Combined Helium and CO2 Isotope Systematics of Turkish Geothermal Systems: Relation to Volcanism and Active Tectonics

Abstract:

Helium and CO₂ isotope compositions and CO₂/³He ratios of Turkish geothermal fluids from various neotectonic provinces suggest genetic relationships with magmatic and tectonic activities. The provinces include western and eastern Anatolia, experiencing extensional and compressional regimes, respectively, and the North Turkish province experiencing strike-slip tectonics associated with the North Anatolian Fault Zone (NAFZ). He-isotope compositions display a range of values from 0.27-1.67 Ra for western Anatolia, 0.85-7.76 Ra for eastern Anatolia, and 0.28-2.19 Ra for NAFZ fluids, with δ¹³C values between -8.04 and +0.35 ‰, -14.59 and +1.30 ‰ , and -1.92 and +1.52‰ , respectively. CO₂/³He ratios lie between 1.66x10⁹-2.35x10¹³, 2.4x10⁵-3.7x10¹³and 32x10⁹-26x10¹³for the fluids of western and eastern Anatolia and the NAFZ, respectively. Combined He-CO₂ systematics suggest that degassing and preferential CO₂ loss significantly affects some samples resulting in the fractionation of elemental CO₂/He ratios.Quantitative assessment of the volatile inventory suggests ~97% and 10% mantle-derived component in the He- and C-inventories, the C-budget being dominated by carbonate contributions in agreement with the composition of basement metamorphics (containing marble) and reservoir lithologies (marble, limestone) in most fields. The highest mantle-He contribution, accompanied by the highest mantle-C, is recorded in eastern Anatolia in the vicinity of the most recently-active Nemrut volcano. Relatively high mantle-He and mantle-C contributions in the Tuzla field of western Anatolia display close spatial association with young, alkaline volcanics. Along the NAFZ, the highest mantle contributions are recorded at the most seismically-active western-central segment. Collectively, these observations suggest the addition of mantle volatiles to crust by young magmatic activities and their transfer to the surface via fault zones of these tectonic provinces.