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higher enrichments are probably due in part to relatively low melt fractions [60],
so they may overestimate the mean enrichments of the associated plumes.
Table 10.3(b) shows the effect of plume enrichments by factors of 2-10 on the
mean composition of the MORB source. If plumes are enriched by factors of 3-5
relative to the 'depleted MORB mantle', then the mean mantle concentrations of
incompatible elements is increased by factors of 1.5-2. Greater enrichments cannot
be ruled out at this stage.
The accuracy of the latter estimates could be considerably improved by carefully
estimating the melt fraction and enrichment of each plume and combining them
with estimates of the melt volume (from erupted volumes) and the plume flow rate
(from hotspot swells) [1], following the approach used in Section 10.4.1.
10.6.6 Chemical disequilibrium
Two recent estimates of the MORB-source composition [185, 186] use the composi-
tion of peridotites in key parts of their arguments. The trace element concentrations
estimated by Salters and Stracke [185] are deduced from major elements through
a correlation between lutetium, a compatible trace element, and CaO in anhydrous
spinel peridotites. However, we would expect the MORB source to have contained
eclogitic or pyroxenitic heterogeneities, and they would be expected to be enriched
in trace elements. By the arguments in Section 10.5.2, the surrounding peridotite
may not have equilibrated with the higher Lu content of the heterogeneities. Thus
the Lu in the peridotites may well underestimate the Lu content of the whole source.
All of the other trace element concentrations are tied to the Lu content, so the trace
element content of the source would be underestimated.
The estimate by Workman and Hart [186] starts from the trace element compo-
sition of clinopyroxenes from abyssal peridotites. Those peridotites likewise may
not have equilibrated with enriched heterogeneities, so their estimates may also
underestimate the trace element content of the whole source.
It is notable that Salters and Dick [211] found that isotopes in some MORBs
from the Indian Ocean could not be explained as products of nearby abyssal peri-
dotites, but implied an additional enriched component. They note that pyroxenites
and eclogites are rare at mid-ocean ridges, though common among xenoliths and
peridotite massifs, and inferred that enriched heterogeneities had been melted out
of the source before it reached the surface. In that case the residual peridotites
would fail to represent the incompatible elements in those heterogeneities.
These estimates are also vulnerable in other respects. They both rely on equi-
librium melting models for a homogeneous source, and both use long chains of
inference. For example, Salters and Stracke's [185] estimate of thorium content
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