EQUILIBRIUM IN GEOLOGICAL SYSTEMS - Chemical Fundamentals of Geology and Environmental Geoscience - page 32

Box 2.5 Partial melting II: melting in the Earth's mantle

Figure 2.5.1 shows in simple terms how the solidus
temperature of peridotite - the temperature at which it
begins to melt - varies with depth in the mantle. 4 This is
in essence a P-T diagram, drawn 'upside-down' in terms of
pressure in order to depict temperature as a function of
depth below the surface. The band between the solidus
and liquidus lines, the 'melting interval', depicts the range
of conditions under which partial melting (Box 2.4) of man-
tle peridotite can occur. These are the conditions neces-
sary to produce basaltic magma.
The curve marked 'Oceanic geotherm' indicates how the
ambient temperature in the upper mantle is believed to vary
with depth beneath a typical sector of mid-ocean ridge. Note
that the geotherm fails to reach the solidus at any depth.
Why then should melting occur at all in the upper mantle?
To see why, we must recognize that the mantle is not a
static body. Because the interior of the Earth is hot, the
solid mantle undergoes continuous (although very slow)
convective motion, with mushroom-like 'plumes' of buoy-
ant hotter material ascending from below (e.g. beneath
hawaii), and dense colder material sinking down (e.g. cold

oceanic lithosphere at subduction zones). In a convecting
mantle, melting may arise purely as a consequence of
upward motion. In Figure 2.5.1a, the solid peridotite at
point X, for instance, can penetrate the solidus and begin
to melt simply by migrating upward to lower pressures along
the path X-Y. This process, known as decompression
melting, is the primary cause of magma generation at
mid-ocean ridges: plate forces continuously pull the lith-
ospheric plates apart and thereby permit passive upwelling
(Figure 2.5.1b) and melting of the underlying astheno-
sphere. No rise in temperature is required; indeed, the
ascending material cools slightly as a result of the work it
has to do in expanding (Box 1.1).
Mantle plumes, on the other hand, are sites of buoyant
upwelling of deeper material that may be 150-300 °C hot-
ter than the surrounding upper mantle. Melting in a plume
is the combined effect of an elevated geotherm and
decompression resulting from upwelling. As many plumes
are located in intra-plate settings, the presence of thick
cool lithosphere confines melting to deeper levels than
beneath a mid-ocean ridge.

(a)

Temperature/°C

1000

50 1500 2000

Surface

0

Y

15

50

30

100

X

Melting not possible
Partially/wholly molten

45

150

Percentage of melting

Mid-ocean ridge
volc ani sm

Ocean island
vo lca nism

Figure 2.5.1 (a) Variation with pressure and depth
of the solidus temperature of dry mantle peridotite.
The heavy curve labelled 'oceanic geotherm' shows
how ambient temperature beneath an oceanic ridge
is believed to vary with depth. X-Y illustrates a
passive upwelling path resulting from lithosphere
extension. (b) Cartoon illustrating where
decompression melting occurs beneath an exten-
sional mid-ocean ridge and in an intraplate mantle
plume head.

(b)

Oceanic
lithosphere

Y

X

+50˚C
+100˚C
+150˚C

Ductile (convecting)
asthenosphere

Plume

Region where upwelling
mantle melts

Net force on plate

4 In the absence of water.