3D quantification of nanolites using X-ray ptychography reveals syn-eruptive nanocrystallisation impacts magma rheology

Domain: earth_science
Published: 2025-08-01
Editorial outcome: minor_revision

Abstract
Nanoscalecrystalsarebecomingincreasinglyrecognisedintheproductsof
volcaniceruptions,spanningarangeofmagmacompositions.Thecrystal-
lisationofnanolitesimpactsmagmarheology,ascentdynamics,anderuptive
style.Theirimpactcanbeenhancedduetotheircapacitytoaggregateand
developneighbouringchemicallydifferentiatedboundarylayers.However,
their3Dinteraction,spatialdistribution,andmorphologyisnotcurrently
understood.Herewepresentacutting-edge,3Dnanometre-scalevisualisation
andquantificationofnanolitesinscoriaeoftheLasSierras-Masayabasaltic
Plinianeruptions,acquiredusingX-rayptychography.WefindthatTi-
magnetitenanolitesaggregate,formingelongate,irregularstructuresin3D.
TheircrystallisationextractsFeandTifromthemelt,resultingindiffer-
entiatedboundarylayerswithhigherviscosity.Syn-eruptivecrystallisationof
nanolitesandtheirinteractionisestimatedtohaveincreasedmagmaviscosity
by2–3ordersofmagnitude,therefore,theylikelyhadastrongcontrolon
magmarheology,increasingthepotentialofmagmafragmentation.

Introduction
ThepotentialofahighlyexplosivePlinianeruptionatavolcanicsys-
tem presents a significant hazard and requires assessment of risk
mitigation. Plinian eruptions eject >1 km3 of tephra into the atmo-
sphere,producingeruptioncolumnswhichexceed10kminheight1,
with considerable impacts on society and the environment. Plinian
eruptions are typically associated with silica-rich magmas1, as their
highviscositycanresultinbrittlefragmentationwithinvolcaniccon-
duits,whichoccursduetotheaccumulationofstressathighapplied
strainratesortheoverpressurewithinentrainedbubbles1–3.However,
sub-PlinianandPlinianeruptionsatbasalticvolcanicsystemsdooccur,
such as the recent 2017–2018 eruption of Ambae (Aoba), Vanuatu,
whichproducedamaximumcolumnheightof20kmandledtothe
evacuation of 11,000 people4,5. Las Sierras-Masaya volcanic system
(Nicaragua),locatedapproximately25kmfromthecapitalcityMana-
gua,hasproducedseveralbasalticPlinianeruptionsinthepast60ka6
and, therefore, may have conditions which favour recurrent highly
explosiveactivity7,8.

Thelowviscosityofbasalticmagmaoftenpreventstheaccumu-
lation of sufficient overpressure to result in fragmentation, unless
subject to exceptionally high strain rates during ascent9. However,
processes such as fracturing10, the inertial expansion of bubbly
magma11 and rapid syn-eruptive crystallisation12,13 can promote the
fragmentationofbasalticmagma.

Syn-eruptive crystallisation increases magma viscosity through
thephysicalpresenceofcrystals,crystal-crystalinteractions14andthe
facilitationofheterogeneousbubblenucleation15.Thefastascentrates
estimatedforbasalticPlinianeruptionsmayinducelargeundercooling
(ΔT) during ascent, favouring rapid, syn-eruptive microlite crystal-
lisation and bubble nucleation7,8,12,16–18. The erupted products often
showheterogeneousandhighlycrystallinemicrotextures7,8,17,19.Crys-
tallinityvariesbetween43and99vol.%inclastsoftheEtna122BC
eruption (Italy)16, although the syn-eruptive crystal content may be
lower.Furthermore,inspectionofmorecrystal-poorregionsofsome
clastsalsoshowsamottledtexture16,17,20,indicatingthatevenappar-
ently glassy areas may show evidence of incipient crystallisation at
highmagnification.

Nano-sized crystals, or ‘nanolites’, are becoming increasingly
recognised21 in both experimental products and natural volcanic
samples, encompassing a range of magma compositions and
explosivity13,20,22–33.Nanolites20-50nminsizehavebeenobservedin
clasts of the Etna 122 BC basaltic Plinian eruption13. Nanolites can
considerablyincreasemagmaviscositythroughtherheologicalimpact
ofthenanoparticlesuspension,byfacilitatingheterogeneousbubble
nucleationorinducingchemicalandstructuralchangesintheresidual
melt,forexample,byincreasingmeltpolymerisationand/orcreatinga
heterogeneous distribution of chemical components13,21,27–29,32,34–40.
They may, therefore, exert an important control on rheology and
ultimately, explosivity. Nanolite crystallisation may also hinder
laboratory-based measurements of melt viscosity, resulting in an
overestimationofthecrystal-freemeltviscosityinmagmacomposi-
tionspronetonanocrystallisation23,35,38,40.

Therheologicalimpactofnanolitesisthoughttobedispropor-
tionate with respect to their size and often low particle content in
magmas13,41.Thehighnumberdensity(N)ofnanoscalecrystalscan
influenceliquidflowbetweenparticles,whilstthelocaldevelopment
of differentiated, high viscosity residual melt surrounding nanolites
caninducerheologicalchangesevenatalowparticlefraction35,36,38,40.
Duetotheirhighsurfacearea,nanolitesarealsosusceptibletoparticle
aggregation through van der Waals interactions, a process which
considerablyincreasestheviscosityofnano-bearingsuspensions13,41.
Asaggregationwillalsoincorporatethemeltbetweennanolites,their
effective volume increases, thereby increasing the viscosity of
nanolite-bearingmagma13.However,sofar,theaggregationofnano-
liteshasbeeninvoked,butnotobserved.

Despite the potential influence of particle interactions on the
rheology of nanolite-bearing suspensions, the interaction between
nanolites has yet to be examined using a three-dimensional (3D)
approach.Itisimportanttoinvestigatethespatialdistribution,inter-
action,andaggregationofnanolitesin3D,intoordertoevaluatethe
impactoftheircrystallisationonconduitdynamicsanderuptivestyle.
However,the technicalchallenge ofresolvingnano-sized crystalsin
multiphase,geologicalmaterials,wherecrystalsandsurroundingglass
havecomparablecontrast,rendersitdifficulttoresolveandvisualise
nanolitesin3Dusingmoreconventionaltomographictechniques42.

X-ray ptychography is a lensless, phase imaging microscopy
techniqueatthenanoscale,alsodescribedasscanningcoherentdif-
fractionimaging(CDI)(Fig.1).Aptychographicacquisitionconsistsof
scanningasampleacrossacoherentbeamwhilstrecordingthedif-
fractionpatterns.Theseareinvertedtoformtheimageofthesample
usinganalgorithminsteadofalens.InstandardCDI,thefieldofviewis
limitedbythelateralcoherencelengthofthebeam.Insteadbyscan-
ning,ptychographyovercomesthislimitation,allowingtheimagingof
extendedobjects.

Ptychographywasfirstconceivedtosolvethelensaberrationsin
electronmicroscopy43,44andthen,in2007,successfullyappliedinthe
X-rayregime45.Sincethen,X-rayptychographyhasachievedareso-
lutionthatexceedsstandardX-rayoptics46andisamongstthemost
used techniques for nanoscale imaging at synchrotron facilities.
Thankstoitsquantitativenature,robustness,andhighsensitivityto
changesinelectrondensity(ρ ),X-rayptychographyhasenabledsci-
entificbreakthroughsacrossawiderangeofdisciplinesfrommagnetic
materialstudies47tomicro-electronics46,48.

Wepresentaninnovative3Dreconstructionandvisualisationof
nanolitesinscoriaeofbasalticPlinianeruptions,acquiredusingX-ray
ptychography.Byreconstructingnanolitesin3D,wecanvisualiseand
quantify their spatial distribution, interaction, and relation with the
residual melt, evaluating their impact on magma rheology during
ascentanderuption.Wefindthatnanoparticlesarepronetoaggre-
gationinbasalticmagmas,increasingtheireffectivevolume.Further-
more, nanolite crystallisation produces chemical changes in the
surrounding melt, which leads to the development of localised
boundarylayersofhigherviscositymeltsurroundingnanolites.The
crystallisation and subsequent aggregation of nanolites markedly
increase their overall contribution to magma viscosity, inducing
rheologicalchangesduringascentthatmaypromotefragmentation
andahighlyexplosiveeruption.Ourresultsprovideinsightintothe
nanoscalestructureofvolcanicproducts,andalsocontributetoour
understandingofthedrivingmechanismsofbasalticPlinianeruptions,
amongstthemosthazardousstylesofvolcanicactivity.

Results
2Dobservationsofnanolites
Arangeofimagingtechniqueswereusedtoexaminetexturalfeatures
inscoriaeoftwobasalticPlinianeruptionsofLasSierras-Masayavol-
canicsystem;theFontanaLapilli(FL)andMasayaTripleLayer(MTL)
eruptions7,8(Methods).High resolutionback-scatteredelectron(BSE)
images obtained using scanning electron microscopy (SEM) show
nanoscalecrystalsinFLscoriae.Duetotheirsize(<1μm),thesecrystals
canbeclassifiedasnanolitesaccordingtotheschemeofMujinetal.25.
Nanolitesarefoundinareaswherethereisinteractionbetweenmelts
oftwodifferentcompositions(Fig.2a,b),graduallyincreasinginsize
towardsthecontact(Fig.2c,d).Nanolitesalsoformstructureswith
concaveinnersurfacesandmoreirregularoutercontacts(Fig.2e,f),as
observed in FL scoriae, which may represent shells of oxides that
nucleateonthesurfaceofbubblesduringvesiculationatthebubble-
melt interface49. The occurrence of multiple shells may reflect a
sequence of detachment from bubbles and subsequent infilling by
melt, crystallising multiple generations of oxides and microlites49.
Theirsizeincreasestowardsthecontactwiththeglassmatrix.Crystal-
poorglassalsoshowsamottledappearance(Fig.2b,d),possiblydue
tothepresenceofnanolites16,17,20,butwherethesizeoftheindividual
crystalsisbelowtheresolutionoftheSEM.

Analysis of the BSE images provides an estimated nanolite
crystalfraction(Φ)of0.27±0.02.SimulationsusingRhyolite-MELTS
(v.1.2)50andtheaverageFLbulkcompositionfindthatthemeasured
nanolitecrystalfraction(0.27±0.02)canbereachedatatempera-
tureof1040°Candapressureof10MPa,withfO equivalenttothe
QFMbuffer.Theseconditionsarewithintheestimatedrangeforthe
FL eruption8. The simulated crystal content consists of both clin-
opyroxene (0.21) and titanomagnetite (0.06), suggesting that the
measuredFLnanolitecrystalfractionof0.27fromthe2DBSEimages
mayconsistofbothphases.Themaximumtitanomagnetitecrystal
fractionestimatedtocrystallisefromthemeltis0.15,asevenforthe
Fe-richFLmeltcomposition,completeextractionofFefromthemelt
during nanolite crystallisation may only crystallise a smallvol.% of
titanomagnetite38. The evolved melt composition (Supplementary
Table1)showsadepletionalsoinMgOandCaO.

By using high angle annular dark field (HAADF) scanning
transmissionelectronmicroscopy(STEM)imaging,wecanresolve
individualnanolitecrystals(Fig.3)withhigherresolution.Imagesof
MTL samples acquired using STEM show spherical-subspherical
nanoscaleparticles 20-30nm in diameter (Fig. 3d), which can be
describedasultrananolites25.Thesespherical-subsphericalparticles
also form clusters with more elongate, irregular shapes (Fig. 3e),
possiblyresultingfromtheaggregationofindividualnanolites.Both
textures are comparable to the spherical and elongate structures
observed in transmission electron microscopy (TEM) and STEM
images of pumice of the Havre 2012 eruption, where irregular,
elongate structures are interpreted as nanolite aggregates30.
Instead,MTLsamplesthatconsistofonlysilicateglass(Fig.3f)do
notshowthesefeatures.

Elemental mapping using energy-dispersive X-ray spectroscopy
(EDS) and STEM shows that the spherical-subspherical clusters
observedintheHAADF-STEMimageshaveahigherconcentrationof
Fe(Fig.3g,h)andTi(Fig.3j,k)thanthesurroundingsilicateglass.Fe
and Ti X-ray intensity maps show the same clustering of spherical-
subspherical nanoscale particles as observed in the HAADF-STEM
images (Fig. 3g, j) and also elongate, irregular Fe-rich and Ti-rich
structureswherenanoliteaggregatesoccur(Fig.3h,k).Instead,X-ray
intensitymapsofthesilicateglasssampleshownorelativeincreasein
theintensityofFeandTi(Fig.3i,l).Similarly,SiX-rayintensitymapsof
thethreesamplesshownoincreaseintheintensityofSirelativetothat
expected for homogeneous silicate glass, as the intensity of Si is
comparableforboththenanolite-bearingandnanolite-freesamples
(Fig.3m-o).

FL and MTL scoriae contain phenocrysts (>40 μm in size) and
microlites (<40 μm) of plagioclase, clinopyroxene, olivine and
titanomagnetite6–8,19. Considering the micron-scale mineral phase
assemblage,X-rayelementalintensitymapsandHAADF-STEMimages,
the observed Fe and Ti-rich, spherical-subspherical nanolites most
likelyrepresenttitanomagnetite.

3DreconstructionofnanolitesusingX-rayptychography
To investigate the morphology and distribution of nanolites in 3D,
threesamplesoftheMTLeruptionwereexaminedusingtheptycho-
graphyendstationoftheI13-1beamline51atDiamondLightSource,
HarwellScience&InnovationCampus(UnitedKingdom)(Methods).
SampleMTL-Gdoesnotcontainnanoscaleparticleswithinthesample
volumeandwasusedasareferencesampleofhomogeneoussilicate
glass(ofbasalticcomposition)tocomparewiththenanolite-bearing
samplesMTL-AandMTL-B.

The 3D images of samples MTL-A and MTL-B show spherical-
subspherical nanolites with a higher electron density (ρ) than the
surroundingsilicateglass.Whiletheρ dependsonboththedensity(ρ)
andcomposition(andtherefore,atomicnumber)ofthecrystalsand
glass42,sincethisisasolidphase,weexpectthemeasuredρ tobe
predominantlyaffectedbythedifferenceincomposition.TheHAADF-
STEMimages,combinedwiththeX-rayintensitymaps,illustratethat
the nanolites are Fe- and Ti-rich (Fig. 3g-h, j-k). In the orthoslices,
spherical-subsphericalnanolitescanberesolvedastheyhaveahigher
ρ thanthesilicateglass(Fig.4).Thespherical-subsphericalshapesof
the nanolites are consistent with previous studies that examine
nanolites in volcanic samples using high-resolution TEM and STEM
imaging13,25,30andalsotheSTEMimagesoftheMTLscoriae(Fig.3d,e).

Forthenanolite-bearingsamplesMTL-AandMTL-B,theptycho-
graphicimagesshowthatρ,and,therefore,composition,varieswithin
the glass. Orthoslices of the ptychographic reconstructions show a
halosurroundingnanolites(Fig.4),correspondingtoanareawitha
lowerρ thanthatofthenanolite-freeglassinsampleMTL-G(Figs.4,5;
SupplementaryFig.1).Thelowerρ suggeststhattheseareashavea
differentcompositionandlowerρthanexpectedforahomogeneous
silicateglass.Thisvariationinρ mayreflecttheformationofdiffer-
entiatedcompositionalboundarylayersthatcandevelopatthemelt-
crystalinterfaceduringcrystalgrowth33,52–57.Astheselayersresultfrom
the crystallisation of Fe-Ti oxide, the layers will be enriched in
incompatible elements such as Si and Al and depleted in Fe and
Ti21,33,40,57.Thisagreeswiththelowerρ ,andthus,ρoftheboundary
layer,asthemoreevolved,residualmeltenrichedinSiO andAl O will
havealowerdensitythanthebulkcomposition58.

ThecommercialsoftwareAvizo(v.2019.1;ThermoFisherScientific,
USA)wasusedtosegmentnanolitesfromthesilicateglassandrecon-
structtheir3Dmorphologyandspatialdistribution.In3D,nanolitesform
aggregatedstructures,whereseveralparticlesareoftenjoinedtogetherto
formelongatestructureswithamoreirregularmorphology(Fig.6a,b).
This morphology is comparable to the 2D observations of nanoscale
aggregatesinclastssampledfromothervolcaniceruptions,suchasthe
Havre 2012 pink pumice30 and the Etna 122 BCeruption13.
Forthenanolite-bearingsamples,nanolitesareapproximately20–300nmindiameterwherethey
are isolated. Instead, aggregates range from 400nm to 6µm in size
(Figs.4,6;SupplementaryTables2-3).Nanoscalestructuresobservedin
theorthoslicesofthe3Dreconstructedvolumesareofcomparablesize
and shape to those observed in HAADF-STEM images (Fig. 3). By per-
formingasecondsegmentation,whichaccountsalsoforthemoredif-
ferentiatedboundarylayerssurroundingnanolites(Fig.6c,d),theinitial
volumeincreasesby60%.

A comparative analysisof2D and 3D characterisationmethods
(Supplementary Table 4) indicates that estimates of the nanolite
volumederivedfrom2Ddatamaybeunderestimatedbyonetotwo
ordersofmagnitude.Thisdiscrepancyarisesasa2Dcross-sectionmay
notfullycapturethenanolitemorphologyin3D,resultinginagreater
underestimationofthevolumewhennanolitesexhibitcomplex,den-
dritic morphologies formed through aggregation-driven growth.
Instead,stereologicalcorrectionsofthevolumeusing2Dimagesmay
providemoreaccuratevolumeestimatesforsmaller,isolatednano-
liteswithspherical-subsphericalshapesandlowaspectratios.

Identificationofnanolites
Thevariationinthegrayscalevalueintheorthoslicesisameasureof
the ρ of the sample. The ρ depends on both the material ρ and
composition,andthus,theatomicnumber.Therefore,thisvariationin
ρ canbeusedtoidentifythenanoliteaggregates.Wecanrelatethe
phaseshiftduetotheX-raybeampassingthroughthematerialcom-
paredtotheair,totheρ oftheunknownphase.Equations1and259
wasusedtocalculatetheρ (m−3)ofthenanoscalephaseusingthe
phaseshift:

ρ = (\pi \lambda / 2 r d) \phi (1)

\delta = (\lambda \phi) / (2 \pi d) (2)

where δ is the refractive index decrement, r the classical electron
radius,λthewavelengthoftheX-ray,ϕthemeasuredphaseshift,andd
thereconstructedvoxelsize.

Toidentifythenanolitephaseandtoquantifytheglasscompo-
sition,theρ wasthenusedtocalculatetheatomicdensity(ρ)(m−3),
forcomparisonwithareferencematerialofknowncompositionusing

Eq.3:

\rho_a = \rho_e / Z (3)

where Z is the sum of the atomic number of each element of the
moleculeunderconsideration,multipliedbytherespectivesubscript.
To calculate the reference ρ for the basaltic glass and titano-
magnetitecomposition,asexpectedforsamplesoftheMTLeruption
(Methods),weusedthemeasuredglassandtitanomagnetitecompo-
sitions acquired using electron probe microanalysis (EPMA) for the
MTLeruption7andtheaverageatomicnumber(Z)calculatedbyProbe
for EPMA software (v.12.9.5) for the glass and titanomagnetite
respectively60.Figure4shows orthoslicesofthe3Dreconstructions
obtained using X-ray ptychography and the range in ρ for both
nanolitesandglass.

Forthenanolites,theestimateofρ showsgoodagreementwith
thereferenceρ calculatedformagnetite,usingaρof5200kgm−361
(Table1).CombinedwiththeresultsoftheX-rayintensitymaps(Fig.3)
andexpectedmineralphaseassemblageforscoriaeoftheMTLerup-
tion, the nanolite phase is most likely titanomagnetite. For sample
MTL-Gwhichconsistsonlyofhomogeneoussilicateglass,thecalcu-
latedglassρ showsgoodagreementwiththereferenceρ estimated
foraglasswithaρof2700kgm−3,comparablewiththeestimatedρfor
theMTLmagmacomposition8.Incontrast,overall,theglassinsamples
MTL-AandMTL-Bdoesnothaveasimilarρ tothehomogeneousglass
sample MTL-G, showing better agreement with a glass of higher ρ
(≥3000kgm−3).

Thisdiscrepancyintheρ ofthenanolite-freeandnanolite-
bearingglasssuggeststhatthevariationinρ observedinthereconstructed3Dvolumes
is controlled by a variation in glass composition and, therefore, ρ.
InterstitialglassbetweennanolitesinsamplesMTL-AandMTL-Bisof
lowerρandadifferentcompositioncomparedtonanolite-freesample
MTL-G.Thismaybeduetoadepletionincompatibleelementssuchas
Fe and Ti and an enrichment in Si and Al following nanolite
crystallisation21,40.Similarly,astudyonglassceramicsfindsvariations
inρ followingthecrystallisationofFe-Mn-Onanoparticles,wherethe
extraction of Fe and Mn from the silicate melt produced a ‘core’
(ρ=4900kgm−3)enrichedinFeandMn,withahigherρ thantheSi-
rich‘shell’(ρ=2200kgm−3)57.

Changesinmeltcompositionduetonanolitecrystallisation
Crystallisation of titanomagnetite nanolites will extract Fe from the
meltstructure,leadingtostructuralmodificationofthemeltdueto
theeffectofFecontentanditsvalencestateonthedegreeofmelt
polymerisation26,62–64. The depletion of iron and its effect on melt
oxidation state, namely the change in the relative proportion of
network-modifying Fe2+ cations relative to network-forming Fe3+
cations,andthesubsequentincreaseinsilicacontent,hasbeenshown
to increase melt viscosity23,35,37,62–65. Crystallisation of pyroxene
nanolitescanhaveasimilareffect,throughthepreferentialextraction
ofnetwork-modifyingcationsFe2+,Mg,andCa65.

The potential for nanolite crystallisation is controlled by melt
composition,particularlyitsTiO andFeO content21,37.Theexperi-
2 tot
ments of Scarani et al.21 demonstrate that the Etna trachybasaltic
composition(FeO =10.05wt.%;TiO =1.67wt.%)ismoreproneto
tot 2
nanolitecrystallisationthantheStrombolibasalticmagmacomposi-
tion (FeO = 7.58wt.%; TiO =0.92wt.%), attributed to its higher
tot 2
transition metal oxide content. As magmas erupted at Las Sierras-
Masaya volcanic system are tholeiitic (MTL FeO =13.67wt.%;
tot
TiO =1.44wt.%)7,8,theyhaveanenrichmentinFeandTirelativeto
2
calc-alkaline melt compositions, and consequently, may be more
pronetonanolitecrystallisation.Theincipientstageofnanolitecrys-
tallisation involves clustering of Fe and Ti, gradually forming a
nanoscaleheterogeneousstructurethateventuallycrystallisesFeand
Ti-bearingnanolites40.ThiscanalsoresultinanenrichmentinAlsur-
roundingnanolites,andSiinthematrix21,35,40.OurHAADF-STEMima-
gesoftheMTLsamplesshowasimilarclusteringofFeandTi,forming
subsphericalparticlesthatare20-30nmindiameter.

Studiesonbothvolcanicsamplesandglassceramicsshowthat
compositional boundary layers form surrounding micron-scale and
nanoscalecrystals,whichdevelopduringtheirgrowth29,33,40,52–54,56,57,66,
indicatingthatnanolitesmayinfluencethestructureandchemistryof
thesurroundingmatrixglassduringcrystallisation13,21,29,36,38,40,67.Asthe
boundarylayers aredepletedincationssuchasFe,Mg,and Ti,yet
enrichedinthenetwork-formingcationsSiandAl,theselayersmay
producealocalincreaseinviscosityrelativetothebulkcomposition,
duetotherelativelyhomogeneousmatrixformedfromastablenet-
work of Si and O bonds21,29,36,38,40,65. These differentiated layers can
increase the effective volume of the nanolite, as their rheological
behaviourwillbeclosertothatofanelasticresponse29,38.Inoursam-
ples, the estimated thickness of the differentiated glass layer sur-
rounding nanolites ranges between 10 and 350nm. Comparing the
segmentation ofthenanolite aggregates both with and withoutthe
incorporationoftheboundarylayer(Fig.6),theinitialvolumeofthe
aggregate increases by up to 60%. Shells surrounding nanolites of
moreevolved,higherviscositymelt21,40,54,68maythenlimittherateof
crystalgrowth,asthelayerswillbedepletedinelementscompatible
withtitanomagnetitecrystallisation29,52.

Asthelayersurroundingnanolitesinoursampleshasalowerρ
thanthesurroundingsilicateglassandglassysampleMTL-G(Figs.4,5),
ithasa lowerρ,reflectingenrichmentinSiand Alintheboundary
layer, and depletion in Fe and Ti following crystallisation of titano-
magnetitenanolites.However,observationsofareaswithanρ inter-
mediate between that of the nanolites and silicate glass may also
indicatethepresence ofultrananoliteswitha sizethatis belowthe
resolutionofthetechnique.Duetothesmallsizeofthelapilli,fast
quenchinginaironejectionfromthevent(secondstoafewminutes69)
mayhave limited chemical diffusion acrossthe boundarylayer and
post-fragmentationmodifications,preservingtheboundarylayers.

Effectonmagmaviscosity
Toevaluatetherheologicalimpactofthenano-particlebearingsus-
pension,weusedseveralrheologicalmodelsandour3Ddatatocal-
culatethemeltviscosityandtherelativeviscosityincreaseattributed
to nanolite crystallisation (Supplementary Table 5), using the mea-
suredcrystalvolumefractionandaspectratio(r ).Belowwepresent
p
theresultsofcalculationswherethecrystal-freemeltviscosityisesti-
matedusingtheGiordanoetal.70model,however,wealsocompare
our calculations with the more recent Langhammer et al.71 model
based on artificial neural networks. Further details on the viscosity
calculations and estimations of uncertainties are provided in the
SupplementaryInformation.

First,weusedtheGiordanoetal.70modeltoquantifythecrystal-
freemeltviscosity,usingtheMTLpre-eruptiveconditions7.Thecal-
culatedmeltviscosityofthebasalticMTLcompositionis6.1×101Pas
at 1080 °C, using the average melt inclusion composition (Supple-
mentaryTable1)andaH Ocontentof2wt.%,asmeasured inmelt
2
inclusionsoftheMTLeruption7.Consideringthemodeluncertainty,
theestimatedmeltviscosityrangesbetween2.1×101and1.8×102Pas.

3.2×102Pasat1080°C,rangingbetween1.1×102and9.2×102Pas
considering the model uncertainty70. Nanolite crystallisation can,
therefore,increasemeltviscosityduetocompositionalchangesinthe
residualmelt,bydecreasingFeOandincreasingSiO.However,recent
2
studiesshowthatviscositymodelsmayoverestimatethemeltviscosity
ofliquids,asnanolitescancrystalliseoverlaboratorytimescalesduring
viscosity measurements in melts prone to nanocrystallisation21,35–38,
andmayformaheterogeneousdistributionofchemicalcomponents
atthenanoscale40.Therefore,ourcalculationsoftheviscosityincrease
duetothechangesinmeltcompositionfollowingnanocrystallisation
mayalsobeanunderestimate.

Nanoliteswillincreasemagmaviscosityduetotheirpresenceas
solidparticles.WeusedtheVonaetal.72modelandtheequationof
Costaetal.73tocalculatetherelativeincreaseinmagmaviscositydue
to the presence of crystals. Incorporating the measured nanolite
crystalfraction(0.27±0.02)increasesmagmaviscosityto1.2×104Pas
for the trachy-andesitic composition. Overall, considering both the
effectonmeltcompositionandcontributionassolidparticles(Fig.7),
nanolitecrystallisationmayincreasemagmaviscositybyafactorof
20–200(SupplementaryTable5).

Experimentsdemonstratethattheinteractionbetweenparticles,
and, consequently, magma viscosity is also strongly influenced by
particle shape74, due to the dependency of the maximum packing
fraction(Φ )and,therefore,theonsetofnon-Newtonianbehaviour,
m
onr 14,72,74,75.Asourptychographicimagesprovideinsightintothe3D
p
morphologyofnanolites,wecanusethesedatatocalculater .From
p
our 3D images, we observe that the measured r for the nanolites
p
rangesbetween1and4(SupplementaryTables2-3).Theflowindex
describesshear-thinningbehaviouranddependsonbothΦ/Φ and
m
r 14, where the transition between Newtonian and non-Newtonian
p
behaviouroccursataflowindexof0.9.AtthemeasuredΦ,thehigher
valuesofr mayleadtotheonsetofnon-Newtonianbehaviour(Fig.8).
p
Finally,theimpactofnanolitesontherheologyofsuspensionsis
alsodependentontheirsizeandN.TheN ofnanolitesisconsiderably
a a
higherthanmicrolitesforthesamecrystalfraction,reducingtheinter-
particledistance,affectingthelocalflowofinterstitialmelt,itsvisc-
osityandpromotingnon-Newtonianbehaviour,evenatlownanolite
contents13. Nanolites may increase magma viscosity by up to two
orders of magnitude21,35,37, even at low particle contents of <5
vol.%13,36,40.

Usingtherelativeviscosityincreasemeasuredfromtheresultsof
analogue experiments on nano-particle bearing suspensions, the
magmaviscosityofEtnatrachy-basaltwasestimatedtoincreasefrom
102to106Paswithananolitecontentof30%,calculatedatpre-eruptive
conditions13. At higher strain rates of 1s−1, as estimated for basaltic
Plinianeruptions9,thisviscosityincreaseoccurswithalowernanolite
contentof10%13.Instead,comparingtheresultsofanalogueexperi-
mentsusingSiO microspheresofasimilarshapeandatstrainratesof
2
1s−1, the same magma viscosity is reached with a higher microlite
contentof60%13,14,74.Asourmeltcompositioniscomparabletothe
trachy-basalticEtnacompositionexaminedbyDiGenovaetal.13,we
canalsosuggestthattheadditionofthenanolitecontentestimatedfor
theMTLmagma(~30%)mayhaveresultedinacomparableincreasein
therelativemagmaviscosity.

Implicationsforeruptiondynamics
Explosiveeruptionsariseduetomagmafragmentationduringascent,
a process which depends on magma viscosity, the timescale of
deformationandthetimescaleofmeltstructuralrelaxation3.Nanolite
crystallisationmayoccurduringmagmaascent24,25,30,37,leadingtosig-
nificantchangesinmagmarheologyduringascentandtransitionsin
eruptivestyle13,24,26–28,30,32,39.

TheFLandMTLscorialapilliwerecollectedfromproximalsam-
plinglocalities6–8andlikelyexperiencedminimalpost-fragmentation
modification following eruption. Their small size (0.3–3cm in
diameter)willhaveallowedrapidcoolingwithintheatmospherefol-
lowing ejection, minimising post-fragmentation crystallisation. The
micro-textures observed within the lapilli may represent features of
which developed prior to magma fragmentation and be used to
interpret conduit processes. This is evidenced by the textures
observedintheBSEimages(Fig.2a–d),whichillustratecomplextex-
tureswithirregular,sinuouscontacts,indicativethatthetwomagmas
were potentially liquid upon emplacement, crystallising nanolites
during mingling. The association between nanolites and vesicles
(Fig.2e,f)mayindicatethatnanolitecrystallisationandvesiculation
occurredatasimilartime.Rhyolite-MELTSsimulationsindicatethat
themeasurednanolitecrystalfractioncanbereachedat1040°Cand
10MPa, likely within the shallow volcanic conduit during magma
ascent.Theρ distribution(Fig.4)alsoindicatesthatnanolitescrys-
e
tallisedinsitufromthesurroundingmelt,duetotheoccurrenceof
haloessurroundingnanoliteswithalowerρ .
e

The in situ observations of Di Genova et al.13 demonstrate that
nanolitecrystallisationcanoccuronrapidtimescales,withnanolites
reaching 8nm in diameter within 2–3min. In these experiments,
nanolitescrystallisedattemperaturesabovetheglasstransitiontem-
perature(T )athighundercooling(ΔT=40–250°C),consistentwith
g
theconditionsestimatedforrapidmagmaascentwithintheconduit
duringbasalticPlinianeruptions12,13.OurSTEMimagesshownanolites
approximately 20–30nm in diameter, which may have crystallised
withinafewminutes.Instead,aggregateswillhavecrystallisedovera
longertimescale.Thistimescaleisconsistentwiththerapidmagma
ascenttimeestimatedfortheMTLeruptionfromnumericalsimula-
tionsof1–13min8.

The FL and MTL Plinian eruptions represent unusually highly
explosiveeventsatbasalticsystems.Ascrystal-poorbasalticmagmais
of low viscosity, gas-melt decoupling can occur during ascent, pre-
ventingmagmafragmentationandpromotingeffusive,orlessintense
explosiveactivity.Theoftenhighlycrystallinegroundmassofsamples
ofbasalticPlinianactivitysuggeststhatrapidmicrolitecrystallisation
mayhaveanimportantroleinincreasingmagmaviscosity,promoting
fragmentation and an explosive eruption, alongside rapid bubble
nucleation and magma ascent6–8,12,16–19,76. Previous work7 which ana-
lysed micron-scale textures in MTL scoriae found that rapid syn-
eruptive microlite crystallisation may have increased the effective
magmaviscosityto106Pas,increasingthepotentialofmagmafrag-
mentation during ascent. Syn-eruptive microlite crystallisation can
provide sites favourable for the heterogeneous nucleation of
bubbles15,whilsttherigidcrystallinenetworkcanincreasethetortu-
osity of connecting channels between bubbles, increasing gas-melt
couplingandrestrictingoutgassing18,76.

FLandMTLclastsshowconsiderabletexturalheterogeneityatthe
micron-scale(Fig.2),wherecrystalfractionvariesbetween0.05and
0.637,8,19.Theresultsofthisstudyshowthatcrystal-poorregionsmay
alsobecrystallisedatthenanoscale,however,inthiscase,thecrystal
sizeisbelowtheresolutionoftheanalyticaltechnique,producingthe
mottledtextureobservedinhigh-resolutionBSEimages(Fig.2b,d).
This mottled texture has also been observed in the groundmass of
clastsofthe1886basalticPlinianeruptionofTarawera,suggestiveof
incipient crystallisation17. Previous work found that the estimated
magmaviscositycalculatedusingthemicron-scalecrystalcontentis
102Pasforcrystal-poorregionsoftheMTLsamples,whilstcrystal-rich
domains have a higher estimated viscosity of 106Pas, due to the
increasedmicrolitecontent7.However,ourestimationoftheviscosity
ofthenanolite-bearingcrystal-poorglassrangesbetween1.4×103to
1.2×104Paswhentheeffectsofnanolitecrystallisationareconsidered.
Weobservethatnanolitecrystallisationcanfurtherincreasemagma
viscositythroughaggregationandtheinfluenceonmeltcomposition
andstructure.Therefore,therheologicalimpactofapparentlycrystal-
poor glassmaybe underestimated by considering onlythemicron-
scalecrystalfraction(Fig.7).Formagmacompositionsthatareprone
tonanocrystallisation21,suchasthetholeiiticbasalticmagmaserupted
at Las Sierras-Masaya volcanic system, assessment of whether the
crystal-poorglassisnanolite-freemaybeimportantfordetermining
therheologicalimpactofcrystallisationandestimatingbulkmagma
viscosity. Alongside syn-eruptive microlite crystallisation and dis-
equilibrium degassing7–9,12,16–19,76, nanocrystallisation would further
increase the viscosity of apparently crystal-poor melt13,24,25,28,36,37,
increasingthepotentialofmagmafragmentationandahighlyexplo-
sivebasalticeruption.

Aggregationofnanocrystals
Aggregatesmayhaveagreaterimpactonmagmarheology13,77,andin
turn,influencethepotentialofmagmafragmentationwithinthecon-
duit.Duetothelargesurfaceareaofnanoscalecrystals,nanolitesare
pronetoaggregationthroughvanderWaalsinteractions13,41.Collisions
betweennanolitesduetoBrownianagitationcanfacilitatenanocrys-
tallisation and aggregation, leading to the formation of structures
100sofnminsize13,41.Involcanicconduits,nanolitesmayinteractwith
eachotherandaggregateundertheinfluenceofshearflow34.

Our3Dptychographicimagesprovideinsightintotheinteraction
betweennanolitesandtheformationandmorphologyofaggregates.
Wherenanolitesareaggregated,theyformstructures(Fig.6)ranging
between400nmand6μminsize(SupplementaryTables2,3),com-
parablewiththeelongate,aggregatedstructuresobservedin2Dima-
ges(Fig.3e,h)30.Thisaggregationwillincreasetheeffectivevolumeof
nanolites13,55.

Nano-sizedcrystalsmaygrowthroughseveralmechanisms,such
as classical nucleation and growth, aggregation and Ostwald
ripening78. All three processes can occur sequentially through time
(Fig.9),contributingtothefinaltextureandnanolitedistribution78,79.
Thesmall(20–30nm)spherical-subsphericalnanolitesobservedinthe
STEMimagesmayreflecttheclassicalmechanismofcrystalnucleation
and growth. However, larger, irregular-shaped nanolites may result
fromparticleaggregationand/orOstwaldripening.

Ostwaldripeningdescribesthegrowthoflargercrystalsfromthe
dissolutionofsmallerones,reducingN andincreasingthemeancrystal
a
size whilst maintaining the volume fraction78,80. Particle morphology
may be used to distinguish between Ostwald ripening and
aggregation78,81. In situ TEM observations of Au and Pt nanocrystal
growth show that in simple growth, a single particle grows whilst
maintaininganearlysphericalshape79,81.Particleinteractionsandcoa-
lescence then produce polycrystalline particles with a more irregular
morphology79,81. In our images (Figs. 2–4) we observe spherical-
subspherical titanomagnetite crystals and more elongate, irregular
shapes,insteadoftheeuhedralcrystalswhichmaybeexpectedfrom
Ostwald ripening80. Therefore, Ostwald ripening likely did not occur,
possiblyduetotheshorttimescaleofcrystalgrowth.Insilicatesystems,
experiments show euhedral olivine and plagioclase can result from
Ostwald ripening, over long timescales and at low undercooling80.
Instead, SAXS and WAXS experiments indicate that nanoscale aggre-
gatesmayformwithinminutesinabasalticcomposition13.

Inigneousrocks,crystalaggregatescanformfrommechanisms
attributedtocrystalgrowthordynamicprocesses49,55,72,82,83.Synneusisis
ahydrodynamicprocessthatcanoccurinconvectingmagmaorduring
settling,leadingtotheattachmentofpreviouslyisolatedcrystals83,84.
Duringsynneusis,meltfilmscanbecometrappedbetweentheattached
crystals,which often have differentcore compositions83.Conversely,
aggregateswhichformfromprocessesrelatedtocrystalgrowth,such
as epitaxy, heterogeneous nucleation or dendritic growth, will com-
priseofcrystalswithasimilarcompositionincrystallographicorien-
tationsrelatedtothemaincrystalhost55,82,83.Ourptychographicimages
illustrate that the interstitial melt between nanocrystals represents a
boundarylayerrelatedtotheircrystallisation52,55.

Heterogeneousnucleationandepitaxialgrowthoftitanomagne-
titeonclinopyroxenehasbeenobservedtoformcrystalaggregatesin
asyntheticFe-richbasalt82.Inepitaxialgrowth,growthofthecrystal-
lisingphaseoccurspreferentiallyonsurfaceswithaspecificcrystal-
lographic orientation82. Although distinguishing between these
mechanismsrequiresdetailedinformationoncrystallographicorien-
tationatthenanoscale,fromourmulti-scaleimagingwecansuggest
that the observed titanomagnetite aggregates may originate from a
mechanismrelatedtocrystalgrowthratherthanadynamicprocess.

Inconclusion,wepresentcutting-edge3Dobservationsofnanolites
intheglassofscoriaefromhighlyexplosivebasalticeruptions,obtained
using X-ray ptychography. Our results provide an innovative, direct
visualisationofnanoliteaggregationinnaturalvolcanicsamplesin3D,a
phenomenondescribedfromtheresultsofexperimentsandmodelling.
TitanomagnetitenanoliteslikelyextractFeandTifromthemeltduring
crystallisation,developinglayersofdifferentiated,Si-richmeltaround
nanolites.Theρ distributionobservedinthe3Dptychographicimages
e
shows nanoscale heterogeneities in glass composition and the devel-
opmentofdifferentiatedboundarylayerssurroundingnanolites.

Our 3D images show that aggregation of individual nanolites
(20–300nm)canproduceaggregatesthatrangebetween400nmand
6μminsize.Growthoftheindividualnanoliteslikelyfollowsaclassical
mechanismofcrystalnucleationandgrowth.Instead,aggregatesmay
formthroughamechanismrelatedtocrystalgrowthsuchasepitaxy,
heterogeneousnucleationordendriticgrowth,ratherthanadynamic
process.EvidenceforOstwaldripeningisnotobservedatthisscale
andmayinsteadleadtotheformationofmicrolites.

Nanolites can, therefore, increase magma viscosity through a
combinationoftheirpresenceassolidparticlesinthemagma,their
interaction and aggregation and the chemical and structural mod-
ificationofthemeltinducedbytheircrystallisation.Thepresenceof
nanolitesinthescoriaeoftheFLandMTLbasalticPlinianeruptions
suggests that the syn-eruptive crystallisation of nanolites during
magma ascent may increase magma viscosity and the potential for
magmafragmentation,leadingtoahighlyexplosiveeruption.

Methods
Sampling
TheFontanaLapilli(FL)eruptionofLasSierras-Masayavolcanicsystem
(60ka)erupted2.9–3.8km3oftephraduringthemainPlinianphase,
reachingamaximumplumeheightof32km85.Themasseruptionrate
of the Plinian phase is estimated as 1.4 × 108 kg s−1. Following the
stratigraphicframeworkofCostantinietal.85,the depositisdivided
into8units(A-G),representingaperiodofcontinuousdepositionover
thedurationoftheeruption(estimatedas4–6h).OurFLsamplesare
scoriae1–3cminsizeandarefromtheopening(UnitA/B)andPlinian
phases(UnitsD-LG),whereFonEreferstounitEoftheFLeruption8,19.
ThesamplinglocalityisthatofBamberetal.8.

The more recent Masaya Triple Layer (MTL) eruption (2.1 ka)
ejected3.4km3oftephra,forminganeruptioncolumnwithanesti-
matedheightbetween21–32km,withepisodesofphreatomagmatic
activity6. The maximum eruption rate is estimated as1 × 108 kg s−1.
ScoriaefromtheMTLeruptionwereobtainedfromPlinianfalloutlayer
TLL2andarescoriae0.3–2cminsize.Thesamplinglocalityisthesame
localitywhichisdescribedinBamberetal.7.

2Dimagingtechniques
High-resolutionBSEimagesofscoriaewereobtainedusingtheJEOL
JXA-8530FFEGelectronmicroprobeatthePhotonScienceInstitute,
UniversityofManchester.Anaccelerationvoltageof15kVandabeam
currentof7–10nAwasusedduringacquisition.Quantitativetextural
analysiswasperformedusing2Dimages(Fig.2b,d)andthesoftware
ImageJ86,to estimate the nanolite crystal fractionover alargerarea
(SupplementaryFig.2).

The high annular dark-field (HAADF) STEM micrographs were
obtained using the FEI Talos scanning transmission electron micro-
scope(STEM)attheDepartmentofMaterials,TheUniversityofMan-
chester,equippedwithanenergydispersiveX-rayspectrometer(EDS)
forcompositionalanalysis.Sampleswerepreparedbygrindingscoriae
oftheMTLeruptionusinganagatemortarandpestle,toproducea
fine-grainedpowderconsistingoffragments~20μmindiameter.The
powderwasthenaddedtoa400μmdiametercopper-carbonmesh
usingapipetteinpreparationforSTEManalysis.

SamplepreparationforX-rayptychography
SampleMTL-Gwasextractedfromacrystal-poorareaofglasswithina
clastof the MTL eruption using a FEINova 600i focused ion beam
(FIB).First,theareawasinspectedusinghighresolutionBSEimagesof
theglasstocheckforthepresenceofmicrolites,obtainedusingtheFEI
Quanta650FEG-SEMattheDepartmentofEarthandEnvironmental
Sciences,TheUniversityofManchester,usinganaccelerationvoltage
of15kVandabeamcurrentof10nA.Thesamplewasthenextracted
fromthisareaofcrystal-poorglassusingthedual-beamFEINova600i
FIB at the Department of Materials, The University of Manchester,
equippedwithaGa+ionbeamandanelectronbeam.Theareawas
sputteredusingtheGa+ionbeamtomillmaterialfromtheglassand
extractthesample,usinganaccelerationvoltageof30kVandabeam
currentof6.5nA.BSEimageswerecollectedusingtheSEMthroughout
themillingprocesstomonitortheextractionofthesample.The10μm
cubicsampleofglasswasthenextractedandmountedonatungsten
needle(Fig.1b)inpreparationforanalysisusingX-rayptychography.

Samples MTL-AandMTL-Bare ~30 μmindividualfragmentsof
scoriae, prepared as a fine powder by grinding scoriae in an agate
mortarandpestle.Theindividualsampleswerethenmountedona
carbonfibre(diameter=7μm)usingamicro-manipulator.Thisfibre
wasthenmountedonawoodenstickandplacedontherotationstage
foracquisitionusingX-rayptychography.

X-rayptychography
Theexperimentwasperformedusingtheptychographyendstationof
theI13-1beamlineatDiamondLightSource51.Theexperimentalsetup
is shown in Fig. 1. The coherent X-ray is focused down to a 3 μm
illuminationatthesampleusingasetofblazedFresnelzoneplates
(FZP in Fig. 1a)87 (with a 400 μm diameter and 150nm outer zone
width).OnlythefirstdiffractionorderoftheFZPreachesthesample.
The higher diffraction orders are blocked using an order sorting
aperture(OSAinFig.1a-10μmdiameteriridiumpinhole)locatedclose
tothefocusofthefirstdiffractionorder.Thezeroorderisblocked
usingacentralstop(CSinFig.1a-50μmdiameter,50μmthickgold)
placedupstreamoftheFZP.Thesampleismountedonthetopofa
3-axis PI-Mars piezo stage which sits above a rotation stage. For
acquiringthe3Dptychographicdataset,thesampleisrotatedovera
rangeof180degrees,withastepsizeof0.18degree.Ateachangular
position,thesampleisscannedacrosstheilluminationina32×40step
grid,witha1μmstepsize,usingthepiezostage.Thediffractionpat-
terns are recorded at each scanning position with a Merlin photon
countingdetector(siliconchip,512×512pixels,55μmpixelsize).

Twodifferentconfigurationswereusedduringtwoexperimental
sessions.Thefirstsessionuseda15keVX-raybeamandasample-to-
detectordistanceof4.2m.Thisconfigurationwasappliedtoimage
sample MTL-B. This configuration corresponds to a reconstructed
pixelsizeof14nm.Thesecondsessionuseda11.7keVX-raybeamand
asample-to-detectordistanceof8m.Thisconfigurationwasappliedto
imagesamplesMTL-GandMTL-A.Thisconfigurationcorrespondstoa
reconstructed pixel size of 60nm. The image reconstruction was
performedusing1000iterationsoftheePIEalgorithm88implemented
in PtyREX code89. The resolution of the instrument was measured
duringtheexperimentbyimagingaSiemensstarstandardtestreso-
lutiontarget(SupplementaryFig.3)andwasdeterminedtobe50nm.

The segmentation of nanolites from the glassy matrix and the
segmentationoftheevolvedglassaroundandbetweennanoliteswas
operated in the 3D domain using Avizo software (v.2019.1; Thermo
Fisher Scientific, USA). Manual bi-level greyscale thresholding was
applied,basedonthegreyscalehistogramoftheselectedVOIsand
visualinspectionoftheslicesindifferentdirections(axial,coronal,and
sagittal).ThisapproachfollowsthatofArzillietal.55.Thereconstructed
segmented3DimageswerethenanalysedusingAvizosoftware,where
thevolumesanddimensionsofnanolitesandtheglassaroundnano-
litesweremeasuredusingthelabelanalysismodule.

3D visualisation (volume renderings) of the reconstructed
volumes wasobtainedusing thecommercialsoftwareVGStudio3.0
(VolumeGraphics).Thevolumerenderingswereusedtoperform3D
textural observations of nanolite morphology and the interaction
between nanolites and the surrounding evolved glass (Fig. 6). The
segmentationofthenanolites,surroundingevolvedglass,andmatrix
glass was based on both the visual inspection of images and the
quantitativeinformationprovidedbytheestimatesofρ .Aseachpixel
e
in the ptychographic images provides information on ρ , and the
e
nanoliteandglassphaseshaveaspecificrangeinρ whichreflectstheir
e
composition (Table 1), the spatial variation in ρ was also used for
e
thresholdingandthesegmentationofeachphase(Fig.4).Theglass
controlsampleMTL-G(Fig.5)wasusedasareferencefortheρ of
e
basalticglassandforthesegmentationofthematrix.

Atomicdensity
Tocalculatetheρ ofthenanolitesandtheglass,thecompositional
a
dataofBamberetal.7wereused,obtainedfromnaturalclastsofunit
TLL2oftheMTLeruption.ProbeforEPMAsoftware(v.12.9.5)wasused
tocalculatetheaverageatomicnumber60forthecrystal-poorbasaltic
glass (n=9) and titanomagnetite (n=10), where n represents the
numberofEPMAanalyses.Theaverageatomicnumberestimatedfor
theMTLbasalticglassis12.91±0.07(1σ),andfortitanomagnetite,itis
20.32±0.04. These analyses were used to determine the reference
valueofρ forbasalticglassandtitanomagnetiteinnaturalclastsof
a
unit TLL2 of the MTL eruption, for comparison with the unknown
valuesofρ obtainedforthenanolitesandglassusingtheorthoslices.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_2": "The key results of this manuscript are that abundant nanolites (crystals <1 µm in size) and clusters of nanolites (up to ~6µm in size) have been imaged in scoria from a basaltic Plinian eruption deposit in Nicaragua. The X-ray ptychography technique used to image the nanolites is exciting and seems to have great potential. The agglomeration process and volume of interstitial melt that can be trapped is discussed, although some of the arguments need clarification. The compositions of the uncrystallized glass, and glass around the nanolites, are used to estimate rheological changes due to crystallization, and due to agglomeration of nanolites. The effects of crystallization on magma rheology and potentially crossing the ductile-brittle transition are undoubtedly very important, although the effects of agglomeration are less convincingly presented. In this section it would be very helpful to explicitly mention the uncertainties in all the models used, from melt viscosity to the effect of crystals, and all the assumptions being made, for example temperature, water content (which should change during crystallization, along with the rest of the melt composition), strain rate, etc.\nIn summary, the results are interesting and add to a growing body of literature suggesting that nanolites must be considered along with microlites when considering evolving magma rheology. This is important in volcanology, and especially to understanding basaltic Plinian eruptions. Comments below are divided into 5 main themes. I hope they will be useful in revising the manuscript.\n[1] Please try to be specific when citing references.\nFor example on line 88, not every reference in the range 19-37 is about nanolites, so there should be some selection applied and consequent re-ordering of the references. Another example is on line 102, where reference 43 is about nanofluid viscosity but it does not mention either agglomeration or van der Waals forces, so this is not the best place to cite it. Please just double check that the appropriate references are cited in each place throughout the manuscript.\n[2] The nature of the chemical zoning in the melt around the nanolites deserves closer scrutiny and possibly a different description.\ne.g. On lines 204-212 it is asserted that the halo has “an ρe intermediate between that of the silicate glass (Fig. 4) and nanolites”. In order to get a melt that was enriched in Fe and Ti relative to the uncrystallized glass would imply either (i) it could contain ultrananolites that are smaller than the spatial resolution of the technique, or (ii) some previously formed nanolites had dissolved into the melt, which would require some explanation.\nBut some of the glass next to the nanolites actually has a lower ρe than the background glass – this is most easily seen by colour-inverting Fig 4a. Look just left of center, and also towards the bottom left hand corner. This makes much more logical sense – if Fe and Ti have been removed from the melt layer from which titanomagnetite nanolites formed, that residual melt should be less dense (consider for example the partial molar volume data of Lange and Carmichael 1987 CMP and more recent studies, which suggest that FeO and TiO2 are the densest major oxides while SiO2 and Al2O3 are much less dense).\nThe reason this is important is that if the nanolites formed in situ, i.e. from the melt with which they are currently associated, this is the pattern one would expect. If they have migrated through the melt they may have been able to shed their FeTi-depleted halo, which one would expect to lead to different properties of the bulk magma.\ne.g. On lines 263-270 it is again suggested that “the glass in the nanolite-bearing samples is likely of higher ρ and a different composition” but the example from the literature used to justify this is one where the nanoparticles are surrounded by a “Si-rich ‘shell’ (ρ = 2.2 g/cm3)”. In that example, the matrix has a density of 2.7 g/cm3, so the Fe-depleted shell is lower density. There is no reason to think that is not also true for the samples presented here, as detailed in the previous paragraphs. For the shell to be higher density than the surrounding glass would require some remarkable geochemical behaviour that would require additional explanation.\ne.g. Line 305. Normal-sized crystals, which are also surrounded by haloes of depleted melt (boundary layers), still manage to grow. So there may be additional justification needed to assert that the halo prevents further growth of nanolites. And does “gas escape” here refer to actual bubbles (nanobubbles?) or to the diffusion of volatiles, which will also be enriched in the halo? This needs clarifying because there are structures in Figure 2 ascribed to the collapse of bubbles that have escaped.\nOne important thing to note is that the halo is depleted in elements that formed the crystal, but enriched in the other elements relative to the adjacent melt reservoir. Chemical diffusion along this gradient will then drive homogenization, so one important question is the timescale of quenching – in the lab or in a Plinian eruption this can be quick, preserving the haloes.\ne.g. lines 308 to 314. The presence of undetectable ultrananolites is one possibility, another is dissolution of previously formed nanolites (although why this would happen would need explaining). A third possibility is that the haloes are in fact less dense than the surrounding melt – as appears to be the case from Fig. 4 – although that would appear inconsistent with the densities calculated in Table 1.\nLines 413 to 416. The melt compositions in Supplemental Table 3 also require cpx crystallization, but this should be mentioned explicitly earlier on, when reporting the melt compositions and attempts to identify the nanolite minerals. Is it possible that the phase contrast between clinopyroxene nanolites and residual glass would be so small as to be essentially undetectable with this technique? Could this potentially resolve the problem of a dense melt halo (lines 204-212)?\n[3] The discussion of the effects of agglomeration probably needs some refining.\ne.g. Line 133 how does agglomeration of existing particles produce “chemical changes in the surrounding melt which markedly increases their overall contribution to magma viscosity”? Nucleation and growth of new particles would cause changes in the surrounding melt due to depletion of Fe and Ti, but is there a reason for agglomeration of existing particles (presumably with their own depleted haloes) to cause further changes?\ne.g. lines 326 to 328. “… agglomeration will increase the effective volume of nanolites. Comparing the segmentation of the nanoscale aggregates both with, and without, the incorporation of the surrounding glass (Fig. 6), the initial volume of the aggregate increases by up to 60 %.”\nBut Fig 6 is comparing the agglomerate with and without its depleted melt halo. The proper comparison is between the agglomerates and the sum of the individual particles with their individual haloes. Agglomeration could result in a smaller volume of enriched melt, because the agglomerated particles presumably have a smaller total surface area than the sum of the individual particles. If the depleted halo compensates for this by thickening, then perhaps there is simply no change in its effective volume.\nThe effective volume could increase if additional (undepleted) melt were trapped between the particles due to agglomeration. For example, random close packing of uniform spheres yields ~36% space between the spheres. So perhaps the effective volume could be 36% higher, but if the particles need to touch to stick (as van der Waals forces decay extremely rapidly with increasing distance), much of the interstitial melt is likely the depleted halo material.\nIn short, if a clearer way can be found to explain how agglomeration leads to an effective volume increase, and what the quantitative magnitude of that increase could be, it would be appreciated.\n[4] The discussion of calculated viscosities needs careful inclusion of uncertainties and better explanation of the assumptions being made\nLines 348 to 352 – please give the model uncertainties on these calculations. For GRD the RMSE for volatile-bearing melts is ±0.46 log units (their Table 3) so combining that with the ±0.17 log unit RMSE for Langhammer et al. (2021) means the calculated melt viscosity is 9-19 Pas (Langhammer, best estimate 13 Pas with ±0.17 log unit rmse) or 21-176 Pas (Giordano with ±0.46 log units rmse). Using the ±0.74 log unit RMSE for the GRD model applied to the Langhammer et al. dataset, gives a range of 11-335 Pas, which is already a substantial overlap with the Langhammer model. Given that RMSE is a one sigma uncertainty, the two models in fact agree quite well, even though that gives a total range of 9-335 Pas (or 0.9-2.5 log units). I do agree that it is useful to show how much uncertainty there still is even for predicting melt viscosity.\nLine 350 – why assume 2 wt.% H2O – how was this water content determined (by previous studies)? For the evolved melt composition, are you also using 2 wt% H2O? Crystallization of 27 vol.% crystals should increase the water content in the melt by ~35%, and this should be factored into the magma viscosity calculations.\nLines 357-361 I think the text is saying that all existing viscosity measurements (and hence models) are suspect because nanolites may have affected them. For the specific point being made, that would likely affect measurements of both the initial liquid and the depleted halo liquid – there is still plenty of Fe and Ti in the halo (Supp Table 3). But it is also worth remembering that crystals will not form below the liquidus, and that fast cooling rates seem to be required to form nanolites rather than microlites – which are easily detected by microscopic inspection. Nanolites form at cooling rates of several˚C/s (Di Genova et al. 2020) but viscosity measurements by e.g. micropenetration or parallel-plate viscometry typically use much lower cooling rates. And even if they were to form, a sudden nanolite nucleation event should be easily detected in the viscosity data as a discrete jump, given the strong effect they can have on viscosity. So on balance this argument does not feel well justified.\nLine 363 please provide the actual equation for relative viscosity as a function of crystallinity that was used here. Which of the approaches used in Vona et al. (2011) was used here? These include a modified Krieger-Dougherty equation from Ishibashi (2009), the Costa et al. (2009) model, and then a new parameterization based on crystal aspect ratio and strain rate. All three of these are strain-rate-dependent and so the strain rate used in the calculations needs to be stated.\nLine 373 To summarize, the calculated evolved melt viscosity is 35 Pas (actually 24-52 Pas allowing for ±1 rmse from Langhammer) or 320 Pas (actually 110-920 Pas allowing for ±1 rmse from Giordano). The calculated magma viscosity is 150-1400 Pas (Mader model) or 1400-12000 Pas (Vona model). The estimated increase is then a factor of ~4-5 (Mader) or 40-50 (Vona), but not 10-100. Bearing in mind the effect of increased water content in the evolved melt, which is not considered here, the actual viscosity change is likely to be less than this. It is still obviously an important change, but the way things are written seems to always be pushing the limits of the maximum effect that could be claimed, while ignoring the uncertainties in all of these models.\nLine 380 there needs to be additional context on the basalt with 30% nanolites reaching a viscosity of 10^6 Pas. What temperature, composition, water content (if any) and what would the crystal-free melt viscosity be at those conditions? Likewise additional context is needed for stating that 60% microcrystals are needed to achieve the same viscosity as 30% nanolites. A quick look at e.g. Figure 2 of Costa et al. (2009) indicates there is a huge uncertainty in the experimentally determined effect of micro-crystallinity on effective magma viscosity. Lejeune and Richet (1995) documented a relative viscosity of more than 1000 at only 40% crystals in one experiment, so at the very least there needs to be acknowledgement of the uncertainties in these statements.\nLine 384 relative viscosity increase is mentioned here but the starting viscosity is not – what was the relative viscosity increase documented by Di Genova et al., and what actual viscosity would that imply starting with the melt compositions and viscosities discussed above?\nLines 393-397 “The micro-textures observed within the lapilli may, therefore, represent features which developed prior to magma fragmentation”.\nIt is tempting to assume that the crystal textures all formed pre-fragmentation, but wouldn’t these clasts have cooled (post-fragmentation) at almost exactly the rates used to stimulate nanolite crystallization in previous experiments such as by Di Genova et al. (2020)? An estimate of the cooling rate could be made based on the pyroclast thermal model of Moitra et al. (2018 G-cubed). For 2-3 cm clasts the cooling rate should be ~4˚C/s from 1000 to 600˚C. Processes in the conduit prior to fragmentation would likely involve much slower cooling, at ranges typically associated with microlite growth. This should perhaps be stated more clearly.\nAlternatively, if the timescale of “a few minutes” applies to both nanolite growth and ascent to the surface this implies that magma mixing caused an immediate eruption (with a delay of less than a few minutes). How could these two magmas mix that quickly? Do they have different compositions? Were they stored at different temperatures? If the answer to either or both of these is yes, the rheological calculations should compare the two magmas. Such rapid eruption triggering would imply a sudden degassing event, but what exactly about the mixing process would have caused it?\nLines 423-425. The assertion that the MTL magma viscosity did reach 10^6 Pas in the conduit, from Bamber et al. (2020), appears based on a microlite fraction of 0.5 and a starting melt viscosity of 10 Pas (section 4.3.2) or 30-40 Pas (section 4.2.3), and the Costa et al. (2009) model. Costa et al. (2009) present a framework for fitting experimental data but not a specific set of recommended parameters. Mader et al. (2013) suggested a set of parameters for use with Costa et al. (2009) but they are somewhat arbitrary – the data in Fig 2 of Costa et al. (2009) show that there is no single equation (or parameters) that can describe relative viscosity as a function of crystal fraction. The MTL magma viscosity quite possibly could have reached 10^6 Pas, but again there are uncertainties that need to be acknowledged.\n[5] Miscellaneous minor points:\nLine 1 The title might be more accurately written as “3D visualisation of nanolite aggregates in basaltic magmas...” because the process of aggregation is not seen in a static image.\nLine 64 “high viscosity facilitates the accumulation of stress due to applied strain”. This may need rephrasing – in volcanic systems the stress is applied and the strain results from that, not vice-versa. High viscosity means a low strain rate and hence low rate of stress dissipation, but it does not cause the high stress in the first place.\nLine 74 how does “fracturing … modify the rheology of basaltic magma”? Isn’t fracturing an indication that the rheology has already changed from the ductile to brittle regime, through some other mechanism (e.g. high strain rate, change in effective viscosity, etc)?\nLines 90-92 is it necessary to list “increasing melt polymerization” separately from “chemical and structural changes in the residual melt”? It may be better worded as “by facilitating heterogeneous bubble nucleation or inducing chemical and structural changes in the residual melt, for example increasing melt polymerisation, …\"\nLines 176-178 I agree that there does not appear to be Si enrichment around the nanolites – and for Fig 3m there does not appear to be much Si depletion at the site of the nanolites either, which is surprising.\nLines 223-225 and caption to figure 6. The caption should also explain why this part of the glass is shown and how it is distinguished from the rest of the glass (lower density). Where we can see the nanolites in Fig 6 c and d, does this imply a lack of the evolved boundary layer in these areas, or has it simply been cut away to reveal the nanolite inside? This is important to know. The 2D orthoslices suggest that the enriched boundary layer is not present everywhere around the nanolites.\nLine 233 why is the unit of rho_e cm-3 and not m-3? cgs units should be replaced with SI units wherever possible.\nLine 242 does rho_a have a name (e.g. is it the atomic density), and are the units m-3?\nLines 297-301 Si4+ is also a cation, so the boundary layers are not “depleted in cations” but they are often depleted in Fe and Mg or Ti. The key is that the boundary layer is enriched in network-forming Si4+ and Al3+ (don’t forget the alumina!). The long sentence on lines 297-301 mentions viscosity increase in two different places, and should be restructured.\nLines 301-304. If you observe shells are 10-350nm in size then that is the size they can reach (at least!). Remove “can reach 10-100 nm in size” on line 301.\nLine 341 “Nanolites can increase magma viscosity by up to two orders of magnitude”. So can larger crystals - it all depends on the volume fraction, which should be specified here. Unless this is meant to imply an additional increase relative to microcrystals at the same volume fraction, in which case that should be specified clearly.\nLine 480 Should the beam diameter be 10 nm rather than 10 microns for the BSE imaging? A 10 micron diameter would be good for a glass microprobe analysis but not for high resolution imaging.\nLines 561-562 should the average atomic numbers be given as 12.91±0.07 and 20.32 ± 0.04? Please specify whether the ± is one standard deviation or two.\nFig. 2 is described in the caption as showing two pieces of scoria from the FL eruption, and the image labels are “FonAB” and “FonE”. These appear to be two different layers of the FL eruption but this should be clarified. Moving the sample descriptions from supplemental material into the main text would help with this.\nFig. 5 it would be helpful to see MTG at the same spatial scale as MTL-A is shown in Fig. 4 (as they were collected using the same geometry and resolution). To help make the comparison described in the text, this could become a third panel of Fig. 4, and the greyscale should be the same in all three panels (currently the greyscale range in Fig 5 maps to a different range of electron densities compared that used in Fig 4). Please also explain the cause of the “thumbprint” pattern evident in Figure 5 – is this some kind of artifact arising from the ptycographic technique or the image processing?\nFor the supplemental information, it would be helpful for the reader to have the sample information in the main article text. Supplementary Tables 3 and 4 would also be useful in the main text, as the glass compositions and calculated viscosities are discussed extensively.", "reviewer_3": "Review of manuscript NCOMMS-24-72256\nTitled: \" 3D visualisation of nanolite aggregation in basaltic magmas using X-ray ptychography: Implications for magma rheology”\nsubmitted for publication in the Journal of Volcanology and Geothermal Research.\nAuthors: Emily C. Bamber, Fabio Arzilli, Silvia Cipiccia, Darren J. Batey, Giuseppe La Spina, Margherita Polacci, Ali Gholinia, Heath Bagshaw, Danilo Di Genova, Richard Brooker, Daniele Giordano, Pedro Valdivia, and Mike R. Burton\nDear Dr. Neely,\nThank you for the opportunity to review this article. The manuscript presents a new approach to volcanic material characterization to provide a careful assessment of the Nanolite content using two explosive basaltic eruptions from the eruption of Las Sierras-Masaya volcanic system as case studies.\nMeasuring and characterizing Nanolite size and geometry is something that traditional volcanological studies have not done and this technique is a promising step toward understanding more about whether and how important these particles are in terms of determining eruption intensity and style. The presented multipronged approach, using a wide range of complementary analytical techniques is exciting and may help advance our understanding of these phenomena. The presented data and analysis are sound and support the main arguments.\nIn the annotated pdfs of the main manuscript and supplements I have attached to this letter I provide a range of comments that include minor suggestions for re-structuring and re-wording and some more in-depth analyses / questions about what could be discussed on the basis of these data. I realize that some of those may go beyond what the authors intend to present in this paper but wanted to include them in case there was capacity to address them here or in future studies. I also provide a few suggestions for where and how this work could be improved and where more detail / better reasoning may help. I hope this can serve to further improve this piece of research. Overall, I suggest minor revisions to address relevant comments before publication.\nBest Regards,\nStephan Kolzenburg"}, "author_response": "We thank both of the reviewers for their constructive and insightful comments which have\ngreatly improved our work. Reported below is a point-by-point response to each comment\nraised by the reviewers, with line references to where the corresponding changes can be\nfound in the revised manuscript. These revisions are colour coded in the updated article file\nand the comments below, where responses to reviewer #2 are highlighted in blue and\nresponses to reviewer #3 are highlighted in orange.\nREVIEWER COMMENTS\nReviewer #2 (Remarks to the Author):\nThe key results of this manuscript are that abundant nanolites (crystals <1 µm in size) and\nclusters of nanolites (up to ~6µm in size) have been imaged in scoria from a basaltic Plinian\neruption deposit in Nicaragua. The X-ray ptychography technique used to image the\nnanolites is exciting and seems to have great potential. The agglomeration process and\nvolume of interstitial melt that can be trapped is discussed, although some of the arguments\nneed clarification. The compositions of the uncrystallized glass, and glass around the\nnanolites, are used to estimate rheological changes due to crystallization, and due to\nagglomeration of nanolites. The effects of crystallization on magma rheology and potentially\ncrossing the ductile-brittle transition are undoubtedly very important, although the effects of\nagglomeration are less convincingly presented. In this section it would be very helpful to\nexplicitly mention the uncertainties in all the models used, from melt viscosity to the effect of\ncrystals, and all the assumptions being made, for example temperature, water content\n(which should change during crystallization, along with the rest of the melt composition),\nstrain rate, etc.\nIn summary, the results are interesting and add to a growing body of literature suggesting\nthat nanolites must be considered along with microlites when considering evolving magma\nrheology. This is important in volcanology, and especially to understanding basaltic Plinian\neruptions. Comments below are divided into 5 main themes. I hope they will be useful in\nrevising the manuscript.\n[1] Please try to be specific when citing references.\nFor example on line 88, not every reference in the range 19-37 is about nanolites, so there\nshould be some selection applied and consequent re-ordering of the references. Another\nexample is on line 102, where reference 43 is about nanofluid viscosity but it does not\nmention either agglomeration or van der Waals forces, so this is not the best place to cite it.\nPlease just double check that the appropriate references are cited in each place throughout\nthe manuscript.\nWe agree with the reviewer and have now revised the citations and references throughout\nthe manuscript. We have made specific changes to the lines highlighted by the reviewer,\nwhich can now be found at lines 89 and 105 in the revised manuscript. We have also revised\nthe citations in the discussion, particularly in the section on melt composition (line 304). Due\nto these changes we have also rearranged the references and restructured the reference\nlist.\n[2] The nature of the chemical zoning in the melt around the nanolites deserves closer\nscrutiny and possibly a different description.\ne.g. On lines 204-212 it is asserted that the halo has “an ρe intermediate between that of the\nsilicate glass (Fig. 4) and nanolites”. In order to get a melt that was enriched in Fe and Ti\nrelative to the uncrystallized glass would imply either (i) it could contain ultrananolites that\nare smaller than the spatial resolution of the technique, or (ii) some previously formed\nnanolites had dissolved into the melt, which would require some explanation.\nBut some of the glass next to the nanolites actually has a lower ρe than the background\nglass – this is most easily seen by colour-inverting Fig 4a. Look just left of center, and also\ntowards the bottom left hand corner. This makes much more logical sense – if Fe and Ti\nhave been removed from the melt layer from which titanomagnetite nanolites formed, that\nresidual melt should be less dense (consider for example the partial molar volume data of\nLange and Carmichael 1987 CMP and more recent studies, which suggest that FeO and\nTiO2 are the densest major oxides while SiO2 and Al2O3 are much less dense).\nWe thank the reviewer for this comment and agree that the areas of glass adjacent to\nnanolites with a lower ρ represent the residual melt depleted in Fe and Ti and enriched in Si\ne\nand Al. Following nanolite crystallisation, the melt will be enriched in Si and Al and will,\ntherefore, have a lower density than the bulk composition, where this change in both melt\ncomposition and density is reflected in the variation in ρ . We have now revised the\ne\npresentation of the results to highlight these areas of lower ρ in Figure 4 and provide a\ne\nrevised interpretation which focuses on the change in melt composition and density following\nnanolite crystallisation (lines 217-228). We have also updated the paragraph which\ndiscusses the ρ data in the later section of the discussion (lines 290-291). Lange (1997) has\ne\nbeen added to the references section.\nThe reason this is important is that if the nanolites formed in situ, i.e. from the melt with\nwhich they are currently associated, this is the pattern one would expect. If they have\nmigrated through the melt they may have been able to shed their FeTi-depleted halo, which\none would expect to lead to different properties of the bulk magma.\nWe have added a more detailed interpretation of the results and the in situ crystallisation of\nnanolites from the melt to the discussion at lines 424-426.\ne.g. On lines 263-270 it is again suggested that “the glass in the nanolite-bearing samples is\nlikely of higher ρ and a different composition” but the example from the literature used to\njustify this is one where the nanoparticles are surrounded by a “Si-rich ‘shell’ (ρ = 2.2\ng/cm3)”. In that example, the matrix has a density of 2.7 g/cm3, so the Fe-depleted shell is\nlower density. There is no reason to think that is not also true for the samples presented\nhere, as detailed in the previous paragraphs. For the shell to be higher density than the\nsurrounding glass would require some remarkable geochemical behaviour that would require\nadditional explanation.\nWe agree with the reviewer and have revised the sentence to highlight the occurrence of\nglass with a lower ρ (observed in Figure 4), which likely corresponds to the SiO and Al O\ne 2 2 3\nenriched residual melt following crystallisation (lines 290-296). This is in agreement with the\nreference provided, where less dense, Si-rich shells are observed surrounding Fe-Mn-O\nnanoparticles.\ne.g. Line 305. Normal-sized crystals, which are also surrounded by haloes of depleted melt\n(boundary layers), still manage to grow. So there may be additional justification needed to\nassert that the halo prevents further growth of nanolites. And does “gas escape” here refer to\nactual bubbles (nanobubbles?) or to the diffusion of volatiles, which will also be enriched in\nthe halo? This needs clarifying because there are structures in Figure 2 ascribed to the\ncollapse of bubbles that have escaped.\nWe have now revised the sentence to indicate that the formation of compositional boundary\nlayers with a more evolved composition, depleted in the elements compatible with\ntitanomagnetite, may reduce the rate of crystal growth and limit further crystallisation (lines\n332-334). What we would like to suggest is that the change in melt composition adjacent to\nnanolites may reduce the rate of crystal growth due to the depletion in compatible elements,\nrather than preventing further growth. We have revised the sentence to clarify this. The\nreference to ‘gas escape’ refers to the decoupling of gas bubbles from the melt, as\ndiscussed in references Di Genova et al. (2020) and Kennedy et al. (2022) which are cited in\nthis section of the manuscript. The crystallisation of Fe-Ti oxide nanolites has been shown to\nfacilitate the heterogeneous nucleation of bubbles and maintain gas-melt coupling due to the\nhigher bubble number density. However, this sentence has now been revised and the\nreference to gas-melt decoupling removed as it is not explored further in the text and does\nnot relate directly to our results on nanoscale aggregates.\nOne important thing to note is that the halo is depleted in elements that formed the crystal,\nbut enriched in the other elements relative to the adjacent melt reservoir. Chemical diffusion\nalong this gradient will then drive homogenization, so one important question is the\ntimescale of quenching – in the lab or in a Plinian eruption this can be quick, preserving the\nhaloes.\nWe thank the reviewer for this comment and have added a sentence at lines 340-343 to\nhighlight that the small size of the lapilli clasts examined in this study likely experienced rapid\nquenching following ejection from the vent, on the order of seconds to minutes, preserving\nthe haloes observed in the matrix glass.\ne.g. lines 308 to 314. The presence of undetectable ultrananolites is one possibility, another\nis dissolution of previously formed nanolites (although why this would happen would need\nexplaining). A third possibility is that the haloes are in fact less dense than the surrounding\nmelt – as appears to be the case from Fig. 4 – although that would appear inconsistent with\nthe densities calculated in Table 1.\nWe agree with the reviewer and have now updated this section of the discussion to better\npresent the results and interpretations of the ρ data, also in response to the comments\ne\nabove (lines 217-228; 290-296). Instead, areas with an ρ intermediate between that of the\ne\nnanolites and silicate glass may indicate the presence of ultrananolites with a size that is\nbelow the resolution of the technique (lines 338-340).\nLines 413 to 416. The melt compositions in Supplemental Table 3 also require cpx\ncrystallization, but this should be mentioned explicitly earlier on, when reporting the melt\ncompositions and attempts to identify the nanolite minerals. Is it possible that the phase\ncontrast between clinopyroxene nanolites and residual glass would be so small as to be\nessentially undetectable with this technique? Could this potentially resolve the problem of a\ndense melt halo (lines 204-212)?\nFrom the BSE images (Figure 2) and results of modelling using Rhyolite-MELTS, the melt\nshould also crystallise clinopyroxene nanolites. The measurement of the nanolite crystal\nfraction from the BSE images is 0.27, which indicates that there may also be nanoscale\nclinopyroxene, as complete extraction from the melt may only crystallise a few vol.% of Fe-Ti\noxide (lines 163-172). This agrees with the melt compositions in Supplementary Table 3\n(now 1), as clinopyroxene crystallisation is also needed to produce the more evolved\ncomposition of the residual melt. Clinopyroxene nanolites have also been observed in\nvolcanic samples by previous studies (Sharp et al., 1996; Mujin et al., 2017). We have added\na sentence earlier in the manuscript which references Supplementary Table 1 and specifies\nhow the change in melt composition may result from crystallisation of both Fe-Ti oxide and\nclinopyroxene nanolites (lines 172-173).\nHowever, we do not observe clinopyroxene in the orthoslices from X-ray ptychography,\nonly titanomagnetite. As it is possible that it may be difficult to resolve clinopyroxene in the\nimages due to the phase contrast, we also calculated the expected atomic density for\nclinopyroxene using the measured atomic number in clinopyroxene from the MTL samples\nand the density for clinopyroxene (3.2 g/cm3), to obtain quantitative information on the\nphase. However, the calculated atomic density of the melt halo surrounding nanolites does\nnot agree with the estimation for clinopyroxene, but is more compatible with a phase of\nhigher density. Therefore, from the images, it is more likely that the dense areas surrounding\nnanolites represent areas where there are Fe-Ti nanolites, but of a scale which is not\nresolvable using the technique (lines 338-340 in the revised manuscript).\n[3] The discussion of the effects of agglomeration probably needs some refining.\ne.g. Line 133 how does agglomeration of existing particles produce “chemical changes in the\nsurrounding melt which markedly increases their overall contribution to magma viscosity”?\nNucleation and growth of new particles would cause changes in the surrounding melt due to\ndepletion of Fe and Ti, but is there a reason for agglomeration of existing particles\n(presumably with their own depleted haloes) to cause further changes?\nWe agree that it is not the agglomeration of the particles which results in changes in the\ncomposition of the surrounding melt, but the crystallisation of the nanolites. We have now\nrevised the sentence (lines 134-138) so that it is clearer that it is the crystallisation of the\nnanolites that results in changes to the melt composition and aggregation is a separate\nprocess, but both processes contribute to their impact on magma viscosity.\ne.g. lines 326 to 328. “… agglomeration will increase the effective volume of nanolites.\nComparing the segmentation of the nanoscale aggregates both with, and without, the\nincorporation of the surrounding glass (Fig. 6), the initial volume of the aggregate increases\nby up to 60 %.”\nBut Fig 6 is comparing the agglomerate with and without its depleted melt halo. The proper\ncomparison is between the agglomerates and the sum of the individual particles with their\nindividual haloes. Agglomeration could result in a smaller volume of enriched melt, because\nthe agglomerated particles presumably have a smaller total surface area than the sum of the\nindividual particles. If the depleted halo compensates for this by thickening, then perhaps\nthere is simply no change in its effective volume.\nThe effective volume could increase if additional (undepleted) melt were trapped between\nthe particles due to agglomeration. For example, random close packing of uniform spheres\nyields ~36% space between the spheres. So perhaps the effective volume could be 36%\nhigher, but if the particles need to touch to stick (as van der Waals forces decay extremely\nrapidly with increasing distance), much of the interstitial melt is likely the depleted halo\nmaterial.\nIn short, if a clearer way can be found to explain how agglomeration leads to an effective\nvolume increase, and what the quantitative magnitude of that increase could be, it would be\nappreciated.\nWe agree with the reviewer and have now considerably revised the discussion of\naggregation in the manuscript to provide a clear description of the process and evaluation of\nthe different mechanisms which may have produced the nanolite aggregates observed in the\nsamples. This revised section can now be found at lines 472-519 and includes the new\nFigure 9. We have added several references which discuss the different mechanisms which\ncan lead to the formation of aggregates, such as growth mechanisms (epitaxy and\nheterogeneous nucleation) and dynamic processes (synneusis). Following the revision of the\nsection which discusses the nanolites and the depleted melt haloes, we also indicate that the\nboundary layer surrounding nanolites relates to their crystallisation and is produced from the\nextraction of Fe and Ti from the melt. Therefore, this interstitial glass relates to the\ncrystallisation of the nanolites and is not entrapped between nanolites as a result of a\ndynamic process such as synneusis. In the revised discussion we present a clearer\nexplanation of the formation of the boundary layer and that the aggregates observed in the\nsamples most likely result from a growth mechanism. Therefore, as also mentioned by the\nreviewer, we acknowledge that the representation of the volume increase in the original\nversion of the manuscript is not consistent with the observations from the images. Instead,\nwhat we would like to indicate is that incorporation of the interstitial melt (or boundary layer)\nincreases the volume of the aggregate and its impact on the magma viscosity (Di Genova et\nal., 2020). As the depleted melt halo surrounding nanolites is enriched in Si and Al, it will\nincrease the effective volume, as the rheological behaviour will be closer to an elastic\nresponse at eruptive temperatures, due to the stable network of Si and O Bonds (Di Genova\net al., 2020b; Kennedy et al., 2022). In the manuscript we have now revised the description\nof this volume increase to clarify this. The specific lines 326-328 highlighted by the reviewer\nin the original version of the manuscript have now been moved to the section which\ndiscusses the formation of the boundary layer, to better highlight that the segmented\nboundary layer is related to the crystallisation of the nanolites and is not only entrapped\nbetween them (lines 330-332, highlighted in orange).\nWe have also revised the section of the discussion which presents the impact on magma\nviscosity to specify how nanolite aggregation may increase magma viscosity, with specific\nreference to the data provided in this study. Nanolite crystallisation and aggregation increase\nthe aspect ratio of the nanoscale particles in the sample, which can be measured in our 3D\nimages and may lead to the development of non-Newtonian rheological behaviour at higher\ncrystal fractions. We have added further references and discussion of this particular impact\non magma viscosity at lines 379-388, including the new Figure 8. We thank the reviewer for\nthis comment as this has resulted in a clearer presentation of the aggregation mechanism\nand impact on magma rheology in the revised manuscript.\n[4] The discussion of calculated viscosities needs careful inclusion of uncertainties and\nbetter explanation of the assumptions being made\nLines 348 to 352 – please give the model uncertainties on these calculations. For GRD the\nRMSE for volatile-bearing melts is ±0.46 log units (their Table 3) so combining that with the\n±0.17 log unit RMSE for Langhammer et al. (2021) means the calculated melt viscosity is 9-\n(Giordano with ±0.46 log units rmse). Using the ±0.74 log unit RMSE for the GRD model\napplied to the Langhammer et al. dataset, gives a range of 11-335 Pas, which is already a\nsubstantial overlap with the Langhammer model. Given that RMSE is a one sigma\nuncertainty, the two models in fact agree quite well, even though that gives a total range of\n9-335 Pas (or 0.9-2.5 log units). I do agree that it is useful to show how much uncertainty\nthere still is even for predicting melt viscosity.\nWe have now added the uncertainties for the calculations of liquid melt viscosity from the\nmodel (lines 358-359; lines 362-364 and Supplementary Information). To better present the\nresults of the viscosity estimation for the MTL and include the uncertainties, accounting also\nfor the effect of nanolite crystallisation, we have also revised this section of the discussion,\nreducing the number of different models and calculations that are presented in the main text.\nAs the results of the GRD and Langhammer models show good agreement for the MTL\ncomposition and the conditions used in the estimation of melt viscosity when considering the\nuncertainty of both models, we have moved this comparison to the Supplementary\nInformation, to provide further information on the uncertainties associated with these\ncalculations. This revision allows a better presentation of the results of the model and the\nassociated uncertainties in the manuscript.\nLine 350 – why assume 2 wt.% H2O – how was this water content determined (by previous\nstudies)? For the evolved melt composition, are you also using 2 wt% H2O? Crystallization\nof 27 vol.% crystals should increase the water content in the melt by ~35%, and this should\nbe factored into the magma viscosity calculations.\nThe water content used in the calculation represents the maximum H O content measured in\n2\nmelt inclusions in scoriae of the MTL eruption using Raman spectroscopy (Bamber et al.,\n2020) and represents the pre-eruptive H O content. This information has been added to the\n2\nmanuscript at lines 355-358.\nRegarding the calculation for the evolved melt composition, the H O content used is 2 wt.%\n2\nas we interpret that the nanolites crystallise during ascent within the conduit. We agree with\nthe reviewer that crystallisation will increase the H O content of the melt, however, for the\n2\ncalculation of the melt viscosity, using this higher H O content assumes that the nanolites\n2\ncrystallise within the magma reservoir before magma ascent. Due to the morphology and\naggregation of nanolites and the results of the Rhyolite-MELTS simulations (lines 164-166;\nlines 422-425), we can suggest that the nanolites crystallise during ascent. During magma\nascent H O will exsolve as crystallisation occurs, so the melt H O content will not be as high\n2 2\nas expected from 35 % crystallisation.\nLines 357-361 I think the text is saying that all existing viscosity measurements (and hence\nmodels) are suspect because nanolites may have affected them. For the specific point being\nmade, that would likely affect measurements of both the initial liquid and the depleted halo\nliquid – there is still plenty of Fe and Ti in the halo (Supp Table 3). But it is also worth\nremembering that crystals will not form below the liquidus, and that fast cooling rates seem\nto be required to form nanolites rather than microlites – which are easily detected by\nmicroscopic inspection. Nanolites form at cooling rates of several˚C/s (Di Genova et al.\n2020) but viscosity measurements by e.g. micropenetration or parallel-plate viscometry\ntypically use much lower cooling rates. And even if they were to form, a sudden nanolite\nnucleation event should be easily detected in the viscosity data as a discrete jump, given the\nstrong effect they can have on viscosity. So on balance this argument does not feel well\njustified.\nIn the manuscript we refer to studies which observe the formation of nanolites during heating\nand hold at high temperature during viscosity measurements at ambient pressure (Bondar et\nal., 2025). Recent studies indicate that during viscosity measurements performed by\nmicropenetration and parallel plate viscometry, restructuring of the melt can occur at the\nnanoscale at temperatures around T in the supercooled liquid state, including the formation\ng\nof nanolites. As our study focuses on 3D observations of nanolites in volcanic samples using\nmulti-scale imaging techniques, we do not describe the results of these works in detail in the\nmanuscript and would like to only highlight the impact of nanocrystallisation during laboratory\nmeasuments and how this may contribute to the uncertainty in estimating melt viscosity. We\nhave now revised part of the discussion (lines 366-369) to instead highlight how these\nobservations may contribute to the uncertainties associated with the estimates of melt\nviscosity provided in this study.\nLine 363 please provide the actual equation for relative viscosity as a function of crystallinity\nthat was used here. Which of the approaches used in Vona et al. (2011) was used here?\nThese include a modified Krieger-Dougherty equation from Ishibashi (2009), the Costa et al.\n(2009) model, and then a new parameterization based on crystal aspect ratio and strain rate.\nAll three of these are strain-rate-dependent and so the strain rate used in the calculations\nneeds to be stated.\nWe used the approach of Vona et al. (2011) and the equation of Costa et al. (2009) to\ncalculate the relative viscosity increase due to crystallisation. In Vona et al. (2011), the\nparameters derived from fitting the data using the Costa et al. (2009) equation are fit\nirrespective of the strain rate. We use the same fit parameters as Vona et al. (2011) in our\ncalculations of the relative viscosity increase. We have provided further details on the\napproach used to calculate the viscosity increase in the manuscript (lines 372-374) and\nSupplementary information, and have added Costa et al. (2009) to the reference list.\nLine 373 To summarize, the calculated evolved melt viscosity is 35 Pas (actually 24-52 Pas\nallowing for ±1 rmse from Langhammer) or 320 Pas (actually 110-920 Pas allowing for ±1\nrmse from Giordano). The calculated magma viscosity is 150-1400 Pas (Mader model) or\n1400-12000 Pas (Vona model). The estimated increase is then a factor of ~4-5 (Mader) or\n40-50 (Vona), but not 10-100. Bearing in mind the effect of increased water content in the\nevolved melt, which is not considered here, the actual viscosity change is likely to be less\nthan this. It is still obviously an important change, but the way things are written seems to\nalways be pushing the limits of the maximum effect that could be claimed, while ignoring the\nuncertainties in all of these models.\nWe have updated the presentation of the evolved liquid viscosity calculation in the main text\nto include the model uncertainty (lines 362-364). We have now revised and rearranged the\nsection of the discussion which presents the results of the viscosity calculations (lines 354-\n388), to provide a clearer presentation of the results of the models, calculating first the liquid\nmelt viscosity, then the incorporation of crystal fraction and aspect ratio, using our 3D data.\nThis section is now also supported by a more expanded discussion of these uncertainties,\ncomparison of the different models used to calculate the liquid viscosity and justification of\nthe values selected for the parameters included in the relative viscosity models, which can\nbe found in the Supplementary Information.\nThe sentence in the original version of the manuscript which summarised the results of the\nviscosity calculation presented the viscosity increase considering both the change in melt\ncomposition resulting from their crystallisation and also their contribution as solid particles.\nThe comparison made was between the initial basaltic composition and the results of the\nMader and Vona models, resulting in a relative increase in viscosity of a factor of 20-200.\nWe have now clarified this in the manuscript to specify that both effects are considered (lines\n376-378). We have also added references to the Supplementary Information in the revised\ndiscussion and stated where the evaluation of the models can be found.\nLine 380 there needs to be additional context on the basalt with 30% nanolites reaching a\nviscosity of 10^6 Pas. What temperature, composition, water content (if any) and what would\nthe crystal-free melt viscosity be at those conditions?\nThe basalt is a basaltic composition of Etna, produced as an experimental starting material\nfrom material of the 2001 eruption. The composition is reported in Polacci et al. (2018). The\ninitial melt viscosity of this composition was estimated as 200 Pa s, at pre-eruptive\nconditions (1070 ˚C; 3 wt.% H O). These conditions are comparable for the MTL example in\n2\nthis study, which has similar estimated pre-eruptive conditions of 1080 ˚C and 2 wt.% H2O.\nThis information has been added to the manuscript at lines 395-398.\nLikewise additional context is needed for stating that 60% microcrystals are needed to\nachieve the same viscosity as 30% nanolites. A quick look at e.g. Figure 2 of Costa et al.\n(2009) indicates there is a huge uncertainty in the experimentally determined effect of micro-\ncrystallinity on effective magma viscosity. Lejeune and Richet (1995) documented a relative\nviscosity of more than 1000 at only 40% crystals in one experiment, so at the very least\nthere needs to be acknowledgement of the uncertainties in these statements.\nThe calculation of the relative viscosity increase due to microcrystals, where 60 %\nmicrocrystals are required to achieve the same viscosity increase as 30 % nanolites was\ncalculated considering SiO microspheres, also in silicon oil. The microspheres used in the\n2\ncalculation have a similar shape to the nanospheres, and, therefore, a similar aspect ratio.\nThe calculation was performed using the approach of Mader et al. (2013) and the data of\nMueller et al. (2010). However, the relative viscosity increase using microspheres was\ncalculated with a strain rate of 1.0 s-1. In Di Genova et al. (2020), less than 10 %\nnanospheres are required to reach the same relative viscosity increase at these higher strain\nrates of 1.0 s-1. We have added further details to the manuscript at lines 398-402 to better\npresent and contextualise the comparison between the impact of nanocrystals and\nmicrocrystals on the relative viscosity increase.\nWe also acknowledge that the calculations of the relative viscosity increase due to\ncrystallisation have uncertainties. Figure 2 of Costa et al. (2009) shows a large variation in\nthe observed viscosity increase when comparing the results of different experimental\ndatasets for the same crystal volume fraction. This variation arises from uncertainties in the\nexperimental datasets used in the model regarding the experimental conditions, such as the\nstrain rate or relative proportion of melt and crystals, leading to a large uncertainty in the\ncalculated effective magma viscosity using the model. We have now added further\ninformation which details the uncertainties associated with this calculation to the\nSupplementary Information and a reference in the main text to where this information can be\nfound at lines 352-353.\nLine 384 relative viscosity increase is mentioned here but the starting viscosity is not – what\nwas the relative viscosity increase documented by Di Genova et al., and what actual\nviscosity would that imply starting with the melt compositions and viscosities discussed\nabove?\nIn Di Genova et al. (2020) the initial viscosity at pre-eruptive conditions for the Etna\ncomposition is 200 Pa s. Incorporating the viscosity increase due to nanolites as calculated\nfrom the results of the analogue experiments, the viscosity increases to 106 Pa s, depending\non the particle content and the strain rate. At strain rates between 1 and 3.5 s-1, this viscosity\nis reached with a particle volume of only 10 %, whilst at lower strain rates of 100 s-1 it is\nreached at a particle volume of 30 %. The calculated liquid viscosity of the MTL composition\nranges between 13 and 61 Pa s depending on the model used. As the MTL composition and\npre-eruptive conditions are comparable to Etna, we may consider that a similar increase in\nviscosity may occur when considering both the nanolite content and the effect of their\ncrystallisation, based on the results of the analogue experiments and recent studies on\nvolcanic melts (Valdivia et al., 2025). A high strain rate during ascent of 1 s-1 is estimated for\nhighly explosive basaltic Plinian eruptions, so we can assume that this viscosity increase is\nreached for the MTL case even at lower nanolite crystal fractions than the 0.27 estimated\nfrom the BSE images. We have added further information to the revised manuscript (lines\n395-405) to better present the comparison with the experimental results of Di Genova et al.\n(2020) and the implications for the MTL composition.\nLines 393-397 “The micro-textures observed within the lapilli may, therefore, represent\nfeatures which developed prior to magma fragmentation”.\nIt is tempting to assume that the crystal textures all formed pre-fragmentation, but wouldn’t\nthese clasts have cooled (post-fragmentation) at almost exactly the rates used to stimulate\nnanolite crystallization in previous experiments such as by Di Genova et al. (2020)? An\nestimate of the cooling rate could be made based on the pyroclast thermal model of Moitra et\nal. (2018 G-cubed). For 2-3 cm clasts the cooling rate should be ~4˚C/s from 1000 to\n600˚C. Processes in the conduit prior to fragmentation would likely involve much slower\ncooling, at ranges typically associated with microlite growth. This should perhaps be stated\nmore clearly.\nAlthough we agree with the reviewer that rapid cooling of clasts post-fragmentation can\noccur, with the potential to crystallise nanolites, nanolite crystallisation has also been shown\nto occur over fast timescales, at higher cooling rates than those estimated by Moitra et al.\n(2018) for cooling of clasts in the atmosphere. The in situ SAXS-WAXS experiments of Di\nGenova et al. (2020) illustrate that nanolites crystallise rapidly at near-liquidus temperatures,\nas nanolites formed at temperatures above 890 ˚C for the Etna basalt composition and not at\ntemperatures close to T . Nanolites crystallised at high cooling rates of 10-20 ˚C/s, which is\ng\nhigher than that expected for cooling of clasts ejected from the conduit. Experiments show\nthat nanolites crystallise under conditions of high undercooling, estimated between ΔT 40\nand 250 ˚C. These conditions are expected to occur for basaltic Plinian eruptions, where\nrapid magma ascent, between 5 and 50 m/s, can drive rapid syn-eruptive crystallisation\nwithin the conduit under conditions of high undercooling (Arzilli et al., 2019; Di Genova et al.,\n2020). Rapid ascent rates (>10 m/s) and high undercooling are also expected to occur for\nthe Plinian MTL eruption, driving rapid crystallisation within the conduit (Bamber et al., 2020;\n2022). Furthermore, several studies on nanolite crystallisation indicate that nanolite\nformation most likely occurs in the conduit, based on the results of experiments and textural\nobservations of natural samples (Mujin and Nakamura, 2014; Mujin et al., 2017; Mujin and\nNakamura, 2020; Knafelc et al., 2022; Okumura et al., 2022). Therefore, it is possible that\nnanolite crystallisation occurred during magma ascent. For the MTL eruption, we can\nassume that it is more likely that these textural features represent syn-eruptive, rather than\npost-fragmentation processes. We thank the reviewer for this comment as we have now\nrevised the manuscript to better clarify our interpretation of the observed textures as syn-\neruptive (lines 414-426; lines 428-431). However, we recognise that it would be beneficial for\nfuture studies to examine the crystallisation behaviour of nanolites during and post-\nfragmentation.\nAlternatively, if the timescale of “a few minutes” applies to both nanolite growth and ascent to\nthe surface this implies that magma mixing caused an immediate eruption (with a delay of\nless than a few minutes). How could these two magmas mix that quickly? Do they have\ndifferent compositions? Were they stored at different temperatures? If the answer to either or\nboth of these is yes, the rheological calculations should compare the two magmas. Such\nrapid eruption triggering would imply a sudden degassing event, but what exactly about the\nmixing process would have caused it?\nScoriae of the Las Sierras-Masaya Plinian eruptions show considerable textural\nheterogeneity, with a crystal-poor and crystal-rich endmember and enclaves which may\nindicate the development of a crystal-rich mush at depth within the storage reservoir\n(Bamber et al., 2020; 2022). The textural heterogeneity observed with the crystal-poor and\ncrystal-rich endmembers has been interpreted as resulting from possible zonation in ascent\nvelocity within the conduit or syn-eruptive mingling. Therefore, the mingling textures\nobserved may represent syn-eruptive processes instead of magma mixing at depth. As\nnanolite crystallisation likely occurs within the conduit during magma ascent, we do not\ndiscuss possible triggering mechanisms for the eruption in this paper. We thank the reviewer\nfor this comment as it is possible that pre-eruptive conditions have an influence on nanolite\ncrystallisation, however, this discussion would require further data than is provided in this\nwork.\nLines 423-425. The assertion that the MTL magma viscosity did reach 10^6 Pas in the\nconduit, from Bamber et al. (2020), appears based on a microlite fraction of 0.5 and a\nstarting melt viscosity of 10 Pas (section 4.3.2) or 30-40 Pas (section 4.2.3), and the Costa\net al. (2009) model. Costa et al. (2009) present a framework for fitting experimental data but\nnot a specific set of recommended parameters. Mader et al. (2013) suggested a set of\nparameters for use with Costa et al. (2009) but they are somewhat arbitrary – the data in Fig\n2 of Costa et al. (2009) show that there is no single equation (or parameters) that can\ndescribe relative viscosity as a function of crystal fraction. The MTL magma viscosity quite\npossibly could have reached 10^6 Pas, but again there are uncertainties that need to be\nacknowledged.\nWe acknowledge that the results of Costa et al. (2009) show that at the maximum crystal\nfraction measured in the samples (0.5), there can be an order of magnitude variation in the\nrelative viscosity increase estimated by varying Φ* and γ, indicating that the selection of the\nparameters can influence the viscosity calculated. For calculation of the MTL viscosity, the\nvalues determined by the experiments of Vona et al. (2011) are used as these values were\ndetermined from experiments performed on a similar basaltic composition, to assess the\nimpact of crystallisation on magma rheology. We have now added a discussion of the\nuncertainty associated with the selection of these parameters in the Supplementary\nInformation.\n[5] Miscellaneous minor points:\nLine 1 The title might be more accurately written as “3D visualisation of nanolite aggregates\nin basaltic magmas...” because the process of aggregation is not seen in a static image.\nWe agree with the reviewer and have now updated the title of the manuscript.\nLine 64 “high viscosity facilitates the accumulation of stress due to applied strain”. This may\nneed rephrasing – in volcanic systems the stress is applied and the strain results from that,\nnot vice-versa. High viscosity means a low strain rate and hence low rate of stress\ndissipation, but it does not cause the high stress in the first place.\nWe have now revised the sentence to instead indicate that high magma viscosity can enable\nbrittle fragmentation, where fragmentation arises from stress accumulation at high strain\nrates and bubble overpressure (lines 64-66).\nLine 74 how does “fracturing … modify the rheology of basaltic magma”? Isn’t fracturing an\nindication that the rheology has already changed from the ductile to brittle regime, through\nsome other mechanism (e.g. high strain rate, change in effective viscosity, etc)?\nWe agree with the comment of the reviewer and have now revised the sentence (lines 74-\n76). In the previous version of the sentence, the phrase ‘modification of magma rheology’\nwas in reference to syn-eruptive crystallisation. However, we acknowledge that by including\nalso fracturing in the same sentence, this phrase was in the wrong place. Instead the\nconnection between syn-eruptive crystallisation and magma rheology is discussed in greater\ndetail in the following paragraph (lines 77-81; highlighted in orange).\nLines 90-92 is it necessary to list “increasing melt polymerization” separately from “chemical\nand structural changes in the residual melt”? It may be better worded as “by facilitating\nheterogeneous bubble nucleation or inducing chemical and structural changes in the\nresidual melt, for example increasing melt polymerisation, …”\nWe thank the reviewer for the suggestion and have revised the sentence (lines 91-94).\nLines 176-178 I agree that there does not appear to be Si enrichment around the nanolites –\nand for Fig 3m there does not appear to be much Si depletion at the site of the nanolites\neither, which is surprising.\nIn Fig 3m, some depletion in Si can be observed in the areas where the Fe-Ti nanolites\noccur, relative to the surrounding distribution of Si in the glass. However, this is not as\nclearly observed as in Fig 3n where there is a more well defined area of Si depletion where\nthe Fe-Ti nanolite aggregates occur. It is possible that it may be more difficult to resolve the\ndifferences in Si at this scale, compared to Fe where there is a large difference in the Fe\ncontent of the nanolite compared to the glass.\nLines 223-225 and caption to figure 6. The caption should also explain why this part of the\nglass is shown and how it is distinguished from the rest of the glass (lower density). Where\nwe can see the nanolites in Fig 6 c and d, does this imply a lack of the evolved boundary\nlayer in these areas, or has it simply been cut away to reveal the nanolite inside? This is\nimportant to know. The 2D orthoslices suggest that the enriched boundary layer is not\npresent everywhere around the nanolites.\nThe part of the glass that is shown in the volume renderings in Figure 6 represents the\nboundary layer depleted in Fe. We quantified a difference in phase shift between the glassy\nboundary layer around the nano-aggregates and the glass not affected by\nnanocrystallisation, which is reported below:\nPhase shift range for the glass not affected by nanocrystallisation: -0.0046 to -0.0079;\nPhase shift range for the boundary layer: -0.010 to -0.013.\nThis difference in phase shift allowed segmentation and separation of the boundary layer in\n3D. Regarding Figure 6c, where some small nanolites are visualised without the boundary\nlayer around them, this is due to the 3D visualisation, as all the nanolites have a boundary\nlayer around them. We have provided a new volume rendering (Supplementary Figure 4)\nthat allows us to visualise the boundary layer around all of the nanolites and updated the\ncaption for Figure 6 (lines 943-945; lines 946-948).\nLine 233 why is the unit of rho_e cm-3 and not m-3? cgs units should be replaced with SI\nunits wherever possible.\nThe unit for rho_e has now been updated to m-3 (line 256) and units replaced with SI units\nthroughout the manuscript. Figures 4 and 5 and Table 1 have also been updated to use SI\nunits.\nLine 242 does rho_a have a name (e.g. is it the atomic density), and are the units m-3?\nRho_a is the atomic density. The units have been replaced as m-3 in response to the\ncomment above. As this information was present only in the methods section in the previous\nversion of the manuscript we have now added it to the main text at line 265.\nLines 297-301 Si4+ is also a cation, so the boundary layers are not “depleted in cations” but\nthey are often depleted in Fe and Mg or Ti. The key is that the boundary layer is enriched in\nnetwork-forming Si4+ and Al3+ (don’t forget the alumina!). The long sentence on lines 297-\n301 mentions viscosity increase in two different places, and should be restructured.\nWe have now revised this sentence and restructured it, so it more clearly focuses on the\nconnection between the chemical changes in the melt and the resulting local increase in melt\nviscosity. We have also specified that it is the depletion of Fe, Mg and Ti and the enrichment\nof Si and Al which can lead to this increase (lines 323-327).\nLines 301-304. If you observe shells are 10-350nm in size then that is the size they can\nreach (at least!). Remove “can reach 10-100 nm in size” on line 301.\nWe have removed the phrase at line 301 (now lines 328-330).\nLine 341 “Nanolites can increase magma viscosity by up to two orders of magnitude”. So\ncan larger crystals - it all depends on the volume fraction, which should be specified here.\nUnless this is meant to imply an additional increase relative to microcrystals at the same\nvolume fraction, in which case that should be specified clearly.\nWe thank the reviewer for this comment and have now revised the sentence to highlight that\nnanolites can increase magma viscosity by up to two orders of magnitude even at low\nparticle contents of < 5 vol.% (lines 392-394). We agree that it is important to state the\nvolume fraction here as the impact of nanolite crystallisation on magma viscosity is\nconsiderable at much lower crystal contents relative to microlites. This is due to their small\nsize, which influences liquid flow between particles and has a larger impact on magma\nviscosity compared to microlites at the same crystal content. Nanolite crystallisation may\nalso lead to an increase in the melt viscosity due to the effect of their crystallisation on the\nmelt chemistry and the formation of nanoscale chemical heterogeneities (Valdivia et al.,\n2025). This comparison with microlites and the impact on viscosity at the same crystal\ncontent is further discussed at lines 395-402.\nLine 480 Should the beam diameter be 10 nm rather than 10 microns for the BSE imaging?\nA 10 micron diameter would be good for a glass microprobe analysis but not for high\nresolution imaging.\nYes, the beam diameter during the image acquisition is ~10 nm. The conditions reported in\nthe manuscript refer to the glass analysis performed on the same area as the image. As this\nis not representative of the conditions during image acquisition, the beam diameter is not\nspecified in the revised manuscript (lines 561-562).\nLines 561-562 should the average atomic numbers be given as 12.91±0.07 and 20.32 ±\n0.04? Please specify whether the ± is one standard deviation or two.\nWe have revised the presentation of the atomic numbers in the Methods section at lines\n642-643.\nFig. 2 is described in the caption as showing two pieces of scoria from the FL eruption, and\nthe image labels are “FonAB” and “FonE”. These appear to be two different layers of the FL\neruption but this should be clarified. Moving the sample descriptions from supplemental\nmaterial into the main text would help with this.\nWe agree with the reviewer and have now moved the sample descriptions from the\nSupplementary Information to the Methods section of the main text, in the section ‘Sampling’\n(lines 544-557), to provide further context for the samples used in the study. We have also\nadded a line to the Methods section to explain the nomenclature used for the samples and\nhow these relate to the Fontana Lapilli eruption sequence and stratigraphic units (lines 556-\n557).\nFig. 5 it would be helpful to see MTG at the same spatial scale as MTL-A is shown in Fig. 4\n(as they were collected using the same geometry and resolution). To help make the\ncomparison described in the text, this could become a third panel of Fig. 4, and the\ngreyscale should be the same in all three panels (currently the greyscale range in Fig 5\nmaps to a different range of electron densities compared that used in Fig 4). Please also\nexplain the cause of the “thumbprint” pattern evident in Figure 5 – is this some kind of artifact\narising from the ptycographic technique or the image processing?\nThe greyscale range is different for Figure 5 (MTL-G) as using the same grayscale range as\nin the other samples results in saturation of the lowest electron density values which\ncorrespond to the air. It is difficult to maintain the same range for the colourbar as in Figure 4\nas there is a different range in electron density values across the samples (due to the\npresence of the nanolites in MTL-A and MTL-B with high values of electron density).\nHowever, we agree with the reviewer that it can help to have a panel with comparable scale\nto MTL-A for comparison of the electron density distribution and heterogeneity in the glass.\nTo help with this comparison, we have now added a new Supplementary Figure\n(Supplementary Figure 1) which shows sample MTL-G at a comparable scale and more\nclearly presents that it is a homogeneous glass also at the same scale as the images of\nMTL-A. This is referenced in the text at lines 218-221.\nThe ‘thumbprint’ pattern observed in Figure 5 is a grid artefact, which is a known artefact in\nptychography that comes from the regular scanning grid. This artefact can then affect the\nalgorithm for producing the 3D reconstructions.\nFor the supplemental information, it would be helpful for the reader to have the sample\ninformation in the main article text.\nWe have now moved the sample information to the Methods section of the main text, in the\nsection ‘Sampling’ (lines 544-557). The reference Costantini et al. (2009) has also been\nmoved from the Supplementary References to the References in the main text.\nSupplementary Tables 3 and 4 would also be useful in the main text, as the glass\ncompositions and calculated viscosities are discussed extensively.\nAs we have now considerably revised the discussion, adding 2 figures and revising the\npresentation of the viscosity calculations, we would prefer to still keep Supplementary Table\n4 in the Supplementary Information. Although we agree with the reviewer that this table can\nbe helpful in the main text, we believe that Figures 8-9 contribute more to the discussion, as\npart of the comparison of the viscosity models has now moved to the Supplementary\nInformation. As Supplementary Table 3 summarises chemical data which has already been\npublished and is used only as the input liquid composition for the calculations, we have\nchosen to keep this table in the Supplementary Information as these data are not new\nresults from this study.\nReviewer #3 (Remarks to the Author):\nReview of manuscript NCOMMS-24-72256\nTitled: \" 3D visualisation of nanolite aggregation in basaltic magmas using X-ray\nptychography: Implications for magma rheology”\nAuthors: Emily C. Bamber, Fabio Arzilli, Silvia Cipiccia, Darren J. Batey, Giuseppe La Spina,\nMargherita Polacci, Ali Gholinia, Heath Bagshaw, Danilo Di Genova, Richard Brooker,\nDaniele Giordano, Pedro Valdivia, and Mike R. Burton\nDear Dr. Neely,\nThank you for the opportunity to review this article. The manuscript presents a new approach\nto volcanic material characterization to provide a careful assessment of the Nanolite content\nusing two explosive basaltic eruptions from the eruption of Las Sierras-Masaya volcanic\nsystem as case studies.\nMeasuring and characterizing Nanolite size and geometry is something that traditional\nvolcanological studies have not done and this technique is a promising step toward\nunderstanding more about whether and how important these particles are in terms of\ndetermining eruption intensity and style. The presented multipronged approach, using a wide\nrange of complementary analytical techniques is exciting and may help advance our\nunderstanding of these phenomena. The presented data and analysis are sound and\nsupport the main arguments.\nIn the annotated pdfs of the main manuscript and supplements I have attached to this letter I\nprovide a range of comments that include minor suggestions for re-structuring and re-\nwording and some more in-depth analyses / questions about what could be discussed on the\nbasis of these data. I realize that some of those may go beyond what the authors intend to\npresent in this paper but wanted to include them in case there was capacity to address them\nhere or in future studies. I also provide a few suggestions for where and how this work could\nbe improved and where more detail / better reasoning may help. I hope this can serve to\nfurther improve this piece of research. Overall, I suggest minor revisions to address relevant\ncomments before publication.\nBest Regards,\nStephan Kolzenburg\nLine 38: What is the absolute volume increase – and the effect of that on rheology? Nowhere\nnear as drastic as analogue materials suggest I suppose (i.e. a 60% increase to 2% crystals\nstill only brings it to 3.2% total solids) – please document and compare.\nThe interstitial melt represents the chemically differentiated boundary layer which forms\nfollowing nanolite crystallisation and is then trapped during the growth of the aggregate. We\nacknowledge that this increase in effective volume of the aggregate is a localised effect. We\nhave now modified the abstract to specify that this is the differentiated boundary layer\nsurrounding nanolites and that this is a localised effect in the glass, as this is a better\nrepresentation of the results. We have removed the reference to the effective volume\nincrease within the abstract and conclusion, also in response to a comment from the other\nreviewer as the explanation of this increase requires further context which is not possible to\nprovide within the shortened abstract and conclusion. In the manuscript we have now\nprovided a further explanation of this effective volume increase and what this represents at\nlines 323-332 and lines 504-512. What we can suggest from our results is that the increase\nin volume of the aggregate due to the differentiated boundary layer may increase their\nimpact on magma rheology, by increasing the effective volume and the local viscosity of the\nmelt. It is difficult to calculate the absolute volume increase attributed to the aggregate\nconsidering the total sample due to their localised effect. However, in the revised manuscript\nwe have provided a clearer explanation of the process and better contextualised the\npresentation of the effective volume increase by moving it to the section of the discussion\nwhich presents the boundary layer.\nLines 40-41: How was that determined? Since it was not measured, maybe reword to. “is\nestimated to have increased…”\nWe agree with the reviewer and have revised the phrase in the abstract (lines 39-40).\nLine 64: Should be called strain RATE. Since there is a time dependence in the relaxation.\nWe have updated the sentence to specify that this is the strain rate (line 65).\nLine 68: Why is SO2 relevant for eruption style – unclear. Erupted volume or volumetric rate\nwould be more meaningful here.\nWe have revised the sentence to instead refer to parameters which better relate to the\neruptive style (lines 66-69). As there are no estimations of the erupted volume for this event\nin the literature, due to the complexity of the eruption (Bani et al., 2025), we have chosen\ninstead to refer to the column height. This recent reference has also been added to the\nreference list.\nLines 77-81: Repetitive – could be simplified\nWe agree with the reviewer and have now revised this section, removing repetitions (lines\n77-81).\nLines 82-83: This quite certainly does not represent the magma during ascent and\nfragmentation – should be highlighted that these values must be lower in syn-eruptive\nscenarios\nWe agree with the reviewer and have revised the sentence to highlight that the high\nmeasured crystallinity may not only represent syn-eruptive crystallisation. We have also\npresented the range in measured crystallinity in the samples rather than just the maximum\nvalue, to show the wider range in estimates for crystal content in these samples (lines 82-\n83).\nLine 86: Strikethrough text\nWe have corrected the sentence (line 87).\nLine 87: Strikethrough text\nWe have corrected the sentence (line 88).\nLines 119-120: Which one? It is implied that it is scale but should be stated explicitly\nThe main limitation of standard CDI is that the field of view is limited by the lateral coherence\nlength of the beam. Instead, by scanning, ptychography can overcome this limitation,\nallowing imaging of extended objects. We have added this more detailed explanation at lines\n120-123.\nLines 136-137: No. rheology or behaviour were not measured here. But this work does\nimprove the understanding of nano structure of volcanic products. Please reword\naccordingly.\nWe agree with the reviewer and have now revised the sentence to focus on the contribution\nof our results to the understanding of both the nanoscale structure of volcanic products and\nthe dynamics of basaltic Plinian eruptions (lines 140-143).\nLine 163: Unclear if this refers to the crystals or the clusters – please reword for clarity\nWe have revised the sentence to specify that it is the clusters which form the more irregular\nshapes (lines 177-178).\nLines 167-168: What causes these differences? Why are some areas showing nanolites and\nothers not – would be interesting to hear some thoughts on that.\nSamples of the MTL and FL eruptions show considerable textural heterogeneity, which is\nobserved at the micron-scale where crystallinity varies between 5 and 63 vol.% (Costantini\net al., 2010; Bamber et al., 2022). This heterogeneity has been interpreted as resulting from\nsyn-eruptive processes such as gradients in ascent velocity within the conduit during magma\nascent or mingling of two endmembers with different thermal histories (Bamber et al., 2020).\nIt is possible that these processes may also affect the nanoscale structure of the magma and\nits potential to crystallise nanolites, leading to the heterogeneity observed in the samples.\nWe observe this heterogeneity at a larger scale in our samples in the BSE images (Figure\n2), which show spatial variations in the size and distribution of nanolites. The distribution of\nnanolites has also been observed to be associated with bubbles, as shown in the\nexperiments of Pleše et al. (2019), where nanolites nucleate on the surface of bubbles\nduring vesiculation (observed in Fig. 2e-f).\nPetrological analysis of basaltic samples also finds that unmixing of magmas into Fe- and Si-\nrich liquids can also lead to compositional variations and the heterogeneous development of\ntextures at the nanoscale (Honour et al., 2019). In these cases, the formation of\ncompositional boundary layers during crystal growth can produce spatial variations in melt\ncomposition, triggering unmixing and/or the development of Fe-rich filaments (Honour et al.,\n2019; D’Oriano et al., 2025). This may lead to heterogeneous crystallisation of nanolites\nwithin the matrix. However, the thermodynamic conditions and kinetics of these processes\ndescribed above are uncertain at the nanoscale, as there are less experimental constraints.\nTherefore, as our work focuses on imaging of the morphology and distribution of nanolites\nwithin natural volcanic samples, we choose not to focus on the processes which may\ngenerate heterogeneities in the ability of the melt to crystallise nanolites without additional\nconstraints. We thank the reviewer for this comment and agree that this is an interesting\naspect of nanocrystallisation which could be investigated in further detail in future work.\nLine 170: Strikethrough text\nWe have corrected the sentence (line 185).\nLines 209-211: Evolved in what way? This should be clarified. With Fe and Ti in the oxide\nnanolites, what is the expected composition of the halo melt. Seeing now that this is a\nsection coming later – maybe refer to that here just to show it will be picked up.\nWe have revised the sentence to specify what melt chemical changes occur following\nnanolite crystallisation (lines 222-226).\nLine 217: How do 2D volume estimates match the reconstructed 3D volumes? This is a real\ndataset to shed more light on the 2D-3D comparison at scales that it was never done before\n– would be good added value to the paper with little to no effort – since the data seem to\nalready exist\nWe thank the reviewer for this comment and agree that it is valuable to investigate if\nquantitative textural measurements performed using 2D and 3D images are comparable at\nthe nanoscale. From our 3D ptychographic images, we observe that below a size of ~ 200\nnm, nanolites have a low aspect ratio and are approximately spherical-subspherical in\nshape. Therefore, for nanolites of this size, 2D images, such as those obtained with TEM or\nSTEM imaging, should provide a good representation of the morphology, also in 3D.\nEstimates of the size and volume comparing 2D and 3D images may in this case be similar.\nInstead, for nanolites larger than this size, the aspect ratio increases and nanolites begin to\nhave a more complex, irregular shape in 3D. This complex morphology may not be captured\nin 2D images and the estimate of the size and volume of the nanolites may, therefore, differ\ncomparing 2D and 3D techniques.\nComparing the results of quantitative textural analysis using two-dimensional (2D) and three-\ndimensional (3D) characterization methods, the volume estimates of the nanolites derived\nfrom 2D data can be underestimated by one to two orders of magnitude. This discrepancy\narises primarily from the limitations inherent in 2D imaging, which may not fully capture the\ntrue volumetric size and morphology of nanolites. As discussed above, the extent of the\nunderestimation depends on the nanolite shape; it is particularly pronounced in cases where\nnanolites exhibit complex, dendritic morphologies formed through aggregation-driven growth.\nIn such instances, the likelihood of a 2D cross-section being representative of the full 3D\nmorphology and size is significantly reduced, leading to misrepresentation of the volume and\ndifficulty in applying an appropriate stereological correction to the 2D data. We have added\ndetails on this comparison of the 2D and 3D characterisation methods to the Supplementary\nInformation (Supplementary Table 4) as this provides a valuable comparison of different\nquantitative textural techniques at the nanoscale. This comparison has been added to the\nmain text at lines 242-249.\nLine 230: Strikethrough text\nWe have corrected the sentence (line 210).\nLines 266-267: State why – to make it very clear to people new to this topic\nWe thank the reviewer for this comment and have added an explanation at lines 291-293\nwhich specifies that the change in melt composition and, therefore, density occurs in the\nresidual melt due to the depletion of elements compatible with titanomagnetite crystallisation.\nThis is now discussed in further detail in the revised manuscript, also in response to a\ncomment from the other reviewer at lines 217-228 (highlighted in blue).\nLine 278: Do you really need all these references to support this simple and well known point\nWe have reduced the number of references here (line 304).\nLines 302-303: It would be valuable to estimate HOW MUCH this changes the viscosity in\nthe boundary layer to place the later comments on volatile and transition metal diffusion on\nmore solid footing\nWe agree with the reviewer and considering the comments of both reviewers, we have now\nmade significant changes to the discussion of the viscosity calculations in the revised\nmanuscript. In the discussion we calculate the viscosity of the evolved melt composition and\nevaluate the impact of this on the magma viscosity. The results of these calculations and the\nanalysis of how the formation of the aggregates can increase the viscosity, considering the\nchange in chemical composition of the boundary layer and the increase in the aspect ratio of\nthe nanoparticles, are provided in lines 346-378 of the revised manuscript.\nLines 306-307: WHY? These higher viscosity zones should have longer relaxation\ntimescales and thus fragment easier at the same change in strain rate than the surrounding\nmelt – please elaborate.\nIn our work we discuss the results of the experiments of Hornby et al. (2024) on how the\npresence of nanoscale crystals in the matrix and the consequent development of\ncompositional boundary layers can influence fracture propagation. In that study, the\npresence of nanoscale crystals is suggested to increase the fracture toughness of the\nmatrix, as fractures branch around nanolites. Instead, fractures propagate preferentially\nthrough the adjacent Mg and Fe depleted compositional boundary layers. In our work, what\nwe intend to highlight in this section of the discussion, is that the development of Fe depleted\nboundary layers following nanolite crystallisation may lead to variations in the fracture\nresistance of the matrix during fragmentation. However, we agree with the reviewer that\nthese heterogeneous zones within the matrix may be more susceptible to fragmentation\nunder the high strain rates expected during magma ascent for a Plinian eruption. The results\nof Hornby et al. (2024) are more applicable to cases where fragmentation results from\nunloading or from secondary processes such as particle collisions, and are more appropriate\nfor less explosive activity, such as Vulcanian eruptions. Although in our manuscript we only\nintend to highlight how variations in melt chemistry may lead to nanoscale heterogeneity in\nproperties of the glass matrix, such as fracture toughness, we recognise that the example\ncited may be limited in its applicability to the Plinian case. We have decided to remove the\nsentence relating to fracture propagation in the revised version of our manuscript (lines\n332-334), as this concept is also not discussed further.\nLine 328: Please propagate this to estimate the effective volume of these agglomerates\n(particles + immobilized melt_high viscosity halo)\nThe calculation of the effective volume of the agglomerate included both the particles and\nthe higher viscosity boundary layer surrounding them. However, in the original version of the\nmanuscript the explanation was not clear. We have now revised this sentence to specify that\nthe glass surrounding the nanolites used in the calculation is the boundary layer (lines 330-\n332).\nLines 353-361: So all this still places you well below where any ‘traditional’ explosive magma\nis thought to fragment in terms of viscosity space – how would you generate the strain rates\nneeded to exceed melt relaxation at such low viscosities? Please elaborate.\nHere, at this point of the discussion, the viscosity is calculated only for the liquid, without\nconsidering the effect of crystals. To calculate the overall rheological impact of the\nnanoparticle bearing suspension, we calculate both the viscosity of the liquid composition\nand the relative viscosity increase from the presence of the nanocrystals. The contribution\nfrom the crystals is calculated later in the discussion. We consider that fragmentation occurs\nthroughout the magma and not only the melt component and to evaluate the role of the\nviscosity increase arising from nanocrystallisation in magma fragmentation and eruption\ndynamics we calculate the mixture viscosity (melt + crystals).\nAccounting also for the presence of crystals (nano- and micron-scale), the magma viscosity\nis 106 Pa s, which reaches the fragmentation threshold, considering the results of\nexperiments and numerical modelling on basaltic magma compositions (Papale, 1999;\nArzilli et al., 2019). Images of fractures and analysis of their distribution in natural samples from\nexplosive basaltic activity indicates that brittle failure of basaltic magma can occur through\nthe propagation of tensional fractures throughout the magma (Taddeucci et al., 2021).\nTherefore, for our MTL composition, we can also consider that brittle fragmentation occurred\nthroughout the magma and account for the role of the mixture viscosity in increasing the\npotential of magma fragmentation. The calculations of liquid viscosity and the relative\nviscosity increase arising from the presence of crystals was present in the original version of\nthe manuscript. However, we have now considerably revised and restructured the\npresentation of the viscosity calculations in the discussion of the revised manuscript (lines\n346-388) to better present our results. We have also added an introductory section to this\npart of the discussion to highlight that our calculations consider both the liquid viscosity and\nthe contribution from crystals and it is the mixture viscosity which overall contributes to the\npotential of magma fragmentation during ascent (lines 346-352).\nLine 365: Nearly 30 vol.% nanolites from 13 wt.% Fe in the melt – somehow that does not\nseem to add up. I think it would be worth to calculate the max volume fraction of Fe-Ti oxides\nthat the melt can produce (assuming all Fe and Ti goes in to these and then see if/how these\nnumbers compare with the 27% reported here).\nAccording to the results of simulations using Rhyolite-MELTS, the maximum volume fraction\nof Fe-Ti oxides which can crystallise from the composition is 0.15. We agree with the\nreviewer that the calculation of 0.27 is not compatible with only the crystallisation of Fe-Ti\noxide nanolites. In our estimate of the nanolite crystal fraction of 0.27, we consider also the\ncrystallisation of clinopyroxene nanolites, which is evidenced from the melt composition in\nSupplementary Table 1 and the crystallising phases predicted by Rhyolite-MELTS\ncalculations. However, the discussion of clinopyroxene nanolite crystallisation occurred in a\nlater part of the manuscript, after this calculation in the original version. We have now\nupdated the presentation of these results in the revised manuscript so they occur in the\nresults section and earlier parts of the discussion (lines 163-169), as this is an important\nresult for the explanation of the measured crystal fraction. We have added the estimate of\nthe maximum volume fraction of Fe-Ti oxide which can crystallise from this melt at lines 169-\n172.\nLine 371: This is not Mader’s model. I think it would be better to say using the approach\npresented in Mader and then cite the model you end up using (which were reviewed and\nplaced in concert in the Mader paper).\nWe agree with the reviewer and have now updated the manuscript and Supplementary\nInformation to specify that the Krieger and Dougherty (1959) equation was used to calculate\nthe relative viscosity, following the approach of Mader et al. (2013). Following the revision of\nthe discussion, this calculation now occurs in the Supplementary Information. We have\nadded the Krieger and Dougherty (1959) reference to the supplementary references list.\nLines 413-414: How do you merge this with the reported nanolite content of 27%? Should be\ncommented on.\nThe results of the simulations suggest that the measured crystal content of 27 % likely\nconsists of both clinopyroxene and titanomagnetite. This is likely as complete extraction of\nFe and Ti from the melt may still only crystallise a few vol.% of titanomagnetite. We have\nupdated the manuscript to compare the results of the simulations and textural observations\nand the implications for the mineralogical assemblage (lines 163-172).\nLine 424: Insert ‘may have’.\nWe have updated the sentence (line 443).\nLines 424-425: Where does the 10^6 come from now? Unclear-please expand on this for\nclarity. Above you only say that in other systems this was suggested but the data presented\nhere provide no foundation for this statement other than a vague comparison – needs to be\nsubstantiated or significantly toned down.\nThe estimate for effective magma viscosity here is from a previous study on the MTL\neruption (Bamber et al., 2020), which instead analysed micron-scale crystals and the impact\non magma viscosity. We have revised the sentence (lines 443-446) to better clarify that this\nis an estimate of magma viscosity from a previous study, which quantifies the impact of the\nmicron-scale fraction on magma viscosity, and also that microlite crystallisation may have\nincreased the potential of magma fragmentation, as we recognise that there are other syn-\neruptive processes during ascent which can influence eruption dynamics. The samples\nexamined in this study and Bamber et al. (2020) are from the same deposit and same unit of\nthe MTL eruption, however, here we examine the microlite-poor regions of scoriae where the\nglass contains nanolites.\nLine 437: But a paragraph before this is estimated at 10^6 – which is it now?\nHere, the estimate for the magma viscosity refers to the microlite-poor regions of the MTL\nsamples. The MTL scoriae exhibit significant textural heterogeneity in terms of crystallinity,\nwhere the microlite crystal fraction varies from 0.2 to 0.5. The estimated viscosity of 10^6\nrefers to the microlite-rich regions of the MTL samples. We have now revised this section\n(lines 455-460) to better differentiate between the different estimates of magma viscosity for\nthe MTL, accounting for the heterogeneity observed in the samples.\nLine 781: There are authors listed that seemingly had no contribution to the paper – at least\nfollowing this statement. Either add their contribution or remove them from the author list.\nFurther details have been added to the author contributions statement.\nLine 821: Add numbers to scale bars in figure\nWe have updated the scale bar for Figure 4.\nLine 826: Add numbers to scale bars in figure\nWe have updated the scale bar for Figure 5.\nSupplementary Information\nSupplementary Table 3: Where did these elements go? What other crystals formed?\nIt is possible that crystallisation of clinopyroxene nanolites also occurred, as shown by the\nresults of the quantitative textural measurements on the 2D BSE images and Rhyolite-\nMELTS modelling, indicating that the measured crystal fraction of 0.27 likely does not\nrepresent only titanomagnetite. Complete extraction of Fe from the melt will only crystallise a\nfew vol.% of titanomagnetite (lines 169-172). We have now added a sentence to the revised\nmanuscript (lines 172-173) which clarifies that the observed change in melt chemistry may\nresult from the crystallisation of both titanomagnetite and clinopyroxene nanolites, with\nspecific reference to Supplementary Table 1 (highlighted in blue)."}, {"round": 2, "reviewer_comments": {"reviewer_2": "The authors have thoroughly revised the manuscript in response to reviewer comments and I now recommend publication.", "reviewer_3": "The rebuttal clearly states that comments raised by myself and other reviewers were considered in detail and the manuscript amended accordingly. I see no further need for revisions and sugest the manuscript be published in the resubmitted form."}, "author_response": ""}]

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