Mantle chemical evolution - Mantle Convection for Geologists - page 165

Figure 10.8. Neodymium isotopic composition versus latitude (degrees north)
along the Mid-Atlantic Ridge. Data have been averaged over 1 ◦ intervals. ε (Nd) is
the variation in 143 Nd/ 144 Nd, measured in parts per 10 000. From Hofmann [60].
Copyright Elsevier Science. Reprinted with permission.
of their emplacement as much as their original condition. Nevertheless, few seem
to doubt that the mantle is indeed a heterogeneous mixture of rock types.
10.4.1 Sources of heterogeneity
There is an obvious source of heterogeneity, in the subduction of differentiated
oceanic lithosphere. The differentiation occurs at mid-ocean ridges, where melting
forms the oceanic crust and leaves a depleted zone, perhaps 60 km thick, from
which the melts have been extracted. Hydrothermal alteration of the near-surface
crust, perhaps down several kilometres, and the deposition of pelagic sediments
add additional chemical heterogeneity. Some of this will be removed by melting
during subduction, but the major heterogeneity will persist.
An impression of the subsequent dispersal of subducted lithosphere is pro-
vided by Figures 9.5 and 9.10. The heterogeneities tend to be reduced down to
smaller scales with time, but the figures show that the heterogeneities persist vis-
ibly throughout the fluid even after the equivalent of billions of years. This will
be true in the real mantle as well, even as heterogeneities are reduced to scales
of kilometres or less, because solid-state diffusion rates are so slow that chemical
homogenisation would only occur over scales less than about one metre even over
billions of years [197].