Volatile element and critical metal cycles beneath theGreenlandCraton

SupervisoryTeam

ProfAdrianFinch,SchoolofEarth&EnvironmentalSciences,UniversityofStAndrews

DrSamiMikhail,SchoolofEarth&EnvironmentalSciences,UniversityofStAndrews

DrEvaStüeken,SchoolofEarth&EnvironmentalSciences,UniversityofStAndrews

DrWillHutchison,SchoolofEarth&EnvironmentalSciences,UniversityofStAndrews

DrLydiaHallis,SchoolofGeographical&EarthSciences,UniversityofGlasgow

KeyWords

Greenland;IsotopeGeochemistry;Petrology,CriticalMetals,PlateTectonics

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Overview

Critical metals, such as Hf, Nb REE, underpin modern technologies and are mined frommantle-derivedigneousrocks.Howeverthegeologicalprocessesinthemantlethatprecedetheformationofsuchdepositsremainpoorlyconstrained.ThisprojectwilldevelopnitrogenisotopesinalkalineigneousrocksasanoveltracerofprocessesinthemantlethatprecedetheformationoforedepositsofcriticalmetalsusingGreenlandasanexample.

Itiswellknownthatisotopicsystemsinmantle-derivedsamplescanoriginatefromdistinctsources.RecentevidencefromGreenlandhasshownArchaeanisotopesignaturesinHf(Finchetal.2019)andS (Hutchisonetal.2019) isotopestrapped inmantlemelts fromthemid-ProterozoicGardar Province. These data suggest that elements, including criticalmetals,were subducted into the mantle during the Archean and have become involved in oreformation.Inotherwords,rift-relatedmagmatismintheGardarProvinceaccessedamantlesourcewhichhaspreviouslybeenenriched(‘fertilised’)bysubductedcrust (Figure1). It islikelythatsuchrecyclingprocessesfromcrusttomantleandbackupsignificantlyimprovethelikelihoodofformingacriticalmetaldeposit.Besidessheddinglightontheformationofanimportantnaturalresource,thismodelalsoimpliesthatanalysesofalkalinerocksprovidesinsightsintothedynamiccyclingofimportantelementsinthedeepEarthandthemechanicsof their concentration in the sub-continental lithospheric mantle (Mikhail et al., 2019;Hutchison et al., 2019), including the remarkable preservation of primordial isotopicsignatures(Hallisetal.,2015).

Figure 1: A schematic sulphur cycle as proposed byHutchison et al. (2019). Nitrogen can

similarlybefingerprintedandcanpotentiallydeconvolvethemanycomponentstonitrogeninalkaline igneous rocks.Suchdatamayprovideevidence forelementcycling fromthecrustthroughthemantleanditsroleincreatingcriticalmetaloredeposits.

This projectwill reconstruct the sub-lithospheric nitrogen cycle,which has a conspicuouspotentialtofingerprintvolatilesourcesfromeithersubductedsediments,theEarth’smantleor primordial (plume-related) sources. Nitrogen is important because each source has adistinct isotopic signature; crustal (𝛿15N values > 0‰ since at least 2.5Ga),mantle (𝛿15Nvaluesof-5±3‰)andprimordial(𝛿15Nvalues<-20‰),andweknowtheSCLMsamplesallthreeviathedatasourcedfrommantlediamonds(seeMikhailetal.,2014foranoverview).Nitrogenisrarelyemployedduetothedifficultyinmeasurements.However,theonestudyofalkalineigneousrocksfromKola(Dauphas&Marty1999)providedtantalisingresultsthathintedat recyclingof surfacenitrogen.TechnologicaladvancesatStAndrews, includingabespoke gas-line and mass spectrometer set-up, allow us to measure routinely Nconcentrations and isotope ratios at abundances <10 ppm. The development ofinstrumentationalongsidethedevelopmentofhypothesesatStAndrewsallowsnitrogenforthefirsttimetobeusedtoadvanceourunderstandingofgeologicalelementcyclesthroughspaceandtime.

Methodology

ThisprogrammeofresearchbuildsonrecentlypublishedworkbythePhDsupervisoryteam(e.g.Hutchisonetal.,2019)andwillexamineseveralsuitsofalkali-andcarbon-richigneousrocks to illuminate the coupling (or lack thereof) between the cycling of critical metalsalongsidecarbonandnitrogeninthedeepEarth.

Thesamplesinthisstudyrepresentarangeofmantle-derivedmeltsofdifferentagessourcedacrosstheGreenlandcraton.FurtherfieldworkinGreenlandwillbolsterthesamplecache.Thisstudentwillapplymajorandtraceelementgeochemistryalongsidestableandradiogenicisotopictracers(13C/12C,15N/14N,18O/16O,34S/32S)toestablishthemechanismsresponsiblefortheformationanddestructionofcratons,usingtheGardarprovinceasacasestudy.Thechapterbreakdownisenvisagedasdescribedbelow:

1. Samples.StAndrewshasoneofthemostcomprehensivesamplearchivesofrift-related

magmatism globally, and it has a particularly comprehensive samples suite for theGreenland craton. Nevertheless, fieldwork in the Nanortalik region of Greenland isenvisaged as part of the project to boost sample suites through collection ofpalaeoproterozoic sediments and granitoids which represent samples of Pre-Gardarsubductedmaterial.

2. Petrography,majorandtraceelementgeochemistryofthesamplesalongsidestableandradiogenic isotope data (13C/12C, 15N/14N, 18O/16O, S) will be used to ascertain thesimilaritiesanddifferencesforthesource(s)ofalkaliandcarbonatiticsamplesuitesusingstate-of-the-artlaboratorieshostedbytheStAndrewsStableIsotopeGroup(STAiG)withmethodsdescribedinMikhailetal.(2014),Hutchisonetal.(2019),Stüekenetal.(2015).Theapplicationofmultiplevolatileisotopeswillbeanovelandilluminatingapproachtostudyingalkalineigneoussystems.

3. TheprojectwillfocusinitiallyontheGardarProvinceofSouthGreenlandtoconstrainNisotopesacrossthedurationofriftingandacrosstheriftprovince.HfisotopeshintatanevolutionofthemantlesourceduringriftingwithArchaeansourcesmoreprevalentattheonsetofrifting.ExistingS,NdandHfisotopedatafortheGardarwillcomplementthenewN,C,Odatasets.Oncethishasbeenfullycharacterised,wewillmoveontoexplorethemantle signatures beneath Greenland as a function of time from 3 Ga (Tupertalikcarbonatite)to0.17Ga(Qeqertaasaqcarbonatite).

4. Collectively,thesethesischapters/paperswillallowthestudenttochart,describe,andmodel the evolution of carbon, and nitrogen isotope systems in the sub-continentallithosphericmantleunderGreenlandasafunctionofgeologicaltime.Itwillprovidekeyinsightsintoglobalelementcyclingandrolethismayhaveinfertilisingthemantlepriortotheformationofcriticalelement-richmantlemelts.

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Timeline

Year 1: Initial training in isotope ratio mass spectrometry, SEM, and LA-ICP-MS. SamplecharacterisationandfilteringusingopticalpetrographyandSEM.Summerof1styear:Fieldseasontobolstersamplesuite(SouthernGreenland).SamplesfromtheGardarProvinceofGreenland.

Year2:C-N-O-Sstableisotopeanalysisofselectedsamples(STAiG).Presentresultsatnationalmeeting(VolcanicandMagmaticStudiesGroup2021,Manchester,UK).AnalysisofmaterialfromelsewhereinGreenland.

Year3:Furtherstableisotopeworkonthenewselectedsamples(STAiG).Presentresultsatinternationalmeeting(Goldschmidt2022,Chicago,Illinois,USA).

Year3.5:Write,submit,anddefendPh.D.thesis.

Training&Skills

ThisIAPETUS2DTPprojectwillprovidetraininginpetrology,stableisotopegeochemistry,andgeochemical modelling. The focus on petrological characterization of minerals, advancedanalytical isotope ratio geochemistry, and the formation of an economically profitable

resource(diamond)willprovidethestudentaskill-settocompetitivelyacquirepostdoctoralresearchpositions, or to transition froman academic to industrial/economic career uponcompletionoftheirPh.D.degree.

Examples of analytical skills include scanning electronmicroscopy, laser-ablation ICP-MS,codingwithPython,gas-sourced isotoperatiomassspectrometry (STAiG).TrainingwillbeprovidedonacontinualbasisthroughoutthePhD.Skillsdevelopmentwillbemonitoredanddeliveredbythesupervisoryteam.

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References&FurtherReading

FinchAAetal.(2019)FromMantletoMotzfeldt:AGeneticModelforSyenite-hostedTa,Nb-mineralisation.OreGeologyReviews,107,402-416

Hallisetal.(2015)EvidenceforprimordialwaterinEarth’sdeepmantle.Science

Hutchison et al. (2019) Sulphur isotopes of alkalinemagmas unlock long-term records ofcrustalrecyclingonEarth.NatureCommunications

Mikhailetal.(2014)Constrainingtheinternalvariabilityofcarbonandnitrogenisotopesindiamonds.ChemicalGeology

Mikhailetal.(2019).Asecretivemechanicalexchangebetweenmantleandcrustalvolatilesrevealedbyheliumisotopesin13C-depleteddiamonds.GeochemicalPerspectiveLettersStüeken EE et al. (2016) The evolution of Earth's biogeochemical nitrogen cycle. Earth-Science Reviews, 160, 220-239. Stüeken EE et al. (2015) Nitrogen isotope evidence for alkaline lakes on late Archean continents. Earth and Planetary Science Letters, 411, 1-10.

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FormoreinformationcontactProf.AdrianFinch(aaf1@st-andrews.ac.uk)