N. Rogers et al., Two mantle plumes beneath the east African rift system: Sr, Nd and Pb isotope evidence from Kenya Rift basalts, EARTH PLAN, 176(3-4), 2000, pp. 387-400
Major and trace element and radiogenic isotope ratios (Sr, Nd and Pb) are p
resented for a suite of Neogene to Recent basalts (MgO > 4 wt%) from the ax
ial regions of the Kenya Rift. Samples have compositions ranging from hyper
sthenenormative basalt through alkali basalt to basanite and are a subset o
f a larger database in which compositions extend to nephelinite. A broadly
negative correlation between Zr/Nb (< 2-7) and Ce/Y (1-8) indicates derivat
ion from a garnet-bearing mantle source region as a result of < 3% melting.
Isotope ratios in basalts from the axial regions of the Kenya Rift have Nd
-143/Nd-144 = 0.51300 to 0.51255, Sr-87/Sr-86 0.7030 to 0.7055 and Pb-206/P
b-204 < 18 to > 20, broadly similar to values from OIB. The Kenya Rift cuts
through basement of different ages and aspects of the composition of mafic
magmas reflect the anisotropy of the underlying lithosphere, Specifically,
those basalts from that part of the rift underlain by the Tanzanian craton
(TC) have higher Ce/Y and lower Zr/Nb ratios than those erupted through th
e Panafrican Mozambique belt (MB) implying an origin either at greater dept
h or from a more trace element-enriched source region. Samples erupted thro
ugh the zone of reactivated craton margin (RCM) share the characteristics o
f mafic lavas from both the craton and the mobile belt. MB samples have Nd-
143/Nd-144 = 0.5130-0.5127, Sr-87/Sr-86 = 0.7030-0.7035 and Pb-206/Pb-204 =
18.3-19.8, defining a steep negative trend on the Nd-Sr diagram and plotti
ng close to the NHRL on conventional Pb isotope diagrams. By contrast TC an
d RCM samples have Nd-143/Nd-144 = 0.5124-0.51275, Sr-87/Sr-86 0.7035-0.705
6 and Pb-206/Pb-204 = 17.6-21.2, defining flat-lying arrays on Nd-Sr plots
and a much greater scatter and spread on Pb isotope diagrams, with many ana
lyses plotting above the NHRL. Both groups of analyses trend towards a comm
on end member on a plot of Nd-143/Nd-144 against Sr-87/Sr-86, at Nd-143/Nd-
144 similar to 0.51275 and Sr-87/Sr-86 similar to 0.7035. These values are
suggested to reflect the isotopic characteristics of the sub-lithospheric K
enyan mantle, inferred to be the Kenya mantle plume. Comparison with data f
rom Afar suggest that the Kenya plume is distinct from the Afar plume, impl
ying that the east African Rift is underlain by at least two distinct mantl
e plumes. Eocene and Oligocene basalts from southern Ethiopia bear a closer
resemblance to the Kenyan basalts than to those from Afar and the Ethiopia
n plateau, suggesting that the Kenya plume has been active for at least 45
Ma. Migration of magmatism from southern Ethiopia southwards is consistent
with the northeastward migration of the African plate over the Kenya plume
over the past 50 My. (C) 2000 Elsevier Science B.V. All rights reserved.