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Review: Samples from the mantleReview: Samples from the mantle Ophiolites Dredge samples from oceanic crustNdldli hiblNodules and xenolithsin some basaltsKimberlite xenolithsKimberlite xenolithsMelting of the mantleMelting of the mantleIncreasing temperature:Intraplate igneousIncreasing temperature: Intraplate igneous activity (OIB, continental basalts etc.)DiDlDecreasing pressure: Divergent plate boundaries (MORBs, rifts, back-arc basins )etc.) Adding volatiles: Convergent plate boundaries (arc lavas, continental margins, etc.)Plate Tectonic - Igneous Genesis g1. Mid-Ocean Ridges5. Back-Arc Basinsg2. Intracontinental Rifts3. Island Arcs6. Ocean Island Basalts7. Miscellaneous Intra-4. Active Continental Margins 7. sce a eous t aContinental Activitykimberlites, carbonatites, th itganorthosites...Primary magmasyg Formed at depth and not subsequently modified by FX or AssimilationCriteriaCe Highest Mg# (100Mg/(Mg+Fe))E i t l lt f lh lit ltExperimental results of lherzolite melts Mg# = 66-75 Cr > 1000 ppm Ni > 400-500 ppmMagmatic diversification Partial meltingMagmatic diversificationg Fractional crystallization Magma mixing Assimilation AFC Soret effectDegree of partial meltingFigure 10.9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160.Fractional crystallization: gravity settling First Æ olivine layer at base of pluton if first olivine sinksolivine sinks Next Æ ol+cpx layer finally Æ ol+cpx+plagCumulate texture:Mutually touching ygphenocrysts with interstitial crystallized id l lresidual meltKilauea Iki lava lake, Hawaii: A b k l f diff i iThe Hawaiian IslandsA textbook example of magma differentiationKilauea Iki lava lakeKilauea Iki lava lake, Hawaii, USABefore eruptionAfter eruptionTeng et al. 08 ScienceCrystallization sequence of Kilaueasequence of Kilauea Iki lavasMgO = 11%: primitive magma11% > MgO > 7.5%: olivineHelz (1987)7.5% > MgO > 5%: augite+ plag5%> MgO: oxides MO 11%MgO > 11%: Olivine + primitive magmaTeng et al. 08 ScienceMagma MixingMagma MixingEnd member mixingfor a suite of rocksEnd member mixingfor a suite of rocks Mixing line: variation on Harker-type diagrams should lie on a straight linebetween the two most extreme compositionsComingled basalt-Rhyolite MtRhyolite Mt. McLoughlin, OregonFigure 11.8 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice HallBasalt pillows accumulating at theaccumulating at the bottom of a granitic magma chamber, Vinalhaven Island, MaineAssimilation Incorporation of wall rocks: e.g., crustal rocks in pg,a basaltic magmaAssimilation by melting is limited by the heatAssimilation by melting is limited by the heat available in the magmaAFC: Assimilation and Fractional CrystallizationAFC: Assimilation and Fractional CrystallizationThe Soret Effect and hiiliffiThermogravitational Diffusion Thermal diffusion, or the Soret effect Heavy elements/molecules migrate toward the colder end and lighter ones to the hotter end of gthe gradient Heavy isotopes migrate toward the colder end and ypglighter ones to the hotter end of the gradientWalker and DeLong (1982) subjected two basalts to h l di f l 50C/ (!)thermal gradients of nearly 50oC/mm (!)  Found that: Samples reached a steady state in a few days y Heavier elements → cooler end and the lighter→hotend and the lighter →hot endThe chemical concentrationThe chemical concentration is similar to that expected from fractionalfrom fractional crystallization Figure 7.4. After Walker, D. C. and S. E. DeLong (1982). Contrib. Mineral. Petrol., 79, 231-240.Richter et al (2008, 2009) show that Soret effect can iifi tlf ti tit hilf ti lsignificantly fractionate isotopes while fractional crystallization generally does


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UT Arlington GEOL 2313 - Mantle

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