Geoscience Reference
In-Depth Information
BUOYANT ASCENT AIDED BY MELTING
PICLOGITE SOLIDUS
BASALT SOURCE REGION
ENTRAINMENT AIDED BY PHASE CHANGES
Basalt
Eclogite
SOLIDUS
BASALT SOURCE REGION
Fig. 7.1
Methods of removing material from a deep, dense
layer. In a chemically layered mantle, the interface between
layers is highly deformed. This may cause phase changes and
melting, and a reduction in density. The deeper layer may also
be entrained. If the deep layer is eclogite, from subduction or
delamination, it will have a lower melting temperature than
ambient mantle and may rise because of melt-induced
buoyancy. Adiabatic decompression leads to further melting.
and 1400
◦
C, averaged over large areas. The adi-
abatic temperature gradient for the solid upper
mantle is approximately 0.3--0.5
◦
C/km. The adia-
batic gradient becomes smaller at very high pres-
sures because the thermal expansivities of solids
are smaller at high pressures. The adiabatic gra-
dient for liquids (
1
◦
C/km) is higher than that
for solids because the thermal expansivities of
liquids are greater than solids.
Melting occurs where the geotherms inter-
sect the mantle solidus. For a dry peridotite
mantle, the geotherm (conduction gradient) in
a region of high surface heatflow, 100 mW/m
2
,
intersects the solidus at
∼
lithosphere. In mid-plate environments, such as
Hawaii and other midplate hotspots, melts will
cool, fractionate and mix with other melts prior
to eruption. Such a mechanism seems capable of
explaining the diversity of hotspot (ocean island,
continental flood) basalts and midocean ridge
magmas.
Material can leave a deep, dense source region
by several mechanisms.
30 km depth. Surface
heat flows of 100 mW/m
2
or greater occur only
at or near midocean ridges. A geotherm with
40 mW/m
2
surface heatflow, characteristic of the
old interiors of continents, never intersects the
dry peridotite solidus, implying that partial melt-
ing does not occur beneath continental interiors.
In suture belts and along arcs there may be low-
melting point constituents such as eclogite in the
shallow mantle. High heatflow, however, in part,
represents intrusion into the plate or thinning of
the thermal boundary layer, rather than intrinsi-
cally high mantle temperatures.
We can induce melting at low temperatures if
we flux the mantle with basalt, eclogite, CO
2
or
water, and plate tectonics does all of these things.
If portions of the mantle upwell rapidly, either
passively in response to spreading at ridges or
displacement by sinking slabs, decompressional
partial melting occurs. Decompressional melting
∼
(1) Melting in the thermal boundary because of
the
high
thermal
gradient
compared
with
melting point gradients.
(2) Melting, or phase changes, due to adiabatic
ascent of hotter regions of a layer and crossing
of phase boundary.
(3) Entrainment of material by adjacent convect-
ing layers.
Some
of
these
mechanisms
are
illustrated
in
Figure 7.1.
The
potential temperature
of the mantle is the
temperature of the mantle adiabat if it were to
ascend directly to the surface. The potential tem-
perature of the mantle is usually between 1300
