First insights into the geochemical and petrological features of the Eger Rift’s plumbing system (Czech Republic)

The western Eger Rift (Czech Republic) is a non-volcanic rift setting of intraplate seismicity that is characterized by abundant degassing of mantle-derived fluids. Since 2019, gases obtained from the free gas phase and volcanic rock samples of Quaternary age have been collected in order to determine the origin and evolution of volatiles in the system and define the magma chamber p-T conditions. Results of the CO₂-dominated gas discharges of the Bublák and Hartoušov mofette fields yield an ~93 % mantle input (considering a subcontinental lithospheric mantle) for He. Ne isotopic ratios range from 9.8 to 11.0 for ²⁰Ne/²²Ne and from 0.0282 to 0.0480 for ²¹Ne/²²Ne. Notwithstanding the samples enriched in ²⁰Ne likely due to mass fractionation, many samples show a mixed atmospheric-mantle type source for Ne. However, an additional crustal input cannot be excluded. ⁴⁰Ar/³⁶Ar ratios also cover a wide spectrum of values (between 300 and 4680). Overall, gas samples typically present a higher-than-atmospheric value with ⁴⁰Ar likely deriving from the mixing of an atmospheric and deep source. The ⁴He/⁴⁰Ar* ratio is moderately constant and falls within the MORB range, suggesting an unfractionated magma that originates from mantle sources.. Results obtained from mineral quantitative analyses and by using thermobarometry of orthopyroxene and clinopyroxene rim pairs of matrix grains yield predominantly temperature and pressure conditions of 700 ±100 °C and 1.1 ±0.5 GPa, respectively, indicating a lithospheric depth that ranges between 40 - 45 km for 1.5 GPa and 20-25km for 0.5 GPa. In addition, pairing cores of mm-sized pyroxenes point to a temperature of 1100 ±100 °C and a pressure of 2.5 ±0.5 GPa that correspond to a lithospheric depth of ~75 km. Those minerals that indicate greater depths are likely the oldest ones as they were able to grow for longer times during their ascent. On the other hand, secondary overgrowths or smaller matrix grains represent younger grains that have grown during magma evolution. Therefore, their diverse chemistry and the wide range of p-T conditions reveal rising (and cooling) of magma.

This research is a part of the “MoRe-Mofette Research” and MoCa - “Monitoring Carbon” projects, which were funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) – 419880416, 461419881.