De Leeuw, G.A.M., Hilton., D.R., Güleç, N., Mutlu, H., 2004: He-C relationships in geothermal waters from the North Anatolian Fault Zone (NAFZ), Turkey, Goldschmidt Conference, Copenhagen, Denmark:
Following the catastrophic earthquakes along the NAFZ in 1999, we initiated a periodic monitoring program targeting geothermal fluids (from boreholes and natural springs) for their chemical and dissolved gas characteristics. Our aim is to examine if chemical variations in the fluids are associated with the timing, location and/or depth of seismic activity along the NAFZ. Here, we report helium and carbon results (from late 2001 through 2003) collected at 9 locations along an 800-km transect - from Yalova on the Sea of Marmara to Resadiye in eastern Anatolia.
The major dissolved gas phase in the fluids is CO2. Its concentration falls between 0.02 – 3.02 cm3 STP/g H2O. For the most part, CO2 abundances are remarkably constant at any given location allowing sample locations to be subdivided into locations with high CO2 (> 0.5 cm3 STP/g H2O: Bolu, Efteni- Gölyaka, Kursunlu-Çavundur, Mudurnu, Resadiye and Seben) and low CO2 (< 0.12 cm3 STP/g H2O: Gözlek, Hamamözü and Yalova). The low CO2 localities show low (mantle-like: ~104) CO2/4He values (from 3.9 × 103 to 1.3 × 105), whereas high CO2 fluids have significantly higher, crustal-like (>105) CO2/4He values (1.1 × 105 to 3.7 × 108).
The isotopic composition of CO2 (δ13C) ranges from –4.5‰ to +5.8‰, with the majority of samples falling between –3.5‰ and 0‰. The low CO2 samples tend to show higher δ13C values (average = –0.5‰ ) versus high CO2 localities (average = –1.7‰). Helium isotope studies are on-going but prior work [Güleç et al. (2002)] has shown that all localities are characterized by a mantle input, with 3He/4He varying between 0.22 RA (Seben) and 2.2 RA (Mudurnu) (RA – air 3He/4He).
Seismic activity throughout the monitoring period remained fairly constant (~11 quakes/month) but there were periods (e.g. Jan. & Oct. 2002 and Jan., Mar., Apr. & Nov. 2003) when the frequency of events doubled to ~26 quakes/month. During the seismicity-active month of October, 2002, most locations showed a 0.5‰ to 3‰ drop in δ13C values and, in Seben, a concomitant drop in CO2/4He was recorded. Presumably, this reflects a change in the balance between mantle-derived and crustal volatiles. We are continuing to search for other trends in the database which can be correlated with seismic activity.