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simple consequence of ferric and ferrous iron being unable to trade electrons; Fe 3 + and
Fe 2 + behave as different elements and their relative proportions are essentially controlled
by their respective compatibility during magma differentiation.
The minerals thus formed may be denser (olivine and pyroxene) or less dense (pla-
gioclase) than the magma from which they crystallize. The denser ones tend to sink and
sediment out, while the buoyant plagioclase floats, with cumulates forming in both cases.
Such crystallization followed by the separation of minerals selectively removes elements
from the parent magma: this is magmatic differentiation by fractional crystallization. This
process is responsible for the formation of magma suites of very variable compositions,
for example the basalt-phonolite-trachyte suite, fine examples of which can be seen in
composite volcanoes such as in the Canary or the Society islands. The chemical evolution
of these suites is generally well understood. It is represented by situations of fractional
crystallization (Rayleigh's law), possibly complicated by the assimilation of surrounding
rocks (AFC
assimilation and fractional crystallization), or recurrent episodes of reser-
voir replenishment. Compatible elements (Ni, Cr, Sc, Sr) will be good tracers of fractional
crystallization as they vary greatly for a low degree of fractionation. Nickel indicates
the precipitation of olivine, Cr and Sc that of clinopyroxene, Eu and Sr that of plagio-
clase. It can be calculated, for example, from (2.30) , that a modest 10% precipitation of
olivine, for a partition coefficient of Ni between olivine and liquid of 15, lowers the par-
ent magma content of this element by 77%. Excess Eu with respect to the neighboring
rare-earth elements Sm and Gd will signal that a mafic rock is a plagioclase cumulate
(gabbro), while a deficit requires plagioclase removal. In contrast, incompatible elements
will be uniformly enriched in the residual liquid. Their relative distribution carries little
or no information about magmatic differentiation: fractionation of 10% olivine enriches
the magma in both Th and La (zero partition coefficient) by 11%. In contrast, the vari-
ability in incompatible elements induced by partial melting is very high compared with
that induced by fractional crystallization. As already alluded to, incompatible elements are
therefore particularly useful for the study of partial melting. In the more viscous granitic
liquids, gravity separation of minerals is less efficient than in the normally much more
fluid basalts. Granitic rocks are generally far more difficult to interpret than basalts in
terms of geochemistry as they are plutonic rocks and fully crystallized: it is practically
impossible to assert from examination of a sample of granite whether it is a sample of
congealed liquid magma, the cumulates derived from it, or some intermediate combination
of both.
=
11.2 Magmatism of the different tectonic sites
Figure 11.7 schematically shows the geographical distribution of various magma types in
relation to their geodynamic site.
The temperature at which rock melts increases with pressure and decreases sharply in
the presence of water. The low water content of the mantle (about 500 ppm) limits its effect
on upper-mantle melting. Mechanical effects of trace amounts of water, however, stand out:
 
 
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