O- and H- Isotope constraints on Neogene high-K calc-alkaline and shoshonitic volcanic rocks and borates in the Kırka (Eskişehir) basin, W Anatolia

Abstract

ABSTRACT The O and H isotopic ratios of borates, clay minerals, and volcanic rocks from the Kırka basin (south of Eskişehir, W Anatolia) have been analysed in order to better constrain the conditions that led to precipitation of the borates. The δ18Owr values of the volcanic rocks range from + 7.6 to + 15.9‰, but many samples are enriched in 18O by post-eruption hydration, as evidenced by the positive correlation between 18O and H2O (LOI up to 3.1 wt.%) contents. The δ18O values of the borate and clay minerals (smectite) range from 10.0 to 18.8‰, and 19.5 to 31.3‰, respectively. The large variation among the δ18O values of the borate minerals either is likely related to oxygen isotope fractionation during their formation and/or indicates that they formed at different stages of evaporation of the original brines over a range of temperatures. Considering that borax was a primary phase in the basin, decreasing δ18O values from borax to colemanite and ulexite is compatible with decreasing formation temperatures. The δD values of borate and clay minerals do not show significant differences (−64 – −88‰, and −106 – −125‰, respectively). The calculated δ18OH2O and δDH2O values of the fluid in equilibrium with the borate minerals (−11.6–5.93‰ and −85 – −65‰, respectively) and in equilibrium with the smectite (14.4–17.0‰ and −126 – −67‰, respectively) indicates that the basin brines were dominated by geothermal fluids. Overall, the oxygen isotope systematics of the borates and smectite in the Kırka basin are consistent with the smectite and borate minerals forming from brines that resulted from the ponding and evaporation of geothermal fluids that had undergone water-rock interaction with the local felsic volcanic rocks.