Crustal Seismology Helps Constrain the Nature of

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Crustal Seismology Helps Constrain the Nature of
AGU Chapman Conference
Ft. William, Scotland, 31/08/2005
CRUSTAL SEISMOLOGY HELPS CONSTRAIN THE
NATURE OF MANTLE MELTING ANOMALIES:
THE GALAPAGOS VOLCANIC PROVINCE
V. Sallarès (1), Ph. Charvis (1), E. Flueh (2), J. Bialas (2)
(1) IRD-Géosciences Azur, Villefranche-sur-mer, France
(2) IFM-GEOMAR, Kiel, Germany
STUDY AREA
Projects:
15 Ma
PAGANINI-1999
2O Ma
IFM-GEOMAR
IRD-GéoAzur
12 Ma
G-PRIME-2000
WHOI
U. Hawaii
SALIERI-2001
IRD-GéoAzur
IFM-GEOMAR
0 Ma
OBJECTIVES
Objectives
• To determine the velocity structure and crustal thickness of the
GVP-volcanic ridges & estimate their uncertainty
 Joint refraction/reflection travel time tomography
 Monte Carlo-type analysis
• To determine upper mantle density structure based on
velocity-derived models
 Gravity and topography analysis
• To connect seismic parameters (H, Vp) with mantle melting
parameters (e.g. Tp, damp melting, composition)
 Mantle melting model
• To contrast model predictions with other observations
 Geochemistry, temperature, mantle tomography…
RESULTS
3-4 km
~19 km
20 Ma
Cocos
Cocos
Carnegie
Carnegie
Veloc. Grad.
~19 km
RESULTS
15 Ma
~18.5 km
Cocos
Carnegie
RESULTS
~16.5 km
12 Ma
Cocos
h~6 km
Carnegie
<Vp, L3>~7.10-7.15 km/s
G-PRIME-2000
~13 km
^^
RESULTS
Overall H-Vp anticorrelation
RESULTS
Mantle?  Gravity and topography
analysis
Cocos
Cocos
Carnegi
GHS e Carnegie
RESULTS
Mantle?  Gravity and topography
analysis
Cocos
Cocos
Carnegi
GHS e Carnegie
RESULTS
Mantle?  Gravity and topography
analysis
Cocos
Cocos
Carnegi
GHS e Carnegie
Airy+Pratt+Crustal dens. correction:
m(x)
mwhw(x)mc(x)hc(x)
Z hw(x)hc(x)
MANTLE MELTING MODEL
Crustal structure  Nature of the anomaly
 (x, z)w(x, z) F u0 (z)(z)
m
z
Crustal thickness, Vp [Tp, active upwelling (x=w/u0), composition]
● 2-D steady-state model for mantle corner flow
● Include deep damp melting
(Forsyth, 1993)
(Braun et al., 2000)
● Active upwelling confined to beneath the dry solidus
(Ito et al., 1999)
MANTLE MELTING MODEL
Connection H  melting parameters
 m M

H
 m
 c u0  c u0
 m ( x, z )dxdz
R
M  Total volume of melt production
.
[*My-1*km-1]  (melt fract./weight)
rm, rc  mantle, crustal density
Estimate H, Vp as a function of
Tp, x, Mp, dz, a, composition,
through
P, FVp  melting parameters
Pyrolite
Connection
F
Korenaga et al., 2002
Vp (F,P)
1
M
 Fm ( x, z)dxdz
R
Z
1
M
 zm ( x, z)dxdz
R
F  Mean fraction of melting
Z  Mean depth (P) of melting
NATURE OF THE GHS
H-Vp Diagrams
a=0.25,
dz=50
MPw=1%/GPa, a=1,
dz=50
kmkm
MPd=15%/GPa, MPw=2%/GPa,
a=0.25,
dz=50
km
MPd=20%/GPa,
a=0.25,
dz=50
km
70% pyrolite
+
30%
MORB
Compositional
anomaly?
Active convection
SUMMARY
Summary
• All GVP-aseismic ridges show a systematic, overall L3
velocity-thickness anti-correlation
This is contrary to the predictions of the thermal plume
model  Need to consider a fertile anomaly, possibly a
mixture of depleted pyrolitic mantle + recycled oceanic crust
• Velocity-derived density models account for gravity
and topography data without need for anomalous upper
mantle density
Upper mantle density anomaly is undetectable at distances
>500 km from GHS (or 10 My after emplacement)
OTHER OBSERVATIONS
Match with other observations?
• Temperature
 GHS-lavas erupt 50-100ºK cooler than Hawaiian lavas 
cooling during ascent through lithosphere (Geist & Harpp 2004)
 Excess temperature estimations:
215ºK (Schilling, 1991)  <200ºK (Ito & Lin 1995)  130ºK (Hooft et
al., 2003)  30-50ºK (Canales, 2003)  <20ºK (Cushman et al., 2004)
• Major element geochemistry
 Fe8 > 13 for individual samples at Galapagos platform
 Fe8 higher than “global MORB array” at the edges of CNSC
 Positive Na8 – crustal thickness correlation along CNSC,
associated to deep, hydrous melting (Cushman et al., 2004) 
smooth Fe8 signature along most of CNSC?
OTHER OBSERVATIONS
• Isotopes geochemistry
 Sr-Pb-Nd isotope and trace element signatures consistent
with derivation from recycled oceanic crust (e.g. Hauff et al., 2000;
Hoernle et al., 2000; Schilling et al., 2003)
 Sm-Nd and U-Pb isotope systematics indicate that the age of
recycled crust is 300-500 My only (Hauff et al., 2000), which seems
to be too short for lower mantle recycling(?)
• Mantle tomography
 P-wave tomography with temporary local network (Toomey et al.,
2001) has resolution to 400 km only
 Receiver functions
transition zone
(Hooft et al., 2003)
show thinner than normal
 P and Pp waves finite-frequency tomography (Montelli et al., 2004)
show anomaly only at upper mantle (S-wave?)
OTHER OBSERVATIONS
P- and Pp- finite-frequency tomography
660 kmdiscontinuity?
ISSUES
Issues
• If there is a regional chemical heterogeneity, why not upper
mantle density anomaly?
• Why is volcanism so focused while global tomography
anomaly appears to be much broader? Why is melt not driven
to CNSC?
• How can the dense, fertile mantle rise to the surface in the
absence of a significant thermal anomaly?
• Where does recycled oceanic crust comes from?
• Why is the GHS apparently a continuous, stable, longlasting melting anomaly?
FUTURE WORK
Future work?
• Seismological petrology + gravity & topography analysis
 Estimate seismic crustal and upper mantle structure with
error bounds
 Compare H-Vp diagrams for other LIPs
 Determine Vp(P,F) for source compositions other than
pyrolite
• Increase geochemical data/melting experiments adequate to
distinguish between thermal/hydrous/chemical origin
• Test consistency of geochemical predictions with alternative
models
• Improve understanding of mantle dynamics

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