Geoscience Reference
In-Depth Information
Table 11.1
Chemical composition of a few representative magmatic rocks: tholeiitic
and alkali basalts, andesite, and granites (in weight percent oxide)
Locality
Rock
SiO
2
TiO
2
Al
2
O
3
FeO MgO CaO K
2
ON
2
OP
2
O
5
Mid-ocean
ridge
Tholeiitic
basalt
50.9
1.2
15.2
10.3
7.7
11.8
0.1
2.3
0.1
Hawaii
Alkali basalt
45.9
4.0
14.6
13.3
6.3
10.3
0.8
3.0
0.4
Hawaii
Tholeiitic
basalt
49.3
2.8
13.4
11.5
7.7
11.0
0.4
2.3
0.3
Siberia
(trap)
Tholeiitic
basalt
48.7
1.4
14.8
10.3
7.3
9.7
0.9
2.4
0.1
Cascades
Andesite
60.4
0.9
17.5
6.4
2.8
6.2
1.2
4.3
0.2
Granite (I)
a
Australia
69.5
0.4
14.2
3.2
1.4
3.1
3.2
3.5
0.1
Granite (S)
b
Australia
70.9
0.4
14.0
3.2
1.2
1.9
2.5
4.1
0.2
the next section. The factors of chemical variability of the primary melts extracted from
the mantle and the crust are multiple and not always independent. Let us consider the most
important of these.
1.
The nature of the source rock
. Partial melting of the peridotitic upper mantle, composed
mainly of olivine, pyroxene, and aluminous minerals (feldspar, spinel, and garnet)
yields basalts. Melting of the continental crust, formed by wet sediments accumulated
by erosion, and by their metamorphic equivalents (schist and gneiss) produces granitic
liquids.
2.
Pressure
. The pressure
P
at any depth of melting
z
is the weight of the overlying rocks;
P
and
z
are related through the hydrostatic condition d
P
=
ρ
g
d
z
(
Fig. 11.3
), where
is the rock density. Different planets have different gravities and therefore different
pressure-depth relationships! At shallow depths, melting of the mantle produces tholei-
itic basalts, i.e. basalts that are somewhat richer in Si and poorer in Mg than other types
of basalts. At greater depths the trend is reversed and magma is richer in Mg and Fe and
poorer in Si; these are alkali basalts.
3.
Temperature
. For a system of a given composition at a given pressure, temperature is
directly related to the
degree of melting
: the major elements that enter the melt most
readily (Si, Al, Ca, K, Na, Ti) and incompatible trace elements (Rb, Zr, Ba, rare-earths,
Th, U, etc.) are relatively abundant in the first melts. As melting proceeds, the liq-
uid becomes richer in refractory elements (Mg, Cr), fresh melt dilutes incompatible
elements, and basalt gives way to picrite. Melt temperature and composition are not
independent. Close to the minimum eutectic temperature, the melt fraction is small and
the liquid composition is buffered by the residual minerals. This is the case not only
for mantle melting at high pressure (
Fig. 11.3
), but also for the crust. At higher temper-
atures, melting progressively consumes residual minerals, and the liquid composition
ρ
