Squeezing: phase changes and mantle mineralogy - New Theory of the Earth - page 292

will approximate an adiabatic temperature gra-
dient, away from thermal boundary layers. The
normal-mantle adiabat is a useful reference
state but it is unlikely to exist anywhere in the
mantle. Different phase assemblages are encoun-
tered as one increases the temperature from the
normal-mantle adiabat. The partial melt field
(not shown) is encountered at low pressure. The
fields of the low-pressure, low-density assem-
blages are expanded at high temperature, but
the change in density is not symmetric about
the average, or normal, temperature. The ther-
mal expansion coefficient increases with temper-
ature, so there is a larger decrease of density for a
given increase in temperature than for the corre-
sponding decrease. For an internally heated man-
tle, upwellings are broader than downwellings,
so the lateral changes in physical properties
areexpectedtobemorediffusethanforthe
slab.

Variations in temperature, at constant pres-
sure, can cause larger changes in the physical
properties than are caused by the effect of tem-
perature alone. In general, the sequence of phase
changes that occurs with increasing pressure
also occurs with decreasing temperature. There
are also some mineral assemblages that do not
exist under normal conditions of pressure and
temperature but occur only under the extremes
of temperature found in cold slabs or near the
solidus in hot regions of the mantle. Generally,
the cold assemblages are characterized by high
density and high elastic moduli. Lithological gra-
dients, such as from peridotite to eclogite, can
result in density increases with seismic velocity
decreases.
The magnitude of the horizontal temper-
ature gradients in the mantle are unknown,
but in slabs and thermal boundary layers the
temperature changes about 800 ◦ C over about
50 km. In an internally heated material the
upwellings are much broader than slabs or down-
wellings. Tomographic results show extensive
low-velocity regions associated with ridges and
tectonic regions, consistent with broad high-
temperature, or low-melting-point, regions. The
cores of convection cells have relatively low ther-
mal gradients. We therefore expect the role of
isobaric phase changes to be most important and
most concentrated in regions of subducting slabs
and delaminating continental crust. The temper-
ature drop across a downwelling is roughly equiv-
alent to a pressure increase of 50 kbar, using
typical Clapeyron slopes of upper-mantle phase
transitions.
In the geophysics literature it is often
assumed that lateral variations in density and
seismic velocity are due to temperature alone. By
contrast, it is well known that radial variations
are controlled not only by temperature and pres-
sure but also by pressure-induced phase changes.
Phase changes such as partial melting, basalt-
eclogite, olivine-- spinel--postspinel , and pyroxene--
majorite-- perovskite dominate the radial variations
in density and seismic velocity. It would be futile
to attempt to explain the radial variations in the
upper mantle, particularly across the 400- and
650-km discontinuities, in terms of temperature
and pressure and a constant mineralogy. All of

Isobaric phase changes and lateral
variations of physical properties

It is important to understand the factors that
influence lateral heterogeneity in density and
seismic velocities. Much of the radial structure
of the Earth is due to changes in mineral-
ogy resulting from pressure-induced equilibrium
phase changes or changes in composition. The
phase fields depend on temperature as well as
pressure so that a given mineral assemblage
will occur at a different depth in colder parts
of the mantle. The elevation of the olivine--
spinel phase boundary in cold slabs is a well
known example of this effect. The other impor-
tant minerals of the mantle, orthopyroxene,
clinopyroxene and garnet, also undergo isobaric
temperature-dependent phase changes to denser
phases with higher elastic moduli. These phases
include majorite, ilmenite , spinel plus stishovite,
and perovskite . The pronounced low-velocity zone
(LVZ) under oceans and tectonic regions and its
suppression under shields is another example
of phase differences (partial melting) associated
with lateral temperature gradients. Deeper LVZs
can be caused by composition, e.g. eclogite vs.
peridotite.