microstructural and rheological contrasts between dunite ...geoverse.eu/data/poster2015.pdf ·...

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Microstructural and rheological contrasts between dunite, harzburgite and pyroxenite 9aD Phasewmixing of olivine and orthopyroxeneDDD jUH Farla, RDJDMD, Karato, SD, and Cai, ZD Proc. Nat. Acad. Sci. 9cU8 j9H Bruijn, HDCDRD and Skemer, PD Geophys. Res. Lett. 9cU5 j8H Lawlis, PhD thesis, DepD of Geosciences, PennD State UnivD, State College, PaD Uää: j5H Linckens, JD, Bruijn, HDCDRD, Skemer, PD Earth Planet. Sci. Lett. 9cU5 j6H Linckens, JD et alD J. Geophys. Res. 9cUU jIH Hirth, GD and Kohlstedt, DD Inside the Subduction Factory, Geophys. Monogr. Ser., 9cc8 References7 Robert Farla 1 1 Bayerisches Geoinstitut, Universität Bayreuth, ä655b Bayreuth, Germany ww Email7 robertDfarlaWuniwbayreuthDde UD Introduction 7odels predicting the strength of the lithosphere fail to produce substantial weakening necessary for plate tectonic convection° Hdditional mechanisms are necessary that can lead to weakeningD most likely in the form of localizing deformation as seen in nature° ?ecent experimental studies Ee°g° Iarla et al° P6G×- report tantalizing clues that a second phase Eorthopyroxene- can help localize deformation via dynamic recrystallization and phase mixing° This study expands the deformation conditions to further investigate the rheological role of pyroxenes in the ductile lower lithosphereCasthenosphere of the Varth with implications for peridotite9pyroxenite shear zones° Key objectivesq Nynthesize various mixtures and end9member compositions of olivineD orthopyroxene Eopx- and clinopyroxene Ecpx- with starting grain sizeq G6 9 ×6 microns° +eform samples in a range of temperatures EB66 9 G×66 °=-D pressures EP 9 < 8Ka- and strain rates between G×G6 94 to 4×G6 95 s 9G ° Vxplore different deformation geometries including pure shear Efor smaller strainsD ε ' ×6)-D and simple shear Efor large strainsD ɣ [ G-° Vxplore viscosity contrasts directly between different compositions in pure shear experiments by stacking P samples° Iully characterize the deformed microstructures using scanning electron microscope ENV7- imaging and electron backscatter diffraction EV*N+- techniquesD and transmission electron microscopy for dislocation analysis Efuture direction-° +espite some recrystallization of grains in the starting materialD especially in the pyroxenite sampleD some preliminary conclusions may be madeq H clear competence contrast between the weaker olivine Edunite- sample and the stronger Erecrystallized- pyroxenite sample° =Kjs suggest dislocation creep was active° The competence contrast between dunite and the olivine9pyroxene mixture Eharzburgite- is less obviousD but early recrystallization suggests the mixture being weaker° jlivine requires a larger strain to recrystallize than opxD but recrystallization of olivine is facilitated in a mixture with opx° The simple shear experiment at lower tempChigher stress shows olivine can recrystallize smaller grains than opxD however this may not apply to nature under the given strain rate° jpx grains unfavourably oriented towards the critical resolved shear stress are preseved and appear to form long /ribbon9like/ shapes as seen in nature° The =Kj patterns in the experiments are typically those expected for olivineO slip on E6G6- in the [G66] or [66G] directions 9 and for opxO slip on EG66- and E6G6- in the [66G] direction° GG46 °= C P 8Ka C av° ~G×G6 94 s 9G 6D Nearlywsimple shear deformation ID Summary Acknowledgements7 Hubert and Raphael for sample preparationD Colleages at BGI for discussionsD Let me know if you have interest to collaborate with me on this projectD = σ exp The rheology of each phase is described by an Hrrhenius equationD such asq EG- where the strain rate is a function of stress σ with power nD grain size d 9p D as well as other parameters summarized in prefactor H° The exponential term contains the activation enthalpy term L: ; V: S KV: and average kinetic energy term ?:T Egas constant and temperature-° The viscosity of a material follows this rate law due to the closeness it plays in the thermally activated motion of atomsCdefects in the material° jpx forms smaller recrystallized grains than olivineD expecially at low stresses EIig° G-° This mayD in partD be due to fast nucleation yet sluggish grain growth kinetics in opx° Jf phase9mixing occursD opx could inhibit grain growth in olivine at opx grain size Eopen circle-D thus causing a switch in deformation mechanism to diffusion creepD which leads to further weakening° Iig° ×° Vmbedded pyroxenite dykes within a mylonitic harzburgite Eol9 opx- demonstrating a competence contrast between the two rock types° Lilti 7assifD jman° T >66 °= Hfter µinckens et al° P6GG° Iig° 5° Ntretching and folding of an opx porphyroclast in a granulite9 facies shear zone ELikadaD Wapan-° T >46 °= C K 6°<4 8Ka Hfter ?aimbourg et al° P6GG° Iig° P° Khase9mixing of olivine Egreen- and opx Eyellow- grains in a natural peridotite shear zoneD Twin sisters 7assifD Washington stateD UNH° T >66 °= C K G 8Ka° Hfter Toy et al° P6G6 G66 μm 466 μm 566 μm ol Mε≈ 29Sw olfopx M3:1w Mε≈ 34Sw 566 μm ol Mε≈ 46S / 7z7×10 f5 s f1 w opxfcpx M3:1w Mε≈ 19S / 3z2×10 f5 s f1 w compr° dir shear dir° Eextension- x P x G x × t/c alumina alumina 46 μm d ol rex %°6 um d opx rex 4°> um d ol G4°6 um 46 μm 6°4 6°5 6°× 6°P 6°G 6°6 jl jljpx Nchmidt factor map 9 jl E6G6-[G66] Nchmidt factor map 9 jl E6G6-[G66]D jpx EG66-[66G] 6°5 6°× 6°P 6°G 6°6 Subfboundaries jl jl jpx d ol rex %°G um 46 μm Nchmidt factor map 9 jl E6G6-[G66] 5D Pure shear jplane strainH deformation GG46 °= C P 8Ka C av° ~4×G6 94 s 9G 6°4 d opxCcpx rex 4°% um 46 μm =px Nchmidt factor map 9 jl E6G6-[G66]D jpxC=px EG66-[66G] =px jpx ×66 μm Nchmidt factor map 9 jl E6G6-[G66]D jpx EG66-[66G] 46 μm 46 μm 6°4 6°5 6°× 6°P 6°G 6°6 jl jpx d ol rex ×°< um d opx rex 5°> um B%6 °= C P 8Ka C ~G6 95 s 9G jl jpx C shear strain ~ G Ligh stressq olivine rex° gs [ opx rex° gs2 8D Starting material d ol G6 um d ol GG°< um d opx B°> um d di rex 5°5 um d opx rex<°P um Iig° 4° 8rain9size sorted powders from Nan =arlos peridotite pre9sintered in piston cylinder apparatus EG< hr at G×66 °= C G°× 8Ka- UnfortunatelyD pyroxene appears to recrystallize easily from deviatoric stresses in the piston cylinder apparatus°°° 46 μm 46 μm 46 μm 466 μm Iig° G° +eformation mechanism maps for orthopyroxene and olivine° The purple9shaded area shows laboratory conditions and the orange9shaded area shows inferred geological conditions without phase9mixing° Ea- Ntrength of opx for dislocation creep based on experiments by µawlis EGBB>- and considerations for diffusion creep by *ruijn and Nkemer EP6G5-° jpx piezometer is from µinckens et al° EP6G5-° Eb- Ntrength of olivine in dislocation creep and diffusion creep regimes based on model by Lirth and Uohlstedt EP66×-° jlivine piezometer is from van der Wal EGBB×-° More work is necessary to understand polyfphase interactions during deformation and whether phasefmixing associated with a change in deformation mechanism is sufficient to cause strain localization in dry mantle regionsz µegends E~ constant displ° rate- E~ constant displ° rate- σ≈ ×66 7Ka σ≈ ×P6 7Ka σ≈ ×66 7Ka ] σ≈ 466 7Ka 9[ σ≈ P%6 7Ka] 10 0 10 1 10 2 10 3 10 4 10 1 10 0 10 1 10 2 10 3 Grain size (micron) Differential stress (MPa) 1012 s1 1010 s1 108 s1 106 s1 104 s1 jlivine +iffusion creep +islocation creep jlivine grain size piezometer disl° creep ; diff° creep GP66 °= jrthopyroxene 10 0 10 1 10 2 10 3 10 4 10 1 10 0 10 1 10 2 10 3 Grain size (micron) Differential stress (MPa) 1012 s1 1010 s1 108 s1 106 s1 104 s1 +iffusion creep +islocation creep jpx grain size piezometer disl° creep ; diff° creep GP66 °= µH* µH* !HTU?V !HTU?V jlivine grain size piezometer cross9over σ ; PB×B d 9G°×6> σ ; G×>6 d 96°%4P σ ; G×>6 d 96°%4P Whilst experimental studies above hint that olivine and orthopyroxene Eopx- exhibit similar strength in dislocation creep regime EIig° G-D some natural observations suggest otherwise° 9bD DDDversus competence contrasts ideal orientation to shear direction jl unfavourable orientation to shear direction jbH Raimbourg et alD Contrib. Mineral. Pet. 9cUU j:H Toy et alD J. Pet. 9cUc jäH van der Wal et alD Geophys. Res. Lett. Uää8 m°u°d° m°u°d° m°u°d° m°u°d° m°u°d° m°u°d° m°u°d° m°u°d°

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Page 1: Microstructural and rheological contrasts between dunite ...geoverse.eu/data/Poster2015.pdf · Microstructural and rheological contrasts between dunite, harzburgite and pyroxenite

Microstructural and rheological contrasts between dunite, harzburgite and pyroxenite

9aDKPhasewmixingKofKolivineKandKorthopyroxeneDDD

jUHKFarlaLKRDJDMDLKKaratoLKSDLKandKCaiLKZDKProc. Nat. Acad. Sci. 9cU8j9HKBruijnLKHDCDRDKandKSkemerLKPDKGeophys. Res. Lett.K9cU5Kj8HKLawlisLKPhDKthesisLKDepDKofKGeosciencesLKPennDKStateKUnivDLStateKCollegeLKPaDKUää:

j5HKLinckensLKJDLKBruijnLKHDCDRDLKSkemerLKPDKEarth Planet. Sci. Lett.K9cU5j6HKLinckensLKJDKetKalDKJ. Geophys. Res.K9cUUjIHKHirthLKGDKandKKohlstedtLKDDKInsideKtheSubductionKFactoryLKGeophys. Monogr. Ser.LK9cc8K

References7

Robert Farla1 1KBayerischesKGeoinstitutLKUniversitätKBayreuthLKä655bKBayreuthLKGermanyKwwKEmail7KrobertDfarlaWuniwbayreuthDdeK

UDKIntroduction7odelsF predictingF theF strengthF ofF theF lithosphereF failF toF produceF substantialF weakeningFnecessaryF forF plateF tectonicF convection°F HdditionalF mechanismsF areF necessaryF thatF canFleadF toF weakeningDF mostF likelyF inF theF formF ofF localizingF deformationF asF seenF inF nature°F?ecentFexperimentalFstudiesFEe°g°FIarlaFetFal°FP6G×-FreportFtantalizingFcluesFthatFaFsecondFphaseF Eorthopyroxene-F canF helpF localizeF deformationF viaF dynamicF recrystallizationF andFphaseF mixing°F ThisF studyF expandsF theF deformationF conditionsF toF furtherF investigateF theFrheologicalFroleFofFpyroxenesFinFtheFductileFlowerFlithosphereCasthenosphereFofFtheFVarthFwithFimplicationsFforFperidotite9pyroxeniteFshearFzones°F

Key5objectivesqFFFFFFNynthesizeFvariousFmixturesFandFend9memberFcompositionsFofFolivineDForthopyroxeneFEopx-FandFclinopyroxeneFEcpx-FwithFstartingFgrainFsizeqFG6F9F×6Fmicrons°FFFFFFF+eformFsamplesFinFaFrangeFofFtemperaturesFEB66F9FG×66F°=-DFpressuresFEPF9F<F8Ka-FandFstrainFratesFbetweenFG×G694FtoF4×G695Fs9G°FFFFFFFVxploreFdifferentFdeformationFgeometriesFincludingFpureFshearFEforFsmallerFstrainsDFεF'F×6)-DFandFsimpleFshearFEforFlargeFstrainsDFɣF[FG-°F F F F VxploreF viscosityF contrastsF directlyF betweenF differentF compositionsF inF pureF shearFexperimentsFbyFstackingFPFsamples°F F F F F IullyF characterizeF theF deformedF microstructuresF usingF scanningF electronF microscopeFENV7-F imagingF andF electronF backscatterF diffractionF EV*N+-F techniquesDF andF transmissionFelectronFmicroscopyFforFdislocationFanalysisFEfutureFdirection-°

+espiteFsomeFrecrystallizationFofFgrainsFinFtheFstartingFmaterialDFespeciallyFinFtheFpyroxeniteFsampleDFsomeFpreliminaryFconclusionsFmayFbeFmadeqFFFFHFclearFcompetenceFcontrastFbetweenFtheFweakerFolivineFEdunite-FsampleFandFtheFstrongerFErecrystallized-FpyroxeniteFsample°F=KjsFsuggestFdislocationFcreepFwasFactive°FFFFTheFcompetenceFcontrastFbetweenFduniteFandFtheFolivine9pyroxeneFmixtureFEharzburgite-FisFlessFobviousDFbutFearlyFrecrystallizationFsuggestsFtheFmixtureFbeingFweaker°FFFFjlivineFrequiresFaFlargerFstrainFtoFrecrystallizeFthanFopxDFbutFrecrystallizationFofFolivineFisFfacilitatedFinFaFmixtureFwithFopx°FFFFFTheFsimpleFshearFexperimentFatFlowerFtempChigherFstressFshowsFolivineFcanFrecrystallizeFsmallerFgrainsFthanFopxDFhoweverFthisFmayFnotFapplyFtoFnatureFunderFtheFgivenFstrainFrate°FFFFjpxFgrainsFunfavourablyForientedFtowardsFtheFcriticalFresolvedFshearFstressFareFpresevedFandFappearFtoFformFlongF/ribbon9like/FshapesFasFseenFinFnature°FFFFTheF=KjFpatternsFinFtheFexperimentsFareFtypicallyFthoseFexpectedFforFolivineOFslipFonFE6G6-FinFtheF[G66]ForF[66G]FdirectionsF9FandFforFopxOFslipFonFEG66-FandFE6G6-FinFtheF[66G]Fdirection°

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