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

The O and H isotopic ratios of borates, clay minerals, and volcanic rocks from the Kirka basin (south of Eskisehir, W Anatolia) have been analysed in order to better constrain the conditions that led to precipitation of the borates. The delta O-18(wr) values of the volcanic rocks range from + 7.6 to + 15.9 parts per thousand, but many samples are enriched in O-18 by post-eruption hydration, as evidenced by the positive correlation between O-18 and H2O (LOI up to 3.1 wt.%) contents. The delta O-18 values of the borate and clay minerals (smectite) range from 10.0 to 18.8 parts per thousand, and 19.5 to 31.3 parts per thousand, respectively. The large variation among the delta O-18 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 delta O-18 values from borax to colemanite and ulexite is compatible with decreasing formation temperatures. The delta D values of borate and clay minerals do not show significant differences (-64 - -88 parts per thousand, and -106 - -125 parts per thousand, respectively). The calculated delta O-18(H2O) and delta D-H2O values of the fluid in equilibrium with the borate minerals (-11.6-5.93 parts per thousand and -85 - -65 parts per thousand, respectively) and in equilibrium with the smectite (14.4-17.0 parts per thousand and -126 - -67 parts per thousand, respectively) indicates that the basin brines were dominated by geothermal fluids. Overall, the oxygen isotope systematics of the borates and smectite in the Kirka 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.