AGU Chapman Conference Ft. William, Scotland, 31/08/2005

1. 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

2. STUDY AREA 15 Ma PAGANINI-1999 2O Ma IFM-GEOMAR IRD-GéoAzur 12 Ma G-PRIME-2000 0 Ma WHOI U. Hawaii SALIERI-2001 IRD-GéoAzur IFM-GEOMAR Projects:

3. 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…

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

5. RESULTS ~18.5 km 15 Ma Cocos Carnegie

6. RESULTS ~16.5 km h~6 km ^^ <Vp, L3>~7.10-7.15 km/s G-PRIME-2000 ~13 km 12 Ma Cocos Carnegie

7. RESULTS Overall H-Vp anticorrelation

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

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

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

11. MANTLE MELTING MODEL Crustal structure  Nature of the anomaly 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)

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

13. NATURE OF THE GHS 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 Hotter Active convection Compositional anomaly? H-Vp Diagrams MPd=15%/GPa, MPw=1%/GPa, a=0.25, dz=50 km

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 fertileanomaly, 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 • 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?

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 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?)

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

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? • 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, long-lasting melting anomaly?

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 • Test consistency of geochemical predictions with alternative models • Improve understanding of mantle dynamics