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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

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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

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:


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

Veloc. Grad.

~19 km

20 Ma

Cocos

Cocos

Carnegie

Carnegie


RESULTS

~18.5 km

15 Ma

Cocos

Carnegie


RESULTS

~16.5 km

h~6 km

^^

<Vp, L3>~7.10-7.15 km/s

G-PRIME-2000

~13 km

12 Ma

Cocos

Carnegie


RESULTS

Overall H-Vp anticorrelation


RESULTS

Cocos

Cocos

Carnegie

Carnegie

GHS

Mantle?Gravity and topography analysis


RESULTS

Cocos

Cocos

Carnegie

Carnegie

GHS

Mantle?Gravity and topography analysis


RESULTS

Cocos

Cocos

Carnegie

Carnegie

GHS

Airy+Pratt+Crustal dens. correction:

Mantle?Gravity and topography analysis


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)


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


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


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)


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 (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?

  • 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?


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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