Helium and other noble gases sampling and analysing from Marmara sea seepages Sylvain Bourlange Pete Burnard Bernard Marty CRPG – CNRS Vandoeuvre-lès-nancy

Scientific Objectives Fluid origin –Is the MOHO permeable across a plate boundary? He and other gas fluxes at a major plate limit Role of the gas fluxes on the seimogenic behavior of the fault

Helium Isotopes: background Background concentrations of He in seawater are extremely low –Concentrations and isotopic compositions of seawater in equilibrium with air are well constrained Input of non-atmospheric He readily detected –sensitive tracer of ‘deep-seated’ fluids –Can be used to estimate total gas fluxes –Isotopic composition indicates gas source Radiogenic (low 3 He/ 4 He) = crustal Non-radiogenic (high 3 He/ 4 He) = mantle

Helium isotope variations in Turkey Helium isotope variations in Turkey: relationship to tectonics,volcanism and recent seismic activities (Gülec, Hilton and Mutlu, 2002) Survey along NAFZ after 1999 earthquake

(Gulec, Hilton and Mutlu, 2002)

Results Presence of mantle-derived helium along the NAFZ –27 % near Duzce (epicenter of magnitude earthquake) –Mantle derived He mostly occurs where the fault intersects volcanic activity However, the relationships between mantle- derived helium and seismic activity are not understood very well. –For example, mantle-derived helium seems to decrease toward Marmara sea (4% near Yalova)

Questions What are the fluxes of noble gases output along the NAFZ (in the Marmara sea) ? What is the amount of mantle-derived helium in the prolongation of the NAFZ in the Marmara sea ? –More representative analyses are possible in the marine environment

fault-weaking fluid pressure Coincidence between areas where helium of mantle origin emerges and areas of active tectonics (Oxburgh and O’Nions (1987)) Release of mantle-derived helium and of CO2 compatible with a mantle origin in the NAFZ (Nagao et al., 1990) Rice (1992): Fluids at sublithostatic pressures could be supplied by a high flux of deep crustal or mantle fluids to the seismogenic zone from the ductile lower crust

Estimated vertical fluid flux in the fault zone

« estimates of fluid flux based on helium isotopes suggest that they may thus contribute directly to fault weakening high fluid pressures at seismogenic zones » (Kennedy et al, 1997) –Much easier to determine fluxes in marine setting Could we suspect such a process in the case of the NAFZ ? What about such a process in NAFZ