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SF State GEOL 426 - Origin of Basalts

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Origin of Basaltic MagmaPowerPoint PresentationSources of mantle materialKimberlite xenolithsSlide 5Lherzolite: A type of peridotite with Olivine > Opx + CpxPhase diagram for aluminous 4-phase lherzolite:How does the mantle melt??Slide 9Slide 10Melts can be created under realistic circumstancesSlide 12Origin of Basaltic MagmaOrigin of Basaltic MagmaTable 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 depthKimberlite xenolithsKimberlite xenolithsPhoto of Kimberley diamond min (South Africa) and two examples of mantle xenoliths (peridotite [top] and garnetPeridotite [bottom] from a kimberlite.1510500.00.20.4 0.60.8Wt.% Al2O3Wt.% TiO2DuniteHarzburgiteLherzoliteTholeiitic basaltPartial MeltingResiduumLherzoliteLherzolite 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 meltingFigure 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.UltramaficrocksLherzoliteLherzolite: A type of : A type of peridotiteperidotite with Olivine > Opx + Cpxwith Olivine > Opx + CpxOlivineOlivineClinopyroxeneClinopyroxeneOrthopyroxeneOrthopyroxeneLherzoliteLherzoliteHarzburgiteWehrliteWebsteriteOrthopyroxeniteOrthopyroxeniteClinopyroxeniteClinopyroxeniteOlivine WebsteritePeridotitesPeridotitesPyroxenitesPyroxenites90401010DuniteDuniteFigure 2-2 C After IUGSFigure 2-2 C After IUGSPhase 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 kmAl-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 temperatureFigure 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 zonesFigure 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.,


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SF State GEOL 426 - Origin of Basalts

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