The solid Earth - Geochemistry: An Introduction - page 217

and their mineral assemblage becomes eclogitic. This is the origin of high-pressure
metamorphism.
The most significant chemical fractionation processes within the Earth are the forma-
tion of oceanic lithosphere, hot spots, and continental crust. The fractionation processes
attending melting and crystallization - the essential mechanisms by which magmatic rocks
originate - are responsible for the diversity of chemical composition of such rocks. They
also determine indirectly the geochemical variability of metamorphic rocks and clas-
tic sedimentary rocks (clays, sands). The essential mixing processes are associated with
hybridization (or contamination) of magmas and mantle convection. The current state of
the mantle and the continental crust is the outcome of competition among all of these
processes.
11.1 The geochemical variability of magmas
Magma, the common term for a molten rock, may contain crystals in suspension, usually
called phenocrysts. Molten magma reaching the surface is known as lava. If the magma
crystallizes completely, most commonly as an effect of slow cooling, the rock produced is
said to be intrusive or plutonic. If cooling is too fast for crystallization to be completed, for
example in a submarine eruption, the liquid is quenched to a glass. After cooling, it forms
an effusive or volcanic rock.
The two most abundant types of magmatic rocks are basalt in ocean areas and granite
in continental areas. A basalt reaches the surface at a temperature of 1150-1250 ◦ C, and a
granitic liquid (rhyolite) at about 1000 ◦ C. Table 11.1 gives the characteristic major element
compositions of magmatic rocks in their standard form, i.e. by oxide weight. Basalt is rich
in iron, magnesium, and calcium, whereas granite is rich in silica and alkaline elements.
We will now look at the two mechanisms responsible for the variability of magmatic rocks,
melting of the mantle and crust, and magmatic differentiation.
11.1.1 Melting of the mantle and crust
To the best of our current knowledge, melting is localized in the upper 100 km of the
mantle, where it produces basalts, and in the crust where it produces granites. There is some
discussion about deeper zones of melting, notably at the bottom of the mantle where some
low seismic velocity anomalies are present, but evidence is still fragmental and ambiguous.
We will leave aside for now the issue of the early Earth and the magma ocean. Melting
occurs on top of mantle upwellings, both under mid-ocean ridges and at hot spots because
(i) the low density of melts with respect to their residue gives the melting curve (solidus)
a positive d P /d T Clapeyron slope, and (ii) silicates are thermal insulators, which reduces
the cooling path to become adiabatic, very nearly isothermal. A primary melt did not lose
mineral phases and opposes a differentiated melt: the process by which crystals are lost
is known as fractional crystallization or magmatic differentiation and will be discussed in