Marine Ecosystems Research Department - jamstec japan agency ...

Japan Marine Science and Technology Center
Institute for Frontier Research on Earth Evolution (IFREE)
143Nd/ 144 Nd
207Pb/ 204 Pb
0.5132
0.5130
0.5128
0.5126
0.5124
0.5122
0.5120
2.0
DMM
HIMU
Pyroxenite
4.0
Px+5% crust
0.5118
0.700 0.702 0.704 0.706 0.708
16
15
14
13
4.0
4.0
Px+5% crust
Pyroxenite
3.0
4.0
Px+10% crust
2.0
4.0
3.0
ment of the felsic partial melt with the residuum, i.e.,
the delaminated component. However, a pyroxenitic
'anti-crust' component with a ~% felsic melt component
can reasonably explain the EMI isotopic signature.
Simple mixing of the bulk silicate Earth component,
which is likely to occupy the deep mantle, and a
- billion-year-old delaminated component could
form the EMI component.
2.0
2.0
4.0
PM
Px+10% crust
87Sr/86Sr
3.0
Px+15% crust
EMI
2.0
PM EMII
EMI
DMM
Px+15% crust
4.0
HIMU
delaminated
anti-continental crust
EMII
12
12 14 16 18 20 22
206Pb/204Pb
Fig.15 Isotopic evolution of delaminated, anti-crust materials.
Isotopic evolution of an inferred pyroxenitic restite with 0-
15% contribution of felsic magmas. The ages of formation
of such delaminated, anti-continental components are
shown in Ga. The pyroxenitic restite was produced by partial
melting of an initial basaltic crust, delaminated from
the crust, and stored in the deep mantle. The isotopic signature
of the EMI reservoir may be explained by mixing of
the primitive mantle, which represents normal mantle
compositions, and delaminated/accumulated pyroxenite
with a 10-15% felsic magma component (stars). Isotopic
compositions of other mantle components such as the
depleted MORB source mantle (DMM), high-µ (HIMU),
and enriched mantle II (EMII) are also shown.
2.2. Mineralogy and phase transitions in the lower
mantle
Laboratory experiments indicate that pressureinduced
phase transitions of the olivine component of
the mantle occur at about . GPa and . GPa, and
are thought to be responsible for the seismic discontinuities
at km and km depths. Recent seismological
studies further indicate the presence of seismic
anomalies in the mid lower mantle. However, an adequate
explanation of the anomalies in the mid lower
mantle has not yet been forthcoming. Peridotitic material
converts to an assemblage of Mg-rich and Ca-rich
perovskites and magnesiowustite by a depth km,
and this lithology probably persists deep into the
lower mantle. Therefore, we explore experimentally
whether phase transitions in subducted oceanic crust
(MORB) might be responsible for these deeper seismic
anomalies. In order to access this problem, we
conducted experiments on densities of minerals in
MORB using diamond-anvil-cell and multi-anvil-type
ultra-high-pressure apparatus and synchrotron radiation
facilities. The pressure-density relationship of Albearing
stishovite, calcium ferrite-type aluminous
phase, and hexagonal aluminous phase may be compared
with those of Mg-perovskite and Ca-perovskite,
which all coexist in the subducted MORB. Densities
of these phases were calculated using an appropriate
equation of state with suitable thermoelastic parameters.
Figure shows the room temperature densities
of high-pressure phases in the subducted MORB.
Although it is known that the compressibility of
Mg-perovskite changes with chemical composition,
Mg-perovskite is denser than the other phases. The
densities of Al-bearing stishovite and the calcium
ferrite-type aluminous phase are lower than those of
Mg-perovskite, Ca-perovskite, and the hexagonal
aluminous phase in the lower mantle. It is known that
natural subducted oceanic crust has a considerable
variation in its chemical composition. Differences in
whole rock composition can change the chemical
composition of the minerals in the subducted oceanic
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