Transcript General
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%)