Geoscience Reference
In-Depth Information
Figure 8.10.
Common
ultramafic rocks. The
three outer corners of the
triangle represent 100%
compositions: dunite is
ultramafic rock with close
to 100% olivine and hardly
any clinopyroxene or
orthopyroxene; wehrlite is
made up of approximately
equal parts of olivine and
clinopyroxene but no
orthopyroxene. The
interior triangle is the 90%
contour. The classification
of igneous rocks is
discussed in Section 9.1.
Clinopyroxene
100%
clino-
pyroxenite
lherzolite
ortho-
pyroxenite
dunite
harzburgite
100%
Olivine
100%
Orthopyroxene
orthopyroxene (Opx) and clinopyroxene (Cpx). Orthopyroxene varies between the
end members enstatite (MgSiO
3
) and ferrosilite (FeSiO
3
), with about the same
Mg to Fe ratio (94-91) in the mantle as olivine. Clinopyroxene also contains
calcium, as Ca(Mg, Fe)Si
2
O
6
.Figure 8.10 illustrates the relationships amongst
the common ultramafic rocks.
Experimental work on olivine has shown that it undergoes phase changes to
denser structures at pressures equivalent to depths of 390-410, 520 and 670 km
(see also Sections 8.2.3 and 9.6.3). This region of the mantle is called the
transition
zone
.Aphase change does not involve a change in chemical composition but
rather a reorganization of the atoms into a different crystalline structure. With
increasing pressure, these phase changes involve closer packing of the atoms
into denser structures (Fig. 8.11(a)). At 390-410 km, olivine changes to a
spinel structure, passing through a mixed
phase region, with a resultant
10% increase in density (pyroxene also changes to a garnet structure at this
depth). This change of olivine to spinel structure is accompanied by a release of
heat (the reaction is exothermic; Clapeyron slope 2-3 MPa K
−
1
). At depth about
520 km the
+
spinel phase changes, again through a mixed-phase region, into the
spinel phase. (This change has much less effect on seismic velocity than do
those at 400 and 670 km.) At 670 km the
spinel structure undergoes another
change to minerals with a post-spinel structure: perovskite (Mg, Fe)SiO
3
and
magnesiowustite (Mg, Fe)O (Fig. 8.11(b)). These reactions, which also involve
increases in density of about 10%, are endothermic (heat is absorbed during the
reaction; Clapeyron slope
(2-3) MPa K
−
1
). The silicon atoms in olivine and
the spinel structure are both surrounded by four oxygen atoms, but this changes
for the perovskite structure. Figure 8.11(a) shows these phase changes alongside
the S-wave velocity profiles of the upper mantle. The depths at which the phase
−
