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SUMMARY:A Tale of Two Plumes – Magmatism and Mantle convection beneath t
 he African Plate - Professor Nick Rogers\, The Open University
DTSTART:20120206T170000Z
DTEND:20120206T180000Z
UID:TALK35690@talks.cam.ac.uk
CONTACT:Ekbal
DESCRIPTION:The African continent is characterised by a dramatic dynamic t
 opography of broad domes and basins that relates to convection in the unde
 rlying mantle. Many of the domes are characterised by basaltic volcanism a
 nd in the case of east Africa\, extensional tectonics heralding the earlie
 st phases of continental break-up. Seismic tomography reveals intriguing s
 tructures in the shallow and deep mantle beneath Africa\, but the relation
 ship between these structures and topography and surface volcanism remains
  unclear. The east African Rift is associated with two of these domes\, th
 e Ethiopian and East African Plateaux. At issue is whether these two featu
 res are related to a single plume beneath the east African Rift or two plu
 mes.\n\nThe geochemical and petrological evidence from the basaltic magmas
  along the length of the African Rift points to distinct features associat
 ed with each of the two domes. Basalts from Kenya are strongly influenced 
 by the nature and age of the underlying lithosphere. Sr and Nd isotopes re
 flect the age of the lithosphere while aspects of their trace element cont
 ents are distinct from MORB-OIB. Identifying the composition of a plume en
 d-member is not straightforward but the data suggest\nan εNd of +2.0. Bas
 alts from Ethiopia\, by contrast\, are more akin to modern OIB-MORB and th
 eir trace element abundances can be broadly related to the amount of exten
 sion in the lithosphere. Isotopically\, they are more systematic and focus
  on a composition with εNd of +5.0\, which appears to be the present day 
 composition of the Afar mantle plume. The existence of separate mantle plu
 mes is also indicated by the geographical variation of 3He/4He ratios\, wh
 ich are high in both lavas and\ngeothermal fluids from the area of the cur
 rent Ethiopian plateau\, but low (i.e. <8 R/Ra) elsewhere.\n\nThe migratio
 n of magmatism through time also presents an intriguing pattern. The oldes
 t basalts occur in southern Ethiopia during the Eocene (35-45Ma). Flood ba
 salts erupt in N Ethiopia at 30 Ma\, whereas basaltic magmatism in Kenya m
 igrated from N to S between 35 Ma and the present day. This pattern is bes
 t explained in the context of two mantle plumes\, the oldest initiating be
 neath S Ethiopia at 45 Ma\, causing magmatism in S Ethiopia and Kenya as t
 he African plate moves north.\nThe second\, the Afar plume initiated with 
 the Ethiopian CFB at 30 Ma and continues to the present day feeding magmat
 ism currently focussed on the Afar depression. The radiogenic isotopes in 
 the erupted basalts are consistent with two distinct sources of magma in t
 hese two provinces.\n\nThe Ethiopian CFB include a sequence of high-Ti bas
 alts and picrites that mark the first eruptives from the Afar plume. Their
  major and trace element compositions are distinctive with low Al2O3 and s
 trongly fractionated REE\, both consistent with an origin at 4-5GPa assumi
 ng derivation from a peridotite source region. Previous investigations rev
 eal that the high-Ti lavas have elevated 3He/4He ratios (up to 19 R/R ) an
 d εNd of +5.0\, further supporting a deep origin. The lavas also have\nel
 evated Fe/Mn ratios\, a characteristic that elsewhere has been used to sig
 nify a core component in the plume source. However\, their Os isotopes rat
 ios are consistently unradiogenic with the lowest values yet recorded from
  any flood basalt province\, and are not consistent with a putative origin
  from the core. The combination of high 3He/4He and low 187Os/188Os ratios
  indicates an origin from a depleted but undegassed source region whereas 
 the high Fe/Mn ratios and TiO2 contents suggest a significant pyroxenite c
 ontribution. However\, the pyroxenites cannot be old recycled oceanic\nlit
 hosphere as the addition of sufficient pyroxenite from such a source to sa
 tisfy the TiO2 and Fe/Mn\ncharacteristics would add too much radiogenic Os
 . Reconciling these two contrasting conclusions remains elusive!
LOCATION:Harker Room 1\, Department of Earth Sciences
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