Geoscience Reference
In-Depth Information
12.2
Evidence for Mantle Heterogeneity
to be the principal source of mid-oceanic ridge
basalts), EM-I and EM-II (enriched mantle compo-
nents, possibly related to the recycling of conti-
nental crust), HIMU (likely related to old recycled
oceanic lithosphere) and FOZO (which suggest a
common component to all oceanic basalts and
has, intriguingly, both depleted and primitive sig-
natures). Figure 12.2 shows the trends of OIB and
MORB basalts between He, Sr and Pb isotopes.
Geophysical observations add some support for
the suggestion of Earth layering and the presence
of reservoirs. While the observation of subducted
slabs in the lower mantle suggests that the Earth
is not completely layered at the 670 km phase
discontinuity, even recent tomographic models
suggest fairly strong separation of structures be-
tween upper and lower mantle (Ritsema
et al
.,
2011) and slabs that appear stagnant in the tran-
sition zone in parts of the circum-Pacific and the
Mediterranean (Schmid
et al
., 2008; Fukao
et al
.,
2009). Deep in the Earth's mantle large shear
wave velocity anomalies are seen that are likely
both warm and compositionally dense (see Mc-
Namara
et al
., 2010, for a recent review). These
The isotopic heterogeneity of midoceanic ridge
basalts (MORB) and ocean island basalts (OIB)
have generally been used to argue for the ex-
istence of multiple reservoirs inside the Earth.
The stronger heterogeneity of OIB compared to
MORB has further led to the suggestion that the
Earth's mantle is at least partly layered and that
the ocean island basalts are derived from thermal
(or thermochemical) plumes rising from a deep
reservoir. Noble gases, and in particular He, have
added weight to the argument for a layered Earth
due to the preservation of ''primitive'' or ''primor-
dial'' components, such as
3
He, that suggest that
significant portions of the Earth have not been
degassed. The analysis of MORBs and OIBs in
multiple isotope systems has suggested the mix-
ing between reservoirs, where the end-members
may be associated with distinct regions, or dis-
tinct processes that lead to mantle heterogeneity.
Following the terminology introduced by Zindler
and Hart (1986), the isotopic end-members in-
clude DMM (depleted MORB source, thought
30
FOZO
Hawaii
Galapagos
Samoa
20
Heard
3
He/
4
He
(R/R
a
)
Pitcairn
Society
Fig. 12.2
(From van Keken
et al
.,
2002). Schematic illustration of the
isotopic variations in OIB and MORB
that suggest the mixing between
end-members (FOZO, EM1, EM2,
MORB, HIMU; see text for
explanation) that could suggest the
presence of distinct reservoirs, or
perhaps, of the activity of various
processes that create heterogeneity.
10
MORB
Australs
EM2
EMI
Azores
HIMU
Canary Is.
0
Increasing
87
Sr/
86
Sr and
206
Pb/
204
Pb
