Mantle Melting
Some slides from Mary Leech
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.
Lherzolite is probably fertile (undepleted) unaltered mantle
Harzburgite ± Dunite are refractory residuum after basalt has been
extracted by partial melting
Tholeiitic basalt
15
Ultramafic
rocks
10
Brown and Mussett, A. E. (1993),
The Inaccessible Earth: An
Integrated View of Its Structure
and Composition. Chapman &
Hall/Kluwer. Slide from Mary
Leech.
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
How does the mantle melt??
1) Increase the temperature
2) Lower the pressure

Adiabatic rise of mantle with no conductive heat loss

Decompression melting could melt at least 30%
Phase diagram of aluminous lherzolite
with melting interval (pink), sub-solidus
reactions, and geothermal gradient.
After Wyllie, P. J. (1981). Geol. Rundsch.
70, 128-153.
3) Add volatiles (especially H2O)
Phase diagram for aluminous 4-phase lherzolite:
Alminous phase =




Plagioclase shallow (< 50 km)
Spinel 50-80 km
Garnet 80-400 km
Si VI coord. > 400 km
Where does mantle melting occur?
Result? Basalt
What is
MO R B ?
id
cean
idge
sasalt

MgO and FeO

Al2O3 and CaO

SiO2

Na2O, K2O, TiO2,
P2O5
Basaltic glasses from the Afar region of the
MAR. Note different ordinate scales. From
Stakes et al. (1984) J. Geophys. Res., 89,
6995-7028.
Ternary Variation Diagrams
Example: AFM diagram
(alkalis-FeO*-MgO)
AFM diagram for Crater Lake
volcanics, Oregon Cascades. From
Mary Leech
Conclusions about MORBs, and the processes
beneath mid-ocean ridges
 MORBs are not the completely uniform
magmas that they were once considered to
be
 They show chemical trends consistent
with fractional crystallization of olivine,
plagioclase, and perhaps clinopyroxene
 MORBs cannot be primary magmas, but
are derivative magmas resulting from
fractional crystallization (~ 60%)