Introduction to Planetary Science

earth: model of planetary evolution 73
Figure 6.5. The melting curve of dry peridotite constrains
the temperature of the mantle of the Earth, which remains
solid even though its temperature rises to 1700 C at a depth
of 400 km where the pressure is 132 kilobars. In addition,
the melting curve illustrates how peridotite at point P can
melt either by increasing the temperature to 1740 C at point
R or by lowering the pressure from 140 to 70 kilobars at
point Q. Melting of rocks in the mantle by decompression is
an important process that leads to the formation of magma,
which can either cause volcanic eruptions on the surface
of the Earth or can result in the formation of plutonic
igneous rocks in the continental crust. Adapted from Press
and Siever (1986, Figure 15.12)
peridotite at point P is considered to be “dry”
which means that it can only melt in response
to an increase in temperature or a decrease in
pressure.
The rocks of the mantle of the Earth do
contain radioactive elements (uranium, thorium,
and potassium) that generate heat when the nuclei
of their atoms decay spontaneously. However,
the data in Table 6.2 demonstrate that peridotite
generates only small amounts of heat 0021 ×
10 −6 cal/g/y by this process compared to
Table 6.2. Production rates of heat by the decay of
radioactive atoms of uranium (U), thorium (Th), and
potassium (K) in different kinds of plutonic igneous rocks
(Emiliani, 1992, Table 11.8)
Rock type Heat production, 10 −6 cal/g/y
U Th K Total
Granite 34 4 108 848
Diorite 19 18 067 437
Gabbro 066 05 012 128
Peridotite 0011 0002 0008 0021
granite 848×10 −6 cal/g/y because peridotites
have much lower concentrations of radioactive
elements than granite. Consequently, peridotite
in the mantle of the Earth is more likely to melt
as a result of a decrease in pressure than because
of an increase in temperature.
Rocks composed of silicate minerals do not
melt completely at a unique temperature the
way ice does. In contrast to ice which melts
congruently, silicate minerals such as olivine,
plagioclase, and pyroxene melt incongruently
over a range of temperature to form a melt
and residual solids. The chemical composition
of the melt depends on the extent of melting
and differs from the chemical composition of
the rock before any melting occurred. Therefore,
the partial melting of peridotite in the mantle
can produce a wide range of melt compositions
depending on the extent of melting. Small
degrees of melting (e.g., 1% or less) yield melts
that are enriched in alkali metals such as sodium,
potassium, and rubidium, whereas large degrees
of partial melting (e.g., 20 to 30%) produce melts
of basaltic composition with elevated concentrations
of iron, magnesium, and calcium,
6.4.3 Mantle Plumes
The volcanic activity on oceanic islands, along
mid-ocean ridges, and along continental rift
zones is caused by decompression melting that
occurs when plumes or diapirs of hot rock rise
through the asthenospheric mantle until they
encounter the underside of the rigid lithosphere
(Faure, 2001). Plumes arise at depth in the
asthenosphere where the irregular distribution of
radioactive elements (explained in Section 6.4.4)
causes localized increases of the temperature.
As a result, the density of the affected rocks