic pulse velocity.