Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteau Peter Kelemen * Roches experimentales, volcaniques et du manteau

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  • Ce que nous apprennent les roches* du manteau sur la migration des magmas dans le manteauPeter Kelemen

    * Roches experimentales, volcaniques et du manteau

  • Minerals in the mantle and lower crust

    OlivineMg2SiO4- Fe2SiO4OrthopyroxeneMg2Si2O6- Fe2Si2O6, etcClinopyroxeneCaMgSi2O6- CaFeSi2O6, etc Spinel(Mg,Fe)(Cr,Al)2O4 , etcGarnet(Mg,Fe,Ca)3Al2Si3O10, etcPlagioclaseCaAl2Si2O8- NaAlSi3O8

    Melting reactions

    P > 20 kilobars (2 Gpa)Ol + Opx + Cpx + Gnt = melt8 kb < P < 20 kbOpx + Cpx + Sp = Ol + meltP < 8 kbOpx + Cpx + Plag = Ol + melt if fertileOpx + Cpx + Sp = Ol + melt if depleted

  • really lowF~3 to 20%meltingreallyhigh F

  • mantle solidusliquidadiabatolivine saturationpyroxene saturation DepthTemperature {peridotite dissolves(even olivine), MgO up{pyroxenes dissolveolivine precipitates, SiO2 up

  • Rare Earth Elementsin order of increasing Zperiodic tablein approximate order of crystal/liquid partitioning

  • Bottom up:

    Diffuse porous flow Melting & diapirsMagma fractureFocused porous flowSills & lenses at topTop down:

    MORB compositionMORB focusingMORB ascent rate

    Arc compositionArc focusingHotspot flux, comp, focusing= WFs/(wf) STEADY STATE!( = 1)w = kg/(f) DARCYS LAW

  • w = kg/(f)k = d23/cVon Bargen & WaffWark, Watson, et al. k = d23/270Faul et al.

  • Von Bargen & Waff

  • Grain size variation: some grains smaller, more melt on triple grain boundaries (= grain edges)At low melt fraction, little or no melt on large grain edgesIf rock is banded in grain size, low permeability to banding

  • hzol+spolHARZBURGITE (+)OL + SP ()OL only ()

  • hzol+spolFaul et al.Von Bargen & WaffWark, Watson, et al.quartzitemarble

  • compositional variation across a large dunite in the Josephine peridotite

    Chart6

    0.060240.02478122950.0247812295

    0.09207727270.02231136660.0223113666

    0.131160.01583976910.0158397691

    0.129681250.04647406760.0464740676

    0.139480.04706422970.0470642297

    0.1086080.04367679510.0436767951

    0.130480.02484810750.0248481075

    0.13483333330.02574209080.0257420908

    0.090350.01429741620.0142974162

    0.10190.03318787430.0331878743

    0.1060.02523914190.0252391419

    CaO in Olivine

    Position (m)

    w.t. %

    REE

    (ppm)3923Z193923Z173923Z163923Z153923Z143923Z133924Z323923Z123924Z243924Z273924Z29

    Position1516.216.319.25212226.526.8527.227.527.9

    Rb

  • upper bound estimate of permeability threshold based on upper bound estimate of trapped melt, based on CaO in whole rock - olivine

  • w = kg/(f)k = d23/cWark, Watson, et al. k = d23/270Von Bargen & WaffWark, Watson, et al. Faul et al.X

  • Wetting angles may vary depending on crystallographic orientation and mineral

    At low melt fractions, unfavorable grain edges have no melt at all

    Positive or negative feedback on permeability?k = d23/cc is a geometric factor

  • ol + meltol + meltol + meltol + meltol + opx + meltol opx NO initial melt6hol + opx + meltol opx NO initial melt6h

  • f3 = 270mWFrs/(d2Drgrf)from= WFs/(wf) STEADY STATE!( = 1)w = kg/(f)DARCYS LAWk = d23/270Wark et al.

  • Bottom up:

    Diffuse porous flow OK, prefer Wark et al.(for now) field evidence? Melting & diapirsMagma fractureFocused porous flowSills & lenses at topTop down:

    MORB compositionMORB focusingMORB ascent rate

    Arc compositionArc focusingHotspot flux, comp, focusing

  • Models of regional pervasive porous flow conflict with structural and seismic evidence that fractures control fluid transportation in the upper mantle. Effects of porous-medium flow have been inferred in studies of mantle peridotite but are well documented only on scales of centimeters or decimeters. In all these [cases], porous flow is fundamentally controlled by proximity to magma-filled fractures.

    Nielsen & Wilshire, 1993

  • melt outresidual porositynothing coming inmelt outmelt coming inresidual porositynothing outMORB coming innothing outlocal melt coming in

  • light REE enrichedlight REE depletedlow Allow Alhigh Al

  • coarse,granular(high T)Porphyroclastic(low T)light REEdepleted(MORB source)Light REEEnriched(addition of low degree melts)

    Partial melts of peridotite have increasingly high SiO2 at lower pressure. This reflects incongruent melting, producing olivine + relatively SiO2 rich melt from pyroxenes + spinel and/or garnet and/or plagioclase. The spread in the data reflects increasing SiO2 at a given pressure as the degree of melting rises from moderate to high. As a consequence of the effect of pressure, as they decompress, mantle melts will be saturated only in olivine, and have the capability of dissolving pyroxene via general reactions in which melt 1 + pyroxene = melt 2 + olivine, where melt 2 has higher SiO2 than melt 1.

    Compositions of partial melts of dry natural peridotite, compiled in the mid-1990s. Hirose = Hirose & Kushiro, EPSL 1993, Kinzler = Kinzler & Grove, JGR 1992a,b & 1993, Baker = Baker & Stolper, GCA 1994, Falloon = Falloon & Green Min & Pet 1987, J Pet 1988, Falloon et al. J Pet 1988

    An-p-to-date compilation would add experimental data from Walter, J Pet 1998, Wasylenki et al J Pet 2003, Pickering et al CMP 2000, and Johnston & Schwab, GCA 2004, among others. Partial melts of peridotite have increasingly high SiO2 at lower pressure. This reflects incongruent melting, producing olivine + relatively SiO2 rich melt from pyroxenes + spinel and/or garnet and/or plagioclase. The spread in the data reflects increasing SiO2 at a given pressure as the degree of melting rises from moderate to high. As a consequence of the effect of pressure, as they decompress, mantle melts will be saturated only in olivine, and have the capability of dissolving pyroxene via general reactions in which melt 1 + pyroxene = melt 2 + olivine, where melt 2 has higher SiO2 than melt 1

    More introductory material: Petrologists & geochemists normalize trace element contents of rocks to solar system, chondritic meteorite, or primitive mantle AKA silicate earth abundances in order to remove the effects of fusion, neutron capture and fission on elemental abundances, and emphasize fractionation due to terrestrial processes.

    Normalized concentrations, showing relatively smooth REE and trace element variation

    Normal MORB, AKA N-MORB, C1 chondrite and primitive mantle compositions from Anders & Grevesse, GCA 1989 and McDonough & Sun, Chem Geol 1995

    Steady state melt flux relationship, indicating that production must equal transport flux, modified from Spiegelman & Kenyon, EPSL 1992 and later (with ) from Kelemen et al., Phil Trans Roy Soc London 1997; Darcys Law holds generally for porous flow (grain boundary, tubes, cracks, etc) and relates melt velocity w to permeability, k, density contrast between melt and solid , the gravitational constant g, melt fraction , and melt viscosity f.

    W = solid upwelling velocity, F = melt fraction at some height, = density of solid s and fluid f, w = melt or fluid velocity, = volume fraction of channels carrying melt; = 1 means that the entire source region transports melt via diffuse porous flow; alternatively, = 0.01 would indicate that 1% of the volume of the source region is composed of channels transporting melt.

    Most scientists believe that partial melt in an olivine rich matrix occupies olivine triple grain boundaries and four grain intersections in a network resembling this network formed by melt (now glass) and gas bubbles. See Wark et al., JGR 2003, for photo and extensive discussion.

    Figures from Wark et al., JGR 2003 and Wark & Watson, EPSL 1998, illustrating various estimates for permeability of grain boundary networks of basaltic melt in an olivine rich matrix.

    Sources for estimates: Faul, JGR 1997; VonBargen & Waff (also VBW), JGR 1986; RK = Riley & Kohlstedt EPSL 1991; C = Cheadle, 1989 thesis at Cambridge Univ(also see Cheadle et al., Geology, 2004); MK = McKenzie EPSL 1989; Wark, Watson et al. = Wark & Watson, EPSL 1998; Liang et al., JGR 2002; Wark et al., JGR 2003

    Figures from Wark & Watson, EPSL 1998, illustrating theoretical grain boundary networks determined by equilibrium wetting angle, which is a function of solid-solid and solid-liquid surface energy, and experimental results on permeability of quartzite+waterNaCl and marble+fluid networks. Explanation from Wark et al., JGR 2003, for slightly lower permeability in experimental quartzite compared to VonBargen & Waff theory, in terms of anistropic melt distribution as a function of grain size.

    Figures from Wark et al., JGR 2003, illustrating the similarity of shapes in 2D samples of grain boundary networks in experimental mixtures of olivinespinel20% orthopyroxene (opx) compared to 2D samples of ideal melt network composed of triangular prisms, and difference between these networks and theoretical networks composed of oblate to prolate ellipsoids. The significance of these observations is that the Faul, JGR 1997 permeability relationship is based on measurements of aspect ratios of melt textures in 2D samples *** interpreted as though they were oblate to prolate ellipsoids ***. Wark et al infer that the conclusions of Faul et al based on this interpretation are incorrect.

    As for previous slide, but also showing similarity of experimental results for melt+olivinespinelopx and results for quartzite+H2ONaCl.

    A percolation threshold, AKA permeability threshold, such as that predicted by Faul, JGR 1997, should be evident in mantle residues as a finite proportion of trapped m