Fırat University – Department of Geology, Elazig Turkey

A geochemical investigation has been carried out on the gas phase associated to thermal fluids discharged along three different segments of the East Anatolian Fault Zone (EAFZ, Turkey) running from Malatya to the Triple Junction area (Karliova) where the East and North Anatolian Faults cross each other. CO2 is always the major gaseous component in both bubbling and dissolved gases with variable amounts of nitrogen helium and CH4. The isotopic ratios of helium range from 0.44 to 4.41Rac (values corrected for the atmospheric contamination) and cover a range spanning from crustal to magmatic-type values. The isotopic composition of carbon (CO2) shows values in the range from -5.6 to -0.2 ‰ vs PDB for the bubbling gases in contrast with the positive values (from 0.3 to 3.4‰ vs PDB) detected for the Total Dissolved Inorganic Carbon (TDIC). Coupling the information from the isotopic and chemical composition, it results that mantle-derived fluids are driven to the surface by lithospheric structures. Despite the absence of outcropping volcanic products, the tectonic setting of the different segments plays a major role in releasing mantle-type fluids . The mantle derived fluids interact at shallower levels with circulating waters and originate geothermal systems which equilibration temperatures are estimated to be up to 360°C. The collected thermal fluids show different geochemical features consistent with processes occurring at two different levels: a deep level where mantle-originated fluids are taken either from the upper mantle and from intruded magma batches, and a shallower level, in the upper crust, where Gas Water Interactions (GWI), secondary CO2 production, fractionation processes induced chemical and isotopic modifications of the pristine gas composition.

Results from a geochemical investigation of fluids (thermal waters and venting gases) discharged along the East Anatolian Fault (EAFZ, Turkey) has been carried out. The EAFZ runs in a northeasterly direction, starting from the northern end of the Dead Sea Transform (Maras Triple Junction) and ending at the Karliova Triple Junction where it crosses the North Anatolian Fault. The EAFZ is a major strike-slip fault zone forming the transform type tectonic boundary between the Anatolian and the Arabian Plates. The East and North Anatolian faults together accommodate the westward motion of the Anatolian Plate as it is squeezed out by the ongoing collision with the Eurasian Plate. The investigations have been carried out over a 250 km-long segment from Malatya to the Karliova Triple Junction area. Samples of both free and dissolved gases were taken from cold and thermal springs displaying outlet temperatures in the range of 17-67°C. The sampling procedures allowed the determination of the chemical and isotopic features of bubbling as well as dissolved gases. The gas phase is always made of CO2-dominated fluids with variable amounts of nitrogen and CH4. The isotopic ratios of helium cover a range spanning from crustal to magmatic-type values in both free and dissolved gases. The isotopic composition of carbon ( CO2 ) shows values in the range of – 5.6 /-0.2 ‰ vs PDB for the bubbling gases in contrast with the positive values (from 0.3 to 3.4‰ vs PDB) detected in the dissolved carbon. Consistently with previous studies on the North Anatolian Fault Zone, the preliminary results show variable contribution of mantle-derived fluids (e.g. from 3 to about 70% of mantle helium) along the fault and the occurrence of intense gas-water interaction (GWI) processes. The intense carbon fractionation during gas bubbling with preferential dissolution of the heavy C isotope (13C) and the highest 3-helium abundance in coincidence with the lowest outlet temperatures, provide useful indications to constrain the GWI processes.