HR: 0800h
AN: V51B-0528    [Abstracts]
TI: How Many Plumes In Africa ? The Geochemical Point of View
AU: PIK, R
EM: rpik@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 France
AU: * Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 France
AU: * Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: ENSG, BP40, Vandoeuvre-Les-Nancy, 54501 France
AU: Hilton, D
EM: drhilton@ucsd.edu
AF: SCRIPPS, 8675 Discovery Way, La Jolla, CA 92093 United States
AB: Since the Oligocene, volcanic activity in Africa was particularly important in the Horn of Africa where ~ 1 million km3 of continental flood basalts (the Ethiopian CFB) erupted 30 Ma ago in a time interval of 1-2 Ma or less. The Afar volcanic province which is still magmatically active is thought to represent the surface expression of a deep mantle plume, a view consistent with ultra-low velocity anomalies at the base of the mantle beneath the African superswell and the Ethiopia-Afar volcanic province. This plume origin is also supported by the occurrence of 3He/4He ratios up to 20 Ra (Ra is the 3He/4He ratio of atmospheric helium) much higher than those of mid-ocean ridge basalts (on average, 8,b1 Ra) and thought to characterize mantle material originating from below the 660 km discontinuity. However, a deep mantle origin for "high 3He" material is currently questioned by some models which rather ascribe a lithospheric or shallow asthenospheric origin for such He component. The origin of this signal can be tested with the distribution of He isotopic signatures and other geochemical tracers among different African volcanic provinces. All these other provinces exhibit 3He/4He ratios that are equal to, or lower than, the mean MORB ratio of 7-9 Ra (Cameroon line: 5-7 Ra; Hoggar: 8 Ra, this work; Darfur 5.4-7.5 Ra; West African rift: 5-8.5 Ra, this work; Comores, 6.5 Ra, this work). Although low 3He/4He ratios in intraplate volcanic provinces could result from crustal recycling in the mantle and remobilisation of recycled crust during plume uprise, the upper range of 3He/4He values within the field of MORB values points to the strong involvement of asthenospheric mantle and limited interactions of magmas with the aged African crust. Furthermore, these "low-3He" volcanic provinces are characterized by strongly alkaline to undersaturated volcanism indicative of low degrees of partial melting and a thermal regime of the asthenosphere cooler than the one that gave rise to transitional to tholeiitic Ethiopian CFBs. These geochemical observations also conflict with models that advocate channelling of the Afar hotspot material by pre-existing tectonic features to account for all these African volcanic provinces.
DE: 8439 Physics and chemistry of magma bodies
DE: 8109 Continental tectonics--extensional (0905)
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8149 Planetary tectonics (5475)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting