Geology Reference
In-Depth Information
Box 2.4 Partial melting I: melting in the laboratory
Phase diagrams provide many insights into the important
process of rock melting. Consider first the progressive
melting of a mixture of diopside and anorthite, for example
the mixture of composition
c
in Figure 2.4.
The mixture will simply heat up until the temperature
reaches 1274 °C. At this point a melt of the eutectic com-
position E begins to form; this is the only melt composi-
tion that can be in equilibrium with diopside and anorthite,
both of which are at this stage present in the solid mix-
ture. Continued heating brings an increase in the propor-
tion of melt at constant temperature, with no change of
melt composition (invariant equilibrium) until the diopside
disappears, having been entirely incorporated in the melt.
(Anorthite has been dissolving too, but has not yet been
used up.) At this stage, from the Lever Rule (Box 2.3), the
ratio of melt:anorthite is XC:EX. Univariant equilibrium
(An + melt) now obtains, and with increasing temperature
the proportion of melt continues to rise, its composition
proceeding up the liquidus curve as more and more anor-
thite dissolves in it. At
x
, the melt has the same composi-
tion as the solid starting mixture, and here the last
remaining crystals of anorthite disappear. The system now
enters the divariant melt field, where the temperature can
continue rising without further change of state.
Similar principles govern the melting of solid-solution
minerals. The olivine system shown in Figure 2.4.1 (analo-
gous to Figure 2.5) provides an example relevant to basalt
production by partial melting in the upper mantle (which
consists largely of olivine). When olivine is heated up to
the solidus (for example, point
c
1
), a small proportion of
melt
m
1
appears. The melt is much less magnesium-rich
than the olivine from which it is produced, a point of great
petrological significance. Continued heating will cause the
temperature to rise, the proportion of melt to increase, its
composition to migrate up the liquidus curve towards
m
2
,
and that of the remaining olivine crystals to migrate up the
solidus towards
c
2
. The system would become completely
molten at just over 1800 °C (
m
3
). Thus a gap exists
between the temperature at which olivine begins to melt
and that at which it becomes completely liquid (as in
Figure 2.5). This gap, called the
melting interval
, is a fea-
ture of all minerals that exhibit solid solution. The every-
day notion of a 'melting point' applies only to pure
end-members, where the liquidus and solidus converge.
The complete melting of rocks like this only occurs in
very unusual circumstances, like meteorite impacts.
Generally, magmas are produced by a process of
partial
melting
, in which temperatures are sufficient to melt a
fraction of the source material but not all of it. Both in the
olivine phase diagram and in actual rocks, partial melting
will generate a melt less magnesium-rich (
m
2
) than the
source material (
c
1
), leaving behind a refractory solid resi-
due which contains more magnesium (
c
2
) than the source
material prior to melting. The composition of both prod-
ucts - melt and residual solid - depends upon the degree
(percentage) of melting, and therefore on the temperature
attained.
We must be careful not to assume that the tempera-
tures shown in the olivine diagram are necessarily charac-
teristic of the upper mantle. We have seen that although
pure anorthite (Figure 2.4) remains solid up to 1553 °C, a
mixture of anorthite with diopside begins to melt below
Crystallization in systems with solid solution
Eutectics are common features in systems of this
kind. They represent the general observation that
mix-
tures of minerals
(in other words, rocks) begin to melt at
lower temperatures than any of the pure constituents
(minerals) would on their own, just as a mixture of ice
and salt conveniently melts at lower temperature than
ice alone. This principle is widely used in industry
when a flux is added to enable a substance to melt at a
lower temperature than it would in the pure state (e.g.
in soldering).
Diopside and anorthite belong to different mineral
groups having different crystal structures, and the ten-
dency for either to incorporate the constituents of the
other into its crystal structure is negligible. But within
many mineral groups it is common to find that crystal
composition can vary continuously between one
end-
member
composition and another. One can visualize
one solid end-member 'dissolving' in the other to form
Search WWH ::
Custom Search