Carbonaceous chondrites contain widespread mineralogical evidence for water–rock interactions, indicating that the C-type asteroids from which they are derived had active hydrothermal systems. In comparison, ordinary chondrites contain secondary minerals that are predominantly anhydrous, suggesting that their parent S-type asteroids were relatively dry. The returned particles from the Hayabusa Mission allow us to probe directly the alteration history of S-type asteroid Itokawa. Here we report nanometre-sized NaCl crystals identified in the interior of an Itokawa particle. These crystals are intimately associated with secondary albitic plagioclase, indicating coupled formation. The NaCl most likely formed through precipitation from an aqueous fluid prior to complete metamorphic dehydration on asteroid Itokawa. Our results therefore imply that asteroid Itokawa supported an active hydrothermal system and suggest that the once-hydrated S-type asteroids could have potentially delivered water to terrestrial planets.

1. Nature Astronomy
natureastronomy
https://doi.org/10.1038/s41550-023-02012-x
Article
Hydrothermalfluidactivityonasteroid
Itokawa
Shaofan Che 1
& Thomas J. Zega1,2
Carbonaceouschondritescontainwidespreadmineralogicalevidence
forwater–rockinteractions,indicatingthattheC-typeasteroidsfrom
whichtheyarederivedhadactivehydrothermalsystems.Incomparison,
ordinarychondritescontainsecondarymineralsthatarepredominantly
anhydrous,suggestingthattheirparentS-typeasteroidswererelatively
dry.ThereturnedparticlesfromtheHayabusaMissionallowusto
probedirectlythealterationhistoryofS-typeasteroidItokawa.Here
wereportnanometre-sizedNaClcrystalsidentifiedintheinteriorofan
Itokawaparticle.Thesecrystalsareintimatelyassociatedwithsecondary
albiticplagioclase,indicatingcoupledformation.TheNaClmostlikely
formedthroughprecipitationfromanaqueousfluidpriortocomplete
metamorphicdehydrationonasteroidItokawa.Ourresultsthereforeimply
thatasteroidItokawasupportedanactivehydrothermalsystemandsuggest
thattheonce-hydratedS-typeasteroidscouldhavepotentiallydelivered
watertoterrestrialplanets.
Primary chondritic materials that formed by high-temperature ther-
mal processing in the early solar nebula1
have experienced second-
ary modifications on the parent asteroids, producing a wide range
of secondary phases and textures2,3
. For ordinary chondrites (OCs),
a group of chondrites that were affected by thermal metamorphism
to various degrees, only some of the low petrological type 3 samples
preserverecordsofinteractionswithlow-temperatureaqueousfluids
asdemonstratedbythedevelopmentofhydrousminerals(forexample,
phyllosilicatesandamphiboles)4,5
.Incomparison,theinvolvementof
waterduringsecondaryalterationofequilibratedOCsislessclearand
ithasbeencommonlyassumedthatmetamorphismofthesesamples
occurredunderdryconditions2
.However,recentstudiesoffeldspars
andphosphatesinequilibratedOCssuggestthataqueousfluidswere
likelypresentduringthermalmetamorphismontheirparentasteroids
and were responsible for the alteration features of these minerals6–8
.
Additional mineralogical evidence for water–rock interactions in
equilibrated OCs could help shed light on the geological evolution
of S-type asteroids.
Apotentialobstacletounequivocalidentificationofparent-body
processingofOCsisthatmosthaveexperiencedterrestrialweathering
that has blurred or completely destroyed some of their indigenous
alteration features9
. This problem can be overcome by the samples
returned from the Hayabusa Mission because they provide a unique
opportunitytostudyindigenousfeatures.Launchedon9May2003,the
Hayabusaspacecraftsuccessfullycollectedover1,500particlesfrom
thenear-EarthS-typeasteroid25413Itokawa.Previousinvestigations
of these particles revealed that their mineralogy and chemistry are
most consistent with LL4-6 OCs10
, establishing a direct link between
OCsandS-typeasteroids.AninterestingdiscoveryinItokawaparticles
istheoccurrenceofNaClgrainsreportedtooccurontheexternalsur-
facesofsomeparticles11–13
.InOCs,NaClwaspreviouslyonlyreported
in Monahans (1998) (H5) (hereafter referred to as Monahans) and
Zag (H3-6)14,15
. Both chondrite samples are observed falls, so notable
terrestrial contamination was eliminated. As suggested by previous
petrologicalandisotopicexaminations,theNaClgrainsinMonahans
andZaghaveapreterrestrialorigin.Incomparison,NaClgrainsprevi-
ously reported in Itokawa samples are much finer-grained and occur
onthesurfacesoftheparticles,whichhasrenderedtheinterpretation
oftheiroriginsverydifficult.
HerewereportonNaClcrystalsinItokawaparticleRA-QD02-0248.
These NaCl grains provide a unique opportunity to probe the role of
aqueous fluids during thermal metamorphism on S-type asteroids.
Received: 9 June 2022
Accepted: 18 May 2023
Published online: xx xx xxxx
Check for updates
1
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, USA. 2
Department of Materials Science and Engineering, University of Arizona, Tucson,
AZ, USA. e-mail: sche@arizona.edu

2. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02012-x
HAADF image. It is thus very likely that these high-Z phases are NaCl
grains, although their Na signals cannot be clearly resolved from the
Na map due to their fine-grained sizes and the Na signal from the sur-
roundingplagioclase.SimilartoSectionno.3,theNaClgrainsinSection
no. 4 occur mainly on the surfaces. To test the hypothesis that NaCl
inclusionscouldoccurwithinthethicknessofthesliceinSectionno.4,
we rethinned it on a Thermo Scientific (formerly FEI) Helios FIB-SEM
under8 kV,removingthegrainsthatwereattachedtothesurfaces.The
sectionwasimagedona200 kVHitachiHF5000scanningtransmission
electronmicroscopeimmediatelyafterionmilling;however,noNaCl
inclusionswerefound(SupplementaryFig.10).
Sectionno.5samplesasinglecrystalofplagioclasewhichcontains
several twin lamellae (50–100 nm in width). The FIB section contains
a vein that transects the section and shows a slightly lower-Z contrast
from plagioclase (Fig. 3). The width of this vein varies from 300 to
500 nm.NaClgrainsarefoundtodecoratetheinterfacesbetweenthe
vein and plagioclase. Some NaCl grains also occur randomly distrib-
uted on plagioclase. Because the vein is sampled by the FIB section
at an angle, it appears to be crosscut by the twin lamellae in the TEM
images. Therefore, the apparent crosscut relationship between the
vein and twin lamellae does not bear information about their relative
formation times. We acquired EDS maps of the vein that show it has
Although halite was reported in Monahans and Zag, its relationship
with other phases is unclear and recent studies suggest that halites
in these meteorites are exogenous and derived from a carbonaceous
asteroid16,17
. In this study, we show that the NaCl grains in Hayabusa
sampleRA-QD02-0248haveaclosetexturalrelationshipwithsecond-
ary albitic plagioclase, indicating that these phases were genetically
linked.Itfurtherindicatesthathydrothermalfluidwaspresentduring
theadvancedstageofthermalmetamorphismontheItokawaasteroid,
andpotentiallytheparentbodiesofequilibratedOCs.
Results
The particle was cross-sectioned using an ultramicrotome, and the
as-sliced surface measures approximately 100 µm × 50 µm (ref. 18
and Supplementary Fig. 1). Three mineral phases are identified on
the energy-dispersive X-ray spectroscopy (EDS) phase map (Supple-
mentary Figs. 2 and 3): olivine, high-Ca pyroxene and albitic plagio-
clase. The grain boundaries are straight or slightly curved, and triple
junctions are observed, which is consistent with the observations of
most Itokawa particles10
. Five regions of interest were selected (Sup-
plementary Fig. 3) for detailed analysis using transmission electron
microscopy(TEM).Electron-transparentsectionswerecreatedusing
focusedionbeam-scanningelectronmicroscopy(FIB-SEM)(seeMeth-
ods for details). Three of the FIB sections (nos. 3 to 5) extracted from
the particle contain plagioclase and NaCl, and were examined over
multiple years, which we describe in detail below.
Section no. 3 is composed of plagioclase and olivine. From the
scanning TEM (STEM) images and EDS X-ray maps obtained in Sep-
tember 2016, plagioclase contained abundant NaCl crystals with a
random distribution (Fig. 1). These grains varied in size from <30 nm
to 300 nm and some of the large ones appeared euhedral, with cubic
or elongate shapes. In comparison, no NaCl grains were found on the
adjacentolivine.TheNaClgrainsdonotcontaindetectableKfromthe
EDSanalysis.Basedonthesecondary-electron(SE)images,itappeared
thatmanyNaClgrainswerepresentonthesurfacesoftheFIBsection.
TorevealiftherewereNaClinclusionswithinthethicknessofthesec-
tion, we compared the STEM images taken from both sides of the sec-
tion. The bright-field (BF) and high-angle annular dark-field (HAADF)
STEMimagingrevealfeaturesinprojection,whiletheSEimagingshows
featuresthatareveryclosetooronthesurfacesofthesection.There-
fore, features within the slice should appear in BF-STEM and HAADF
but not SE images. After careful counting and comparing NaCl grains
intheimagesofbothsides,alloftherelativelycoarse-grained(>50 nm)
andmostofthesmallerNaClgrainsoccurredonthesurfaces(Supple-
mentaryFig.4).Itremainedpossiblethatsomeofthesmaller(<50 nm)
NaClgrainsmightoccurwithinthethicknessofthesection;however,
furtherconfirmationwasimpossiblebecausethesesmallergrainswere
barely resolved on the images. After the initial imaging and analysis
in September 2016, Section no. 3 was stored in an N2-filled desiccator
until we revisited it in September 2021 to evaluate if the NaCl grains
had experienced any modifications. Our STEM results show that the
amount and overall distribution of these grains did not change, but
minor modification was identified mainly in grains >100 nm—that is,
somegrainsgrewlargerandmoreeuhedral,andsomeadjacentgrains
mergedintolargerones(Fig.2andSupplementaryFig.5).BesidesNaCl,
plagioclasealsocontainsthin(15–60 nm)K-feldsparexsolutionlamel-
lae mostly within a ~500 nm distance from its interface with olivine
(SupplementaryFig.6).Notexturalrelationshipisobservedbetween
NaClandK-feldspar.
Sectionno.4alsotransectsplagioclaseandolivinedomains.The
STEM analysis shows abundant high-Z grains (<30 nm in size) on the
surface of plagioclase (Supplementary Fig. 7). We conducted EDS
elementalmappingonanapproximately1.5 × 1.5 µmregionofplagio-
clase(SupplementaryFig.8).Inthesummedspectrum,thereisasmall
but clearly resolved Cl Kα peak (Supplementary Fig. 9). The hotspots
on the Cl Kα map are closely associated with the high-Z grains on the
HAADF
Ca K
Na Cl
Ol
Pl
NaCl
Kfs
1 µm
1 µm 1 µm
1 µm
1 µm
Fig.1|HAADFimageandassociatedEDSX-raymapsofFIBSectionno.3.
ThedistributionofNaClgrainsisbestobservedontheClmap.SmallerNaCl
grains(<~30 nm)tendtobepoorlyresolvedontheNamapduetothehigh
concentrationofNainplagioclase(Pl).Therearethin(15–60 nm)K-feldspar(Kfs)
lamellaethatoccurmostlywithina ~500 nmdistancefromtheinterfacebetween
plagioclaseandolivine(Ol).
Become more euhedral
09–16
09–21
Grow larger
Merge into larger crystals
0.5 µm 0.5 µm
Fig.2|Higher-magnificationSEimagesofNaClgrainsinFIBSectionno.3.
TheseimagesshowNaClgrainsfromFIBSectionno.3asacquiredinSeptember
2016(left)andSeptember2021(right)andarecomparedtorevealtheminor
modificationstograinsizesandshapes.SomeNaClgrainsgrewlargerandmore
euhedral,andsomeadjacentgrainsmergedintolargerones.Notethatthese
changesmainlyoccurredinrelativelylargegrains.

3. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02012-x
a plagioclase-like composition but is depleted in Na compared with
the surrounding plagioclase. To reveal the compositional difference
betweentheveinandplagioclasemorequantitively,wecomparetheir
EDS spectra after normalization to the Si Kα peak (Supplementary
Fig. 11). The EDS data confirm the Na depletion of the vein, which
contains an average Na/Si atomic ratio of 0.18 compared to 0.25
for plagioclase. Such an Na depletion of the vein also results in an
obvious deviation from the plagioclase stoichiometry. The average
(Na + K + Ca)total oftheveinis0.57,comparedto0.84fortheplagioclase
(SupplementaryTables1and2).Wenotethattheapparentnonstoichi-
ometryofplagioclaseiscausedbyelectron-beamirradiation.Nonethe-
less,theobviouslydifferentalkalicationcontentsbetweentheveinand
plagioclaseimplythattheNadepletionintheveinisanindigenousfea-
tureofthesample.Additionally,wetiltedthesectiontothe[102]zone
axis of plagioclase and obtained selected-area electron-diffraction
(SAED) patterns from the vein and plagioclase (Fig. 4). The pattern of
theveinshowsthesamelong-rangeorderastheplagioclase;however,
thediffractionspotsfortheveinarefainteranditshigher-indexreflec-
tionsaremuchweakerorevenabsent.
Discussion
NaCl is highly soluble in water and can be easily affected by deliques-
cence,aprocessinwhichNaCltransformsintoasolutionbyabsorbing
moisturefromtheair19
.Theseeffectscouldoccurduringsamplecura-
tion,storageandpreparationofextraterrestrialmaterials.Therefore,
we consider several possibilities for NaCl as terrestrial contaminants
in RA-QD02-0248. (1) Introduction of NaCl during sample recovery
and curation is unlikely because sulfates as efflorescence products
commonlyfoundonmeteoritefinds20
areabsentinItokawaparticles,
and NaCl grains in RA-QD02-0248 are not associated with previously
reported contaminants—for example, metallic Al, tin oxide, quartz
andpossiblycarbonaceousparticles21
thatcouldbeintroducedduring
curation. (2) NaCl precipitation from the moisture in our laboratory
canbeexcludedduetothefollowingreasons.Anyliquidthatcontacted
the external surface of the particle was unlikely to penetrate to the
interiorduetothelackofsurfacefractures.Evenifsuchcontaminants
were present on the particle, they should have been removed during
cross-sectioning by ultramicrotome slicing and ion milling. It is also
implausible that NaCl was introduced onto the FIB sections from the
moisture, based on the observation that NaCl grains in Section no. 3
werewellpreservedinthedesiccatorforfiveyears.Inaddition,acon-
taminationscenariocannotexplainwhyNaClgrainsinRA-QD02-0248
areonlyassociatedwithplagioclasebutabsentonadjacentolivine.(3)
TotestifNaClcanformbyareactionbetweenNainplagioclaseandCl
in epoxy under room conditions, we conducted an experiment using
a terrestrial albite sample (Methods and Supplementary Fig. 12) that
has a similar composition to the plagioclase in RA-QD02-0248 (the
averagemeasuredEDScomposition:(Na0.75K0.08Ca0.11)Al1.01Si2.98O8)and
albitic plagioclase in other Itokawa particles10
. The TEM results show
thattheFIBsectionsofthealbitearefeaturelessandNaCldidnotform
underlaboratoryconditions(SupplementaryFig.13),indicatingthata
plagioclase–epoxyreactionwashighlyunlikely.(4)Uponexposureto
theelectronbeam,Na+
plagioclasecanbecomemobile,diffuse22
,and
couldreactwithClintheepoxytoformNaCl.However,NaClgrainsdo
nothaveanyspatialrelationshipwiththethinepoxylayersbeneaththe
capping layers of FIB sections, which would be expected if mobilized
Na+
combined with Cl in the epoxy. In addition, we did not find any
NaClgrainsontheFIBsectionsthatwereliftedoutfromtheterrestrial
albite sample. Therefore, we can rule out the electron-beam damage
as a source of NaCl contamination.
WeacknowledgethattheobservationofNaClgrainsmainlyonthe
surfaces of FIB sections of RA-QD02-0248 appears to be inconsistent
with an indigenous origin. However, as discussed above, potential
sources of contamination cannot account for the formation of NaCl
on the FIB sections. An alternative and more likely explanation for
thesurface-adheringnatureismigrationofNaClinclusionsthatwere
originally present within the thickness of the FIB sections to the sur-
faces. Such a migration process could be driven by water uptake and
recrystallizationofNaClinthepresenceofmoistureinthedesiccator.
ManyoftheNaClgrainshavesizesthatarecomparabletothethickness
BF
Na
HAADF
Cl
Twin
lamellae
Vein
Pl
Pl
NaCl
NaCl
Pl
Pl
Vein
1 µm 1 µm
1 µm 1 µm
a b
c d
Fig.3|TEMimagesandEDSX-raymapsofFIBSectionno.5showingthe
presenceofaveinthattransectsthesectionandassociatedNaClgrains.
a,BF-TEMimageoftheFIBsectionshowingthattheveintraversesalmostthe
entiresectionandoccursatanobliqueangletothetwinlamellaeofplagioclase
(Pl).b,HAADFimageshowingthatNaClismainlypresentasfine-grained,
high-Zcrystalsdecoratingtheinterfacesbetweentheveinandthesurrounding
plagioclase.TherearealsoNaClgrainsthatarerandomlydistributedinthe
plagioclase.c–d,TheNa(c)andCl(d)EDSmapsoftheregionshowninb show
thetexturalrelationshipbetweenNaClandthevein,andrevealthattheveinis
depletedinNacomparedwiththeplagioclase.
Vein
Plagioclase
020
201
221
201
020
221
a b
Fig.4|ComparisonofSAEDpatternsacquiredfromplagioclaseandthevein
inFIBSectionno.5. a,b,SAEDpatternsacquiredfromplagioclase(a)andthe
vein(b).Bothpatternswereacquiredatthesamecameralength(40 cm)andtilt
angles(tx = 6.3°,ty = 8.8°),andcanbeindexedasalbitewiththe[102]zoneaxis.
Thepatternoftheveinpreservesthelong-rangeorderofplagioclase;however,
thediffractionspotsaregenerallyweakerthanthoseofplagioclase.Inaddition,
thehigher-indexreflectionsoftheveinaremuchweakerorevenabsent.

4. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02012-x
oftheFIBsections,andthereforeoriginalNaClinclusionsinplagioclase
couldhavecrystalsurfacesexposedtotheairinthedesiccator,further
facilitatingwaterabsorption.NaClgrainscanabsorbwatermolecules
onto their surfaces, forming water monolayers or thin films depend-
ingontherelativehumidity23,24
.NaClmaypartiallydissolveintothese
monolayers or thin films24
. For the N2-filled desiccator in our lab, the
relativehumidityismaintained≤10%,whichiswellbelowthevaluefor
NaCltodeliquesce.Therefore,wateruptakeofNaClontheFIBsections
might not impose considerable changes on the morphology of the
grains;instead,NaClcouldpartiallydissolveintothewatermonolayers,
forming monovalent Na+
and Cl−
ions which later recrystallize on the
surface.OnlyminormorphologicalchangesofNaCl(graincoarsening
andminorchangesofshapes)areobservedinFIBSectionno.3,which
arelikelydrivenbyOstwaldripening25
.Theabovedynamicprocess,in
effect,dragspreexistingNaClinclusionstothesurfacesofFIBsections,
whichisillustratedbytheschematicdiagraminSupplementaryFig.14.
Thedegreeofwateruptakeissizedependentduetotheincreasingcon-
tributionofsurfaceenergytothetotalfreeenergyforsmallergrains26
,
whichexplainswhyminormodificationsinducedbyOstwaldripening
mainlyaffectedNaClgrains>100 nminSectionno.3.
Therefore,ourTEMobservationssuggestthattheNaClgrainsare
native to asteroid Itokawa. Further, the NaCl grains in RA-QD02-0248
occur exclusively on plagioclase, which indicates that these phases
aregeneticallylinked.Comparisonwithmeteoriteanaloguesprovides
additional insights. Recent studies of feldspars in different types of
OCs7,8
suggest that during prograde metamorphism, feldspars were
alteredbyahydrous,alkali-halogen-bearingfluidthatwasgeneratedvia
meltingofH2O-HClsolidmixtures27
.Basedonthesestudies,feldspars
werepartiallyreplacedbyphyllosilicates,scapolitesandfeldspathoids
at the early stage of alteration, and albitic plagioclase became the
dominant alteration product of feldspars towards higher metamor-
phic temperatures. Ref. 8 argued that albitization ended before peak
metamorphism,whichwaspossiblycausedbythelossofaqueousfluid.
The textural association of NaCl with albitic plagioclase in RA-QD02-
0248 therefore suggests that NaCl could have precipitated from the
hydrothermal fluid (a hot aqueous solution) that altered plagioclase
duringadvancedthermalmetamorphismonasteroidItokawa(Supple-
mentary Text). Porosity is commonly observed in altered plagioclase
inOCs,whichresultedfromleachingofplagioclasecomponents(such
asCa,SiandAl)intothefluid8,28
.Theformationofporesprovidedopen
pathways for the fluid to further infiltrate into plagioclase, which is
consistent with the interface-coupled dissolution-reprecipitation
mechanismforfluid–mineralinteractions29
.Duringthisprocess,fluid
might be trapped in these pores and partially equilibrate with plagio-
clase.Thosefluidinclusionstrappedatthelatestageofhydrothermal
alteration prior to peak metamorphism could potentially crystallize
saltcrystals.AlthoughequilibratedOCscouldexperienceapostpeak
metamorphismanhydrousfluideventasproposedbyref.8toexplain
the heterogeneous distribution of K-feldspar exsolution in albite, we
suggest that such an anhydrous fluid event is not responsible for the
formationofNaClinRA-QD02-0248.ThisisbecauseNaClgrainsinthe
FIBsectionsdonothaveanytexturalrelationshipwithK-feldsparexso-
lutionlamellae,andthealterationveininSectionno.5doesnotshowK
enrichment.Bothobservationsindicatethattheformationprocesses
ofNaClandK-feldsparsinRA-QD02-0248werelikelydecoupled.
AnotherpieceofevidencecomesfromSectionno.5,whereabun-
dant NaCl crystals are present at the interfaces between the vein and
host plagioclase, implying a genetic relationship between NaCl and
the vein. The vein has a similar composition to albitic plagioclase but
is markedly more depleted in Na and shows a partially amorphized
structure. This vein could represent an alteration product during the
infiltration of the same aqueous fluid that produced NaCl inclusions
into the plagioclase through a fracture prior to peak metamorphism.
A plausible hypothesis for the absence of veins in olivine grains in
RA-QD02-0248 is that olivine did not contain preexisting fractures
prior to fluid alteration. The different responses of olivine and pla-
gioclase to the changing fluid chemistry could also play an impor-
tant role: the alteration rate of olivine decreases as pH increases30
,
whereas albitization occurs in an alkaline environment31
. We hypoth-
esize that the plagioclase was dissolved stoichiometrically upon its
contact with the fluid, which is consistent with the interface-coupled
dissolution-reprecipitationmechanism29
.Thedissolvedspecies(Na+
,
SiO2 and Al2O3) were concentrated in the interfacial boundary layer,
resulting in precipitation of an Na-depleted, plagioclase-like phase.
Suchaphasemightbemetastableduetoitspartiallyamorphousnature
andwouldeventuallytransformintoacrystallinephasewithcontinued
alteration and dissolution of the host plagioclase. In this context, the
preservation of the amorphous phase in the vein suggests that the
limited availability of fluid inhibited further equilibration. That the
veinandhostplagioclasehavethesameorientation(Fig.4)isalsocon-
sistent with interface-coupled dissolution-reprecipitation processes
and characteristic of epitaxial nucleation of the alteration product
on the parent phase29,32
. The alteration front progressively migrated
towards unaltered plagioclase, and therefore the vein grew increas-
ingly wider. The composition of the fluid in the interfacial region of
alteration became increasingly rich in aqueous Na+
and Cl−
, and once
its solubility constant was reached, NaCl precipitated from the fluid.
Asimilaralterationmicrostructurewaspreviouslyreportedinhydro-
thermal experiments of albite, in an SiO2-Al2O3-NaCl-H2O system33
.
Theproposedalterationprocessoftheveinisshownschematicallyin
SupplementaryFig.15.
NaCl could have survived peak metamorphism on the Itokawa
asteroid. The estimated formation temperature of Cl-scapolite
from albite and halite at dry and low-pressure conditions is ≳820 °C
(refs. 34,35). Itokawa particles may have experienced peak meta-
morphism with temperatures of ~800 °C (ref. 10), which are within
the thermal stability field of halite (solidus of ~800 °C at 1 bar) in the
binary NaCl-KCl system with high NaCl activity36,37
. A previous study
of a halite-siderite-anhydrite-chlorapatite assemblage in the Nakhla
Martianmeteoritealsoimpliedthathaliteandplagioclasecouldcoexist
atelevatedtemperaturesandlowpressures35
.Postpeakmetamorphism
disruption of S-type asteroids, as depicted in a rubble-pile model38–41
,
could also help preserve NaCl in the sample. NaCl is thermally more
stablecomparedwithphyllosilicatesandamphibolesthatwereprevi-
ously reported in unequilibrated OCs (such as Semarkona, Bishun-
pur, Chainpur and Tieschitz4,5
). Phyllosilicates and amphiboles can
beeasilydehydratedatelevatedmetamorphictemperaturesrelevant
to equilibrated OCs4,5
. Therefore, NaCl grains in RA-QD02-0248 pro-
vide direct mineralogical evidence for the involvement of an aqueous
fluid during advanced thermal metamorphism on asteroid Itokawa. It
was conventionally assumed that water was essentially absent during
thermal metamorphism of equilibrated OCs2
, which seems to be sup-
ported by their extreme low bulk water contents42
. This conventional
viewwaschallengedbyrecentstudiesofplagioclaseandphosphatesin
equilibratedOCs8,43
,whichsuggestthattheseOCsexperiencedhydrous
alteration.Ourstudyprovidesfurthersupportthattheparentasteroids
of equilibrated OCs could be more hydrated than previously thought
(see Supplementary Text for further discussion). This is in line with
recentmeasurementsofthewatercontentofItokawapyroxene,which
indicatehighwaterabundanceinItokawa44
.Previousstudiessuggested
thatpeakmetamorphicconditionsofOCswereachievedatafewmillion
yearsaftertheformationofcalcium-aluminium-richinclusions3,45
.The
formation of planetary embryos for terrestrial planets could be com-
pletedatasimilartimescale:forexample,isotopicevidencefromMars
suggestedthatitaccretedhalfofitsfinalsizewithin~2 millionyearsafter
calcium-aluminium-rich inclusions46
. Therefore, our study suggests
thattheonce-hydrousS-typeasteroidscouldhavepotentiallydelivered
water to Earth and other terrestrial planets at their very early stage of
accretion. The identical hydrogen isotopic compositions between
Itokawa pyroxenes and Earth44
support this hypothesis. We note that

5. Nature Astronomy
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othertypesofasteroidscouldalsodeliverwatertoterrestrialplanets47
,
andestimationoftherelativecontributionsofdifferentmaterialstothe
terrestrialvolatilebudgetrequiresfuturework.
Methods
Itokawa particle RA-QD02-0248
We were allocated Itokawa particle RA-QD02-0248 from the Haya-
busa sample collection of NASA’s Johnson Space Center in 2016. This
particle was originally located in Room A of the sample catcher on
the Hayabusa spacecraft. Based on the SEM and EDS analyses at the
Japan Aerospace Exploration Agency (http://darts.isas.jaxa.jp/pub/
curation/hayabusa/RA-QD02-0248/RA-QD02-0248.pdf) and John-
son Space Center (https://curator.jsc.nasa.gov/hayabusa/samples/
hayabusa_spectra.cfm?sample=RA-QD02-0248),RA-QD02-0248meas-
ured approximately 148 µm across and contains olivine, plagioclase,
high-Capyroxeneandtroilite.Thisparticlewasembeddedinanepoxy
bullet that is prepared using Araldite 502 (glycerol-based aromatic
epoxyresin)andcuringagents(DDSAandDMP-30).TheAraldite502
is bisphenol-A-(epichlorohydrin) that has a composition of (C15H16O2·
C3H5ClO)x.Thetopoftheepoxybulletwaslatertrimmedandslicedon
a Leica EM UC7 ultramicrotome, following the procedure described
byref.48,sothataflatsurfacewascreatedontheembeddedparticle.
TerrestrialalbitesampleR040129
Because the epoxy resin contains a trace amount of chlorine as men-
tioned above, it could have reacted with sodium in plagioclase in
RA-QD02-0248toproduceNaClcontaminants.Totestthishypothesis,
wewereallocatedapieceofterrestrialalbitesampleR040129(RRUFF
ID:https://rruff.info/Albite/R040129)fromProfessorRobertDownsin
theDepartmentofGeosciences,UniversityofArizona(UA).Thealbite
was collected from the LaChance Quarry, Brunswick, Cumberland
County, ME, USA. The measured chemistry using an electron micro-
probe is (Na0.99K0.01)Al1.00Si3.00O8. We separated the albite sample into
two fragments: (1) R040129-01 with a flat cleavage plane was directly
attached to a scanning electron microscope pin mount using carbon
tape; (2) R040129-02 was embedded in a Cl-bearing epoxy, cured in
a bullet geometry and ultramicrotomed before being attached to a
scanningelectronmicroscopepinmount.
Analyticalmethods
To characterize the mineralogy of Itokawa particle RA-QD02-0248,
weimageditusingaThermoScientific(formerlyFEI)HeliosNanoLab
660 G3
FIB-SEM located in the Kuiper Materials Imaging and Charac-
terizationFacility(KMICF)attheLunarandPlanetaryLaboratory,UA.
The FIB-SEM is also equipped with an EDAX EDS system and electron
backscatter diffraction analysis system. The electron-imaging condi-
tionswere5 kVand0.8 nA.AsuiteofEDSmapswasacquiredat20 kV,
with a resolution of 512 × 400 pixels and a dwell time of 50 μs. An EDS
phasemapwasgeneratedbytheThermoScientificMAPSsoftware.
AtotaloffiveFIBsectionswerepreparedfromRA-QD02-0248:four
of them (Section nos. 1–4) were extracted using the Helios FIB-SEM,
whichisequippedwithanEasyLiftmicromanipulatorintheKMICF,and
theotherone(Sectionno.5)waspreparedatHitachiHigh-Technologies
Co.inNaka,Japan.Additionally,wepreparedoneFIBsectionforeach
albitefragmentontheHelios.TheFIBpreparationproceduresonthe
Heliosgenerallyfollowthosedescribedinref.49,exceptthatsections
weremountedinflagpole-stylegeometryonCuhalfgrids.Thefinalion
millingwascarriedoutunder8 kV,whicheffectivelyremovedtheamor-
phousdamagelayersonFIBsurfacescreatedbyhigher-voltagemilling.
The characterization work of the FIB sections was primarily con-
ductedonthe200 kVHitachiHF5000scanningtransmissionelectron
microscope located in the KMICF. The HF5000 is equipped with a
cold-field emission gun, a third-order spherical aberration corrector
for the STEM mode, BF-, DF- and SE-STEM detectors. The SE detector
is very useful in detecting surface-adhering grains on the FIB sections.
The phase structure and orientation information were determined
using SAED patterns which were measured and indexed with Crystal-
lographic Image Processing Software50
and SingleCrystal Software,
based on calibrated camera constants. The HF5000 is also equipped
with an Oxford Instruments X-Max N 100 TLE EDS system with dual
100 mm2
windowlesssilicon-driftdetectorsprovidingalargesolidangle
of up to 2.0 sr, which translates into improved counting statistics over
conventionalEDSsystems.Asaresult,sampledriftingduringmapping
was minimized, and we acquired X-ray maps within minutes. The large
solidanglealsomeansthatbeam-sensitiveminerals,suchasplagioclase
andhalitethatarerelevanttothisstudy,canbereadilymappedwithout
causingconsiderablebeamdamage.EDSmapswereobtainedat200 kV
inSTEMmodeusinga300 pmprobewitharesolutionof512 × 512 pixels,
adwelltimeof7.2 msandaprocesstimeof4.Someadditionalstructural
andcompositionaldatawereacquiredona200 kVaberration-corrected
HitachiHF5000transmissionelectronmicroscopelocatedattheHitachi
High-Technologies Factory (Hitachinaka, Ibaraki Prefecture, Japan),
whichisequippedsimilarlytotheHF5000intheKMICF,UA.
DataAvailability
All data supporting this study are provided in Results section of this
paperandthesupplementaryinformationaccompanyingthispaper.
Sourcedataareprovidedwiththispaper.
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Acknowledgements
This research was supported by the NASA Laboratory Analysis
of Returned Samples grant no. 80NSSC19K0509 awarded to
T.J.Z. We acknowledge NASA grant no. NNX12AL47G and no.
NNX15AJ22G, and NSF grant no. 1531243 and no. 0619599 for
funding the instrumentation in the KMICF at the Lunar and
Planetary Laboratory, UA. We thank R. Downs in the Department

7. Nature Astronomy
Article https://doi.org/10.1038/s41550-023-02012-x
of Geosciences, UA for the loan of the terrestrial albite sample.
We greatly appreciate members of the Planetary Materials
Research Group at the Lunar and Planetary Laboratory for very
helpful feedback and suggestions. S.C. would like to thank P.-M.
Zanetta for helpful conversations on quantification of EDS spectra
and L. Keller, J. Lewis and M. Zolotov for useful information and
discussions. S.C. also appreciates Z. Zeszut, T. Ramprasad and Y.-J.
Chang for their assistance with FIB and TEM operations.
Authorcontributions
S.C. prepared the FIB sections of the terrestrial albite sample, carried
out part of the TEM analyses and wrote the paper. T.J.Z. prepared the
FIB sections of Itokawa particle RA-QD02-0248 and conducted part of
the TEM analyses. Both authors contributed to the data interpretation
and discussions and revision of the paper.
Competinginterests
The authors declare no competing interests.
Additionalinformation
Supplementaryinformation The online version contains supplementary
material available at https://doi.org/10.1038/s41550-023-02012-x.
Correspondence and requests for materialsshould be addressed to
Shaofan Che.
Peer review information Nature Astronomy thanks Makoto Kimura
and the other, anonymous, reviewer(s) for their contribution to the
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