Micromagnetic and microstructural analyses in … · 162 Flores-Gutiérrez et al. Micromagnetic and microstructural analyses in chondrules of the Allende meteorite Daniel Flores-Gutiérrez1,*,

Flores-Gutiérrez et al.162

MicromagneticandmicrostructuralanalysesinchondrulesoftheAllendemeteorite

Daniel Flores-Gutiérrez1,*, Jaime Urrutia-Fucugauchi2, Ligia Pérez-Cruz2, and Carlos Linares-López3

1 Instituto de Astronomía, Universidad Nacional Autónoma de México, Cd. Universitaria, Del. Coyoacán, 04510 México D.F., Mexico.

2 Laboratorio de Paleomagnetismo y Geofísica Nuclear, Instituto de Geofísica, Universidad Nacional Autónoma de México,Cd. Universitaria, Del. Coyoacán, 04510 México D.F., Mexico.

3 Laboratorio Universitario de Petrología, Instituto de Geofísica. Universidad Nacional Autónoma de México, Cd. Universitaria, Del. Coyoacán, 04510 México D.F., Mexico.

* [email protected]

ABSTRACT

Results of micromagnetic and microstructural studies of individual chondrules from the Allende carbonaceous meteorite are presented. Allende is a member of the CV3 carbonaceous chondrites, and part of the oxidized meteorites with iron in silicates and oxides. Magnetic hysteresis data in terms of plots of parameter ratios give relationships with chondrule size and shape, in particular with magnetization ratio (Mr/Ms) and coercivity (Hc). Morphology, internal structure and elemental composition are investigated by scanning electron microscopy and spectrometric analyses. Chondrules show low ranges of magnetization ratios (Mr/Ms from 0 to 0.22) and coercivity (Hc from 3 to 24 mT). Low values suggest that chondrules are susceptible to alteration and re-magnetization, which affects paleointensity determinations for the early planetary magnetic fields A linear relation of Mr/Ms as a function of Hc is found up to values of 0.17 and 17 mT, respectively. This relation correlates with internal microstructure and composition, with compound chondrules showing higher hysteresis ratio and parameter values. Chondrules with hysteresis parameters falling outside the major trend show internal structures, composition and textures indicative of compound chondrules, and fragmentation and alteration processes. Microprobe analyses show distinct mineralogical assemblages with spatial compositional variation related to chondrule size, shape and microstructure.

Key words: chondrules, magnetism, structure, chemical composition, Allende meteorite.

RESUMEN

En este trabajo se presentan los resultados de estudios micromagnéticos y microestructurales en condros individuales separados del meteorito Allende. Allende es una condrita carbonácea CV3, oxidada y con hierro en los silicatos y óxidos. Los datos de histéresis magnética analizados en términos de graficas de cocientes de los parámetros de histéresis revelan relaciones con la forma y tamaño de los condros, en particular con los cocientes de magnetización y de coercitividad. La morfología, estructura interna y composición elemental se investigan con análisis de microscopía electrónica de transmisión y espectrometría. Los condros muestran valores bajos de cocientes de magnetización, entre 0 y 0.22,

Revista Mexicana de Ciencias Geológicas, v. 27, núm. 1, 2010, p. 162-174

Flores-Gutiérrez,D.,Urrutia-Fucugauchi,J.,Pérez-Cruz,L.,Linares-López,C.,2010,MicromagneticandmicrostructuralanalysesinchondrulesoftheAllendemeteorite:RevistaMexicanadeCienciasGeológicas,v.27,núm.1,p.162-174.

Analyses in chondrules of the Allende meteorite 163

y de coercitividad, entre 3 y 24 mT. Los valores bajos sugieren que los condros fueron afectados por alteración y remagnetización, lo cual limita los estudios de paleointensidad para la determinación de campos magnéticos en la nebulosa. Una relación lineal es observada entre los cocientes de magnetización y la coercitividad, para el rango entre 0.17 y 17 mT. Esta relación muestra correlación con los datos microestucturales y la composición, con condros compuestos mostrando valores altos de histéresis. Los condros cuyos parámetros salen fuera de la tendencia encontrada presentan estructuras y texturas que indican condros compuestos, fragmentación y alteración. Los análisis de microscopia electrónica y espectrométricos muestran arreglos mineralógicos relacionados al tamaño de condro, forma y microestuructura.

Palabras clave: condros, magnetismo, estructura, composición química, meteorito Allende.

INTRODUCTION

Studiesontheoriginofthesolarsystemhavelongbeenbasedontheoreticalmodelsandexperimentalinves-tigationsofchondriticmeteorites,whichrepresentthemostprimitivematerialavailablefromtheearlystagesofthesolarnebula(AndersandEbahara,1982;Cameron,1988;Hewinset al.,1996;ConnollyandLove1998;Scott,2007).Recently,highresolutionastronomicalobservationsofpro-toplanetarydisksandapparentlysimilarplanetarysystemshaveprovidedvaluableinsightsontheinitialconditions,characteristicsandearlyevolutionofplanetarysystems.Amongtheprimitivemeteorites,carbonaceouschondriticmeteoriteshaveprovedparticularlyimportant(Wood,1988).Theyareformedbychondrulesandcalcium-alumi-num inclusions (CAIs) in a fine-grained aphanitic matrix. Chondrulesaremillimeter-scalemeltdropletsofigneouscompositionandtogetherwithCAIshavebeenintensivelystudiedfortheirchemicalandmineralogicalcomposition,presenceofradioactiveisotopes,alterationfeatures,etc.(e.g.,BlanderandFuchs,1975;ChenandWasserburg,1981;FegleyandPalme,1985;McSween,1985,1987;Hewinset al.,1996;Scott,2007).Allthisineffortstounderstandthesource(s)oftheinitialmatter,mattersourcedistribution,conditions,processesandmechanismsofchondruleforma-tion,earlyandlatealterationprocesses,andplanetesimalformationandtiming.

StudiesonindividualchondrulesandCAIsandtheirinternalstructures,compositionandtexturesprovideevi-denceformorecomplexscenariosfortheearlysolarsystem,withheterogeneousconditionsandgradientsinthesolarnebula, suggesting fluctuating conditions, various heating mechanisms,complexmatterdistributions,varyingoxidiz-ingconditions,alterationeffectsandextendedoverlappingchronologiesforchondrule,CAIandchondruleformation(e.g.,FegleyandPalme,1985;Prinzet al.,1986;Kring,1991;HoodandKring,1996;HongandFegley,1998;Scott,2007).OurinterestintheAllendemeteoritearousedfromstudiesofmagneticpropertiesandquantitativepaleointen-sity determinations of the early magnetic fields in the solar nebula(Flores-GutiérrezandUrrutia-Fucugauchi,2000,2002).Analysesofmagneticmineralogyandmagnetic

propertiesinindividualAllendechondrules,whileevidenc-ing the internal complexity, also indicated first-order rela-tionshipsofmagnetichsyteresispropertieswithchondrulesizeandshape.Interpretationofthelinearandhigherorderrelationsrequiresadditionaldetailedstudiesofchondrulemicrostructuresandmorphologies.InthisnotewepresentresultsofmorphologicalandmicrostructuralanalysesinchondrulesoftheAllendecarbonaceouschondrite(Figure1)anddiscusstheirimplicationsfortheironoxidesandopaquemineralsandchondruleformation.

THE ALLENDE METEORITE

TheAllendemeteoritehasbeencriticalinmeteoriticandsolarsystemoriginstudiessincetheinitialinvestiga-tionsfollowingitsfallinChihuahua,northernMexico,onFebruary8,1969.Allendebelongstothegroupofcar-bonaceouschondrites,whicharerelativelyrareonEarthascomparedtoordinarychondrites.TheAllendemeteoritefallwaswelldocumentedandlargequantitieswererecoveredandrapidlydistributedandanalyzedinmanylaboratoriesaroundtheworld(Clarkeet al.,1970).

Allende has been classified as a CV3 carbonaceous chondritethatconsistsmainlyofchondrules(ca.43%vol)andmatrix(ca.38.4%vol),withtheadditionsofCAIs(ca.9.4%vol),olivineinclusions(ca.3.2%vol),opaqueminerals(ca.3.1%vol),andlithicandmineralfragments(ca.2.9%vol)(McSween,1977).Themacroscopictextureof Allende reflects the abundance of chondrules of sub-mil-limetersizewithintheaphaniticblackmatrix(Figure1).Allendeispartoftheoxidizedchondriteswithananhydrousmineralogy,withhydrousphasesrestrictedtochondrulesandCAIs.OxidixedchondritesshowolivinerichinFeOandnoironmetalblebs,incontrasttothereducedchon-drites.StudieshavealsoindicatedsecondaryalterationprocessesaffectingAllendemineralogyandcompositioninvariousways(McSween,1979,1987;Krotet al.,1995;Scott,2007).

Studies on the Allende have resulted in significant advancesinunderstandingtheearlyconditionsandforma-tionofthesolarsystem(BlanderandFuchs,1975;Chenand


roscopicallyanalyzed.Basiccharacterizationparametersdeterminedincluderoundness,weight,diameter,andvol-ume.Chondrulediametersrangefromabout0.04to0.32mm.Chondruleweightrangesfromabout0.001milligramsto0.12milligrams.Hysteresisparametersweremeasuredwith the MicroMag system using maximum direct fields of 1Tesla(T).Theresultsdisplaysimilartrendstothosefoundintheinitialstudy.Fromthediagramsofmagnetizationratiosplottedasafunctionofmagneticcoercivity(Figure2) we selected five chondrules for detailed study of internal structure. These chondrules (identified as M23, M27, M40, M53andM55)showhysteresisparameterpropertiesfromlowtohighcoercivityandlowtohighremanentandsatura-tionmagnetizations.Analysesofthedensityasafunctionofchondrule size show no significant variation, with densities rangingfromabout2to3.5g/cm3.Somechondrulesshowlowerdensities,particularlythelargeroneswithradiuslargerthan0.07mmandupto0.16mm.TheparameterrangesandrelationshipsofthemagnetizationratioasafunctionofchondrulesizeandmassareillustratedinFigures3and4.Thechondrulesselectedforstudyoftheinternalstructure are also identified in the diagrams. In general, these chondrulesspantherangesobservedinthedifferentbulkandhysteresisproperties,aswellasshapesandsizes.

Thechondruleswereslicedtoinvestigatetheinter-nalstructureandcompositionusingaportablesawwithnon-magneticthindisks.Theinternalmorphologyandmicrostructuresinthechondruleshavebeenexaminedus-ingtheJEOLJXA-8900RmicroprobeintheLaboratorioUniversitariodePetrología,UNAM.Thescanningelectronmicroprobeisequippedwithfourwavelengthdispersivespectrometers(WDS)andanenergydispersivespectrometer(EDS)systemforsemi-quantitativeandquantitativeelemen-talanalyses.Scanningelectronmicroscope(SEM)observa-

Tilton,1976;Leeet al.,1976;Sugiuraet al.,1979;ChenandWasserburg,1981;AndersandEbahara,1982;Hewinset al.,1996;KavnerandJeanloz,1998).Forinstance,studiesinAllendehaveprovidedthedataonelementalabundanceinthesunandplanetarysystem,documentationofshort-livedisotopes,notablyaluminumwithimplicationsforanearbysupernova explosion, and presence of strong magnetic fields intheprotoplanetarydisk.

MORPHOLOGIES AND MICROSTRUCTURES IN CHONDRULES OF ALLENDE

Ourinitialstudyofmagneticmineralogyandmag-neticpropertiesofchondrulesrevealedapparentsimplerelationshipsbetweenchondrulesizeandmorphologyand magnetic hysteresis parameters, pointing to first-order property relations. In particular, analyses quantified rela-tionshipsbetweenthemagnetizationratio(Mr/Ms)andcoercivity(Hc)(Flores-GutiérrezandUrrutia-Fucugauchi,2002).Chondruleswithnon-roundshapeswereoutliersinthediagrams,suggestingthattheirexternalshapeshadaneffectonthehysteresisparameters,possiblyassociatedwiththepresenceofextraneousmaterialorsecondaryprocessalterations.Magneticdomainstatesdeterminedfromthehysteresisparameterratioplots(Dayet al.,1977;Dunlop,2002)arepseudo-singledomain(PSD)andmultidomain(MD).TheDayplotsdonotshowanyapparenttrendsandseveralchondrulesdisplayhighcoercivityratios(Hcr/Hc)butslightlyhighermagnetizationratios(fallingabovetheMD field).

Forthestudy,aboutone-hundredindividualchon-druleswereseparatedfromthematrix,cleanedandmac-

Figure1.FragmentoftheAllendechondriticmeteorite,showingitstexturewiththecharacteristicchondrulesandaluminum-calciumCAIinclusionsincorporatedintoablackaphaniticmatrix.

0.25

0.20

0.15

0.10

0.05

5 10 15 20 25 30 35

Mr/

Ms

Hc

M40

M55

M23

M53

M27

Figure2.Plotofthemagnetizationratio(Mr/Ms)asafunctionofcoer-civity(Hc)forindividualchondrulesfromAllende.NotethelineartrendforHclessthan17mTwithMr/Msupto0.17.Chondrulesanalyzedforinternalmicrostructurearemarked,coveringtherangeofcoercivityandmagnetizationcharacteristics.


tionsandscanelementaldataaresummarizedinFigures5to9,showingthevariationinsizes,shapes,internalstructuresandelementalcompositioninthechondrules.

Elementalanalysesofthechondruleinteriorpermittoinvestigateindetailthemicrostructuresandcompositionalvariations.Resultsintermsofscanimagesaresummarizedforeachchondrule,withsemi-quantitativedataforSi,Ca,Al,Fe,Mg,CrandScompositionalspatialvariations.

TheAllendesilicatechondritehashighcontentsofmagnesiumandiron.Majormineralsareolivine(MgO47–49%,FeO7–9%,SiO242–44%),awaruite(FeO28–32%,NiO68%),andenstatite(MgO35–36%,FeO0.8%,Al2O31.2%,SiO260%),withkamacite,taenite,troilite,mackinawite,antigorite,majoriteandpentlandite.Magnetite, chromite, Ni-Fe alloys, sulfides and sulfates and anorthiteoccurringasaccessoryminerals.ChondrulesintheAllendepresentdifferenttexturesandcompositions,includ-inganhedral,euhedralandrecrystallizedolivinechondrules,

barredolivinechondrules,pyroxeneolivinechondrulesandpyroxene chondrules. Magnetite, taenite and iron sulfides arethemainmagneticminerals.

Mostchondrulesanalyzedpresentnearlysphericalshapes,asshownforchondrulesM53andM55(Figures5aand6a).Thesechondrulesshowincreasingmagnetizationratiosasafunctionofcoercivity(Figure2).M53showslowHcandMr/Ms,whereasM55showshigherHcandhigherMr/Ms.ChondruleM53hasanearlysphericalshape,withathinrimandasmallbulge(Figures5a-5e),whichshowsdifferentelementalcontentanddistributionintheCa,Si,AlandScompositionalimages(e.g.,higherMgcontentandlowerFecontent;Figures5g,5h).M53isaporphyriticoli-vinechondrule,S-poor,thatcontainsafewFe-richroundedparticles.ChondruleM55presentsanirregularroundedshapewithathinrim(Figure6a),whichcanbedistinguishedmoreclearlywithrespecttoitsironcontent(Figure6h).Itsinternalmorphologyismorecomplex,withlargercrystalsthan those in M53 and a central zone of iron sulfides (Figures 6e,6h),whichcanbeobservedinthetextureoftheCa-andAl-richminerals(Figures6b,6d)andMgcontent(Figure6g).ItpresentsalowCrcontent(Figure6f).

Chondruleswithhighermagnetizationratiosandco-ercivities(Figure2)showmorecomplexmorphologiesandinternalstructures.ChondruleM27hasanirregularshapewithathickincompleterim(Figure7a),whichisFe-richandwithdistinctCa,SiandAlcomposition(Figures7b-7d).M27isacompositechondrulewithacentralMg-richpartsurroundedbyadditionalchondrulematerialre-meltedandalteredwithelementdiffusion.ItisCaandAlpoor(Figures7b,7d),whichisalsoacharacteristicoftheigneousFe-richrim(Figure7h).ChondruleM40showsanirregularshape(Figure8a),whichfromtheinternalmicrostructureandelementalcompositionalmapscanberelatedtofragmenta-tion.M40hasabarredolivinetexturemarkedbyMg-richpyroxeneolivinebarredpatterns(Figure8g),withCa-Alrichbands(Figures8b,8d).M40hasnosulfur(Figure8e)andalowhomogenouscontentofFeO.ChondruleM23hasanirregularshape(Figure9a),whichisassociatedwithasmallerroundedsilicateFe-poorchondruleprecursorwithhomogeneoustexture(Figures9a,9h).ItsrimismarkedbytheCa,SiandAlrichigneousmaterial(Figures9b-9d).ThecompoundmaterialisFe-richandofigneoustexture(Figure9h).ThischondrulepresentsahigherCrandMgabundanceinistcentralpart(Figures8e,8g).

DISCUSSION

Theconspicuouswidespreadabundanceofchondrulesintheprimitivematerialrepresentedinchondriticmeteoritessuggested,sincetheearlystudies,thatchondruleformationwasakeymechanismforaccretionoftheinterplanetarydustmaterial.Theproto-chondruleswerethenheatedtomeltingpointandacquiredtheircharacteristicroundeddroplet shapeswith igneouscomposition.Formation

Figure3.PlotofmagnetizationratioMr/Msasafunctionofdensity.Chondrules analyzed by scanning electron microscopy are identified by thenumbers.

Figure4.PlotofmagnetizationratioMr/Msasafunctionofmass


Figure5.a:ScanningelectronmicroscopeimagesforchondruleM53withscanelementaldataforb:Si,c:Ca,d:Al,e:S,f:Mg,g:Cr,andh:Fe.


Figure6.a:ScanningelectronmicroscopeimagesforchondruleM55withscanelementaldataforb:Ca,c:Si,d:Al,f:Cr,g:Mg,andh:Fe.






modelsofthesolarsystempredictformationofchondriticbodieswithinamatrixincorporatingthechondrulesandCAIs.Proto-planetesimalswereformedbyaccretionofchondriticbodiesofvarioussizesinalong-termprocessofcollisionsandfragmentation/growth.Recently,studiesonindividualchondrulesandCAIsandtheirinternalstructures,compositionandtexturesprovideadditionalevidenceformorecomplexscenarioswithheterogeneousconditionsand gradients in the solar nebula, suggesting fluctuating conditions,variousheatingmechanisms,complexmatterdistributions,varyingoxidizingconditions,alterationeffectsandextendedoverlappingchronologiesforchondrule,CAIandchondruleformation(e.g.,FegleyandPalme,1985;Prinzet al.,1986;Kring,1991;HoodandKring,1996;HongandFegley,1998;Scott,2007).

Theopaqueandmagneticmineralogyinmeteor-iteshasbeenlongstudied,withattentiontothereduc-ing/oxidizingconditionsinthesolarnebula.Chondriticmeteoritesformacontinuousbroadtrendfrom‘reduced’withFeinmetallicformto‘oxidized’withFeinsilicatesandoxides.MagneticmineralsaredominantphasesinCV3 chondrites, including FeNi, iron oxides and silicates. Studieslinkthemagnetitetoaqueousalterationinproto-planetarybodies;ourobservationsshowthatmagnetiteisa dominant phase responsible for the high field magnetic propertieswithPSDandMDdomainstates,alongwithtaeniteandironsulfidesandoccurrenceofironinthesilicates(Figures5to9).

Analysesofmagnetichysteresisparametersshowasimplelinearrelationshipbetweenthemagnetizationratioandcoercivity(Figure2).Magnetizationratiosarelow,lowerthan0.225,withthelinearrelationshippresentupto0.17.Coercivityislow,lowerthan24mT.Thelinearrelationshipcoverstherangefromabout2mTto17mT.Theselowvaluessuggestthatchondrulesaresusceptibletoalterationandre-magnetization,whichhasimplicationsforretrievalofpaleointensityestimatesfortheearlyplan-etary magnetic fields. Analysis of chondrules within the linear relation field and outliers may show some further indicationsforunderstandingthemagneticmineralogyandmagnetichistory.Theindicationsfromthemicrostrucuralobservationssuggestthatmagneticpropertiesmayrelatetochondruleinternalstructureandcomposition.Chondrulesfallingoutsidetherelationarecompound,withaggrega-tionrims,andfractured(M40,M27andM23),incontrasttothosecharacterizedbylowHcandMr/Msthatappearhomogeneous(M53andM55).Figures5a,6a,7a,8aand9ashowthesilicamapsforfourofthechondrules;theirinternal structure can be observed, which is also reflected intheirexternalmorphologyandshape.

SamplesfromAllendehavebeenlongusedtoin-vestigate the early magnetic fields in the protoplanetary nebula(Sugiuraet al.,1979).However,paleointensitydeterminationsusingdifferentmethodsincludingmag-netizationratiosandthedoubleheatingThelliertechniquegivecontrastingresults.Paleointensitymethodsrequiring

laboratoryheatingtohightemperaturesarelimitedbyalterationprocessesaffectingthemagneticminerals(e.g.,BanerjeeandHargraves,1972;Urrutia-Fucugauchi,1979).Methodsusinglaboratoryinducedmagnetizationssuchasisothermalremanentmagnetization(IRM)andanysther-eticremanentmagnetization(ARM)havebeendeveloped(e.g.,Wasilewski,1981;GattaccecaandRochette,2004).Effectsofremagnetization,withacquisitionofremanentcomponentsassociatedwithchemical,thermalandmag-neticviscosityprocesses,aremajorlimitationsinreteivingthe early nebula magnetic fields (e.g.,Lanoixet al.,1978;Sugiuraet al.,1979;Urrutia-Fucugauchi,1981;Urrutia-Fucugauchiet al.,1984).Studieshaveusedwholesamples,aswellasindividualchondrules.Forinstance,BanerjeeandHargraves(1972)reportedestimatesof110TfromThellierexperiments.Lanoixet al.(1978)reportedvaluesfrom100to700T,andWasilewski(1981)reportedestimatesof1.2to15T.Thecoercivitydistributioninchondriticmeteoritessuggestthatthesematerialsaresusceptibleofmagneticoverprinting,resultingfromdifferentprocessesduringcol-lisions,alterationwithinplanetesimalbodies,expositiontohigh magnetic fields, lighting strikes, etc. This has suggested thatreliablepaleointensityestimatesaremorelikelytobeobtainedfromthehighcoercivityspectrainchondrules.Sugiuraet al.(1979)reporteddata,obtainedbyapplyingaformofpaleomagneticconglomeratetest,whichsuggestthatchondrulesretainaremanentmagnetizationacquiredpriortotheirincorporationintothechondriticmeteorite.Therelationshipuncoveredforthemagnetizationratioandcoercivity(Figure2)maypermittobetterunderstandthecoercivitydistribution.Theconnectiontotheinternalchondrulestructure,withcompoundchondrules,altera-tionrims,fracturingcanbeusedforsampleselectionforpaleointensityexperiments.

Analysesofmagneticmineralogy,originandaltera-tionprocessessuggestalongchronologyextendingfrominterstellardustaccretion,chondruleformationwithrepeat-edmeltingevents,formationofplanetesimals,metamorphicandheatingeventsofplanetesimalsandsecondaryalterationafterwardsbeforeandafterarrivaltoEarthasmeteorites.ThisextendedchronologyisillustratedinFigure10,wherethermalprocessesprocessedamorphousinterstellarsilicatedustsinthenebulardiskthatwereaccretedintochondrulesandCAIs.RepeatedmeltingeventsproducedthechondrulesthatwerelaterincorporatedwithCAIs,silicatefragmentsintochondritesandcometarybodiesindifferentpartsofthenebula.Planetesimalsformedduringalongperiod,involvingmetamorphicandthermalprocessesandthesebodiesalsoaddeddebrisfromthecollisionsbetweentheplanetesimals(Scott,2007).Thecomplexityofchondruleformation processes, which are reflected in their internal structures,isillustratedinFigure11.Chondruleswithdiffer-entmorphologiesandinternalstructuresincludecompoundchondrules,igneousrimmedchondrules,chondruleswithmetallicandforeignfragmentsandAl-richchondruleswithCAIinclusions.




Protoplanetary disk zone of crystalline and amorphous condensates

Rep

eate

d

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ting

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+Chondrulefragment

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exchange

+CAIfragment

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Al-rich chondrulewith relict CAI

Collisionsbetween

partly meltedchondrules

+Dust

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amorphousdust

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Sun

Modelsforchondruleformationshowthatearlyconditionsintheplanetarynebulaincludedawiderangeofinteractingmechanisms,resultingincomplexformationofchondrulesfromtheinitialcrystallinerefractorydustandin-clusions(Figures10and11).Iftheseprocessesapplywidelyin the protoplanetary disk, it is hard to define a controlling mechanismforchondrulesizeandgrowth.Chondrulesizerangesaredifferentforthedifferentchondriticmeteorites,butshowrelativelynarrowrangesforgiventypes.Theapparentrestrictedrangeinchondrulesizeshasbeenlittle

discussedinmodelsofchondruleformation,anditrequiresfurtherinvestigation.Thedifferentmeteoritetypesshowdistinctchondrulesizeranges.Microanalysesofindividualchondrulesrevealtheeffectsofaccretionprocesseswithseveralphasesofadditionofmaterialandalsoindicateeffectsofsecondaryalterationprocesses.Therelativelynarrowrangesandthecharacteristicrangesinchondritetypessupportthepresenceofcontrolling/limitingprocessesinvolvedinchondruleformationandsubsequentaccretioninchondriticbodies

Figure10.Schematicrepresentationofprotoplanetarynebulawithrelativepositionofchondruleformationregionplottedasafunctionofrelativedistance,inastronomicalunits(AU),awayfromtheproto-sun(adoptedfromScott,2007).

Figure11.Schematicrepresentationofchondruleformingprocessesfromprecursordustandpartlymeltedandsinteredaggregatematerial.Diagramillustratesformationofcompoundchondrules,chondruleswithigneousrims,chondruleswithforeignparticlesandchondruleswithrelictcalcium-alu-minuminclusions,CAIs(adoptedfromScott,2007).


CONCLUSIONS

ResultsofourmicromagneticandmicrostructuralstudyofindividualchondrulesfromtheAllendecarbona-ceousmeteoriteprovidenewdataforanalysesofinternalchondrule structure and high field magnetic properties. Chondrulesshowwiderangesofmagnetizationratios(Mr/Msfrom0to0.22)andcoercivity(Hcfrom3to24mT),characterizedbyalinearrelationofMr/MsasafunctionofHc(Figure2).Thisrelationcorrelateswithinternalmicrostructureandcomposition,withcompoundchondrulesshowinghigherhysteresisratioandparametervalues.Chondrulesthatpresenthysteresisparametersfallingoutsidethemajortrendshowinternalstructures,composi-tionandtexturesindicativeofcompoundchondrules,andfragmentationandalterationprocesses(Figures5to9).Resultshaveimplicationsforoxidizing/reducingconditionsandformationmechanismsofmagneticmineralogyintheearly solar nebula. Allende is a member of the CV3 carbona-ceouschondrites,andpartoftheoxidizedmeteoriteswithFeO-richolivinesandironcontainedinsilicatesandoxides.Micromagneticanalysesinthechondrulesidentifymagnet-iteasamainmagneticphasewithpseudo-singledomainandmultidomainstates.Magnetichysteresisdataintermsofplotsofparameterratiosshowdistinctrelationshipswithchondrulesize,density,massandshape.Therelationshipscorrelatewiththeinternalmicrostructuresandelementalcompositionalvariation.Morphology,internalstructureandelementalcompositioncharacterizedistinctmineralogicalassemblageswithspatialcompositionalvariationrelatedtochondrulesizeandshapeandmagneticproperties.

ACKNOWLEDGEMENTS

ThisstudyformspartoftheGeophysicsMeteorPlanProject.ThepieceoftheAllendechondriteformspartoftheMeteoriticCollectionoftheInstitutodeAstronomía,UNAM.WegratefullyacknowledgethecollaborationofDr. Margarita Reyes in the microprobe analyses, and Víctor MacíasAviñaandMartínEspinosawiththemagneticanalyses.Weacknowledgecommentsonthepaperbythetwojournalreviewers.

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Manuscriptreceived:December6,2008Correctedmanuscriptreceived:March24,2009Manuscriptaccepted:November23,2009

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Micromagnetic and microstructural analyses in … · 162 Flores-Gutiérrez et al. Micromagnetic and microstructural analyses in chondrules of the Allende meteorite Daniel Flores-Gutiérrez1,*,