Mantle Melting

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Mantle Melting Mantle Melting Some slides from Some slides from Mary Leech Mary Leech

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Page 1: Mantle Melting

Mantle MeltingMantle Melting

Some slides from Mary LeechSome 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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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

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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

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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.

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Result? BasaltResult? Basalt

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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??

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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

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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%)