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 3He/4He) = crustal • Non-radiogenic (high 3He/4He) = 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
Results • Presence of mantle-derived heliumalong the NAFZ • 27 % near Duzce (epicenter of 1999 7.2 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
What about such a process in NAFZ • « 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 ?