Geology Reference
In-Depth Information
(a)
Slowly-changing flow
(b)
Unsteady flow
Figure 10.9. Stirring rates in different types of flow. (a) Flow that changes only
slowly, in the manner of slowly evolving plates. (b) Flow that changes from
one cell to three and back, in the manner of unsteady convection in constant-
viscosity fluids. The tracers began in compact clumps, and have been stirred for
the equivalent of about 2 Gyr in each case. From Davies [1].
flows change rapidly with time and how important the three-dimensionality of
flow is [198]. Some of the early models assumed that convection was confined to
the upper mantle and used constant-viscosity fluid with no plates, so the scale of
flow was only a few hundred kilometres, flows were quite time-dependent, and
homogenisation indeed required only a few hundred million years [199]. However,
models with plates yield much larger scales of flow that change only slowly, and
mixing can take many billions of years in such flow [200].
Figure 10.9 illustrates one aspect of this, showing that a flow with an unreal-
istically variable flow homogenises tracers much more quickly than a flow that
changes in the manner of slowly evolving plates. Much of this debate was moti-
vated by attempts to explain the age of heterogeneities inferred from lead isotopes
(Figure 10.3) as reflecting the homogenisation timescale [201], but we will see later
that a different explanation has come to the fore, and the timescale of homogenisa-
tion is a less important consideration. In any case the observations of geochemical
variations cited earlier give a strong basis for assuming that the mantle is indeed
heterogeneous on all scales.
10.4.3 How much primitive mantle?
The process of melting under mid-ocean ridges, to form oceanic crust and its
depleted mantle complement, is the main source of the heterogeneities that are
later subducted and stirred into the mantle. If this process has been operating for
several billion years, how much old oceanic crust would have accumulated in the
