Transcript Présentation PowerPoint

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 mantlederived 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
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 ?