Geoscience Reference
In-Depth Information
PC1
EM1
PC2
EM2
FOZO
HIMU
MORB
OIB
−
5
0
5
IC1
Fig. 13.8
Independent components (IC1 and IC2) in the
204
Pb/
206
Pb-
207
Pb/
206
Pb-
208
Pb/
206
Pb-
87
Sr/
86
Pb-
143
Nd/
144
Nd
system of the oceanic basalts (modified from Iwamori
et al
., 2010). The isotopic compositions of the oceanic basalts
are decomposed into the two ICs, where crosses and solid circles indicate MORB and OIB, respectively. The dark
symbols indicate the data from the Southern Atlantic Ocean, the Indian Ocean, and the Central Pacific Ocean,
whereas the light symbols indicate the data from the rest of the oceans. The two principal components (PC1 and
PC2) are within almost the same plane spanned by IC1 and IC2, while clear obliquity between ICs and PCs shows
that the two PCs are not independent of each other (Iwamori & Albar ede, 2008). Approximate location of conven-
tional mantle geochemical end-members is shown for HIMU, FOZO or C, average depleted MORB mantle
(A-DMM), depleted DMM (D-DMM), enriched-DMM (E-DMM), EM I and EM II. Reproduced with permission of
Elsevier.
fluid-rock interaction and the subsequent radio-
genic ingrowth with an average recycling time of
0.3-0.9 Ga.
A high IC1 value indicates a mantle source
enriched in ''melt'' component such as recy-
cling MORB (Hofmann & White, 1982) or OIB
(McKenzie
et al
., 2004) inherited in the mantle.
The melt eclogitic component is heavier than
the ambient mantle, and may accumulate at the
base of the convecting mantle (e.g., Christensen
& Hofmann, 1994; Nakagawa & Tackley, 2005).
This accumulated material is enriched in U, Th
and K, which may generate heat to cause plumes
and OIB magmas (Figure 13.9). On the other hand,
the shallow part of the mantle is depleted in
the melt component to be the MORB source re-
gion. Therefore, IC1 naturally discriminates OIB
(positive IC1) from MORB (negative IC1). In this
sense, IC1 may reflect a vertical accumulation
structure in terms of the heavy melt component
(Figure 13.9).
On the other hand, IC2 may represent horizon-
tal segments. A mantle source with a high IC2
value could represent a domain contaminated by
the anciently subducted aqueous fluid compo-
nent (Figure 13.9) associated with the HBL, as
was discussed in the previous sections. It has been
further argued that the high-IC2 domains are re-
lated to extensive subduction towards the super-
continents, possibly Pangea and Rodinia which
have created the high-IC2 domains now beneath
the Eastern Hemisphere and the Pacific Ocean
(Iwamori
et al
., 2010; Iwamori and Nakamura,
2012). These high IC2 regions include the two
large low shear-wave velocity provinces (LLSVPs)
near the core-mantle boundary (CMB) beneath
