Geoscience Reference
In-Depth Information
35
35
30
800
T
≈
1500
°
C
PEROVSKITE
+
(Mg,Fe)O
+
2 Perovskites
(
+
MgO
+
St?)
PEROVSKITE
+
St
30
(Mg,Fe)O
30
2Pv
+
Gt
700
25
(Mg,Fe)O
(
+
MgO
+
St?)
+
St
PEROVSKITE
+
25
25
g
g
+
St
+
Cpx
Pv
+
Gt
600
g
+
St
+
Gt
+
Cpx
20
Garnet-Majorite ss
1600
°
C
20
20
g
+
(Mg,Fe)O
b
+
g
+
St
500
+
St
+
Cpx
+
Gt
b
+
St
+
Cpx
β
+
Gt
β + γ
γ
-SPINEL
b
+
St
+
Opx
15
+
Cpx
15
15
g
+
St
+
Opx
+
Cpx
+
Gt
400
α +
g
1500
°
C
10
10
10
300
Opx
+
Cpx
+
Gt
−
Mj ss
Opx
+
Cpx
α
-OLIVINE
5
5
200
5
100
80 60 40
Mole fraction pyroxene (percent)
20
0
2
2
1.0
.9
.8
.7
.6
.5
.4
.3
.2
.1
0.0
Mg
2
SiO
4
Fe
2
SiO
4
Fig. 22.2
Phase relations in the opx--gt system.
Mole fraction Mg
2
SiO
4
Phase relations in the (Mg
,
Fe)
2
SiO
4
Fig. 22.1
system.
magnesium and silicon:
VIII
(Mg
Fe)
3
VI
Mg
VI
Si
VI
Si
3
O
12
,
is crystallographically orthorhombic, but the oxy-
gen atoms are in approximate cubic close pack-
ing.Itissometimesreferredtoasa
distorted
or
modified spinel structure
.Itisapproximately
7% denser than olivine.
where the Roman numerals signify the coordi-
nation. This can be viewed as a garnet with
MgSi replacing the Al
2
. This is a high-temperature
transformation. Phase changes in olivine and
orthoproxene both contribute to the 410-km dis-
continuity (Figures 22.1 and 22.2) but the seismic
velocity jump at this depth is much less than pre-
dicted if these are the only two minerals in the
mantle. Other mantle minerals, garnet (gt) and
clinopyroxene (cpx), do not transform at 410 km
(Figure 22.3). In principle, this can be used to
constrain the gt
-
spinel
is an elastically
anisotropic mineral. The transformation to
γ
-
spinel
results in a density increase of about 3%
with no overall change in coordination.
β
Orthopyroxene, opx
Enstatite is the magnesium end-member of the
orthopyroxene series. Hypersthene is the inter-
mediate member with about 50% iron and fer-
rosilite is the iron-rich end member of the
series. These minerals have low seismic veloc-
ities. Orthopyroxene, (Mg,Fe)SiO
3
transforms to
a
cpx, i.e. eclogitic or fertile,
fraction of the mantle.
+
MgSiO
3
--Majorite
Pyroxene enters the garnet structure at high pres-
sure via the substitution
distorted
garnet-like
phase,
majorite
,w th
VI
[MgSi]
an
increase
in
coordination
of
some
of
the
→
Al
2
