Phase transition and density of subducted MORB crust in the lower ...

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K. Hirose et al. / Earth and Planetary Science Letters 237 (2005) 239–251
of CaFe 2O 4-type Al-phase is larger the PREM density
even at high temperatures (Fig. 5d). It is noted, however,
that chemical composition of CaFe 2O 4-type Alphase
is variable depending on the bulk MORB composition
as summarized by Guignot and Andrault
[36], which significantly affects the mineral density.
Ono et al. [7] calculated remarkably lower densities
for this mineral primarily due to the difference in
chemical composition.
The net density of MORB composition is calculated
from the density of each constituent mineral and
mineral proportions. The density profile of MORB
composition is presented in Fig. 6. Fitting of pressure–density
data for perovskite-dominant assembly
at 300 K to the Birch–Murnaghan equation of state
gives K0=222(F6) GPa and q0=4.22(F0.02) g/cm 3
when KV is fixed at 4 (Fig. 6a). The density of
MORB crust increases by about 1% at ~2400-km
depth, if Al substitutes Si by using octahedral vacancy
(without forming oxygen vacancy) in a-PbO2-type
SiO2 phase. In contrast, if oxygen vacancy-type substitution
is assumed, the calculation shows that density
of MORB crust marginally decreases at the
pressure-induced phase transition, which is physically
unreasonable. It indicates that the former Al substitution
mechanism is dominant in a-PbO 2-type SiO 2
phase.
The subducted MORB crust is denser than the
average lower mantle at all depths greater than 720
km where MORB crust becomes perovskite-dominant
lithology [2,5,6], even after thermal equilibrium is
attained. The same conclusions are obtained when
pressure is estimated by using different EOS of gold
proposed by Jamieson et al. [27] (Fig. 6b).
This conclusion contrasts with earlier predictions
[4,23]. The mineral volumes were not measured at
high P–T in these previous studies, and instead compression
and thermal expansion data of the end-member
composition were used for each constituent
mineral. This resulted in a serious underestimate of
the density of MORB crust in the lower mantle [4,23].
It is true that MORB crust has extensively wide
chemical variations. Present study demonstrated that
only the SiO2 phases, especially a-PbO2-type phase,
contribute to the buoyancy of subducted MORB crust
relative to the surrounding mantle, using the particular
MORB sample. Ono et al. [7] showed that CaFe2O4type
Al-phase is also less dense than the normal
Rock density (g/cm 3 ) Rock density (g/cm 3 )
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
a
40 60 80 100 120 140
b
Depth (km)
1200 1600 2000 2400 2800
Pressure (GPa)
Depth (km)
1200 1600 2000 2400 2800
40 60 80 100 120 140
Pressure (GPa)
PREM
Au_Tsuchiya
PREM
Au_Jamieson
Fig. 6. Net density profile of MORB composition. Pressure was
calculated based on EOS of gold proposed by (a) Tsuchiya [25] and
by (b) Jamieson et al. [27]. Circles, MgPv+St+CaPv+CF; triangles,
MgPv+CaCl 2-type SiO 2+CaPv+CF; squares, MgPP+a-
PbO2-type SiO2+CaPv+CF. Closed and open symbols indicate
300 K and high temperature (1750–2290 K) data, respectively.
Broken lines indicate the PREM density [42]. The error of density
is typically 0.02 g/cm 3 , derived from the uncertainties in volumes
of coexisting phases and in mineral proportion. (a) Solid line shows
a density profile at 300 K for perovskite-dominant assembly fitted
to the Birch–Murnaghan equation of state. (b) Data by Ono et al.
[7] using Jamieson’s gold scale were shown for comparison (pluses).
Slightly lower density reported by [7] is primarily due to the
lower density of CaFe 2O 4-type Al-phase with a different chemical
composition.
mantle (Fig. 5d) using the different MORB composition.
However, both reached the same conclusion on
the density relationship between MORB and the nor-