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CRUSTAL SEISMOLOGY HELPS CONSTRAIN THE NATURE OF MANTLE MELTING ANOMALIES: THE GALAPAGOS VOLCANIC PROVINCE AGU Chapman Conference Ft. William, Scotland,

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Presentation on theme: "CRUSTAL SEISMOLOGY HELPS CONSTRAIN THE NATURE OF MANTLE MELTING ANOMALIES: THE GALAPAGOS VOLCANIC PROVINCE AGU Chapman Conference Ft. William, Scotland,"— Presentation transcript:

1 CRUSTAL SEISMOLOGY HELPS CONSTRAIN THE NATURE OF MANTLE MELTING ANOMALIES: THE GALAPAGOS VOLCANIC PROVINCE AGU Chapman Conference Ft. 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

2 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

3 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…

4 Cocos Carnegie 20 Ma Cocos Carnegie RESULTS ~19 km Veloc. Grad. 3-4 km

5 Cocos Carnegie 15 Ma RESULTS ~18.5 km

6 Cocos 12 Ma Carnegie RESULTS ~16.5 km ^^ ~13 km G-PRIME-2000 ~ km/s h~6 km

7 RESULTS Overall H-Vp anticorrelation

8 Cocos Carnegi e Cocos Carnegie GHS RESULTS Mantle?  Gravity and topography analysis

9 Cocos Carnegi e Cocos Carnegie GHS RESULTS Mantle?  Gravity and topography analysis

10 Cocos Carnegi e Cocos Carnegie GHS RESULTS Mantle?  Gravity and topography analysis Airy+Pratt+Crustal dens. correction:

11 Crustal structure  Nature of the anomaly MANTLE MELTING MODEL Crustal thickness, Vp [Tp, active upwelling ( x=w/u 0 ), 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)

12 MANTLE MELTING MODEL Connection H  melting parameters M  Total volume of melt production. [ * My -1 *km -1 ]  (melt fract./weight) r m, r c  mantle, crustal density Connection Vp  melting parameters F  Mean fraction of melting Z  Mean depth (P) of melting Vp (F,P) Korenaga et al., 2002 Pyrolite Estimate H, Vp as a function of Tp, x, Mp, dz, a, composition, through P, F

13 H-Vp Diagrams NATURE OF THE GHS Hotter Active convection MPd=15%/GPa, MPw=1%/GPa, a=0.25, dz=50 km MPd=15%/GPa, MPw=1%/GPa, a=1, dz=50 km MPd=15%/GPa, MPw=2%/GPa, a=0.25, dz=50 km MPd=20%/GPa, MPw=1%/GPa, a=0.25, dz=50 km 70% pyrolite + 30% MORB Compositional anomaly?

14 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)

15 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 ºK 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)

16 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 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?)

17 OTHER OBSERVATIONS P- and Pp- finite-frequency tomography 660 km- discontinuity?

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19 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?

20 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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