scanning electron microscopy characterization of iron

Flores-Gutiérrez et al.338

ABSTRACT

We analyze the relative contents and internal spatial elemental distribution of Fe, Ni and S in minerals within selected individual chondrules of a fragment of the Allende chondrite, as derived from scanning electron microscopy (SEM) - energy dispersive X-ray spectroscopy (EDS) analyses. Chondrules were selected to cover the range of micromagnetic properties in terms of hysteresis coercivity and magnetization ratios. Variation in micromagnetic properties appear correlated to the external and internal morphologies and mineralogical assemblages. In order to identify the distribution of mineral assemblages characterized by Fe, Ni and S, a mathematical technique for image analysis was applied to the SEM-EDS images. The resulting compositional images display the complexity in composition and microstructural morphologies of individual chondrules. Chondrule c27b is characterized by Fe-rich minerals and is Ni and S poor, although Ni and S appear concentrated within a Fe-rich rim. Chondrule c40b, formed by barred olivine, is Fe poor and has low contents of Ni and S; Fe is homogeneously distributed within the olivine crystals. Chondrule c53b has a spherical shape, except for a partly preserved Fe-rich rim that forms a bulge; minerals with higher contents of Fe, Ni and S are observed in the chondrule interior. Chondrule c55b, of porphyritic olivine type, has a more complex morphology, characterized by variable Fe, Ni and S compositional distribution and characteristic mineral assemblages. Results confirm that closely spaced chondrules within given sectors of Allende meteorite have major differences in elemental content and distribution, in addition to differences in shape, size, texture and mineralogy, which support the idea that chondrules underwent distinct thermal, shock, alteration and evolutionary histories, related to chondrule formation and alteration processes, in the early stages of evolution of the planetary system.

Key words: chondrules, chemistry, elemental distribution, mineralogy, Allende meteorite.

Scanningelectronmicroscopycharacterizationofiron,nickelandsulfurinchondrulesfromtheAllendemeteorite–furtherevidencefor

between-chondrulesmajorcompositionaldifferences

Daniel Flores-Gutiérrez1,*, Jaime Urrutia-Fucugauchi2, Ligia Pérez-Cruz2, Raquel Díaz-Hernández1, and Carlos Linares-López3

1 Instituto de Astronomía, Universidad Nacional Autónoma de México, Delegación Coyoacán, 04510 México D.F., Mexico.2 Proyecto Universitario de Perforaciones en Océanos y Continentes, Laboratorio de Paleomagnetismo y Paleoambientes,

Instituto de Geofisica, Universidad Nacional Autónoma de México, Delegación Coyoacán, 04510 México D.F., Mexico.3 Laboratorio Universitario de Petrología, Instituto de Geofisica, Universidad Nacional Autónoma de México,

Delegación Coyoacán, 04510 México D.F., Mexico.* [email protected]

Revista Mexicana de Ciencias Geológicas, v. 27, núm. 2, 2010, p. 338-346

Flores-Gutiérrez,D.,Urrutia-Fucugauchi,J.,Pérez-Cruz,L.,Díaz-Hernández,R.,Linares-López,C.,2010,Scanningelectronmicroscopycharacterizationofiron,nickelandsulfurinchondrulesfromtheAllendemeteorite–furtherevidenceforbetween-chondrulesmajorcompositionaldifferences:RevistaMexicanadeCienciasGeológicas,v.27,núm.2,p.338-346.

Iron, nickel and sulfur in chondrules from the Allende meteorite 339

RESUMEN

En este trabajo se analiza la distribución espacial y los contenidos relativos de hierro (Fe), níquel (Ni) y azufre (S) en condros individuales separados de un fragmento del meteorito Allende, por medio de observaciones y análisis con microscopía electrónica de barrido (SEM) – espectroscopía de rayos X de dispersión de energía (EDS). Los condros seleccionados presentan un rango amplio en sus propiedades micromagnéticas en términos de cocientes de coercitividad y magnetización, las cuales han sido correlacionadas con variaciones morfológicas y texturales. A fin de identificar la distribución de ensambles minerales caracterizados por su contenido de Fe, Ni y S, se aplicó una técnica matemática de análisis a las imágenes obtenidas por SEM-EDS. Las imágenes composicionales resultantes muestran la complejidad en la composición y la morfología microestructural de condros individuales. El condro c27b se caracteriza por minerales ricos en Fe y pobres en Ni y S, aunque presenta una capa externa rica en Fe que concentra Ni y S. El condro c40b, formado por olivino barrado, es pobre en Fe y tiene contenidos bajos de Ni y S; el Fe se encuentra distribuido de manera homogénea dentro de los cristales de olivino. El condro c53b tiene forma esférica con una capa parcial externa rica en Fe y arreglos internos de minerales ricos en Fe, Ni y S. El condro c55b, del tipo porfírico con olivino, tiene una morfología más compleja, caracterizada por una distribución variable de Fe, Ni y S. Los resultados confirman que condros espacialmente cercanos del meteorito Allende presentan marcadas variaciones en la distribución de Fe, Ni y S, además de diferencias en su forma, tamaño, textura y mineralogía, las cuales se asocian a una evolución compleja que involucra varios eventos de calentamiento y alteración durante los procesos de formación en las etapas tempranas de evolución del sistema planetario.

Palabras clave: condros, química, distribución elemental, mineralogía, meteorito Allende.

INTRODUCTION

Chondriticmeteoritesareformedbyabundantchon-drulesandcalcium-aluminuminclusions(CAIs) inanigneous-texturedmatrix.Chondrulesaremillimetricround-shapedsilicatespherules,whichformedbymeltingofsoliddustprecursorsandsubsequentrapidcoolinginadomi-nantlyreducingenvironmentinthesolarnebula(Hewins,1997;Wood,1988;ConnollyandLove,1998;Scott,2007).Twodistincttypesofchondruleshavebeenfound:(a)thereducedtypeIchondrules,whichareFeOandvolatilepoorandmetalrich,and(b)theoxidizedtypeIIchondrules,whichareFeOandvolatilerichandmetalpoor(McSween,1979;Wood,1988).ThechondrulesandCAIsformedattheearlystagesofevolutionofthesolarsystem,andthereforethechronologyandprocessesinvolvedintheirformationarekeyelementsthathavebeenlonginvestigated.Here,weconcentrateonadetailedstudyofthecontentanddistribu-tionofFe,NiandSinchondrulesoftheAllendemeteorite,asderivedfromresultsofscanningelectronmicroscopy(SEM)-energydispersivespectroscopy(EDS)analyses.Resultswereanalyzedforthespatialcompositionalandtexturaldistribution,andtheirimplicationsforchondruleformationandsubsequentalterationprocessesintheearlystagesofevolutionoftheplanetarysystem.

Inthispaper,wereportnewresultsfrompetrographicandgeochemicalstudiesofasuiteofindividualchondrulesisolatedfromafragmentoftheAllendemeteorite.Allendemeteoriteisamemberofthegroupofcarbonaceouschon-driticmeteorites,whichhasbeenintensivelystudiedsince

itsfallonFebruary8,1969inChihuahua,northernMexico(Clarkeet al.,1970).Carbonaceouschondritesarecom-posedofprimitiveundifferentiatedmaterialandrepresenttheoldestpreservedmatterinthesolarsystem(e.g.,Wood,1988;Cameron,1988;Hewinset al.,1996;Scott,2007).

ALLENDE CHONDRULES

TheAllendemeteoriteisanoxidizedCV3carbo-naceouschondrite.CV3chondritesarerelativelyrareascomparedtoothertypesofchondrites.Allendefallin1969waswelldocumented,andalargeamountoffragments(upto2tons)wasrecovered(Clarkeet al.,1970).Asaresult,considerableattentionhasbeendevotedtotheAllendechon-driteandnumerousgroupsandlaboratorieshavebeenabletoanalyzeit,makingAllendeoneofthemostintensivelyinvestigatedmeteorites.

Allendeiscomposedmainlyofchondrules(ca.43%vol.),matrix(ca.38.4%vol.),andCAIs(ca.9.4%vol.);itcontainsolivineinclusions(ca.3.2%vol.),opaqueminerals(ca.3.1%vol.),andlithicandmineralfragments(ca.2.9%vol.)(McSween,1977).Allendedisplaysamacroscopic texture that reflectsabundanceofchon-drulesofsub-millimetersizewithintheaphaniticblacksilicatematrix(Figure1).Allendebelongstotheoxidizedchondriteswithananhydrousmineralogy,withhydrousphasesrestrictedtochondrulesandCAIs(BlanderandFuch,1975).OxidizedchondritescontainolivinesrichinFeOandnoironmetalblebs,incontrasttothereduced


[Mg,Fe]2SiO4]andCa-poorpyroxenes[(Mg,Fe)2Si2O6].Chondrulec40brepresentsabarredtype,witholivinebarredpatterns,andc53bisachondruleoftheporphyriticolivinetype.Otherequilibratedchondrulesarecharacterizedbyradialpyroxenebarredpatterns.Complexchondrulesarecharacterizedbydistinctmineralassociationswithfayalite,spinels,cromites,magnetiteandtrolite,andFe-Nialloys.Complexchondrulessuggestarangeoffusiontemperatures,whichareindicatedbythedistinctconstituentminerals.Thepresenceofinclusionsisevidenceoffurthercomplexitiesinthemediumforchondruleformationandinthemechanismsandtemperature-pressure-fugacityconditions.Forinstance,theoccurrenceofCr-richspinelssuggeststhepresenceof fine dust in high relative proportion. Comparison with presentconditionssuggestsconcentrationsseveralordersofmagnitudelarger,whichisbeinginterpretedintermsofprimordialcondensatesfromthepre-solargasanddustnebula.

typechondrites.Allendeappearstohavebeenpartlyal-teredandmetamorphosed,withametamorphicgradeof3.7.Studiesindicatesecondaryalterationprocessesaffect-ingAllendemineralogyandcompositioninvariousways(e.g.,McSween,1979,1987;Krotet al.,1995;KavnerandJeanloz,1998;Scott,2007).

Forthisstudy,wepreparedthinslabsoffourselectedchondrules identified as c27b, c40b, c53b and c55b from a largerchondrulesetthathasbeenstudiedintermsofsize,morphology,geochemistryandmagneticmineralproperties(Flores-GutierrezandUrrutia-Fucugauchi,2002;Flores-Gutiérrezet al.,2010).Theselectedchondrulescovertherangeofmicromagneticpropertiesfromlowtohighcoercivitiesandmagnetizations,andinthepreviousstudyhavebeenalsoshowntopresentdistinctmorphologiesandinternalmineralogicalassemblages.Magneticpropertiesofchondriticmeteoriteshavebeenlongstudiedtodeterminethe characteristics of magnetic fields in the protoplanetary nebula. Determination of the strength of paleofields from chondriteshasbeenproblematic,mainlybecauseofmeth-odologicallimitationsandnatureofthepaleomagneticrecordinmeteorites.Paleointensitydeterminationsbasedonlaboratoryacquisitionofthermoremanentmagnetizationsareaffectedbyalterationofmagneticcarriers(e.g.,Suiguraet al.,1979;Urrutia-Fucugauchi,1979,1981;Wasilewski,1981).Rockmagneticanalysesusingmagnetichyster-esis,acquisitionoflaboratoryremanences,coercivityandunblockingtemperaturespectra,temperaturevariationofsusceptibility,remanentmagnetizationsandhysteresismayprovideusefulindicationsonthemagneticcarriers(e.g.,Urrutia-Fucugauchiet al.,1984).MagnetiteformationinoxidizedCVchondriteshasbeeninvestigatedandrelatedtoaqueousalterationprocesses(e.g.,Choiet al.,1997).InAllende,magnetitehasbeenshowntoconstituteamajormagneticphase.Thehighlyenergeticconditionsandmul-tipleheatingeventsinthesolarnebulamayhavecontributedtocomplexmineralassemblagesandpaleomagneticrecord.The paleomagnetic record was later modified by impacts, thermalandchemicalalterations,weathering,andaltera-tionsintheterrestrialenvironment.Micromagneticanaly-sesofchondrulesofferalternativehighresolutiontoolstoinvestigateonthenatureofthepaleomagneticrecordandrecordingminerals.

Thediametersofthestudiedchondrulevaryfromabout0.4mmto1.2mm,withmorphologiesfromalmostsphericaltoirregularshapes.Forslicing,individualchon-drules were fixed with resin in 2.5 cm cylindrical containers andslicedwithnon-magnetic,thin(0.03mm)cuttingdisks.Threeormorethinslabswerepreparedfromeachchondrulefortheexperimentalanalyses.

Allendechondrulesarecharacterizedbydistinctmineralogicalandcompositionalassemblages,rangingfromsimpleequilibrated2-3mainsilicatemineralassemblagestocomplexarrangementswithaccessoryminerals,includingFe,NiandSmineralsandalloys.AllendechondrulescontainabundantFe-Mgsilicateminerals,mainlyolivine

Figure1.FragmentoftheAllendecarbonaceouschondriticmeteorite(6cmslabcutfromAllendeshowingdistributionofchondrulesandrefractoryinclusions in the fine grained silicate matrix).


C55bC53bC40bC27b

CP

Fe

Ni

S

Thechondrulesanalyzedhavebeendescribedinapreviousstudy,whereadditionalcompositionaldataarereported(Flores-Gutiérrezet al.,2010).Chondrulec27bhasanirregularshapewithathickincompleterimthatisFerichandwithdistinctSiandMgcomposition.c27bisacompositechondrulewithaMg-richcentralpartsur-roundedbyadditionalchondrulematerialremeltedandalteredbyelementdiffusion;itisCaandAlpoor,whichisalsoacharacteristicoftheigneousFe-richrim.Chondrulec40bdisplaysanirregularshape,whichonthebasisoftheinternalmicrostructureandelementalcompositionalmapsisrelatedtofragmentation.c40bhasabarredolivinetexturemarkedbyMg-richpyroxeneandolivinebarredpatterns,

withCa-Alrichbands,lowsulfurandlowFeOcontents.Chondrulec53bshowsanearlysphericalshape,withathinrimandasmallbulge,whichshows,intheSi,MgandFecompositionalimages,differentelementalcontentanddistribution(e.g.,lowerMgcontentandhigherFecontent).c53bisaporphyriticolivinechondrule,Spoor,andcontainsafewFe-richroundedparticles.Chondrulec55bpresentsanirregularroundedshapewithathinrim,whichcanbedistinguishedmoreclearlywithrespecttoitsironcontent.Itsinternalmorphologyismorecomplex,withlargercrystalsthan those in c53b and a central zone of iron sulfides that canbedistinguishedinthetextureoftheCa-andAl-richmineralsandbyitsMgcontent.

Figure2.Scanningelectronmicroscopy(SEM)andSEM-Energydispersivespectroscopy(EDS)imagesforindividualchondrulesfromAllendeme-teorite.SEMcompositepictures(CP)andfalsecolorcompositionalimagesforFe,NiandSareshownforchondrulesc27b,c40b,c53bandc55b(seetextfordescription).


SCANNING ELECTRON MICROSCOPY AND MICROPROBE ANALYSES

ThechondruleslabswereanalyzedusingaJEOLmicroprobeat theLaboratoriodePetrología,UNAM.ElementalcompositionalmappingwasobtainedwithenergydispersiveX-rayspectroscopy(EDS)analyses.Resultsforchondrulesc27b,c40b,c53bandc55baresummarizedinFigure2,whichalsoincludesaSEMcompositepicture(CP)foreachsample;theCPimages(totalX-rayemission)displaythemorphologiesandinternalstructuresofthechon-drules,withc27bshowingbimodaldistributionandc40b,c53bandc55bshowingunimodaldistributions.

DistributionofFe,NiandSgiveninthefalsecolorimagesdocumentthecomplexityincompositionandmi-crostructuralmorphologiesofindividualchondruleswithinasinglefragmentoftheAllendemeteorite.Thediversityofthesecompositionaldistributionscorrelatewithpreviousob-servationsonwidevariationinchondrulesizes,types,den-sity,andmagnetichysteresisproperties(Flores-Gutiérrezet al.,2010).Chondrulec27bischaracterizedbyFe-richmineralsandisNiandSpoor.NiandSareconcentratedinthechondrulerim,whichisalsoFerich;therimcanbeclearlyrecognizedintheCPimageandiswelldelineatedbythespatialdistributionofFe).Chondrulec40bisFepoor,andhaslowcontentsofNiandS;itisformedbybarredolivineswithFedistributedmorehomogeneouslywithintheolivinecrystals.Chondrulec53bhasasphericalshape,exceptforapartlypreservedFe-richrim.MineralswithhighercontentsofFe,NiandSarepresentinthechondruleinterior.Chondrulec55bhasamorecomplexmorphologyandischaracterizedbymoreirregulardistributionofFe,Niand that define characteristic mineralogical assemblages.

Inordertoidentifythemineralassemblagescharacter-izedbyFe,NiandS,wehaveappliedamathematicaltech-niqueforimageanalysis,whichisdescribedinthefollowingsection. The analysis permits an accurate identification of themineralogicaldiversityanditsspatialdistributionwithinthechondrules.Intersectionzonesisolatedintheimageanalysescharacterizemineralsinthechondrules:troilite(FeS),magnetite(Fe2O3),awarite(Ni3Fe),tetranite(FeNi),ilmenite(FeTiO3),haezlewoodite(Ni3S2),millerite(NiS),polydymite(Ni3S4)andvaesite(NiS2).

SEM-EDS IMAGE ANALYSIS

Weanalyzedthemineralboundariesinthechondrules,focusingontheelementsFe,NiandS.Themathematicaltoolsemployedarepartofacommercialintegrateddevelop-mentenvironment(IDE)formachinevision.

For the analyses, we first read the original images, whichweretransformedfromRGBcolourspacetogreyscales. Then we applied filters in order to achieve a best signal to noise ratio. A shock filter was applied to the input imagetosharpentheedgesthefeaturescontainedinit.The

principle of the shock filter is based on the consideration ofacontinuousimagef :R2 → R. Then a class of filtered imagesoff (x, y)maybecreatedbyevolvingfundertheprocess

ut =-sign(Δu)|∇u|, (1) u(x,y,0)=f(x,y). (2)

wheresubscriptsdenotepartialderivatives,and∇u=(ux,uy)┴isthe(spatial)gradientofu.Theinitialcondition(2)ensuresthattheprocessstartsattimet=0withtheoriginalimagef(x,y)(Weickert,2003).

Thenthealgorithmsegmentstheimageintoregionsofthesameintensity(rasteredintorectanglesofsize5×5).Inordertodecidewhethertwoadjacentrectanglesbelongtothesameregion,onlythegreyvalueoftheircentrepointsisused.Ifthegreyvaluedifferenceislessthenorequalto20therectanglesaremergedintooneregion,whichmeans:Ifg1andg2aretwogreyvaluesofthecentrepointstobeexamined,theyaremergedintothesameregionif:

|g1–g2|<Tolerance

whereToleranceissetto20inourcase.Next,theregionsareconnected.Thealgorithmde-

termineswhichregionsareconnected,fromthesetofinputregionsgeneratedinthepreviousstep,analysingtheirneighbourhood.Weusedan8degreeneighbourhood,whichisusefulfordeterminingtheconnectedcomponentsoftheforeground.

Next,thealgorithmselectsregionswiththeaidofshapefeatures.Foreachinputregionasetofshapefeaturesiscalculated.Ifeachcalculatedfeatureiswithinaselectedrange,theregionisaddedtotheendresult.Inourcasewehaveusedtheareaandsizefeaturesforeachregion.

Thelaststepofouralgorithmistocalculateadditionalfeaturesthathelpustoclassifytheregions.Thosefeaturesareregionnumber,area,mean,minimumandmaximumintensity,standarddeviationandrank.Finalresultofthealgorithm is integrated into a file containing all informa-tioncalculatedforeachselectedregion.InFigures3to6,resultsintermsofthespatialassemblagesofFe,NiandSforchondrulesc27b,c40b,c53bandc55bareshowed.

INTEGRATED MINERAL ANALYSES

ThemineralogicalcompositionaldifferencesandwidevariationrangesinthechondrulesaredocumentedintheSEM-EDSimageanalyses(Figures3to6).Mineralsandalloys are identified by different colours, which display the spatialdistributionandrelativecontents.

Chondrulec27b(Figure3),whichshowsacomplexassemblageofFemineralswithsome10distinctassocia-tions,ischaracterizedintheintersectionimageanalysesin


termsofcompositionaldistributionbyawelldelineatedrimthatextendstocoveralmosttheentirechondrule(incontrasttothepartialrimrecognizedinCPandFeimages),andcharacteristicpatternsinthechondruleinterior.Niismainlyconcentratedwithintherim,whereasSispresentintherimmineralsandinthemineralassemblagesoftheinterior.

Chondrulec40b(Figure4)ischaracterizedbyawelldevelopedbarredolivinepattern,whichdominatestheinternalstructure,althoughamineralassemblageisalsoidentified. The minerals identified are characterized by sizesintheorderofafewmicronsandaredistributedintheinterstitialzonesofthechondruleinteriorandformapartialrim.SomeofthemineralsthatstandoutinwhitecolorintheCPimageapparentlycorrespondtometallicinclusionsrichinFe,NiandS.

Chondrulec53b(Figure5),whichshowsanearlysphericalmorphologyandanapparenthomogeneousdistributionofFe,ischaracterizedbyacomplexmineralassemblage.Feisdistributedthroughthechondruleandisconcentratedingranularassemblageswithapparentpolygonalshapes.Fe-richmineralsandFe-NiandFe-Ni-Salloysareobserved,whichcanbedistinguishedbythewhitestructuresintheCPimage.Niisenrichedinmineralsintheinterior,whicharemarkedbythegreyinclusionsintheCPimage.Sismoreabundantinthelargerinclusionsandisalsodistributedintheinterstitialspaces.

Chondrulec55b(Figure6)showsacomplexassem-blageofmineralswithhighrelativeconcentrationsofFe,NiandS,whichisdistinctfromtheotherchondrulesanalyzed.Thecharacteristicmineralpatternsaregivenbytheirregu-larzonesofelongatedandclusteredmineralassemblagesofmetallicinclusionsaroundtheolivines,pyroxenesandotherMgsilicates.FeisenrichedintwolargeinclusionsthatalsoshowhighcontentsofNiandS,andwhichcanbedistinguishedaswhitezonesoftheCPimage.

TheFe-,Ni-andS-richmineralassemblageshavebeen identified in Raman spectra (Flores-Gutiérrez et al.,inpreparation).TheRamanspectraforc53bindicatethepresenceofNimineralsandalloys,withNiOorNi(OH)2,NiS2andNi3S4.

Thesmallpartialriminc53bappearstohaveasimilarmineralogicalassemblageasthethickerandmorecompleterimobservedinc27b.Thesemineralassemblagesarealsopresent,albeitinlessconcentration,inthepartialrimofchondrulec40b.Chondrulerimsarethoughttobeformedatlatertimesbyprocessesaffectingthechondrules.Theanalysesshowthatrimsinc27bandc40bareenrichedinNiandS.

DISCUSSION

ChondrulesandCAIsformedattheearlystagesofthesolarsystemevolution,andthusthechronologyandprocessesinvolvedintheirformationarekeyelements

tounraveltheoriginofthesolarsystem(Wood,1988;Cameron,1988;Hewinset al.,1996;Scott,2007).BothchondrulesandCAIsformedearlybyshort-livedheatingeventsthataffectedthedustandgasnebula.Studiesindi-catethattemperaturesmayhavereachedsome1400oCto1800oCforshortperiodsofheating/cooling.Differentmechanisms have been proposed for the melting of fine-grainedsolidmatter.ChondrulesappeartohaveformedatalatertimethanCAIs,assuggestedfromisotopicdataandpetrographicandmicrostructuralobservations.IsotopicanalysesoftheAlandMgsystematicshavelongbeencar-riedoutinAllendesamples,andprovidedconstraintsforthechronologyofeventsformingCAIsandchondrules,andforsolarnebulaconditions(e.g.,Leeet al.,1976;Zinner,2002;Scott,2007).Analysesof26AlisotopicdifferencesinCAIsandchondruleshavebeendiscussedintermsoforderlyoroverlappingsequenceofevents(e.g.,Prinzet al.,1986;Wood,1988;ItohandYurimoto,2003;Bizarroet al.,2004;Krotet al.,2005).AdditionaldetailedstudiesofCAIsandchondrulesarerequiredtoconstrainthechronologyofearlyevents,processesandtheirspatialrelationships(ItohandYurimoto,2003;Bizarroet al.,2004;Krotet al.,2005).ExperimentalevidenceonanearlyoriginofCAIsincludesthe findings of relict CAIs inside chondrules. Studies have also reported findings of a chondrule inside a CAI, which maysuggestcontemporaneousformationofCAIsandchon-drules,andanextendedperiodofchondruleformation(ItohandYurimoto,2003;Krotet al.,2005).StudiesfavorthatCAIsformedsome1to4millionyearsearlier(e.g.,Scott,2007),thoughchondrulesmayhavestartedtoformtogetherwithCAIs,butchondruleformationlastedlonger.

Severalstudieshavefocusedonthemineralogyofchondrules,CAIsandmatrix,insearchofconstraintsfortheformationmechanisms,conditions,spatial-temporalzoningandscalerangeswithinthesolarnebula.InthisstudywehaveconcentratedinstudyingthecontentsandspatialdistributionofFe,NiandSwithinindividualchondrules.Femineralsaremajorconstituentsinchondrules,wheretheymight occur in oxides, sulfides and metal alloys. Troilite (FeS),forinstance,occursfrequentlyinchondrules,anditsdistributionandrelativecontentprovidesconstraintsforformationconditionsandsubsequentalterationprocesses.Sulfurisvolatile,condensingastroiliteatabout983oCthroughreactionsbetweenFe-Nialloysandhydrogensul-fide (H2S).Sulfurmaybeeasilylostatchondruleformingtemperatureranges.Additionally,troilitemayhaveformedatlowertemperaturesduringcooling,associatedwithsul-furmobility.Distinctionofprimaryhightemperatureandsecondarylowtemperaturetroilitegivesindicationsfortheformationandalterationofchondrules(YuandHewins,1998;Rubin,2000;Joneset al.,2005).

Chondrulec40brepresentsanolivinebarredtype,whichwasfragmentedbeforeincorporationintothematrix.Thisprovidesevidenceoncollisionsinthechondruleform-ingregionoccurringbeforeincorporationintothematrixandperhapscontemporaneouslywiththechondruleformation.


chondrules,igneous-rimmedchondrules,fragmentedchon-drulesandrelativelyhomogeneousnearlysphericalchon-druleswitholivinebarredandolivineporphyritictypes.

Chondruleshavebeenfoundtoseparateintotwodif-ferenttypes:(a)thereducedtypeIchondrulescharacterizedbytheoccurrenceofironmetalblebsandFeO-poorolivine,and(b)theoxidizedtypeIIchondruleswithnometalblebs

Fragmentedchondrulesandrefractoryinclusionshavebeentakenasindicationsforcollisionsandturbulentconditionsinthesolarnebula.Occurrenceofcompoundchondrulesandigneousrimshasbeenconsideredintermsofmultipleheatingeventsinregionsofthenebula.ThechondrulesexaminedinthesestudiesoftheAllendechondriteillustratearangeofchondrulemorphologies,includingcompound

Figure3.Metalboundariesinsidec27bchondrule,determinedusinganimageprocessingtechnique,focusinginthenickel,ironandsulphurelements.C27bisanexampleofaniron-richchondrulewithiron-andnickel-richrim.

Figure4.Metalboundariesinsidec40bchondrule,determinedusinganim-ageprocessingtechnique,focusinginthenickel,ironandsulphurelements.C40bisanexampleofiron-,nickel-andsulphur-poorchondrules.

Figure5.Metalboundariesinsidec53bchondrule,determinedusinganim-ageprocessingtechnique,focusinginthenickel,ironandsulphurelements.C53bisanexampleofiron-andnickel-poorchondruleswithiron-richrims.Observethenickelandsulphurrichmetallicinclusions.

Figure6.Metalboundariesinsidec55bchondrule,determinedusinganimageprocessingtechnique,focusinginthenickel,ironandsulphurele-ments.C55bisanexampleofiro-,nickel-andsulphur-richchondrules.Notetheinternalmineralogicalassemblageandspatialarrangement,withsulphur-richminerals.


andFeO-richolivine(McSween,1979,1985,1987).SEM-EDSanalysesshowthatchondrulesinAllendecorrespondtotypeI,characterizedbyFeO-poorandmetal-richlowvolatilematerial.Thesub-groupsintheCVchondritesmainlydependontherelativecontentsofmagnetite,whichreflects on the modal ratios of magnetite to metal.

FemineralsandinparticularmagnetiteintheAllendechondritehavebeenintensivelystudied,aswellasinotheroxidizedandreducedchondrites.Studieshaveproposeddifferentscenariosformagnetiteformation, includingmagnetitecrystallizationfromcoolingoxidizedchondrulemelts,oxidationofmetallicnodulesinthesolarnebula,andlateroxidationofmetallicchondrulenodulesinasteroids.Magnetiteformationandtimingareimportantintermsofchondriteoriginsandevolution,withprocessesseparatingintothesolarnebulaand,atlatertimes,inprotoplanetsorasteroidalsettings(Krotet al.,1995).IntheAllende,mag-netiteappearsmainlyinporphyriticolivinechondrulesassphericalnodules,inassociationwithNi-richalloysandsulfides. In our analyses, Fe appears with Si and Mg (fos-teritetofayalite),withhigherconcentrationsinchondrulerims(e.g.,c27b,c53b).Inthosecases,FeoccurswithNiandS,formingFe-Nialloysandtrolite.Silicaisamajorelementinchondrulecomposition;inlowconcentrationsittendstobeuniformlydistributedandinhigherconcentra-tionsitisdistributedinwideregionsandinrims.Inc55b,itshowshigherrelativecontentsintheinterstitialzones(Flores-Gutierrezet al.,2010andinpreparation).NickeldistributionandNi-richmineralsandalloysmayalsoconstituteanimportantindicatorofalterationprocessesinthesolarnebulaandatlatertimes.MetamorphicprocessesaffectingchondritesallowNitomigrate;thismechanismhasbeenusedtoestimatemetamorphictemperaturesinpentlandite-bearingchondrites.Theseassemblagesandtheirdistributionsuggestlatersecondaryprocessesafterinitialchondruleformation,butcanstilltakeplaceinthesolarnebula.Kring(1991)hasinterpretedthehightemperaturechondrule rims in terms of fluctuating conditions occur-ringinthesolarnebula.Accretionofdustrimshasbeenshowntooccurunderturbulentconditions,particularlyinparticle-richregionsofthesolarnebula,whichcanbeseveralhundredofkilometersinsize(CuzziandAlexander,2006).Inthisrespect,Scott(2007)hasrecentlyconcludedthatmostchondrites,includingtheAllende,wereaffectedbyaqueousmetamorphicprocessesintheasteroidbelt.ThisemphasizestheneedforfurtherdetailedstudiesofadditionalCV3primitivechondritesandfurtheranalysesofalterationandmetamorphicprocessesthatcouldoccurinthesolarnebulaand/ortheasteroidalbelt.

Our results confirm that closely spaced chondrules withingivensectorsofAllendecanshowvariabledistinctel-ementalFe,NiandScontentsanddistributions.Chondrulesexamineddisplaymajordifferencesinrockmagneticprop-erties,inadditiontovariablecomplexsize,texturalandmineralogicaldifferences.Theseresultsandthespatialandtexturalcompositionaldistributionandmineralogysupport

chondrulesunderwentdistinctthermal,shock,alterationandevolutionaryhistoriesduringchondruleformationintheearlystagesofevolutionoftheplanetarysystem.

ACKNOWLDEGMENTS

This study forms part of the MeteorPlanetaryInternationalProject.ThepieceoftheAllendechondriteformspartoftheMeteoriticCollectionoftheInstituteofAstronomy,UNAM.WethankJoseFloresFlores,RobertoSatoBerru,LeticiaAlbaAldaveandMargaritaReyesforassistancewiththelaboratoryanalyses.Usefulcommentsbytworeviewersaregratefullyacknowledged.

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Manuscriptreceived:December13,2008Correctedmanuscriptreceived:January14,2010Manuscriptaccepted:May19,2010

scanning electron microscopy characterization of iron

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