The subcontinental lithosphere is extremely heterogeneous in
its isotopic and trace element composition. This reflects the stable
nature of the continental lithosphere: variations in parent-daughter
ratios are preserved over long time periods and produce a larger range
in radiogenic isotope ratios than in the asthenosphere, where
convective mixing destroys heterogeneity. For example, Deng
and Macdougall [1992] found that clinopyroxenes in xenoliths
from Pleistocene volcanics of Inner Mongolia define a 1.64 Ga (1 Ga
is 10
years) isochron, which appears to record incompatible
element depletion of this part of the mantle then. Johnson and
Beard [1993] found that basalts from the Rio Grande Rift define
a shallower 
-- 
slope than do
oceanic basalts, which they attribute to prior incompatible
element depletion through melting.
Several mechanisms may produce incompatible element enrichment of
the subcontinental lithosphere, including metasomatism by hydrous
fluids released from subducting lithosphere and freezing of
mantle plume-derived or asthenosphere-derived melts. Such
processes appear to have affected much of the lithosphere beneath
the Western U.S. [ Johnson and Thompson, 1991; Menzies et
al., 1991; Fitton et al, 1991]. Within the last 5 Ma (Ma
is 10
yrs) depleted asthenosphere appears to have replaced
lithosphere as the principal magma source within the basin and range
[ Fitton et al., 1991].
Volcanic rocks of Eastern China have isotopic characteristics that fall almost entirely within the range of oceanic island basalts [ Basu et al., 1991]. Xenoliths in these basalts show a wider range of isotopic compositions than the basalts [ Tatsumoto et al., 1992]. These authors suggested that the incompatible element enrichment recorded by these isotope ratios occurred through metasomatism by fluids released from subducting lithosphere in Precambrian time.
The oceanic mantle lithosphere can apparently also be metasomatized
by infiltrating melts. Hauri et al. [1993] reported
finding xenoliths in lavas from Samoa and Tubuai (Austral Islands)
that contained secondary assemblages of clinopyroxene 
spinel
apatite 
glass and were highly enriched in incompatible
elements, with notable relative depletions of Ti, Zr, Sr, and Nb.
This suggests they formed by reaction with or precipitation from
a carbonatite melt. The Samoan xenoliths lie on an extension of the
Sr-Nd isotopic array defined by Samoan basalts. Hauri et al.
[1993] thought the metasomatizing carbonatite was produced by
melting of the plume in both cases, probably an early, very low degree
melt that migrated upward and reacted with the overlying lithosphere.