| Title | Isotope Composition of Geothermal Fluids along the North Anatolian Fault Zone: Spatial and Temporal Variations in Relation to Seismic Activities |
|---|---|
| Authors | Nilgün Güleç, David R. Hilton, G.A.M. de Leeuw, Halim Mutlu, Selin Süer, Candan Çifter |
| Year | 2005 |
| Conference | World Geothermal Congress |
| Keywords | Geothermal Fluids, Isotopic Composition, Monitoring, Seismicity, North Anatolian Fault Zone |
| Abstract | This study presents results of an on-going monitoring programme aimed at characterizing compositional variations in geothermal fluids along the North Anatolian Fault Zone (NAFZ). The study was initiated through a preliminary He-isotope survey in 2000 in response to 2 catastrophic earthquakes in 1999 (see results in Chem. Geol. 2002). The present programme was started in late 2001 and covers 9 geothermal sites which are monitored 3 times per year. The sites are located along an 800 km long segment of the NAFZ, from Yalova near the Sea of Marmara in the west, to Resadiye in the east. The data reported here cover the results obtained to date from the analyses of i) tritium contents and 18O/16O and D/H ratios of the hot and cold waters, along with their major anion-cation contents, and ii) CO2/He gas ratios and ?13C values of the CO2 gas in geothermal fluids. The geothermal waters associated with NAFZ are dominantly Na-HCO3, whereas the cold waters are Ca-HCO3 type. The 18O and D values (-8.29 â to -13.44 â and -54.67 â to -96.47 â, respectively) reveal meteoric origin for both hot and cold waters. Tritium contents range between 0-12.55 TU for the hot and 3.00-15.70 TU for the cold waters. The slightly higher 18O values and tritium contents of cold waters suggest recharge of cold water aquifers from higher altitudes with more recent precipitation. CO2 concentrations in geothermal fluids range between 0.02 - 3.02 cm3 STP/g H2O, and the CO2/4He ratios fall between 3.85 x 103 - 3.72 x 108. 13C values cover the range - 4.51 â to + 5.79 â, although most of the values are < 0 â. Low CO2 concentrations tend to be associated with low CO2/4He and high 13C values. Although no major earthquakes (M>5) occurred along NAFZ over the monitoring period, the temporal variations observed in chemical and isotopic compositions appear to correlate with seismicity occurring close to sampling sites. Important to note is in this respect the prominent Cl decrease and concomitant tritium increase (by a factor of about 1.5 for both) in Yalova hot waters in July 2002, which correlate well with the earthquakes recorded on 3.7.2002 and 13.7.2002 (epicentre: Armutlu-Yalova, M: 3.1). Similarly, although 13C values in Yalova are < 0 â in almost all sampling periods, an anomalous value (+ 5.79 â) is recorded in March 2002, on the same day (23.3.2002) as an earthquake (M: 4.7) occurred in the Sea of Marmara. Yet another point to note is the 0.5 - 2 â drop in 13C values at most sampling sites in October 2002 which is one of the most seismically active months of the monitoring programme. These compositional variations in geothermal fluids probably reflect the effects of seismicity-induced changes in i) mixing ratios of hot waters with cold, shallow groundwaters, and ii) the balance between mantle-derived and crustal volatiles. Continuing monitoring of fluid compositions should lead to a better understanding of their relationship to seismic activities. |