crustal seismology helps constrain the nature of mantle melting anomalies: the galapagos volcanic...
TRANSCRIPT
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CRUSTAL SEISMOLOGY HELPS CONSTRAIN THE NATURE OF MANTLE MELTING
ANOMALIES: THE GALAPAGOS VOLCANIC PROVINCE
AGU Chapman ConferenceFt. William, Scotland, 31/08/2005
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
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STUDY AREA
2O Ma
15 Ma
12 Ma
Projects:
0 Ma
SALIERI-2001
IRD-GéoAzur IFM-GEOMAR
IFM-GEOMAR IRD-GéoAzur
PAGANINI-1999
G-PRIME-2000
WHOI U. Hawaii
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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
OBJECTIVES
• 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…
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Cocos
Carnegie
20 Ma
Cocos
Carnegie
RESULTS
~19 km
~19 kmVeloc. Grad.
3-4 km
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Cocos
Carnegie
15 Ma
RESULTS
~18.5 km
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Cocos
12 Ma
Carnegie
RESULTS
~16.5 km
^^
~13 km
G-PRIME-2000
<Vp, L3>~7.10-7.15 km/s
h~6 km
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RESULTS
Overall H-Vp anticorrelation
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Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
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Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
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Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
)()()()()(
)(xhxhZ
xhxxhx
cw
cmcwmwm
−−ΔΔ+ΔΔ=Δ ρρρ
Airy+Pratt+Crustal dens. correction:
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Crustal structure Nature of the anomaly
MANTLE MELTING MODEL
Crustal thickness, Vp [Tp, active upwelling (x=w/u0), composition]
● 2-D steady-state model for mantle corner flow (Forsyth, 1993)
● Include deep damp melting (Braun et al., 2000)
● Active upwelling confined to beneath the dry solidus (Ito et al., 1999)
)()(),(),( 0 zzuzFzxwzxm χΓ=∂∂=&
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MANTLE MELTING MODEL
∫∫==Rc
m
c
m dxdzzxmuu
MH ),(
00
&&
ρρ
ρρ
Connection H melting parameters
M Total volume of melt production . [*My-1*km-1] (melt fract./weight)rm, rc mantle, crustal density
Connection Vp melting parameters
F Mean fraction of meltingZ Mean depth (P) of melting
∫∫=R
dxdzzxmFM
F ),(1 && ∫∫=
R
dxdzzxmzM
Z ),(1
&&
Vp (F,P)
Korenaga et al., 2002
Pyrolite
Estimate H, Vp as a function of Tp, x, Mp, dz, a, composition,
through P, F
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H-Vp Diagrams
NATURE OF THE GHS
Hotter
Active convection
MPd=15%/GPa, MPw=1%/GPa, a=0.25, dz=50 kmMPd=15%/GPa, MPw=1%/GPa, a=1, dz=50 kmMPd=15%/GPa, MPw=2%/GPa, a=0.25, dz=50 kmMPd=20%/GPa, MPw=1%/GPa, a=0.25, dz=50 km70% pyrolite + 30% MORB
Compositional anomaly?
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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)
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OTHER OBSERVATIONS
• 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?
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)
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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 (Hooft et al., 2003) show thinner than normal transition zone
P and Pp waves finite-frequency tomography (Montelli et
al., 2004) show anomaly only at upper mantle (S-wave?)
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OTHER OBSERVATIONS
P- and Pp- finite-frequency tomography
660 km-discontinuity
?
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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?
• Why is the GHS apparently a continuous, stable, long-lasting melting anomaly?
• 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?
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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
• Improve understanding of mantle dynamics
• Test consistency of geochemical predictions with alternative models
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