A HIMU-like component in Mariana Convergent Margin magma sources during initial arc rifting revealed by melt inclusions

Domain: earth_science
Published: 2024-05-14
Editorial outcome: accept

Abstract
Compositionsofislandarcandback-arcbasinbasaltsareoftenusedtotrace therecyclingofsubductedmaterials.However,thecontributionofsubducted componentstothemantlesourceduringinitialarcriftingbeforeback-arc basinspreadingisnotyetwellconstrained.ThenorthernmostMarianaarcis idealforstudyingthisbecausethetransitionfromriftingtoback-arcspreading ishappeninghere.HerewereportmajorandtraceelementandPbisotopic compositionsofolivine-hostedmeltinclusionsfromlavaseruptedduring initialriftingat24°N(NSP-24)andcomparethemwiththoseinactivearcfront at21°Nandmatureback-arcbasinat18°N.NSP-24high-Kmeltinclusionshave highlyradiogenicPbcompositionsandareclosetothoseoftheHIMUend- member,suggestingthepresenceofthiscomponentinthemagmasource. TheHIMU-likecomponentmaybestoredintheover-ridingplateandreleased intoarcmagmawithrifting.HIMU-typeseamountsmaybesubductedelse- wherebeneaththeMarianaarc,butobviousHIMU-typecomponentsappear onlyintheinitialstagesofarcriftingduetothelowmeltingdegreeandbeing consumedduringtheprocessofback-arcspreading.

Introduction
Subductionzonesareimportantplacesforchemicalrecyclinginthe Earth1.Oceanicislandarcs,wherecontinentalcrustisabsent,arekey locations for identifying various subduction components and recy- clingprocesses2.Arcmagmasarecharacterizedbytheenrichmentof large-ion lithophile elements (LILE) and light rare-earth elements (LREE),relative tomid-oceanridge basalts(MORB),whichismainly attributedtothedehydrationandmeltingofsubductedcomponents, includingsediments,alteredoceaniccrust(AOC)3–5andseamounts6–8. TheIzu-Bonin-Mariana(IBM)arcisatypicaloceanicislandarc,where sediment,AOC,andseamountsofthePacificPlatesubduct,makingit anidealsystemtostudysubductedcomponentinfluencesonmagma genesis4,9–12. Geochemical studies of IBM lavas show that the con- tribution of subduction components varies along the length of the arc12. In the Izu-Boninarc,hydrous fluid released frompelagic sedi- mentandAOCdominatecontributionstothemantlewedge,whilein thenorthernMarianaarc,thereisagreatercontributionofsediment melt4,10,12.ThemantlesourcebeneaththeMarianaback-arcbasingra- duallychangesfromsubduction-modifiedtoMORB-likemantlewith decreasinglatitudeandbasinwidening13–16.Inaddition,afewarcvol- canoes(IwoJimaandKitaIwoJima)inthesouthernIzu-Boninarchave lavas with highly radiogenic Pb compositions (206Pb/204Pb>19.4), reflecting the result of subducted seamounts with HIMU (high-μ or hightime-integrated238U/204Pb)characteristics10,12,17.

TheNorthNorthernSeamountProvince(N-NSP)oftheMariana arcbetween23°Nand24°NisimmediatelynorthofwheretheMariana Troughterminates(Fig.1)andisundergoingnorthward-propagating back-arcrifting,butseafloorspreadingtoformoceaniccrusthasnot yetbegun.Sternetal.18suggestedthatmagmasthateruptedinthis location justprior to initialarc rifting have different mantle source compositions than normal arc magmas. High-precision Pb isotopic studies of whole-rocks show that the compositions of the mantle source beneath this area are complex, involving subducted pelagic sediment, AOC and HIMU-like Cretaceous seamounts, and volcaniclastics10,19. Compared to whole-rocks, olivine-hosted melt inclusions are less affected by later magmatic processes such as magma mixing and fractional crystallization, because they are pro- tected by their host minerals, thus retaining more complete compositional information of the primitive magma and the mantle source20–23.Inaddition,meltinclusionsalsorecordinformationabout magmatic evolution prior to eruption20, which can help us better understandhowsubductioncomponentsmigratethroughthemantle wedge.Therefore,intandemwithstudyingmagmaticproductsduring arcrifting,identifyingthecompositionofmantlesourcesbeforerifting begins allows us to better understand lithosphericprocesses atthe initiation of rifting, and sheds light on the various subduction components19.Inthispaper,wereportmajorandtraceelementsand Pbisotopicanalysesofolivine-hostedmeltinclusionsfrombasaltsat three sites in the northern Mariana arc. These include samples col- lectedfromtheeasternscarpoftheremnantarcWestMarianaRidge (WMR)whichistheconjugatepairofthenorthernmostNorthMariana SeamountProvincearound24°N(NSP-24;Fig.1),theMarianaarcat 21°N(NSP-21)andtheback-arcbasinat18°N(MT-18).Weusethese datatoidentifytraceelementandPbisotopiccompositionsofmantle sources,compareandanalyzetheirspatialandtemporalvariabilitiesat differenttectonicevolutionarystages,andilluminatethecausescon- trollingthesevariations.

TheIBMconvergentmarginmarkstheeasternedgeofthePhi- lippineSeaPlateandformedinresponsetothewestwardsubduction ofthePacificPlate(Fig.1).TheMarianaArcandMarianaTroughback- arc basin are different but related magma-tectonic systems. The magmaticarcissubdividedintofoursegments,fromnorthtosouth: the southern Volcano Arc (VA, 24–28°N), the Northern Seamount Province (NSP, 20.7–24°N), the Central Islands Province (CIP, 16–20.7°N), and the Southern Seamount Province (SSP, south of 16°N)11,24.TheMarianaTroughissubdividedintotheNorthernMariana Trough (NMT, 21–24°N), the Central Mariana Trough (CMT, 17.6–21°N),andtheSouthernMarianaTrough(SMT,12.5–17.5°N)25,26. The Mariana Trough south of 19°15′N is dominated by seafloor spreading which is propagating northward into the NMT, which is characterizedbyrifting14,27,28.WefocusourstudyontheCMTandNSP (Fig. 1), which have different mantle sources affected by different proportionsandtypesofsubductioncomponents.TheCMTincludesa matureback-arcspreadingridgewithspreadingratesrangingfrom3.0 to4.4cm/a29,30;volcanismoccursnearthemainspreadingaxis27,31.The CMThasamaximumwidthof~250kmat18°Nandback-arcspreading startedatabout6Ma32,33.CMTcrustalstructureissimilartothateofa slow-spreadingmid-oceanicridge;seismicprofilesshowthatthiscrust isabout6kmthick11.CMTlavashavegeochemicalandisotopicchar- acteristics of MORB with minor involvement of subduction components14,28,34–37.ThedepthtothesubductedslabbeneathMariana arcvolcanoesalongthemagmaticfrontis~130km38.TheNSPcanbe furthersubdividedintonorthNSP(N-NSP,23–24°N)andsouthNSP(S- NSP,20.7–23°N)reflectingdifferencesinenrichedcomponentsinthe magma source25,26,39,40 (Fig. 1). The N-NSP is where the propagating MarianaTroughriftintersectstheMarianaarc,andtheWMR(remnant arc) terminates here19,41. The NMT extensional zone terminates at Nikkovolcano(23.1°N)14.ThenorthwardterminationoftheMariana Trough mayalso have been caused by the ongoing collision of the OgasawaraPlateauwiththeIBMarcat26°N42(Fig.1).

N-NSP and southern VA arc lavas are shoshonitic with distinct enrichmentsinincompatiblemajorandtraceelementsandradiogenic isotopes;thisarcsegmentisalsocalledtheAlkalicVolcanoProvince (AVP)26.Incontrast,S-NSPlavasarenotasenrichedinincompatible trace elements as those of the N-NSP10,26,40,43. This study focuses on samples from around 18°N in the CMT, 21°N in the S-NSP, and 24°N in the WMR near the N-NSP (hereafter MT-18, NSP-21, and NSP-24, respectively) (Fig. 1). These correspond to magmatism associated with back-arc spreading, active arc, and arc rifting, respectively14.Allstudiedsamplesarebasalts,witholivine,clinopyr- oxene,andplagioclaseasthemainphenocrysts(SupplementaryFig.1 andSupplementaryData1,2).Olivinephenocrystsinallsamplesare euhedral to subhedral and show no obvious compositional zonation (Supplementary Fig. 1). Round orellipticalmeltinclusions 20–60 μm across are randomly distributed in some olivine pheno- crysts(SupplementaryFig.1),indicatingaprimaryorigin44.Composi- tions of all studied samples plot within the range of the previous whole-rockdata(SupplementaryFigs.2–4),suggestingthattheyare representative.

Results
Chemicalcompositionsofhostolivinesandmeltinclusions
The chemicalcompositions ofhost olivines and melt inclusions are presented in Supplementary Data 3. Olivine compositions vary sig- nificantly in different sampling areas, with Forsterite contents [Fo, atomic100×Mg/(Mg+Fe)]decreasingfromNSP-24(83.8–90.7)toMT- 18(76.6–87.8)toNSP-21(70.8–78.5).Almostallhostolivineshavehigh CaO contents (average = 0.18 wt.%, n=300), indicating that they crystallizedfrommagma,ratherthanbeingmantle-derivedxenocrysts (CaO<0.1wt.%45).

Meltinclusioncompositionsmaybemodifiedbycrystallization and/orre-equilibrationafterbeingentrappedbytheirhostminerals, i.e.,post-entrapmentcrystallization(PEC)20,46,47.Themeasuredmajor elementcompositionsofmeltinclusionswerecorrectedforPECFe- loss using the method described by ref. 46 and recalculated with PETROLOG3software47.Olivine-meltequilibriumwassetbyapplying themodelofref.48.TheinitialFeOtcontentofthetrappedmeltcanbe obtained by using the FeOt fractionation trend of whole-rock samples20,46,whichwereassumedtobe8.0wt.%inMT-18,9.0wt.% inNSP-21and9.0wt.%inNSP-2426,49.Thesecorrectionsmainlyaffect MgOandFeOconcentrationsbuthavelittleeffectonotherelements, thustheoverallgeochemicaltrendsandcompositionchangesremain unaffected.Wefocusontheratiosofincompatibletraceelementsthat arenotsignificantlyaffectedbyPECFe-loss.

CompositionsofmeltinclusionsaftercorrectingforPECFe-loss arelistedinSupplementaryData3anddisplayedonthetotalalkali- silica(TAS)diagram.OntheTASdiagram,meltinclusionsshowlarger compositionalvariationthantheirhostwhole-rocks(Supplementary Fig. 2). Most MT-18 and NSP-21 melt inclusions have compositions similartothoseoftheirhostwhole-rocks,whereassomeNSP-24melt inclusioncompositionsaremoreprimitive(SupplementaryFig.2).MT- 18meltinclusionsaresubalkaline,low-Ktholeiites.SomeNSP-21melt inclusionshavehighenoughalkalicontentstobeclassifiedasalkaline series; these are the first alkaline compositions reported from the S-NSP.Accordingtotheiralkalicontents,weidentifiedtwogroupsof bothNSP-21andNSP-24meltinclusions.NSP-21andNSP-24group1 melt inclusions plot inthe medium-K calc-alkaline series field (Sup- plementaryFig.2b).NSP-24group2meltinclusionsplotinthehigh-K calc-alkalineseriesfield,whereasNSP-21group2meltinclusionsplotin high-Kcalc-alkalinetoshoshoniticfields(SupplementaryFig.2b).The majoroxidesversusMgOcontentsofdifferentgroupmeltinclusions donotcovarysystematically(SupplementaryFig.3),indicatingthat fractionalcrystallizationwasnottheonlycauseofmagmaticvariability butthatdifferentmeltsourceswerealsoimportant.

In primitive mantle-normalized trace element “spider”diagrams, MT-18 melt inclusions are similar to E-MORB, except for slightly depletedNbandTaandenrichedPbcontents(Fig.2a).Incontrast, NSP-21andNSP-24meltinclusionsaremarkedlyenrichedinLILEsand REEsanddepletedinhigh-field-strengthelements(HFSEs),exhibiting arc-liketraceelementdistributionpatterns(Fig.2c,e).NSP-21group1 meltinclusionsarechemicallysimilartotheirhostwhole-rocks(Fig.2 and Supplementary Fig. 5), whereas group 2 melt inclusions are stronglyenrichedinLi,Rb,Ba,andKbutdepletedinCu,Zn,andPb (Fig.2andSupplementaryFig.4).NSP-24group1meltinclusionsalso havesimilartraceelementdistributionpatternstotheirhostwhole- rocksandotherAVPbasalticlavas(e.g.,NorthHiyosh),butincompa- tibletraceelementcontents(e.g.Rb,K,P)ofgroup2meltinclusions arehigher(Fig.2andSupplementaryFig.4,5).Chondrite-normalized REEpatternsofMT-18meltinclusionsaresimilartothoseofE-MORB, showinglittlefractionationofLREEsandHREEs(Fig.2b,d,f).Incon- trast,NSP-21andNSP-24meltinclusionsshowsimilarstrongfractio- nationofLREEsfromHREEs(Fig.2d,f).NSP-24group2meltinclusions have higher LREE contents than NSP-24 group 1 melt inclu- sions(Fig.2f).

Pbisotopecompositionsofmeltinclusions
The Pb isotopic compositions of olivine-hosted melt inclusions are listedinSupplementaryData3.Eachofthefivetypesofmeltinclusions defined above hasa narrow range of Pb isotope compositions.The total range that encompasses most of the global variation seen in oceanicbasaltsandthatisfarbeyondthehostwhole-rocks(Fig.3a). ThereisnosignificantrelationshipbetweenPbisotopevariationsof differentgroup meltinclusions and theextentof magmatic fractio- nation as captured by MgO contents or by the Fo content of host olivines(SupplementaryFig.6).

The 207Pb/206Pb and 208Pb/206Pb values of MT-18 melt inclusions rangefrom0.8344to0.8612and2.0607to2.1145,withmeanvaluesof 0.8467±0.0117 (2SD, n=34) and 2.0809±0.0268 (2SD, n=34), respectively.ThesevaluesallplotwithintheIndianOcean-typemantle field (Fig. 3a). Actually, the radiogenic isotope composition of the mantlewedgeisverycommonlyobservedtobesimilartoIndianOcean- type mantle for almost all Western Pacific Subduction zones (e.g., Kamchatka, Kuriles, IBM, Ryukyu)50–54. The 207Pb/206Pb and 208Pb/206Pb ratiosofNSP-21meltinclusionsrangefrom0.8223to0.8780and2.0380 to2.1503.NSP-21group2meltinclusionshavemoreolduranogenicand more thorogenic Pb (average 207Pb/206Pb=0.8653±0.0215, 208Pb/206Pb=2.1243±0.0494,2SD,n=11)thangroup1meltinclusions (average 207Pb/206Pb=0.8253±0.0041, 208Pb/206Pb=2.0430±0.0069, 2SD,n=31)andtendtowardstheenrichedmantletypeI(EMI)field (Fig.3a).ThetwogroupsofNSP-24meltinclusionsalsohaveverydif- ferentPbisotopiccompositions.Group1meltinclusionshavearangeof 207Pb/206Pband208Pb/206Pbratiosfrom0.8202to0.8300and2.0343to 2.0508 with mean values of 0.8246±0.0054 (2SD, n=24) and 2.0433±0.0096(2SD,n=24),respectively.Thesevaluesfallwithinthe rangeofpreviouslystudiedNSP-24whole-rocks(Fig.3a).NSP-24group1 meltinclusionPbisotopesareremarkablysimilartoNSP-21group1melt inclusions.The207Pb/206Pband208Pb/206PbratiosofNSP-24group2melt inclusionsarelowerthanthoseofNSP-24group1meltinclusions,which rangefrom0.7979to0.8078and2.0062to2.0253,withmeanvaluesof 0.8028±0.0043 (2SD, n=33) and 2.0119±0.0087 (2SD, n=33), respectively. Pb isotopic compositions of NSP-24 group 2 melt inclu- sionsfalloutsidethePacific-andIndianOcean-typemantlerangeand trendtowardstheHIMUend-member(Fig.3a).

Influenceoffractionalcrystallizationandpartialmeltingon meltinclusionelementalvariations
Trace elementvariationsbetweenthedifferentgroupsofmeltinclu- sionsinNSP-21andNSP-24mightresultfromthefractionalcrystal- lizationofmineralsandpartialmeltingofthemantlesource.Although theMgOcontentsofdifferentgroupmeltinclusionsvary,theydonot differsignificantlyinthemajormineralslikelytobeinvolvedinfrac- tional crystallization (such as olivine, pyroxene, and plagioclase) (SupplementaryFig.3).Norcanfractionalcrystallizationexplainthe large and systematic variations in Pb isotopic compositions seen betweengroup1,NSP-21group2andNSP-24group2meltinclusions.

Partialmeltingmodelcalculationsattributedgeochemicalvaria- tionsalongtheMarianamagmaticfronttovaryingdegreesofmelting ofahomogeneousmantle43.GeochemicalstudiesofNMTlavasalso show that the degree of mantle partial melting gradually increases northward as the rift propogated in that direction14. In contrast, Bloomeretal.26arguedthatmeltsoftheMarianaarcmagmaticfront were produced by similardegrees of mantle partialmelting. In this study,weestimatethepartialmeltingdegreeofdifferentgroupsof meltinclusionsfromratiosofincompatibletraceelements(Supple- mentary Fig. 7). Although the results may vary as the simulation parameters change, the overall modeling trend shows that Mariana back-arcbasinbasaltismainlyderivedfrompartialmeltingofspinel- bearing mantle peridotite, which is consistent with the results of ref.43.Incontrast,theMarianaarcmagmasourcecontainsresidual garnet,andtheroleofgarnetincreasesnorthward(i.e.,NSP-24>NSP- 21)(SupplementaryFig.7).DifferentgroupsofNSP-21meltinclusions showsimilardegreesofpartialmelting,whereasNSP-24high-Kmelt inclusions appear to represent lower degrees of melting than do medium-Kmeltinclusions(SupplementaryFig.7).However,thedif- ferenceinmeltingdegreeisnotenoughtocontrolobservedincom- patibleelementvariations(suchasK,Li,Rb,andBa).Accordingly,the distinctive element compositions of these different groups of melt inclusionsmayreflectsourcesaffectedbyvariablesubductioncom- ponentsratherthandifferentdegreesofpartialmeltingofahomo- geneous mantle source. This conclusion is also consistent with Pb isotopicvariations.

SubductioncomponentsinMT-18,NSP-21,andNSP-24mantle sources
The source of mostarc and back-arc magmasis the mantlewedge, which is affected by the addition of subduction components and exhibitsenrichmentofLILEsrelativetoHFSEsandREEs1.Onthebasis of trace element data, Peate and Pearce40 proposed that the NSP subductioncomponentis dominated bysilicatemeltsderivedfrom subducted pelagic sediments and AOC. NSP-21 and NSP-24 melt inclusions are enriched in LILEs and depleted in HFSEs, including negativeNb,Ta,andTianomalies(Fig.2).Thesecanbeexplainedby residualorfractionatingmineralscontainingtheseelementsorbya depleted mantle wedge. Some elements, owing to their differential compatibilityinmagma,canbeusedtofurtherconstrainthetypesof subductioncomponents.We useincompatible trace elementratios, whichchangelittleduringfractionalcrystallizationandpartialmelting orduringpost-entrapmentcrystallization,toconstrainthecontribu- tionsofsubductioncomponentstothemantlesource.Forexample, ThislesssolublethanREEsandHFSEsinslab-derivedfluids,whereas Baisstronglypartitionedintofluids55,56.Inaddition,ThandBaabun- dances in subducted sediments are much higher than their abun- dancesinthemantle5,57.

AsisillustratedinFig.4,NSP-21andNSP-24meltinclusionshave highly variable Th/Yb and Ba/Yb ratios, suggesting that subducted sediment contributions are more variable than those of subducted AOC(Fig.4).AlthoughMT-18meltinclusionshaveE-MORB-liketrace elementdistributionpatterns,theystillshowsomeinfluenceofsub- duction components, such as negative Nb, Ta and positive Pb anomalies(Fig.2),slightlyelevatedTh/YbandBa/Ybratios(Fig.4)and highBa/Nbratios(>7;SupplementaryData3)asdiscussedbyref.58.In addition,thePbisotopiccompositionsofMT-18meltinclusionscovers almostthefullrangeof16-22°NMarianaTroughbasaltcompositions (Fig.3a).Thissuggeststhatawiderangeofsubductioncomponents and small-scale mantle heterogeneity interact beneath the Mariana Trough, which also explains the large variation of whole-rock geo- chemicalcompositionsinthestudyarea(SupplementaryFigs.2–4). Nevertheless,subductioncomponentshavelessinfluenceontheMT- 18magmasourcecomparedtotheNSPmagmasource(Figs.2,4).

NSP-21K-richmeltinclusionshaveespeciallyhighalkalicontents andcanbeclassifiedasalkalineseries(SupplementaryFig.2a),similar to the high-K melts of Kasuga 2 and 3 seamounts from the Kasuga Cross-ChaininthenorthernMarianaback-arcbasin2,49,suggestingthat ahigh-Kmagmaticcomponentispresentbeneaththevolcanicfront, aswellasbehinditintheKasugaCross-Chain(Fig.1).Inaddition,NSP- 21K-richmeltinclusionshavehigher208Pb/206Pband207Pb/206Pbratios andLi,Rb,andBacontents,butlowerCu,Zn,andPbcontents,thando coexistingNSP-21medium-Kmeltinclusions(Figs.2,3,andSupple- mentaryFig.4).Slab-derivedfluidswithhighLi,Rb,andBacontents canmetasomatizethesubarcmantleandcontributetheseelementsto arcmagmas.Potassiumabundancesinsomearclavasareconsistently high59,60,suggestingthatalargeamountofpotassiumisretainedin subductedsedimentsbymica(biotiteand/orphengite)toconsider- able depth, where it may be released by partial melting61,62. When sulfides(e.g.,chalcopyrite,sphalerite,and galena) crystallize and/or separatefrommagma,Cu,Zn,andPbpartitionintothem63,resultingin residualmagmasthatarehighlydepletedinchalcophileelements64,65. ItshouldbenotedthatNSP-21high-KmeltinclusionshavelowMgO content(<3.07-3.58wt.%)andaslightcorrelationbetweenPbisotopes andMgO(SupplementaryFigs.3,6),indicatingthatthesemeltswere products of fractional crystallization. Therefore, it is reasonable to speculatethatfractionalcrystallizationand/orsulphideimmiscibility control the element anomalies coupled with subducted K-rich slab componentsthatalsoaffectthePbisotopiccompositionsoftheNSP- 21K-richmeltinclusions.

Exceptforafewelements(e.g.,Rb,K,P),thetwogroupsofNSP-24 meltinclusionshavesimilartraceelementdistributionpatterns,while their Pb isotopes differ significantly (Figs. 2, 3a). Medium-K melt inclusionsinNSP-24andNSP-21haveverysimilartraceelementcon- tents and Pb isotopic compositions, which are consistent with NSP whole-rockcompositions(Figs.2,3a).Incontrast,NSP-24high-Kmelt inclusionsshowunusuallylow207Pb/206Pband208Pb/206Pbratios(note that the lowest 207Pb/206Pb and 208Pb/206Pb ratios correspond to the mostradiogenicPb),whichisnotreflectedinNSPbasalticwhole-rock compositions but is close to andesitic-dacitic lavas from Iwo Jima (24°45′N)andKitaIwoJima(25°30′N)inthenearbyVA10,12,17(Figs.1, 3a).However,NSP-24high-Kmeltinclusionsareallbasalticandhave differenttraceelementsdistributionpatternsthanIwoJimaandKita IwoJimalavas(SupplementaryFigs.2,5),indicatingdifferentmantle sourcecomponentsand/ormagmaticevolutionprocesses.Thissug- geststhatmedium-Kmeltinclusionsrepresentpartialmeltsofambient NSP mantle, while the high-K melt inclusions come from different mantlesourceregions.Figure3ashowsthattheselow207Pb/206Pband 208Pb/206PbratioscannotbeexplainedbymixingofdepletedMORB mantle(DMM),subductionsediment,andAOC,butratherpointtoa HIMU end-member component. This is a key observation from our meltinclusiondata.

AHIMU-likecomponentintheNSPmagmasource
Asoneofthemantleend-members,theHIMUreservoirisdefinedby the composition of lavas from Mangaia and Tubuaii islands in the south-westPacificOceanandSt.HelenainthesouthAtlanticOcean, whichhaveradiogenicPbratios(206Pb/204Pb>20.5)66,67.Todetermine howtheHIMUcomponentwasaddedtothemagmasourceofNSP-24 high-Kmeltinclusions,wecarriedoutasimulationcalculationusinga bulkcomponentsmixingmodel68,69.Theresultsshowthattheinjection oflessthan1.5%HIMUcomponentintoacompositeof99%depleted mantleand1%mixtureofsubductedsedimentandAOC(70:30)can explainthePbisotopiccompositionsofNSP-24high-Kmeltinclusions (Fig.3a).Tofurthertestthepossiblepetrogeneticlinkbetweenthe HIMUmantlecomponentandtheNSPmagmasource,wecalculated traceelementdistributionpatterns(Fig.3b).UsingresultsfromthePb isotopesimulation(Fig.3a),thecalculatedmelttraceelementpatterns showthatabout1–3%partialmeltingofthiscompositeproducestrace element patterns that arestrikinglysimilarto those of NSP-24 melt inclusions(Fig.3b).

TheformationoftheHIMUend-memberwithahigh206Pb/204Pb ratio requires long-term isolation(~ 2 to 3 Ga),and time-integrated enrichmentofThandUrelativetoPb70,71.TworegionsintheEarth wherethiscanhappenarethemantletransitionzone(MTZ)72,73andthe baseofthelowermantlenearthecore-mantleboundary74,75.Geophy- sicalevidencedoesnotrevealanydeep-rootedmantleplumesbeneath theMarianaarc,whichmeansthatadirectcontributionofHIMUfrom thedeepmantletotheNSPmagmaticsourceisunrealistic.Incontrast, therearemanyoceanislandbasalt(OIB)-typeseamountswithHIMU- likePbisotopiccharacteristicsoutboardoftheMarianaTrench(e.g., Wake and Magellan seamount chains; Figs. 1, 3)76–79 which are sub- ducted and provide more likely sources of the HIMU component (Fig.3a).Therefore,weproposethatthesubductionoftheseHIMU seamountsisresponsibleforgeneratingNSPenrichedmagmas(Fig.5).

ImplicationsforsubductedHIMUcomponentrecyclinginthe Marianasubductionzone
For the IBM system, there are abundant seamounts in the western PacificoutboardoftheMarianaarc(Fig.1),manyofwhichhaveHIMU characteristics and are subducting with the Pacific Plate76,80,81. In addition, the HIMU-type seamount subduction signal has also been recognizedinthecentralMarianaforearcmetabasalts(16–18°N)8,82.Pb isotopiccompositionsofNSP-21andNSP-24medium-Kmeltinclusions areclose tothoseofthe FOZOend-member(Fig.3a).FOZO(FOcal Zone83)isusuallyinterpretedtohaveformedinamannersimilarto that of HIMU, but its overall formation age is younger thanthat of HIMU79,84.AsshowninFig.3a,theadditionofasmallamountofHIMU components(~0.5%)tothemantlesourcecanexplainthePbisotopic variationofNSP-24andNSP-21medium-Kmeltinclusionsandmost CIP,NSP,andAVPwhole-rockcompositions.Therefore,wespeculate thataslightHIMU-likecomponentexistswidelyintheMarianasub- ductionsystem,aspreviouslysuggested10,19,85.

In contrastto arc magmassuch as NSP-21 melt, obvious HIMU characteristicsonlyappearinNSP-24meltinclusionsassociatedwith the earliest stage of rifting (Fig. 1). This suggests that the HIMU componentisonlymobilizedintoarcmagmabyrifting.HIMUcom- ponentsfromsubductedseamountsmaymigrateandbestoredinthe lithosphericmantle.Riftingthins,heatsanddecompressesthelitho- sphere,remobilizingsomecomponentsinit,includingriftingHIMU- bearingcomponents(Fig.5a).Thisinterpretationisconsistentwiththe highNb/TaratiosinNSP25andtheanomalouslow-velocityzoneinthe uppermantlebeneaththeregionofriftpropagation86.Asriftingpro- gresses, asthenospheric magma sources dominate over lithospheric sources, moreMORB-type components areinvolved with the HIMU component gradually becoming less important. Once spreading begins, the HIMU component is swamped by upwelling astheno- spheric MORB-type magma and the HIMU signature disappears (Fig.5b).TheelementandisotopecompositionsofMT-18meltinclu- sionsshowMORB-likecharacteristics(Figs.2,3).Thismaybebecause the back-arc basin spreading center is located far away from the trench,andismainlyfedbyalargeamountofMORB-likemagma.Ifso, then no lithospheric HIMU component is present, or it has been diluted.Ourmeltinclusionstudydemonstratesthatearlyriftmagma retainsmorecompleteinformationaboutsubductedcomponentsand that a significant HIMU-like component in the Mariana subduction systemmaybemorewidespreadthanpreviouslythought,atleastto thenorthof24°N,whichmaybethekeytoproducingarcpotassic/ shoshoniticmagmas.

Methods
Selectionandpreparationofmeltinclusionswerecarriedoutatthe GuangzhouInstituteofGeochemistry,ChineseAcademyofSciences. Melt inclusions were re-homogenized and quenched using a high- temperature furnace following procedures in ref. 87. A platinum packetcontainingolivinecrystalswasheatedat1250°Cfor10minto meltthecrystal-glassmixture.Then,thepacketwasquicklyraisedto the top of the furnace tube to achieve quenching. Melt inclusion- bearing olivines were selected, mounted on epoxy resin disks, and polisheduntilthemeltinclusionswereexposedforanalysis.Themajor and trace element analysis of host whole-rocks are presented in theSupplementaryInformation.

Olivineandmeltinclusionmajorelementanalysis
Major elemental compositions of melt inclusions and host olivine phenocrysts were analyzed with a JEOL JXA-8230 electron probe microanalyzer (EPMA) at the Key Laboratory of Submarine Geos- ciences and Prospecting Techniques, Ministry of Education, Ocean University of China, Qingdao. The operating conditions were 15kV acceleratingvoltage,20nAbeamcurrent,and2μmbeamdiameterfor meltinclusionsandhostolivines.NaandKweremeasuredfirstwith 10sforthepeakand5sforthebackgroundtominimizevolatilization loss.Analyticalprecisionsformajorandtraceelementswerebetter than1%and5%,respectively.

Meltinclusiontraceelementanalysis
After EPMA analysis of melt inclusions, their trace elements were measuredusinginductivelycoupledplasmamassspectrometry(ICP- MS)equippedwitha193-nm(ArF)laserablationsystemintheInstitute ofOceanology,ChineseAcademyofSciences.Thecarboncoatingwas completelyremovedfromeachepoxyresindiskbeforelaserablation. Laserconditionsweresetto25μmbeamsize,~4Jcm−2energydensity, and5Hzrepetitionrate.Eachanalysisconsistedofa20sgasblank withthelaseroffanda20ssampleablationsignalwiththelaseron. Every eight sample analyses were followed by an analysis of the externalreferenceglassesofBCR-2G,BHVO-2G,andBIR-1Gtocorrect forsensitivitydriftandmassdiscrimination.SideterminedbyEPMA wasusedastheinternalstandardelementforcorrection.Allanalyzed datawereprocessedusingtheofflinesoftwareICPMSDataCal11.888. Resultsshowthatthemeasuredvaluesofreferencestandardsarein good agreement with the recommended values (Supplementary Data4).Theaccuracyandprecisionofmosttraceelementanalyses werebetterthan10%and20%,respectively(SupplementaryData4).

MeltinclusionPbisotopeanalysis
AfterEPMAandLA-ICP-MSmeasurements,meltinclusionswithdia- meters >40 μm were chosen for in situ Pb isotope analysis using a Neptune Plus multiple collector inductively coupled plasma mass spectrometer (MC-ICP-MS, Thermo Fisher Scientific, Bremen, Ger- many)coupledtoa193nm(ArF)COMPexPro102laserablationsystem in the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, Guilin, China. Priorto thisprocedure,eachdiskwaswashedwithpurified0.1%nitricacidand ultrasonically cleaned. Detailed procedures for Pb isotope analysis followedthoseofref.89.Thelaserparametersweresetwitharepe- tition rate of 6Hz, laser beam diameter of 32 μm, and energy of 5Jcm−2.Heliumwasthecarriergas(650mlmin−1).Beforesampleswere lasered,30swereusedtodetectthegasblank,andthen30swere used to laser ablate the samples and collectthe ablation signals. In ordertoimproveinstrumentalsensitivity,anX-skimmerconeandaJet sample cone were used. All isotopic signalswere detected with ion countersunderstaticmode.TheNKT-1Ginternationalbasalticglass standard90 was chosen as the external standard. Basaltic glass stan- dards NIST 614 and BHVO-2G were analyzed before and after each batchofeightmeltinclusionanalysestomonitorinstrumentaldrift. Here,wepresenttheratiosof20xPb/206Pb(x=208,207)and20yPb/204Pb (y=208,207,206)ofthemeltinclusions.Dataprocessingfollowed ref. 91. Instrumental mass bias was corrected using the standard–sample–standard bracketing method and any outliers (>2SD) were excluded. The Pb isotope measurement of every unknownsamplewascarriedoutbeforeandafterthemeasurementof standard glass for mass bias correction. Analyses of NIST 614 and BHVO-2G during this study yielded averaged 20yPb/204Pb and 20xPb/206Pb consistent with the recommended values. For most ana- lyses,precisionswerebetterthan0.39%for20xPb/206Pband1.22%for 20yPb/204Pb(2RSD,n=20),whereasaccuracieswerebetterthan0.25% for20xPb/206Pband0.28%for20yPb/204Pb(SupplementaryData5).Due to the small melt inclusion lasered size (32μm) and low relative abundance of 204Pb, precisions of 20yPb/204Pb are lower than the 20xPb/206Pb(SupplementaryData5).Therefore,weemphasizeratiosof 20xPb/206Pbformeltinclusionsinthisstudy.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "This is an interesting and well-written paper with a LOT of new major and trace element and Pb isotope data for olivine-hosted melt inclusions (OHMI) and their host rocks from three locations in the Mariana arc and backarc. The new data extend the geographic range of previously known things, and include something altogether new: 12 weird andesitic OHMI (7% of those analyzed) that have unusually low Pb contents and EM1-like Pb isotope ratios in a basalt from a seamount in the northern volcanic front of the arc. The data seem reliable and warrant publication, and much of the interpretation is reasonable, but several fundamental topics need attention during revision.\nFirst, the paper presents itself as a study of geochemical change during the transition from rifting to spreading in a backarc basin. However, this study cannot separate the along-strike spatial variations in the Mariana system (i.e., what is being subducted and the depth of the slab beneath eruption sites) from the effects of the rifting-to-spreading process. Second, the OHMI are less novel than claimed. Third, the paper over-states the importance, and over-interprets the origin, of the weird OHMI at NSP21. I will elaborate on those three points below.\nAn alternative emphasis of the paper, which is there already but under-played, might be how using OHMI can discover extreme melt mixing components that are diluted in erupted magmas in three different tectonic settings in the Mariana system. The second half of the paper is mostly about this topic; it’s only the title and introduction that lead readers (at least this reviewer) to expect the paper to be about how and why magma changes from backarc rifting to spreading. Revision could be mostly about organization and emphasis.\nThe first fundamental topic is how to separate the spatial effect from the stage of backarc evolution. Latitudinal variations in the Marianas have been known for many decades and attributed by some of the co-authors of this paper, and others, to subduction of enriched seamount chains in the vicinity of Io-To (aka Iwo-Jima). That enriched signal decreases southward along both the volcanic front and the Mariana Trough (i.e., from 25° to 21° and beyond) and northward to 26°. The authors need to explain why their observations cannot be explained just by differences in what is subducted at those latitudes, or how much is added to the mantle at various distances behind the volcanic front or above the slab. That is, the slab component may have always differed in kind or amount or ubiquity between at 24°, 21° and 18°N. The rift-to-spread argument also begs the question of when and how a HIMU-type component got as far south as 18°N beneath the MT where there is no slab currently. Did appropriate seamounts get subducted there too when the slab dip was shallower? Or did the slab-contaminated mantle flow south as the arc unzipped? Or was the HIMU character inherited from the Oki-Daito plume under the Philippine Sea Plate (Ishizuka et al., 2022) and is unrelated to subduction? Answering this may require careful comparison of the enriched components in the Mariana Trough and arc in light of recent data.\nThe NSP24 host rocks and their OHMI are very similar to the Hiyoshis with the principal difference being their lower Ba/La, Pb/Ce, and Sr/Nd ratios despite higher La/Yb. Those differences may reflect their reararc setting, such as in the Kasuga and Guguan cross-chain (Stern et al., 1993, 2006). All three samples are from the eastern scarp of the remnant arc, ~40 km behind the volcanic front. They are 2.5-3.3 Ma and may pre-date rifting. At best, they are from the earliest stage of extension. I agree with the authors that their mantle source probably underlay the arc just before, or as, it started to rift, but these are samples of the reararc 3 m.y. ago, not the rift now.\nIn light of this, I suggest that the paper be re-structured to focus more on the differences in slab components and less on the rift-to-spread process. If the authors retain an emphasis on the latter, then they must address other places where that process can be, and has been, studied more thoroughly than here. These are primarily in the southern hemisphere including the Manus, North Fiji, and Lau Basins, and Kermadec Trough. Spreading at the East Lau and Futuna Spreading Centers in the Lau Basin reaches as close to the volcanic front as the WMR is to Hiyoshi. If anything, the nature of the slab component and its effects on mantle melting during initial rifting can be studied there even better than in this paper (e.g., Bezos et al., 2009; Escrig et al., 2012). The rifting stage is known in the greatest detail in the Havre Trough (e.g., Gill et al., 2021). The current paper is provincial in this respect.\nSecond, the OHMI are less novel than claimed. The title proclaims that a HIMU-like has been revealed by their melt inclusions, but it was discovered almost 60 years ago by Tatsumoto at Io-To, then called Iwo Jima, in one of the first papers about Pb isotopes in volcanic rocks! The Pb isotopes in the Group 2 NSP24 OHMI are similar to those in Io-To (Tatsumoto, 1966; Sun and Stern, 2001) and Kita-Iwojima (Ishizuka et al., 2010) even though the major and trace elements differ. The opposite is true for the Group 2 NSP21 OHMI that are similar to the high-K basalts of the nearby Kasuga 2 and 3 seamounts in everything except their weird Pb isotopes and low chalcophile element contents.\nThird, the paper over-states the importance, and over-interprets the origin, of the weird Group 2 OHMI at NSP21 that are a new discovery in this paper. By “weird” I mean their low chalcophile element contents and their Pb isotope ratios. The host rock (T1-3) is similar to the high-K basalts at the nearby Kasuga 2 and 3 seamounts (Fryer et al, 1997). The OHMI are similar in MgO and some other oxides to the dacites from Kasuga 3. It seems like special pleading to infer an entirely different slab source for these 3 wt% MgO OHMI in Fo70 olivine when so much is similar to nearby rocks. Why not appeal to something that just affects chalcophile elements during differentiation? For example, sulfides are known to fractionate Pb isotopes such that coexisting pyrite-sphalerites can have at least the range of Pb isotopes as in these OHMI (Yuan et al., 2015: Science China: Earth Sciences). After all, the Sr-Nd-Pb-Mo isotopes all vary considerably as a function of MgO at this location (Li et al., 2021), so it is an open system. Adding an exotic hydrothermal fluid to differentiated, Pb-poor melt might be an easier explanation than the 80 lines of speculation that start at l.331.\nMoreover, whatever their origin, they are a tiny fraction of the OHMI that were studied, and they can be only a small mass fraction of magma at the volcanic front including their host rock. Try mass-balancing the OHMI to equal the host rock. The claim that this component “dominates the Mariana subarc mantle” (l. 542) is a vast over-statement. These are an intriguing discovery on the flanks of one seamount, but there is no evidence that they are a common component of Mariana volcanic front lavas.\nAn alternative emphasis for the paper might be how OHMI preserve slab component end-members at each site that are diluted in whole rocks (i.e., the subduction components section in l.289ff). To me, their most striking discovery is one that they do not discuss even though it is the best evidence for their claim that a wide range of slab components, including volcaniclastic as well as pelagic sediments, might be present beneath the Mariana Trough as far south as 18°N. The Pb in their OHMI from 18°N covers the full range of Pb isotopes of the entire Mariana Trough from 16-23°N, extending from the ambient mantle of Woodhead et al. (2005) to their most enriched MT basalts. There is even a negative correlation between 208Pb/206Pb ratios and Ba and Th-enrichment in both the OHMI and whole rocks. For NSP24, the OHMI data mean that they found the extreme HIMU component as far south of Io-To as Ishizuka et al. (2007) found it to the north at Kita-Iwojima, and that it occurs even in the reararc where Pb/Ce ratios are lower, K2O is higher, and the slab component is melt-like. This may be important but will require careful consideration of published data for all samples from 23°-26°N. As the authors already say, the NSP21 OHMI data mean that small amounts of a high-K component are present even at the volcanic front as well as behind it in the adjacent Kasuga cross-chain.\nThis paper’s discussion of the nature and origin of slab components is most persuasive about the HIMU component in l.411ff, and that topic is in the paper’s title. The discussion seems reasonable to me, but I do not think that the data in this paper add new insight into the general origin of HIMU. It’s not like Zhang et al.’s (2022) discovery of high δ66Zn values in HIMU lavas from the Cook-Austral islands and St. Helena. Consequently, while I found nothing wrong in that section, I do not think that it warrants such length. The overall model for NSP24 melts is essentially the same as in Ishizuka et al. (2010).\nIn addition to those main points, here are some more specific things to address during revision.\nThe paper needs much greater clarity about exactly what samples were studied.\n• MT18. Are the analyses of L1 and L3 in Supp.Table2 new analyses of the samples by that name in Zhao et al. (2016) that in turn came from Tian et al. (2005)? If so, then by what analytical methods? The data differ between this paper and Zhao/Tian by almost 2 wt% MgO and 25 relative% Na2O, for example, and by a factor of 4 in some trace elements. Or, are these new analyses of the same or similar samples from 18°12’N that have a different name (T2-1,2,3) in another recent paper about Mo isotopes by the same first author of this paper (Li et al., 2021:Geology)? Give the lat/long and water depth at which the samples in this study were collected, details of their collection, and how they compare to other samples from the site. Then discuss how these two samples relate to the heterogeneity known along the MT between 18-19°N (segment 6 of Pearce et al., 2005). Trace element rand Pb isotope ratios can vary significantly in <1 km in this segment, and all the samples in the Li and Zhao/Tian papers plus those from the same area in Chen et al. (2021) are more enriched than many (cf. Pearce et al., 2005; Woodhead et al., 2015). Neither sample in this paper meets the criterion (Ba/Nb<7) for unmodified (ambient) mantle wedge of Woodhead et al. (2015) whose Pb isotopes plot about where the letters CMT are in Fig. 3a of this paper. That strengthens the authors’ inference that even their MT18 samples have a significant slab component but they chose an especially enriched example to study. They also might note that everything north of 19.8°N -- where the seafloor fabric deteriorates at the north end of Pearce et al.’s (2005) segment 3, west of Asuncion and Maug at the volcanic front -- appears enriched in a slab component based on the recent results of Chen et al. (2021) using old SIO dredges. Whether that change reflects melting out of a uniform slab component during rifting-to-spreading, or a change in the amount or kind or ubiquity of a slab component, is the question.\n•NSP21. In reply to questioning, the authors said that sample T1 in SuppTable2 is a new analysis of a sample like T1-3 in Li et al. (2021), and they provided a map showing its location between Eifuku and Kasuga on the volcanic front. That reference and map need to be in the paper. Also, it should be noted that Eifuku has the lowest Nd and Hf isotopes south of the NSP, consistent with the addition of the most sediment (mixed pelagic-volcaniclastic) to its mantle source, according to Woodhead et al. (2015). The sample from this location with the lowest Nd isotope ratios in Li et al. (2021) also has the lightest Mo, consistent with that information.\n•NSP24. In reply to questioning, the authors said these samples are from the West Mariana Ridge (Ishizuka et al., 2010) including D13 that was not discussed in that paper. Again, this information and map need to be in the paper, and the map should identify the Hiyoshis.\nl.59. Few if any MT basalts cannot be distinguished from MORB using criteria like H2O/Ce or Ba/Nb. Even Th/Yb vs Nb/Yb (Fig. 4a) distinguishes most.\nl.68. What does “typical” mean? That requires comparison to other examples like those in the southern hemisphere. This is one example of un-necessary over-reach because of emphasizing the process of backarc basin evolution.\nl.79. Why is the N-NSP “ideal”? As noted above, the chosen samples may even pre-date rifting.\nl. 173. Why “arc” tholeiites?\nl.186. The OHMI are NOT more enriched in K2O/TiO2 than E-MORB, but they are enriched in Ba/Nb when MgO <7 wt%. The latter is how Woodhead et al. (2015) distinguished their source as having a slab component. Most also are enriched in Th relative to ambient mantle, although it is important to compare them to “Indian”-type MORB.\nl. 261ff. Consider breaking this long paragraph into two, with the first being about fractional crystallization, and the second starting at l.266 about partial melting. For NSP24, the latter depends on Supp.Fig.6 that in turn depends on the oddly low HREE in all the NSP24 OHMI that is rare in arc rocks. That accounts for the high Sm/Yb despite low La/Sm in these OHMI. The authors attribute that atypicality to 20% residual garnet in a depleted peridotite mantle source. That much garnet is unlikely even for fertile DM. Moreover, it seems impossible to mass balance the heavy REE in these OHMI relative to their host or other nearby rocks. Therefore, the model in Supp.Fig.6 is unconvincing. If not an analytical artifact, then might such low HREE result from adding eclogite melt from the slab to the reararc mantle?\nl.288. By “heterogeneous sources” do you mean a variable addition of slab component to a homogeneous ambient mantle? That seems like the simplest, and most conventional, option.\nl.293. Why not say silicate melts instead of silicic fluids?\nl.302. Why not say that Th is less soluble than REE and HFSE in “fluid”, but then cite experimental evidence rather than inferences from other field studies.\nl.312. Wouldn’t adding the same mass of sediment melt to variably depleted mantle have the same effect?\nl.315-318. I haven’t chased down all these references, but I didn’t think that the first three appealed to a sediment melt component in MTB. The classic Stolper and Newman (1994) paper was about a low-density fluid, which this paper’s authors seem to prefer too.\nl.340. What differs from the high-K basalts at Kasuga 2 and 3? The Pb isotope ratios are higher and chalcophile element contents are lower, but what about everything else?\nl.411. NSP21 and 24 rocks have quite different Nd and Hf isotope ratios so they cannot have similar sources even if their Pb is similar.\nl.420. The Pb isotopes of the high-K OHMI aren’t then same a in the Hiyoshis or WMR, but they are like Iwo-Jima, which requires a de-coupling of Pb from K between those sites, which is cool.\nl.478. This model for the HIMU component is essentially the same as in Ishizuka et al. (2010) and seems reasonable. However, please also apply it to the Group 1 OHMI in the same sample. Presumably they have less HIMU (carbonatite?), but why then do they have lower Nb/Yb and higher Ba/La and Th/Nb than Group 2?\nTables:\n• The uncorrected K2O and P2O5 entries for Type 2 NSP21 inclusions are transposed in Supp.Table1.\n• I strongly urge the authors to obtain and use IGSN numbers for their samples prior to publication. Otherwise, as more data are published for these and other samples, it will be come increasingly difficult to keep track of what is being studied using just a D- or T-number.\n• Is there a rationale for the sequence of samples numbers in Supp.Table1?\n> Specify the host rock of the melt inclusions in MT18 and NSP24, and group the OHMI in each by host rock.\n> If any of the OHMI are from the same olivine, then group them together too.\n> Beyond those two principles, why not present the data in order of decreasing Mg# of the host or the melt inclusion? That would help readers to see the data in a geochemical context.\nFigures:\n• Figure 1 or a Supplementary equivalent needs to have much more detailed maps showing the location of the samples; something like Fig. 1c of Ishizuka et al. (2010) for each site.\n• Figure 2. Perhaps as Supplemental equivalents, I wanted to see a least-squares best fit to mass balance between OHMI and host rock for c+d, and how the OHMI compared to whole rock compositions for the Hiyoshis and Io-To (Iwo-Jima) for e+f.\n• Figure 3a. This is such a small scale and has so much white space that it is hard to see details. Consider reducing the x-axis to 0.80-0.88 and then showing HIMU and EM1 in an inset because the data are more important than the model. I assume that the dashed yellow line is for NSP, but is there also a buried dashed red line for CIP, and is the white dashed line for CMT? The AVP, Iwo-Jima, and Kita-Iwojima, need to be added. All this should be explained in the caption. Why is the MORB source placed outside even the Pacific field? Why not place it inside the Indian field since the MT ambient mantle is Indian-type?", "reviewer_2": "review of Li et al: A HIMU-like component in the magma source during initial arc rifting revealed by melt inclusions\nThis manuscript presents new major and trace element concentrations and Pb isotope compositions for melt inclusions from lavas collected in three different regions of the Mariana arc: the mature back-arc, the northern portion of the main arc front, and the northern edge of the back-arc (NSP-24) where rifting is taking place. The authors show that all three locations exhibit distinct chemical and Pb isotopic compositions, largely confirming previous bulk rock studies from the same regions. The main difference to previous bulk rock work is that a group of melt inclusions from the NSP-24 region exhibit anomalous Pb isotope ratios that bear some similarities to so-called HIMU ocean island basalt lavas. After considering different scenarios of the HIMU origin, the authors conclude that it most likely represents seamounts that were subducted together with the Pacific plate. As such, no HIMU mantle source is present in the sub-arc Mariana mantle wedge, it is simply carried by the incoming subducted slab.\nThe quality of the data is good and, overall, I agree with the authors that their general interpretation is the most likely to account for a HIMU source in subduction-related lavas. In general, the paper certainly is worthy of eventual publication, but I am less sure of just how novel the paper really is given that the HIMU story feels more like an addendum to the generally well-studied Mariana arc system. I suppose it is up to the editor to determine if that merits publication in Nature Communications.\nOne major problem I had while reading this paper was that all the figures were presented in low resolution with very small font sizes. I, therefore, really struggled to see some of the details of the plots. I would urge the authors to fix this and recommend the Nature Communications do not allow papers with almost illegible figures to go out for review.\nWhen diving into a more detailed view of the manuscript as written I have some issues, both with the way the interpretations are presented as well as the processes that are implied to drive the subduction processes. I outline these in more detail below:\n1. The paper is far too long and some of the interpretations, while I generally agree with them, are argued in a convoluted way. For example, the section on the origin of the NSP-21 K rich lavas is almost four pages long. Yet, the authors end up concluding that the origin of these lavas is mostly due to assimilation-fractional-crystallization (AFC) type processes. I think this makes sense, but the authors could likely have stated this in less than a page. Instead of providing a long-winded discussion of other processes, I don't understand why they didnt just go straight to the strong evidence for their conclusion (low chalcophiles, low MgO, correlations between Pb isotopes and MgO, and more). Supplementary figures 3, 4, 5 do a great job of documenting all this.\nAnother example is the very long discussion the authors present on the origin of HIMU mantle sources. This includes a detailed account of arguments supporting carbonate involvement in HIMU generation. However, since the main conclusion of the paper is that the HIMU signature comes from a seamount, I do not see any reason to even mention carbonates or carbonatites. The only evidence the authors present that they claim support a carbonatite involvement in the HIMU source is figure 5, which I find very tenuous. While the trace element ratios shown there are distinct from the other melt inclusion groups, they are only very slightly offset towards a carbonatite source and what they fail to mention is that the observed values fully overlap with normal mantle values. Hence, there is nothing particularly 'carbonatite-like' about the observed trace element data. In the end, none of that matters for the interpretation of the origin of the HIMU signature in the NSP-24 samples. It makes sense that it is a subducted seamount and whether that seamount had a carbonatite or an altered oceanic crust influenced mantle source is irrelevant.\n2. There are some inconsistencies between the processes the authors seem to infer occurring during subduction and what has been proposed in the literature. For example, the authors claim that the mantle wedge is metasomatized by subducted sediments. To me it looks like these sediments are simply just bulk sediments. But typically, studies have suggested that sediments will experience partial melting before entering the mantle wedge and resulting in metasomatism. If the authors want to pursue such an interpretation, they would need to use sediment melts calculated based on one of the available experimental studies that have investigated such a process (eg Hermann and Rubatto, 2009). Including this calculation will primarily affect the trace element ratios of the resulting mantle source as well as the proportions of sediment required to explain the Pb isotopes.\nFurthermore, I find it difficult to reconcile the HIMU seamount as a bulk component in the mixing calculations because that would require high degrees of partial melting of what is essentially basaltic oceanic crust. I am very doubtful about such a partial melting process since oceanic crust becomes eclogitic at sub-arc depths. In turn, melting of eclogite only happens at very elevated temperatures and also results in very specific geochemical signatures, such as high Sr/Y ratios (eg adakites). As far as I can tell, this is not observed in the studied samples.\nIf the authors prefer to consider bulk sediment and oceanic crust mixing processes for their Pb isotope and trace element calculations, then they are basically inferring so-called melange formation (eg Marschall and Schumacher 2012; Nielsen and Marschall 2017). Instead of the long-winded discussions of the NSP-21 samples and the origin of HIMU, it might make more sense to present a discussion of different subduction processes like mantle metasomatism and melange formation. To me that would be more directly related to the presented data set and would also, in my opinion, make the paper of broader more general interest that might be better suited to Nature Communications.\nI also have some more specific line-by-line comments that the authors should address prior to resubmitting a revised version:\nLine 108: I looks like the latitudes are wrong for the SMT\nLine 155: what about re-equilibration? You just mention it here but do not further explore it. This process is the only one that is likely to affect isotopes and incompatible trace element ratios, so that seems more important to discuss than the PEC\nLine 194: delete significance\nLine 306-308: you should note that some sediments can have very large Ba enrichments with Ba/La and Ba/Th ratios that exceed those of arc lavas. Hence, without knowing the Ba contents of subducting sediments outboard of your arc lavas, you cannot necessarily attribute Ba enrichments to fluids (neither from oceanic crust nor from sediment). As shown in recent Ba isotope papers on arc lavas, there is little evidence to suggest that Ba in arc lavas is primarily supplied by ocean crust derived fluids.\nLine 310: I dont see this. Ba/La is quite variable, whereas Ba/Th is more constant. This may imply that Ba and Th are both coming from sediment and that fluids are less important in supplying Ba to the mantle source.\nLine 315-317: I do not follow what evidence you have presented that supports this conclusion about the slab component in the mature back-arc. The subduction component is generally very small and, therefore, is difficult to determine the origin of.\nLine 436-439: I dont think the paper by Zhang et al makes a strong case for carbonatite origin of HIMU. Zn isotopes are essentially unexplored in subduction zones and the paper presents no information about what might happen to Zn isotopes in a residual slab. So to me it is equally possible that small amounts of Zn isotope fractionation occurs during slab dehydration. I also cannot recognize the description of the Zn isotope values as 'exceptionally high' since they are only about 0.1 permil heavier than MORB and even less compared with other OIBs.\nLine 441-443: I dont really agree that this is clear. The NSP-24 samples are only slightly plotting towards the carbonatite field and I dont see a clear reason for why carbonatite needs to be implicated.\nLine 436-455: I do not understand the reason for the authors wanting to push the argument of HIMU being a carbonatite source. I dont think it has much to do with their data set.\nLine 477: why use a generic marine sediment field instead of sediments outboard of the Mariana arc?\nLine 478-481: This text seems to imply bulk mixing processes were considered. As I note above, such a process is not compatible with mantle metasomatism but instead more reminiscent of melange formation followed by partial melting.\nLine 492-494: I still see no particular evidence to support that the HIMU component in the NSP has anything to do with carbonatite or carbonated mantle.\nLine 509-527: I think the authors would be better served by deleting this whole paragraph.\nLine 546-554: Alternatively, it is more of a random chance problem. There will not be HIMU seamounts subducting all the way along the arc. Maybe there is a HIMU seamount subducting under the NSP-24, but not under NSP-21? Also, the mature back arc samples only carry a very minor slab component, which is very difficult to identify the exact nature of because it only causes a small modification away from the MORB field. So that, in itself, might not have anything to do with a progressive depletion of the mantle wedge. In any case, the NSP-24 samples only reveal a quite dilute HIMU signature, which, when further strongly diluted, would be essentially impossible to distinguish from any other slab component.\nLine 556: I dont see how this reveals it is more common. It basically requires subduction of seamounts with a very specific HIMU composition. Whether these types of seamounts are more common than we think is another question. If yes, then it is possible that more arcs could include such a component.\nLine 634: the athors need to describe how they corrected or instrumental mass bias. And how variations in mass bias were accounted for.\nLine 634-641: Was the laser spot size the same for samples and reference materials?\nBased on the SE of individual melt inclusions and the refefence materials, it looks like they must have been similar. However, there is a large number of melt inclusion measurements that have significantly higher SE counting statistics than the reference materials. So these likely have greater error bars than the 0.39% indicated for the 208/206 and 207/206 ratios and 1.22% indicated for ratios with 204 involved. I think the authors need to consider if some of their Pb isotope variations (especially within a specific group) might be due to analytical scatter. This is not the case for the variation between groups which is much larger than what can be attributed to analytical scatter."}, "author_response": "Response to reviewer’s comments:\nReviewer #1 (Remarks to the Author):\nThis is an interesting and well-written paper with a LOT of new major and trace element and Pb\nisotope data for olivine-hosted melt inclusions (OHMI) and their host rocks from three locations in\nthe Mariana arc and backarc. The new data extend the geographic range of previously known things,\nand include something altogether new: 12 weird andesitic OHMI (7% of those analyzed) that have\nunusually low Pb contents and EM1-like Pb isotope ratios in a basalt from a seamount in the\nnorthern volcanic front of the arc. The data seem reliable and warrant publication, and much of the\ninterpretation is reasonable, but several fundamental topics need attention during revision.\nFirst, the paper presents itself as a study of geochemical change during the transition from rifting to\nspreading in a backarc basin. However, this study cannot separate the along-strike spatial variations\nin the Mariana system (i.e., what is being subducted and the depth of the slab beneath eruption sites)\nfrom the effects of the rifting-to-spreading process. Second, the OHMI are less novel than claimed.\nThird, the paper over-states the importance, and over-interprets the origin, of the weird OHMI at\nNSP21. I will elaborate on those three points below.\nAn alternative emphasis of the paper, which is there already but under-played, might be how using\nOHMI can discover extreme melt mixing components that are diluted in erupted magmas in three\ndifferent tectonic settings in the Mariana system. The second half of the paper is mostly about this\ntopic; it’s only the title and introduction that lead readers (at least this reviewer) to expect the paper\nto be about how and why magma changes from backarc rifting to spreading. Revision could be\nmostly about organization and emphasis.\nResponse: We are very grateful for your professional and constructive comments on our manuscript,\nwhich are very helpful to improve the quality of our manuscripts. We have taken some of your\nsuggestions and made substantial revisions to the manuscript. We have changed the title to “A\nHIMU-like component in Mariana Convergent Margin magma sources during initial arc rifting\nrevealed by melt inclusions”. We hope you will be satisfied with our response and efforts in the\nrevision process.\nThe first fundamental topic is how to separate the spatial effect from the stage of backarc evolution.\nLatitudinal variations in the Marianas have been known for many decades and attributed by some\nof the co-authors of this paper, and others, to subduction of enriched seamount chains in the vicinity\nof Io-To (aka Iwo-Jima). That enriched signal decreases southward along both the volcanic front\nand the Mariana Trough (i.e., from 25° to 21° and beyond) and northward to 26°. The authors need\nto explain why their observations cannot be explained just by differences in what is subducted at\nthose latitudes, or how much is added to the mantle at various distances behind the volcanic front\nor above the slab. That is, the slab component may have always differed in kind or amount or\nubiquity between at 24°, 21° and 18°N.\nResponse: Thank you for your comment. We agree with you that subduction of different components\nat different latitudes may produce different geochemical signatures. However, if we look at the arc\nwhole rock data, most AVP, NSP, and CIP lavas have similar Pb isotopic compositions and all fall\nin or near the FOZO region (Fig. 3a in revised version). This indicates that these mantle source\nregions may be affected by similar proportions (~0.5%) of HIMU components, assuming that they\nall have the same Philippine Sea MORB mantle as the dominant source. In addition, subduction\ncomponents should be similar in such a small area as the NSP-24 source region, but only the high-\nFo (>88) olivine hosted melt inclusions have obvious HIMU characteristics (Supplementary Table\n3). This may be due to a small degree of partial melting during rifting producing a primitive melt\nwith clear HIMU characteristics. The Mariana Trough at 18°N may not be affected by HIMU\ncomponents because of the distance from the trench; alternatively, HIMU components in the mantle\nsource were consumed with increased partial melting needed to create MORB-type magma beneath\nthe back-arc basin. In the revised version, we emphasize that different regions are affected by\ndifferent types and extents of subducted slab components (lines 255-324). In addition, we have\nrevised the introduction section to emphasize the differences in subduction components in different\nregions of the IBM arc (lines 49-68), and highlight the unique geological background and the\nadvantages of melt inclusions research for understanding melt formation beneath the NSP-24 region\n(lines 69-94). We hope you will be satisfied with our revisions.\nThe rift-to-spread argument also begs the question of when and how a HIMU-type component got\nas far south as 18°N beneath the MT where there is no slab currently. Did appropriate seamounts\nget subducted there too when the slab dip was shallower? Or did the slab-contaminated mantle flow\nsouth as the arc unzipped? Or was the HIMU character inherited from the Oki-Daito plume under\nthe Philippine Sea Plate (Ishizuka et al., 2022) and is unrelated to subduction? Answering this may\nrequire careful comparison of the enriched components in the Mariana Trough and arc in light of\nrecent data.\nResponse: Thank you for your questions and comments. In the revised version, we suggest that there\nis no HIMU component in the mantle at 18°N beneath the Mariana Trough, and no HIMU signal is\nreflected in trace elemental and Pb isotopic compositions (lines 379-382). This may be because the\nMariana Trough at 18°N is located far away from the trench and no subducted seamount material\nexists beneath here, but this area is influenced by subducted sediments as indicated by its trace\nelement characteristics and some element ratios (e.g., Ba/Nb>7). In addition, the nearby Oki-Daito\nmantle plume does not have very low 207Pb/206Pb (0.83-0.86) and 208Pb/206Pb (2.05-2.11) ratios\n(Ishizuka et al., 2013), so the HIMU features in NSP-24 melt inclusions may not be inherited from\nthe Oki-Daito mantle plume.\nIshizuka, O., Taylor, R. N., Ohara, Y., & Yuasa, M. (2013). Upwelling, rifting and age-progressive\nmagmatism from the Oki-Daito mantle plume. Geology, 41(9), 1011–1014.\nThe NSP24 host rocks and their OHMI are very similar to the Hiyoshis with the principal difference\nbeing their lower Ba/La, Pb/Ce, and Sr/Nd ratios despite higher La/Yb. Those differences may\nreflect their reararc setting, such as in the Kasuga and Guguan cross-chain (Stern et al., 1993, 2006).\nAll three samples are from the eastern scarp of the remnant arc, ~40 km behind the volcanic front.\nThey are 2.5-3.3 Ma and may pre-date rifting. At best, they are from the earliest stage of extension.\nI agree with the authors that their mantle source probably underlay the arc just before, or as, it started\nto rift, but these are samples of the reararc 3 m.y. ago, not the rift now.\nIn light of this, I suggest that the paper be re-structured to focus more on the differences in slab\ncomponents and less on the rift-to-spread process. If the authors retain an emphasis on the latter,\nthen they must address other places where that process can be, and has been, studied more\nthoroughly than here. These are primarily in the southern hemisphere including the Manus, North\nFiji, and Lau Basins, and Kermadec Trough. Spreading at the East Lau and Futuna Spreading\nCenters in the Lau Basin reaches as close to the volcanic front as the WMR is to Hiyoshi. If anything,\nthe nature of the slab component and its effects on mantle melting during initial rifting can be studied\nthere even better than in this paper (e.g., Bezos et al., 2009; Escrig et al., 2012). The rifting stage is\nknown in the greatest detail in the Havre Trough (e.g., Gill et al., 2021). The current paper is\nprovincial in this respect.\nResponse: Thank you for your suggestion, which we have adopted. In the revised version, we have\nreduced the discussion of rifting-to-spreading process. In the introduction section, we have removed\nsome description of rifting-to-spreading process and re-emphasized the differences in slab\ncomponents in the mantle source in different regions (lines 49-76). The NSP-24 region predates or\nbegan close to the time of rift propagation, providing an excellent opportunity for us to understand\nmantle source compositions during rifting. Our study does not focus on the mantle composition\nduring rifting but on or before the earliest stages of extension to rift formation (i.e., initial arc rifting).\nWe have changed the title to “A HIMU-like component in Mariana Convergent Margin magma\nsources during initial arc rifting revealed by melt inclusions”. In this region, the evolution from\nisland arc to rifting occurred, the crust thinned, and the mantle composition should be more complex\nthan that of mature island arc and back-arc basin. In addition, this region can be compared with\nback-arc basin and island arc activity in neighboring areas. They are a continuous transition of\ndifferent stages of evolution of an island arc. This makes NSP-24 an ideal region for melt inclusion\nstudies to identify complex components in the mantle source. \nSecond, the OHMI are less novel than claimed. The title proclaims that a HIMU-like has been\nrevealed by their melt inclusions, but it was discovered almost 60 years ago by Tatsumoto at Io-To,\nthen called Iwo Jima, in one of the first papers about Pb isotopes in volcanic rocks! The Pb isotopes\nin the Group 2 NSP24 OHMI are similar to those in Io-To (Tatsumoto, 1966; Sun and Stern, 2001)\nand Kita-Iwojima (Ishizuka et al., 2010) even though the major and trace elements differ. The\nopposite is true for the Group 2 NSP21 OHMI that are similar to the high-K basalts of the nearby\nKasuga 2 and 3 seamounts in everything except their weird Pb isotopes and low chalcophile element\ncontents.\nResponse: Thank you for your comment. We agree with you, but both Iwo Jima and Kita-Iwo Jima\nareas are located about 200 km north of our study area, and belong to the Volcano Arc (VA) region.\nThe NSP-24 study area is close to the north Northern Seamount Province (N-NSP), is in the stage\njust prior to initial rifting, and thus has different tectonic setting than the Volcano Arc. In addition,\nthere is no significant HIMU components in West Mariana Ridge whole-rock data near the N-NSP.\nThe obvious HIMU nature of NSP-24 melt inclusions is reported for the first time here, indicating\nthat HIMU components also exist in the mantle this far south. Compared with andesitic-dacitic\nvolcanic rocks in Iwo Jima and Kita-Iwo Jima, the melt inclusion data are primitive enough to\napproximate mantle source compositions. The high-K and Pb isotope anomaly components of NSP-\n21 were also found for the first time in the volcanic front near 21°N, as a result of melt inclusion\nstudies. Therefore, we believe that the study of melt inclusion in Mariana arc and Trough updates\nour previous understanding that some subduction components exist on a larger scale than indicated\nby whole rock studies, which is one of the novelties of our study. In addition, this study focuses on\nthe melt inclusion characteristics before or during the earliest stage of arc rifting, which provides\nsupport for our understanding of the mantle composition under this special geological background.\nThird, the paper over-states the importance, and over-interprets the origin, of the weird Group 2\nOHMI at NSP21 that are a new discovery in this paper. By “weird” I mean their low chalcophile\nelement contents and their Pb isotope ratios. The host rock (T1-3) is similar to the high-K basalts at\nthe nearby Kasuga 2 and 3 seamounts (Fryer et al, 1997). The OHMI are similar in MgO and some\nother oxides to the dacites from Kasuga 3. It seems like special pleading to infer an entirely different\nslab source for these 3 wt% MgO OHMI in Fo70 olivine when so much is similar to nearby rocks.\nWhy not appeal to something that just affects chalcophile elements during differentiation? For\nexample, sulfides are known to fractionate Pb isotopes such that coexisting pyrite-sphalerites can\nhave at least the range of Pb isotopes as in these OHMI (Yuan et al., 2015: Science China: Earth\nSciences). After all, the Sr-Nd-Pb-Mo isotopes all vary considerably as a function of MgO at this\nlocation (Li et al., 2021), so it is an open system. Adding an exotic hydrothermal fluid to\ndifferentiated, Pb-poor melt might be an easier explanation than the 80 lines of speculation that start\nat l.331.\nResponse: Thank you for your suggestion. In the revised version, we have simplified the discussion\nof the source of NSP-21 K-rich magma (lines 285-306). We think that the fractional crystallization\nof sulfide can control the element contents of the melt and the assimilation of differentiated, Pb-\npoor and K-rich slab-derived components can explain the isotopic variations of the NSP-21 K-rich\nmelt inclusions.\nMoreover, whatever their origin, they are a tiny fraction of the OHMI that were studied, and they\ncan be only a small mass fraction of magma at the volcanic front including their host rock. Try mass-\nbalancing the OHMI to equal the host rock. The claim that this component “dominates the Mariana\nsubarc mantle” (l. 542) is a vast over-statement. These are an intriguing discovery on the flanks of\none seamount, but there is no evidence that they are a common component of Mariana volcanic\nfront lavas.\nResponse: Thank you for your comment. We agree with you and have removed this sentence in the\nrevised version. We believe that some HIMU signals in the mantle source may be obscured by\ndifferent types of magmas and modified by late magmatic evolution, resulting in the absence of a\nclear HIMU signal in most whole rocks. On the contrary, under special geological circumstances,\nsuch as lithospheric thinning caused by rifting, the magmas derived from HIMU mantle sources that\nundergo a small degree of partial melting and may be captured by olivine, forming the melt\ninclusions in our study. This further indicates that magma in the initial rift stage has the potential to\nretain more complete information about mantle sources affected by subduction. We have changed\n“Our melt inclusion study demonstrates that the existence of a HIMU-like component in the oceanic\narc settings may be more common than previously thought…” to “Our melt inclusion study\ndemonstrates that early rift magma retain more complete information about subducted components\nand that a significant HIMU-like component in the Mariana subduction system may be more\nwidespread than previously thought, at least to the north of 24°N,…” in lines 383-386.\nAn alternative emphasis for the paper might be how OHMI preserve slab component end-members\nat each site that are diluted in whole rocks (i.e., the subduction components section in l.289ff). To\nme, their most striking discovery is one that they do not discuss even though it is the best evidence\nfor their claim that a wide range of slab components, including volcaniclastic as well as pelagic\nsediments, might be present beneath the Mariana Trough as far south as 18°N. The Pb in their OHMI\nfrom 18°N covers the full range of Pb isotopes of the entire Mariana Trough from 16-23°N,\nextending from the ambient mantle of Woodhead et al. (2005) to their most enriched MT basalts.\nThere is even a negative correlation between 208Pb/206Pb ratios and Ba and Th-enrichment in both\nthe OHMI and whole rocks. For NSP24, the OHMI data mean that they found the extreme HIMU\ncomponent as far south of Io-To as Ishizuka et al. (2007) found it to the north at Kita-Iwojima, and\nthat it occurs even in the reararc where Pb/Ce ratios are lower, K2O is higher, and the slab\ncomponent is melt-like. This may be important but will require careful consideration of published\ndata for all samples from 23°-26°N. As the authors already say, the NSP21 OHMI data mean that\nsmall amounts of a high-K component are present even at the volcanic front as well as behind it in\nthe adjacent Kasuga cross-chain.\nResponse: Thank you for your suggestion. In the revised version, we have added the comparison\nand discussion of melt inclusion data with existing whole rock studies in the “subduction\ncomponents” section (line 255), including Mariana Trough from 16-22°N (lines 277-282), Iwo Jima\nand Kita-Iwo Jima north to 25°N (lines 311-321), and Kasuga cross-chain near 21°N (lines 285-\n289). Our results show that olivine-hosted melt inclusions in the Mariana arc and Trough can reflect\nmore end-member components, which is impossible to identify in whole-rock data, which is also\none of the important results of melt inclusion research.\nThis paper’s discussion of the nature and origin of slab components is most persuasive about the\nHIMU component in l.411ff, and that topic is in the paper’s title. The discussion seems reasonable\nto me, but I do not think that the data in this paper add new insight into the general origin of HIMU.\nIt’s not like Zhang et al.’s (2022) discovery of high δ66Zn values in HIMU lavas from the Cook-\nAustral islands and St. Helena. Consequently, while I found nothing wrong in that section, I do not\nthink that it warrants such length. The overall model for NSP24 melts is essentially the same as in\nIshizuka et al. (2010).\nResponse: Thank you for your suggestion. In the revised version, we have removed further\ndiscussion of the general origin of HIMU. HIMU signals in our OHMIs are more obvious than those\nin the whole rock studied by Ishizuka et al. (2010) in NSP-24, which is a key observation from our\nnew melt inclusion data. In addition, a bulk components mixing model (mélange model) is used to\nsimulate the source of NSP-24 melt inclusions.\nIn addition to those main points, here are some more specific things to address during revision.\nThe paper needs much greater clarity about exactly what samples were studied.\n• MT18. Are the analyses of L1 and L3 in Supp.Table2 new analyses of the samples by that name\nin Zhao et al. (2016) that in turn came from Tian et al. (2005)? If so, then by what analytical methods?\nThe data differ between this paper and Zhao/Tian by almost 2 wt% MgO and 25 relative% Na2O,\nfor example, and by a factor of 4 in some trace elements. Or, are these new analyses of the same or\nsimilar samples from 18°12’N that have a different name (T2-1,2,3) in another recent paper about\nMo isotopes by the same first author of this paper (Li et al., 2021:Geology)? Give the lat/long and\nwater depth at which the samples in this study were collected, details of their collection, and how\nthey compare to other samples from the site. Then discuss how these two samples relate to the\nheterogeneity known along the MT between 18-19°N (segment 6 of Pearce et al., 2005). Trace\nelement rand Pb isotope ratios can vary significantly in <1 km in this segment, and all the samples\nin the Li and Zhao/Tian papers plus those from the same area in Chen et al. (2021) are more enriched\nthan many (cf. Pearce et al., 2005; Woodhead et al., 2015). Neither sample in this paper meets the\ncriterion (Ba/Nb<7) for unmodified (ambient) mantle wedge of Woodhead et al. (2015) whose Pb\nisotopes plot about where the letters CMT are in Fig. 3a of this paper. That strengthens the authors’\ninference that even their MT18 samples have a significant slab component but they chose an\nespecially enriched example to study. They also might note that everything north of 19.8°N -- where\nthe seafloor fabric deteriorates at the north end of Pearce et al.’s (2005) segment 3, west of Asuncion\nand Maug at the volcanic front -- appears enriched in a slab component based on the recent results\nof Chen et al. (2021) using old SIO dredges. Whether that change reflects melting out of a uniform\nslab component during rifting-to-spreading, or a change in the amount or kind or ubiquity of a slab\ncomponent, is the question.\n•NSP21. In reply to questioning, the authors said that sample T1 in SuppTable2 is a new analysis of\na sample like T1-3 in Li et al. (2021), and they provided a map showing its location between Eifuku\nand Kasuga on the volcanic front. That reference and map need to be in the paper. Also, it should\nbe noted that Eifuku has the lowest Nd and Hf isotopes south of the NSP, consistent with the addition\nof the most sediment (mixed pelagic-volcaniclastic) to its mantle source, according to Woodhead et\nal. (2015). The sample from this location with the lowest Nd isotope ratios in Li et al. (2021) also\nhas the lightest Mo, consistent with that information.\n•NSP24. In reply to questioning, the authors said these samples are from the West Mariana Ridge\n(Ishizuka et al., 2010) including D13 that was not discussed in that paper. Again, this information\nand map need to be in the paper, and the map should identify the Hiyoshis.\nResponse: Thank you for your comments and suggestions on our sample information. L1 and L3\nhave the same sampling information as Zhao et al. (2016). We acknowledge that in our previous\nversion we focused on the data quality of melt inclusions and neglected to review the whole rock\ndata. In order to more accurately characterize the geochemical composition of the whole rocks, we\nre-analyzed MT-18 and NSP-24 whole rock samples during the revision of the manuscript. During\nthe analysis of the whole rocks, we conducted a rigorous review of the testing process and standard\nsample detection, and added duplicate samples for monitoring precision. The new whole-rock data\nare presented in Supplementary Table 2, and analysis methods are described in the Supplementary\ntext. The gap between the new analytical data and the previous data may be due to the low accuracy\nof older analyses (nearly 20 years) and/or due to sample heterogeneity. In addition, the wide\nvariability in different melt inclusions in the same sample also indicates that the MT-18 mantle has\nsmall-scale heterogeneity, similar to that reported in Pearce et al. (2005), which may be caused by\nsubduction of different slab components, as you mentioned. Compared to other samples at the same\nlocation, our MT-18 samples may be enriched in subducted slab components due to their high Ba/Nb\nvalues. However, no matter which component (lithosphere, deep-subduction, shallow-subduction)\nis enriched, it does not show any HIMU signature, indicating that there was no HIMU material\npresent beneath the Mariana back-arc region or that the HIMU component was strongly diluted.\nChen et al. (2021) discussed the variation of mantle source compositions from north to south in the\nMariana Trough and shown that back-arc spreading center magmatism is characterized by\nincreasing asthenospheric mantle melting, decreasing slab material contribution and diminishing\nsub-arc lithospheric mantle participation. This is outside the scope of our study because we only\nstudied Mariana Trough samples from 18°N. In this paper, we use different melt inclusion\ncompositions in different convergent margin tectonic settings (arc, rift, back-arc basin) to reflect\ntheir different mantle sources. When plotting whole rock data, we collected all published data for\nigneous rocks of the study area, which span a compositional range covering our whole rock samples\nand indicating that our new whole rock compositional data is credible. In the revised version, we\nadded a detailed map (Fig. 1) and brief description of samples (Supplementary Table 1) and related\nreferences. We hope you will be satisfied with our revisions.\nPearce, J. A., Stern, R. J., Bloomer, S. H., & Fryer, P. Geochemical mapping of the Mariana arc‐\nbasin system: Implications for the nature and distribution of subduction components. Geochem.\nGeophys. Geosyst. 6(7) (2005).\nChen, Y., Niu, Y., Xue, Q., Gao, Y. & Castillo, P. An iron isotope perspective on back-arc basin\ndevelopment: Messages from Mariana Trough basalts. Earth Planet. Sci. Lett. 572, 117133 (2021).\nl.59. Few if any MT basalts cannot be distinguished from MORB using criteria like H2O/Ce or\nBa/Nb. Even Th/Yb vs Nb/Yb (Fig. 4a) distinguishes most.\nResponse: Thank you for your comment. In the revised version, we have removed this sentence.\nAccording to your suggestion, we have made extensive changes to the introduction section, deleted\nsome descriptions about rifting-to-spreading process, and explained that different IBM arc segments\nare differently affected by subduction components.\nl.68. What does “typical” mean? That requires comparison to other examples like those in the\nsouthern hemisphere. This is one example of un-necessary over-reach because of emphasizing the\nprocess of backarc basin evolution.\nResponse: We have removed this sentence in the revised version.\nl.79. Why is the N-NSP “ideal”? As noted above, the chosen samples may even pre-date rifting.\nResponse: In the revised version, we have changed “The north Northern Seamount Province (N-\nNSP) between 23°N and 24°N is immediately north of where the Mariana Trough terminates (Fig.\n1) and is an ideal area for studying how back-arc extension begins and evolves. This region is\nundergoing back-arc rifting that is propagating northward…” to “The north Northern Seamount\nProvince (N-NSP) of the Mariana arc between 23°N and 24°N is immediately north of where the\nMariana Trough terminates (Fig. 1) and is undergoing northward-propagating back-arc rifting” in\nlines 69-71.\nl. 173. Why “arc” tholeiites?\nResponse: We have removed “arc” in this sentence (line 166).\nl.186. The OHMI are NOT more enriched in K2O/TiO2 than E-MORB, but they are enriched in\nBa/Nb when MgO <7 wt%. The latter is how Woodhead et al. (2015) distinguished their source as\nhaving a slab component. Most also are enriched in Th relative to ambient mantle, although it is\nimportant to compare them to “Indian”-type MORB.\nResponse: Thank you for you comment. We have changed “enriched K and Pb” to “enriched Pb” in\nthis sentence (line 179). In addition, in the spider diagram, the enrichment of Ba/Nb cannot be shown,\nso we added the description of enrichment of Ba/Nb and Th in the discussion section about\nsubduction components (lines 275-277).\nl. 261ff. Consider breaking this long paragraph into two, with the first being about fractional\ncrystallization, and the second starting at l.266 about partial melting. For NSP24, the latter depends\non Supp.Fig.6 that in turn depends on the oddly low HREE in all the NSP24 OHMI that is rare in\narc rocks. That accounts for the high Sm/Yb despite low La/Sm in these OHMI. The authors attribute\nthat atypicality to 20% residual garnet in a depleted peridotite mantle source. That much garnet is\nunlikely even for fertile DM. Moreover, it seems impossible to mass balance the heavy REE in these\nOHMI relative to their host or other nearby rocks. Therefore, the model in Supp.Fig.6 is\nunconvincing. If not an analytical artifact, then might such low HREE result from adding eclogite\nmelt from the slab to the reararc mantle?\nResponse: Thank you for your suggestion. We have broken this paragraph into two, with the first\nbeing about fractional crystallization (lines 226-233), and the second about partial melting (lines\n234-254). In the revised version, the mineral proportions of garnet lherzolite were re-selected\n(Ol + Opx + Cpx + Gt ) and the degree of partial melting was simulated\n0.600 0.200 0.100 0.100\n(Supplementary Fig. 7). The results show that the NSP-24 high-K melt inclusions have less degree\nof partial melting and more garnet residue than those in NSP-21 and NSP-24 medium-K melt\ninclusions. For the HREE, we re-checked the data, and the analysis results of the reference samples\nwere relatively good, with a relative error of less than 10% (Supplementary Table 4). The ranges of\nHREE in NSP-24 melt inclusions are large, but their distribution patterns are similar to that of host\nwhole rocks. The lower HREE content may be due to differences in the degree of partial melting,\nor to the addition of eclogite melt from the slab. However, due to the significant difference in\nelement distribution patterns with adakites, we think the addition degree of eclogite melt should not\nbe high, and it is difficult to change our simulation results of partial melting.\nl.288. By “heterogeneous sources” do you mean a variable addition of slab component to a\nhomogeneous ambient mantle? That seems like the simplest, and most conventional, option.\nResponse: Yes, we exclude the influence of magmatic processes such as fractional crystallization\nand partial melting. Based on the differences in element abundances and Pb isotopes of melt\ninclusions from different groups, we believe that these inclusions come from an originally\nhomogeneous mantle that was made heterogeneous by addition of different subduction components.\nWe have changed “heterogeneous sources” to “sources affected by variable subduction components”\nin line 252.\nl.293. Why not say silicate melts instead of silicic fluids?\nResponse: Thank you for your suggestion. We have changed “silicic fluids” to “silicate melts” in\nthis sentence (line 259).\nl.302. Why not say that Th is less soluble than REE and HFSE in “fluid”, but then cite experimental\nevidence rather than inferences from other field studies.\nResponse: Thank you for your suggestion. We have changed “Th is insoluble relative to REEs and\nHFSEs in slab-derived fluids” to “Th is less soluble than REEs and HFSEs in slab-derived fluids”\nand cited Brenan et al. (1995) and Kessel et al. (2005) in lines 267-269.\nl.312. Wouldn’t adding the same mass of sediment melt to variably depleted mantle have the same\neffect?\nResponse: This sentence expresses that the contribution of sediments in subduction components is\ng is\ngreater than that of the altered oceanic crust. In the simulation calculation, we assume that NSP-21\nand NSP-24 have similar depleted mantle composition, that is, Indian Ocean type mantle. The\ndifference between the compositions of NSP-21 and NSP-24 melt inclusions is mainly controlled\nby subduction component types and partial melting degree.\nl.315-318. I haven’t chased down all these references, but I didn’t think that the first three appealed\nto a sediment melt component in MTB. The classic Stolper and Newman (1994) paper was about a\nlow-density fluid, which this paper’s authors seem to prefer too.\nResponse: The subduction components have less influence on the MT-18 magma source compared\nto contributions to the NSP magma source. We have revised these descriptions in lines 273-282.\nl.340. What differs from the high-K basalts at Kasuga 2 and 3? The Pb isotope ratios are higher and\nchalcophile element contents are lower, but what about everything else?\nResponse: We would like to emphasize that the samples of NSP-21 have a different geological\nbackground from the samples of Kasuga 2 and 3, with NSP-21 located in the arc front and Kasuga\nin the Cross-Chain extending to the back arc. In the revised version, we have changed “NSP-21 K-\nrich melt inclusions have especially high alkali contents and can be classified as alkaline series\n(Supplementary Fig. 2a), which are the first reported alkaline compositions in S-NSP and different\nfrom the high-K melts of Kasuga 2 and 3 seamounts from the Kasuga Cross-Chain in the northern\npart of the Mariana back-arc basin” to “NSP-21 K-rich melt inclusions have especially high alkali\ncontents and can be classified as alkaline series (Supplementary Fig. 2a), similar to the high-K melts\nof Kasuga 2 and 3 seamounts from the Kasuga Cross-Chain in the northern Mariana back-arc basin”\nin lines 285-288.\nl.411. NSP21 and 24 rocks have quite different Nd and Hf isotope ratios so they cannot have similar\nsources even if their Pb is similar.\nResponse: Thank you for your comment. We have removed “suggesting similar mantle sources” in\nthis sentence (line 310).\nl.420. The Pb isotopes of the high-K OHMI aren’t then same a in the Hiyoshis or WMR, but they\nare like Iwo-Jima, which requires a de-coupling of Pb from K between those sites, which is cool.\nResponse: We have added comparison with Pb isotopes of Iwo-Jima in lines 311-321.\nl.478. This model for the HIMU component is essentially the same as in Ishizuka et al. (2010) and\nseems reasonable. However, please also apply it to the Group 1 OHMI in the same sample.\nPresumably they have less HIMU (carbonatite?), but why then do they have lower Nb/Yb and higher\nBa/La and Th/Nb than Group 2?\nResponse: Thank you for your suggestion. In the revised version, simulation calculations are also\napplied to the group 1 melt inclusions in NSP-24 (Fig. 3a). We believe that these two groups of\nNSP-24 melt inclusions have similar proportion of depleted mantle, subducted sediment and altered\noceanic crust because they come from the same region. The difference between these two groups of\nmelt inclusions lies in the different proportion of the HIMU components. In the NSP-24 group 1\nmelt inclusions, HIMU components are negligible, suggesting a higher proportion of subductied\nsediments and altered oceanic crust. This also explains their higher Ba/La and Th/Nb and lower\nNb/Yb ratios compared to NSP-24 group 2 melt inclusions. In addition, the difference of the partial\nmelting degree may also control the contents and ratios of trace elements in different groups of melt\ninclusions.\nTables:\n• The uncorrected K2O and P2O5 entries for Type 2 NSP21 inclusions are transposed in\nSupp.Table1.\nResponse: Thank you for your comment. We have revised it.\n• I strongly urge the authors to obtain and use IGSN numbers for their samples prior to publication.\nOtherwise, as more data are published for these and other samples, it will be come increasingly\ndifficult to keep track of what is being studied using just a D- or T-number.\nResponse: Thank you for your suggestion. We fully agree with your suggestion. I have obtained 6\nIGSN numbers of the samples, which are 10.58052/IE9080001 to 10.58052/IE9080006, and the\ndata have been published on EarthChem Library (Xiaohui, L., 2024. Melt inclusion data from the\nMariana subduction zone, Version 1.0. Interdisciplinary Earth Data Alliance (IEDA).\nhttps://doi.org/10.60520/IEDA/113107).\n• Is there a rationale for the sequence of samples numbers in Supp.Table1?\n> Specify the host rock of the melt inclusions in MT18 and NSP24, and group the OHMI in each\nby host rock.\n> If any of the OHMI are from the same olivine, then group them together too.\n> Beyond those two principles, why not present the data in order of decreasing Mg# of the host or\nthe melt inclusion? That would help readers to see the data in a geochemical context.\nResponse: Thank you for your suggestion. We have modified the tables according to your suggestion.\nFigures:\n• Figure 1 or a Supplementary equivalent needs to have much more detailed maps showing the\nlocation of the samples; something like Fig. 1c of Ishizuka et al. (2010) for each site.\nResponse: In the revised version, we have added detailed maps (Fig. 1) showing the sample\nlocations.\n• Figure 2. Perhaps as Supplemental equivalents, I wanted to see a least-squares best fit to mass\nbalance between OHMI and host rock for c+d, and how the OHMI compared to whole rock\ncompositions for the Hiyoshis and Io-To (Iwo-Jima) for e+f.\nResponse: In the revised version, we have added whole-rock data of these regions for comparison\nand described them in the figure caption (Supplementary Fig. 5).\n• Figure 3a. This is such a small scale and has so much white space that it is hard to see details.\nConsider reducing the x-axis to 0.80-0.88 and then showing HIMU and EM1 in an inset because the\ndata are more important than the model. I assume that the dashed yellow line is for NSP, but is there\nalso a buried dashed red line for CIP, and is the white dashed line for CMT? The AVP, Iwo-Jima,\nand Kita-Iwojima, need to be added. All this should be explained in the caption. Why is the MORB\nsource placed outside even the Pacific field? Why not place it inside the Indian field since the MT\nambient mantle is Indian-type?\nResponse: Thank you for your suggestion. We have modified the Figure 3a to make it look clearer.\nWe also have added the fields of AVP, Iwo-Jima, and Kita-Iwo Jima. In addition, we have changed\nthe DMM end-member to Indian Ocean-type mantle field.\nReviewer #2 (Remarks to the Author):\nreview of Li et al: A HIMU-like component in the magma source during initial arc rifting\nrevealed by melt inclusions\nThis manuscript presents new major and trace element concentrations and Pb isotope compositions\nfor melt inclusions from lavas collected in three different regions of the Mariana arc: the mature\nback-arc, the northern portion of the main arc front, and the northern edge of the back-arc (NSP-24)\nwhere rifting is taking place. The authors show that all three locations exhibit distinct chemical and\nPb isotopic compositions, largely confirming previous bulk rock studies from the same regions. The\nmain difference to previous bulk rock work is that a group of melt inclusions from the NSP-24\nregion exhibit anomalous Pb isotope ratios that bear some similarities to so-called HIMU ocean\nisland basalt lavas. After considering different scenarios of the HIMU origin, the authors conclude\nthat it most likely represents seamounts that were subducted together with the Pacific plate. As such,\nno HIMU mantle source is present in the sub-arc Mariana mantle wedge, it is simply carried by the\nincoming subducted slab.\nThe quality of the data is good and, overall, I agree with the authors that their general interpretation\nis the most likely to account for a HIMU source in subduction-related lavas. In general, the paper\ncertainly is worthy of eventual publication, but I am less sure of just how novel the paper really is\ngiven that the HIMU story feels more like an addendum to the generally well-studied Mariana arc\nsystem. I suppose it is up to the editor to determine if that merits publication in Nature\nCommunications.\nResponse: We are grateful for your professional and constructive comments that helped us\nstrengthen our arguments. We have revised the manuscript according to your comments and\nsuggestions. In the revised version, we use melt inclusions to study magmatic source compositions\nat the initial stage of back-arc rifting. These NSP-24 high-K melt inclusions are primitive enough\n(MgO ~10 wt.%) to be directly representative of primary mantle melts. These data indicate the\npresence of extreme HIMU components that were not identified by previous whole-rock studies in\nthis region, expanding our understanding of slab components, especially seamount subduction. This\nhighlights the influence of seamount subduction on arc magma compositions, which is often\noverlooked in conventional studies. In addition, we present the compositional characteristics of melt\ninclusions in different tectonic evolution stages, and these data reflect different contributions of\nsubduction components to the mantle source in three different tectonic settings of the Mariana\nsystem, which provides a new perspective for us to understand convergent margin magma\ngeneration. We believe that these studies are novel and worthy of publication in Nature\nCommunications. We also hope you are satisfied with our explanations and efforts in the manuscript\nrevision.\nOne major problem I had while reading this paper was that all the figures were presented in low\nresolution with very small font sizes. I, therefore, really struggled to see some of the details of the\nplots. I would urge the authors to fix this and recommend the Nature Communications do not allow\npapers with almost illegible figures to go out for review.\nResponse: Thank you for your suggestion. We have made substantial modification to all the figures\nto improve their resolution and readability.\nWhen diving into a more detailed view of the manuscript as written I have some issues, both with\nthe way the interpretations are presented as well as the processes that are implied to drive the\nsubduction processes. I outline these in more detail below:\n1. The paper is far too long and some of the interpretations, while I generally agree with them, are\nargued in a convoluted way. For example, the section on the origin of the NSP-21 K rich lavas is\nalmost four pages long. Yet, the authors end up concluding that the origin of these lavas is mostly\ndue to assimilation-fractional-crystallization (AFC) type processes. I think this makes sense, but the\nauthors could likely have stated this in less than a page. Instead of providing a long-winded\ndiscussion of other processes, I don't understand why they didnt just go straight to the strong\nevidence for their conclusion (low chalcophiles, low MgO, correlations between Pb isotopes and\nMgO, and more). Supplementary figures 3, 4, 5 do a great job of documenting all this.\nResponse: Thank you for your suggestion. In the revised version, we have simplified the discussion\nof the source of NSP-21 K-rich magma (lines 285-306), which makes our manuscript more concise\nand readable. The generation of these K-rich magmas is mainly attributed to the contamination of\nsubducted K-rich and sulfides separated slab components during magma evolution.\nAnother example is the very long discussion the authors present on the origin of HIMU mantle\nsources. This includes a detailed account of arguments supporting carbonate involvement in HIMU\ngeneration. However, since the main conclusion of the paper is that the HIMU signature comes from\na seamount, I do not see any reason to even mention carbonates or carbonatites. The only evidence\nthe authors present that they claim support a carbonatite involvement in the HIMU source is figure\n5, which I find very tenuous. While the trace element ratios shown there are distinct from the other\nmelt inclusion groups, they are only very slightly offset towards a carbonatite source and what they\nfail to mention is that the observed values fully overlap with normal mantle values. Hence, there is\nnothing particularly 'carbonatite-like' about the observed trace element data. In the end, none of that\nmatters for the interpretation of the origin of the HIMU signature in the NSP-24 samples. It makes\nsense that it is a subducted seamount and whether that seamount had a carbonatite or an altered\noceanic crust influenced mantle source is irrelevant.\nResponse: Thank you for your suggestion. In the revised version, we have removed the discussion\nof involvement in HIMU generation and also removed the Figure 5 (Zr/Hf vs. Ce/Pb and Sm/Yb vs.\nCe/Pb). After modification, we emphasize that the HIMU signal is caused by seamount subduction\n(lines 326-351).\n2. There are some inconsistencies between the processes the authors seem to infer occurring during\nsubduction and what has been proposed in the literature. For example, the authors claim that the\nmantle wedge is metasomatized by subducted sediments. To me it looks like these sediments are\nsimply just bulk sediments. But typically, studies have suggested that sediments will experience\npartial melting before entering the mantle wedge and resulting in metasomatism. If the authors want\nto pursue such an interpretation, they would need to use sediment melts calculated based on one of\nthe available experimental studies that have investigated such a process (eg Hermann and Rubatto,\n2009). Including this calculation will primarily affect the trace element ratios of the resulting mantle\nsource as well as the proportions of sediment required to explain the Pb isotopes.\nFurthermore, I find it difficult to reconcile the HIMU seamount as a bulk component in the mixing\ncalculations because that would require high degrees of partial melting of what is essentially basaltic\noceanic crust. I am very doubtful about such a partial melting process since oceanic crust becomes\neclogitic at sub-arc depths. In turn, melting of eclogite only happens at very elevated temperatures\nand also results in very specific geochemical signatures, such as high Sr/Y ratios (eg adakites). As\nfar as I can tell, this is not observed in the studied samples.\nIf the authors prefer to consider bulk sediment and oceanic crust mixing processes for their Pb\nisotope and trace element calculations, then they are basically inferring so-called melange formation\n(eg Marschall and Schumacher 2012; Nielsen and Marschall 2017). Instead of the long-winded\ndiscussions of the NSP-21 samples and the origin of HIMU, it might make more sense to present a\ndiscussion of different subduction processes like mantle metasomatism and melange formation.\nTo me that would be more directly related to the presented data set and would also, in my opinion,\nmake the paper of broader more general interest that might be better suited to Nature\nCommunications.\nResponse: Thank you for your suggestion. We fully agree with you that it makes sense to clarify the\ntransfer process of subducted slab materials to mantle wedge. However, we do not know the details\nof how the HIMU-like component was transferred from the subducting plate to the zone of melt\ngeneration; this is an important question but it is not the major point of our study. The focus of this\nstudy is to reveal the composition of the mantle source region in the initial arc rifting stage of the\nMariana subduction zone, and to clarify the storage mechanism of this HIMU-like component under\nthis special background. Only the melt inclusions hosted by primitive olivine show HIMU-like\ncharacteristics, while the whole rocks in this region do not. In fact, both the metasomatized mantle-\nwedge melting model and the mélange-diapir melting model can transfer the subducted HIMU\ncomponents to the over-riding plate, so the melt transformation process will not affect the overall\nlogic of this paper, and does not change the fact that HIMU components are present in the Mariana\nisland arc source region. Nevertheless, in order to facilitate the calculation, in the revised version,\nwe chose the bulk components mixing model, that is, mélange model, to draw the Figure 3a. If the\nmetasomatic model is used to calculate, only the proportion of end-element components changes\nand the overall conclusion does not change. In addition, we have removed the description of\nmetasomatism in the revised manuscript. We hope you will be satisfied with our modification.\nI also have some more specific line-by-line comments that the authors should address prior to\nresubmitting a revised version:\nLine 108: I looks like the latitudes are wrong for the SMT\nResponse: Thank you for your comment. We have changed “20.5-17.5°N” to “12.5-17.5°N” in line\n105.\nLine 155: what about re-equilibration? You just mention it here but do not further explore it. This\nprocess is the only one that is likely to affect isotopes and incompatible trace element ratios, so that\nseems more important to discuss than the PEC\nResponse: As described by Danyushevsky et al. (2000), the re-equilibration process, referred to as\n‘‘Fe-loss’’, can operate during natural pre-eruptive cooling of host magma and results in lower FeOt\nand higher MgO contents within the initially trapped volume of inclusion. This process can be\ncorrected using the Petrolog 3 software. We have described the correction process in lines148-159.\nDue to being captured by olivine, the re-equilibration process mainly affects MgO and FeO\nconcentrations of melt inclusions, but has little effect on other major-element oxides and isotopes,\nthus the overall geochemical trends and composition changes of melt inclusions remain little or\nunaffected.\nDanyushevsky, L.V., Della-Pasqua, F.N. & Sokolov, S. Re-equilibration of melt inclusions trapped\nby magnesian olivine phenocrysts from subduction-related magmas: petrological implications.\nContrib. Mineral. Petrol. 138, 68-83 (2000).\nLine 194: delete significance\nResponse: We have deleted it.\nLine 306-308: you should note that some sediments can have very large Ba enrichments with Ba/La\nand Ba/Th ratios that exceed those of arc lavas. Hence, without knowing the Ba contents of\nsubducting sediments outboard of your arc lavas, you cannot necessarily attribute Ba enrichments\nto fluids (neither from oceanic crust nor from sediment). As shown in recent Ba isotope papers on\narc lavas, there is little evidence to suggest that Ba in arc lavas is primarily supplied by ocean crust\nderived fluids.\nResponse: Thank you for your comment and suggestion. In the revised version, we have removed\nthe plot of “Ba/La vs. Th/Yb” and “Ba/Th vs. Th/Nb” and added a plot of “Ba/Yb vs. Th/Yb”. We\nhave described the high Th/Yb and Ba/Yb ratios as the contribution of subducted sediments (lines\n271-273).\nLine 310: I dont see this. Ba/La is quite variable, whereas Ba/Th is more constant. This may imply\nthat Ba and Th are both coming from sediment and that fluids are less important in supplying Ba to\nthe mantle source.\nResponse: Thank you for your comment. We agree with you and have changed “NSP-21 and NSP-\n24 melt inclusions have highly variable Th/Yb and Th/Nb ratios but relatively constant Ba/La and\nBa/Th ratios, suggesting that the contributions of subducted sediments or sediment melts are more\nvariable than those of subducted altered oceanic crust-derived fluids” to “NSP-21 and NSP-24 melt\ninclusions have highly variable Th/Yb and Ba/Yb ratios, suggesting that the contributions of\nsubducted sediments are more variable than those of subducted AOC” in lines 271-273.\nLine 315-317: I do not follow what evidence you have presented that supports this conclusion about\nthe slab component in the mature back-arc. The subduction component is generally very small and,\ntherefore, is difficult to determine the origin of.\nResponse: Thank you for your comment. We have removed the descriptions of the origin of\nsubduction components and suggested that the subduction components have less influence on the\nMT-18 magma source compared to contributions to the NSP magma source.\nLine 436-439: I dont think the paper by Zhang et al makes a strong case for carbonatite origin of\nHIMU. Zn isotopes are essentially unexplored in subduction zones and the paper presents no\ninformation about what might happen to Zn isotopes in a residual slab. So to me it is equally possible\nthat small amounts of Zn isotope fractionation occurs during slab dehydration. I also cannot\nrecognize the description of the Zn isotope values as 'exceptionally high' since they are only about\n0.1 permil heavier than MORB and even less compared with other OIBs.\nResponse: We have removed the description of Zn isotopes research in HIMU origin in the revised\nversion.\nLine 441-443: I dont really agree that this is clear. The NSP-24 samples are only slightly plotting\ntowards the carbonatite field and I dont see a clear reason for why carbonatite needs to be implicated.\nResponse: We have removed this discussion.\nLine 436-455: I do not understand the reason for the authors wanting to push the argument of HIMU\nbeing a carbonatite source. I dont think it has much to do with their data set.\nResponse: Thank you for your comment and suggestion. In the revised version, we have removed\nfurther discussion of carbonatite and HIMU sources.\nLine 477: why use a generic marine sediment field instead of sediments outboard of the Mariana\narc?\nResponse: In the revised version, we used the sediments from ODP 801 outboard the Mariana arc\nas the sediment end member for the simulation calculations (Supplementary Table 6).\nLine 478-481: This text seems to imply bulk mixing processes were considered. As I note above,\nsuch a process is not compatible with mantle metasomatism but instead more reminiscent of\nmelange formation followed by partial melting.\nResponse: Thank you for your suggestion. We agree with your comments about mélange mixing.\nIn the revised version, we have used a bulk components mixing model to simulate the calculation (lines\n329-334), which is the mélange-diapir melting model and could reasonably explain our data.\nLine 492-494: I still see no particular evidence to support that the HIMU component in the NSP has\nanything to do with carbonatite or carbonated mantle.\nResponse: We have removed the “characterized by carbonated mantle peridotite” in this sentence\n(line 365).\nLine 509-527: I think the authors would be better served by deleting this whole paragraph.\nResponse: Thank you for your suggestion. We have removed this paragraph in the revised version.\nLine 546-554: Alternatively, it is more of a random chance problem. There will not be HIMU\nseamounts subducting all the way along the arc. Maybe there is a HIMU seamount subducting under\nthe NSP-24, but not under NSP-21? Also, the mature back arc samples only carry a very minor slab\ncomponent, which is very difficult to identify the exact nature of because it only causes a small\nmodification away from the MORB field. So that, in itself, might not have anything to do with a\nprogressive depletion of the mantle wedge. In any case, the NSP-24 samples only reveal a quite\ndilute HIMU signature, which, when further strongly diluted, would be essentially impossible to\ndistinguish from any other slab component.\nResponse: Thank you for your comment. We agree with you. However, for Mariana arc, there are\nabundant HIMU-seamounts distributed in the west Pacific outboard of the Mariana arcs. The Wake\nseamount chain and its volcaniclastic components near 21°N have HIMU characteristics and are\nsubducting into the mantle together with the Pacific Plate. In addition, whole-rock data indicate that\nmost Mariana arc volcanic rocks have FoZo-characteristic Pb isotopic compositions. This isotopic\ncomposition can be obtained by the injection of HIMU component in the mantle wedge. Therefore,\nwe infer that more HIMU seamounts may be subducting than previously thought (lines 354-366).\nThese HIMU signals may be diluted by subarc mantle magma and generally are not well presented.\nUnder some special conditions, such as low degree partial melting of the mélange, the melt of the\ntypical HIMU signal can be captured and form the NSP-24 high K melt inclusions. This means that\nmelt inclusions can reveal more information about magmatic source composition than the whole\nrock. Hope you are satisfied with our response.\nLine 556: I dont see how this reveals it is more common. It basically requires subduction of\nseamounts with a very specific HIMU composition. Whether these types of seamounts are more\ncommon than we think is another question. If yes, then it is possible that more arcs could include\nsuch a component.\nResponse: Thank you for your comment. We have changed “the existence of a HIMU-like\ncomponent in the oceanic arc settings may be more common” to “a significant HIMU-like\ncomponent in the Mariana subduction system may be more widespread than previously thought,\nat least to the north of 24°N…” in this sentence (lines 384-386). As we answered in the previous\nquestion, the subduction of HIMU-type seamounts in the Mariana system may be more widespread\nthan previously thought, and we have also described this in lines 354-366.\nLine 634: the authors need to describe how they corrected or instrumental mass bias. And how\nvariations in mass bias were accounted for.\nResponse: Thank you for your suggestion. We have added the relevant description in lines 449-451.\nThe instrumental mass bias was corrected using the standard–sample–standard bracketing method\nand any outliers (>2SD) were excluded.\nLine 634-641: Was the laser spot size the same for samples and reference materials?\nBased on the SE of individual melt inclusions and the reference materials, it looks like they must have\nbeen similar. However, there is a large number of melt inclusion measurements that have significantly\nhigher SE counting statistics than the reference materials. So these likely have greater error bars\nthan the 0.39% indicated for the 208/206 and 207/206 ratios and 1.22% indicated for ratios with 204\ninvolved. I think the authors need to consider if some of their Pb isotope variations (especially within\na specific group) might be due to analytical scatter. This is not the case for the variation between\ngroups which is much larger than what can be attributed to analytical scatter.\nResponse: Thank you for your comment. We used the same laser beam spot size for the samples and\nreference glass. We have reviewed the data and believe it to be accurate and reliable. The analysis\nof reference glass is performed after every 8 melt inclusions, and the results show that the analysis\naccuracy of reference glass is excellent. Therefore, we believe that some Pb isotope variations in\nmelt inclusions are due to their own characteristics rather than analytical scatter. In addition, when\nplotting, we added the error bars of the Pb isotopes, and the Pb isotopes of different groups of melt\ninclusions showed sufficient differences to allow comparison of end-member components (Fig. 3).\nWe hope you will agree with our explanation.\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors are to be congratulated and rewarded for their willingness and ability to revise their paper\nso thoroughly in light of reviewers’ comments. At the same time, they significantly improved the quality\nof prose and figures in many places beyond reviewer input. Perhaps the co-authors engaged more this\ntime. The result is a great improvement that could be accepted with only minor further revision. The\nimprovements allow reviewers to focus on less central matters that could improve the manuscript\nfurther.\nThe description of samples needs further improvement either in Supplemental Information or\nSupplemental Table Sheet 1. How exactly are the samples in this study related to samples and data in\npreviously published papers? This is discussed in the Rebuttal but I didn’t find it in the revision. The IGSN\nnumbers also need to be inserted in Supplemental Table Sheet 1, and the new ECL should be cited in the\nmain manuscript.\nOne new idea introduced during revision is that the HIMU-like slab-derived component is restricted to\nthe lithosphere and, therefore, evident only or mostly when the lithosphere melts during rifting. I see no\nreason not to make this suggestion, and the authors could cite Pearce et al. (2005) for support of a\nlithospheric component in the NSP due to high Nb/Ta ratios there. However, conceptually, why would a\nslab-derived HIMU component be restricted to the lithosphere? Instead, might it just be restricted\nspatially to where HIMU seamount chains are being subducted, thereby constricting backarc spreading?\nThe new figure 1 is a vast improvement over the original. It would have saved me many hours had it\nbeen submitted initially. The model in Fig. 3b is much improved, and the new Fig. 5 cartoon is lovely\nalthough it seems odd to show such thick lithosphere (if any) beneath the Mariana Trough after millions\nof years of spreading. L.380 even says that none needs to be present.\nThe authors missed the chance to estimate the mass fraction of melt inclusion types in their host rocks,\nallowing readers to dismiss the importance of their discovery of previously unknown melt compositions\nin these locations.\nLine numbers below refer to the revised and merged document.\nl.73: Consider change to “…erupted IN THIS LOCATION just prior to…” I know of no evidence that this\ndifference occurs elsewhere.\nl.89: Consider inserting “remnant arc” before West Mariana Ridge, to alert readers to the tectonic setting\nof this ridge earlier than l.120 as now. You could also replace “corresponding to” with “that is the\nconjugate pair of” to further emphasize the setting.\nl.109: Consider removing “as a result of propagating rifting”. This was added during revision but is a\ncarry-over from the original focus of the paper. The authors provide no evidence that the different\nproportions and types of subduction components in the mantle source result from rift propagation\nrather than, for example, what was subducted at different latitudes or times.\nl.191: Consider replacing “and” with “from” because I think you mean strong relative LREE enrichment;\ni.e., fractionation of LREEs from HREEs.\nl.196: This sentence falsely implies that there are five separate fields of Pb isotope ratios in Fig. 3\nwhereas there are only three. It might be better to say that “Each of the five types of melt inclusions\ndefined above has a narrow range of Pb isotope compositions. The total range encompasses…”\nl.244-246. I continue to find the model of melting garnet peridotite unconvincing and the HREE data\nsuspect, but the authors’ rebuttal is reasonable. Variable subduction components, as discussed in the\nfollowing section, seem a more likely explanation to me.\nl.266. Use words instead of the PEC acronym.\nl.272-273. Do you mean that the relative proportion of sediment and AOC in the subduction component\nis highly variable?\nL277: Consider replacing “proposed” by “discussed” because the high Ba/Nb ratios were discovered and\ndiscussed, not proposed, in ref. 53.\nl.302: Consider replacing “these experienced…” with “these melts were products of “ fractional\ncrystallization.\nl.336: Remove final “s” from patterns.\nl.352ff: Although the model is reasonable, as usual there is ambiguity whether an intermediate\nFOZO-like source component would suffice without the leverage of a HIMU extreme. However, no need to revise.\nl.357: Should “metabasites” be “metabasalts”?\nl.380: This run-on sentence needs revision. Do you mean “This may be because the back-arc basin\nspreading center is located far away from the trench, and is mainly fed by a large amount of MORB-like\nmagma. If so, then no lithospheric HIMU component is present, or it has been diluted.”\nReviewer #2 (Remarks to the Author):\nI have had another read of the revised manuscript on melt inclusions from the Mariana arc, back-arc and\nnorthern rift zone. As outlined in my initial review, I found that the main conclusions were well\nsupported by the data presented and my main criticism involved the length of the manuscript as well as\na few conceptual modifications.\nI think the authors have done a very nice job of addressing all my comments and suggestions. The paper\nis now much easier to read both due to shortening and tightening up of a few sections.\nI have no major issues that I think need to be addressed. I have a single comment to line 205 where\nthe authors state that the MT-18 lavas have Pb isotope compositions similar to Indian MORB. The authors\nmight add here that the radiogenic isotope composition of the mantle wedge is very commonly\nobserved to be similar to Indian MORB for almost all Western Pacific Subduction zones (Kamchatka,\nKuriles, IBM, Ryukyu). See for example the following papers:\nPearce et al J. Petrol. 40, 1579–1611 (1999)\nMcCarthy et al Geochimica et. Cosmochimica Acta 296, 170–188 (2021)\nMartynov, et al Island Arc 19, 86–104 (2010)\nShu et al Geochimica et. Cosmochimica Acta 217, 462-491 (2017)\nShu et al Nat Commun 13, 4467 (2022)\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors are to be congratulated and rewarded for their willingness and ability to revise their\npaper so thoroughly in light of reviewers’ comments. At the same time, they significantly improved\nthe quality of prose and figures in many places beyond reviewer input. Perhaps the co-authors\nengaged more this time. The result is a great improvement that could be accepted with only minor\nfurther revision. The improvements allow reviewers to focus on less central matters that could\nimprove the manuscript further.\nResponse: We appreciate your positive comments and important suggestions. We have revised them\nin our new submission.\nThe description of samples needs further improvement either in Supplemental Information or\nSupplemental Table Sheet 1. How exactly are the samples in this study related to samples and data\nin previously published papers? This is discussed in the Rebuttal but I didn’t find it in the revision.\nThe IGSN numbers also need to be inserted in Supplemental Table Sheet 1, and the new ECL should\nbe cited in the main manuscript.\nResponse: Thank you for your suggestion. We have added a description of the differences between\nthis study and the previous paper in the Supplemental Table Sheet 1. The IGSN numbers also have\nbeen inserted in Supplemental Table Sheet 1. The ECL has been cited in “Data availability” section\nin the manuscript.\nOne new idea introduced during revision is that the HIMU-like slab-derived component is restricted\nto the lithosphere and, therefore, evident only or mostly when the lithosphere melts during rifting. I\nsee no reason not to make this suggestion, and the authors could cite Pearce et al. (2005) for support\nof a lithospheric component in the NSP due to high Nb/Ta ratios there. However, conceptually, why\nwould a slab-derived HIMU component be restricted to the lithosphere? Instead, might it just be\nrestricted spatially to where HIMU seamount chains are being subducted, thereby constricting\nbackarc spreading?\nResponse: Thank you for your suggestion. We have cited this reference and added “high Nb/Ta\nratios in NSP” in line 367. The density of the lithosphere is relatively small, no melting occurs, and\nthis HIMU component can be retained in the lithosphere. In the asthenosphere, HIMU components\nmay be diluted and not present. The occurrence of HIMU components is mainly due to the\nsubduction of HIMU type seamount in NSP. However, samples from NSP-24 region do not all show\nthe presence of HIMU components, which may be mainly due to the difference of melting degree\nin the process of lithosphere decompression.\nThe new figure 1 is a vast improvement over the original. It would have saved me many hours had\nit been submitted initially. The model in Fig. 3b is much improved, and the new Fig. 5 cartoon is\nlovely although it seems odd to show such thick lithosphere (if any) beneath the Mariana Trough\nafter millions of years of spreading. L.380 even says that none needs to be present.\nResponse: Thank you for your positive comment. We have thinned the lithosphere in Figure 5b.\nThe authors missed the chance to estimate the mass fraction of melt inclusion types in their host\nrocks, allowing readers to dismiss the importance of their discovery of previously unknown melt\ncompositions in these locations.\nResponse: Thank you for your comment. Indeed, it is difficult to give an exact proportion of HIMU-\nlike melt at this stage. However, according to the data, it is clear that HIMU-like melt inclusions are\nmainly stored in olivine with high Fo values, which represents a sufficiently primitive melt\ncomponent.\nLine numbers below refer to the revised and merged document.\nl.73: Consider change to “…erupted IN THIS LOCATION just prior to…” I know of no evidence\nthat this difference occurs elsewhere.\nResponse: We have changed “… that magmas erupted just prior to initial arc rifting…” to “… that\nmagmas erupted in this location just prior to initial arc rifting…” in line 65.\nl.89: Consider inserting “remnant arc” before West Mariana Ridge, to alert readers to the tectonic\nsetting of this ridge earlier than l.120 as now. You could also replace “corresponding to” with “that\nis the conjugate pair of” to further emphasize the setting.\nResponse: Thank you for your suggestion. We have changed “These include samples collected from\nthe eastern scarp of the West Mariana Ridge (WMR) corresponding to the northernmost North\nMariana Seamount Province around 24°N” to “These include samples collected from the eastern\nscarp of the remnant arc West Mariana Ridge (WMR) that is the conjugate pair of the northernmost\nNorth Mariana Seamount Province around 24°N” in lines 80-83.\nl.109: Consider removing “as a result of propagating rifting”. This was added during revision but is\na carry-over from the original focus of the paper. The authors provide no evidence that the different\nproportions and types of subduction components in the mantle source result from rift propagation\nrather than, for example, what was subducted at different latitudes or times.\nResponse: Thank you for your suggestion. We have removed “as a result of propagating rifting” in\nthis sentence (line 100).\nl.191: Consider replacing “and” with “from” because I think you mean strong relative LREE\nenrichment; i.e., fractionation of LREEs from HREEs.\nResponse: Thank you for your suggestion. We have changed it in line 183.\nl.196: This sentence falsely implies that there are five separate fields of Pb isotope ratios in Fig. 3\nwhereas there are only three. It might be better to say that “Each of the five types of melt inclusions\ndefined above has a narrow range of Pb isotope compositions. The total range encompasses…”\nResponse: Thank you for your comment. We have changed “The 5 different groups of melt\ninclusions fall into different fields with little overlap (Fig. 3a), with a total range that\nencompasses …” to “Each of the five types of melt inclusions defined above has a narrow range of\nPb isotope compositions. The total range encompasses …” in lines 188-190.\nl.244-246. I continue to find the model of melting garnet peridotite unconvincing and the HREE\ndata suspect, but the authors’ rebuttal is reasonable. Variable subduction components, as discussed\nin the following section, seem a more likely explanation to me.\nResponse: Thank you for your comment. The description of this part only shows that partial melting\nwill not significantly change the element composition of different groups of melt inclusions\naccording to the simulation results. Therefore, the chosen melting model is not the main concern.\nl.266. Use words instead of the PEC acronym.\nResponse: Thank you for your comment. We have changed it in lines 260-261.\nl.272-273. Do you mean that the relative proportion of sediment and AOC in the subduction\ncomponent is highly variable?\nResponse: Yes, what we mean is that the proportion of sediment contribution is variable.\nL277: Consider replacing “proposed” by “discussed” because the high Ba/Nb ratios were discovered\nand discussed, not proposed, in ref. 53.\nResponse: Thank you for your suggestion. We have changed it in line 271.\nl.302: Consider replacing “these experienced…” with “these melts were products of “fractional\ncrystallization.\nResponse: Thank you for your suggestion. We have changed it in line 296.\nl.336: Remove final “s” from patterns.\nResponse: Thank you for your comment. We have changed it in line 330.\nl.352ff: Although the model is reasonable, as usual there is ambiguity whether an intermediate\nFOZO-like source component would suffice without the leverage of a HIMU extreme. However, no\nneed to revise.\nResponse: Thank you for your comment. We will consider this question.\nl.357: Should “metabasites” be “metabasalts”?\nResponse: Thank you for your comment. We have changed “metabasites” to “metabasalts” in line\n351.\nl.380: This run-on sentence needs revision. Do you mean “This may be because the back-arc basin\nspreading center is located far away from the trench, and is mainly fed by a large amount of MORB-\nlike magma. If so, then no lithospheric HIMU component is present, or it has been diluted.”\nResponse: Thank you for your suggestion. We have changed this sentence in lines 374-377.\nReviewer #2 (Remarks to the Author):\nI have had another read of the revised manuscript on melt inclusions from the Mariana arc, back-\narc and northern rift zone. As outlined in my initial review, I found that the main conclusions were\nwell supported by the data presented and my main criticism involved the length of the manuscript\nas well as a few conceptual modifications.\nI think the authors have done a very nice job of addressing all my comments and suggestions. The\npaper is now much easier to read both due to shortening and tightening up of a few sections.\nI have no major issues that I think need to be addressed. I have a single comment to line 205 where\nthe authors state that the MT-18 lavas have Pb isotope compositions similar to Indian MORB. The\nauthors might add here that the radiogenic isotope composition of the mantle wedge is very\ncommonly observed to be similar to Indian MORB for almost all Western Pacific Subduction zones\n(Kamchatka, Kuriles, IBM, Ryukyu). See for example the following papers:\nPearce et al J. Petrol. 40, 1579–1611 (1999)\nMcCarthy et al Geochimica et. Cosmochimica Acta 296, 170–188 (2021)\nMartynov, et al Island Arc 19, 86–104 (2010)\nShu et al Geochimica et. Cosmochimica Acta 217, 462-491 (2017)\nShu et al Nat Commun 13, 4467 (2022)\nResponse: Thank you for your comment. We have added this sentence “Actually, the radiogenic\nisotope composition of the mantle wedge is very commonly observed to be similar to Indian Ocean-\ntype mantle for almost all Western Pacific Subduction zones (e.g., Kamchatka, Kuriles, IBM,\nRyukyu)” in lines 197-200."}]

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