Distribution of dissolved water in magmatic glass records growth and resorption of bubbles

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Distribution of dissolved water in magmatic glass records growth and resorption of bubbles

I.M. Mcintosh, E.W. Llewellin, M.C.S. Humphreys, A.R.L. Nichols, Alain Burgisser, C. Ian Schipper, J.F. Larsen

To cite this version:

I.M. Mcintosh, E.W. Llewellin, M.C.S. Humphreys, A.R.L. Nichols, Alain Burgisser, et al.. Distri-

bution of dissolved water in magmatic glass records growth and resorption of bubbles. Earth and

Planetary Science Letters, Elsevier, 2014, 401, pp.1-11. �10.1016/j.epsl.2014.05.037�. �insu-01010754�

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Contents lists available atScienceDirect

Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Distribution of dissolved water in magmatic glass records growth and resorption of bubbles

I.M. McIntoshâ^,^c^,^∗,E.W. Llewellinâ,M.C.S. Humphreysâ^,^b,A.R.L. Nichols^c, A. Burgisser^d^,ê, C.I. Schipper^d^,ê^,^f,J.F. Larsen^g

aDepartmentofEarthSciences,DurhamUniversity,ScienceLabs,Durham,DH13LE,UK¹ bDepartmentofEarthSciences,UniversityofOxford,SouthParksRoad,Oxford,OX13AN,UK

cResearchandDevelopmentCenterforOceanDrillingScience,JapanAgencyforMarineEarthScienceandTechnology(JAMSTEC),2-15Natsushima-cho, Yokosuka,Kanagawa237-0061,Japan

dInstitutdesSciencesdelaTerre,CNRS–IRD–UniversitédeSavoie,CampusScientiﬁque,73376LeBourgetduLac,France

eInstitutdesSciencesdelaTerre(ISTO),CentreNationaldelaRechercheScientiﬁque(CNRS),l’Universitéd’Orléans,1aruedelaFérollerie,OrléansCedex2, 45071,France²

fSchoolofGeography,EnvironmentandEarthSciences,VictoriaUniversityofWellington,POBox600,Wellington6140,NewZealand³ gGeophysicalInstitute,UniversityofAlaska,Fairbanks,AK99775,UnitedStates

a r t i c l e i n f o a b s t ra c t

Articlehistory:

Received15September2013 Receivedinrevisedform12May2014 Accepted21May2014

Availableonlinexxxx Editor:T.Elliott Keywords:

bubblegrowth bubbleresorption diffusion quencheffect waterspeciation disequilibrium

Volcanic eruptions aredrivenby thegrowthofgas bubblesinmagma. Bubbles growwhendissolved volatilespecies,principallywater, diffusethroughthesilicatemeltand exsolveatthebubblewall. On rapid cooling, the melt quenches to glass, preserving the spatial distribution ofwater concentration aroundthebubbles(nowvesicles),offeringawindowintopre-eruptiveconditions.Wemeasurethewater distributionaroundvesiclesinexperimentally-vesiculatedsamples,withhighspatialresolution.Wefind that, contraryto expectation,water concentration increasestowards vesicles,indicating that water is resorbedfrombubblesduringcooling;texturalevidencesuggeststhatresorptionoccurs largelybefore the melt solidifies. Speciationdata indicatethat the molecularwater distributionrecords resorption, whilstthehydroxyldistributionrecordsearlierdecompressivegrowth.Ourresultschallengetheemerging paradigmthatresorptionindicatesfluctuatingpressureconditions,andlaythefoundationsforanewtool forreconstructingtheeruptivehistoryofnaturalvolcanicproducts.

©²⁰¹⁴^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license (http://creativecommons.org/licenses/by/3.0/).

1. Introduction

Bubblesnucleatewhenmagmaticvolatiles(speciessuchaswa- ter, CO2 and SO2, that are only weakly soluble in the silicate melt)exsolvefroma supersaturatedmelt. Wateristhemostim- portantvolatile because itis usually the mostabundant and be- causeitstronglyaffectsmelt viscosity(HessandDingwell,1996).

Itisdissolvedinthemeltastwoprincipalspecies:molecularwater,H2Om, andhydroxyl groups, OH. As magmaascends, bubbles growthroughdecompressiveexpansionandcontinuingexsolution

* Correspondingauthorat:ResearchandDevelopmentCenterforOceanDrilling Science,JapanAgencyforMarine EarthScienceandTechnology(JAMSTEC),2-15 Natsushima-cho,Yokosuka,Kanagawa237-0061,Japan.Tel.:+81468679766;fax:

+81468679625.

E-mailaddress:i.m.mcintosh@jamstec.go.jp(I.M. McIntosh).

1 CurrentaddressoftheauthorM.C.S.Humphreys.

2 CurrentaddressoftheauthorA.Burgisser.

3 CurrentaddressoftheauthorC.I.Schipper.

ofvolatilesfromthemelt(Sparks,1978).Togethertheseprocesses control the bubble growth ratewhich, in turn,controls orinﬂu- encesalmostevery aspectofmagmaascentanderuption,includ- ing:magmavesicularity,buoyancy,rheologyandpermeability;the pressuregradientthatdrivestheeruption;andtheonsetofmagma fragmentation. Understanding and quantifying bubble growth is, therefore,oneofthemostfundamentalchallengesinphysicalvol- canology.

Waterexsolvesfromthemelt,intoabubble,whenitssolubility inthemeltdecreases,andresorbs intothemelt whenitssolubility increases.The resulting change inthe water concentration at thebubblewallcreatesachemical potentialgradientinthemelt, whichdrivesdiffusiontowardsagrowingbubbleandawayfroma shrinking bubble(Fig. 1). Thewaterconcentration proﬁlemaybe preservedwhenthe meltquenchesto glass,offeringthe tantalis- ing prospectofreconstructing thebubble’shistoryofgrowthand resorption.Wequantifythespatialdistributionofdissolvedwater anditsspeciesinexperimentally-vesiculatedmagmaticglasses,us- ingsecondaryionmassspectrometry(SIMS)-calibratedbackscatter http://dx.doi.org/10.1016/j.epsl.2014.05.037

0012-821X/©²⁰¹⁴^TheÂuthors.^Published^byÊlsevier^B.V.^Thisîsânôpenâccessârticleûnder^the^CC^BY^license(http://creativecommons.org/licenses/by/3.0/).

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Fig. 1.Bubblegrowthandresorption.Schematicﬁguretoshowthepressureand temperature(P,T)conditionsrequiredforabubbletoremain inequilibrium,to grow,ortoresorb,withexampleH2Otconcentrationproﬁlesthatresult.Solidblack linesindicatecurrentbubblesize;dashedgreylinesindicatepreviousbubblesize, hencewhetherbubbleisgrowingorresorbing.

scanning electron microscope (BSEM) images (Humphreys et al., 2008) and Fourier-transforminfra-red(FTIR)imaging(e.g.Nichols andWysoczanski,2007),inordertotestthishypothesis.

Two recent studies apply a similar conceptual framework to draw significant conclusions about conduit processes.Watkins et al. (2012)analysevolatiledistributionsaroundvesiclesinobsidian clastsandfindwaterconcentrationprofilesconsistentwithbubble resorption(cf.Fig. 1).Theyinferapressureincreaseinthevolcanic conduitpriortoeruption.Careyetal. (2013)studyvesicledistribu- tionsinbasalticpyroclastsandfindindirectevidenceofresorption ofbubblespriortoeruption,whichtheyalsointerpretasevidence ofapressureincrease intheconduit.Basedon ourdata,we pro- pose that bubble resorption may occur during the quench from melttoglassasH2Osolubilityincreaseswithdecreasingtempera- ture,andpresentanalternativeinterpretationofthesefindingsin Section4.4.

Otherworkershaveusedtexturalevidencefromexperimental- ly-vesiculatedmagmasamplesto investigateinteractions between bubbles (Castro et al., 2012). They observe dimpled and sinuous glassﬁlmsbetweenvesicles,whichtheyinterpretaspreservedev- idence of incipient coalescence of growing bubbles. We analyse water distribution in the same samples and offer an alternative interpretationfortheirobservations,whichisconsistent withour conceptualmodel(Section4.3).

1.1. Waterinsilicatemelts

Interpretationofwaterdistributionsinglassreliesonquantita- tivemodelsforwatersolubilityanddiffusivity.Experimentalstud- iesofvariousmagmacompositionsshowthat,forcrustalpressures relevanttomagmatic degassing,solubilityincreaseswithincreas- ingpressureanddecreasingtemperature(Baker andAlletti,2012;

Newman and Lowenstern, 2002) while diffusivity (D) increases with increasing temperature, decreasing pressure, and increasing waterconcentration(NiandZhang,2008) (Fig. 2).Temperatureex- ertsadominantcontrolonwaterdiffusivityand,toalesserextent, onsolubility;however,thereremains a gapindatabetweenam- bient and magmatic temperatures, which includes the transition betweenmeltandglass.

Thetwospeciesofwaterpresentinglass(H₂O_mandOH)inter- convertviatheequilibriumreaction

H₂O_m₍_melt₎+^O^o₍_melt₎^2OH(melt), (1)

Fig. 2.Controlsondiffusivityandsolubilityofwater.Variationinwatersolubility (upper) anddiffusivity (lower)with pressure andtemperature forrhyolitecom- position.A datagapexistsbetweenmagmaticandambienttemperatures.High-T solubilitymodelisfromNewmanandLowenstern (2002) showingproportionof H2Om andOHatequilibriumspeciation;diffusivity dataarefromNiandZhang (2008)assuming4wt%H2Otwithboth H2Ot andH2Om diffusivityshown. Low temperaturesolubility(diamond)istakenfromAnovitzetal. (1999);lowtempera- turediffusivitydata(for0.1 MPa)arefromAnovitzetal. (2006).

inwhichmolecularwaterreactswithbridgingoxygens(O^◦)inthe melt to produce hydroxyl groups that are bound to the silicate polymerframework (Stolper,1982a).The‘totalwater’ (H2Ot)con- tentofameltorglassisthesumofthecontributionsfromH₂O_m andOH.Thepositionoftheequilibrium ofEq.(1)(the‘equilibrium speciation’) changes withpressure, temperature, H2Ot concentration and melt composition (Hui et al., 2008; Silver et al., 1990;

Stolper, 1989, 1982a) (Fig. 2). The bound OH groups are effec- tively immobile andH₂O_m is the diffusingspecies;consequently, OH concentration gradients formindirectly by diffusion ofH₂O_m and subsequent readjustment towards equilibrium speciation via Eq. (1) (Zhang et al., 1991). For identical conditions, DH2Om is thereforehigherthanDH2Ot(Fig. 2).Atexperimental(ormagmatic) temperatures the rate of the species interconversion reaction is suﬃcientlyfastthat,followingaperturbationtothesystem,equi- libriumspeciationisre-establishedovertimescalesofmilliseconds.

As aresultofthe strongtemperature-dependenceofthe reaction ratehowever,thetimetakentoachieveequilibriumspeciationbe- comes much longer astemperature decreases, taking minutes to hoursat∼⁶⁰⁰^◦^Când^daysât∼⁴⁰⁰^◦^C^(Zhangêtâl.,^1995,^1991).

2. Materialsandmethods

Samples are obtained from pre-existing experimental suites, and were manufactured under controlled conditions of pressure (P) and temperature (T). P and T conditions are given in Table 1,alongwithreferencestotheoriginalstudies;samplecom- positionsare giveninTableS1intheSupplementaryInformation.

Theexperimentswerealldesignedtoproducebubblepopulations witheitherequilibriumproﬁles(solubilityexperiments) orbubble growthproﬁles(decompressionexperiments)(Fig. 1).

2.1. Sampleproduction

Allsamplesweresynthesisedathighpressure(P_syn) andtemperature(T_exp,constantthroughoutexperiment)withexcesswater to forma startingmelt that waswater-saturatedandfully equili-

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Table 1

Sampleproductiondetails.

Sample Texp

(^◦C) Psyn (MPa)

Pi (MPa)

Timeheld atPi (hr)

Pf (MPa)

d P/dt (MPa/s)

Typeof experiment

Expectedbubble stateatquench

Experimental apparatus

Rhyolite^a Quench: 3–10 s

ABG1 825 150 100 0.17 100 N/A Decompression Equilibrium Externally-heatedpressure

vessel;water-cooledrapid quenchattachment;3mm

ABG2 825 150 100 0.17 60 0.1 Decompression Growth

Phonolite^b Quench: 3–10 s

IS14 1050 200 200 N/A 100 0.024 Decompression Growth Internally-heatedpressure

vessel;Ar-cooledrapid quenchattachment;5mm Rhyolite^c

Quench: 30–60 s

MCN13 825 80 80 72 80 N/A Solubility Equilibrium Externally-heatedpressure

vessels,quenchedusing compressedairthen submersioninwater;4mm Rhyolite^d

Quench: 3–10 s

MCN15 825 80 80 73 56 0.4^† Decompression Growth Externally-heatedpressure

vessel;water-cooledrapid quenchattachment;2mm Abbreviations:Texpisexperimentaltemperature,Psynispressureofsamplesynthesis,Pi isintermediatepressure(ifinitialstepdecompressionusedtonucleatebubbles), Pf isﬁnalpressure(NBquenchisisobaric),d P/dtisdecompressionrate.Experimentalapparatuscolumnsummariseskeydetails,includingsamplecapsulediameter(given inmm).Quenchtimeisestimatedandvarieswithexperimentalquenchapparatusused.Sinceallexperimentswereconductedisothermally,expectedbubblestateatquench isdeterminedbypressurehistoryonly.

Samplesource:

a BurgisserandGardner (2005).

b C.I.Schipper(unpublished),compositionasinIacono Marzianoetal. (2007),experimentalapparatusasinDiCarloetal. (2006).

c J.F.Larsen(unpublished),experimentalapparatusasinLarsen (2008).

d J.F.Larsen(unpublished),experimentalapparatusasinLarsenetal. (2004).

† SamplewasdecompressedtoPf at4MPa/s,thenheldat Pf for60s beforequenching.

brated.Fordecompressionexperiments,somesamplesthenunder- wentasinglestepdecompressiontoalowerintermediatepressure (Pi) to nucleate bubbles and were held at Pi for some time to createa bubblepopulation inequilibriumwiththe melt prior to further controlled decompression to the ﬁnal pressure (P_f); for sampleswithoutaseparatenucleationstep(e.g.IS14)decompres- sionbegan at P_i=^Psyn.For MCN13,an undecompressed sample from a solubility experiment, pressure was constant throughout, i.e.Psyn=^Pi=^Pf.Allsampleswerequenchedisobaricallyat Pf; sampleswerequenchedimmediatelyuponreaching P_f exceptfor sample MCN15, which was held at P_f for 60 s before quench- ing.Experimentalquenchrateswerenotdirectlymeasuredbutare estimatedtovaryfrom∼³^to∼^{60 s}^to^the^glass^transition^temperature(Table 1).FullproductiondetailsforABGsamplesarefound inBurgisserandGardner (2005).IS14wasproducedfollowingthe procedureofDiCarloetal. (2006);MCN13followingtheprocedure ofLarsen (2008)andMCN15followingtheprocedureofLarsenet al. (2004).

2.2.Quantifyingwatercontent

H₂O_t data are obtained following Humphreys et al. (2008):

SIMSH2Ot analysesareusedtocalibrategreyscalevaluesinBSEM imagesofthesamearea(Fig. 3a, b).

2.2.1. BSEMimaging

Sampleswerepreparedbyembeddinginepoxyresinandgrind- ingtoexposeaflatsurface.Additionalresinwasthenaddedtothe surface to fillin exposed vesicles and reduce topographic effects during SEM imaging. Re-grinding down to the original exposed surface creates a flat surface of exposed glass and infilled vesicles.Surfacetopographyfollowingsamplepreparationwaschecked viaconfocalmicroscopy attheNationalPhysical Laboratoryusing an Olympus LEXT OLS4000 microscope (Wertheim andGillmore, 2014). The join between the glass and infilling resin is smooth withtypically lessthan 1 μmdifference in height betweenglass andresin, withnogradientinthe slopeofthe glassapproaching

Fig. 3. Extractionof H2Ot data usingSIMS-calibratedBSEM. (a)BSEM imageof vesiclesinphonoliteglassshowingdarkwater-richhalos.Notethinbrightwhite edgeeffectat vesiclewalls. (b)CalibrationofSIMSH2Ot datafromprofileA–A withcorrespondingBSEMgreyscaleintensityvalues.Errorsshownresultfromer- rorsonSIMSH2Otanalyses(y-axis,∼±10% relative)anderrorsonmeangreyscale valueoftheprofilesegmentsduetonoiseintheBSEMimage(x-axis,onestandard deviationfrom themean). (c) ProfileA–A showsH2Ot measuredbySIMS(dia- monds)andhighspatialresolutionH2Otconcentrationsextractedfromcalibration ofgreyscalevalues(solidline).Shadingshowserrorresultingfromthecalibration equationshownin(b).ProfileB–BshowsH2Otdataextractedfromgreyscaleval- uesusingthecalibrationequationderivedfromA–A.

thevesicleedge.‘Edge effects’(Newbury,1975) arethus conﬁned to thin (<2 μm) bright white rims at vesicle boundaries. As an additional measure to rule out a topographic causefor greyscale

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Fig. 4.Resorptionproﬁlesandtextures.BSEMimageofABGsample.(a)Dark,water- richhalossurroundvesicles.Circulardarkpatchbetweenvesicles(rightarrow)isa

‘ghost’bubble,i.e.thewater-richhaloofabubbleaboveorbelowtheplaneofview.

Marginsofdark,water-richglassalsoseenalongcracksbutwithsmallerwidththan water-richhalos.Cracksextendbetweenvesiclesbutoftenterminateattheedgeof water-richhalos.(b)Meltﬁlmsbetweenneighbouringvesiclesareoftendeformed (boxisenlargedin(c)).Leftarrowin(a)showsbrokenbuckledﬁlm.Notethatcon- trasthasnotbeen adjustedtohighlighthalos in(b)and(c).Brightpatchesare gold-coatremnantsorFe–Tioxides.TexturesarediscussedinSection4.3.

variationsapproachingvesicleedgesandcrackmargins,thesample stage was rotated and the same area imaged in a different ori- entationwithrespecttothe incidentelectronbeamanddetector.

Greyscale variations caused by sample topography (analogous to shadowsinaphotograph)wouldbereversedwhenthesamplewas rotated by 180^◦; the consistency of the greyscale variations ob- servedhere, irrespective ofsampleorientation, demonstratesthat ourresultsarenotaffectedbysamplesurfacetopography.Insome images taken after SIMS analysis, residual gold coat remains in cracksandvesicleinteriorsandappearsbrightwhiteinBSEM(e.g.

Fig. 3a). Images were acquired at Durham University (GJ Russell MicroscopyFacility) witha HitachiSU-70AnalyticalHighResolu- tionSEMwithattachedGatanMonoCL andassociatedDigitalMi- crograph software using a 15 keV electron beam in backscatter mode witha workingdistance of15 mm. Qualitative H2Ot vari- ationsareseeningreyscalevariations:darkglassisH2Ot-richand lightglassisH2Ot-poor(Fig. 3a,Fig. 4a).

2.2.2. SIMSanalysis

Samples were prepared as for BSEM work. ¹H⁺, ²³Na⁺ and 28Si⁺ were analysed in radial proﬁles towards vesicles using a CAMECAims-4fion microprobeattheEdinburgh IonMicroprobe Facility.A 10.7 keV,6 nA,O⁻ primarybeamwasacceleratedonto thesamplewithanetimpactenergyof15keV.Positivesecondary ionswereacceleratedto4.25 keVandcollectedsequentiallyonthe electronmultiplierdetector,usinga75 eVoffsetwitha40 eVen- ergy window. Proﬁles were run inscan mode witha 5 μmstep size,witheachanalysedspotapproximately5×^{5 μm and}<3 μm deep.H2Ot concentration wascalculatedfromaworkingcurve of 1H⁺/²⁸Si⁺whichwascalibratedtwicedailyusingwellconstrained standardsofvaryingSiO2 andH2Otcontent.Errorsare±^10%^relative.

2.2.3. SIMS-BSEMcalibrationanddataprocessing

FollowingthetechniqueofHumphreysetal. (2008),SIMSH2Ot datawereusedtocalibrateBSEMimagesinordertoextractquan- titative H2Ot data at high spatial resolution (<1 μm). Greyscale valueswere extractedalonga 5 μm-wideprofileimmediatelyad- jacent to the visible SIMS track, using Gatan DigitalMicrograph software. Mean greyscale values of5 μm segments were plotted againstthecorrespondingSIMSmeasurements(Fig. 3).A linearre- gressionfitwasappliedandtheresultingcalibrationequationused to extract quantitative data in the same image, each newimage requiring aseparate calibration. ‘Edgeeffects’ atvesiclewallsare narrowandanomalouslybrightandthuseasilyremovedfromex- tracted profiles;presentedprofiles thereforebegina fewmicrons from the vesicle wall. Presented images are enhanced to make greyscalevariationsmoreapparentbyvaryingbrightness,contrast andgammasettings;thesesettingsdonotaltertherawgreyscale valuesusedforcalibrationanddataextraction.

Foreachsample,multipleradialprofileswereextractedaround multiplevesicles.Onceallextractedgreyscaledatawereconverted to H₂O_t data using the relevant image calibration equation they werecompiledtocreateacompositedatasetofH2Otasafunction of distance from vesicle wall for the sample. An example figure which shows this profile-averaging methodology is presented in Supplementary Information.The meanH2Ot value was calculated for all data within 2 μm segments; these meanvalues form the H₂O_t profileforeachsample.Errorsshownaretwicethestandard errorofthismean.Averagingallextractedprofilesforonesample thusaccountsforthevariationresultingfromvesiclesthataresec- tionedatdifferentdistancesfromtheirequator (whichaffectsthe profilegradient,withsteepest profilesforthosesectioneddirectly atthe equator,see SupplementaryInformation), andforvariation inH2Ot resultingfromtheuseofmultipleSIMStrackspersample (whichaffectsthepositiononthe y-axisbutnottheshapeofthe profile).

2.2.4. FTIRanalysis

Samples were prepared as free-standing wafers polished on bothsides.TheirhighH2Otcontentsrequiredthinwafers(<20 μm) toavoidsaturatingthedetector.Samplesweremountedonaglass slidewithCrystalbond509andgroundwithsilicacarbidegritbe- fore polishingwith3 μm and1 μmdiamond pasteto produce a ﬂat, polished surface. Samples were then ﬂipped and remounted polishedside downandgroundandpolishedfromtheother side.

Thickness was monitored during polishing using a micrometer on the glass surface. As target thickness was approached, micro- metermeasurementswereconductedontheadjacentcrystalbond to avoid damaging the delicate sample. Final thickness was de- termined using interference fringes on reﬂectance FTIR spectra.

Samples were ﬁnally removed from the slide by dissolving the crystalbond withacetone, then a paintbrush (rather than tweez- ers)wasusedtoremovethefragilewafersfromtheacetonebath.

FTIR analyses were acquired at the Institute for Research on EarthEvolution(IFREE),JapanAgencyforMarine-EarthScienceand Technology (JAMSTEC), using theVarian FTS Stingray 7000Micro Imager Analyzer spectrometer withan attached UMA600micro- scope. Mid-IR (6000–700 cm⁻¹) transmittance spectroscopic im- ageswerecollectedover512scansataresolutionof8 cm⁻¹using aheatedceramic(globar)infra-redsource,a Ge-coatedKBrbeam- splitter, andthe Varian Inc. Lancer Focal Plane Array (FPA) cam- era housedinthemicroscope,whichconsistsofaliquid-nitrogen cooledinfraredphotovoltaicHgCdTe2(MCT)arraydetector.Thear- ray detector is comprised of 4096 channels (arranged 64×⁶⁴⁾ acrossa350×350 μm areagivingachannel,orspectral,resolution of5.5×⁵.5 μm.TheFPAcamerawascalibratedregularly.Samples wereplacedonaKBrwindowunderN₂ purgeandareaswerese- lectedforanalysisusingthemicroscope.Initiallyabackgroundim-