crustal seismology helps constrain the nature of mantle melting anomalies: the galapagos volcanic...
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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
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
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…
Cocos
Carnegie
20 Ma
Cocos
Carnegie
RESULTS
~19 km
~19 kmVeloc. Grad.
3-4 km
Cocos
Carnegie
15 Ma
RESULTS
~18.5 km
Cocos
12 Ma
Carnegie
RESULTS
~16.5 km
^^
~13 km
G-PRIME-2000
<Vp, L3>~7.10-7.15 km/s
h~6 km
RESULTS
Overall H-Vp anticorrelation
Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
Cocos
Carnegie
Cocos
CarnegieGHS
RESULTS
Mantle? Gravity and topography analysis
)()()()()(
)(xhxhZ
xhxxhx
cw
cmcwmwm
−−ΔΔ+ΔΔ=Δ ρρρ
Airy+Pratt+Crustal dens. correction:
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=∂∂=&
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
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?
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
• 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)
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?)
OTHER OBSERVATIONS
P- and Pp- finite-frequency tomography
660 km-discontinuity
?
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?
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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