Experimental studies of equilibrium iron isotope fractionation in ferric aquo–chloro complexes

16 Pages·2009·0.64 MB·English

Available online at www.sciencedirect.com
GeochimicaetCosmochimicaActa73(2009)2366–2381
www.elsevier.com/locate/gca
Experimental studies of equilibrium iron isotope fractionation in
ferric aquo–chloro complexes
Pamela S. Hilla,*, Edwin A. Schaublea, Anat Shaharb, Eric Tonuia,
Edward D. Younga,c
aDepartmentofEarthandSpaceSciences,UniversityofCalifornia,LosAngeles,CA90095,USA
bGeophyscialLaboratory,CarnegieInstitutionofWashington,WashingtonDC20015,USA
cInstituteofGeophysicsandPlanetaryPhysics,UniversityofCalifornia,LosAngeles,CA90095,USA
Received7July2008;acceptedinrevisedform22January2009;availableonline4February2009
Abstract
Herewecomparenewexperimentalstudieswiththeoreticalpredictionsofequilibriumironisotopicfractionationamong
aqueousferricchloridecomplexes(Fe(H O) 3+,FeCl(H O) 2+,FeCl (H O) +,FeCl (H O) ,andFeCl –),usingtheFe–Cl–
2 6 2 5 2 2 4 3 2 3 4
H Osystemasasimple,easily-modeledexampleofthelargervarietyofiron–ligandcompounds,suchaschlorides,sulfides,
2
simple organic acids, and siderophores. Isotopicfractionation (56Fe/54Fe) among naturally occuring iron-bearing species at
Earthsurfacetemperatures(upto 3&)isusuallyattributedtoredoxeffectsintheenvironment.However,theoreticalmod-
!
eling of reduced isotopic partition functions among iron-bearing species in solution also predicts fractionations of similar
magnitudeduetonon-redoxchangesinspeciation(i.e.,ligandbondstrengthandcoordinationnumber).Inthepresentstudy,
fractionationsaremeasuredinaseriesoflowpH([H+]=5M)solutionsofferricchloride(totalFe=0.0749mol/L)atchlo-
rinities ranging from 0.5 to 5.0mol/L. Advantage is taken of the unique solubility of FeCl – in immiscible diethyl ether to
4
createaseparatespectatorphase,usedtomonitorchangingfractionationintheaqueoussolution.D56Fe =d56Fe(total
aq-eth
Feremaining inaqueous phase) d56Fe(FeCl – inetherphase) isdetermined foreach solutionvia MC-ICPMSanalysis.
Bothexperimentsandtheoret"icalcalculatio4nsofD56Fe showadownwardtrendwithincreasingchlorinity:D56Fe
aq-eth aq-eth
isgreatestatlowchlorinity,whereFeCl (H O) +isthedominantspecies,andsmallestathighchlorinitywhereFeCl (H O) is
2 2 4 3 2 3
dominant.TheexperimentalD56Feaq-ethrangesfrom0.8&at[Cl–]=0.5Mto0.0&at[Cl–]=5.0M,adecreaseinaqueous–
etherfractionationof0.8&.ThisisveryclosetothetheoreticallypredicteddecreasesinD56Feaq-eth,whichrangefrom1.0to
0.7&, dependingonthe abinitio model.
Therateofisotopicexchangeandattainmentofequilibriumareshownusingspikedreversalexperimentsinconjunction
withthe two-phase aqueous–ethersystem. Equilibrium underthe experimental conditionsis established within30min.
Thegeneralagreementbetweentheoreticalpredictionsandexperimentalresultspointstosubstantialequilibriumisotopic
fractionationamongaqueousferricchloridecomplexesandadecreasein56Fe/54FeastheCl–/Fe3+ionratioincreases.The
effects on isotopic fractionation shown by the modeling of this simple iron–ligand system imply that ligands present in an
aqueous environment are potentially important drivers of fractionation, are indicative of possible fractionation effects due
tootherspeciationeffects(suchasiron–sulfidesystemsorironbondingwithorganicligands),andmustbeconsideredwhen
interpreting ironisotopefractionation in the geologicalrecord.
!2009Elsevier Ltd. Allrights reserved.
1. INTRODUCTION
Isotopic fractionation among iron-bearing species is
* Correspondingauthor.
found in both geological and biological systems at Earth-
E-mailaddress:[email protected](P.S.Hill).
0016-7037/$-seefrontmatter!2009ElsevierLtd.Allrightsreserved.
doi:10.1016/j.gca.2009.01.016
FeisotopefractionationexperimentswithFe–Clcomplexes 2367
surface temperatures. Large fractionations have been to study the effects of solution chemistry and iron–ligand
attributed to redox reactions between ferrous and ferric speciation on equilibrium mass-dependent iron isotopic
species. Both theoretical studies (Polyakov and Mineev, fractionation. We measured the extent of Fe-isotope frac-
2000; Schauble et al., 2001; Anbar et al., 2005; Polyakov tionation among aqueous Fe–Cl complexes relative to a
et al., 2007; Tang and Liu, 2007; Hill and Schauble, 2008) monospecific etherspectator phase,under arangeofchlo-
andexperimentalstudies(e.g.,Anbaretal.,2000;Johnson ride concentrations (0.5–5.0mol/L) in low pH solutions,
etal.,2002)havedemonstratedFe3+/Fe2+fractionationof andthencomparedourresultsagainstthetheoreticalequi-
atleast 2–3&atroom temperature. librium mass-dependent isotopic fractionations predicted
Theoretical studies also indicate that changes in bond byseveralsetsofabinitiomodelsdevelopedforthesecom-
partners (i.e., different ligands or coordination numbers) plexes byHillandSchauble (2008).
shouldresultinsignificantisotopicfractionation(Polyakov These studies demonstratea potentially significantcon-
and Mineev, 2000; Schauble et al., 2001; Polyakov et al., tributiontoironisotopesignaturesinnaturalsystemsfrom
2007; Hill and Schauble, 2008). The omnipresence of dis- thenon-redoxeffectsofiron–ligandspeciation.Further,the
solved aqueous species (e.g., chlorides, sulfides, organic li- techniques used in the present study, viz. separation of a
gands, and siderophores) and their potential influence on spectator phase using diethyl ether and the use of spiked
thesolubility,mobility,andavailabilityofiron-bearingspe- two-phase equilibrium experiments, can be used to make
cies make the isotope geochemistry of aqueous iron com- reversible measurements of both Fe3+/Fe2+ fractionation
plexesanimportant areaof study. andfractionationsassociatedwithsmall,labileinorganicli-
Chloride,aubiquitousaqueoussolute,isofinterestasa gands analogousto biological molecules. These techniques
bondingpartnerwithironbecauseofitssimilaritytoother shouldalsobeapplicabletomeasurementsofisotopicfrac-
likelyiron–ligandpartners,anditsfacilityinaqueous-phase tionation of otherheavy elements in solution.
experiments.Forexample,S2–,acommonpartnerofironin
bothnaturalandbiologicalsystems,hasanelectronicstruc- 1.1.Speciationandstructuresoftheferricchloridecomplexes
ture similar to that of Cl– and can form similar structures
withsimilar56Fe/54Fetrends(Schaubleetal.,2001;Polya- According to several spectroscopic studies (e.g., Lind,
kovetal.,2007).Ferricchloridesandsulfides/sulfatesoccur 1967; Cotton and Gibson, 1971; Sharma, 1974; Best
naturallyinsomehighlyacidicextremeterrestrialenviron- et al., 1984; Murata and Irish, 1988; Murata et al., 1989),
mentsiftheelementalcomponentsareavailable(e.g.,acidic themajorcomplexesinanacidicaqueoussolutionofferric
sulfate–chloridelakesorhotspringsassociatedwithsubaer- chloride are Fe(H O) 3+, Fe(H O) Cl2+, Fe(H O) Cl +,
2 6 2 5 2 4 2
ial volcanic activity (e.g., Delmelle and Bernard, 1994), Fe(H O) Cl 0,andFeCl –(Table1).Followingtheconven-
2 3 3 4
acidic evaporative basins or lakes (e.g., Bowen et al., tioninHillandSchauble(2008),wehaveabbreviatedthese
2007), and highly acidic mine tailings such as Spain’s Rio complexes according to the number of Fe–Cl bonds (i.e.,
Tinto(e.g.,Amilsetal.,2007)ortheRichmondmine,Iron C0=Fe(H O) 3+, C1=(Fe(H O) Cl)2+, etc.). The com-
2 6 2 5
Mountain, California (e.g., Jamieson etal., 2005)). plexes C0, C1 and C2 have octahedral geometries, while
The formation of Fe3+–Cl––H O complexes is expected C4isclearlytetrahedral(MaginiandRadnai,1979;Apted
2
to generate inter-species fractionations nearly as large as etal.,1985).ThestructureofferrictrichlorideC3islesscer-
the 3& fractionation between Fe3+(aq) and Fe2+(aq) tain: it may be either octahedral or trigonal bipyramidal
!
(Schauble et al., 2001; Hill and Schauble, 2008). However, (Lind, 1967; Bjerrum and Lukes, 1986) and was modeled
the experimental record is mixed. In low-chlorinity experi- both ways by Hill and Schauble (2008). The octahedral
ments([Cl–]60.1mol/L),noisotopiceffecthasbeenfound structuresofC2andC3weremodeledrespectivelyastrans
(Welch et al., 2003); at higher chloride concentrations (i.e.,withCl–onoppositecornersoftheoctahedron)andas
( 2MHCl)inresin/solutionpartitioningexperimentsAn- fac(i.e.,withCl–ionsonthethreecornersofonefaceofthe
!
bar et al. (2000) found a modest ( 0.1&) fractionation, octahedron).WeusedvaluesforC2astransandC3asfac
!
while Fujii et al. (2006) found fractionations from 1.6 to octahedral whencalculating ourtheoretical predications.
0.6&betweenthedominantFe3+–Cl–speciesinanaqueous There is also evidence for the existence of further com-
HCl solution and FeCl – in crown ether at [HCl]=1.6– plexes such as FeCl (H O)2– and FeCl 3– (e.g., Shamir,
4 5 2 6
3.5M. 1991; Inada and Funahashi, 1999); however, the relative
Thisstudy integrates experimental studies withtheoret- abundances of these complexes in solution are too low to
ical predictions in the aqueous ferric aquo–chloro system be considered in our experiments. Ferric hydroxide com-
Table1
Aqueousferric–chloridecomplexes.
FerricComplex Abbreviation(#Clatoms) #Fe–Clbonds Charge Structure
Fe(HO) 3+ C0 0 +3 Octahedral
2 6
FeCl(HO) 2+ C1 1 +2 Octahedral
2 5
FeCl(HO)+ C2 2 +1 Octahedral(trans)
2 2 4
FeCl(HO)0 C3 3 0 Octahedral(fac)
3 2 3
FeCl– C4 4 1 Tetrahedral
Fe(H4O) 3+NO– 0 +"2 Nitrateinouterhydrationsphere
2 6 # 3
2368 P.S.Hilletal./GeochimicaetCosmochimicaActa73(2009)2366–2381
plexes are also considered negligible due to the low pH of more C4 from the aqueous phase at low chlorinities than
the solutions ( 0); a preliminary report by Liu and Tang does isopropyl ether (Dodson et al., 1936). Nachtrieb and
!
(2006) suggests that fractionation of the ferric hydroxides Conway (1948)noted thatthe timerequired formaximum
is similar to that of C0. Nitric acid (HNO ) is used in the extractionofironintotheetherdependsappreciablyupon
3
experiments to control the pH and ionic strength of the theamountofmixingoftheetherandtheaqueoussolution;
solution while allowing for a range of chloride activities. morethan98%ofthetotalirondissolvedintheetherwas
Nitrate is believed to interact weakly with the ferric ionin extracted within 20min after gentle inversions of the solu-
the outer hydration sphere as Fe(H O) 3+.NO – (Horne tion container. We gave each of our ether–aqueous mix-
2 6 3
etal.,1964;MorrisandSturgess,1969);preliminaryabini- tures a vigorous shake and then let them equilibrate for
tiomodelingbyHillandSchauble(2008)indicatesthatthe at least 40min before proceeding, so that the ether and
reduced partition function ratio of Fe(H O) 3+.NO – aqueous phases would have a chance to separate after the
2 6 3
should besimilar to, orslightlysmaller than,that of C0. initial emulsification.
Speciationmodelingoftheaqueousferricchloridecom- Thereisaslightdissolutionofhydrochloricacid(i.e.,the
plexes (this work and others such as Rabinowitch and aqueous phase) into the ether phase upon addition of the
Stockmayer, 1942; Bjerrum and Lukes, 1986; Bethke, ether.Dodsonetal.(1936)reportedadecreaseintheaque-
1996; Lee et al., 2003) indicates that the initial concentra- ous volume 62% with the addition of an equal amount of
tion of chloride, the acidity, and the ionic strength of the ether. For a concentration of 0.0794M Fe aq, the 2%
solution determine the relative concentration of each Fe– reduction in volume results in an increased Fe concentra-
Cl complex. As the Cl–/ Fe3+ ratio increases, the relative tionequalto0.0810MFe,notenoughtoinfluencethelevel
abundances of species with more Fe–Cl bonds increases. of accuracy of our experiments. We ran a control experi-
Thedesignofourexperimentsmakesuseofthesechanging ment with [Cl–]=0.0M to ensure that no measurable
relative abundances. amountofaqueousironwasextractedintotheetherphase
without the presence ofHCl (seeSection 3).
1.2.Ether withdissolved FeCl –as spectatorphase
4
1.3. Abinitio models ofequilibrium mass-dependent Fe
We took advantage of the selective solubility of isotope fractionation in Fe–Cl species
C4 (FeCl –) in organic solvents at low pH to create a
4
separable spectator phase. The use of ether to extract Four different ab initio models of equilibrium mass-
FeCl –fromasolutionofferricchloridedissolvedinhydro- dependent Fe isotopic fractionation among the ferric
4
chloricacidisanoldtechnique,developedbyJ.W.Rothein chloride complexes, (from Hill and Schauble, 2008), are
thelate1800s(NachtriebandConway,1948).Becauseether used to generate theoretical predictions for the aque-
andwaterareimmiscible,theethersolutioncontainingthe ous–ether fractionations resulting from our experiments.
dissolvedC4formsaseparatephase,whichrestsontopof The models employ Unrestricted Hartree Fock (UHF)
the aqueousphase. and hybrid Density Functional Theory (DFT), B3LYP
A monospecific ether phase can easily be equilibrated (Becke, 1993), methods paired with different basis sets.
withanaqueousferricchloridesolution,andquantitatively The UHF model is paired with the 6-31G(d) basis set
separatedfromthatsolutionwithoutdisturbingtheoverall (Rassolov et al., 1998), and the hybrid DFT models, with
equilibrium of the two-phase solution. This procedure is basis sets 6-31G(d), 6-311G(d) (Frisch et al., 2004), and
analogoustotheuseofCO gastomonitortheoxygen-iso- Ahlrich’s VTZ (Scha¨fer et al., 1992). 6-31G(d) is a double
2
topefractionationbehaviorofaqueoussolutionsofvarying split-valence basis set; 6-311G(d), a triple split-valence ba-
chemistries(e.g.,O’NeilandTruesdell,1991).Theexistence sis set; and Ahlrich’s VTZ, a triple zeta basis set. The
ofatwo-phasesystemalsofacilitatesreversalexperiments, four models are abbreviated: (1) UHF/6-31G(d); (2)
as described below. B3LYP/6-31G(d); (3) B3LYP/6-311G(d); and (4)
Ether extraction of a spectator phase has advantages B3LYP/VTZ. The latter model uses Ahlrich’s VTZ for
over precipitation methods (e.g., Johnson et al., 2002) or Fe and Cl, and basis set 6-31G(d) for O and H. The
columnelutionmethods(e.g.,Anbaretal.,2000;Mathews models are in vacuo, i.e., modeled with only an inner
et al., 2001). Once the system comes to equilibrium, one hydration sphere as gas phase molecules.
neednotworryaboutkineticeffectsorfurtherfractionation The reduced partition function ratios (b) of the four
duringthesamplingprocess.Conversely,bothprecipitation models for each complex are listed in Table 2 as 1000 lnb
methods and column elution methods involve potential b =56Fe/54FereducedpartitionfunctionratioofAXrel-
AX
fractionation in order to separate the phases being com- ativetoadissociatedFeatomX,i.e.,ironvapor.1000lnb
pared, the effects of which must be subtracted from the valuesare in parts permil (&).
measured fractionation. The isotopic fractionations of each complex Ci relative
Earlystudiesshowedthatthe amountof ironextracted to C4 (denoted as a ) are shown in Fig. 1 where
(Ci C4)
by the ether phase (most likely as FeCl .H O.nH O) in- 1000 lna =1000" lnb 1000 lnb (i.e.,
creasesastheconcentrationofironinthe4aci3dsolut2ionin- a b(Ci"/Cb4) ). The frac(tCioi)"nation facto(rC4)between
(Ci C4)= (Ci) (C4)
creasesandastheamountofHClinthesolutionincreases each"complex and C4 decreases as the number of Fe–Cl
(Dodsonetal.,1936;Sandell,1944;NachtriebandFryxell, bonds increases (Table 3). The ranges of fractionation be-
1948). We chose diethyl ether overthe less volatile isopro- tween C0 and C4 predicted by the models are 1.5–
!
pyletherasasolvent forC4becausediethyl etherextracts 2.5&.
FeisotopefractionationexperimentswithFe–Clcomplexes 2369
Table2
Equilibrium Fe isotopic fractionation (1000 lnb) for the Fe–Cl
complexesat293.15K(20"C)fromfouroftheabinitiomodelsof
HillandSchauble(2008).
Complex UHF/6- B3LYP/ B3LYP/ B3LYP/
31G(d) 631G(d) 6311G(d) VTZ
C0 9.73 8.93 9.41 9.59
C1 9.12 8.04 8.54 8.57
C2 8.73 7.61 8.1 8.13
C3a 7.74 7.05 7.13 7.13
C4 8.25 7.46 7.17 7.14
a C3modeledasFeCl(HO) (facoctahedralstructure).
3 2 3
Fig.2. Separationofetherandaqueousphasesinaferricchloride
solution.Theetherphaseisonthetop.
2.1.4.). In each series the chlorinity was varied from 0.5
to 5.0M by the addition of HCl. Total acidity was held
constant ([H+]=5mol/L), as well as the initial ionic
strength ( 5 – see Section 3.2 on speciation), by adding
!
appropriate amounts of HNO . All measurements were
3
madewithanEppendorfTMauto-pipette.Thesolutionswere
gentlyshakenandthenallowedtorestfor10mininasep-
aration funnel.
2.1.2. Etherphase
Anequalvolumeofimmisciblediethyletherwasadded
to the aqueous solution. The ether–aqueous mixture was
briskly shaken, and then allowed to rest for at least
40min. The ether almost immediately formed a separate,
lessdense,liquidphaseontopoftheaqueousphase.Dur-
ingthistime,aportionofFeCl –passedfromtheaqueous
Fig. 1. Predicted Fe isotopic fractionation of ferric chloride 4
complexes (Ci) relative to C4 expressed as 1000 lna where 1000 phaseintotheetherphase,turningtheetherphaseapale–
lna (Ci C4)=1000 lnb (Ci)–1000 lnb (C4) (Hill and Schauble, yellow color. A sharp boundary between the aqueous and
"
2008).(ComparewithTables2and3.) ether phases was visible almost immediately (Fig. 2). No
second ether phase was seen; the formation of a second
phase, mentioned by Dodson et al. (1936), appears to re-
quirechlorinityP7M.SomeFeCl –isalsoexpectedtore-
4
Table3 mainin the aqueousphase.
PredictedFractionationofC0RelativetoC4expressedas1000lna When the ether–aqueous mixture is shaken, there is a
wSchhearueb1le0,02000ln8)a.(C(S0"eeC4F)i=g.110.0)0 ln b(C0)" 1000 lnb(C4) (Hill and tferonmdenthceytfoopraofvtehryessempaalrlaatimonoufnutnnoeflt,hdeumetixotuthreetinocersecaaspede
UHF/ B3LYP/ B3LYP/ B3LYP/ vaporpressureoftheether.Lossofsolutionwasavoidedby
6-31G(d) 6-31G(d) 6-311G(d) VTZ gentlereleaseandthenreinsertionofthefunnelcap.Volu-
metricethermeasurementsweremadewitha25mlgradu-
1000lna(C0 C4) 1.48 1.47 2.24 2.45
" atedglass cylinder.
2.1.3. Separationofphases
2.METHODS After the designated equilibrium time had elapsed in
each experiment, separate aliquots of the ether (1ml) and
2.1.Fractionation experiment procedures aqueous(0.25ml)phaseswereremovedimmediatelybydis-
posablepipettefromthetopofthefunnelandthroughthe
2.1.1.Aqueous phase stopcockatthebottomofthefunnel,respectively,toSavil-
Series of aqueous ferric chloride solutions (20ml each) lexTM beakers for isotopic analysis. One milliliter of 1M
were created from stock solutions of crystalline ferric ni- HCl was added to each ether sample to ensure dissolution
trate Fe(NO ) .9(H O), ([Fe]=0.0794mol/L). The ferric ofallironastheetherevaporated.Thesamplesweredehy-
33 2
nitrate crystals were ultra pure (>99.9%) in order to avoid drated on a hot plate (100"C for aqueous samples; 50"C
contamination by Cr in the MC-ICPMS (See Section forethersamples)toremoveanyremainingHCl,andthen
2370 P.S.Hilletal./GeochimicaetCosmochimicaActa73(2009)2366–2381
rehydrated with dilute nitric acid. Further dilutions were intensity of the color depends on the concentration of
madewith2%nitricacidasnecessarytomatchtheconcen- Fe2+. The reducing agent hydroxylamine hydrochloride
tration of the Fe isotope standard used for mass was added to the samples to reduce all ferric species. Ten
spectrometry. timestherequiredamountsofreducingagentandofferro-
Afewmillilitersofaqueoussolutionwereallowedtorun zinewereaddedtothesampletoensurethatallferriciron
through the stopcock before samples were gathered for reacted.
analysis. Theaqueoussample wasthengathered in a glass Maximum absorbance for iron occurs between pH val-
beaker, from which measurements were made with an ues of 4 and 9 at a wavelength of 562nm (Stookey,
auto-pipette. 1970).Weaddedanaceticacid–ammoniumhydroxidebuf-
Thevolatilityofethercausesproblemswithanauto-pip- ferofpH 5.7tothesamplestoensurethecorrectrangeof
!
ette.Therefore,measurementsoftheethersolutionwereta- pHvalues.Reducer,ferrozine,andbufferwerepremixedin
ken with a disposable pipette with gradations at 0.25ml. a1:2:4ratioandthenaddedtothesamplestofacilitatethe
We found that the ether in the disposable pipette has less measurement process.
tendency to shoot out prematurely if the pipette is held Sincetheabsorbencyofsamplesdriftsslightlyovertime,
vertically. each sample with added ferrozine, reducer, and buffer was
allowedtorestauniformtenminutesbeforemeasurement
2.1.4.Isotopic measurements in the spectrophotometer.
The Fe isotope ratios 56Fe/54Fe and 57Fe/54Fe of both Themolarabsorptivityconstantforourspectrophotom-
theaqueousphaseandtheetherphaseforeachsamplewere eter,determinedempiricallybymeasurementofaliquotsof
measured on the UCLA Thermo Finnegan Neptune MC- knownstandards,is26107±1441,inreasonableagreement
ICPMS (Multiple Collector Inductively Coupled Plasma with measurements by other workers (e.g., 27,000 molar
Mass Spectrometer). The instrument has a fixed array of absorbtivity, Stookey, 1970). Samples were diluted to
nine Faraday collectors with 1011X amplifiers. The mass [Fe]= 40lM(absorbancy= 1)via2to3separatedilu-
! !
spectrometer was operated at a mass resolving power of tionsteps,usinganEppendorfTMauto-pipette.Thefirstdilu-
12,000 (M/dm) to eliminate ArO+, ArOH+, and ArN+ tion(s)werewithdeionizedwater;thelastwiththereducer–
m!assinterferencesatcardinalmasses56,57and54,respec- ferrozine–buffermixture.Allaqueoussampleswerediluted
tively(e.g.,WeyerandSchwieters,2003).Solutionswerein- and measured independentlyatleast threetimes.
spected for 52Cr+ to monitor possible interference from Measuring the iron concentration in the ether phase
54Cr+.Inallcases,Crwasfoundtobebelowdetectionlev- posedpotentialproblemsbecausetheetherevaporatesrap-
els. Potential instrumental mass bias due to inter-element idly, leaving behind an unknown volume of solution. To
matrix effects was not an issue since samples consisted of avoidthisproblem,weremoved3mlofethersolutionfrom
pureFe.Sampleswererunindryplasmausingadesolvat- the separation funnel to a 10ml graduated glass cylinder
ingnebulizer(CetacAridus)at 1ppmFein 2%HNO . with a disposable pipette. We immediately added 1ml of
Ironisotoperatioswerecorr!ectedforinstru!mentalfrac3- 1M HCl to the cylinder and then set the cylinder aside
tionation using standard-sample-standard bracketing. Iso- for a few days, lightly covered with ParafilmTM, until all
tope ratios are reported as d56Fe and d57Fe values relative the ether had evaporated. Deionized water was added to
to astandard: the remaining liquid to return the total volume to 3ml.
The resultant iron solution concentration, with ether re-
iFe=54Fe moved and with a known dilution factor, was then mea-
diFe¼103 ð iFe=54FÞeSample"1! ð1Þ sured with the spectrophotometer as described above.
ð ÞStd Eachsamplewasdilutedandmeasuredthreetimesindepen-
dently.However,measurementsofetherealironhavelarger
where i represents either 56 or 57, Std represents the in-
errors than aqueous iron measurements due to the addi-
house standard, and Sample signifies the sample. In this
tionalmeasurementerrorfromtheuseofbothadisposable
study all Fe isotope ratios are reported relative to
pipette and a graduated cylinder in the first steps of the
the working standard Spex-1; the isotopic composition
measurement process. All three sets of separate dilution
of this standard is related to the IRMM14
measurements for the ethereal ironsample will include the
iron standard by the experimentally determined
d56Fe(IRMM14)=0.99979 (d56Fe(Spex-1)) 0.20845. same error resulting from the graduated cylinder and dis-
" posablepipette.Therefore,allthreeetherealmeasurements
IronisotopefractionationbetweenaqueousFeandethe-
real FeCl – is expressed as d56Fe =d56Fe (total Fe willhavethesamecomponentofthisbiasintheirresultant
inaqueou4sphase) d56Fe (FeC(laq–-eitnh)etherphaasqe).Values measurement errors.
forD56Fe we"redeteertmhinedfo4raqueous–etherpairsof Two other measuring techniques for the ether phase
eachmola(arqit-eytho)f Cl–. were also tried: (1) A known amount of ether phase was
added immediately to a premeasured amount of reducer–
buffer–ferrozine mix and covered with parafilm for 10min
2.1.5.Fe concentration measurements
before going to the spectrophotometer. (2) A known
The concentration of total iron in both the ether and
amount of ether phase with added 1M HCl was put into
aqueous phases for each sample was measured with a
GENESYS20spectrophotometerusingtheferrozinemeth-
od(Stookey,1970;Vollieretal.,2000).Ferrozineinteracts
with ferrous iron to create a purple–hued complex; the 1 Standarderroronesigma;arelativestandarderrorof 0.5%.
!
FeisotopefractionationexperimentswithFe–Clcomplexes 2371
aPetridishandpartiallycoveredwiththePetridishlidto sive 54FeCl –) was added to the ether of the normal - iso-
4
hasten evaporation of the ether (due to a larger surface tope forward experiment, creating the reversal
area). Then an additional measured amount of 1M HCl experiment. Thus, the reversal experiment began at time
was added to the dish and gently swirled to pick up any t=0 with 20ml aqueous phase and 30ml ether phase.
remnants of Fe. However, measurements from these two We measured the rate and progress of the reversal experi-
methodswerenotasreproducibleasthegraduatedcylinder ment toward equilibrium by removing small aliquots from
method describedabove. both the ether and aqueous phases after 20, 30 and
40min (without disturbing the mixtures) and measuring
2.2.Reversal experiments the aqueous–etherisotopicfractionationateachtimestep.
In order to quantify progress towards equilibrium, we
Reversal experiments demonstrate equilibrium by run- usedthethree-isotopeexchangemethodtobracketequilib-
ning the same reaction in opposite directions (forward rium(Matsuhisaetal.,1978;Mathewsetal.,1983;Shahar
andreverse),therebybracketingtheequilibriumpointfrom etal.,2008).Thismethodisbasedontheexponentialrela-
two different paths. In the forward experiment, the direc- tionshipbetweentheequilibriummass-dependentfraction-
tion of iron transport is from the aqueous phase into the ation factors ab=a and ac=a between phases i and j for three
i j i j
ether phase as some FeCl –(aq) dissolves into the ether differentisotope"sa,b,an"dcofagivenelement(Matsuhisa
4
phase et al.,1978; Young etal.,2002).Inthiscase,
3 a56=54 a57=54 c 6
n 0FeClnðH2OÞ6"n3"nðaqÞþFeCl4"ðaqÞ c¼ð1¼=mð54"1Þ=m56Þ=ð1=m54"1=m57Þ ðð7ÞÞ
X¼
3 and 103ln a56=54 c103ln a57=54 8
)n 0FeClnðH2OÞ6"n3"nðaqÞþFeCl4"ðaqÞþFeCl4"ðethÞ where atomic ðmasseÞs¼mi areð: m54Þ=53.93961, m56ð=Þ
X¼
2 55.93494, and m57 =56.93540amu (Baum et al., 2002).
ð Þ Using these values the exponent c=0.678. In d notation
Thiscreatesanisotopicfractionationbetweentheaque- the above equilibrium relationship between the three iso-
ousandetherphases,themagnitudeofwhichisdependent topes becomes
uponthe chlorinity of thesolution.
In the reversal experiment, the direction of iron trans- d56Fe ¼ ð1000 þ d56FerefÞ½ð1000 þ d57FeÞ
portisfromtheetherphaseintotheaqueousphase.Ether = 1000 d57Fe c 1000 9
containingspikedC4(i.e.,54FeCl –)isaddedtotheetherof ð þ refÞ) " ð Þ
4
a forward, isotopically normal experiment, which has al- where ref refers to any point on the above fractionation
ready achieved equilibrium. This forces isotopic transport line.Inthree-Fe-isotopespace(d56Fevs.d57Fe)Eq.(9)de-
ofspikedC4fromtheetherphaseintotheaqueousphase, fines a slightly curved, primary (terrestrial) mass fraction-
where speciation and isotopic exchange with the aqueous ation line, along which naturally occurring, iron-bearing
iron occur. Further exchange will take place between the terrestrial samples lie (Matsuhisa et al., 1978; Young
etherandtheaqueousphasesuntilanisotopicequilibrium et al.,2002).
isreached. Atthestartofthereversalexperiment,whenthespiked
etherisaddedtotheetheroftheunspikedforwardexperi-
3 ment,theresultingethermixtureliesofftheterrestrialmass
54FeCl"4ðethÞ) 54FeCl4"ðaqÞ) n 054FeClnðH2OÞ6"n3"nðaqÞ fractionation line (TFL)due to excess54Fe (Fig. 3).Isoto-
þ54FeCl4"ðaqÞ X¼ ð3Þ pkiecdexacqhuaenoguesbepthwaeseentwhiellspeiskteadbleisthherepqhuailsiberaiunmdthaelounngspia-
3
54FeCl4"ðaqÞþn 056FeClnðH2OÞ6"n3"nðaqÞ()56FeCl4"ðaqÞ stheceonfidrsatry(Ffirga.ct3io).naTthioenploinseiti(oSnFLin),tpharerae-lliesolttoop,ebusptabceeloowf
3 X¼ the new SFL will pass through the bulk composition of
þn 054FeClnðH2OÞ6"n3"nðaqÞ ð4Þ tehteal.m,1ix9t8u3r)e.,Thsiegnloificaedtionasofdt5h6Feeb0ulakncdomdp57oFsiet0ion(Minat3h-eiswos-
X¼
54FeCl"4ðethÞ þ56FeCl"4ðaqÞ()56FeCl"4ðethÞ tope spaceisdefined byisotopic mass balancesuch that
þ54FeCl"4ðaqÞ ð5Þ ð56Fe=54FeÞ0¼ð56Fe=54Þaq*ðfÞ* ½Feaq)=½Fe0)
Ifequilibriumisattainedintheseexperiments,theresul- þð56Fe=54FeÞeth*ð1"fÞ!*ð½Feeth)=½F"e0)Þ ð10Þ
tant fractionation between the aqueous and ether phases
and Fe f Fe 1 f Fe 11
shouldbethesameastheinterphasefractionationofafor- ½ 0)¼ *½ aq)þð " Þ*½ eth) ð Þ
wardexperimentofthesamechlorinity,acidityandnitrate Note 56Fe=54Fe ; 56Fe=54 ,and 56Fe=54Fe arethe
ð Þ0 ð Þaq ð Þeth
concentration. isotopicratiosofthebulkcompositiontheaqueousphase,
TheetherwithspikedC4forthereversalexperimentwas and the ether phase respectively; f is the fractional volume
prepared via a forward experiment prepared with aqueous oftheaqueousphasetothetotalvolume;and[Feaq],[Feeth],
54Fe spike. Both the spiked and normal solutions had and[Fe0]aretheconcentrationsofFeintheaqueousphase,
[Cl–]=1.0M. Once each mixture equilibrated, half the the ether phase, and the entire solution in moles/liter
ether(10ml)fromthespikedexperiment(containingexces- (Faure, 2005).
2372 P.S.Hilletal./GeochimicaetCosmochimicaActa73(2009)2366–2381
of d56Fe) on the SFL should ideally be equal to the
D56Fe of theforward experiment.
aq eth
Equi"librium is thus demonstrated in two ways: (1) Fe
isotopic composition in both the ether and the aqueous
phases of the reversal experiment moves to the same mass
fractionation line, and 2) the D56Fe values of both
(aq eth)
theforwardandthereversalexperiment"sarethesamewith-
in analytical uncertainties.
Fe concentrations of each phase of the reversal experi-
mentweremeasuredat40min(inordertominimizedisrup-
tionofthereversalexperiment)andareusedincalculating
the bulk isotopic composition ofthe mixture.
3. RESULTS
3.1. Dissolution ofFeCl –intothe ether phase
4
Fig.3. Schematicdiagramofprogressionofareversalexperiment
toward equilibrium in three-isotope space. At time=0, the Thepercentageoftotalironextractedfromtheaqueous
aqueousphase(toprightbluehexagon)sitsontheterrestrialmass phase by dissolution into the diethyl ether phase increases
fractionation line (TFL) while the ether phase, with excess
as the chlorinity of the solution increases, changing the
54FeCl–, rests below the TFL (red triangle in lower left corner).
4 ether from a pale yellow to a deep amber color. In our
Asthemixtureequilibratesisotopically,bothphasesmovetoward
experiments,inwhichbothpHandionicstrengthareheld
equilibriumonthesecondarymassfractionationline(SFL)(lower
roughly constant ( 0 and 5, respectively), the ethereal
line), each along its own mixing line. The ether phase moves up ! !
along the lower (red) line while the aqueous phase moves down iron climbs from 0.0% at 0.0 M Cl" to >95% at 5.0M
alongtheupper(blue)lineinthedirectionofthearrows.TheSFL (Table 4and Fig. 4).
mustpassthroughthebulkcompositionofthetwophases,shown Whenacidity,ionicstrength,andchlorinityincreaseto-
bythegreendiamond.Equilibriumisachievedwhenbothphases gether, as a function of increasing concentrations of HCl,
lieontheSFL.Thedistancebetweenthetwophasesonceonthe the amount of initial iron dissolved into diethyl ether in-
SFL(intermsofd56Fe)isD56Feaq eth.(Forinterpretationofthe creasesmoreslowly(Dodsonetal.,1936).Apparentlydis-
rthefeerweenbcevsetrosiocnolooufrtshiisnptahpeefir.g)urele"gend,thereaderisreferredto solutionofFeCl4"increasesasbothmoreH+andCl"are
madeavailable because theironismost likelyextracted as
An analogous relationship holds for 57Fe. d56Fe and stoichiometric HFeCl4" (i.e., FeCl4.H3O.nH2O see section
o
d57Fe are thencalculatedfrom Eq.(1). 1.2).Dodsonetal.’setherealironconcentrationsareshown
o
in Fig. 4 for comparison.
Asisotopicexchangeprogressestowardequilibrium,the
Mass balance calculations of aqueous and ether iron
etherphasewillmoveupamixinglinetowardapositionon
concentration measurements are on average within 8% of
the SFL while the aqueous phase will travel down its own
thetotalinitialiron(comparablewiththetotaluncertainty
mixing line toward itsposition on the same mass fraction-
obtainedbypropagationofmeasurementerror–seeTable
ationline(Fig.3).Equilibriumisreachedwhenbothphases
4). This is similar to mass balance molarity measurements
arrive at the new equilibrium mass fractionation line (i.e.,
by Nachtrieb and Conway (1948). Measurement errors of
the SFL). The distance between the two phases (in terms
Table4
Divisionoftotalironbetweenaqueousandetherphasesfordifferentchlorinities.Eachconcentrationwasmeasuredindependentlyatleast
threetimes.Reportederrorsarestandarderror1randaretheoriginalmeasurementerrorspropagatedthrough2–4separatedilutionsteps.
Unitsaremol/liter.TotalFeisthecalculatedconcentrationoftheoriginalironsolutionbeforetheadditionofether.FeaqandFeethare
measuredquantities.Feaqerrorsresultfrommeasurementwithanauto-pipettewhileFeetherrorscontainmeasurementserrorsfroman
auto-pipette,adisposablepipette,anda10mlgraduatedcylinder(seeSection2.1.5).(SeeFig.4.)
Cl– TotalFe Feaq Feeth Feaq+Feeth Feeth/(Feaq+Feeth) Feaq/(Feaq+Feeth)
0.0 0.0794 0.079±0.0003 0.000±0.0000 0.079±0.0003 0.0000 1.0000
0.5 0.0794 0.078±0.0003 0.003±0.0001 0.080±0.0003 0.0359 0.9641
0.5 0.0794 0.073±0.0007 0.001±0.0000 0.074±0.0007 0.0129 0.9871
1 0.0794 0.085±0.0001 0.005±0.0001 0.090±0.0001 0.0546 0.9454
1 0.0794 0.068±0.0011 0.008±0.0002 0.075±0.0011 0.1032 0.8968
1.5 0.0794 0.057±0.0000 0.029±0.0008 0.086±0.0008 0.3386 0.6614
1.75 0.0794 0.042±0.0001 0.035±0.0008 0.076±0.0008 0.4524 0.5476
3 0.0794 0.018±0.0000 0.074±0.0018 0.092±0.0018 0.8017 0.1983
3 0.0794 0.019±0.0000 0.066±0.0016 0.085±0.0016 0.7742 0.2258
3 0.0794 0.019±0.0002 0.065±0.0039 0.084±0.0039 0.7748 0.2252
4 0.0794 0.012±0.0001 0.073±0.0019 0.085±0.0019 0.8616 0.1384
5 0.0794 0.004±0.0002 0.086±0.0023 0.090±0.0023 0.9513 0.0487
5 0.0794 0.004±0.0001 0.079±0.0042 0.083±0.0042 0.9555 0.0445
FeisotopefractionationexperimentswithFe–Clcomplexes 2373
etherealironconcentrationingeneralarelargerthanthose
ofaqueousironduetothevolatilityoftheetherandtodif-
ferences in the measurement process (see Section 2.1.5 for
further error discussion). Measurements may also be
slightlyaffectedbyminutechangeinvolumeduetothedis-
solution of some HCl (i.e., the aqueous phase) into the
etherphase (see Section1.2).
3.2. Speciationof FeClcomplexes with varyingchlorinities
The relative abundances of the aqueous ferric chloride
complexes depend on the amount of available chloride,
theacidity,andtheionicstrengthoftheaqueoussolution.
Asthechlorinitytoironratioincreases,Fe–Clspecieswith
Fig.4. Percentageoftotalinitialironextractedintoetherphasefor more Fe3+–Cl" bonds become more plentiful (Fig. 5 and
Table 5). According to our speciation models, with pH
variouschlorinities.Theamountofetherealironfromthisworkis
0andionicstrength 5,C2isthemostabundantcomplex
comparedtoexperimentsfromDodsonetal.(1936)inwhichboth
! !
[H+] and [Cl–] were increased together (as [HCl]); in our experi- between0.5and3Mchlorinity,whileC3ismostabundant
ments[H+]=5Mforall[Cl–].ThegreaterdissolutionofFeinto from 3.2 to 5M chlorinities. C4 abundances are initially
!
theetherphaseatlowerchlorinitiesinourexperimentsshowsthat low but increase continuously with rising chlorinity. C0
[H+]isalimitingfactorsincetheetherealironisprobablydissolved and Fe(H2O)63+.NO3" and, to a lesser extent, C1, are
asstoichiometricHFeCl4(seeSection1.2).(SeeTable4.) important at [Cl"]=.5–1M,but decease rapidly at higher
[Cl"].
Our models for the ferric chloride complexes at each
chlorideconcentrationwerecalculatedusingtheFeClasso-
ciation equilibrium constants for C1, C2, C3 and C4 (30,
4.5,0.15and0.0078respectively)fromBjerrumandLukes
(1986), the FeNO 2+ association equilibrium constant (10)
3
from Delany and Lundeen (1990), and the activity models
forHClandHNO fromBromley(1973).Calculationswere
3
iterated to determine final ionic strength and species con-
centrations.Ateachchlorinity,theamountofaqueousiron
(i.e.,theamountofironleftintheaqueousphaseafterdis-
solution of some iron into the ether phase – see Table 4)
was used as the total aqueous iron concentration. Since
FeCl4" is dissolved into the ether phase as stoichiometric
HFeCl (FeCl .H O+.nH O see Section 1.2), the initial
4 4 3 2
aqueous chloride concentration was decreased by four
times the amount of ethereal iron while the initial
aqueous H+ concentration was decreased by the amount
of ethereal iron. (For example, at [Cl"]=3M and
[Fe ]=0.0794M, [Fe ]=0.017M so the amounts of
total aq
Fe3+, Cl" and H+ sequestered from the aqueous to the
aFvqaigruy.eion5ug.s[MCploh–l]ae,sreefrlaaaftctitevireontdosistsohofelutttohioteanlinaomdfiovsiuodnmutaeolfFFiereCo–nlC–rleimcnotamoinpitnlhegexeines,ththaeert [eHtheethr+]p=ha0se.06aMre.[FTeheuths],=[C0.l0aq6"M]=, 2[C.9l7et6h"M]=a0n.d024[HMaq+a]n=d
4 4.94M). These small changes in aqueous concentrations
phase.(SeeTable5.)
Table5
CalculatedmolefractionsofeachFe–Clcomplex,forvaryingchlorinities,relativetothetotalmolesofaqueousiron,afterequilibrationwith
the ether phase. Ave. Fe aq is calculated from the average percentage of Fe aq (Table 4, column 7) based on an initial total calculated
Fe=.0794M.(SeeFig.5.)
AveFeaq(M) Cl" Fe(H2O)63+ FeCl(H2O)52+ FeCl2(H2O)4+ FeCl3(H2O)3 FeCl4– Fe(H2O)6#NO32+
0.077 0.5 0.005 0.130 0.543 0.076 0.001 0.246
0.073 1.0 0.001 0.074 0.664 0.199 0.003 0.058
0.053 1.5 0.001 0.047 0.636 0.289 0.007 0.021
0.043 1.75 0.000 0.038 0.612 0.327 0.009 0.014
0.017 3.0 0.000 0.017 0.492 0.465 0.023 0.002
0.011 4.0 0.000 0.011 0.417 0.536 0.036 0.001
0.004 5.0 0.000 0.007 0.359 0.584 0.049 0.000
2374 P.S.Hilletal./GeochimicaetCosmochimicaActa73(2009)2366–2381
of Fe, Cl and H did not have a noticeable impact on
speciation.
Somespeciationmodelsofferricchloridecomplexesvs.
chlorinity are shown in which chlorinity is increased by
adding increasing amounts of HCl (e.g., Roe et al., 2003;
Fujiietal.,2006).Inthesemodels,incontrasttoourmodel,
pH decreases and the ionic strength increases in the aque-
ous solution as chlorinity increases, resulting in a slightly
differentdistributionofferricchloridecomplexabundances
(discussedfurther in Section4.5).
The presence of nitrate ions in the aqueous solution at
lowchlorinitiesinfluencesthespeciationofironbycausing
formationofFe(H2O)63+#NO3"inadditiontoFe(H2O)63+.
In our experiments the NO3" ion is most abundant at the
verylowestchlorinities,sincetheamountofnitricacidnec- Fig. 6. Experimental data for D56Feaq eth=d56Fe (total Fe
essary to keep the pH constant decreases as more HCl is remaining in aqueous phase)"d56Fe (FeCl"4– in ether phase) (&).
(pH 0,T=20"C).(SeeTable6.)
added. !
Complexation of the nitrate ion with some of the iron
sequesterspart of the ironthat mightotherwise have gone
intoironchloridespecies,butthisapparentlyhasnosignif-
icant effect on the relative abundances of the non-nitrate
complexes northe dissolution of Feinto the ether.
3.3.Experimental fractionation betweenaqueousandether
phaseswith increasing chlorinity
The measured fractionation between the aqueous and
ether phases, defined as D56Fe =d56Fe (total Fe
(aq eth)
remaining in aqueous phase)"d5"6Fe (FeCl4" in ether
phase), shows a clear downward trend with the increasing
chlorinity (Table 6 and Fig. 6). At [Cl"]=0.5M,
D56Feaq eth=0.8&; at 5.0M, 0.0&. The d56Fe and d57Fe
experim"entalvaluesplotalongtheterrestrialmassfraction- Fig.7. Experimentalvaluesofd56Fe(&)andd57Fe(&)pairsfrom
ationline,ademonstrationofmassdependentfractionation D56Feaq eth (&) plotted along the terrestrial mass fractionation
anda lackof Crinterference (Section 2.1.4)(Fig. 7). line. Err"or bars represent 1-sigma standard error. The error bars
IsotopicFemassbalancecalculationsonaqueous–ether aresmallerthantheplottedsymbolsonthegraph.(SeeTable6.)
pairswerewithin±0.05&oftheinitialferricnitratestock
D56Fe 1000 lnb X
solution. aq"eth ¼ ð FeCliÞð iaqÞ
i
X
1000 lnb 12
3.4.Predicted D56Fe fromthe abinitio fractionation " FeCl"4ether ð Þ
aq-eth
models whereXiisthefractionofFeincomplexi(relativetototal
Feintheaqueoussolution)andb isthereducedparti-
PredictionsofD56Fe usingthefourabinitiomod- tion function ratio for the ith FeCFleCcolimplex. This assumes
aq eth
els werecalculated as " there is no fractionation between C4aq and C4eth when C4
Table6
Measuredfractionation(in&)betweenaqueousandetherphases,definedasD56Fe(aq eth)=d56Fe(totalFeremaininginaqueousphase)–
d56Fe (FeCl4– in ether phase), versus chlorinity. d56Fe and d57Fe for both ether and a"queous phasesare shown, expressedrelative to the
inhousestandardFeSpex-1(seeSection2.1.4).Standarderrorsarefor1sigma.Theetherresttimeisthetimeinminutesbetweentheaddition
ofethertotheaqueoussolutionandtheremovalofaqueousandethersamplesforisotopicmeasurement.(SeeFig.6.)
Exp’t Cl D56Fe(aq eth) Aqd56Fe Etherd56Fe Aqd57Fe Etherd57Fe Etherresttime(min)
"
FN30 0.5 0.817±0.011 0.611±0.013 0.206±0.007 0.882±0.020 0.323±0.023 41
" "
FN23 0.5 0.793±0.011 0.509±0.011 0.284±0.016 0.777±0.017 0.402±0.023 45
" "
FN31 1 0.614±0.010 0.614±0.003 0.000±0.011 0.909±0.017 0.005±0.024 45
"
FN20 1 0.540±0.012 0.644±0.016 0.104±0.010 0.978±0.015 0.130±0.016 42
FM32 3 0.186±0.016 0.717±0.004 0.531±0.017 1.051±0.010 0.747±0.013 41
FN21 3 0.365±0.008 0.868±0.011 0.503±0.008 1.280±0.017 0.684±0.008 42
FN33 5 0.014±0.017 0.679±0.018 0.666±0.006 0.956±0.015 0.918±0.016 45
FN22 5 0.011±0.003 0.553±0.011 0.542±0.011 0.837±0.015 0.836±0.011 45
FeisotopefractionationexperimentswithFe–Clcomplexes 2375
the forward and the reversal experiments achieved similar
values of D56Feaq-eth: 0.828±0.0415& (1 sigma std err)
and 0.783±0.0249&, respectively (Table 8 and Fig. 9).
Second, both the aqueous and ether phases of the reversal
experimenthavearrivedatthesameequilibriummassfrac-
tionationline(SFL–seesection2.2)inthree-isotopespace
in time t, where 20<t630min. On a plot of d57Fe vs.
d56Fe, at the start of the reversal experiment (t=0), the
unspiked aqueous phase and the spiked ether phase are
on different mass fractionation lines. At t=20min, both
phases have moved much closer; the ether phase has
reached 99.8%2 of its equilibrium d56Fe , D56Fe at
eth aq eth
20minisat97.4%2ofitsequilibriumvalue,whilethe"aque-
Fig.8. Comparisonofabinitiomodelpredictionsandexperimental ousphasehasachieved71%2ofitsfinald56Feat20min(see
dataforD56Feaq ethvs.chlorinity.Thesolid(red)lineisalogarithmic Section4.4).At30min,bothphasesareonthesameSFL.
bestfittotheex"perimentaldata;bestfitequationsforexperimental Thelocationofthebulkcompositionoftheequilibrated
dataandmodelpredictionsarelistedinTable9.(SeeTables6and7.) mixturein3-isotopespace,ascalculatedfromthemeasure-
(Forinterpretationofthereferencestocoloursinthefigurelegend, ments of Fe concentrations and delta values at t=40min
thereaderisreferredtothewebversionofthispaper.) (Table 8), also lies on the SFL as it should (see Section
2.2). (d57Fe, d56Fe (bulk composition))=( 3.50±.02&,
"
Table7 3.70±.02&, std err1 sigma) (Fig. 9).
PredictionsofD56Feaq eth(&)foreachchlorinity,calculatedfrom "
the fractional concen"trations of each Fe–Cl complex and the 4. DISCUSSION
correspondingabinitio1000lnbfactors.Thepredicteddifferences
inD56Feaq eth(&)for[Cl–]=0.5and5.0Mareshowninthelast 4.1. Fractionation amongtheFe–Cl complexes
row.(SeeF"ig.8.)
Cl"(M) UHF/ B3LYP/ B3LYP/ B3LYP/ Themostsignificantfeatureofboththetheoreticaland
6-31G(d) 6-31G(d) 6-311G(d) VTZ theexperimentalD56Fe dataistheprominentsmooth
aq eth
0.5 0.71 0.49 1.24 1.34 decreaseinaqueous–ether"isotopicfractionationwithrising
1.0 0.37 0.15 0.84 0.91 chlorinity(Fig.8).Experimentsshowadecreaseinfraction-
1.5 0.23 0.04 0.69 0.75 ationfrom0.8&at0.5MClto0.0&for5.0MCl.Thethe-
1.8 0.18 0.00 0.64 0.69
oretical models predict a decrease of 0.7 to 1.0 &,
3.0 0.02 0.10 0.47 0.51
" dependingonthedetailsoftheabinitiomethod(Table7).
4.0 0.06 0.15 0.38 0.42
" " Thecloseagreementoftheexperimentalresultswiththe-
5.0 0.12 0.18 0.32 0.36
D56Feaq eth(Cl= "0.82 "0.68 0.92 0.98 oreticalpredictionsstronglysupportsthepresenceofnota-
0.5M") D56 ble fractionation resulting from speciation and/or
Fe ("Cl=5.0M) complexing in the ferric iron–chloride–water system. This
aq eth
" is significant for two reasons: (1) it is of similar order of
magnitude to Fe3+/Fe2+ fractionation; and (2) it implies
thatligandspresentinanaqueousenvironmentarepoten-
dissolves into the ether. All four ab initio models and the
tially important drivers of fractionation, and must also be
experimentalresultsdisplayasimilardecreaseintheaque-
consideredwheninterpretingironfractionationingeologi-
ous–ether fractionation with increasing chlorinity (Fig. 8,
cal records.
Tables 6 and 7). Eq. (12) is derived from the following
Thereductioninaqueous–etherphasefractionationwith
relationships.
increasing chlorinity is most simply explained by the
D56Fe d56Fe d56Fe 13 changes in aqueoussolution chemistry.As chlorinity rises,
aq"eth ¼ AX " BX ð Þ
I000lna d56Fe d56Fe 14 species with higher Cl/Fe ratios become more abundant,
ðAX"BXÞ! AX " BX ð Þ thus decreasing the overall isotopic fractionation of the
b
aðAX"BXÞ ¼ bABXX"XX ð15Þ a(sqeueeoFuisgsp.ha1searnedlat5iv).e tTohiCs4t(rFenedCl4t"o)wianrdthereedtuhcetriopnhaosef
"
1000 ln aðAX"BXÞ ¼ 1000 lnbAX"X "1000 lnbBX"X ð16Þ 56Fe/54Feoftheaqueoussolutionduetosolutionchemistry
should also be apparent in fractionation between the solu-
Preliminarycalculationsindicatethatthereducedparti-
tion function ratio for Fe(H O) 3+.NO – is similar to that tion and any associated minerals (see Hill and Schauble,
ofC0;hence,Fe(H O) 3+.NO2 –6wastre3atedasC0(seeHill 2008).
2 6 3 Acomparison ofthetheoretical modelswiththe exper-
andSchauble, 2008).
imentaldatashowsthattheyallhavethesamequasi-loga-
3.5.Reversal experiments
2 We define the % equilibrium value of X=100 (X
@20min-
demTohnestartattaeidnminenttwoof weqauyisli.bFriiurmst,waitthtihnirtthyirmtyinmutinesutbesotihs DX5@6F0meianq)/(etXh.@30min"X@0min) where X is d56Feeth, d56Feaq, or
"