Origin of Basaltic MagmaOrigin of Basaltic Magma

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.

Sources of mantle materialSources of mantle material OphiolitesOphiolites

Slabs of oceanic crust and upper mantleSlabs of oceanic crust and upper mantle Thrust at subduction zones onto edge of continentThrust at subduction zones onto edge of continent

Dredge samples from oceanic fracture zonesDredge samples from oceanic fracture zones Nodules and Nodules and xenolithsxenoliths in some basalts in some basalts Kimberlite xenolithsKimberlite xenoliths

Diamond-bearing pipes blasted up from the Diamond-bearing pipes blasted up from the mantle carrying numerous xenoliths from depthmantle carrying numerous xenoliths from depth

Kimberlite xenolithsKimberlite xenoliths

Photo of Kimberley diamond min (South Africa) and two examples of mantle xenoliths (peridotite [top] and garnetPeridotite [bottom] from a kimberlite.

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 mantleDuniteDunite and and harzburgiteharzburgite are refractory residuum after basalt has been are refractory residuum after basalt has been

extracted by partial meltingextracted by partial melting

Figure 10-1 Figure 10-1 Brown and Mussett, Brown and Mussett, A. E. (1993), A. E. (1993), The Inaccessible The Inaccessible Earth: An Integrated View of Its Earth: An Integrated View of Its Structure and Composition. Structure and Composition. Chapman & Hall/Kluwer.Chapman & Hall/Kluwer.

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

Phase diagram for aluminous Phase diagram for aluminous 4-phase lherzolite:4-phase lherzolite:

PlagioclasePlagioclase shallow (< 50 km)shallow (< 50 km)

SpinelSpinel 50-80 km50-80 km

GarnetGarnet 80-400 km80-400 km

Si Si VI coord. VI coord. > 400 km> 400 km

Al-phase =Al-phase =

Figure 10-2 Figure 10-2 Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus Phase diagram of aluminous lherzolite with melting interval (gray), sub-solidus reactions, and geothermal gradient. After reactions, and geothermal gradient. After Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.Wyllie, P. J. (1981). Geol. Rundsch. 70, 128-153.

How does the mantle melt??How does the mantle melt??1) 1) Increase the temperatureIncrease the temperature

Figure 10-3. Figure 10-3. Melting by raising the temperature.Melting by raising 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%

Figure 10-4. Figure 10-4. Melting by (adiabatic) pressure reduction. Melting begins when the adiabat crosses the solidus and traverses the shaded melting interval. Dashed lines represent approximate % melting.

3) 3) Add volatilesAdd volatiles (especially (especially HH22OO))

Figure 10-4. Figure 10-4. Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.

Melts Melts cancan be created under be created under realistic circumstancesrealistic circumstances

Plates separatePlates separate and mantle rises at mid- and mantle rises at mid-ocean ridges, or at ocean ridges, or at continental riftscontinental rifts Adibatic rise Adibatic rise decompression melting decompression melting

Hot spotsHot spots localized plumes of melt localized plumes of melt Fluid fluxingFluid fluxing

Important in Important in subduction zonessubduction zones

Figure 9-8.Figure 9-8. (a)(a) after Pearce and Cann (1973), after Pearce and Cann (1973), Earth Planet, Sci. Lett., Earth Planet, Sci. Lett., 1919, 290-300, 290-300. . (b)(b) after Pearce (1982) after Pearce (1982) in Thorpe (ed.), in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548 , Coish et al. (1986), , Coish et al. (1986), Amer. J. Sci., Amer. J. Sci., 286286, 1-28, 1-28.. (c)(c) after Mullen (1983), after Mullen (1983), Earth Planet. Sci. Lett., Earth Planet. Sci. Lett., 6262, 53-62., 53-62.