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

(Gulec, Hilton and Mutlu, 2002)

(Gulec, Hilton and Mutlu, 2002)

Results

• Presence of mantle-derived helium along 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

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