The Fina Nagu Volcanic Complex: Unusual submarine arc

TheFinaNaguVolcanicComplex:UnusualsubmarinearcvolcanismintherapidlydeformingsouthernMarianamarginMaryjoN.Brounce1,KatherineA.Kelley2,RobertStern3,FernandoMartinez4,andElizabethCottrell51DivisionofGeologicalandPlanetarySciences,CaliforniaInstituteofTechnology,PasadenaCA,USA2GraduateSchoolofOceanography,UniversityofRhodeIsland,NarragansettRI,USA3GeosciencesDepartment,UniversityofTexasatDallas,RichardsonTX,USA4Hawai’iInstituteofGeophysicsandPlanetology,UniversityofHawai’iatManoa,HonoluluHI,USA5DepartmentofMineralSciences,NationalMuseumofNaturalHistory,SmithsonianInstitution,WashingtonDC,USAKEYPOINTS‐TheFinaNaguvolcanicchainisanunusualchainofsmallvolcanoesmarkingthesouthernextensionoftheMarianaarc‐ThesubductioncomponentresponsibleformeltgenerationatFinaNagudoesnotproduceoxidizedmagmas‐Amphiboleand/orserpentinemineralbreakdownmaybecriticalforformingoxidizedarcmagmasABSTRACT

IntheMarianaconvergentmargin,largearcvolcanoesdisappearsouthof

GuameventhoughthePacificplatecontinuestosubductandinstead,smallcones

scatterontheseafloor.Thesesmallconescouldformeitherduetodecompression

meltingaccompanyingback‐arcextensionorfluxmelting,asexpectedforarc

volcanoes,orasaresultofbothprocesses.Here,wereportthemajor,trace,and

volatileelementcompositions,aswellastheoxidationstateofFe,inrecently‐

dredged,freshpillowlavasfromtheFinaNaguvolcanicchain,anunusualalignment

ofsmall,closely‐spacedsubmarinecalderasandconessouthwestofGuam.Weshow

thatFinaNagumagmasaretheconsequenceofmantlemeltingduetoinfiltrating

Research Article Geochemistry, Geophysics, GeosystemsDOI 10.1002/2016GC006457

This article has been accepted for publication and undergone full peer review but has not beenthrough the copyediting, typesetting, pagination and proofreading process which may lead todifferences between this version and the Version of Record. Please cite this article asdoi: 10.1002/2016GC006457

© 2016 American Geophysical UnionReceived: May 25, 2016; Revised: Sep 27, 2016; Accepted: Sep 27, 2016

This article is protected by copyright. All rights reserved.

aqueousfluidsandsedimentmeltssourcedfromthesubductingPacificplateintoa

depletedmantlewedge,similarinextentofmeltingtoacceptedmodelsforarc

melts.FinaNagumagmasarenotasoxidizedasmagmaselsewherealongthe

Marianaarc,suggestingthatthesubductioncomponentresponsibleforproducing

arcmagmasiseitherdifferentornotpresentinthezoneofmeltgenerationforFina

Nagu,andthatamphiboleorserpentinemineraldestabilizationreactionsarekeyin

producingoxidizedarcmagmas.IndividualFinaNaguvolcanicstructuresare

smallerinvolumethanMarianaarcvolcanoes,althoughtheestimatedcumulative

volumeofthevolcanicchainissimilartonearbysubmarinearcvolcanoes.We

concludethatmeltgenerationundertheFinaNaguchainoccursbysimilar

mechanismsasunderMarianaarcvolcanoes,butthatcomplexlithospheric

deformationintheregiondistributesthemeltsamongseveralsmalledificesthatget

youngertothenortheast.

INDEXTERMS

1031SubductionZoneProcesses;1037Magmagenesisandpartialmelting;1065

Majorandtraceelementgeochemistry;8485Remotesensingofvolcanoes;8427

Subaqueousvolcanism

KEYWORDS

Marianasubductionzone;arcvolcanoformation;oxidizedbasalt;back‐arc

spreading;hydrothermalvents

1.INTRODUCTION


Arcvolcanoesgrowlargebecausetheyreflectlongperiodsofintense

eruptiveactivityatasinglelocationandtheirlavasaredemonstrablyinfluencedby

meltsandfluidsfromsubductingoceanicplatesthatmovethroughthemantleon

geologicallyrapidtimescales[i.e.,fasterthanplatetectonictimescales;e.g.,Elliottet

al.,1997;Morrisetal.,1990;PlankandLangmuir,1993;Turneretal.,2001].These

observationsindicatethatfluidsfromthesubductingslabenterthemantlewedge

andcauseittomelt[e.g.,GaetaniandGrove,2003;Kelleyetal.,2010].Mantlemelts,

inadditiontomeltsfromthesubductingslab,formconduitnetworksandbuoyant

diapirsinthemantlewedgethattransportmeltsrapidlytothesurface[e.g.,Halland

Kincaid,2001;MarshandCarmichael,1974].Thisphenomenonbroadlydescribes

themechanismsresponsibleforproducingthevolcanicarcintheMarianas,where

U‐Th,Ra‐Th,andU‐Padisequilibriarequire<150,000yearsbetweenslab

dehydrationanderuptionofarcmagmas[Avanzinellietal.,2012;Turneretal.,

2001].

Anatahan(~16.35°)andTraceySeamount(~13°40’N,westofGuam)are

thesouthernmostsubaerialandsubmarinestratovolcanoes,respectively,that

clearlydefinetheMarianamagmaticarc[Fig.1;e.g.,Sternetal.,2013],despitethe

continuedsubductionofthePacificplatetothesouth.Theproposedextensionofarc

volcanismfurthersouthistheAlphabetseamountvolcanicprovince[~13°00’to

13°40’N,southwestofTracey;Sternetal.,2013],buttheseseamountsaresmallin

volumeandirregularlyspaced.Furthermore,Alphabetseamountshavelava

compositionswithcharacteristicsofbothback‐arcandarclavas,whichare

inconsistentwiththemodelforarcvolcanoformationdescribedabove[Fryeretal.,


1998;Sternetal.,2013].TheFinaNaguvolcanicchainextendsevenfurthersouth

fromtheAlphabetseamountvolcanicprovinceto~12°20’N,andconsistsofa~60

kmalignmentoffourlargesubmarinecalderasandfivecone‐shapededifices(Fig.

1b,Fig.2).Likethearcvolcanoestothenorth,andunlikethescatteredvolcanoesof

theAlphabetseamounts,thosethatmakeuptheFinaNaguvolcanicchainalign

linearlyandparalleltothetrenchandtheMalaguana‐GadaoRidge[referredtoas

theSouthernMarianaTroughbyBrounceetal.,2014],thesouthernmostspreading

segmentoftheMarianaTrough.

TheFinaNaguvolcaniccentersareinsomewaysunliketraditionalarc

volcanoes.Thecalderafeaturesarespacedanaverageof12kmapartonthe

seafloor,comparedto75kmspacingofthesubaerialarc.Theyarelocated20‐30km

fromtheback‐arcspreadingcenter,whichisunusuallyclosecomparedtocentral

Marianavolcanoes,whichare90‐100kmfromtheback‐arcspreadingridge.The

FinaNaguvolcanoesarealsounusuallyclosetothetrench,~90km,whereasnormal

Marianaarcstratovolcanoesare~220kmwestofthetrench.Giventhisunusual

location,FinaNaguvolcanoescouldrepresentthesouthernmostexpressionofthe

Marianavolcanicfront,orcouldberelatedtoextensionattheMarianaTroughback‐

arcspreadingcenter–orboth.TheunusualnatureandpositionoftheFinaNagu

volcanicchainprovidesanopportunitytoevaluatetherelationshipbetweenarcand

back‐arcbasinmagmatism,especiallytointerrogatetherelativeimportanceof

upperplatestressesversusdowngoingplategeometryanddehydrationonmagma

generation.


Here,wepresentmajor,trace,andvolatileelementcontentsandFe3+/∑Fe

ratiosoffreshsubmarinepillowglassesdredgedfromfoursubmarinevolcanoesof

theFinaNaguvolcanicchain,inordertodeterminewhetherthesereflectarcor

back‐arcmagmaticprocesses,orsomehybrid.Wethencomparetheseresultswith

thecompositionofsoutheasternMarianaforearcrift(SEMFR,Fig.1)lavasinorder

toconstraintherelationshipbetweenmeltsandfluidsfromthesubductingPacific

plateandvolcanisminthesouthernMarianaconvergentmargin.

2.GEOLOGICBACKGROUND

TheFina‐Naguvolcanicchainconsistsoffourdistinctsubmarinecalderas

andfivevolcanicconesinthesouthernmostMarianasubductionsystem,southwest

ofGuam(Fig.1,Fig.2).ItislocatedbetweenthesouthernterminusoftheMariana

Troughback‐arcspreadingcenter,calledtheMalaguana‐GadaoRidge[referredtoas

theSouthernMarianaTroughbyBrounceetal.,2014],andtheSoutheastern

MarianaForearcRift,anunusualregionofforearcextensionandrelatedvolcanism

[Fryer,1995;Ribeiroetal.,2013b].ThesouthernmostMarianas,betweenthe

Malaguana‐GadaoRidgeinthewest,theMarianatrenchintheeastandthesouth,

andtheAlphabetseamountvolcanicprovinceinthenorth,isatectonicallycomplex

andrapidlydeformingregion[Fryer,1995;Kato,2003;Martínezetal.,2000].The

upperplateisextendingsoutheast‐northwest,accommodatedbyspreadingalong

theMalaguana‐GadaoRidge.Inaddition,arecent,northeast‐southwestextensionin

theforearcaccommodatedyoungvolcanicactivity(<3Ma)intheSoutheastern

MarianaForearcRift[Kato,2003;Martínezetal.,2000;Ribeiroetal.,2013a;Ribeiro


etal.,2013b].ThePacificslabisalsosubductingorthogonallytotheMarianatrench

inthisregionatapproximately30mm/year[Bird,2003].Theslabprogressively

deepensfrom50kmbeneaththeforearcrift,to75‐125kmbelowtheFinaNagu

volcanicchain,andto150‐200kmundertheMalaguana‐GadaoRidge[Syracuseand

Abers,2006].

LavaseruptedintheforearcriftandalongtheMalaguana‐GadaoRidgeare

compositionallysimilartooneanother,enrichedinincompatibletraceelements

thatarecharacteristicofinputfrommeltsorfluidsfromthesubductingPacificplate

[e.g.,Ba,Th;Fig.3;Fig.4;Fig.5;Brounceetal.,2014;Ribeiroetal.,2013b].The

forearcriftlavaserupted2.7‐3.5Ma,inatectonicsettingthatmayhavebeen

differentfromthepresentconfigurationduetorapidtectonicreorganizationinthe

southernMarianassince3Ma[Ribeiroetal.,2013b].Therearenoradiometricages

availableforMalaguana‐GadaoRidgelavas,butthefreshnatureofpillowlavas,

includingintactglassyrims,theinflatedmorphologyoftheridge,andthepresence

ofageophysicallyimagedmagmachamber[Beckeretal.,2010]andactive

hydrothermalvents[Yoshikawaetal.,2012]demonstratesthatthisisanactive

spreadingcenterandthatthelavasrecoveredfromtheaxialhighareessentially

zeroage.

GlassypillowlavasfromfourvolcanoesintheFinaNaguvolcanicchainwere

dredgedfromtheseafloorbetween12°20.3’Nand12°48.0’Nduringexpedition

TN273oftheR.VThomasG.Thompsonin2011‐2012.Informationaboutsample

locationsandwaterdepthsareprovidedinSupplementaryTable1.Handsample

descriptionscanbefoundinSupplementaryTable2.Seafloorobservationswere


madeduringROVdives5,6,and7duringExpedition1605L1ofNOAAOkeanos

ExplorerinApril2016.

3.ANALYTICALMETHODS

3.1MajorelementandS,Clanalysis

Weanalyzedfreshsubmarineglasschipsformajorelement,S,andCl

contentsbyelectronmicroprobeattheSmithsonianInstitution.Duringmajor

elementanalysis,thebeamwasoperatedat10nA,15kVanda10µmbeam

diameter.Sodiumandpotassiumweremeasuredfirstwith20secondpeakcount

timestominimizealkaliloss.Subsequently,Si,Ti,Al,Fe*,Mn,Ca,andPwere

measuredwith30‐40secondpeakcounttimes.AlldataweresubjecttoZAF

correctionprocedures.PrimarycalibrationstandardsincludeVG‐2glass,Kakanui

hornblende,anorthite,microcline,ilmentite,andapatite[Jarosewichetal.,1980].

TheVG‐2andVG‐A99glassesweremonitoredassecondarystandardsduringeach

run[Jarosewichetal.,1980].Sulfurandchlorineweremeasuredseparatelyusinga

beamoperatedat80nA,15kV,and10µmbeamdiameter.Scapolitewasusedasthe

primarycalibrationstandard(0.529wt%S,1.49wt%Cl).TheVG‐2(1400ppmS,

300ppmCl)andNIST620(1121ppmS)glasseswereusedassecondarystandards

ineachrun[CarrollandRutherford,1988;Jarosewichetal.,1980;Wallaceand

Carmichael,1991].FurtherdetailsareprovidedintheSupplementarymaterials.

3.2Traceelementanalysis


Abundancesof33traceelements(Sc,V,Cr,Co,Ni,Cu,Zn,Rb,Sr,Y,Zr,Nb,Cs,

Ba,La,Ce,Pr,Nd,Sm,Eu,Gd,Tb,Dy,Er,Tm,Yb,Lu,Hf,Ta,Pb,Th,U)were

determinedinglasschipsbylaser‐ablationinductively‐coupledplasmamass

spectrometryattheUniversityofRhodeIslandonaThermoX‐SeriesIIquadrupole

ICP‐MScoupledwithaNewWaveUP213Nd‐YAGlaserablationsystemfollowing

techniquesoutlinebyKelleyetal.[2003]andLytleetal.[2012],normalizingto43Ca

astheinternalstandard.Thelaserenergywas0.2‐0.3mJatthesamplesurfacefora

referencespot(60µm,10Hz)onNIST612glassandtherepeatrateforglasseswas

reducedto5Hztoachieveaslowdrillingratethroughthinsamples.Spotsizes

rangefrom40‐80µm.UnitedStatesGeologicalSurveyglassstandardsBCR‐2g,

BHVO‐2g,BIR‐1g,andMaxPlanckInstituteglassstandardsGOR‐132‐G,StHls‐G,T1‐

G,ML3B‐GandKL2‐Gwereusedtocreatelinearcalibrationcurves(R2>0.990)for

eachanalyticalsession[Jochumetal.,2006;Kelleyetal.,2003].Sampleswere

analyzedintriplicateandconcentrationswerereproducibletowithin4%rsdforall

elements.

3.3DissolvedH2OandCO2

WaferedglasschipswereanalyzedfordissolvedH2OandCO2concentrations

byFourier‐transforminfraredspectroscopyattheSmithsonianInstitutionandthe

UniversityofRhodeIsland,onsimilarinstrumentsandfollowingidentical

procedures.AllsampleswereanalyzedwithaThermo‐Nicolet6700oraThermo

NicoletiS50spectrometercoupledwithaContinuummicroscope.Spectrawere

collectedbetween1000‐6000cm‐1usingatungsten‐halogensource,KBr


beamsplitterandaliquid‐nitrogencooledMCT‐Adetector.Thebench,microscope,

andsampleswerecontinuouslypurgedwithairfreeofwaterandcarbondioxide

usingapurgegasgenerator.Aperturedimensionswereselectedforeachsample

dependingonthegeometryoffreeglasspathways,ranginginsizefrom20x20µmto

150x150µm.DissolvedtotalH2Oconcentrationsweredeterminedusingthe3530

cm‐1band.DissolvedCO32‐concentrationswerealwaysbelowdetectionatthe

thicknessofglasswafersnecessarytogeneratelargepoolsofglassfreefrom

crystalsorvesicles,sonodataarereportedforCO2contentsoftheseglasses.

Thicknessesofeachsampleweremeasuredusingapiezometricdigimaticindicator.

Glassdensitiesandabsorptioncoefficientsrelevanttoeachabsorptionbandwere

calculatedusingmethodsfromDixonetal.[1995]andLuhr[2001].

3.4Fe3+/∑Feratios

WaferedglasschipswerealsoanalyzedinsituforFe3+/∑Feratiosviamicro

x‐rayabsorptionnearedgestructure(‐XANES)spectroscopyfollowingthe

methodsandtechniquesofCottrelletal.[2009]atbeamlineX26A,National

SynchrotronLightsource,BrookhavenNationalLaboratory.Spectrawerecollected

influorescencemodefrom7020eVto7220eVusinga4‐elementVortexME‐4

silicondriftdiodedetectorwithtwosingleelementVortex‐Exdetectors(Hitachi)

coupledtoanXMapdigitalspectrometersystem,aSi[311]monochromatoranda

nominalbeamsizeof9x5µm.Aberylliumwindowwasplacedoverthedetectorto

attenuatehighcountratesabovethemainFeK‐alphafluorescencepeak.Reference

glassLW_0wasmonitoredcontinuouslyduringeachexperimentalsessionto


correctforinstrumentaldrift.Furtherdetailsrelatedtothiscorrectioncanbefound

inCottrelletal.[2009].Spectrawerescrutinizedforinfluencefromphenocrystsor

microphenocrystsintheglasschips.Detailsofthisprocedurecanbefoundin

Brounceetal.[2014].

4.COMPOSITIONOFFINANAGULAVAS

Thepillowlavasincludedinthisstudyareaphyricorolivine+plagioclase

+/‐clinopyroxenephyric,variablyvesicularbasalts.Submarinepillowglassesfrom

theFinaNaguvolcanicchainarebasalttobasalticandesite,with2.73–7.23wt%

MgO.ThemajorelementandvolatilecompositionsandFe3+/∑Feratiosofthese

glassesarebroadlysimilartolavasfromthenearbyMalaguana‐GadaoRidgeand

southeastMarianaforearcrift,withthenotableexceptionthatFinaNaguandsome

southeastMarianaforearcriftglasseshavelowerTiO2andNa2Oconcentrations

thanMalaguana‐GadaoRidgeglasseswithsimilarMgOcontents(Fig.3,Fig.4b).

VariationsinmajorelementsforglassesfromMalaguana‐GadaoRidge,the

southeastMarianaforearcrift,andFinaNaguareconsistentwitholivine+

plagioclase+/‐clinopyroxenefractionation[Fig.3;Brounceetal.,2014].Theglasses

haveuniformH2Oconcentrations(1.9+/‐0.2wt%),CO2contentsbelowdetection,

andSconcentrationsbelowsulfidesaturation(60‐353ppm).Thesampleswere

collectedin2358–3800meterswaterdepth,correspondingtoconfiningpressures

fromthewatercolumnof315‐400bars.Thewatercontentsoftheglassesindicate

thattheFinaNagumagmasdegassedwaterconcomitantlywithcrystalfractionation

tomaintainamagmaticwaterconcentrationthatisequaltotheconfiningpressure


fromthewatercolumnatthetimeoferuption,whichisconsistentwiththedepthof

lavacollectionontheseafloor.ThelowCO2contentsoftheseglassesindicatethat

CO2degassingalsotookplace.ThelowScontentsoftheseglassescouldbetheresult

ofthelossofsulfurtoavaporphaseduringdegassing,howeverthewaterdepthsof

collection(2358‐3800mbsl)donotpermitSdegassing(Sdegassingoccursatwater

depths<1000meters;e.g.,Burgisseretal.,2015).Weemphasizethat,(1)theH2O‐

CO2contentsareconsistentwithvaporsaturationatthewaterdepthsofsample

collection,(2)thatthepeaksofthereconstructededifices(i.e.,theestimatedheight

abovetheseafloorofeachofthevolcanicconesbeforecalderaformationorother

masswastingevents)intheFinaNaguvolcanicchainremainin≥2000meterswater

depth,and(3)thehandspecimensofsamplesinthisstudy,thoughmoderately

vesicular,donothavereticuliticorotherwisevolcaniclastic‐typetexturestypicalof

explosiveunderwatervolcanism.WeconcludethatthelowScontentsoftheglasses

inthisstudyareunlikelytobetheresultofSdegassing,andthattheglass

chemistriesareconsistentwithdegassingofH2O+CO2concomitantwithcrystal

fractionation(SupplementaryTable1,Fig.3).FinaNagusubmarineglassesare

depletedinmoderatelyincompatibletraceelements(Fig.4),relativetobothnormal

MORBandMalaguana‐GadaoRidgelavas,butare~2‐70timesenrichedovernormal

MORBinselectincompatibletraceelements[e.g.,Sr,Pb,K2O,U,Ba,Cs,Rb;Elliottet

al.,1997]thatarecommonlyassociatedwithfluidsormeltsoriginatingfromthe

subductingplate(Fig.4a,5).FinaNaguglassesareslightlyenrichedinLREEand

depletedinHREEandhigh‐fieldstrengthelementsrelativetonormalMORB

compositions(Fig.4a).FinaNagusubmarineglasseshaveFe3+/∑Feratiosof0.177–


0.223andfO2~QFM+0.7,thataresimilaracrossMgOcontents,andareslightly

oxidizedrelativetoMORBglass,butnotasoxidizedasmodernMarianaarcmagmas

(Fig.6a,d);theyaresimilartoMarianaTroughglasses(includingtheMalaguana‐

GadaoRidgesegment).AlthoughwebelieveSdegassingtobeanunlikely

explanationforthelowScontentsofFinaNagusubmarineglasses(seeabove),ifS

degassingdidtakeplaceitispossiblethattheundegassedmagmashadhigherFe3+/

∑Feratiosthanthosemeasuredinthefinaleruptedglasses.Thereducingeffectsof

SdegassinghavebeenobservedatAgrigan,Erebus,andKilaueavolcanoes[Kelley

andCottrell,2012;Moussallametal.,2014;Moussallametal.,2016],andcould

accountforachangeinFe3+/∑Feratiosof~0.05units(absolute),foramagma

chemistrysimilartothatobservedatAgriganvolcano[KelleyandCottrell,2012].

ThiswouldmeanthattheundegassedFinaNagumagmashaveFe3+/∑Feratios

near0.227‐0.273,similartothatobservedforthelargesubaerialandsubmarine

edificesinMarianaarc.

TheFe3+/∑FeratiosandfO2ofMarianaTrough(includingtheMalaguana‐

GadaoRidgesegment)andarcbasalticglassescorrelatewithenrichmentsinfluid‐

mobiletraceelements(e.g.,Ba/Laratio;Fig.6b,e),whichhasbeeninterpretedto

ariseduetotheoxidizinginfluenceofslabfluidsinthemantlewedge[Brounceetal.,

2014;Brounceetal.,2015].FinaNagusubmarineglassesextendtomoderately

elevatedBa/Laratios(~30)butonlyslightlyoxidizedFe3+/∑Feratios(0.19)and

fO2(QFM+0.7),whichdonotoverlapwiththeobservedtrendbetweenoxidation

andtraceelementenrichmentforback‐arcandarcbasalticglassesintheMarianas.

FinaNagusubmarineglassesalsohavelowerTiO2/VratiosthanMORBand


Malaguana‐Gadaoridgeglasses,andoverlapentirelywithTiO2/Vratiosobservedin

Marianaarcbasalticglasses(Fig.7).

5.DISCUSSION

Inthefollowing,weintegrateourgeochemicaldatawithvolumeandage

estimatesfortheFinaNaguvolcanicchaintoevaluatethereasonsfortheunusual

distributionofmagmatismresultinginmanysmallvolcanicedificesatFinaNagu.

A.GEOCHEMICALCONSTRAINTS

ThesouthernmostportionoftheMarianaconvergentmarginistectonically

complex,andasthenosphericmantleflowandpathwaysforcommunication

betweentheslabandthesurfacearelikelytobesimilarlycomplex[Ribeiroetal.,

2013a;Ribeiroetal.,2013b;Sternetal.,2013].Malaguana‐GadaoRidgelavasare

slightlyenrichedinheavyrareearthelementsandhigh‐fieldstrengthelements

relativetonormalMORBcompositions,suggestingthatthemantlethatflowsinto

themeltingtrianglebeneaththeback‐arcbasinspreadingridgeissimilartonormal

orenrichedMORBsourcemantle[Brounceetal.,2014;MasudaandFryer,2015].

FinaNagulavasaremoredepletedinheavyrareearthandhighfieldstrength

elementsthanMalaguana‐GadaoRidgelavas(Fig.4a,b),showingsimilarlevelsof

mantledepletionasthemantlesourceforsubaerialarcvolcanoes(e.g.,Sarigan

volcano,Fig.4a),evenwhenvariableextentsofmeltingaretakenintoconsideration

(e.g.,variableNa2Ocontents;Fig.4b).Giventhecloseproximity(~20‐30km)ofthe

FinaNaguvolcanicchainandtheMalaguana‐GadaoRidge,thissuggeststhatmantle


undergoesmeltextractionattheridgeandthedepletedresidueflowstowardthe

trench,andismeltedagaininthesourceregionforFinaNaguvolcanism.

SoutheasternMarianaforearcriftlavasareintermediatebetween

Malaguana‐GadaoRidgeandFinaNagulavasinheavyrareearthandmoderately

incompatibletraceelementdepletion[Ribeiroetal.,2013a;Ribeiroetal.,2013b].

Thisisanimportantobservation,becauseitsuggeststhatatthetimeoferuptionof

theforearcriftlavas,thetwice‐depletedresidueofmeltextractionatthe

Malaguana‐GadaoRidgeandtheFinaNaguvolcanicchaindidnotcontinuetoflow

towardtheforearcrifttoserveastheprimarymantlesourcecomponentfor

southeasternMarianaforearcriftvolcanism.Instead,eithertheFinaNaguvolcanic

chainisyoungerthanthesoutheasternMarianaforearcrift(i.e.,≪3Ma),or,ifFina

Naguvolcanismwascoincidentorolder,theremusthavebeenasourceofmantle

flowingintotheforearcriftregionthatwasnotsignificantlydepletedbypriormelt

extraction.Inthepresenttectonicconfiguration,thesubductingPacificplateis≤50

kmbelowthesurfaceofthesoutheasternMarianaforearcrift[SyracuseandAbers,

2006],whichmaycreatetoosmallavolumeundertheforearcriftforsignificant

asthenosphericcirculation.However,iftherewasasthenospherepresentbelowthe

forearcriftatthetimeofactivevolcanismintherift,themoderatelydepletedheavy

rareearthelementcompositionsoftheforearcriftlavassuggestthatthemantlehad

acomponentofflowthatwasparallelorsub‐paralleltothetrench,inordertobring

mantleintothesourceregionofforearcriftlavasthathasnotpreviouslymelted

undertheMalaguana‐GadaoRidgeandtheFinaNaguvolcanicchain[Ribeiroetal.,

2013a].


Thetraceelementandisotopiccompositionsofglobalback‐arcandarclavas

arecommonlyinfluencedbycontributionsfromsedimentmeltsoraqueousfluids

fromsubductingslabs[Elliottetal.,1997;Morrisetal.,1990;PlankandLangmuir,

1993].Thecompositionalsignaturesofsedimentmelts(i.e.,silicateliquids)and

aqueousfluids(i.e.,H2O‐richliquids)aredemonstratedintheMarianasbyback‐arc

andarclavasthatareenrichedinBa,U,andsometimesThrelativetomoderately

incompatibleorheavyrareearthelements.Terrigenoussedimentsthatare

subductedwiththePacificplateareenrichedinthoriumthatismobilizedintothe

wedgebysedimentmelts[JohnsonandPlank,2000].Bariumanduranium,onthe

otherhand,partitionstronglyintoaqueousfluids,andaremobilizedintothewedge

influidsgeneratedbydehydrationofthesubductingslab.Uraniuminparticularis

immobileasU4+,butU6+isfluid‐mobile,suchthatUbecomesincreasinglymore

mobileinaqueousfluidsasfO2increases[Balietal.,2011].Malaguana‐GadaoRidge

lavashavevariableNb/Ybratiosthatreflectmantledepletionbypriormelt

extraction.HoweverTh/Yb,andtoagreaterextentBa/YbandU/Ybratiosinthese

lavasareelevatedrelativetoMORBandtheirvaluesindicatethatBa,U,andThhave

beenaddedtothemantlesourcefortheMalaguana‐GadaoRidgebybothaqueous

fluids(inthecaseofBaandU)andsilicatemelts(i.e.,meltsoftheoverlying

sedimentoroceaniccrust,inthecaseofTh)fromthesubductingPacificplate[Fig.

5a,b,c;e.g.,Pearceetal.,2005].TheelevatedU/YbratiosindicatethatthefO2inthe

zoneofmeltgenerationmusthavebeenelevatedoverMORB(Fig.5c).Thelevelof

Ba,U,andThenrichmentforMalaguana‐GadaoRidgelavasissimilartothatof

centralandnorthernMarianaTroughlavas.FinaNagulavasshowevidencefor


largeradditionsofBaandThinthemantlesourcethanintheMalaguana‐Gadao

Ridgemantlesource,butnotaslargeasthoseforthesubaerialarcvolcanoes(Fig.

5a,b).TraceelementcompositionsofMalaguana‐GadaoRidgeandFinaNagulavas

areconsistentwithmixingbetweenvariablydepletedMORB‐likemelts(white

invertedtriangles,Fig.5a,b,c)andanarc‐likemelt(whitestar,Fig.5a,b,c).

AlthoughFinaNagulavasarenotasenrichedinBa,U,orThassomeofsubaerial

Marianaarc,theyareinfluencedbysubductioncomponentstoagreaterextentthan

theMalaguana‐Gadaoridge.

AtagivenMgOconcentration,theconcentrationsofTiO2andNa2Ocan

reflectdifferencesinprimarymeltTiO2andNa2Ocompositions,whichare

generatedbyvariabledegreesofmantlemelting.TheNa2OandTiO2concentrations

ofFinaNagulavasarelowerthanthoseoftheMalaguana‐Gadaoridge,andareon

thelowendoftherangeofarclavasatMgO=6.0wt%[atMgO=6.0wt%,TiO2~0.8

wt%forFinaNagu,~1.0wt%forMalaguana‐Gadaoridge,~0.6‐2.0wt%forMariana

arc;Fig.3,Fig.4b;Kelleyetal.,2010].Thissuggeststhat,ifthemantlesource

compositionissimilarbeneaththecentralarcandFinaNagu,themantleunderthe

FinaNaguvolcanicchainproducesextentsofmeltingsimilartoorhigherthanthe

mantleundercentralMarianaarcvolcanoes.Thisissurprising,giventhemuch

smallersizeofFinaNaguedifices.

Marianaback‐arcbasinandarcbasalticglassesaremoreoxidizedthanmid‐

oceanridgebasalticglasses[Fig.6a,b,c,d,e,f;e.g.,Brounceetal.,2014].Thereisa

simplerelationshipbetweentraceelementenrichment(e.g.,Ba/Laratios)and

oxidationinMarianabackarcbasinandarcglasses,suggestingthataqueousslab


fluidsareoxidizedandgeneratevariablyoxidizedmantlemelts,dependentonthe

ratiooffluidtomantleinvolvedinmeltgeneration[Fig.6b,e;Brounceetal.,2014;

Brounceetal.,2015;KelleyandCottrell,2012].ThisistrueoftheFe3+/∑Feratios

andalsoofthecalculatedmagmaticfO2values,thelatterofwhichtakesinto

considerationtheeffectsofpressure,temperature,andanhydrousmajorelement

chemistryontherelationshipbetweentheoxidationstateofFeoffO2(Fig.6e).

Surprisingly,FinaNaguglassesdonotfollowtherelationshipbetweenincreasing

Ba/LaandFe3+/∑Feratiosthatisdefinedby>150submarineglasschipsandolivine

hostedmeltinclusionsfromMarianaarcvolcanoes,theMarianaTrough,andglobal

MORB,includingsubmarineglasschipsthatrecordtheinitiationofsubduction

alongtheIzu‐Bonin‐Marianaconvergentmargin[Fig.6b,e;Brounceetal.,2014;

Brounceetal.,2015;CottrellandKelley,2011;KelleyandCottrell,2009].

VolcanicdegassinghasbeenshowntohaveaneffectontheFe3+/∑Feratios

andfO2ofolivine‐hostedbasalticmeltinclusionsfromErebus,Agrigan,andKilauea

volcanoes[KelleyandCottrell,2014;Moussallametal.,2014;Moussallametal.,

2016].Inthesecases,volcanicdegassingledtoadecreaseinFe3+/∑Feratios,and

thiswastiedspecificallytoSdegassingatmagmatictemperatures.IfSdegassinghas

notoccurredfromtheFinaNagumagmasbeforeeruptionontheseafloor(see

above),thennosignificantshiftinFe3+/∑Feratiosisexpected.Ithasalsobeen

proposedthatdegassingandpreferentiallossof(1)H2[e.g.,Holloway,2004],or(2)

CO[MathezandDelaney,1981]canoxidizemagmas,buttwolinesofevidence

demonstratethattheeffectissmall.First,basalticmeltinclusionsandsubmarine

glassesfromtheMarianaarcthatspan>4wt%H2Oand>1000ppmCO2donot


demonstrateanysignificantvariationsinFe3+/∑FeratiosorfO2thatindicatethat

H2O‐CO2degassingimpactstheredoxstateoftheirdegassingmeltssignificantly

[Brounceetal.,2014].Second,observationsofnaturalandesitictorhyoliticeruptive

productsdemonstratethatthelossofseveralweightpercentofwaterdoesnot

changethemeasuredFe3+/∑FeratiosorinferredfO2oftheassociatedmagmas

studied[CrabtreeandLange2012;WatersandLange,2016].

Intheabsenceofdegassing,theredoxstateofFinaNagulavasmayreflecta

mantlesourceormeltgenerationprocess.TheelevatedU/Ybratios(Fig.5c)and

lowerTiO2/Vratios(Fig.7)ofFinaNagulavasareconsistentwithmeasured

Fe3+/∑FeratiosandcalculatedmagmaticfO2valuesthatareelevatedoverMORB,

andthoughTiO2/Vratiosoverlapwitharcsamples,slightlylowerU/Ybratiosin

FinaNagulavasindicatethatfO2maynothavebeenashighasforthemostoxidized

arcsamples(Fig.5c,Fig.6c,f).Further,itappearsasthoughmixingbetween

variablydepletedMORB‐likemelts(invertedtriangles,Fig.6b,c)andanarc‐like

melt(whitestar,Fig.6b,c)cannotexplainthelackofrelationshipbetweentrace

elementenrichmentandoxidationobservedinFinaNagulavas,suggestingthatthe

subductioncomponentresponsibleforoxidationalongtheMarianaarcisdifferent

ornotpresentinthezoneofmeltgenerationforFinaNagulavas.Thisissimilarto

observationsoftheValuFaridge,wherethereisevidenceforasubduction

componentintraceelementcompositions,butnoobviousrelationshipbetweenthe

subductioncomponentandtheoxidationstateofthelavas[Jenneretal.,2015].

ThesubductingPacificplateisbetween75and125kmdepth(2.5‐4GPa)

belowtheFinaNaguvolcanicchain,whichisshallowerthantheaveragedepthto


theslabundersubaerialMarianaarcvolcanoes[150‐175km,5‐5.8GPa;Syracuse

andAbers,2006].TherelativelyshallowdepthsofthesubductingslabunderFina

Nagucorrespondtosedimentsurfacetemperaturesof~380–800°C,whichare

sufficientlyhightodehydrateandmeltthesedimentcolumn[vanKekenetal.,

2011].Theupper1.5kmofoceaniccrustbetween75‐125kmhastemperaturesof

275‐800°C,wherehydrousmineralssuchasamphiboleandlawsonitearestable,

thoughthehighendoftherange(T>600°C),shouldcauselawsonitebreakdown

[vanKekenetal.,2011].Theslab’slowercrustandmantlerocksremaincool

enoughthathydrousmineralphasessuchaslawsonite,chlorite,amphibole(lower

crust)andserpentine,chlorite(mantle)remainstable[vanKekenetal.,2011].The

greaterdepthsofthesubductingslabundersubaerialMarianaarcvolcanoesleadto

amphibolebreakdownintheupper1.5kmoftheoceaniccrustand

chlorite/serpentinebreakdowninthesubductinglithosphere[vanKekenetal.,

2011].ThelackofcorrelationbetweenBa/LaandFe3+/∑FeratiosinFinaNagu

glassesmayindicatethereforethatdeserpentinizationand/oramphibole

breakdownreactionsinthesubductingslabareimportantingeneratingfluidsthat

arecapableofoxidizingthemeltsproducedinthemantlewedge.Serpentinization

reactionsontheseafloortransformolivinetomagnetite,releasingH2‐richfluidsand

leavingtheoceaniclithospherewithamoreoxidizedmagnetite‐bearingassemblage.

Ifhighpressure,hightemperaturedeserpentinizationreactionstransform

magnetitebacktoolivine,thefluidsproducedfromthatreactionmustcontain

oxidizedcomponents(e.g.,SOxspecies;Debretetal.,2016).Similarly,amphibolecan

containFe3+andthebreakdownreactionsthatproduceH2O‐richfluidsthatwould


equilibratewiththatoxidizingassemblage.Thebreakdownofanoxidizing

assemblagecouldpotentiallyoxidizedslabcomponentstofluid‐mobileagentsof

oxidation(e.g.,S2‐toSO3;Gaillardetal.,2015).ThefluidthatcontributestoFina

Naguvolcanism,unliketherestofthearc,mayhaveoxygenfugacitysimilartothe

ambientuppermantle[i.e,nearthequartz‐fayalite‐magnetiteoxygenbuffer;Cottrell

andKelley,2011],ormaynothavesignificantconcentrationsofspeciescapableof

producingoxidizedmantlemelts[e.g.,S,Fe;Mungall,2002].ThatmagmaticfO2

remainedrelativelylowfrommantlemeltingthroughtoeruptionissupportedby

thelackofmagnetitefractionationfrommeltswithhigherthan~3‐4wt%MgO(as

monitoredbyVcontents,Fig.3),despiteallsampleshavingdissolvedH2Ocontents

elevatedovertypicalMORB[e.g.,Osborn,1959,SissonandGrove,1993,Saaletal.,

2002].

B.VOLUMEANDAGEESTIMATES

Despiteinferencesfromgeochemicalobservationsthattheextentsofmelting

requiredtogeneratethecompositionofFinaNagulavasaresimilartoorgreater

thanthoseresponsibleforformingarclavas,thevolumeofindividualvolcanic

edificesintheFinaNaguvolcanicchainareremarkablysmall~24km3.Thisisan

orderofmagnitudesmallerthannearbyMarianaarcvolcanoTracyseamountor

Anatahan(Fig.2).AlthoughtheFinaNagusampleslackradiometricageconstraints,

theFinaNaguvolcanoesappeartobecomeyoungerfromSWtoNEalongthechain.

Severallinesofevidencesupportthisconclusion:seafloorfabrics,theappearanceof


mudandmanganesecoatedlavas,pillowlavaappearanceontheseafloor,and

hydrothermalactivity.

Althoughinterpretationsbasedsolelyonseafloorfabricscanbetenuous,

morphologicallymoremature,andpossiblyoldervolcaniccentersinthesouthwest

appearascalderaswithrelativelyshallowlyslopingcalderawallsthatmayhave

erodedanddegradedovertime(Caldera#1,Fig.1).Thecalderasinthenortheast

havesteepercalderawallsthatmayreflectmorerecentvolcanism(Caldera#4,Fig.

1),alongwiththepresenceofintactvolcanicconesinthenortheast(Cones#4,#5,

Fig.1).AnapparentageprogressioninFinaNaguvolcanismisbroadlysupportedby

recentobservationsoftheseafloorduringEX1605oftheNOAAshipOkeanos

ExplorerandROVDeepDiscoverer(locationsmarkedwithgray,fourpointedstars,

Fig.1).VideoimageryofexposuresonthenorthwallofCaldera#3showsangularto

sub‐angulartalusandoccasionalpillowflowordikeoutcropwithlighttomoderate

sedimentcover(Fig.8a).Caldera#3appearstohaveundergoneepisodesofmass

wastingsincethemostrecentlavaflow,revealingdikeoutcropsonthecalderawall

andfragmentedpillowflowstoproducetalusfields.Thissuggestssometimehas

passedsincethemostrecentlavaflowintheregionexploredbyDeepDiscoverer.

VideoimageryoftheeastwallofCaldera#4revealsabundantandintactpillow

lobesandsheetflowswithlightsedimentcover,suggestingthattheseundisturbed

flowshappenedrecentlyenoughtopreservetheiroriginalemplacementonthe

seafloor(Fig.8b).Videoimageryoftworesurgentdomestructuresinthecrater

floorofCone#5revealsextinctbutintact,large,hydrothermalchimneystructures

(Fig.8c)andlowtemperature,activefluidseepingwithassociatedremnantworm


tubes(Fig.8d).Thecoherenceandapparentlyemplacednatureoftheserelatively

delicatestructures(e.g.,wormtubes,flangeandchimneystructures)suggeststhat

Cone#5wasactiveuntilveryrecently,oratleastmorerecentlythanCaldera#3and

Caldera#4.Thesumofbathymetric,dredgesample,andROVseafloorimages

supportsthehypothesisthatFinaNaguvolcanoesareolderintheSWandyounger

intheNE.

C.FINANAGUVOLCANICCHAIN:ANARCVOLCANO,DISTRIBUTED?

TheestimatedcumulativevolumeoftheFinaNaguvolcanicchainis

approximately217km3,whichisslightlylargerthanTracyseamount(~154km3)

androughlyhalfthesizeofAnatahanvolcano(~463km3;Fig.2).Inconjuctionwith

informationthattheregionisactivelyextending,thissuggeststhatwhilethemain

aspectsofmeltgenerationundertheFinaNaguvolcanicchainaresimilarto

Marianaarcvolcanoes,themeltsweredistributedalongthevolcanicchainrather

thanbeingfocusedinasinglevolcanicedifice.Ifthemeltsproducedunderthe

volcanicchainwerefocusedintoasinglevolcano,theedificewouldbe

approximatelythesizeofTracyseamount.Itislikelythatcomplexlithospheric

deformationpatterns,generatedbyperpendicularspreadingdirectionsatthe

Malaguana‐GadaoridgeandthesoutheasternMarianaforearcrift,arecontrolling

melttransportthroughthelithospheretoamagnitudenotseenelsewherealongthe

Marianavolcanicarc.Thisdeformationencouragestheformationofridge‐like,or

volcanicchainfeatures,andexplainsboththesmallvolumeofFinaNaguvolcanic

structuresandtheirclosespacingontheseafloor.


Itispossiblethatmeltfocusinginthecrust[e.g.,Sternetal.,2013]mayhave

progressedtowardsthenortheastthroughtime.TheFinaNaguvolcanicchainis

alignedinthedirectionofriftinginthesoutheasternMarianaforearcrift,andthis

generatesstretchingorriftingoftheoverridingplatelithosphere[e.g.,Martinezet

al.,2014]relativetodiapir‐likesourcesofarcmeltstiedtothesubductingPacific

plate,whichgeneratedthechainofvolcanoes.Alternatively,thediapir‐likesources

ofarcmeltsmaythemselvesmigraterelativetothesubductingslab,andtherefore

alsorelativetotheoverridingplate(i.e.,thepathofdiapirsthroughthewedgecould

migratetrench‐paralleltowardsthenortheast),andthiscouldgeneratethe

observedmorphologyoftheFinaNaguvolcanoesontheoverridingplate.Achange

inthepathofdiapirsinthewedgecouldariseduetoachangeinshapeofthe

subductingPacificplateinthisregion,producingachangeintheconditionsoffluid

ormelttransferfromtheslabsurfaceandthroughthemantlewedge.Itisalso

possiblethatacombinationofbothprocessesisresponsiblefortheformationofthe

FinaNaguvolcanicchain.

6.CONCLUSIONS

Theresultspresentedheresuggestthat,priortoinfiltrationofsediment

meltsandaqueousslabfluids,FinaNagulavaswereproducedfromadepleted

mantlethatisbroadlysimilarincompositiontothedepletedmantlethatisthe

sourceforsubaerialarcvolcanismintheMarianas.FinaNagulavasareintermediate

betweenMalaguana‐GadaoRidgelavasandsubaerialMarianaarclavasintrace

elementratiosthattracktheinputofaqueousslabfluidstothemantlewedge.The


lackofrelationshipbetweenincreasingBa/Laratiosandoxidationsuggeststhatthis

arisesbecausetheaqueousfluidpercolatingthroughthemantlesourceisdifferent

incompositionthanelsewhereintheMarianas.TheTiO2contentsofFinaNagu

lavasindicatemeltfractionssimilartothoseproducedatMarianaarcvolcanoes,

that,whencombinedwithvolumeestimatesfortheFinaNaguvolcanicedifices,

suggestmeltgenerationinthemantlewedgetakesplacewithsimilarmechanisms

toarcmeltgeneration,andthatthesmallsizeofFinaNaguvolcanicedificesarises

duetothedistributionofthesemeltsamongmanyedificesontheseafloor.A

possibleprogressionofvolcanismfromthesouthwesttothenortheastthroughtime

maybegeneratedastheresultoflithosphericextensioninthesoutheastern

Marianaforearcrift,orachangeinthepathofmeltdiapirsthroughthewedge,or

both,generatingtheunusualchainofsmallvolume,submarinearcvolcanicfeatures

inthesouthernMarianaconvergentmargin.

ACKNOWLEDGEMENTS

WewouldliketothankFrancesJennerandtwoanonymousreviewersfortheircarefulattentiontoourmanuscript.WewouldliketothankthecaptainandcrewaboardtheThomasG.ThompsonduringexpeditionTN273,andaboardtheNOAAvesselOkeanosExplorerandROVDeepDiscovererduringexpeditionEX1605L1.WethankM.Lytle,B.Covellone,J.Ribeiro,W.Lieu,andE.JordanforleadershipandassistanceduringdredgingoperationsforTN273.WethankA.Lanzirotti,W.Rao,andS.WirickforassistanceinbeamlineoperationsatNSLSBNL.AccesstoNSLSBNLwassupportedbytheUSDepartmentofEnergyundercontractDE‐AC02‐98CH10886.WeacknowledgesupportfromNSFgrantOCE‐0961811toMartinez,NSFOCE‐0961559andNSFEAR‐1258940toKelley,NSFEAR‐0841006toCottrell,andNSFOCE‐0961352toStern.NSFOCE‐1258771providescuratorialsupportformarinegeologicalsamplesattheUniversityofRhodeIsland.Supportingdataareincludedasdatatablesinasupplementaryinformationfile.Additionalinformationmaybeobtainedfromthefirstauthoratmbrounce@ucr.edu.REFERENCESCITED


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FIGURECAPTIONS


Figure1.Alocationmapforsamplesinthisstudy.[A]AmapoftheMarianamargin,

withthelocationofthesubmarinearcandsubaerialarcmarkedwithashortdashed

andwhitecircles,respectively.TheblackboxshowsthepositionofpanelB.[B]A

bathymetricmapshowingtheFinaNaguvolcanicchainindetail.White,fivepointed

starsmarkthepositionofdredgesthatrecoveredlavasincludedinthisstudy.Gray,

fourpointedstarsmarkthepositionofROVDeepExplorerdivesfromexpedition

EX1605L1oftheOkeanosExplorer.Calderasaremarkedwithdashedlinecirclesand

conesaremarkedwithsolidlinecircles.Labelssuchas“22D”markthedredge

numberfromexpeditionTN273.Thesmallblackstarmarksthepositionofsample

KH98D06[MasudaandFryer,2015].ThebasemapforpanelsAandBwascreated

usingGeoMapApp[http://www.geomapapp.org;Ryanetal.,2009].

Figure2.LineprofilestakenthroughthevolcanicedificesoftheFinaNaguVolcanic

complex,Tracysubmarineseamount,andAnatahanvolcano,extractedfromthe

basemapofRyanetal.,2009,providedbyGeoMapApp

[http://www.geomapapp.org].Thevolumesofvolcanicedificeswereestimated

usingtwo‐dimensionalbathymetryprofiles.Profileswerechosentotransectthe

centerofeachvolcanicedificeandtoextendto3500meterswaterdepth,whichis

takenastheapproximatedepthofthebaseofeachedifice.Thegeometryofeach

edificewasestimatedusingsimplegeometricalshapes,andvolumecalculations

wereperformedbyassumingradialsymmetryaboutthepeakheightofeach

volcanicedifice.


Figure3.Severalplotsofmajorelementvariationsinsamplesinthisstudy.Thegray

fieldshowsdatafromMarianaTroughlavas[Pearceetal.,2005;Stolperand

Newman,1994].Arrowsinthetoptwopanelsshowtheexpecteddirectionthat

fractionalcrystallizationofolivineandplagioclase[lefttoppanel]andmagnetite

[righttoppanel,bottomrightpanel]willcauseglasscompositionstogotowards.

Thearrowinthebottomleftpanelshowstheexpecteddirectionthatcombined

crystalfractionationandwaterdegassingwillmovemagmacompositionstowards.

Olv=olivine,plag=plagioclase,mgt=magnetite.

Figure4.[a]Aspiderdiagramshowingthetraceelementcompositionsoflavasin

thisstudy,normalizedtoNMORB.ThedarkgrayfieldshowsdatafromSarigan

volcano[Brounceetal.,2014].ThelightgrayfieldshowsdatafromtheMalaguana‐

GadaoRidge[Brounceetal.,2014].Thethickmediumgraylineshowstheaverage

traceelementcompositionoflavasfromthesoutheasternMarianaforearcrift

[SEMFR;Ribeiroetal.,2013a].Theblacklinesshowtraceelementcompositionsof

individuallavasfromtheFinaNaguvolcanicchain[thisstudy].[b]AplotofTiO2/Y

ratiosversusNa2OcontentsforFinaNaguvolcanicchainlavasandMalaguana‐

Gadaoridgelavas.Theblacklineshowsthetrajectoryofameltproducedbybath

meltinganormalMORBcompositionmantle,fromKelleyetal.,2006.Arrowsshow

thegeneraleffectsofincreasingmeltfractionandincreasingsourcedepletiononthe

compositionofmelts.


Figure5.Aplotof[A]Th/Ybratios,[B]Ba/Ybratios,and[C]U/Ybratiosversus

Nb/Ybratiosforsamplesinthisstudy.Thedarkgrayfieldshowsdataforglobal

MORB[Jenneretal.,2012].ThelightgrayfieldshowsdatafortheMarianaTrough

[Pearceetal.,2005;StolperandNewman,1994].Thewhitefieldwithhorizontal

linesshowsdatafortheMarianaarc[Brounceetal.,2014].Whitecirclesaredatafor

theMalaguanaGadaoridge[Brounceetal.,2014].Whitediamondswithgray

outlinesaredatafortheSEMFR[Ribeiroetal.,2014b].Smallgreycirclesarebulk

pillowglassdataforFinaNagusamplesdredgedduringcruiseHakuho,1998,and

describedbyHasudaandFryer,2015.Thewhiteinvertedtrianglesrepresentthree

MORBcompositions.Thewhitestarrepresentsanaveragearccomposition.The

dash,dash‐dot,andsolidblacklinesaremixinglinesbetweentheMORB

compositionsandthearccomposition.

Figure6.AplotofFe3+/∑Feratiosversus[A]MgOcontentsand[B]Ba/Laratios,as

wellasfO2(calculatedusingthealgorithmofKressandCarmichael,1991)relative

totheQFMoxygenbuffer(accordingtoFrost,1991)at1200°Cversus[C]MgO

contentsand[D]Ba/Laratios,forsamplesinthisstudy.Datafieldsandsymbolsare

asinFigure5.

Figure7.AplotofTiO2versusVcontents.SymbolsareasinFigure5.

Figure8.StillimagesfromvideofeedcapturedbyROVDeepExplorerduring

expeditionEX1605L1.VideocredittoNOAAOER,NOAAvesselOkeanosExplorer.


Figure 1.


dredge locations (this study)

146°E144°E10°N

12°N

14°N

16°N

18°N

20°N

22°N subaerial submarine

Mariana arc

Malaguan

a -

Gadao

Ridge

SEMFR

Toto caldera

METERS BELOW SEA LEVEL

5000 4000 3000 2000 1000 0

A.

B.

B.

Anatahan

Tracey seamount

Alphabetseamounts

Caldera#1

Caldera#2

Caldera#3

Caldera#4

Cone#1

Cone#2, #3 Cone

#4, #5Sarigan

Guam

KH98D06

25D

29D

24D

44D

42D

41D

22D

32D12º20’N

12º30’N

12º40’N

12º40’N

143º20’ E 143º30’ E 143º40’ E 143º50’ E 144º00’ E

Fig. 1


Figure 2.


-4000 -3000 -2000 -1000

0

Anatahan

Tracy

Cone 5: 19 km3 Cone 4: 24 km3 Caldera 4:

27 km3 Cone 3:26 km3

met

ers

belo

w s

ea le

vel

kilometers distance across surface

154 km3

463 km3

Fig. 2

Cone 223 km3

Caldera 3:33 km3

Cone 1:15 km3

Caldera 2:26 km3

Caldera 1:26 km3


Figure 3.


12

13

14

15

16

17

18

19

Malaguana-Gadao Ridge(Brounce et al., 2014)

FNVC glass (this study)

5 6 7 8 9 10 11 12 13 14

FeO

* (w

t%)

0.0

0.5

1.0

1.5

2.0

2.5

2 3 4 5 6 7 8 9 10

MgO (wt%)

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

2 3 4 5 6 7 8 9 10

Na 2O

(w

t%)

MgO (wt%)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

0

100

200

300

400

500

600

V (p

pm)

mgt

mgtolv+plag

crystallization at H2O saturation pressure

Alphabet seamounts(Stern et al., 2013)

0.2

0.3

0.4 0.5

0.6

0.7

0.8 0.9

1.0

FNVC whole rock(Matsuda and Fryer., 2015)

SEMFR glass(Ribeiro et al., 2013b)

Fig. 3


Figure 4.


0.1

1

10

100

Rb

Cs

Ba

Th

U

Ta

Nb La

Ce

Pb

Pr

Sr

Hf

Zr

Eu

Gd

Tb

Dy

Ho Y Li

Er

Tm

Yb

Lu

Sam

ple/

NM

OR

B

K 2O

TiO

2

Malaguana-Gadao ridge

FNVC (this study)

Nd

Sm

Sarigan

Southeast Mariana forearc rift

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

1.0 1.5 2.0 2.5 3.0 3.5 4.0

batch melting, NMORB

Na2O (wt %)

increasing melt fraction

increasing source

depletion


FNVC

a.

b.

SEMFR(Riberio et al., 2013b)

Fig. 4


Figure 5.


0.01

0.1

1 Th

/Yb

1

10

100

0.1 1 10

Ba/

Yb

Nb/Yb

FNVC

Marian

a arc

Depletion from melt extraction


FNVC whole rock

SEMFR

Fig. 5

MORB

Marian

a Trou

gh

addition of Ba rich subduction component

addition of Th rich subduction component

Malaguana-Gadao Ridge

U/Y

b

0.004

0.04

0.4


addition of U rich subduction component, increasing ƒO2

a.

b.

c.


Figure 6.


Ba/La6050403020100

-0.25

0.25

0.75

1.25

1.75

a. b. c.

d.

0.12

0.14

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

2 3 4 5 6 7 8 9 10 MgO (wt%)


FNVC MORB

Mariana arc Mariana Trough

Fig. 6

U/Yb0.05 0.10 0.15 0.20 0.25

e. f.


Figure 7.


TiO

2 (w

t %)

V (ppm)100 200 300 400 500 600

Fig. 7

0.0

0.5

1.0

1.5

2.0

2.5

3.0

TiO2

(ppm

)/V =

100

TiO 2 (ppm)/V

= 50

TiO2 (ppm)/V = 17

MORB

Malaguana-Gadao Ridge

Mariana arc


Figure 8.


a. Caldera #3, north wall

b. Caldera #4, east wall

c. Cone #5, dome 1

worm tubes

low T (~5ºC) fluid seep

d. Cone #5, dome 2

intact chimneys

intact flange

Fig. 8


Documents

The Fina Nagu Volcanic Complex: Unusual submarine arc