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Formation of low-T hydrated silicates in modern microbialites from Mexico and implications for microbial fossilization

Abstract : Microbialitesareorgano-sedimentaryrocksfoundinabundancethroughoutthegeologicalrecordbackto∼3.5Ga.Interpretationsofthebiologicalandenvironmentalconditionsunderwhichtheyformedrelyoncomparisonswithmodernmicrobialites.Therefore,abettercharacterizationofdiversemodernmicrobialitesiscrucialtoimprovesuchinterpretations.Here,westudiedmodernmicrobialitesfromthreeMexicanalkalinecraterlakes:Quechulac,LaPreciosaandAtexcac.Thegeochemicalanalysesofwatersolutionsshowedthattheyweresupersaturatedtovaryingextentswithseveralmineralphases,includingaragonite,calcite,hydromagnesite,aswellashydratedMg-silicates.Consistently,X-raydiffraction(XRD)andFouriertransforminfraredspectroscopy(FTIR)analysesrevealedthatmicrobialitesarecomposedofadiversityofmineralphasesincludingaragoniteandsometimescalcite,hydromagnesite,andmoreinterestingly,apoorly-crystallinehydratedsilicatephase.Couplingofscanningelectronmicroscopy(SEM)withenergydispersiveX-rayspectrometrymicroanalysesonpolishedsectionsshowedthatthislatterphaseisabundant,authigenic,magnesium-richandsometimesassociatedwithironandmanganese.Thismineralphaseissimilartokerolite,ahydratedpoorlycrystallinetalc-likephase(Mg3Si4O10(OH)2·nH2O).Diversemicrofossilswerepermineralizedbythissilicatephase.Someofthemwereimagedin3DbyFIB-tomographyshowingthattheirmorphologywasexquisitelypreserveddowntothefewnm-scale.ThestructuralandchemicalfeaturesofthesefossilswerefurtherstudiedusingacombinationoftransmissionelectronmicroscopyandscanningtransmissionX-raymicroscopy(STXM)atthecarbonandmagnesiumK-edgesandironL2,3-edges.Theseresultsshowedthatorganiccarbonispervasivelyassociatedwithkerolite.Overall,itissuggestedthatthepoorly-crystallinehydratedmagnesium-richsilicateformsinmanyalkalinelakesandhasastrongpotentialforfossilizationofmicrobesandorganicmatter.Moreover,thefrequentoccurrenceofsuchanauthigenicsilicatephaseinmodernlacustrinemicrobialitescallsforareappraisalofitspotentialpresenceinancientrocks.
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Nina Zeyen, Karim Benzerara, Jinhua Li, Alexis Groleau, Etienne Balan, et al.. Formation of low-T hydrated silicates in modern microbialites from Mexico and implications for microbial fossilization. Frontiers in Earth Science, Frontiers Media, 2015, 3, pp.64. ⟨10.3389/feart.2015.00064⟩. ⟨hal-01226254⟩

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