Page 188 - Global Tectonics

174 CHAPTER 7

(a) T C (b) T C
1000 2000 1000 2000
0 0
Plagioclase Plagioclase
Spinel Liquid Spinel Liquid
100 60% 100
P (GPa) 50 Solid phases Geotherm Liquidus 200 50 Solid phases Geotherm Adiabat 15% Liquidus 200 Depth (km)
30%

100 Solidus 300 100 Solidus 300

Figure 7.18 (a) Melting by raising temperature. (b) Melting by decreasing pressure (from Winter, John D., An
Introduction to Igneous and Metamorphic Petrology, 1st edition © 2001, p. 195. Reprinted by permission of Pearson
Education, Inc., Upper Saddle River, NJ). In (b) melting occurs when the adiabat enters the shaded melting zone.
Percentages of melting are shown.

Compositional variability also refl ects the assimilation that mafic magmas in the Eastern branch of the East
of crustal components and magma mixing. The bimodal African Rift system were derived from at least two
basalt-rhyolite eruptions are thought to refl ect combi- mantle sources, one of sublithospheric origin similar to
nations of mantle and silica-rich crustal melts. that which produces ocean island basalts and one within
A comparison of trace element concentrations and the subcontinental lithosphere. Contributions from the
isotopic characteristics indicates that basalts generated subcontinental mantle are indicated by xenoliths of
in continental rifts are broadly similar to those of lithospheric mantle preserved in lavas, distinctive rare
oceanic islands (Section 5.5). Both rock types preserve earth element patterns, and by the mineralogy of basal-
evidence of a mantle source enriched in incompatible tic rock. In southern Kenya, the presence of amphibole
trace elements, including the LILE, and show relatively in some mafic lavas implies a magma source in the

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high radiogenic strontium ( Sr/ Sr) and low neodym- subcontinental lithosphere rather than the astheno-
144
143
ium ( Nd/ Nd) ratios. These patterns are quite differ- sphere (le Roex et al., 2001; Späth et al., 2001). This
ent to those displayed by mid-ocean ridge basalts, which conclusion is illustrated in Fig. 7.19 where the experi-
are depleted in incompatible trace elements (Fig. mentally determined stability field of amphibole is

7.17b,c) and display low strontium and high neodym- shown together with a probable continental geotherm
ium ratios. Trace elements are considered incompatible and adiabats corresponding to normal asthenospheric
if they are concentrated into melts relative to solid mantle and a 200°C hotter mantle plume. It is only in
phases. Since it is not possible to explain these differ- the comparatively cool lithospheric mantle that typical
ences in terms of the conditions of magma genesis and hydrous amphibole can exist. The additional require-
evolution, the mantle from which these magmas are ment of garnet in the source, which is indicated by
derived must be heterogeneous. In general, the asthe- distinctive rare earth element patterns, constrains the
nosphere is recognized as depleted in incompatible ele- depth of melting to 75–90 km. These and other studies
ments, but opinions diverge over whether the enriched show that the generation of lithospheric melts is
sources originate above or below the asthenosphere. common in rifts, especially during their early stages of
Undepleted mantle plumes offer one plausible source development. They also indicate that the identifi cation
of enriched mantle material. Enrichment also may of melts derived from the subcontinental lithosphere
result from the trapping of primitive undepleted asthe- provides a potentially useful tool for assessing changes
nosphere at the base of the lithosphere or the diffusion in lithospheric thickness during rifting.
of LILE-rich volatiles from the asthenosphere or deeper In addition to compositional variations related to
mantle into the lithosphere. source regions, many authors have inferred systematic
On the basis of trace element concentrations and relationships between basalt composition and the depth
isotopic characteristics, Macdonald et al. (2001) inferred and amount of melting in the mantle beneath rifts