La limite lithoshère-asthénosphère continentale:
piège gravitaire pour certains basaltes
Chrystèle Sanloup1, Céline Crépisson2, Guillaume Morard3, Hélène Bureau3,
Gaëlle Prouteau4, Y. Morizet4, et Sylvain Petit-Girard5
1ISTeP,
Université Pierre et Marie Curie
2Ecole Normale Supérieure et ISTeP
3IMPMC, Université Pierre et Marie Curie
4ISTO, Université d’Orléans
5ESRF, Grenoble
Outline
▪ Framework:
in situ experimental studies on magmas at extreme conditions
▪ Preliminary study on the density of molten basalts
The continental lithosphere-asthenosphere boundary: a melt trap?
Seismology
Nettles and Dziewonski JGR 2008
Continental averages
Yuan and Romanowicz, Nature 2010.
Depth (km)
100
LVZ
200
orogenic
300
phanerozoic
precambrian
4.3
4.4
4.5
V (km/s)
s
4.6
4.7
0
1
2
3
Seismic anisotropy (%)
4
5
The continental lithosphere-asthenosphere boundary: a melt trap?
Magnetotelluric data
Lathi GJI 2005
Fennoscandian shield
Petrological models
Foley Nat. Geo. 2008
T (°C)
0
1000
2000
crust
0
100
cratonic lithospheric mantle
100
200
QuickTime™ and a
decompressor
are needed to see this picture.
Melts or olivine
with >1000ppm H+?
200
asthenosphere
100
101
102
103
104
105
Resistivity (ohmm)
 The elusive nature of the lithosphere-asthenosphere boundary:
Presence of melts proposed since long, but pb: they should migrate
Eaton et al., Lithos 2009.
Density of molten basalt
data
Shock wave
C. Agee PEPI 1998:
‘The resulting neutral buoyancy horizon
could account for the observed low
seismic
velocities between 160 and 220 km depth’
Density of molten basalt
data
Sink-float
Ex situ measurements: sink-float method
Choice of marker :
→ density crossover
→ no reaction marker/sample
Markers for silicate melts:
Olivine, ruby, garnet, diamond
 Density too low to account for the LVZ
but:
 crystals/melt density inversion at depth
Agee, PEPI 1998
Density of molten basalt
absorption
I
I

o
In situ X-ray
 exp( ( d)liq + (d)sp + (d)env )
b ea m
7.0 mm
dx
, density , absorption coefficient d, thickness
Single-crystal diamond capsules minimize:
 x-ray absorption/diffraction
 chemical interactions and deformation
 crucial for ‘light’ silicate melts
P-T range: 10 GPa-2300 K
Katayama JNCS 1996, 1998; Sanloup et al., GRL 2000, 2004, EPSL 2011
Sakamaki, 2006, Nat. Geo. 2013; Malfait et al., Nat Geo. 2014
Density of molten basalt
absorption
In situ X-ray
Absorption scan
ESRF, ID27
QuickTime™ et un
décompresseur
sont requis pour visionner cette image.
X-ray diffraction of molten basalt at 6 GPa- 2300 K
Density of molten basalt
In situ X-ray
diffraction
Structure and density obtained simultaneously
F (r )  4r g (r )  1 
 Initial slope gives 
2

Qmax
 QS (Q)  1sin(Qr)dr
0
Kaplow et al., Phys. Rev. 1965
Density of molten basalt
 Melts compressible enough
to be trapped at the continental
lithosphere-asthenosphere boundary
Legend:
  absorption/diffraction this work
 Shock-wave
Rowan 1993
 Sink-float
Agee 1998
--- Shock-wave
Rigden 1984
PREM
Dziewsonski&Anderson 1981
Pacific ocean
Ito 2011
 harzburgites
James et al. 2004
 lherzolites
id.
Crepisson et al., EPSL 2014
Density of molten basalt
0.9 wt% CO32-
1.04 wt% H2O
Pt-Rh caps
glass
diamond
capsule
crystals
Melt composition: volatile-containing alkali basalt (Stromboli)
Density of molten basalt
QuickTime™ and a
decompressor
are needed to see this picture.
Sobolev, Science 2007.
Melt composition: volatile-containing alkali basalt (Stromboli)
 Gravitationnally and thermally stable at continental LAB conditions
Conclusions
▪ volatile-rich alkali basalts can be trapped
at the continental lithosphere-asthenosphere boundary
 Geodynamical consequences (e.g. preservation of cratonic roots, plate lubrification)
▪ At higher mantle T: basalts become buoyant
 Density trap in the LAB probably not present before mid-Archean times
 Role in the initiation of plate tectonics?
Density of molten basalt
absorption
Anisotropy Resistivity
Depth
Vs
In situ X-ray
Eaton et al., Lithos 2009.
Sobolev, Science 2007.