The depleted upper mantle is perhaps the best understood of the
mantle reservoirs, as MORB provide a global sample of it. Although it
is relatively homogeneous in comparison with other mantle
reservoirs, significant heterogeneity does exist within it. The
Indian Ocean upper mantle is isotopically distinct from the upper
mantle beneath the Atlantic and Pacific. The boundary between the
Indian and Pacific upper mantle provinces, known from earlier work to
be located within the Australian-Antarctic Discordance was found by
Pyle et al. [1992] to be only 40 km wide and be migrating
westward at about 25 mm/yr. In contrast to the sharp eastern boundary
of the Indian province, the southwestern boundary appears to be
more diffuse, and occurs between 17
E and 26
E along the
Southwest Indian Ridge [ Mahoney et al., 1992]. In the northeast Indian
Ocean, Schilling et al. [1992] observed incompatible element
ratios such as La/Sm and isotope ratios of Sr and Pb in basalts from
the Gulf of Aden decreasing with distance from the Afar plume
center, located at the junction of the Red Sea, Gulf of Aden, and
African Rift. In addition to plume and depleted mantle components
(the latter is Indian type), they also identified possible
Pan-African continental lithosphere in the isotope systematics.
Hf isotopes in MORB have been somewhat problematic because they
are more variable in MORB for a given 
than are
oceanic island basalts. This was confirmed by the first new Hf isotope
data on MORB in nearly a decade [ Salters and Hart, 1991].
Salters and Hart argue that this requires the presence of both
residual garnet and clinopyroxene in the melting events that depleted
the mantle in Hf relative to Lu and Nd relative to Sm.