Alexander, B. W., Bau, M., Andersson, P., and Dulski, P., 2008. Continentally-derived solutes in ...

17 Pages·2007·0.5 MB·English

Available online at www.sciencedirect.com
GeochimicaetCosmochimicaActa72(2008)378–394
www.elsevier.com/locate/gca
Continentally-derived solutes in shallow Archean seawater:
Rare earth element and Nd isotope evidence in iron
formation from the 2.9 Ga Pongola Supergroup, South Africa
Brian W. Alexander a,*, Michael Bau a, Per Andersson b, Peter Dulski c
aEarthandSpaceSciences,CampusRing1,ResearchIII102,JacobsUniversityBremen,D-28759Bremen,Germany
bLaboratoryforIsotopeGeology,SwedishMuseumofNaturalHistory,SE-10405Stockholm,Sweden
cGeoForschungsZentrum,D-14473Potsdam,Germany
Received27November2006;acceptedinrevisedform25October2007;availableonline17November2007
Abstract
ThechemicalcompositionofsurfacewaterinthephoticzoneofthePrecambrianoceanisalmostexclusivelyknownfrom
studiesofstromatoliticcarbonates,whilebandedironformations(IFs)haveprovidedinformationonthecompositionofdee-
perwaters.HerewediscussthetraceelementandNdisotopegeochemistryofveryshallow-waterIFfromthePongolaSuper-
group,SouthAfrica,togainabetterunderstandingofsolutesourcestoMesoarcheanshallowcoastalseawater.ThePongola
SupergroupformedonthestablemarginoftheKaapvaalcraton(cid:2)2.9Gaagoandcontainsbandedironformations(IFs)that
representtheoldestdocumentedSuperior-typeironformations.TheIFsarenear-shore,purechemicalsediments,andshale-
normalizedrareearthandyttriumdistributions(REY )exhibitpositiveLa ,Gd ,andY anomalies,whicharetypical
SN SN SN SN
featuresofmarinewatersthroughouttheArcheanandProterozoic.Themarineoriginofthesesamplesisfurthersupported
by super-chondritic Y/Ho ratios (average Y/Ho=42). Relative to older Isua IFs (3.7Ga) from Greenland, and younger
Kuruman IFs (2.5Ga) also from South Africa, the Pongola IFs are depleted in heavy rare earth elements (HREE), and
appeartorecordvariationsinsolutefluxesrelatedtosealevelriseandfall.Sm–Ndisotopeswereusedtoidentifypotential
sedimentandsolutesourceswithinpongolashalesandIFs.The(cid:2) (t)forPongolashalesrangesfrom(cid:3)2.7to(cid:3)4.2,and(cid:2) (t)
Nd Nd
values for the coeval iron-formation samples (range (cid:3)1.9 to (cid:3)4.3) are generally indistinguishable from those of the shales,
althoughtwoIFsamplesdisplay(cid:2) (t)aslowas(cid:3)8.1and(cid:3)10.9.ThesimilarityinNdisotopesignaturesbetweentheshale
Nd
andiron-formationsuggeststhatmantle-derivedREYwerenotasignificantNdsourcewithinthePongoladepositionalenvi-
ronment,thoughthepresenceofpositiveEuanomaliesintheIFsamplesindicatesthathigh-Thydrothermalinputdidcon-
tribute to their REY signature. Isotopic mass balance calculations indicate that most (P72%) of the Nd in these seawater
precipitateswasderivedfromcontinentalsources.IfpreviousmodelsofFe–NddistributionsinArcheanIFsareapplied,then
the Pongola IFs suggest that continental fluxes of Fe to Archean seawater were significantly greater than are generally
considered.
(cid:2)2007Elsevier Ltd. Allrights reserved.
1. INTRODUCTION ably reflect seawater composition. Such studies have
examined sedimentary rocks and minerals which form di-
Studies to determine the characteristics of seawater in rectly from seawater solutions, including inorganic precip-
the geologic past are dependent upon proxies which reli- itates (e.g., evaporites) or biologically mediated
precipitates (e.g., CaCO ). Attempts to identify reliable
3
proxies for paleo-seawater rely on chemical tracers, par-
* Correspondingauthor.
ticularly elements with known and predictable behavior
E-mail address: [email protected] (B.W.
in modern seawater and modern seawater precipitates.
Alexander).
0016-7037/$-seefrontmatter (cid:2)2007ElsevierLtd.Allrightsreserved.
doi:10.1016/j.gca.2007.10.028
Ndisotopesin2.9GaArcheansurfaceseawater 379
These elemental tracers must also be relatively unaffected TwobroadtypesofIFaregenerallyrecognized:theAlg-
by geologic processes which commonly affect marine pre- omatypewhichisoftenfoundingreenstonebelts,andthe
cipitates, such as diagenesis and metamorphism. One Superiortypeassociatedwithstablesedimentarybasinsand
such group of chemical tracers are the rare earth ele- cratonicmargins(Gross,1983).Whenconsideringthemass
ments and yttrium (REY), which have been used as sea- of sediment deposited, Superior-type IFs contain far more
water proxies in a variety of sedimentary rocks of known Fe than Algoma-type IFs (James and Trendall, 1982;
marine origin, including limestones, phosphorites, and Klemm, 2000), although this may represent a preservation
banded iron formations. The usefulness of the REYs as biasduetotheirproposedtectonicsettings.Theassociation
seawater proxies has been discussed extensively by Bau ofSuperior-typeIFswithshelfdepositssuchasquartzites,
and Dulski (1996), Webb and Kamber (2000), Shields carbonates, andshalesisconsistent withtheirdepositional
and Stille (2001), Nothdurft et al. (2004), Shields and model, and Superior-type IFs are particularly useful for
Webb (2004), and Bolhar et al. (2004). These workers constrainingtraceelementdistributionsinshallowArchean
have concluded that REY distributions in marine chemi- seawater.
cal sediments can accurately reflect the REY distribution The Kaapvaal craton in South Africa is widely recog-
of the seawater from which they formed, and that this nizedasoneoftheearlieststablecratonsformed(Tankard
relationship can be reliably extended to sediments from etal.,1982),andhoststheoldestknownSuperior-typeIFs
throughout the geologic record. intheworldwithinthe(cid:2)3.0GaWitwatersrandandPongo-
The information provided by the REYs regarding sea- la Supergroups. Work by Beukes and Cairncross (1991)
watercompositionisafunctionoftheprocesseswhichcon- indicates that some units within both the Witwatersrand
troltheirdistributioninseawater.Inmodernmarinewaters andthePongolaarecorrelative,however,thePongoladif-
thevariationinREYdistributionsisdominatedbyvarying fers most significantly from the Witwatersrand in that it
degrees of carbonate complexation, with REY principally experiencedlowerdegreesofmetamorphismandstructural
sourced from weathering of continents, as suggested by deformation. The age, depositional environment, and the
theisotopiccompositionofNdinmodernseawater(Piepg- relatively pristine preservation of the Pongola Supergroup
ras and Wasserburg, 1980). High-temperature hydrother- provide unique opportunities for the investigation of Ar-
mal inputs (e.g., black smoker fluids) are generally chean marine chemical sediments deposited in relatively
negligible REY sources to modern seawater, though this shallow waters. Rare earth and yttrium distributions and
may not have been the case in Earth’s early oceans, a sce- Sm–Nd isotope systematics in Pongola Supergroup IFs
nario assessed using Eu, which is enriched in high-temper- indicate significant continentally-derived solute fluxes to
ature fluids emanating from basaltic, mid-ocean ridge shallow Archean seawater.
sources (MORB, Michard, 1989; Bau and Dulski, 1999).
This approach, coupled with Nd isotope systematics, has 2. GEOLOGIC SETTING
been extended to Archean banded iron formations (e.g.,
Derry and Jacobsen, 1990; Danielson et al., 1992), in at- The 2.9–3.0Ga Pongola Supergroup is located in east-
tempts to constrain continental- versus MORB-derived Fe ern South Africa and southwestern Swaziland and crops
inputs to Earth’s early oceans (Miller and O’Nions, 1985; out over a 270-km by100-km area(Weilers, 1990; Fig. 1).
Jacobsen and Pimentel-Klose, 1988a). Therefore, the The extent of the Pongola Supergroup is consistent with a
REY are particularly well suited not only for identifying minimum depositional area of 32,500km2 (Button et al.,
Archeanseawaterproxies,butalsoforofferinginsightinto 1981), and the sequence as a whole consists of two strati-
the importance of mid-ocean ridgesolute fluxes relative to graphicunits;theNsuzeGroupandtheoverlyingMozaan
solutes sourced from continentalweathering. Group. Sedimentary structures within Nsuze siliciclastic
Numerous workers have used Precambrian iron forma- beds such as lenticular/flaser bedding and herringbone
tions to study ancient seawater indirectly (e.g., Fryer, cross-lamination (vonBrunnandHobday, 1976)andstro-
1977;JacobsenandPimentel-Klose,1988a;DerryandJac- matolitebearingNsuzecarbonates(MasonandvonBrunn,
obsen, 1990; Shimizu et al., 1990; Towe, 1991; Bau and 1977; Beukes and Lowe, 1989) indicate a near-shore shal-
Mo¨ller, 1993; among others). The similarity of REY pat- low water depositional environment (see also Matthews,
ternsbetweenmodernseawaterandbothArcheanironfor- 1967; von BrunnandMason, 1977;Tankardet al.,1982).
mationsandcarbonateshasbeennoted,andthesechemical ThisstudyfocusesonsamplesfromtheSinqeniForma-
sediments have been used to constrain Archean seawater tionoftheMozaanGroup,whichoutcropswithintheWit
composition (e.g., Bau and Dulski, 1996; Kamber and Mfolozi inlier (Fig. 1). The Sinqeni Formation is the most
Webb, 2001; Bolhar et al., 2004). Though certain features laterally extensive formation within the Mozaan Group
are common for many Precambrian IFs (e.g., alternating (Nhleko, 2003) and is dominated by quartz arenite and
bands of Si- and Fe-rich oxide layers), variations are ob- shalewithminorconglomerateandbandedironformation
served in chemical composition and mineralogy and are (Matthews, 1967; Beukes and Cairncross, 1991). Specifi-
generallyattributedtodistinctdifferencesintectonicsetting cally, the iron formation is hosted within the Ijzermijn
(e.g.,Gross,1983),orfacieschangeswithinasingledeposi- Member,anapproximately15-m-thickunitwhichhasshale
tional basin (Klein and Beukes, 1989). As such, Precam- at the base overlying with a gradational contact coarse
brian IFs formed in different marine environments may sandstoneoftheolderDipkaMember.TheIjzermijnshale
reflectpotentialvariationsinseawatercompositionbetween gradesupwardinto3-to5-m-thickironformationinterca-
theseenvironments. lated with shale, followed again by shale which is capped
380 B.W.Alexanderetal./GeochimicaetCosmochimicaActa72(2008)378–394
30o 31o 32o
25o
SWAZILAND MOZAMBIQUE
REPUBLIC
OF
SOUTHAFRICA
27o
LESOTHO
INDIAN
OCEAN 30o
30o PONGOLA
REPUBLIC SUPER GROUP
OF LITHOLOGY AND SAMPLE POSITIONS
SOUTHAFRICA shale 19
18
28o conglomerate
13x
WHITE MFOLOZI INLIER
11x
iron formation
12x
10x
shale 8
7x
6x
iron formation
5x
Mozaan Group INDIAN 29o 4x
Nsuze Group OCEAN shale 2
0 50 km
30o 31o 32o conglomerate
# = sample number (see caption for details)
Fig. 1. Map of study area showing extent of Pongola Supergroup, location of White Mfolozi inlier, and relative positions of sampling
locationswithinasimplifiedsequencestratigraphy.Thexfollowingsomesamplenumbersindicatesthattwosampleswerecollected,and
numberedaccordingly(e.g.,4xreferstosamples41and42).Thicknessofthelithologicunitsisnottoscale.
with a sharp erosional contact formed by supermature unconformityandasaseriesofsillsintheMozaanGroup
orthoquartzite (Nhleko, 2003). Sedimentary structures (Button et al., 1981). Granitic plutons emplaced at 2.5–
found within the Mozaan Group are similar to those de- 2.7Ga bound the Pongola Supergroup primarily along its
scribed in the Nsuze Group, and Beukes and Cairncross eastern margins, resulting in narrow contact aureoles with
(1991) recognized only two major depositional environ- relatively little deformation of Pongola strata (Button
mentsfortheMozaan:fluvialbraidplainandshallowmar- et al., 1981; Tankard et al., 1982). Mineral assemblages
ineshelfsystems.StudiesoftheMozaanGroup(Watchorn, withinthePongolagenerallyreflectgreenschistfaciesregio-
1980; Weilers, 1990) describe the IFs as being deposited nal metamorphism (Tankard et al., 1982; Hegner et al.,
withinadistalshelfenvironment,conclusionsthatarecon- 1984; Hunter and Wilson, 1988; Beukes and Cairncross,
sistentwiththedetailed analysisofBeukesandCairncross 1991).
(1991),whocharacterizedtheMozaanIFsasformingona
shallowstarvedoutercontinentalshelfduringthepeakofa 3.SAMPLING AND ANALYTICAL METHODS
marinetransgression.
Age constraints for the Mozaan are not directly avail- Fourshalesamplesand16ironformationsampleswere
able from units within the group, though it appears that collectedfromtheSinqeniFormationintheWhiteMfolozi
deposition occurred between 2940 and 2870Ma (Beukes Inlier (Fig. 1).TheshaleandIFbedsareintercalated,and
andCairncross,1991).ThisisinferredfromaU–Pbzircon areboundedatthetopandbottombybedsofsmallpebble
ageof 2940±22Maforrhyolite withintheNsuzeGroup, conglomerate.Desiccationcracksarecommonintheclastic
and a 2871±30Ma Sm–Nd mineral isochron (Hegner units immediately above and below the sequence studied,
etal.,1984)forpyroxenitefromthepost-Pongolaintrusive indicating that IF deposition was preceded and followed
Usushwana Complex. Following deposition, the Pongola byrelatively lowerseawaterlevels. Fieldsampling avoided
Supergroup was intruded by differentiated gabbroic sills obvious fault zones and alteration features, and fresh
(the Usushwana Complex), primarily along the basal unweathered samples were subsequently crushed and ana-
Ndisotopesin2.9GaArcheansurfaceseawater 381
lysed at the GeoForschungsZentrum, Potsdam, Germany. (3Pr(cid:3)2Nd), Ce/Ce*=Ce/(2Pr(cid:3)Nd) Eu/Eu*=Eu/
MajorelementconcentrationswereobtainedbyX-rayfluo- (0.67Sm+0.33Tb),andGd/Gd*=Gd/(0.33Sm+0.67Tb).
rescence (XRF), and mineral phase determinations were
obtained by X-ray diffraction (XRD). Trace element con- 4.2. Mozaan shales
centrations were determined with a Perkin-Elmer/Sciex
Elan Model 5000 inductively coupled plasma mass spec- Of the four shale samples (WM2, WM8, WM18, and
trometer (ICP-MS) following the procedures of Dulski WM19), three bound the bottom (WM2) and top (WM18
(2001). Samarium and Nd concentrations were also deter- and WM19) of the section studied, and differ from shale
minedusingthermalionizationmassspectrometry(TIMS). WM8, which was sampled between two adjacent bands of
Agreement between TIMS and ICPMS data is better than IFandpossessesdistinctlydifferentgeochemicalcharacter-
2% for Nd (except shale WM2, 3.7%), and better than 4% istics (discussed in detail below). Samples WM2, WM18,
for Sm (except shale WM2 and IF WM41, 6.1% and and WM19 exhibit Fe, Mn, Al, K, and Rb concentrations
6.0%,respectively).Iron-formationsampleWM121isatyp- whichcomparewellwithpublisheddataforMozaanshales
icalisdisplayingdifferencesof7.2%and10.8%inrespective andpelites(McLennanetal.,1983;Wronkiewicz,1989;see
Nd and Sm concentrations as determined by TIMS and Table1).SamplesWM2,WM18,andWM19appeartorep-
ICPMS, yet Sm/Nd for WM121 differs by only 2.8% be- resent typical clastic sediment derived from the source re-
tweenthetwoanalyticalmethods,andSm/Ndcomparisons gion for the Mozaan shales, and therefore it is
between TIMS and ICPMS datanever exceeds 5.3%, indi- appropriate to normalize data from the IF samples to the
cating that consistent REE ratios were determined regard- average of these three shale samples (hereafter indicated
less of the method used. Accuracy for other ICPMS withsubscriptWMS).ThisapproachofcomparingArche-
analyses is conservatively estimated to be ±10%, and rare an marine chemical sediments (IFs) to contemporaneous
earth element ratios are estimated to be accurate within clasticsedimentisnotnovel,andhasbeenusedbyprevious
±5%. Samples for Sm–Nd isotope determinations were workers(e.g.,Fryer,1977;Barrettetal.,1988).WhileREE
spiked using a mixed 147Sm/150Nd spike, and processed at concentrationsintheMozaanshaleaverageWMSaregen-
the Laboratory for Isotope Geology, Swedish Museum of erally higher than PAAS by (cid:2)30%, the PAAS-normalized
Natural History. Samarium and Nd were separated using pattern ofWMSisflat(Sm/Yb=0.97),exceptforaslight
ion exchange techniques, and isotopic ratios were deter- enrichment in Eu, a common feature in Archean shales
minedwithafivecollectorFinniganMAT261TIMS.Total (Taylorand McLennan, 1985).
analytical blank for Nd was approximately 40pg, and Nd
was analysed as NdO+ in multidynamic mode, with the 4.3. Mozaan ironformations
data being reduced assuming exponential fractionation
and normalized to 146Nd/144Nd=0.7219. External preci- Element concentrationsfor 15ofthe 16ironformation
sionwasobtainedbyrepeatedanalysesoftheLaJollastan- samples are typical for Archean and Paleoproterozic IFs,
dardwith143Nd/144Nd=0.511853±0.000025(2r,n=10). with Si and Fe dominating the major element abundances
Samariumwasanalyzedasanoxideinstaticmodeandnor- (Table 1). The geochemistry of sample WM42 is unique
malized assuming 149Sm/152Sm=0.51686. Uncertainties in in that it contains >1% Al O and almost 15% MnO and
2 3
TIMSdeterminationofSmandNdconcentrationsareesti- will be discussed in detail below. Unless otherwise noted,
matedto about<1% and2%, respectively. results refer only to the remaining 15 IF samples. Norma-
tive quartz and iron-oxides combined represent 96–99%
4.RESULTS (wt.) of the samples, with Fe ranging from 10% to 81%.
Aluminium, Ca, Mg, Ti, and P are present only in trace
4.1.Normalization ofREY andcalculation ofREY amounts, and Na and K are absent or not measurable.
anomalies Other than Si and Fe, only Mn is present in appreciable
amounts of the major elements studied, and averages
In this study, normalized REY data will refer to Post (cid:2)1%. The mineralogy of the iron oxide component for all
Archean Average Shale (PAAS, subscript SN, McLennan, samples varies between magnetite and hematite, with the
1989), C1 chondrite (subscript CN, Anders and Grevesse, relativeproportion ofmagnetite increasingwithincreasing
1989),orlocalArcheanshale(discussedbelow).Rareearth Fe content. The absence of chlorite or secondary minerals
elementswhichexhibitanomalousbehaviorwithrespectto indicatesthatthesampleshaveexperiencedlittlealteration
normalized data have those anomalies quantified as or weathering. Immobile trace element concentrations in
[REE ]/½REE (cid:4)(cid:5), where [REE] is the measured concentra- theIFsamplesarelow(<10ppm,Table2),consistentwith
X X
tionofrareearthelementX,and½REE (cid:4)(cid:5)istheconcentra- the Al contentof the samples.
X
tion of element X predicted by extrapolation or Distributions of the REY in all samples are generally
interpolationfromadjacentREEs.Quantifyingthemagni- sub-parallel from the LREEs to the middle rare earth ele-
tudeoftheseanomaliesinREYdistributionsisoftencom- ments (MREEs). The REY distributions are not similar,
plicated because adjacent REYs may themselves display however, betweenthemiddleandtheheavyrare earthele-
anomalous behavior, as is the case with redox sensitive ments, and the patterns display two distinctly different
Ce and Eu. Normalized REY anomalies will be quantified trends (Fig. 2). Eight of the samples have relatively high
following the methods of Bau and Dulski (1996) and Bol- (Dy/Yb) (average 1.93) and generally higher RREY,
WMS
haret al. (2004), and are defined as follows: La/La*=La/ whereas (Dy/Yb) of the remaining eight samples are
WMS
3
8
2
Table1 B
Majorelementdataforironformationandshales,withmineralphasedeterminationsforIFsamples .W
.
Iron-formationsamples Shalesamplesa A
le
x
WM41 WM42 WM51 WM52 WM61 WM62 WM71 WM72 WM101 WM102 WM111 WM112 WM121 WM122 WM131 WM132 WM2 WM8 WM18 WM19 peliteb a
n
d
XRF(wt%) er
SiO2 75.4 26.2 75.7 16.8 19.7 20.7 58.8 70.8 80.9 78.4 80.3 81.0 85.8 88.0 84.4 83.6 51.6 47.9 49.1 48.6 58 eta
Al2O3 0.13 1.22 0.18 0.07 0.67 0.39 0.21 0.02 0.52 0.22 nd nd 0.06 0.06 0.34 0.10 23.3 11.9 24.7 26.8 18.8 l.
/
Fe2O3 22.2 57.0 21.0 81.0 76.4 76.3 37.5 26.1 15.2 17.3 17.7 18.1 12.0 10.4 14.7 15.0 10.5 29.0 9.16 6.55 9.4 G
MnO 0.92 14.5 1.85 0.34 1.54 2.50 1.93 2.39 2.11 2.57 1.59 1.27 1.14 1.30 0.35 0.63 0.12 0.43 0.13 0.11 0.27 eo
MgO 0.12 0.25 0.26 0.13 0.20 0.22 0.23 0.27 0.33 0.37 0.30 0.26 0.21 0.19 0.13 0.13 3.01 3.57 3.65 3.18 1.68 ch
CaO 0.15 0.18 0.18 0.18 0.11 0.17 0.24 0.17 0.14 0.21 0.16 0.18 0.14 0.12 0.11 0.11 <0.14 0.30 <0.14 <0.14 0.08 im
ic
NaO nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 0.43 <0.11 0.15 <0.11 0.28 a
2 e
KO nd nd nd nd nd 0.01 nd nd 0.02 0.01 nd nd nd nd nd nd 4.07 <0.05 6.54 7.97 5.4 t
2 C
TiO2 0.02 0.08 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.02 0.90 0.72 1.35 1.21 0.82 os
PO 0.02 0.08 0.03 0.05 0.05 0.05 0.02 0.02 0.04 0.04 0.03 0.03 0.03 0.02 0.02 0.03 0.07 0.05 0.08 0.04 0.09 m
2 5 o
LOI nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 5.01 5.17 5.15 4.96 4.5 ch
im
Total 98.96 99.51 99.22 98.59 98.69 100.36 98.95 99.79 99.28 99.14 100.10 100.86 99.40 100.11 100.07 99.62 99.01 99.04 100.01 99.42 99.32 ic
a
XRD(wt%) A
c
Quartz 87.0 85.0 88.0 37.0 39.0 39.0 80.0 92.0 94.0 94.0 92.0 91.0 95.0 96.0 87.0 88.0 ta
7
Magnetite Trace Trace 7.0 46.0 39.0 38.0 9.0 3.0 3.0 3.0 5.0 7.0 2.0 1.0 nd nd 2
Hematite 13.0 15.0 5.0 17.0 22.0 22.0 12.0 5.0 3.0 4.0 4.0 2.0 4.0 3.0 13.0 12.0 (2
0
Chlorite nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd nd 08
)
3
nd,notdetected. 7
8
a AverageofshalesamplesWM2,WM18,andWM19usedasrepresentativeMozaanshale(WMS). –3
b Averageof15pelitesamplesfromMozaanGroup(datafromWronkiewicz,1989). 94
Table2
Traceelementconcentrationsiniron-formationandshalesamples(mg/kg)
Iron-formationsamples Shalesamples
WM41 WM42 WM51 WM52 WM61 WM62 WM71 WM72 WM101 WM102 WM111 WM112 WM121 WM122 WM131 WM132 WM2 WM8 WM18 WM19 pelitea
Rb <0.5 4.9 <0.5 <0.5 1.0 1.5 <0.5 <0.5 2.2 1.2 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 125 0.98 236 263 235
Sr 1.9 139 25.5 6.3 10.4 20.2 13.8 13.1 20.8 34.2 20.6 19.8 8.8 7.6 7.9 13.8 49.4 7.6 16.3 13.0 28
Y 1.35 10.5 1.15 2.51 3.43 3.61 1.12 1.11 1.66 1.56 1.22 1.12 0.73 0.72 1.22 4.82 40.4 10.3 29.2 19.3 36
Zr <1 29.0 <1 1.3 1.5 2.1 1.7 1.3 <1 <1 <1 <1 <1 <1 <1 <1 208 145 239 207 270
Cs 0.189 0.513 0.084 0.063 0.069 0.085 0.035 0.027 0.118 0.142 0.396 0.425 0.127 0.118 0.282 0.318 3.24 0.146 5.84 8.05 9.3
Ba <2 14.3 5.4 3.2 10.6 49.5 11.8 13.1 42.5 34.4 14.4 6.1 2.5 2.1 13.7 23.8 331 39.4 919 991 510
N
La 0.538 7.12 0.482 1.73 2.33 2.27 1.01 0.706 0.897 0.971 0.625 0.531 0.370 0.298 0.631 0.782 34.3 17.6 95.2 42.3 38 d
Ce 1.08 12.9 0.905 3.01 4.04 3.93 1.63 1.19 1.65 1.47 1.13 0.988 0.509 0.415 1.24 1.21 67.9 27.9 181 82.5 80 iso
Pr 0.134 1.58 0.118 0.385 0.481 0.486 0.185 0.138 0.190 0.179 0.148 0.129 0.055 0.044 0.173 0.195 8.30 2.69 19.4 9.48 — to
p
Nd 0.524 6.25 0.466 1.47 1.76 1.79 0.672 0.538 0.762 0.670 0.550 0.502 0.208 0.175 0.762 0.884 29.6 8.08 65.5 34.2 — es
Sm 0.157 1.46 0.111 0.321 0.375 0.366 0.121 0.108 0.169 0.141 0.122 0.111 0.041 0.036 0.221 0.278 5.96 1.35 9.62 5.87 6.2 in
2
Eu 0.066 0.567 0.052 0.139 0.141 0.139 0.042 0.038 0.061 0.054 0.047 0.044 0.017 0.018 0.088 0.129 1.36 0.320 1.96 1.09 1.42 .9
Gd 0.227 2.05 0.142 0.394 0.457 0.476 0.134 0.128 0.215 0.188 0.160 0.146 0.059 0.058 0.273 0.524 7.22 1.23 6.12 3.91 — G
a
Tb 0.038 0.277 0.023 0.059 0.073 0.075 0.021 0.018 0.033 0.026 0.021 0.020 0.007 0.007 0.037 0.089 1.24 0.201 0.96 0.60 0.89 A
Dy 0.223 1.54 0.141 0.350 0.459 0.463 0.128 0.120 0.219 0.178 0.130 0.117 0.052 0.049 0.204 0.574 7.82 1.29 5.63 3.79 — rc
h
Ho 0.043 0.294 0.031 0.067 0.092 0.095 0.028 0.026 0.047 0.040 0.026 0.026 0.012 0.011 0.035 0.121 1.55 0.304 1.17 0.783 — ea
n
Er 0.116 0.785 0.086 0.170 0.238 0.257 0.085 0.083 0.143 0.118 0.077 0.075 0.038 0.039 0.085 0.317 4.45 1.04 3.62 2.58 —
s
u
Tm 0.015 0.104 0.012 0.021 0.029 0.031 0.012 0.012 0.021 0.018 0.011 0.010 0.005 0.005 0.010 0.037 0.651 0.159 0.58 0.39 — r
fa
Yb 0.082 0.596 0.070 0.097 0.154 0.165 0.077 0.076 0.148 0.111 0.060 0.057 0.029 0.031 0.051 0.188 4.19 1.16 4.27 2.76 3.4 c
e
Lu 0.011 0.091 0.010 0.013 0.022 0.025 0.012 0.012 0.022 0.019 0.010 0.009 0.005 0.005 0.007 0.026 0.599 0.182 0.65 0.46 0.54 s
e
a
Hf <0.05 0.75 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 5.68 3.85 6.12 5.34 8.4 w
a
Pb 1.42 9.23 1.14 1.06 0.86 0.96 0.87 0.89 1.14 2.33 3.67 3.54 1.54 1.38 4.04 4.75 3.43 2.38 3.8 1.90 13 te
r
Th <0.05 0.78 <0.05 0.06 0.07 0.08 0.07 0.06 0.07 0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 10.5 6.39 16.3 10.2 8.6
U 0.55 1.95 0.06 0.15 0.08 0.10 0.08 0.05 0.09 0.11 0.03 0.03 0.02 0.02 0.25 0.36 1.58 0.916 5.24 2.68 2.3
RREY 4.6 46 3.8 10.7 14.1 14.2 5.3 4.3 6.2 5.7 4.3 3.9 2.1 1.9 5.0 10.2 216 74 425 210 —
ðLa=La(cid:4)Þ b 1.16 1.34 1.21 1.21 1.17 1.15 1.3 1.46 1.47 1.39 1.06 1.17 1.76 2.06 1.69 2.21 0.94 1.11 1.00 1.04
WMS
ðGd=Gd(cid:4)Þ b 1.15 1.34 1.15 1.20 1.14 1.17 1.13 1.25 1.19 1.30 1.35 1.31 1.49 1.51 1.30 1.19 1.08 1.04 0.94 0.97
WMS
ðEu=Eu(cid:4)Þ b 1.11 1.10 1.30 1.24 1.06 1.05 1.01 1.03 1.02 1.10 1.12 1.14 1.21 1.41 1.17 1.10 0.63 0.72 0.67 0.60 0.70
CN
a DatafromWronkiewicz(1989)andequalstheaverageof15pelitesamplesfromMozaanGroup,whereallconcentrationsdeterminedbyINAA,exceptitalicizeddatawhichweredetermined
byXRF.
b SubscriptforcalculatedLa/La*,Gd/Gd*,andEu/Eu*referstodatanormalizedeithertoaverageofMozaanshalesamplesWM2,WM18,andWM19(WMS)orchondrite(CN).Seetextfor
details.
3
8
3
384 B.W.Alexanderetal./GeochimicaetCosmochimicaActa72(2008)378–394
considerably lower (average 1.13). The REY patterns are studied,andlow(Dy/Yb) isfoundinsamplescollected
WMS
not correlated with major element chemistry, nor with from the middleof the sequence.
immobiletraceelementcontent,butthetwogroupsofpat- The REY patterns of all the IF samples exhibit WMS-
terns can be distinguished on the basis of relative strati- normalizedpositiveLa,Eu,andGdanomalies.Calculated
graphic position; i.e., high (Dy/Yb) corresponds to (La/La*) and(Gd/Gd*) are aboveunity forall16
WMS WMS WMS
samplescollectedatthebaseandatthetopofthesequence IFsamples(Fig.3),and(Eu/Eu*) rangesfrom1.51to
WMS
2.09 (average1.69).Themagnitudeof theLa,Eu,andGd
anomaliesareindistinguishableonthebasisofstratigraphic
position. Y/Ho averages 42 for all samples, and higher Y/
1
Hovaluesareobservedforsamplescollectedfromthemid-
dleof thesequence, thoughthe difference betweenthetwo
groupsissmallwhensamplesWM121andWM122(Y/Ho
WM42 of 61and66, respectively) are notconsidered.
TheREYpatternofsampleWM42issimilartosamples
0.1 obtained stratigraphically above and below it, possessing
S positive La, Gd, and Y anomalies. The RREY content is
M
W the highest of the IFsamples, and though most major ele-
Y/ ment concentrations for sample WM42 are comparable to
E
R the other IF samples, Al O is enriched at 1.2% and MnO
2 3
0.01 is significantly higher (14.5%). WM42 also contains some-
top of sequence WM132 what greater concentrations of Rb, and significantly more
WM131
Zr, Hf andTh thananyof theother 15IF samples.
WM62
WM61 Seven iron-formation samples and three shale samples
bottom of sequence WWMM5521 were analyzed using TIMS, and Nd and Sm isotope data
WM42
are presented in Table 3. Six of the samples (one shale
WM41
0.001 and five IF samples) display similar 147Sm/144Nd between
LaCePrNdPmSmEuGdTbDy Y HoEr TmYbLu
0.1164and0.1486,valuescommontootherArcheanshales
1 andironformation(e.g.,MillerandO’Nions,1985;Alibert
WM122 and McCulloch, 1993; Jahn and Condie, 1995; Bau et al.,
WM121
WM112 1997).Oftheremainingfoursamples,twoareironforma-
middle of sequence WWMM111012 tions displayingthehighest 147Sm/144Ndobserved(>0.17),
WM101 whereasthelowest147Sm/144Ndvaluesareobservedinthe
WM72
WM71 remainingtwoshalesamples(<0.10).Theisotopicbehavior
0.1
of Ndisexpressedusing(cid:2) (t)(DePaoloandWasserburg,
Nd
S 1976), which reflects the relative difference between
M
W
Y/
E
R
2.5
0.01
2.0
0.001
LaCePr NdPmSmEuGdTbDy Y HoEr TmYbLu MS
W
Fig.2. REYdistributionsiniron-formationsamplesnormalizedto d*) 1.5
Mozaan shale average WMS to facilitate comparison with G
d/
contemporaneous continental crust. Samples generally exhibit G
positive La, Gd, and Eu anomalies which are characteristic of (
marine chemical sediments regardless of age. All samples display
1.0
generally parallel patterns from the light- to middle-rare-earth
elements. From the MREE to the HREE, the patterns are not
parallel,andtwogroupsofpatternscanbedistinguished.Samples
from the bottom and top of the sequence (top figure) have
relatively higher REY concentrations, smaller Y anomalies, and 0.5
decrease from the MREE to the HREE, compared to samples 0.5 1.0 1.5 2.0 2.5
obtainedfromthemiddleofthesequencestudied(bottomfigure). (La/La*)
WMS
The REYpatternstrack afull transgressive–regressivecycle, and
variationsinthesepatternsmirrorinferredwaterdepthsbasedon Fig. 3. La andGd anomalies of ironformationsamplesnormal-
lithology (i.e., transition from deposition of conglomerate to IF ized to WMS. All samples display La/La* and Gd/Gd* above
andbacktoconglomeratedeposition). unity,featurescharacteristicofmarinechemicalsediments.
Ndisotopesin2.9GaArcheansurfaceseawater 385
Table3
NdandSmconcentrations(mg/kg)andisotopicdatadeterminedbyTIMSfor2.9GaMozaanshalesandiron-formations
Sample Nd(ppm) Sm(ppm) 143Nd/144Nd±2ra 147Sm/144Nd±2r (cid:2) (0) (cid:2) (2.9Ga)
Nd Nd
Shale
WM2 30.7 6.34 0.511120±25 0.1247±6 (cid:3)29.61 (cid:3)2.7±0.3
WM8 8.19 1.33 0.510543±25 0.0979±5 (cid:3)40.87 (cid:3)4.0±0.6
WM18 64.7 9.70 0.510389±25 0.0905±5 (cid:3)43.87 (cid:3)4.2±0.3
Ironformation
WM41 0.520 0.148 0.511754±25 0.1721±9 (cid:3)17.24 (cid:3)8.1±0.7
WM51 0.457 0.112 0.511587±25 0.1486±7 (cid:3)20.50 (cid:3)2.6±0.8
WM61 1.76 0.362 0.511029±25 0.1242±6 (cid:3)31.39 (cid:3)4.3±0.6
WM72 0.533 0.104 0.511203±25 0.1302±7 (cid:3)27.99 (cid:3)3.2±0.6
WM102 0.659 0.146 0.511322±25 0.1330±7 (cid:3)25.67 (cid:3)1.9±0.7
WM121 0.194 0.037 0.510921±25 0.1164±6 (cid:3)33.49 (cid:3)3.5±0.5
WM131 0.762 0.215 0.511698±25 0.1766±9 (cid:3)18.34 (cid:3)10.9±0.6
WM131rb 0.748 0.218 0.511734±25 0.1763±9 (cid:3)17.63 (cid:3)10.1±0.8
BCR-2(3mg) 28.8 6.6 0.512619±25 0.1378±7
a The errors in the 143Nd/144Nd refer to the last digits and are estimated from repeated analysis of the La Jolla standard which gave
143Nd/144Nd=0.511853±0.000025(2r,n=10).
b Replicatedigestion,columnseparation,andanalysisofsampleWM131.
143Nd/144Ndofthesample,and143Nd/144Ndwithinachon- amounts of metasomatic fluids during metamorphism
dritic uniform reservoir (CHUR) at any given time t. For (Grauch, 1989; Bau, 1993). Iron-formation samples from
the shale samples, (cid:2) (t) falls within a narrow range from regions that have experienced high grade metamorphism
Nd
(cid:3)2.74to(cid:3)4.24,whereastheIFsamplesdisplayabimodal (e.g., Isua, Greenland) do not exhibit Eu or LREE deple-
(cid:2) (t)distribution,withmostsamplesexhibiting(cid:2) (t)sim- tion,andingeneralcoevaldetritusfreeIFsamplesdisplay
Nd Nd
ilar to the shales, while the two IF samples with similar REY patterns regardless of metamorphic grade
CN
147Sm/144Nd>0.17(WM41andWM131)havesignificantly (Bau, 1991, 1993). Therefore, the relatively low-grade
morenegative(cid:2) (t)valuesof(cid:3)8.1and(cid:3)10.9,respectively. greenschist facies metamorphism which affected Mozaan
Nd
IFsampleswouldnothavehadasignificanteffectontheir
5. DISCUSSION REY distribution.
Forsyn-depositionalprocesses,REYpatternsinchemi-
5.1.Depositional controlsonREYsin Mozaan IFs calprecipitatesmaybefractionatedwithrespecttocontem-
poraneous seawater due to the presence of detrital
TheMozaanIFsREYdistributionsmighthavebeenaf- aluminosilicates, or as a result of exchange effects between
fectedbyanumberofprocessesincluding:(i)post-andsyn- REYscavengingparticulatesandmarinewaters.Theeffect
depositional processes, (ii) the degree to which the REY of clastic contamination can be assessed using trace ele-
were scavenged by particulate matter in the water column ments that are essentially immobile in aqueous solutions,
prior to deposition, and (iii) also by the various inputs to and hence occur only at ultra-trace levels in seawater. The
the seawaterfrom which the IFsprecipitated. low abundances in the Mozaan IFs of such elements as
Potential post-depositional mechanisms affecting trace Al,Ti,Zr,Hf,Y,andThdemonstratesthatthesearevery
metalandREYdistributionintheMozaanIFsincludedia- pure chemical sediments and that clastic contamination is
genetic and metamorphic processes. During diagenesis, negligible,whichisconsistentwithasediment-starvedcon-
mobilityoftheREYwouldtendtoaverageoutREYdistri- tinentalshelfdepositionalenvironmentfortheMozaanIFs
butions in banded iron formations, yet Bau and Dulski (e.g., Beukes andCairncross, 1991).
(1992) observed highly variable REY ratios in individual The influence of solution complexation, fractionation,
Fe-andquartz-richbandswithinthe2.46GaSuperior-type and scavenging within the marine environment on REY
Kuruman IF. Such variation is also present in REY data distributions in IFs is more difficult to assess. The REY
for IF mesobands from the Dales Gorge Member in the concentration in modern seawater is controlled primarily
Hamersley Group of Australia (Morris, 1993). Bau (1993) byscavengingparticulatematter(e.g.,Elderfield,1988;Erel
studiedtheimpactofpost-burialprocessesonREYsigna- andStolper,1993),totheextentthatseawaterabundances
turesinPrecambrianIFsfromtheHamersleyBasininWes- of REYs are extremely low. The association between Fe-
tern Australia, the Kuruman and Penge IFs in South rich colloids and REYs has led manyworkers to conclude
Africa, and the Broomstock IFs of Zimbabwe, and con- thatFe-oxyhydroxideparticlesdominatedREYscavenging
cluded thatREYs are effectively immobile duringdiagene- during the formation of ancient metalliferous sediments
sisand lithification. (e.g., Derry and Jacobsen, 1990). This scavenging should
TheeffectsofmetamorphismonREYmobilityisafunc- represent the balance between two competing effects; the
tion of water/rock ratios, with LREE depletion and nega- solutioncomplexationoftheREYsandtheFe-oxyhydrox-
tive Eu anomalies expected in rocks that host significant idesurfacecomplexation.Thismodelassumesthatthereac-
386 B.W.Alexanderetal./GeochimicaetCosmochimicaActa72(2008)378–394
tionkineticsofadsorption/desorptionaresignificantlyfas- anomalyinnormalizeddataindicatesthatnoprocessoper-
terthantheparticleresidencetimeinanoxicwatercolumn. atedduringFe-sedimentationthatwascapableoffraction-
This assumption is supported by experimental evidence ating Ce. This scenario is clearly different from the oxic
indicating that particle-surface/solution REY exchange modernmarinesystem,inwhichCeisoxidizedtoimmobile
equilibrium occurs within minutes (Bau, 1999), suggesting Ce(IV)onparticlesurfaces,resultinginfractionationfrom
that most Fe-oxyhydroxide particles settling through an theotherREYandproducingthestrongnegativeCeanom-
oxicwatercolumn(ofrelativelyconstantREYdistribution) aly observed in modern seawater. The lack of significant
should bein equilibrium withthe solutionphase. negative-Ce anomalies in many Archean chemical sedi-
Natural systems that provide insight into such equilib- ments has been discussed by numerous authors in terms
riumexchangeprocessesareavailableintheformofmod- of the oxidation historyof the Earth’s surface (e.g., Fryer,
ern marine hydrogenetic ferromanganese crusts and 1977; Derry and Jacobsen, 1990; Kato et al., 1998; among
terrestrialspring-waterprecipitates.SeafloorFe–Mncrusts others). While the exact process that converted large
have trace metal distributions that reflect equilibrium amountsofFe(II)toFe(III)inseawaterduringsedimenta-
adsorption–desorption processes between REYs and sea- tionoftheMozaanIFsisunknown,theCedistributionsin
water,processesthatstronglyfractionateREYs.Verysim- these chemical precipitates indicate Archean seawater did
ilar fractionation is observed between Fe-oxyhydroxides not possess a negative Ce anomaly, and correspondingly,
and terrestrial spring water (Bau et al., 1998). The most redox levels were lower than those observed in modern
striking difference between these precipitates and the marine systems.
respectivefluidswhichformedthemisthelowerY/Hoob- UnlikeCe,theMozaanIFsdisplaypronouncedpositive
served in the precipitates, which display negative Y anom- Eu anomalies (Fig. 4), which in modern marine environ-
alies. This Y–Ho fractionation is due to the preferential ments are only observed in high-temperature (>250(cid:3)C)
sorption of Ho relative to Y on the scavenging Fe(–Mn) hydrothermal fluids, such as those typically found at mid-
particles (e.g., Bau et al., 1996, 1998). Preferential adsorp- ocean ridges and back-arc spreading centers. It is reason-
tion of Ho over Y on Fe-oxyhydroxides has been demon- able that the REY patterns of these high-T fluids in
CN
strated experimentally (Bau, 1999) and is completely theEarlyPrecambrianweresimilartothoseobservedtoday
consistent with the observed fractionation of Y and Ho and exhibited Eu anomalies >1 and enrichment in LREEs
during precipitation of hydrogenetic Fe–Mn crusts from (Bau and Mo¨ller, 1993), features which are characteristic
seawater. We emphasize that such Y–Ho fractionation is
notobserved in Archeanironformations.
10.0
If redox conditions in Archean seawater permitted the
precipitation of ferric iron, and if scavenging Fe-oxyhy-
Pongola pelites
droxide particles achieved exchange equilibrium with sur-
rounding seawater, the above observations predict sub-
chondriticvaluesofY/HointheIFs.Thesignificantlyhigh-
erY/HoobservedinIFsofdifferentagesstronglysuggests PAAS
N
thatFe-oxyhydroxideparticlesthatscavengedREYscould b)C
notbeatornearexchangeequilibriawithrespecttoambi- m/Y
ent seawater (Bau and Dulski, 1996). Although the actual S
( 1.0
reason why IFs display the REY seawater pattern has not Pongola IF
Penge IF
beensatisfactorilyexplainedandrequiresfurtherinvestiga- Kuruman IF
Isua IF
tion,thestrikingsimilaritybetweenREYpatternsinArche- Pacific seawater
an marine carbonates and IFs suggests that pure Archean high-T hydrothermal
fluids
IFsfaithfullyrecordtheREYdistributionsofcontempora- 0.3
neous seawater. 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 5 10 15 20 25
(Eu/Eu*)
CN
5.2.Archean seawater
Fig. 4. Plot of chondrite-normalized Sm/Yb and Eu/Eu* for
MozaanIFs,andincludingdatafor3.7GaIsuaIFs(Bolharetal.,
5.2.1.High-temperature hydrothermalinput 2004), 2.5Ga Kuruman and Penge IFs (Bau and Dulski, 1996),
The geologic setting and low detritus content, coupled shallow(<500m)Pacificseawater(AliboandNozaki,1999),and
with the observed LREE depletions and positive La, Gd, high-Thydrothermalfluids(>350(cid:3)C,BauandDulski,1999).Note
andYanomalies(Figs.2and5),stronglysuggeststheMoz- break in horizontal axis. Isua samples are those considered by
aan IFs represent (cid:2)3.0Ga seawater. Working under the Bolharetal.(2004)toreflectcontemporaneousseawater.Thegrey
assumption that these sediments possess trace element dis- shadedarearepresentstherangeofvaluesfor62pelitesfromthe
Pongola Supergroup sampled by Wronkiewicz (1989), and the
tributions reflective of contemporaneous seawater, some
crossed-square represents Post-Archean Average Shale (PAAS,
general constraints may be placed on the composition of
McLennan,1989).TheMozaanIFsexhibitEu/Eu*betweenthatof
Archean seawater.
the Isua and Kuruman IFs, yet display Sm/Yb values similar to
Theoccurrenceoftheoxide-faciesMozaanIFssuggests
continentalcrustandsignificantlyhigherthananyoftheotherIFs
thattheredoxlevelofevenveryshallowseawaterpermitted (except for two Isua and one Kuruman sample). All IF samples
nearly continuous precipitation of significant amounts of have significantly lower Sm/Yb and Eu/Eu* than high-T hydro-
Fe(III)-bearing (hydr)oxides, while the lack of any Ce thermalfluids.
Ndisotopesin2.9GaArcheansurfaceseawater 387
oftheMozaanIFsamples(Fig.4).Thepresenceofpositive (Fig. 6a). This is comparable with the model of Klein
Eu anomalies is clear evidence that REY from high-T and Beukes (1989), who noted that a 1000:1 mixing ratio
hydrothermal inputs influenced the continental shelf envi- of typical North Atlantic seawater with deep sea hydro-
ronment thathosted theMozaan IFs. thermal fluids produced REY patterns similar to that of
For constraints on ancient hydrothermal fluxes, mod- numerous Precambrian IFs. An identical seawater/hydro-
ernblacksmokerfluidsofferthebestanalogsforthe trace thermal fluid mixing ratio of 1000:1 was also calculated
element compositions of high-T fluids which contributed by Khan et al. (1996) as sufficient to account for the
to the REY distribution in Archean iron formations. REY distributions in IFs from the 2.6–3.0Ga Kushtagi
For the Mozaan IFs, the magnitude of the positive Eu schist belt. Therefore, it appears that relatively minor in-
anomalies generally compare well with data for other puts (<1%) of high-T hydrothermal fluids during conser-
IFs (Fig. 5). However, the general REY distributions of vative mixing may reasonably account for Eu/Sm
the Mozaan IFs are significantly different when compared observed within a wide variety of Archean IFs.
to well studied examples such as the 3.7Ga Isua IF of The differences between the REY distribution of the
Greenland (Bolhar et al., 2004) and the 2.5Ga Kuruman MozaanIFsrelativetootherIFsdoessuggestagreaterin-
IF from South Africa (Bau and Dulski, 1996), though put of ‘Archean black smoker’ fluids, particularly with re-
regardless of age, theseironformations display the seawa- spect to Sm/Yb. These fluids possess higher Sm/Yb and
ter characteristics of marine chemical sediments (Fig. 5). chondritic Y/Ho values ((cid:2)28) when compared to modern
Conservative mixing calculations indicate that a high-T seawater, and these ratios provide additional constraints
hydrothermal fluid contribution of less than 0.1% is ade- onthemixingmodel(Fig.6b).Mixingcalculationsindicate
quate for producing the Eu/Sm ratios observed within that a 1–5% contribution of hydrothermal fluids may ac-
the Pongola and Kuruman IFs, and reasonably accounts count for the Y/Hoand Sm/Yb observed withinthe Moz-
for most of the Eu/Sm ratios observed in the Isua IFs aan IFs. However, this mixing ratio is inconsistent with
the0.1%contributionofhigh-Tfluidsnecessarytoaccount
for Eu/Sm (Fig. 6c). It is worth noting that all of the
Mozaansamples,regardlessofstratigraphicposition,exhi-
101 bitSm/YbhigherthanaverageKurumanorIsuaIFs(0.68
and 0.86, respectively). Except for two Isua samples and
one Kuruman sample, the general HREE depletion exhib-
ited within the Mozaan IFs is not observed in the older
or younger iron formations. The poor fit of the mixing
100 model in explaining the relatively high Sm/Yb of the
Mozaan IFs suggests different processes controlled the
REYdistributions intheKurumanandIsuaIFsandindi-
cates the possible existence of additional REY sources to
MS the Mozaansamples.
W10-1
EY/ 5.2.2. Low-temperature fluidinput
R
In addition to the high-T component expected in deep
Archean seawater, the marine bottom water component
would be influenced by low-T solutions (<200(cid:3)C) altering
10-2 oceaniccrustandfluxesfromporewaterssourcedfromsed-
iments. Wheat et al. (2002) examined <100(cid:3)C hydrother-
mal fluids venting from the flank of the Juan de Fuca
high-T hydrothermal fluid (x105) ridge and did not observe either a significant enrichment
Isua IF (x2)
in Eu or a significant fractionation of Sm/Yb in the low -
Kuruman IF
10-3 top/bottom IFs Tfluidendmemberrelativetoseawater.Traceelementdata
middle IFs for near-shore, low-T (6100(cid:3)C) hydrothermal vent fluids
<500 m seawater (x105)
(Pichler et al., 1999) suggests REY distributions similar to
La Ce Pr NdPmSmEuGd Tb Dy Y Ho Er TmYb Lu
seawater and negligible Eu enrichment compared to host
Fig.5. REYdistributionsofaverageMozaan,Isua,andKuruman rocks.Therefore,comparedtohigh-Tfluids,low-Thydro-
IFs, as well as average high-T hydrothermal fluid and average thermalfluidswouldlikelyexhibitnopositiveEu anom-
CN
shallow (<500m) Pacific seawater (see Fig. 4 caption for data aly and lower (Sm/Yb) . Therefore, even though the
CN
sources). Mozaan data represent averages of the two distinct Mozaan IFs formed in a shallow near-shore depositional
groupsofREYpatternsshowninFig.2:top/bottomistheaverage
environment, the positive Eu anomalies observed indicate
ofsevensamplesfromthetopandbottomofthesequencestudied
that high-T hydrothermal inputs were one REY source
(excluding sample WM42), and middle refers to the eight IF
for the Mozaan IFs. Whereas constraints can be placed
samples from the middle of the sequence. All of the IFs show
on the relative contribution of these high-T hydrothermal
positive shale-normalized La, Gd, and Y anomalies typical of
seawaterandageneralenrichmentofHREEsoverLREEs.Note fluids in an effort to explain Sm/Yb and Eu/Sm within
thegenerallyflatpatternoftheMozaanIFaveragesfromSmtoYb the Mozaan IF, this is not possible for the low-T compo-
comparedtotheotherArcheanironformation. nent. In a reducing Archean ocean deep water benthic