Geology Reference
In-Depth Information
Table 10.3. Plume contributions to mantle inventories.
(a) Enrichments of plume components a
Nb
Th
U
Hawaii/NMORB
2.2
2.9
2.4
EM1/NMORB
13
24
15
EM2/NMORB
11
42
24
HIMU/NMORB
17
28
18
(b) MORB-source enrichments due to plume components b
Plume enrichment
MORB-source enrichment
2
1.25
3
1.5
5
2
10
3.25
a Values taken from Figure 10.1. NMORB: 'normal MORB';
EM1: enriched mantle 1; EM2: enriched mantle 2; HIMU:
high 238 U/ 204 Pb [177, 178].
b Enrichment factors are relative to 'depleted MORB mantle'
inferred from 'normal MORB'. Plume material is assumed to
comprise one-quarter of MORB-source material (see text).
MORB ratios are Th/U
47 [60], so these are suitable proxies.
All-MOR Th values are 2-3 times larger than previous estimates, while Nb values
are 1.6-2.6 times greater. These factors may be minima, because plume-affected
ridge segments may have been excluded from the 'all-MOR' category of PetDB,
though this is not clear from the PetDB summaries.
=
2.6 and Nb/U
=
10.6.5 Heterogeneities from plumes
A potentially more robust estimate of mean MORB-source composition, though
with rather large uncertainties at present, comes from the composition of OIBs,
inferred to be produced by plumes. In Section 10.4.1 it was estimated that about
25% of heterogeneities in the mantle may come from plume material that did
not melt, and that has been stirred back into the mantle. This material could add
significantly to the inventory of trace elements.
Table 10.3(a) shows the enrichments of various classes of OIBs relative to
MORBs for Nb, Th and U. Hawaiian OIB is enriched by a factor of 2-3, whereas
the other OIBs are enriched by factors of 10-40, with a mean of around 20. The
 
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