Mantle Melting
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Transcript of Mantle Melting
Mantle MeltingMantle Melting
Some slides from Mary LeechSome 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.
15
10
5
00.0 0.2 0.4 0.6 0.8
Wt.
% A
l 2O3
Wt.% TiO2
DuniteHarzburgite
Lherzolite
Tholeiitic basalt
Partia
l Melt
ing
Residuum
LherzoliteLherzolite is probably fertile (undepleted) unaltered mantle is probably fertile (undepleted) unaltered mantleHarzburgite ± Dunite Harzburgite ± Dunite are refractory residuum after basalt has been are refractory residuum after basalt has been
extracted by partial meltingextracted by partial melting
Brown and Mussett, A. E. (1993), Brown and Mussett, A. E. (1993), The Inaccessible Earth: An The Inaccessible Earth: An Integrated View of Its Structure Integrated View of Its Structure and Composition. and Composition. Chapman & Chapman & Hall/Kluwer. Slide from Mary Hall/Kluwer. Slide from Mary Leech.Leech.
Ultramaficrocks
LherzoliteLherzolite: A type of : A type of peridotiteperidotite with Olivine > Opx + Cpxwith Olivine > Opx + Cpx
OlivineOlivine
ClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxene
LherzoliteLherzoliteH
arzb
urgi
teW
ehrlite
Websterite
OrthopyroxeniteOrthopyroxenite
ClinopyroxeniteClinopyroxenite
Olivine Websterite
PeridotitesPeridotites
PyroxenitesPyroxenites
90
40
10
10
DuniteDunite
Figure 2-2 C After IUGSFigure 2-2 C After IUGS
How does the mantle melt??How does the mantle melt??1) 1) Increase the temperatureIncrease the temperature
2) 2) Lower the pressureLower the pressure AdiabaticAdiabatic rise of mantle with no conductive heat loss rise of mantle with no conductive heat loss Decompression meltingDecompression melting could melt at least 30% could melt at least 30%
3) 3) Add volatilesAdd volatiles (especially (especially HH22OO))
Where does mantle melting occur?Where does mantle melting occur?
Phase diagram for aluminous 4-phase lherzolite:Phase diagram for aluminous 4-phase lherzolite:
Plagioclase Plagioclase shallow (< 50 km)shallow (< 50 km) Spinel Spinel 50-80 km50-80 km Garnet Garnet 80-400 km80-400 km Si VI coord. > 400 kmSi VI coord. > 400 km
Alminous phase =Alminous phase =
Phase diagram of aluminous lherzolite Phase diagram of aluminous lherzolite with melting interval (pink), sub-solidus with melting interval (pink), sub-solidus reactions, and geothermal gradient. reactions, and geothermal gradient. After After Wyllie, P. J. (1981). Geol. Rundsch. Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.70, 128-153.
Result? BasaltResult? Basalt
MgO and FeOMgO and FeO
AlAl22OO33 and CaO and CaO
SiO2SiO2
NaNa22O, KO, K22O, TiOO, TiO22, P, P22OO55
Basaltic glasses from the Afar region of the Basaltic glasses from the Afar region of the MAR. Note different ordinate scales. From MAR. Note different ordinate scales. From Stakes et al. (1984) Stakes et al. (1984) J. Geophys. Res., 89, J. Geophys. Res., 89, 6995-7028.6995-7028.
What is What is MMid id OOcean cean RRidge idge BBsasaltsasalt??
Ternary Variation Diagrams Ternary Variation Diagrams Example: AFM diagramExample: AFM diagram
(alkalis-FeO*-MgO)(alkalis-FeO*-MgO)
AFM diagram for Crater Lake volcanics, Oregon Cascades. From Mary Leech
Conclusions about MORBs, and the processes Conclusions about MORBs, and the processes beneath mid-ocean ridgesbeneath mid-ocean ridges
MORBs are not the completely uniform MORBs are not the completely uniform magmas that they were once considered to magmas that they were once considered to bebe
They show chemical trends consistent They show chemical trends consistent with fractional crystallization of olivine, with fractional crystallization of olivine, plagioclase, and perhaps clinopyroxeneplagioclase, and perhaps clinopyroxene
MORBs MORBs cannot be primary magmascannot be primary magmas, but , but are derivative magmas resulting from are derivative magmas resulting from fractional crystallization (~ 60%)fractional crystallization (~ 60%)