Geoscience Reference
In-Depth Information
the water concentration of the magma is primarily controlled by pressure. The presence
of abundant water also changes the composition of the melt: a water-rich source rock
produces a magma that is richer in silica than a dry rock (
Fig. 11.3
). Carbon dioxide
at pressures
3 GPa (90 km) has a behavior similar to water, except that its effect on
melting is smaller due to its larger molar weight (44) and its lower solubility in melts.
At higher pressures, CO
2
reacts with silicates to form carbonates according to reactions
such as:
<
Mg
2
SiO
4
+
CO
2
⇔
MgSiO
3
+
MgCO
3
(11.1)
(olivine)
(vapor)
pyroxene magnesite
Melting of carbonated mantle at high pressure tends to produce melts with a high car-
bonate content that may eventually form low-silica carbonate-rich melts (nephelinites)
or even carbonate melts (carbonatites).
5.
Fertility
. This is the abundance of minerals with a lowmelting temperature, and it affects
the capacity of the rock to produce magma (melt productivity). At a given temperature,
a mantle rock rich in pyroxene and aluminous minerals will produce more liquid than a
peridotite with abundant refractory olivine.
Geochemical observations of lavas, notably of trace elements, can be interpreted quan-
More sophisticated models, mentioned earlier, draw on the percolation of fluids through
rock pores or the compaction of molten rock. These provide a more realistic picture of
the geochemistry of melt products. The mantle melting mechanisms themselves are still
poorly understood. The liquid first appears at the grain boundaries. Because it is less dense
than the solid mantle, buoyancy tends to expel the melt upwards as would a sponge full
of water under its own weight. So long as the porous matrix can be deformed, liquids per-
colate, producing chromatographic fractionation effects. When they penetrate the colder,
more rigid parts of the mantle, they collect in fractures and are channeled quickly toward
the surface forming volcanic systems.
Melting in the continental crust, most often in the presence of water, produces granitic
liquid: the composition of the melt is buffered by a mineralogical assemblage of quartz and
feldspar (eutectic melting), so that the variability of granite composition is fairly narrow.
The presence in the granite source of sedimentary material or material weathered at low
temperature is attested to by many indicators, the most obvious being the commonly high
level of their
18
O value (8-14
). During orogenic periods, substantial crustal melting
may occur: if we imagine that a granite intrusion, some 150 km long, 50 km wide and 6
km thick (a common enough dimension in many shield areas), represents a partial melt rate
of 30%, there must have been an enormous molten layer of nearly 20 km of crust at the time
these granites formed. Crustal melting is therefore a large-scale process. Narrow swarms
of granitic intrusions, occasionally elongated over thousands of kilometers (batholiths),
often mark the edge of subduction/collision zones: hybrid magmas formed by mixing of
mantle-derived basalts and andesites with felsic anatectic melts dominate these structures.
Some granites, known as S-type granites, form primarily when sediments and their meta-
morphic equivalents melt deep in the continental crust. Their high
δ
18
O values (9-15
)
imply that their source rock formed at low temperature in the presence of water. The
δ
