Early evolution of the solar accretion disk inferred from Cr-Ti-O isotopes in individual chondrules

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Early evolution of the solar accretion disk inferred from

Cr-Ti-O isotopes in individual chondrules

Jonas Schneider, Christoph Burkhardt, Yves Marrocchi, Gregory Brennecka,

Thorsten Kleine

To cite this version:

Jonas Schneider, Christoph Burkhardt, Yves Marrocchi, Gregory Brennecka, Thorsten Kleine. Early

evolution of the solar accretion disk inferred from Cr-Ti-O isotopes in individual chondrules. Earth

and Planetary Science Letters, Elsevier, 2020, 551, pp.116585. �10.1016/j.epsl.2020.116585�.

�hal-03009471�

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Contents lists available atScienceDirect

Earth

and

Planetary

Science

Letters

www.elsevier.com/locate/epsl

Early

evolution

of

the

solar

accretion

disk

inferred

from

Cr-Ti-O

isotopes

in

individual

chondrules

Jonas

M. Schneider

a

,

∗

,

Christoph Burkhardt

a

,

Yves Marrocchi

b

,

Gregory

A. Brennecka

c

,

Thorsten Kleine

a

a_Institut_für_{Planetologie,}_University_of_Münster,_{Wilhelm-Klemm-Straße}_10,_48149,_Germany b_CRPG,_CNRS,_Université_de_Lorraine,_UMR_7358,_{Vandoeuvre-lès-Nancy,}_54501,_France

c_Nuclear_and_Chemical_Sciences_Division,_Lawrence_Livermore_National_Laboratory,_Livermore,_CA_94550,_United_States_of_America

a

r

t

i

c

l

e

i

n

f

o

a

b

s

t

r

a

c

t

Articlehistory:

Received30January2020

Receivedinrevisedform26August2020 Accepted12September2020

Availableonline23September2020 Editor:W.B.McKinnon Keywords: chondrules isotopicanomalies meteoritedichotomy nebularmixing

Isotopic anomalies in chondrules hold important clues about the dynamics of mixing and transport processesinthesolaraccretiondisk.The meaningoftheseanomaliesisdebatedand theyhavebeen interpretedto indicateeither disk-wide transportof chondrulesor localheterogeneities ofchondrule precursors. However, all previous studies relied on isotopic data for asingle element (either Cr, Ti, orO), whichdoes not allowdistinguishing betweensource and precursor signaturesas the causeof the chondrules’isotope anomalies. To overcome thisproblem, we obtained the ﬁrst combined O, Ti, and Cr isotope data for individual chondrules from enstatite, ordinary, and carbonaceouschondrites. We ﬁndthat chondrules from non-carbonaceous (NC) chondriteshave relatively homogeneous17_O,

ε

50_Ti,_and

_ε

54_Cr,_which_are_similar_to_the_compositions_of_their_host_chondrites._By_contrast,_chondrules

from carbonaceouschondrites (CC) have more variable compositions, someof which differfrom the host chondrite compositions.Although the compositions ofthe analyzed CC and NC chondrules may overlapforeither

ε

50_Ti,

_ε

54_Cr,_or17_O,_in_{multi-isotope} _space,_none_of_the_CC_chondrules_plot_in_the

compositional field of NC chondrites, and no NC chondrule plots within the field of CC chondrites. Assuch, our data reveal afundamental isotopic differencebetween NC and CC chondrules,whichis inconsistent with adisk-wide transport ofchondrules acrossand between theNC and CC reservoirs. Instead,theisotopicvariationsamongCCchondrulesreflectlocalprecursorheterogeneities,whichmost likely result from mixing between NC-like dust and a chemically diverse dust component that was isotopicallysimilartoCAIsandAOAs.Thesamemixingprocesses,butonalarger,disk-widescale,were likelyresponsibleforestablishingthedistinctisotopiccompositionsoftheNCandCCreservoirs,which representininnerandouterdisk,respectively.

1. Introduction

Chondritic meteorites provide fossilized remnants of the ma-terials present in the early Solar System, and are key samples to unravel the early dynamical evolution of the solar accretion disk(Scott andKrot, 2013).Chondrites areunequilibrated assem-blagesofvarious low- andhigh-temperaturecomponents, includ-ingvolatile-rich,ﬁne-grainedmatrix,Fe-Nimetal,refractory inclu-sions,andtheeponymouschondrules,whichconstituteup to90% ofbulkchondrites(e.g.,Scott,2007).Althoughtheconditions, pro-cess(es), and chronology of chondrule formation remain debated (forareviewseeConnollyandJones,2016),theirubiquitous

pres-*

Correspondingauthor.

E-mailaddress:jonas.m.schneider@uni-muenster.de(J.M. Schneider).

enceinthe diskmakesthema primetracer oftransport,mixing, andaccretionprocessesintheearlySolarSystem.

Utilizingchondrulesinthismannerrequiresunderstandingthe genetic relationships among individual chondrules, and between chondrulesandtheir hostchondrite.Thisinformationmaybe ob-tainedfromisotopeanomalies inindividual chondrules. Ofthese, O isotopes are the most extensively studied, but the O isotope variability among chondrules likely represents a combination of precursor heterogeneity, open-system exchange with the nebular gas,andﬂuid-assistedalterationontheparentbody(e.g.,Clayton, 2003; Kita et al., 2010; Pirallaetal., 2020). As such, the genetic heritageofchondrulescanbediﬃculttointerpretusingOisotopes alone.

Otherisotopetracersthathavebeenusedtoassessthegenetic heritageofchondrulesincludevariationsintherelativeabundance of50Ti and54Cr.Anomalies inthesetwo isotopes are

nucleosyn-https://doi.org/10.1016/j.epsl.2020.116585

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theticinoriginandare powerfultoolstoinvestigategenetic rela-tionships betweenindividual solids, bulk meteorites, and planets (e.g., Gerber et al., 2017; Trinquier et al., 2009; Warren, 2011). Forinstance,isotopicanomaliesin 50_Ti_and54_Cr,_together _with_O

isotopevariations,wereinstrumentalinidentifyingafundamental isotopic dichotomybetweennon-carbonaceous (NC)and carbona-ceous(CC)meteorites(Warren,2011).Subsequentworkhasshown that this dichotomy extends to several other elements, most no-tably Mo(Buddeetal.,2016a;Pooleetal.,2017;Worshametal., 2017). The NC and CC meteorites represent two genetically dis-tinct disk reservoirs thatremained spatially separatedforseveral millionsofyears(Ma),mostlikelythrough theearlyformationof Jupiter(Kruijeretal.,2017).TheNCreservoiristhereforethought torepresenttheprimordialinner,andtheCCreservoirthe primor-dialouterSolarSystem.

The existence oftheNC-CC dichotomy amongbulk meteorites raisesthequestionofwhetherthisdichotomyalsoexistsatthe in-dividualchondrulescale.Thismightbeexpected,giventhatthe di-chotomywaslikelycausedbydistinctdustcompositionsbetween the tworeservoirs. As chondrules providea snapshot ofthe dust composition ata particular time and location in the disk, chon-drules fromthe NC and CC reservoir should have fundamentally different isotopic compositions, at least for elements that deﬁne theNC-CCdichotomy.However,theisotopiccompositionof chon-drules may also vary because the chondrules themselves derive fromdifferentareasofthediskandweresubsequentlytransported to the accretion region of their host chondrite. Currently avail-able CrandTi isotopic datado not allowdistinguishing between thesetwodisparateinterpretationsandhavebeeninterpretedboth assourcesignatures(Olsen etal., 2016) andlocalheterogeneities amongchondruleprecursors(Gerberetal.,2017).

A major problemcommon to previous isotopic studies on in-dividual chondrules is that the isotopic datawere only obtained for a single element. Although such studies reveal isotopic het-erogeneities amongchondrulesforeachinvestigatedelement, the interpretation of a single isotopic signature in terms of large-scale disk processes remains inherently uncertain. To overcome thisshortcoming,weobtainedcombinedCr,Ti,andOisotopicdata foracomprehensivesetofindividualNCandCCchondrules.These data provide new and ﬁrm constraints on the origin of isotope anomaliesinchondrulesandtheirrelationtotheNC-CCdichotomy observedamongbulkmeteorites.Combined,thesenewconstrains haveimportantimplicationsforunderstandingtheearlyevolution ofthesolaraccretiondisk.

2. Materialandmethods

2.1. Samples

Allsamples ofthisstudywere partofthe Tiisotopestudy by Gerberetal.(2017) andincludesinglechondrulesaswellasbulk rocksfromthecarbonaceouschondrites Allende(CV3),GRA06100 (CR2), and NWA 801 (CR2), the enstatite chondrite MAC02837 (EL3) andthe ordinary chondrite Ragland (LL3.4). Forthis study, sevenchondrulesfromAllende,sixfromGRA06100,onefromNWA 801,ninefromRagland,andfourfromMAC02837wereselected.In addition,matrixandpooledAllendechondruleseparates, consist-ingofhundredsofchondrulesandchondrulefragments(Buddeet al.,2016a),werealsoanalyzed.

Aftergentlycrushingpiecesofbulkchondriteinanagate mor-tar,individual chondrules were hand-pickedwithtweezersunder abinocularmicroscope.Wheneverpossible,whole,spherical chon-drules wereselected.Adheringdustwas removedfromchondrule rimsbysonicationinacetonefor15–30min.Theindividual chon-drules were photographed witha Keyence VHX-500F digital mi-croscope(SMFig.S1),andthenbrokeninanagatemortar.A

frag-ment comprising between 5-20 wt.% of the bulk chondrule was embedded into epoxy for petrographic characterization by SEM (Fig. S1) andin-situ Oisotopic analyses by SIMS. The remaining chondrule material, comprising 80-95 wt.% of the original bulk chondrule, was dissolved in purified mineral acids, and Ti was separatedby ion exchange chromatography(Gerber etal., 2017). TheCrisotopemeasurementsofthisstudyweremadeonpurified Cr fromthe column washes of the Ti chemistry of Gerber et al. (2017).Chromiumconcentrationsweredeterminedonthecolumn washespriortoCrseparationbyionexchangechromatography, us-ing aThermoScientific X-SeriesII quadrupole inductivelycoupled plasma-massspectrometer(ICP-MS).

2.2.OxygenisotopemeasurementsbySIMS

Thinsections of the chondrulefragments were examined mi-croscopicallyintransmitted andreﬂectedlight. Scanningelectron microscope observations were performed at CRPG-CNRS (Nancy, France)usingaJEOLJSM-6510with3nAprimary beamat15kV. Cathodoluminescence (CL) imaging of chondrules was performed usingan RGBCL-detection unit attached to a ﬁeld emission gun secondary electron microscope JEOL J7600F at the Service Com-mun de Microscopie Électronique (SCMEM, Nancy, France). We measured the O isotopic compositions of chemically character-izedolivine crystals withaCAMECA ims1270 E7atCRPG-CNRS.

16_O−_,17_O−_,_and18_O− _ions_produced_by_a_Cs+ _primary_ion_beam

(

∼

15 μm,

∼

4nA) were measured inmulti-collection mode with two off-axis Faraday cups (FCs) for 16,18_O− _and _the _axial _FC _for 17_O−_(see _Marrocchi_et_al.,_2018a_)._To_remove16_OH−_interference

on the 17_O− _peak _and _to _maximize _the _ﬂatness _atop _the 16_O−

and18_O−_peaks,_the_entrance_and_exit_slits_of_the_central_FC_were

adjusted to obtain a mass resolution power (MRP) of

∼

7000 for

17_O−_._The _{multi-collection} _FCs_were _set_on _slit₁ _(MRP

₌

_2500).

Totalmeasurementtimeswere240s (180smeasurement

+

60s pre-sputtering).We usedthree terrestrialstandardmaterials (San Carlosolivine,magnetite,anddiopside)todeﬁnetheinstrumental massfractionation lineforthe threeoxygen isotopes andcorrect for instrumental mass fractionation due to the matrix effect of olivine. Toincrease the precision onthe measurements, we con-secutivelyanalyzedfourstandards,eightchondruleolivinecrystals, andfourstandards.TypicalcountratesobtainedontheSanCarlos olivine standards were 2

.

5

×

109 _cps _for 16_O, ₁

_.

₀

_×

₁₀6 _cps _for 17_O, _and₅

_.

₄

_×

₁₀6 _cps _for18_O. _Data _are_given_in _the_delta

nota-tion

δ

17,18O (

=

[(17,18O/16Osample/17,18O/16OSMOW)

−

1]

×

1000), and

capital delta notation

17O (

= δ

17_O

₋

_0.52

_×δ

18_O). _Depending _on

samplehomogeneityandsize,between2and25individual mea-surements were obtained on each chondrule fragment. Only one measurementperolivinegrainwascarriedoutandeachSIMSspot was then checkedby SEM.Analyses fromcracks orgrain bound-aries were not included. The 2

σ

errors were

∼

0.2

for

δ

18O,

∼

0.4

for

δ

17O,and0.9

for

17O.Theerroron

17Owas cal-culatedbyquadraticallysummingtheerrorson

δ

17Oand

δ

18Oand thestandarddeviationof

17Ovaluesofthefourterrestrial stan-dards.

2.3.ChromiumseparationandisotopicmeasurementbyTIMS

Chromium was separated from the sample matrix using a three-stageionexchangechromatography.Theﬁrsttwostepswere similarto the procedures fromYamakawaetal.(2009) and Trin-quier et al. (2008), employing both anion and cation exchange resins.Theelutionprocedureonthethirdcolumn(cationexchange resinAG50W-X8,200-400 mesh)was slightlymodiﬁed compared to the two aforementioned studies. The Cr cut from the second columnwasloaded in0.5MHNO3 andresidual matrixelements

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1MHCl,beforeCrwas elutedwith10mlof2MHCl.Toremove residualorganics, theCrcut was thentreatedwith2

×

4 dropsof

aquaregia at 120◦C, 2

×

2 dropsof concentrated HNO3 at120◦C,

and2

×

1drop ofH2O2 at80◦C.The sample was then converted

withconcentratedHCltwice beforeit wastakenup inan appro-priate volumeof6MHCltoobtaina

∼

500 μg/gloadingsolution. The Cr yield of the full chemical procedure was determined on aliquotsbyICP-MS,andwasgenerally

>

70%.Aliquots oftheNIST 3112astandardprocessedthroughthesamechemistryyielded in-distinguishableisotopicresultsasunprocessedstandards.

TheCrisotopemeasurementswereperformedona ThermoSci-entiﬁcTritonPlus thermalionizationmassspectrometer (TIMS)at theInstitutfürPlanetologieinMünster.Approximately1 μgCrper sample was loaded onto single outgassed Re ﬁlaments and cov-ered with an Al-Si-gel emitter. The emitter was made from 3 g of Aerosil 300 (Evonik) Si-Gel in 60 ml MQ water, a portion of which was then mixed 1:1 with a 5 g/L H3BO3 solution and a

1000 μg/g Al solution. Ion beams were simultaneously collected instaticmodeforallfourchromiumisotopes(50Cr,52Cr,53Cr,and

54_Cr)_using_Faraday _cups _connected_to₁₀11

_feedback_resistors.

The signalintensitywas usually

∼

10 Von52Crand200-250 mV on54Cr.Possibleinterferencesfrom50Ti,50V,and54Fewere mon-itored using 49_Ti, 51_V, _and 56_Fe _beams _in _cups _L4, _L1, _and _H4,

respectively. However, Ti and V signals were not detected above baselineduring themeasurementsofthisstudy,andinterferences fromFewereminimal.Measured56_Fe/52_Cr_ratios_were_typically

_<

10−5_,_well_below_the₁₀−4_threshold_for_successful_Fe_interference

correction(Qinetal.,2010).WetestedtheaccuracyoftheFe inter-ference correction bymultipleanalyses ofFe-dopedCrstandards. Repeatedmeasurementofindividualﬁlaments showedadecrease intheFesignalovertime,butnochangeintheﬁnal interference-corrected54Cr/52Crratio.

Dataacquisitionconsistedof50blocksof30cycleseach(1500 totalcycles),4s integrationtimepercycle, anda baselineand re-focuseverythreeblocks.Instrumentalmassfractionationwas cor-rectedbyinternalnormalizationto50_Cr/52_Cr

₌

_0.051859_(Shields

et al., 1966) usingthe exponential law(Russell et al., 1978). All data are reported in

ε

-units as the parts-per-ten-thousand de-viation from the 53_Cr/52_Cr _and 54_Cr/52_Cr _measured _for _the _NIST

SRM3112aCrstandardineachsession.Repeatedmeasurementsof the NISTSRM3112a Crstandard ineach session(n

=

7–14) pro-vided an external reproducibility (2s.d.) of

±

0.2

ε

53_Cr _and

_±

_0.3

ε

54_Cr. _Depending _on _the _total _amount _of _Cr _available, _samples

were loaded on 1-3 filaments and each filament was measured multipletimesifpossible,resultinginuptosevenindividual mea-surements for individual chondrules. For the pooled chondrule separates,matrixfractions,andbulk Allende,thenumberof mea-surementswaslarger asmoreCrwasavailable forthesesamples. Reported uncertainties for sample measurements are 95% confi-dencelimitsofthemean(n

≥

4),ortwicethestandarddeviation obtained for the NIST SRM3112a Cr standard that was analyzed duringthesamesession(n

<

4).

3. Results

3.1. PetrographyandOisotopes

Petrographic characteristics of the individual chondrules are provided in Table 1and inFig. S1. Exceptforone barred olivine (BO)chondrule,and oneAl-rich chondrule,all investigated chon-drules fromtheCVchondrite Allendeareporphyriticolivine (PO) andolivine-pyroxene(POP)type I(lowFeO)chondrules.All inves-tigatedchondrulesfromtheCRchondritesGRA06100andNWA081 are porphyritic olivine-pyroxene-rich (POP)type Ichondrules. All butonechondrulefromtheordinarychondritesRaglandaretypeII chondrules(75

<

Mg#

<

90)andincludeporphyriticchondrules(PO,

Fig. 1. Oxygenthree-isotopeplot.CCchondrulesplotalongtheCCAMlinewitha slopeofapproximately1.Thecomparativelylargeerrorsareduetointra-chondrule heterogeneitywithsingleolivinegrainsbeingisotopicallydistinctfromoneanother. OCandECchondrulesdonotdisplaysigniﬁcantintra-chondrulevariationandplot onorabovetheterrestrialfractionationline(TFL)withaslopeof0.52.

POP),radialpyroxenechondrules(RP,withsystematicpresence of smallolivine grains)andone crypto-crystalline (C) chondrule. Fi-nally,thefourinvestigatedchondrulesfromtheenstatitechondrite MAC02837 aretype Iporphyriticpyroxene(PP) andradial pyrox-ene (RP) chondrules. The diameters of the analyzed chondrules rangefrom0.7to 1.9mm forCC chondrules,from1.0to2.7mm forOC,andfrom0.9to1.7mmforECchondrules.

TheOisotopiccompositionsofthechondrules areprovidedin Table 1, and plotted in Fig. 1. The data of the individual analy-sesfor each chondrule are givenin the supplementary materials (SM, Table S1). For most chondrules, measured olivine grainsof agivenchondruleexhibithomogeneous

17Owithinuncertainty. Forthesesamples averagingthe individual olivine measurements providesagood estimateofthebulk chondrule’sOisotopic com-position(TableS1).However,threeCVandtwoCRchondrules con-tain relict grains,which are deﬁnedashavinga

17Ovalue that differsfromthatoftheirhostchondruleoutsideof3

σ

(Ushikubo etal., 2012). An SEM image of the CV chondrule fragment with themost16O-richrelictgrain(

17O

= −

14

)isshowninFig.2, alongwithanCLimageofthesamesample.Forthesampleswith relictgrains,averagingoftheolivine

17Ovaluesprovidesabiased estimateofthebulkchondrulecompositiontowardsthe composi-tion of the relict grains. This is because we have not randomly sampledthe chondrules, andchondrules do not consist solelyof olivine. Prior studies haveshown that low-Capyroxenes are iso-topicallyhomogeneousforO, andthatthey havethesameO iso-topiccompositionsasthehostolivines(Tenneretal.,2018,2015). Wethereforecalculatedthe“bulk”O-isotopiccompositionof chon-drules having relict grainsby considering the host olivines only. Importantly,theOisotopic compositionsofhostmineralsreﬂects contributionsfromOinheritedfromchondruleprecursors(through relict mineraldissolution) andfromO comingfrom the gas(i.e., SiO,H2O;LibourelandPortail,2018;Marrocchi etal.,2019). Our

bulk estimate, therefore, accountsfor the effectof relict mineral dissolutionandshouldbeclosetotheactualbulkOisotopic com-position(TableS1).

In an O three-isotope diagram the bulk CC chondrules plot alongaslope

∼

1linebelowtheterrestrialfractionationline(TFL), whereas the NC chondrules are clustered on or above the TFL (Fig. 1). The ordinary chondrite chondrules display indistinguish-able

17Ovaluesaveragingat1.4

±

0.7

.Similarly,theenstatite chondritechondrulesexhibithomogeneous

17_O_with_an_average

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Table 1

Chromium,titaniumandoxygenisotopicandconcentrationdataforterrestrialsamples,bulkchondritesandchondrules. Sample Typea _∅ [mm]b Weight [mg]c Cr [ppm]d ne _ε53_Cr_±₂_σf _ε54_Cr_±₂_σf _Ti [ppm]b ε50_Tib_±₂_σ 17_O_±₂_σ Terrestrial DTS2b Silicatesg ₂₇ ₁₅₅₁ ₈ _0.12_±_0.07 _0.16_±_0.13 DTS2b Chromitesg ₇ ₋_0.03_±_0.20 ₋_0.02_±_0.34 JA-2 161 423 5 0.03±0.16 0.13±0.28 BIR1a 240 392 3 0.08±0.17 0.07±0.30 Average 0.05±0.13 0.08±0.16 OC (Ragland - LL3.4)

MSc42 Type II, POP 2.7 17 4108 1 0.40±0.10 −0.24±0.24 601 −0.43±0.14 1.28±1.24 MSc43 Type II, RP 2.1 14 4613 5 0.45±0.17 −0.11±0.18 327 −0.41±0.13 0.82±0.63 MSc44 Type II, RP 2.4 13 4693 3 0.25±0.15 −0.34±0.31 550 −0.48±0.13 1.58±2.25 MSc45 Type II, RP(O) 2.6 18 3959 4 0.37±0.16 −0.14±0.34 803 −1.00±0.12 1.83±0.97 MSc49 Type II, RP 1.4 1.8 4316 3 0.74±0.13 −0.39±0.32 439 −0.67±0.56 1.43±0.60 MSc50 Type II, PO 1.4 2.4 3870 741 −0.53±0.08 1.15±0.39 MSc51 Type II, C 1.0 1.2 4593 2 0.47±0.13 −0.51±0.32 553 −0.58±0.08 1.79±0.83 MSc52 Type II. POP 1.4 2.8 800 −0.60±0.09 1.09±1.29 MSc53 Type II, RP 1.5 1.2 472 −0.24±0.15 1.07±1.09 Average 1.8 4307 0.45±0.33 −0.29±0.31 587 −0.55±0.42 1.34±0.69 Std8 Whole rock 51 3034 4 0.21±0.06 −0.31±0.19 606 −0.70±0.13 EC (MAC02837 - EL3) MSc55 Type I, PP 1.3 1.3 2200 443 −0.13±0.11 −0.20±0.54 MSc57 Type I, PP 1.2 0.9 2217 546 −0.21±0.13 −0.02±0.44 MSc58 Type I, RP 0.9 0.7 1865 408 −1.47±0.19 −0.08±0.81 MSc61 Type I, PP 1.7 4.4 796 1 0.18±0.06 0.01±0.19 137 −0.28±0.08 0.38±0.94 Average 1.3 1769 384 −0.52±1.27 0.02±0.51 Std9 Whole rock 54 2843 1 0.16±0.06 0.02±0.19 486 −0.40±0.12 CV (Allende - CV3) MSc2 Type I, PO 1.5 1.5 2493 5 0.12±0.25 1.24±0.14 1190 4.85±0.16 −5.21±1.29 MSc3 Type I, PO(P) 1.2 1.7 2670 5 0.07±0.16 0.71±0.19 793 −0.15±0.14 −5.95±0.75 MSc5 Type I, BO 1.2 1.3 3295 4 0.05±0.17 1.16±0.21 1401 1.73±0.18 −4.82±1.22 MSc8 Type I, POP 1.9 5.7 3121 7 0.13±0.10 1.43±0.16 775 −0.05±0.08 −2.98±1.21 MSc10 Type I, PO 1.4 2.0 2844 6 0.07±0.12 0.82±0.15 1015 1.30±0.11 −4.99±1.90 MSc12 Type I, PO 1.8 2.9 3307 6 0.07±0.12 0.09±0.11 953 1.56±0.20 −5.43±1.45 MSc14 Al-rich 1.8 4.2 2987 3 −0.10±0.13 0.44±0.32 1760 7.32±0.09 −5.74±1.13 Average 1.5 2960 0.06±0.15 0.84±0.95 1127 2.36±5.48 −5.02±1.97 AL 25 Chondr. fraction 0.25−1.6 302 17 0.07±0.09 0.55±0.23 1204 2.50±0.12 AL 32 Chondr. fraction 0.5−1.0 384 10 0.03±0.08 0.65±0.19 1161 2.17±0.15 AL 36 Chondr. fraction 0.5−1.0 518 15 0.05±0.08 0.55±0.18 1029 2.24±0.14 Average 0.05±0.04 0.58±0.11 1131 2.30±0.35 AL 26 Matrix fraction 506 14 0.30±0.07 1.18±0.14 656 2.68±0.07 AL 27 Matrix fraction 394 17 0.15±0.04 0.98±0.13 663 1.91±0.19 AL 42 Matrix fraction 469 8 0.17±0.09 1.01±0.19 696 2.25±0.15 Average 0.21±0.16 1.06±0.22 672 2.28±0.78

Allende 2 Whole rock 36 6 0.12±0.12 0.96±0.22 Allende 2 Whole rock 41 4 0.11±0.10 1.06±0.30

Ti1 Whole rock 37 3970 4 0.13±0.20 1.06±0.34 731 3.31±0.14 Ti2 Whole rock 32 4013 12 0.20±0.06 0.99±0.11 826 3.22±0.34 Std1 Whole rock 10 3198 6 0.19±0.07 1.18±0.18 729 3.20±0.16 Average 3727 0.15±0.08 1.05±0.17 762 3.24±0.12 CR (GRA06100 - CR2) MSc72 Type I, POP 0.9 0.9 3984 2 0.06±0.13 1.25±0.30 873 2.21±0.16 −2.28±0.60 MSc73 Type I, POP 0.7 0.4 4952 842 2.13±0.26 −2.58±1.69 MSc74 Type I, POP 1.1 0.9 3880 2 0.27±0.13 1.39±0.30 624 1.98±0.23 −1.72±0.47 MSc75 Type I, POP 1.1 1.0 5617 977 1.79±0.16 −3.65±0.72 MSc76 Type I, POP 1.2 1.9 3778 2 0.21±0.13 1.20±0.30 782 1.41±0.15 −3.88±0.86 MSc77 Type I, POP 1.8 4.6 3706 2 0.24±0.13 1.37±0.30 789 1.21±0.08 −0.89±1.00 Average 1.1 4320 0.20±0.19 1.30±0.18 814 1.79±0.80 −2.50±1.14 Std10 Whole rock 51 3096 3 0.26±0.13 1.23±0.30 715 3.26±0.09 CR (NWA 801 - CR2) MSc78 Type I, POP 1.6 1 3393 642 3.39±0.17 −1.75±1.10

a _Type_I_chondrules_have_Fa-content_<_10,_type_II_chondrules_>_10;_{PP—porphyritic}_pyroxene,_{PO—porphyritic}_olivine,_{POP—porphyritic}_{olivine-pyroxene,}_BO—barred_olivine,

RP—radialpyroxene,C—cryptocristalline.

b _Data_from_Gerber_et_al.₍₂₀₁₇_).

c _Mass_denotes_digested_sample_mass,_for_chondrules_this_represents_between₈₀_and_95%_of_chondrule_mass. d _Error_on_Cr_{concentration}_measurements_is_estimated_to_be_10%.

e _Number_of_individual_Cr_isotope_{measurements.}

f _Chondrule_data:_for_n_≥₄_the_95%_conﬁdence_interval_is_used,_for_n_≤₄_the_2s.d._of_the_NIST3112a_standard_of_the_respective_session_is_used,_for_averages_the_2s.d._is_given. g _Bulk_powders_of_DTS2b_were_digested_in_aqua_regia_and_subsequent_HF-HNO

3.TheresidualchromitesweredigestedinParrbombswith2mlconc.HNO3at190◦Cfor4

(6)

Fig. 2. (a)Back-scatteredelectronimageofPOchondruleMSc10fromAllende(CV3) showingthelocationsofionprobemeasurements(coloredellipses).Thecolors in-dicatetherangeof17Ovalues.(b)CLimagerevealinginternalstructuresofolivine grains.16_O-rich_relict_olivine_grains_show_no_CL_emission_due_to_low_abundances_of

CLactivators(e.g.,Ca,Al,Ti)whilehostolivinegrainsshowCLemissioninducedby higherconcentrationofCa,Al,andTi.

value of 0.02

±

0.51

. The CR chondrules display more variable

17Owithvaluesfrom

−

3.9to

−

1.5

,andforchondrulesfrom Allende

17Ovariesfrom

−

3to

−

6

.Itisimportanttorecognize that theuncertaintyonthebulk

17_O_for_some_of_the_individual

CCchondrules islargerbecausethechondrulesareinternally het-erogeneous.Thus, withinandamongindividual CCchondrules(in particularCVchondrules)thereisOisotopevariability.Bycontrast, different grains within individual NC chondrules are isotopically homogeneous,andvariabilityamongNCchondrulesislimited.

3.2. Chromiumisotopes

The Cr isotope data are reported in Table 1. In addition to the chondrules andbulk chondrites, we also report data for the terrestrialrockstandardsBIR1a,DTS2b,andJA-2.Thesewere pro-cessed through the full analytical procedures alongside the me-teorite samples to assess the accuracy and precision of the Cr isotopemeasurements.The

ε

53_Cr_values_of_the_terrestrial_samples

are indistinguishablefromNIST SRM3112a,buttheir

ε

54_Cr _seems

tobeoffsettoslightlyhighervalues.Thisisconsistent with simi-larsmalloffsetsreportedformeasured

ε

54_Cr_of_terrestrial_basalts

inpriorstudies(Mougeletal.,2018;Qinetal.,2010).Similar off-setsofnaturalrocksfromstandardsolutionsareobservedforother

Fig. 3.ε54_Cr_values_of_individual_chondrules._Compositions_of_respective_bulk

chon-dritesareshown assolid lines;shaded areasaround theselinesindicate2s.d. uncertaintiesofthebulkvalues.Boldsymbolsrepresentdatafromthisstudy,open symbolsareliteraturedata(Trinquieretal.,2008;Yamashitaetal.,2010;Connelly etal.,2012;Olsenetal.,2016;Zhuetal.,2019).Bulkchondritevaluesarefromthis study(EC,OC,CR,andCV),andQinetal.(2010) (CB,CO).

elements(e.g.,Ti,Mo) andarelikelyduetonon-exponential frac-tionation processes, possibly induced during standard production (Zhangetal.,2011;Buddeetal.,2019).

TheCrisotopicdataforthebulkchondritesamplesarealsoin good agreement with previously reported data for samples from thesamechondritegroups(Trinquieretal.,2007;Qinetal.,2010; Göpeletal.,2015).Combined,thedataforterrestrialsamplesand bulkchondritesthereforeshowgoodinter-laboratoryagreement.

Individualordinarychondritechondrulesdisplayvariable

ε

53_Cr

rangingfrom

∼

0.25to

∼

0.74, whichformostchondrulesarealso higher than the bulk ordinary chondrite value. Chondrules from the other chondrite groups investigated in this study show less variabilityand are more similar to the valuesof their respective hostchondrites. Thevariable

ε

53_Cr _among_the _OC_chondrules

al-mostcertainlyreﬂectsradiogenic variationsresultingfromthe de-cay of short-lived 53Mn (t1/2

∼

3

.

7 Ma). However, asthe Mn/Cr

ratios of the chondrules were not determined in this study, the magnitudeofpotentialradiogenic

ε

53_Cr_variations_cannot_be

quan-tiﬁed. Regardless, the main focus of this study is on nucleosyn-thetic

ε

54_Cr _variations, _and _the

_ε

53_Cr _will _therefore _not _be

(7)

The

ε

54_Cr _values _of_the _EC, _OC, _and_CC _chondrules _and_their

hostbulk chondrites areshowninFig. 3. TheOCchondrules dis-playnoresolvable

ε

54_Cr_variations_and_their_mean

_ε

54_Cr_are

indis-tinguishable from thehost chondrite’s

ε

54_Cr. _Likewise, _the

_ε

54_Cr

ofthe singleEC chondruleofthisstudy isindistinguishablefrom the

ε

54_Cr _of _bulk _enstatite _chondrites. _Compared _to _the _EC _and

OCchondrules,allCRchondruleshaveelevated

ε

54_Cr,_and_most_of

them areindistinguishable fromthe

ε

54_Cr _of_bulk _CR_chondrites.

By contrast,individual Allende chondrulesexhibit highly variable

ε

54_Cr, _with _a _continuum _from

_∼

_0.1 _to

_∼

_1.4

_ε

54_Cr _(Fig. ₃_). _The

ε

54_Cr_results_for_CR_and_CV_chondrules _are_consistent _with_those

of Olsen et al.(2016), although these authors reported a some-what larger range of

ε

54_Cr _values_among _CR _and_CV _chondrules

(Fig.3).Finally,thethreepooledchondruleseparatesfromAllende, each consisting of hundreds of chondrules (Budde etal., 2016b), areindistinguishablefromoneanotherandhaveanaverage

ε

54_Cr

of0.58

±

0.12.ThisvalueisresolvedfromthebulkCVcomposition (

ε

54_Cr

_∼

₁₎ _and_from_Allende_matrix,_which_has_an _average

_ε

54_Cr

of

∼

1.

3.3. Titaniumisotopes

To facilitate direct comparison of the O and Cr isotopic data withpreviouslyobtainedTiisotope dataon thesamechondrules, theTi concentrationand

ε

50_Ti_data_from_Gerber_et_al.₍₂₀₁₇_{) are}

also provided in Table 1,and the resultsare brieﬂy summarized here.WhereasECandOCchondrulesexhibitonlysmall

ε

50_Ti

vari-ations relative to the composition of their host chondrites, CC chondrules are characterized by a broad range of

ε

50_Ti _values,

rangingfromtheslightlynegative

ε

50_Ti_typically_observed_for_OC

chondrules tothelarge

ε

50_Ti _excesses_known_from_Ca–Al-rich

in-clusions (CAIs) (Fig. 4). The CC chondrules also display a broad correlation of

ε

50_Ti _and_Ti _{concentration,} _indicating _the _addition

ofisotopicallyheterogeneousandTi-rich—i.e.,CAI-like—materialto enstatiteandordinarychondrite-likechondruleprecursors(Gerber etal.,2017).

4. Discussion

WhilethenewOandCrisotopedataforsingle chondrulesare consistent with results of previous studies, the major advantage of thisstudy is that,for theﬁrst time, the Cr, O, andTi isotopic data havebeen obtainedforthe same individual chondrules.We willshow belowthat althoughtheisotopicdataforeachelement alone reveal isotopic heterogeneity among chondrules, only the combineddataofallthreeelementsmakeitpossibleto disentan-gletheprocess(es)responsiblefortheseheterogeneities.

ThekeyobservationsofthecombinedCr,Ti,andOisotopicdata maybesummarizedasfollows(Fig.4).First,OCandECchondrules displayrelativelyhomogeneous

17O,

ε

50_Ti,_and

_ε

54_Cr,_which_are

indistinguishable from the compositions of their respective host chondrites.Second,CCchondruleshave,onaverage,moreelevated

ε

50_Ti_and

_ε

54_Cr,_and_lower

17_O_compared_to_NC_(i.e.,_OC_and_EC)

chondrules. Although the compositionsof CC and NCchondrules mayoverlapforeither

ε

50_Ti,

_ε

54_Cr,_or

17_O,_in_{multi-isotope}_space

noneoftheinvestigatedCCchondrulesplotsinthecompositional ﬁeldofNCchondrites,andnoneoftheinvestigatedNCchondrules plotswithintheCCﬁeld(Fig.4a-c).Finally,incontrasttoNC chon-drules, CC chondrules—and in particular CV chondrules—exhibit variableisotopeanomaliesforallthreeelements.Thedistinct iso-topic compositionsof NC andCC chondrules for a range of geo-andcosmochemiallydifferentelements,combinedwiththe obser-vation of isotopic homogeneity in NC chondrules versus isotopic heterogeneity in CC chondrules represents a fundamental differ-encebetweenNCandCCchondrites.

Below, we ﬁrst attempt to identify the carrierof the isotopic anomaliesamongthechondrules,andthenusethecombined iso-topicdatatoevaluate mixingandtransport processesbefore and duringchondrite parentbody accretion.Finally,we willplacethe chondrulecompositionsintothebroadercontext oftheevolution ofdistinctdiskreservoirs,namelytheNCandCCreservoirs.

4.1. Originofisotopicheterogeneitiesamongchondrules

When the

ε

54_Cr _data _for _chondrules _from _NC _and _CC

chon-dritesare consideredtogether,chondrulesfromchondriteswitha higherabundanceofCAIs(i.e.,CV,CO)exhibitmore

ε

54_Cr

variabil-ity compared to chondrites with lower CAI abundances (i.e., EC, OC,CR). The same observation holds for

ε

50_Ti _variations _among

chondrules,whichhavebeenattributedtotheheterogeneous dis-tribution of CAIs among the chondrule precursors (Gerber et al., 2017). Speciﬁcally, the

ε

50_Ti _variations _among_the_CV _chondrules

are correlated withtheir Ti content, consistent withthe variable additionof50Ti- andTi-rich CAIstochondruleprecursors(Gerber etal.,2017).Bycontrast,thereisnoobviouscorrelation between

ε

54_Cr _and_Cr _{concentration} _among_the _CV _chondrules, _neither_in

thedata ofthis study,nor when datafor CVand COchondrules frompreviousstudiesarealsoincluded(Olsenetal.,2016;Zhuet al., 2019). The

ε

54_Cr _anomalies _in_the _chondrules_may, _therefore,

reﬂectadmixtureof anisotopicallyhighly anomalous component, in which case small amounts of admixed material are suﬃcient togeneratetheobserved

ε

54_Cr_variations,_while_the_Cr

concentra-tionswouldremainunaffected.Alternatively,theadmixedmaterial isisotopicallylessanomalous andhada Cr concentration similar tothechondrulesthemselves.

Materialsknown to have signiﬁcant 54Cr excesses andwhich may be capable of shifting bulk isotopic compositions include presolargrainsandCAIs.Forinstance,Cr-bearingnanospinels iden-tiﬁedinthematrixofprimitivecarbonaceouschondriteshave el-evated 54Cr/52Cr ratios of up to 50x the solar composition (Qin etal., 2010; Dauphaset al., 2010; Nittler etal., 2018). Toassess whether these grains can account for the observed

ε

54_Cr

varia-tionsamongchondrules, wecalculatedthe numberofnanospinel grainsthat wouldneed to be added toan averageCV chondrule togeneratetheobserved1-2

ε

54_Cr_variations._Using_typical_values

forCV chondrules(1 mg; 3000 μg/gCr)andpresolarnanospinels (80-200 nm; 2.5x solar 54_Cr/52_Cr; _Dauphas _et _al., ₂₀₁₀_; _Nittler

etal., 2018), we ﬁnd that tens of thousands ofgrains would be neededtogeneratethe observed 1-2

ε

54_Cr _variations _among_the

chondrules. The basicpicture doesnot change even when grains withthe highest reported anomalies (56x solar) are used in the calculation,sincethe 54Cr anomaliesofthenanospinelgrainsare inverselycorrelatedwiththeirsize(Nittleretal.,2018).Therefore, if presolar nanospinels were the only source of

ε

54_Cr _variations

amongchondrules,thentheamountofpresolargrainsmixedinto differentchondrules wouldhave to varyby tens of thousandsof grainsfromone chondrule tothe next. Such a scenariois highly unrealisticintermsofmixingdynamicsofdustin thesolar neb-ula,andcontrasts withtheoverall rarityofpresolargrainsinthe meteoriterecord.Assuch,presolarnanospinelsdonotseemtobe responsibleforthe

ε

54_Cr_variability_among_individual_chondrules.

Aswas shownbyGerber etal.(2017), Tiisotope variationsin CCchondrulesweremostlikelycausedbyincorporationofvariable amountsofrefractory inclusionssuch asCAIs.However, although CAIshavewellresolved

ε

54_Cr_excesses,_they_contain_too_little_Cr_to

signiﬁcantlyaffecttheCrisotopic compositionofchondrules, and soadmixtureofCAIstochondruleprecursorsresultsinastrongly curvedmixinghyperbolain

ε

50_Ti_vs.

_ε

54_Cr_space_with_little

over-allvariationsin

ε

54_Cr_(Fig.₄_d)._Accordingly,_the_chondrules_of_this

study do not plot on a mixing line betweenNC chondrules and CAIs (Fig. 4d-f). Moreover, there are chondrules with

ε

54_Cr

(8)

ex-Fig. 4.17_O,_ε50_Ti,_and_ε54_Cr_of_individual_chondrules_in_comparison_to_bulk_NC_and_CC_meteorites_(a-c)_and_CAIs/AOAs_(d-f)._The_NC_(red)_and_CC_(blue)_ﬁelds_represent

therangeofanomaliesmeasuredforbulkmeteoritesfromtheNCandCCreservoir,respectively.TheNCandCCﬁeldsaswellasrepresentativeCAIisotopiccompositions arebasedondatacompiledinBurkhardtetal.(2019).Bulkchondritedatapoints(opensymbols)areonlyshownforsamplesfromwhichindividualchondrules(ﬁlled symbols)wereanalyzedinthisstudy.(a)-(c)NCchondrulesdisplayrelativelyhomogeneous17O,ε50_Ti,_and_ε54_Cr,_which_are_{indistinguishable}_from_the_compositions_of

theirrespectivehostchondrites,whileCCchondruleshavemorevariableε50_Ti,_ε54_Cr,_and17_O._Note_that_although_the_compositions_of_CC_and_NC_chondrules_may_overlap

foreitherε50_Ti,_ε54_Cr,_or17_O,_in_{multi-isotope}_space_no_CC_chondrule_plots_within_the_NC_ﬁeld,_and_no_NC_chondrule_plots_within_the_CC_ﬁeld._Also_note_that_most

CCmeteoritesareaffectedbyaqueousalterationwhichmaymodifytheirpristine17_O_through_the_interaction_with16_O_poor_water(-ice)_(Piralla_et_al.,₂₀₂₀_)._Alteration

corrected17_O_values_for_bulk_carbonaceous_chondrites_would_thus_plot_at_slightly_lower17_O._(d)-(f)_The_composition_of_all_chondrules_can_be_reproduced_by_mixing

betweenanNCcomponentwithchondriticCr/Ti/OratiosandanisotopicallyCAI/AOA-likecomponent(IC)having morevariableCr/Ti/Oratios.Crossesonmixinglines indicate20%stepsofICcomponentinNC-ICmixture.(Forinterpretationofthecolorsintheﬁgure(s),thereaderisreferredtothewebversionofthisarticle.)

cesses butno

ε

50_Ti _anomalies,_indicating _that _the

_ε

54_Cr_excesses

are unrelatedtotheadditionofCAIs.Theseobservationsruleout CAI admixture as a signiﬁcant driver for

ε

54_Cr _variations _among

chondrules,andshowthat the50_Ti_and54_Cr_anomalies_are_carried

bydistinctmaterials.

Amoeboid olivine aggregates (AOAs) are another component that isabundant incarbonaceous chondrites andmaybe respon-sible for the

ε

54_Cr _variations _among _chondrules. _Of _note, _AOAs

are characterized by similar

ε

54_Cr _and

_ε

50_Ti _excesses _than _CAIs

(Trinquieretal.,2009),butcanhavemuchhigherCr/Ti(Komatsu et al., 2001; Sugiura et al., 2009), reﬂecting their less refractory character comparedto CAIs.Moreover, there isgrowing evidence thatCCchondrulesinheritedprimitive16_O-rich_precursors

includ-ingnotonlyCAIsbutalsoAOA-likerelictsilicategrains,suggesting admixtureofnon-CAIbutisotopicallyanomalousmaterialto chon-druleprecursors(Kita etal.,2010;Marrocchietal., 2019,2018a). This isalso consistent with the observation that samples ofthis studyincludechondrulescontainingrelictolivinegrainswith

(9)

neg-ative

17_O _of _up _to

₋

₁₄

_. _Moreover, _when _the _Cr, _Ti, _and _O

isotope variations among the chondrules are considered together (Fig.4d-f),itisevidentthatadditionofCAIsalonecannotaccount fortheobservedvariations.Instead,componentswithCAI-like iso-topiccompositionsbutlessrefractorychemicalcompositions,such asAOAs,musthavebeenpartofchondruleprecursors.

To more quantitatively assess as to whether the addition of AOA-like material is consistent with the observed isotopic vari-abilityamongchondrules,wecalculatedmixinglinesbetweentwo chemicallyandisotopicallydistinctdustcomponents.One compo-nent is characterized by a chondritic chemical and NC-like iso-topic composition. The other component hasa CAI/AOA-like iso-topic composition and is characterized by variable refractory to non-refractory element ratios; this range in compositions is re-ﬂectedinincreasingCr/TifromtypicalCAI-likevaluestothemore elevatedvaluesobservedforAOAs.Theresultsofthemixing calcu-lationsshowthatheterogeneousmixingofdifferentchemical com-ponentsfromtwobulkdustreservoirsreproducestheentirerange ofobserved chondrulecompositionsquitewell (Fig. 4d-f).For in-stance, while the extreme composition of the Al-rich chondrule MSc14 fromAllende (

ε

50_Ti_of

_∼

_7,

_ε

54_Cr

_∼

_0.4)_reﬂects _admixture

ofrefractory,CAI-likematerial,chondruleswith

ε

54_Cr_excesses_but

NC-like

ε

50_Ti_(e.g.,_Allende_chondrules_MSc₃_and_MSc₈₎_are

con-sistent with addition of material with AOA-like Cr/Ti (Fig. 4d). The mixing calculationsinvolving Oisotopes are less straightfor-ward because the O concentrations of the mixing endmembers are not well known. Nevertheless, assuming a reasonable range of O/Cr and O/Ti ratios for the mixing endmembers [O concen-trations are based on the typical modal mineralogy ofOCs (Mc-Sween et al., 1991), CAIs (Grossman, 1975) and AOAs (assuming pure forsterite)], we find that all CC chondrules fall within the range of possible mixing trajectories (Fig. 4e). Finally, it is note-worthy that although the NC chondrules of this study fully plot within thecompositionalfieldsofbulkNCbodies,they neverthe-lessseemtodefineaweaktrendtowardstheisotopiccomposition ofCAIs/AOAs(Fig. 4).Thismaysuggestthatsmallamountsof iso-topicallyCAI-likematerialwerealsopresentamongtheprecursors ofNCchondrules.However,investigatingtherelevance andorigin ofthistrendwillrequireTi andCrisotopemeasurements ofrare OCchondruleswithrelict16_O-rich_grains_(Kita_et_al.,₂₀₁₀_).

In summary, the Cr-Ti-O isotopic heterogeneity among single chondrulescanbeaccountedforbymixingofanNCdust compo-nentwithchemicallyvariabledustcomponentshavinga CAI/AOA-likeisotopiccomposition.Thismixingmodelisconsistentwiththe higher abundance of CAIs and AOAs in carbonaceous chondrites, andtherarityofthesecomponentsinNCchondrites.

4.2. Chondruletransportversusprecursorheterogeneity

Because the range of

ε

54_Cr _values _observed _among _CV

chon-drules is similar to the range measured among bulk meteorites, Olsenetal.(2016) arguedthatachondrule’s

ε

54_Cr_reﬂects_its

spe-ciﬁcformationlocationwithinthedisk.Thisinterpretationimplies that chondrules with distinct

ε

54_Cr _formed _in _different _regions

of the disk. As such, the variable

ε

54_Cr _of _CV _chondrules _would

in turn require formation of chondrules from a given chondrite group across large areas of the disk, and the subsequent disk-wide transport ofthe chondrules tothe accretionregion oftheir respective host chondrites. Thisinterpretation of the chondrules’ isotopic compositions assource signatures is very different from the interpretation put forwardabove, in which the isotopic het-erogeneities are inherited signatures of chondrule precursors. As notedabove,whileitisdiﬃculttodistinguishbetweenthesetwo interpretations basedsolelyonisotopic datafora single element, the combined Cr-Ti-O isotopicdata from thisstudy allow distin-guishingbetweensourceandprecursorsignatures.

Ifthe isotopic variations wouldreﬂect distinct source regions ofchondrules,thentheir

17O,

ε

50_Ti,_and

_ε

54_Cr_signatures_should

becorrelated.Forexample,a chondrulefromtheinner Solar Sys-tem(i.e.,the NC reservoir)that was transportedto the accretion regionofCVchondrites shouldnot onlyexhibit anNC-like

ε

54_Cr,

butalsoNC-like

17Oand

ε

50_Ti._Conversely,_a_CC_chondrule_from

theouterSolarSystem(i.e.,theCCreservoir)shouldhaveCC-like

17O,

ε

50_Ti,_and

_ε

54_Cr_anomalies._However, _among_the_CV

chon-drulestherearesampleswithNC-like

ε

50_Ti_and_CC-like

_ε

54_Cr,_and

therearealsoCVchondruleswithCC-like

ε

50_Ti_but_NC-like

_ε

54_Cr.

Moreover,in

17_O_versus

_ε

54_Cr_space,_all_CC_chondrules_plot_close

to the ﬁeld of bulk CC chondrites (Fig. 4b,e). Thus, although CV chondrules with NC-like

ε

54_Cr _exist, _the _combined

17_O–

_ε

54_Cr

of these chondrules nevertheless places their origin in the CC reservoir. Combinedthese observationshighlight that theCr-Ti-O isotopicvariabilityamongCVchondrulescannotbetiedtoan ori-ginfromdistinctdiskregionsinaself-consistent way.Thesedata are,therefore, inconsistentwith variableformation regions ofCV (andotherCC)chondrulesandsubsequenttransportofchondrules through the disk.By contrast,the Cr-Ti-O isotopic variations can readilybeattributedtolocalheterogeneitiesamongchondrule pre-cursors,resultingfromtheincorporationofchemicallydiverseand isotopicallyCAI-likematerialintothedustaggregatesthatbecame chondrules.Thisinterpretationisalsoconsistentwithmixing mod-elsthat employbulkchemical compositionsofchondrules,which alsofavor local precursorheterogeneity over chondrule transport (HezelandParteli,2018).

4.3.RelationbetweenisotopicanomaliesinchondrulesandtheNC-CC dichotomyofthedisk

Althoughtheisotopeanomaliesinchondrulesdonottrace spa-tially distinct origins andlarge-scale transport of the chondrules themselves,they nevertheless provide evidencefor transport and mixingamongtheir precursors.Thisisbecausethedegreeof iso-topicvariabilityamongchondrules ofagivenchondritegroup re-ﬂects the amount of isotopically anomalous precursor materials presentinagivenregion ofthedisk,andhowwell thismaterial was mixedatthechondrule precursorscale.The isotopic hetero-geneitiesamongchondrules,therefore,provideinformationonthe spatialandtemporalevolutionofthesolarprotoplanetarydisk.

Recently, it has been argued that the isotopic dichotomy be-tweentheNCandCCreservoirsoftheaccretiondiskresultsfrom a change in the isotopic composition of infalling material from theSun’sparentalmolecular cloud(Burkhardtetal., 2019; Nanne et al., 2019). In this model, early infalling material was charac-terizedby an isotopiccomposition asmeasured inCAIs, whereas theisotopiccompositionofthelaterinfallhadan NC-likeisotopic composition.The outward transport ofearly infalling material by viscousspreading(HuesoandGuillot,2005;Pignataleetal.,2018; YangandCiesla,2012)resultedinan earlydiskhavingaCAI-like isotopiccomposition(termedICforInclusion-likeChondritic reser-voir;Burkhardtetal., 2019).Laterinfall onto thedisk,now with anNC-likeisotopic compositionthen changedthe compositionof theinnerdisk,whiletheouterdiskretainedamemoryofthe iso-topiccompositionoftheearlydisk.Ultimately,theseprocessesled to theformation of the CC reservoir, representing theouter disk whose composition can be regarded as a mixture of IC and NC material,andtheNCreservoir,representingtheinnerdisk,whose compositionisdominatedbythelaterinfall.

Thecombined Cr-Ti-Oisotopicdata forchondrules are consis-tentwiththismodel,becausetheyreveallimitedvariabilityinNC chondrulesandvariable

δ

16O(i.e.,low

17O),

ε

54_Cr,_and

_ε

50_Ti

en-richmentsinCC chondrules.Since chondrules providea snapshot ofthe dust composition at their formation location (Scott,2007; Buddeetal., 2016b;Bollard etal., 2017; MorrisandBoley,2018),

(10)

therelativelyhomogeneousisotopiccompositionofNCchondrules indicatesthattheyformedinawell-mixedreservoirdominatedby NCdust.This isconsistentwiththeidea that theinner disk pre-dominantlyconsistsofmaterialfromthelaterinfall(Burkhardtet al.,2019;Nanneetal.,2019),andwiththerarityofCAIsand CAI-like refractory material in NC chondrites (Ebert et al., 2018). By contrast,CCchondrulesformedinaregionofthediskwhich con-tainsmaterialfromboththeearlyandlate infall.The dataofthis study indicatethat thismixture ofearly and late infalling mate-rial was heterogeneous atthe level of individual CV chondrules, whichdidnot onlyincorporatevariousamounts ofCAIs,butalso lessrefractorymaterialwithCAI-likeisotopiccompositions.Thisis consistentwiththehigherabundanceofCAIsandAOAsinCCover NCmeteorites,andwiththeideathatthesecomponentsrepresent remnants of theIC reservoir, which hadformed during theearly infall and contained not only CAIs, but also less refractory ma-terial (Burkhardt etal., 2019; Nanneet al., 2019). Finally,within theCCreservoir,theisotopicvariationsamongchondrulesseemto show aprogressive homogenizationfromCV andCOtoCR chon-drules(Gerberetal.,2017;Zhuetal.,2019).Thisisconsistentwith thelate formationtimesof

∼

3.7MaafterCAIsforCRchondrules compared to

∼

2.2 Ma after CAIs for CV chondrules (Schrader et al., 2017; Nagashimaetal.,2017;Buddeetal., 2016b,2018),and mayindicatethe progressivehomogenizationofICandNC mate-rialwithintheouterdisk.

4.4. Originofisotopeanomaliesinbulkcarbonaceouschondrites

The evidence for mixing between IC and NC components in the outer disk hasimplications for understanding thegeneration ofisotopeanomalies amongbulk carbonaceouschondrites.In the

ε

50_Ti–

17_O_and

_ε

50_Ti–

_ε

54_Cr_diagrams_(Fig.₄_),_CC_chondrules_plot

within andoutsidetheﬁeldsofbulk CCchondrites,reﬂecting the strong control of CAIs on particular

ε

50_Ti _(Gerber _et _al., ₂₀₁₇_).

VariableCAIabundancesalsoaccountformuchofthe

ε

50_Ti

varia-tionsamongbulkcarbonaceouschondrites(Trinquieretal.,2009). Bycontrast,in

17_O–

_ε

54_Cr_space, _CC_chondrules_plot_close_to_the

compositional ﬁeldofbulk CC chondrites.As such,the chondrule datacanhelpassesstheoriginof

ε

54_Cr_variations_among_bulk_CCs.

Trinquier et al. (2007) observed a correlation between

17_O

and

ε

54_Cr _among _bulk _carbonaceous _chondrites _(see _Fig. ₄₎ _and

argued that this correlation reﬂects mixing between two iso-topically distinct reservoirs. However, the origin of these reser-voirs, and whether they can be tied to speciﬁc meteorite com-ponents, remained ambiguous. More recently, it was argued that the

17O–

ε

54_Cr_correlation _results_from_mixing_CI-like_dust_with

a hypothetical 16O-rich (i.e.,CAI-like

17O)but 54Cr-poor (

ε

54_Cr

≈ −

10)component(Alexander,2019;Zhuetal.,2019).Direct ev-idencefortheexistenceofsucha54_Cr-poor_component_is_lacking,

however. None of the CC chondrules of this study plot outside therangeofmixingtrajectoriesbetweentheNCandICreservoirs, and so these data also provide no evidence for the existence of material with strongly negative

ε

54_Cr. _However, _mixing_between

NC andIC materials doesnot easily account forthe

17O–

ε

54_Cr

array of bulk carbonaceous chondrites, which is almost perpen-dicular to the NC-ICmixing trajectories(Fig. 5). Either thisarray is inherited from the NC reservoir, where bulk meteorites plot along a similar, parallel trend, orthe array reﬂects variable mix-ing between CI-like material, characterized by the highest

ε

54_Cr

and

17O, and chondrules withlower

ε

54_Cr _and

17_O. _Of _note,

for pooled Allende chondrule separates, each consisting of hun-dreds ofchondrules, we obtained

ε

54_Cr

₌

₀

_.

₅₈

_±

₀

_.

_{11 (Table} ₁_).

Combinedwiththeaverage

17O

= −

5

.

4

±

0

.

9 ofthisstudy,this showsthatCVchondrulesplotatthelowerleftofthe

ε

54_Cr–

17_O

array deﬁnedby bulkCCs.As such,mixingbetweenCI-like mate-rial anda common “chondrule-component” may account for the

Fig. 5.17_O_vs._ε54_Cr_of_chondrules_and_bulk_meteorites._NC_(red)_and_CC_(blue)

bodies(opensymbols)definedistinctfields.WhiletheoffsetbetweenNCandCC meteoritesmostlikelyreflectsmixingbetweenNCandICmaterials,thebroad cor-relationof17Oandε54_Cr_among_CC_materials_most_likely_reflects_mixing_between

averageCCchondrulesandCI-likedust.Bulkmeteoritedatafromcompilationin Burkhardtetal.(2019).Forclarityerrorbarsonbulkmeteoritedatawereomitted. NotethatmostCCmeteoritesareaffectedbyaqueousalterationwhichmay mod-ifytheirpristine17_O_through_the_interaction_with16_O_poor_water(-ice)_(Piralla_et

al.,2020).Alterationcorrected17Ovaluesforbulkcarbonaceouschondriteswould thusplotatslightlylower17O.Datafor17OofCVmatrixwastakenfromPiralla etal.(2020) (Marrocchietal.,2018b).TL:TagishLake.

correlated

ε

54_Cr–

17_O_variations_among_bulk_carbonaceous

chon-drites. Such a mixing scenario would be consistent with chemi-calvariationsamongCCchondrites (e.g.,Braukmülleretal.,2018; Alexander,2019), andwiththecorrelation ofmass-dependent Te isotope compositions and

ε

54_Cr _anomalies _observed _among _bulk

carbonaceouschondrites(Hellmannetal.,2020).

5. Conclusions

The ﬁrst combined Cr-Ti-O isotope data set for individual chondrules reveals a fundamental difference between chondrules from non-carbonaceous (NC) and carbonaceous chondrites (CC). Whereas NC chondrules display relatively homogeneous

17O,

ε

50_Ti, _and

_ε

54_Cr, _which _are _{indistinguishable} _from _the

composi-tionsoftheirrespectivehostchondrites,CCchondruleshavemore variable

ε

50_Ti,

_ε

54_Cr, _and

17_O. _Importantly, _although _the

com-positions ofCC andNC chondrules may overlapfor either

ε

50_Ti,

ε

54_Cr, _or

17_O, _in_{multi-isotope} _space_none _of_the _measured _CC

chondrulesplots inthecompositional field ofNCchondrites,and none of the NC chondrules plots within the field of CC chon-drites. These data are inconsistent with a prior proposal for the disk-wide transport of chondrules across the NC and CC reser-voirs, and indicate that there has been no significant exchange of chondrules betweenthese reservoirs. Instead the entirerange ofisotopic anomalies inchondrules is attributableto the hetero-geneous nature oftheir precursors, andcan be accountedfor by mixingbetweenNC-likedust andachemically diversedust com-ponentthat was isotopically similar toCAIs andAOAs.The same mixingprocesses,butonalarger, disk-widescale,werelikely re-sponsibleforestablishingthedistinctisotopiccompositionsofthe NC and CC reservoirs, which represent in inner and outer disk, respectively.