Inner Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 206

identical composition. A selection of values for L f is shown
in Box 5.1. Because, melting of a given volume of solid
cannot be achieved instantaneously, even if a homogenous
mineral or elemental solid is involved, we need concepts
to express the onset of melting and its completion: these
are solidus and liquidus respectively. We generally draw the
solidus and liquidus as lines on temperature : pressure
graphs or on phase diagrams. The solidus line thus indi-
cates the temperature at which a rock begins to melt
(or conversely becomes completely solid on cooling) and
the liquidus line is the temperature at which melting is
complete (or conversely at which solidification begins on
cooling). As an example, we can follow the solidus of
basalt on the P-T diagram of Fig. 5.6.
Since most rocks are chemically different and may be
comprised of various mineral species or minerals free to
vary in composition, the onset of melting or the process of
crystallization on cooling is complex. Major progress in
understanding the processes of melting and crystallization
of natural silicates were made by N.L. Bowen in experi-
ments conducted in the early twentieth century (Figs 5.7
and 5.8). To illustrate this, consider one of Bowen's earli-
est triumphs, an explanation of the variation in behavior of
the simplest possible rock made up of only olivine , an
iron-magnesium silicate, whose composition is free to vary
between 100 percent iron silicate (representing a mineral
phase called fayalite ) and 100 percent magnesium silicate
(the mineral forsterite ). The olivine system is obviously of
major importance because it makes up a major mineral
phase of the Earth's ocean crust. Minerals like olivine that
are able to vary in their solid composition between two
end-members like this are quite common in nature
(the common feldspar minerals are another) and are said
to exhibit solid solution . A solid solution is like any alloy,
bronze, solder, or pewter for example, where the metal
ions can mix freely in most proportions since they are of

similar size and charge. However, since the Mg 2 ion in
forsterite is somewhat smaller than the Fe 2 ion in fayalite,
it is held more tightly by atomic bond energy into the
silicate crystal lattice and therefore melts at a higher
temperature; olivines composed of pure Mg 2 and Fe 2
thus melt at about 700 C apart. Now, take a 50 : 50

combination of Fe 2 and Mg 2 silicate in an olivine solid
volume and heat it up at atmospheric pressure to 1400

C
(Figs 5.7 and 5.8). The composition of the initial melt, or
partial melt, produced from such an olivine will tend to be

Temperature (°C)

Melt
collection

Onset
melting

Upwelling

Liquidus

Solidus

Fig. 5.6 To show solidus, liquidus, and an adiabatic melting curve as
mantle rock is elevated by convection, partially melts and rises to
surface.

N.L. Bowen

Mineral phase A Mineral phase B

Initial
melt

Box 5.1 Latent heat of melting
(cal g 1 ) for some important silicate
minerals.

Mg-olivine 208

Fe-olivine 108

Clinopyroxene 146

Orthopyroxene 85

Garnet 82

Ca-Feldspar 67

100% A 100% B

50 : 50
Mixture

Na-Feldspar 52

K-Feldspar 53

Fig. 5.7 Melting relations in a binary silicate solid solution series.