The solid Earth - Geochemistry: An Introduction - page 221

abundance of muscovite and the low Fe 3 + /Fe 2 + ratio of the S-granites indicate that this
source contained large proportions of oxygen-starved clay minerals. Their high 87 Sr/ 86 Sr
and low 143 Nd/ 144 Nd ratios reflect the fact that this source has evolved over hundreds
of millions, even billions of years, with a high Rb/Sr ratio typical of metasedimentary
rocks. In contrast, other granites, known as I-type granites, have low

18 O values (6-9

)
indicative of a source dominated by igneous rocks that were only slightly affected by low-
temperature alteration (typically older plutonic rocks). Ubiquitous hornblende reflects the
abundance of Na and Ca in the source rock. More than 25% of the iron is oxidized, while
Sr is less radiogenic and Nd more radiogenic than in S-granites: all these features point to
the melting of deeper levels of the continental crust.
Granitic magmatism is almost exclusively limited to the Earth's continents: a sample
of lunar granite has been described, a few lavas of granitic composition are known in
Iceland and the islands of Réunion and Ascension, and on mid-ocean ridges, but their
isotopic characteristics (O, Sr, Nd) make them more like basalts than continental gran-
ites. These granitic rocks must have formed by remelting of the basaltic pile, although, in
rare cases, extreme differentiation of basaltic magmas may also leave felsic lavas as the
end-product.

δ

11.1.2 Differentiation of magmatic series

We have seen that one of the causes of magma chemical variability is differentiation,
already referred to at the end of Chapter 2 . As magmatic liquids are less dense, at least
at upper mantle pressure, they are subjected to the buoyancy forces of the surround-
ing rocks and therefore rise, initially by percolation when the matrix is deformable and
then, in the lithosphere, along fractures. As they rise, they cool by contact with the
walls and the solubility limit is reached for each mineral in turn ( Fig. 11.4 ) in rather
straightforward sequences. In basalts at high pressure, olivine saturation is reached first,
followed by that of clinopyroxene and, finally, plagioclase. If cooling occurs at low
pressure ( 15 km), as under the mid-ocean ridges, the order of appearance of plagio-

clase and pyroxene is reversed. In some cases, an intermediate reservoir may temporarily
accommodate the magma. This is the mythical magma chamber, a concept that pervades
petrological literature. Such magma chambers are actually uncommon in island volca-
noes, but are ubiquitous beneath ridges. This contrasting pressure-dependent behavior is
clearly visible in the evolution of the major element concentrations in various types of
basalts ( Figs. 11.5 , 11.6 ) . Since Mg is much more compatible than Fe in mafic minerals
(olivine and pyroxene), the FeO/MgO ratio (a convenient differentiation index) decreases
when the degree of crystallization increases. Most ocean island basalts (except from those
islands that lie on the ridge axis, such as Iceland, or adjacent to it, such as the Gala-
pagos) differentiate at medium pressure and remain undersaturated in plagioclase: when
plotted against FeO/MgO, the Al 2 O 3 ,orTiO 2 concentrations attest to the essentially
incompatible behavior of Al and Ti and increase in differentiated melts. In contrast, mid-
ocean ridge basalts fractionate plagioclase: Al 2 O 3 is compatible and decreases, while
FeO and TiO 2 , rejected by plagioclase, remain incompatible and increase in residual

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