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Origin of Basaltic Magma
Table 18-4. A
Classification of
Granitoid Rocks Based
on Tectonic Setting.
After Pitcher (1983) in
K. J. Hsü (ed.),
Mountain Building
Processes, Academic
Press, London; Pitcher
(1993), The Nature and
Origin of Granite,
Blackie, London; and
Barbarin (1990) Geol.
Journal, 25, 227-238.
Winter (2001) An
Introduction to Igneous
and Metamorphic
Petrology. Prentice Hall.
Sources of mantle material

Ophiolites
 Slabs
of oceanic crust and upper mantle
 Thrust at subduction zones onto edge of continent



Dredge samples from oceanic fracture zones
Nodules and xenoliths in some basalts
Kimberlite xenoliths
 Diamond-bearing
pipes blasted up from the
mantle carrying numerous xenoliths from depth
Kimberlite xenoliths
Photo of Kimberley diamond min (South Africa) and two
examples of mantle xenoliths (peridotite [top] and garnet
Peridotite [bottom] from a kimberlite.
Lherzolite is probably fertile (undepleted) unaltered mantle
Dunite and harzburgite are refractory residuum after basalt has been
extracted by partial melting
Tholeiitic basalt
15
Ultramafic
rocks
10
Figure 10-1 Brown and Mussett,
A. E. (1993), The Inaccessible
Earth: An Integrated View of Its
Structure and Composition.
Chapman & Hall/Kluwer.
5
Lherzolite
Harzburgite
Dunite
0
0.0
0.2
Residuum
0.4
Wt.% TiO2
0.6
0.8
Lherzolite: A type of peridotite
with Olivine > Opx + Cpx
Olivine
Dunite
90
Peridotites
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
10
Orthopyroxene
Websterite
Clinopyroxenite
Figure 2-2 C After IUGS
Clinopyroxene
Phase diagram for aluminous
4-phase lherzolite:
Al-phase =

Plagioclase
 shallow

Spinel
 50-80

km
Garnet
 80-400

(< 50 km)
km
Si  VI coord.

> 400 km
Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus
reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.
How does the mantle melt??
1) Increase the temperature
Figure 10-3. Melting by raising the temperature.
2) Lower the pressure
 Adiabatic
rise of mantle with no conductive heat loss
 Decompression melting could melt at least 30%
Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the
solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.
3) Add volatiles (especially H2O)
Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
Melts can be created under
realistic circumstances



Plates separate and mantle rises at midocean ridges, or at continental rifts
 Adibatic rise  decompression melting
Hot spots  localized plumes of melt
Fluid fluxing
 Important in subduction zones
Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett., 19, 290-300. (b) after Pearce (1982) in Thorpe (ed.),
Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548, Coish et al. (1986), Amer. J. Sci., 286, 1-28. (c)
after Mullen (1983), Earth Planet. Sci. Lett., 62, 53-62.