Geoscience Reference
In-Depth Information
magnesiowustite are stable throughout most of
the lower mantle. The details of the stable assem-
blages depend on composition and temperature.
It is usually assumed that the radial stucture of
the mantle, and mantle discontinuities, are due
to equilibrium solid--solid phase changes, and
that lateral changes in seismic velocity -- tomog-
raphy -- are due to changes in temperature. This
limited view has produced some strange models
of mantle dynamics and chemistry.
Lateral variations in velocity due to
temperature-induced phase changes can be
as important as pressure-induced phase changes
are in the radial direction. Phase equilibria is
usually discussed in terms of the olivine system,
the pyroxene system and the pyroxene--garnet
system. However, at high pressures, these sys-
tems can interact with each other. Pyroxenes
can tolerate a certain amount of Al
2
O
3
and
garnets, at high pressure, dissolve pyroxene, so
pyroxenesandgarnetsmustalwaysbetreated
together. Olivine and orthopyroxene also interact
at high-temperature, exchanging iron with one
another.
Phase equilibria in mantle systems are sum-
marized below in a series of figures based on
available experiments and calculations. Although
these are useful and informative to specialists,
the main point for non-specialists -- seismolo-
gists, geodynamicists, students -- is that one can-
not wander very far in P--T-composition-depth
space without encountering large changes in
density and other physical properties. Interpre-
tations of tomographic models, and design of
convection simulations must take phase changes
into account in order to be realistic. On the other
hand, radial or 1D Earth models may also be
affected by chemical layering as well as by phase
equilibria.
The main mantle systems are shown in
Figures 22.1 through 22.6. More complete
phase diagrams for mantle minerals are readily
available:
http:/www.mpi.stonybrook.edu/
ResearchResults/PhaseRelations
Gasparik/figures.htm
http:/www.springeronline.com/sgw/
cda/frontpage/0,11855,1-10010-
22-2155856-0,00.html
http:/www.mantleplumes.org/
Transition-Zone.html
Pyroxene system at high pressure
Enstatite (en) and diopside (di) do not form a
complete solid-solution series, but en dissolves a
certain amount of di, and di contains an appre-
ciable amount of en at moderate temperature
and pressure. The amount of mutual solubility
increases with temperature and decreases with
pressure, and this provides a method for estimat-
ing the temperature of equilibration of mantle-
derived xenoliths. Pyroxenes also react with gar-
net. In principle, the measurement of the com-
positions of coexisting pyroxenes and garnets
provides information about pressures and tem-
peratures in the mantle. At higher pressure gar-
net dissolves the enstatite, and this requires a
change in coordination of one-fourth of the Mg
and Si. Depths in excess of about 300 km are
required for this change in coordination.
Natural clinopyroxenes, particularly in eclog-
ites, are solid solutions between diopside and
jadeite called omphacite. At modest pressure,
equivalent to about 50--60 km depth, the solid
solution series is complete (Figure 22.3). Natural
clinopyroxenes from kimberlite eclogites contain
up to 8 wt.% Na
2
O. Clinopyroxenes from peri-
dotites typically have much less Na and jadeite.
Most garnets contain very little sodium; how-
ever,athighpressureNa
2
O can enter the gar-
net lattice. Natural garnets associated with dia-
monds in kimberlite pipes contain up to 0.26%
Na
2
O. In the transition region, the sodium is
probably contained in a complex garnet solid
solution.
The other high-pressure forms of pyroxene
include
ilmenite, spinel
plus stishovite, and
per-
ovskite
, depending on pressure, temperature and
content of calcium, aluminum and iron. The
pressures at which clinopyroxene and orthopy-
roxene
disappear
are
strong
functions
of
the
other variables.
The
phase
behavior
of
garnet
+
clinopy-
roxene
orthopyroxene, the peridotite assem-
blage, is substantially different from the behav-
ior of garnet
+
+
clinopyroxene, the basalt--eclogite
assemblage. When only clinopyroxene
garnet
are present, the clinopyroxene dissolves in the
+
