Mantle Melting

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Mantle Melting
Some slides from Mary Leech
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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.
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Lherzolite is probably fertile (undepleted)
unaltered mantle Harzburgite Dunite are
refractory residuum after basalt has been
extracted by partial melting
Tholeiitic basalt
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Ultramafic rocks
Partial Melting
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Wt. Al2O3
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Brown and Mussett, A. E. (1993), The Inaccessible
Earth An Integrated View of Its Structure and
Composition. Chapman Hall/Kluwer. Slide from
Mary Leech.
Lherzolite
Harzburgite
Residuum
Dunite
0
0.8
0.4
0.6
0.2
0.0
Wt. TiO2
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Lherzolite A type of peridotite with Olivine gt
Opx Cpx
Olivine
Dunite
90
Peridotites
Wehrlite
Harzburgite
Lherzolite
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
Websterite
10
Clinopyroxenite
Orthopyroxene
Clinopyroxene
Figure 2-2 C After IUGS
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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 (lt 50 km)
• Spinel 50-80 km
• Garnet 80-400 km
• Si VI coord. gt 400 km

Where does mantle melting occur?
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Result? Basalt
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What is Mid Ocean Ridge Bsasalt?

• 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.
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Ternary Variation Diagrams

• Example AFM diagram
• (alkalis-FeO-MgO)

AFM diagram for Crater Lake volcanics, Oregon
Cascades. From Mary Leech
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• 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)