Characterisation of deep intra-continental magma reservoirs – Insights from noble gases and p-T estimates into the western Eger Rift (Czech Republic)

Abstract

The western Eger Rift (Czech Republic) is a currently inactive volcanic rift that is characterised by earthquake swarm activity and degassing of mantle-derived fluids. Free gases and (ultra-) mafic phenocrysts and xenoliths are used to determine the origin and evolution of volatiles in the system and to define the magma reservoir p-T conditions. Isotope compositions of helium in both fluids and minerals are up to 6.79 R_A. ²⁰Ne/²²Ne ratios range up to ∼11.0 and ²¹Ne/²²Ne ratios up to ∼0.048. Correction for isotopic mass fractionation of Ne suggests a source of mixed atmospheric and mantle type components with a significant crustal input. The slightly lower-than-mantle ³He/⁴He ratios and the Ne isotopic compositions indicate that these gases may have been impacted by a subduction-related crustal component during the Variscan (or Hercynian) Orogeny. ⁴⁰Ar/³⁶Ar ratios range up to 4680. Such higher-than-atmospheric values in most mineral and free gas samples indicate that Ar likely derives from the mixing of atmospheric and mantle sources as well. Thermobarometry of ortho- and clinopyroxene rim pairs of matrix grains yields predominantly p-T conditions of 1.1 ± 0.5 GPa and 700 ± 100 °C, respectively, indicating a lithospheric depth of 40–45 km for 1.5 GPa and 20–25 km for 0.5 GPa. Conversely, pairing cores of mm-sized pyroxenes points to p-T conditions of 2.5 ± 0.5 GPa and 1100 ± 100 °C, corresponding to a lithospheric depth of ∼75 km. The diverse chemistry and the wide range of p-T conditions characterising the crystals indicate magma ascent.