Geoscience Reference
In-Depth Information
the parameters and assumptions that are used in
the following chapters.
it defines the potential temperature of the man-
tle at that point. Potential temperatures of the
mantle vary by about 200
◦
C. Such variations in
temperature are also required to cause the man-
tle to convect and are the result of plate tectonic
processes such as subduction cooling and conti-
nental insulation.
Temperature
A rise in temperature generally decreases the den-
sity and the seismic velocity and it is usually
assumed that temperature increases with depth.
It does not follow that seismic velocity is a proxy
for temperature and density. Rates of magmatism
and crustal thicknesses are also not proxies for
temperature.
The largest temperature variations in the
mantle are across conduction boundary layers,
plates and slabs; the high-temperature gradient
allows the internal heat to be conducted away
or the surrounding heat to be conducted in.
The temperature rise across the upper thermal
boundary layer (TBL) is about 1400
◦
C. Geophys-
ical and petrological data are consistent with a
variation of temperature of about 200
◦
Catany
given depth in the mantle, averaged over several
hundred km.
Petrologically inferred temperature variations
in the upper mantle are much lower than geo-
physically plausible estimates for the tempera-
ture change across the core--mantle boundary --
CMB. This implies that deep mantle material does
not get into the upper mantle or contribute to
surface volcanism.
There are trade-offs between composition and
temperature. Equally satisfactory fits to seismic
data can be obtained using a range of plausi-
ble compositions and temperatures. If the mantle
is homogenous in composition and if the lower
mantle composition is the same as estimates of
upper mantle peridotites, then the lower man-
tle is cold and iron-poor. On the other hand, if
the lower mantle has chondritic Mg/Si ratios then
the same data require a hot and iron-rich lower
mantle. In most cases, the inferred geotherms are
subadiabatic.
In the calculation of seismic velocity, there
are several temperature effects to be considered;
intrinsic, extrinsic, anharmonic, anelastic, quan-
tum and Arrhenius or exponential. In some of
these, the associated density effect is small.
Homologous temperature
The homologous temperature is the absolute tem-
perature divided by 'the melting temperature.'
Many physical properties depend on this scaled
temperature rather than on the absolute temper-
ature. The homologous temperature varies from
place to place in the mantle and this may be con-
fused with variations of absolute temperature.
This confusion has led to the hotspot hypothe-
sis. There are other parameters that also control
the locations and rates of excess magmatism at
places called melting anomalies or hotspots.
Melting temperature
Rocks begin to melt at the solidus and are com-
pletely molten at the liquidus. The addition of
water and CO
2
and other 'impurities' such as
potassium decrease the solidus temperature. The
materials in the mantle have quite different melt-
ing temperatures; eclogite, for example, melts
at temperatures about 200 K below the melting
point of peridotite. Partially molten eclogite can
be denser and colder than unmelted peridotite.
Variations of seismic velocity in the upper mantle
can be due to variations in lithology and melting
temperature, in addition to variations in absolute
temperature. A cold dense sinking eclogite blob --
perhaps a bit of delaminated lower continental
crust -- can have low seismic velocities and high
homologous temperature.
Adiabatic gradient
The temperature gradient in the mantle is fun-
damental in prescribing properties, such as the
composition, density, seismic velocity, viscosity,
thermal conductivity and electrical conductivity.
It is also essential in discussions of whole man-
tle vs. layered or stratified mantle convection.
An adiabatic temperature gradient is commonly
assumed. The adiabatic gradient is achieved by a
homogenous self-compressed solid. A fluid heated
from
Potential temperature
If a parcel of material at depth is brought to the
surface on an adiabatic path (rapid upwelling)
below
will
not
convect
until
the
radial
