Gas Geochemistry of Turkish Geothermal Fluids: He- and C-Isotope and Relative Abundance Studies

Abstract

We examine the He-isotope record of geothermal fluids from various neotectonic provinces of Turkey along with the results of a recent study concerned with the combined He-CO2 characteristics of volatiles in the western Anatolian region. The distribution of mantle-He in Turkey has close connections with the distribution of heat and the age of tectono-magmatic activity. The mantle-He contribution is high (>50 % of total composition) in regions of historically-active volcanoes (eastern and central Anatolia) and most recent seismic activity (western central segment of the North Anatolian Fault Zone) where low-to moderate-enthalpy geothermal fields are located. In the high-enthalpy fields of western Anatolia, mantle-He contribution is relatively less. The negative correlation between 3He and heat distribution in Turkey suggests different mechanisms for the transfer of heat and helium in different provinces. While recent and extensive volcanism in central and eastern Anatolia is consistent with the release of both helium and heat from cooling magmatic systems, the low 3He/enthalpy ratio (1.7 x 106 atoms 3He/J) in western Anatolia appears to result from lithospheric stretching which provides a regional and significant source of heat but comparatively low helium additions from localised magmatic activity. The western Anatolian fluids are characterized by 3He/4He ratios ranging from 0.27 to 1.67 RA, δ13C (CO2) values between –8.04 and +0.35‰, and the CO2/3He ratios varying from 1.6 x 109 to 2.3 x 1014. Evaluation of He-C abundances indicates that degassing and calcite precipitation have significantly fractionated the elemental ratio (CO2/3He) in geothermal waters, while gas phase samples better represent the source characteristics of volatiles. Mixing between mantle and various crustal sources appears to be the main control on the observed He-C systematics, and crustal inputs dominate the CO2 inventory. As to the He-inventory, the mantle-derived component is found to vary up to 21 % of the total He content and is probably transferred to the crust by fluids degassed from deep mantle melts generated in association with adiabatic melting accompanying current extension in the region.