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Abstract

The long mining history of the Werra-Fulda potash district is characterized by highly frequent gas-induced rock bursts during the extraction of potassium and magnesium salts. The gas was trapped by magmatic alterations of the Permian salt deposit during the Miocene Rhön volcanism. Large gas inclusions in the salt rocks pose a hazard to man and machinery during mining activities. The investigation of the gas content of the magmatic and saline rocks of the Werra-Fulda potash district combines applied research on hazard prevention in active mining with fundamental research on gas formation by contact metamorphism of intra-plate volcanism. In this context, the Werra-Fulda potash district offers the rare opportunity of sampling magmatic rocks underground and at the surface. The aim was to obtain fundamental information on the mechanisms of gas formation, storage, and release. The highest gas concentrations (up to 1.8 mg*g-1), which consist mainly of CO2, could be detected in magmatic rocks and magmatite-associated salt alterations. The trace gas distributions (CH4, CH3Cl, N2O) in magmatite-salt profiles provide evidence for petrological relationships with high gas contents. The stable carbon isotopic value of CO2 in extremely gasenriched salt rocks (Knistersalze) allowed the development of a novel mathematical-analytical method for hazard prevention, by which the proportions of three CO2 sources in active mining can be determined in real time. The applicability of this method must first be tested extensively in active mining. In this study, the four potent climate gases CO2, CH4, CH3Cl and N2O were detected in the magmatic rocks of the Werra-Fulda potash district. By comparing the gas content of the magmatic rocks of the surface with equivalent rocks from the subsurface mining area, it could be inferred that the gases were released from the magmatic melts when they reached the surface. The stable carbon isotopic value of CO2 in rock samples with high gas accumulations showed a typical magmatic source signature. Additionally, the stable isotopic values in CH4 and the Bernard ratio (C1/C2+C3) indicate a thermogenic CH4 formation. Studies of organic material in the footwall area indicate contact metamorphic formation of CH4 during the intrusions. Furthermore, with CH3Cl, a compound with high O3-depleting properties was detected which was supposedly at least partly responsible for the largest mass extinction in earth's history. II The formation of CH3Cl by the Siberian Trap volcanism during the Permian-Triassic boundary led to the destruction of the stratospheric O3 layer. The Conversion of hydrocarbons into CH3Cl could have occurred via the Hoechst process, by redox-sensitive minerals, or in the presence of highly activated surfaces during crystallization. Further information about the formation mechanism could be provided by future isotopic characterizations of CH3Cl. In addition to CH3Cl, N2O as another strong O3-depleting compound was found in the context of intra-plate volcanism for the first time, which additionally has a high climatic impact. If the so far unrecognized N2O formation is indeed common in intraplate volcanism settings, the environmental effect of such volcanism might have been underestimated in the past. Combining the detected gas concentrations of the four climate gases in the magmatic rocks with their GWP, the relative contributions to climate warming by Miocene volcanism of the Werra- Fulda potash district can be accounted for 60 % to CO2, 7 % to CH4, < 1 % to CH3Cl and 33 % to N2O. Thus, based on this work, N2O has the second highest climate impact of the detected greenhouse gases in the studied area, even though it has previously been unrecognized in magmatic systems. The nitrogen source for the formation of N2O may be located within rocks in the footwall of the salt deposit. The formation mechanism(s) could not be identified within the scope of this work. However, in the magmatic rocks, nitrogen compounds could be detected in the oxidation states from -III to +V, which means that the formation of N2O may have occurred by both reduced and oxidized nitrogen compounds. Moreover, the isotopic fingerprint of the newly discovered N2O source was determined to be 15NBulkN2O = 10.6 ± 5.3 ‰ (Air) and 18ON2O = 47.6 ± 5.9 ‰ (VSMOW). If the formation of N2O occurred throughout the CEVP, it could have contributed massively to the Miocene temperature maximum. The results also show that N2O could cause up to 70 times greater O3 destruction during intra-plate volcanism due to its higher enrichment compared to CH3Cl. If N2O was also formed during the Siberian Trap intrusions at the Permian-Triassic boundary, it could be responsible for the largest mass extinction in Earth's history.