Depositional age of the early Paleoproterozoic Klipputs Member, Nelani Formation (Ghaap Group,...

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Precambrian Research 237 (2013) 1– 12

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Precambrian Research

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Depositional age of the early Paleoproterozoic Klipputs Member,Nelani Formation (Ghaap Group, Transvaal Supergroup, South Africa)and implications for low-level Re–Os geochronology andPaleoproterozoic global correlations

Brian Kendall a,b,∗, David van Ackenb, Robert A. Creaserb

a Department of Earth and Environmental Sciences, University of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canadab Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB T6G 2E3, Canada

a r t i c l e i n f o

Article history:Received 18 October 2012Received in revised form 20 July 2013Accepted 11 August 2013Available online 30 August 2013

Keywords:Rhenium–osmium geochronologyOrganic-rich sedimentary rocksKlipputs MemberGhaap GroupPaleoproterozoic glaciationsHamersley Group

a b s t r a c t

We present new Re–Os isotope data from the early Paleoproterozoic Klipputs Member (Nelani Formation,Ghaap Group, Transvaal Supergroup, South Africa) to further evaluate low-level Re–Os geochronologyof organic-rich sedimentary rocks and improve global correlations of Paleoproterozoic sedimentaryrocks. The Klipputs Member contains Re (0.3–0.8 ppb), total Os (34–177 ppt) and common Os (192Os;10–66 ppt) abundances that overlap upper crustal abundances of 0.2–2 ppb, 30–50 ppt, and 12–21 ppt,respectively. Five samples yield a Model 1 Re–Os date of 2479 ± 20 Ma (±22 Ma if the 187Re decayconstant uncertainty is included, 2�, Mean Square of Weighted Deviates [MSWD] = 0.01) using theCrVI-H2SO4 digestion protocol. Dissolution of the same sample powders using inverse aqua regia yields aModel 3 Re–Os date of 2862 ± 190 Ma (2�, MSWD = 148). Comparison of Re–Os isotope data between thetwo methods reveals that inverse aqua regia liberated a component of radiogenic detrital Os whereasthe CrVI-H2SO4 medium did not. The Re–Os date of 2479 ± 22 Ma for the Klipputs Member is consistentwith published geochronological ages from underlying and overlying stratigraphic units, suggesting theCrVI-H2SO4 protocol yields accurate Re–Os depositional ages from ORS containing very low levels ofhydrogenous Re and hydrogenous + radiogenic Os. Hence, there is potential for widespread applicationof low-level Re–Os ORS geochronology, including sedimentary successions deposited before the GreatOxidation Event. The initial 187Os/188Os (0.12 ± 0.01) from the Klipputs Member CrVI-H2SO4 regressionindicates that dissolved Os in the contemporaneous local seawater originated from hydrothermalalteration of unradiogenic oceanic crust and/or cosmic dust rather than oxidative weathering of theradiogenic upper crust. In light of the Re–Os depositional age for the Klipputs Member, we propose thatthe stratigraphic units in the upper Hamersley Group above the Brockman Iron Formation (WesternAustralia) are younger than the Ghaap Group. Diamictite from the Doradale Formation in the GhaapGroup is constrained to be older than 2479 ± 22 Ma and thus may not be correlative with glacial depositsassigned to the oldest Paleoproterozoic glaciation.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Sedimentary geochronology has advanced significantly withthe emergence, then refinement, of the 187Re–187Os radioisotopesystem as a deposition-age geochronometer for organic-richsedimentary rocks (ORS) (Ravizza and Turekian, 1989; Cohenet al., 1999; Kendall et al., 2009a). In the presence of atmospheric

∗ Corresponding author at: Department of Earth and Environmental Sciences, Uni-versity of Waterloo, 200 University Avenue West, Waterloo, ON N2L 3G1, Canada.Tel.: +1 226 972 9217; fax: +1 519 746 7484.

E-mail address: [email protected] (B. Kendall).

O2, Re and Os are oxidatively mobilized from the upper crustduring weathering as oxyanions (ReO4

−, HOsO5−, H3OsO6

−),which are transported by rivers to the oceans (Colodner et al.,1993; Peucker-Ehrenbrink and Blum, 1998; Levasseur et al., 1999;Peucker-Ehrenbrink and Ravizza, 2000; Miller et al., 2011). Both Reand Os accumulate in oxygenated seawater, with residence timesof ∼130 kyr and ∼5–50 kyr in the modern ocean, respectively(Levasseur et al., 1999; Oxburgh, 2001; Oxburgh et al., 2007; Milleret al., 2011). Both Re and Os are enriched in anoxic sedimentstogether with organic matter and sulfide, resulting in accumulationof hydrogenous Re and Os in ORS (Ravizza et al., 1991; Ravizzaand Turekian, 1992; Crusius et al., 1996; Selby and Creaser, 2003;Yamashita et al., 2007; Helz and Dolor, 2012). Closure of the Re–Os

0301-9268/$ – see front matter © 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.precamres.2013.08.002


2 B. Kendall et al. / Precambrian Research 237 (2013) 1– 12

Fig. 1. Geological map of the Griqualand West basin, Kaapvaal craton, South Africa. The position of drill core GEC01 is shown.Modified from Schröder et al. (2011).

isotope system occurs during or very soon after sediment deposi-tion (Cohen et al., 1999) and the subsequent �− decay of 187Re to187Os forms the basis of a deposition-age geochronometer for ORS.

Confidence in the reliability of the Re–Os ORS geochronome-ter comes from a precise and accurate determination of the 187Redecay constant (� 187Re = 1.666 ± 0.005 × 10−11 a−1; Smoliar et al.,1996; Selby et al., 2007) and several examples of excellent agree-ment between Re–Os ORS ages and U–Pb, Pb–Pb, and Ar–Ar ageconstraints for late Archean and Phanerozoic sedimentary succes-sions (Selby and Creaser, 2005; Anbar et al., 2007; Selby, 2007;Turgeon et al., 2007; Xu et al., 2009; Georgiev et al., 2011).The Re–Os ORS geochronometer is known to be robust duringsome post-depositional processes, including hydrocarbon matu-ration/migration (Creaser et al., 2002; Selby and Creaser, 2005;Kendall et al., 2009b; Rooney et al., 2012), flash pyrolysis causedby igneous intrusions (Rooney et al., 2010), and lower greenschistfacies metamorphism (Kendall et al., 2004; Rooney et al., 2011).Hence, Re–Os ORS geochronology has become a valuable tool forPhanerozoic timescale calibration (Selby and Creaser, 2005; Selby,2007; Turgeon et al., 2007; Xu et al., 2009; Georgiev et al., 2011) andfor placing time pins within sedimentary sequences that otherwiselack geochronological data, a problem that is particularly acute forPrecambrian sedimentary basins (Hannah et al., 2004; Kendall et al.,2004, 2006, 2009b,c; Creaser and Stasiuk, 2007; Yang et al., 2009;Rooney et al., 2010, 2011).

Traditionally, ORS containing low Re and Os abundances (closeto upper crustal averages of ∼0.2–2 ppb and 30–50 ppt, respec-tively; Esser and Turekian, 1993; Peucker-Ehrenbrink and Jahn,2001; Hattori et al., 2003; Sun et al., 2003) are not considered goodcandidates for geochronology, because a significant fraction of Reand Os in these rocks would be located in the detrital fraction.

Mixing of detrital and hydrogenous components with differentOs isotope compositions can result in a spuriously young or oldRe–Os date (Ravizza et al., 1991). Even in ORS containing moderatehydrogenous Re and Os enrichments, detrital Os can affect theprecision and accuracy of Re–Os dates derived from standardwhole-rock acid digestions like inverse aqua regia (Creaser et al.,2002; Selby and Creaser, 2003; Kendall et al., 2004). To combatthis problem, a whole-rock CrVI-H2SO4 digestion medium wasdeveloped to minimize inclusion of detrital Os, by preferen-tially attacking organic matter and sulfide minerals that hosthydrogenous Re and Os in ORS (Selby and Creaser, 2003; Kendallet al., 2004). The CrVI-H2SO4 protocol has produced many preciseand accurate Re–Os depositional ages from ORS with at least mildenrichments of hydrogenous Re and Os (>2 ppb and >80 ppt, respec-tively). However, this technique may not quantitatively excludedetrital Os (Ravizza et al., 1991) and hence its utility for ORS withvery small amounts of hydrogenous Re and Os may be limited.

Using the CrVI-H2SO4 protocol, Rooney et al. (2011) reporteda precise Re–Os date of 659.6 ± 9.6 Ma from the NeoproterozoicBallachulish Slate Formation (Dalradian Supergroup, Scotland),which contains very low, crustal-like Re and Os abundances of0.3–1.8 ppb and 26–52 ppt, respectively. In contrast, an impreciseRe–Os date of 655 ± 49 Ma was obtained using inverse aqua regiadigestion of the same sample powders. The high quality of the CrVI-H2SO4 regression led Rooney et al. (2011) to infer that the Re–Osdate of 659.6 ± 9.6 Ma represents a depositional age and hencecan be used as a time pin for the Dalradian Supergroup, whichotherwise is nearly devoid of reliable absolute age constraints.This finding raises the possibility that ORS with small amounts ofhydrogenous Re and Os could be generally amenable for sedimen-tary geochronology using the CrVI-H2SO4 protocol, but the lack of


B. Kendall et al. / Precambrian Research 237 (2013) 1– 12 3

external age control for the strata studied by Rooney et al. (2011)limits a complete evaluation of accuracy for this possibility.

To further evaluate low-level Re–Os ORS geochronology, wepresent new Re–Os isotope data for the early PaleoproterozoicKlipputs Member of the Nelani Formation (Griqualand West basin,Transvaal Supergroup, South Africa). We compare Re–Os isotopedata for the inverse aqua regia and CrVI-H2SO4 digestion methodsand show that a precise Re–Os date derived from the CrVI-H2SO4technique most likely records a depositional age for the KlipputsMember. Our data provide further evidence that Re–Os geochronol-ogy of “low-level” ORS is feasible, and can yield accurate as wellas precise dates. The new Re–Os age from the Klipputs Mem-ber provides a new time pin for the upper Ghaap Group, leadingto revised correlations between the early Paleoproterozoic suc-cessions of South Africa and Western Australia. We discuss theimplications of the new Re–Os age for global correlations of Paleo-proterozoic glacial deposits. Our data also enable an estimate for thelocal 187Os/188Os isotope composition of earliest Paleoproterozoicseawater in the Griqualand West basin.

2. Geological setting and previous age constraints

2.1. Geological setting

Sedimentary rocks of the late Archean to early Paleopro-terozoic Transvaal Supergroup are exceptionally well-preservedin the Griqualand West and Transvaal basins on the KaapvaalCraton, South Africa. In the Griqualand West basin, the lowerTransvaal Supergroup is represented by the minimally metamor-phosed (sub-greenschist facies) Ghaap Group (Fig. 1). The foursubgroups of the Ghaap Group record the development of amixed siliciclastic-carbonate ramp following initial flooding of theKaapvaal Craton (Schmidtsdrif Subgroup), an extensive carbon-ate platform (Campbellrand Subgroup), a transgressive–regressivecycle of chert and banded iron formation following the drowningof the carbonate platform (Asbestos Hills Subgroup), and multi-ple transgressive–regressive cycles along a prograding delta orsubmarine fan system (Koegas Subgroup) (Beukes, 1983, 1987;Sumner, 1997; Sumner and Grotzinger, 2004; Schröder et al.,2006, 2011; Sumner and Beukes, 2006; Beukes and Gutzmer,2008). A switch from predominantly chemical to mixed siliciclastic-chemical deposition marks the conformable contact betweenthe Koegas Subgroup and the underlying Griquatown Iron For-mation (Asbestos Hills Subgroup; Beukes and Gutzmer, 2008;Schröder et al., 2011). The contact between the Koegas Subgroupand the overlying glaciogenic Makganyene Formation (Postmas-burg Group) is often an erosional unconformity in shallow-waterplatform sections (Schröder et al., 2011 and references therein).However, a conformable relationship in deep-water sections sug-gests continuous deposition between the Ghaap and PostmasburgGroups (Moore et al., 2001; Polteau et al., 2006).

The Koegas Subgroup was deposited in the southwest part ofthe Griqualand West basin (Beukes, 1983). Schröder et al. (2011)subdivided the Koegas Subgroup into five sequences, which arerepresented by the Pannetjie Formation, the Doradale-Kwakwas-lower Naragas formations, the upper Naragas Formation, theHeynskop Formation, and the Rooinekke-Nelani formations(Fig. 2). Lateral facies relationships and stratigraphic patternsindicate that regressive upper units of siltstones and sandstonesprograded over transgressive lower units of mudstones and ironformations. These five sequences form part of two larger deposi-tional sequence sets that record a longer-term regression followingdeposition of the deep-water Kuruman Iron Formation (lowerAsbestos Hills Subgroup). Following relatively shallow-waterdeposition of the granular Griquatown Iron Formation, the firstsequence set in the Koegas Subgroup starts with the Pannetjie

Fig. 2. Simplified stratigraphic column for the upper Ghaap and Postmasburggroups. Geochronological dates are displayed to the right of the stratigraphiccolumn, with the Re–Os date from the Klipputs Member in bold. Those datesdenoted with a * are unpublished (see text for further discussion). The Re–Os ageof 2479 ± 22 Ma from the Klipputs Member implies the date of 2415 ± 6 Ma fromthe older Rooinekke Formation is erroneously young. Published dates are from: (1)Sumner and Bowring (1996), (2) Altermann and Nelson (1998), (3) Pickard (2003),(4) Moore et al. (2012), (5) Cornell et al. (1996), (6) Fairey et al. (2013), and (7) Bauet al. (1999).

Modified from Schröder et al. (2011).

Formation, culminating in maximum flooding during deposition ofthe Doradale iron formation, and ending with the nearshore coarsesiliciclastics of the Naragas Formation. A diamictite lens of possibleglacial origin occurs near the base of the Doradale Formation(Polteau et al., 2006) and is suggested to represent the oldestPaleoproterozoic glaciation (Hoffman, in press). Thereafter, thesecond sequence set commences with mudstones and sandstonesof the Heynskop Formation and was followed by deposition ofsilicate iron formation, ferruginous shales, and bioherm carbonatesof the Klipputs Member (basal Nelani Formation) during maximumflooding. Overlying granular iron formation, ferruginous shales,sandstones, and rip-up clast breccias of the Nelani Formation



Grey/black mudstone

Siltstone

Sandstone

Ferruginous mudstone

Olive-brown mudstone

Carbonate

360

320

280

240

200

160

120

80

40

Depth(m)

Rooinekke

Formation

Nelani

Formation

Klipputs

Member

Naragas/

Heynskop

Formations

2479 ± 22 Ma

Fig. 3. Stratigraphic column for drill core GEC01, modified from the pub-licly available stratigraphic column shown on the website for the Agouron– Griqualand Paleoproterozoic Drilling Project (http://general.uj.ac.za/agouron/well summary.aspx?coreid=3). The arrow denotes the stratigraphic intervals sam-pled for Re–Os geochronology (174.53–174.83 m and 176.06–176.35 m). The Re–Osage of 2479 ± 22 Ma was derived from the 176.06 to 176.35 m interval.

constitute the early phase of a high stand systems tract (Beukes,1983; Beukes and Gutzmer, 2008; Schröder et al., 2011). Theoverlying Makganyene Formation likely represents the product ofa widespread glaciation extending to low-latitudes at ca. 2.40 Ga(age constraints are summarized in the next section; Evans et al.,1997; Moore et al., 2012). It is suggested to be the second of threePaleoproterozoic glaciations and had a close temporal relationshipto the Great Oxidation Event (Hoffman, in press).

As part of the Agouron-Griqualand Paleoproterozoic DrillingProject, drill core GEC01 (S 28◦39′33.8′′; E 22◦41′33.2′′) inter-sected the upper part of the Koegas Subgroup (Fig. 3). In thiscore, the Klipputs Member is nearly ∼35 m thick and con-sists primarily of flat bedded gray and black mudstone (seehttp://general.uj.ac.za/agouron/). Deposition likely occurred ina quiet-water environment below storm wave base (Schröderet al., 2011). We obtained Re–Os isotope data from four sam-ples at 174.53–174.83 m and five samples at 176.06–176.35 m inGEC01. Our samples show no visible evidence for post-depositionalhydrothermal fluid flow (e.g., quartz-carbonate-pyrite veinlets)that is known to disturb the Re–Os isotope systematics in ORS(Kendall et al., 2009b; Rooney et al., 2011).

2.2. Previous age constraints for the Koegas Subgroup

Published age constraints for the Koegas Subgroup are absent.Citing an unpublished report (Gutzmer and Beukes, 1998), Beukesand Gutzmer (2008) mention a “poorly constrained” U–Pb SHRIMPzircon date of ca. 2413 Ma from a tuffaceous mudrock in the lowerRooinekke Formation. A number of papers quote an unpublishedPb–Pb date of 2415 ± 6 Ma for the Rooinekke Formation (see Fig. 2;Kirschvink et al., 2000; Polteau et al., 2006; Moore et al., 2012).Citing the same unpublished report by Gutzmer and Beukes (1998),Schröder et al. (2011) indicate this 2415 ± 6 Ma age was derivedfrom zircons in a tuffaceous bed.

In the older Asbestos Hills Subgroup, a large number of U–PbSHRIMP zircon dates have been reported from tuffaceous mudrocksthroughout the Riries Member of the upper Kuruman Iron Forma-tion (Pickard, 2003). Several dates are statistically identical and areinterpreted to record the age of syndepositional volcanism. The bestestimate for the depositional age of the upper Riries Member is con-sidered to be 2460 ± 5 Ma. An unpublished U–Pb SHRIMP zircondate of 2465 ± 7 Ma was also cited for the Riries Member by Martinet al. (1998). Older U–Pb dates from the Riries Member are thoughtto reflect reworked/resedimented deposits or are derived from pri-mary pyroclastic deposits containing inherited zircons entrainedduring volcanism. A small proportion of younger U–Pb dates (ca.2450–2370 Ma) are suggested to reflect Pb loss or post-depositionalevent(s) (Pickard, 2003). Unpublished U–Pb SHRIMP zircon datesof 2431 ± 31 Ma and 2489 ± 33 Ma were obtained from the baseand 18 m above the base of the overlying Griquatown Iron For-mation, respectively (cited by Nelson et al., 1999; Trendall et al.,1990). Another unpublished U–Pb zircon date of 2480 ± 6 Ma wascited by Nelson et al. (1999) for the lower Penge Iron Formation inthe Transvaal basin (a stratigraphic correlative of the Kuruman andGriquatown Iron Formations; Beukes, 1983).

In the younger Postmasburg Group, a whole-rock Pb–Pb dateof 2222 ± 13 Ma was obtained for the volcanic Ongeluk Forma-tion (Cornell et al., 1996). However, this date is at odds with aPb–Pb carbonate-leach date of 2394 ± 26 Ma and a U–Pb discordia-intercept date of 2392 ± 23 Ma from the younger MooidraaiFormation (Bau et al., 1999; Fairey et al., 2013). The two Mooidraaicarbonate dates may represent diagenetic ages and thus a mini-mum age constraint for deposition of the upper Postmasburg Group(Moore et al., 2012; Fairey et al., 2013). In contrast, the youngerOngeluk Pb–Pb age is attributed to isotopic resetting during a latermetamorphic/metasomatic event (Bau et al., 1999; Moore et al.,2001, 2012; Fairey et al., 2013). Recently, Moore et al. (2012)obtained a youngest U–Pb detrital zircon date of 2436 ± 7 Ma fromthe Makganyene Formation, thus providing a maximum deposi-tional age for the Postmasburg Group. Collectively, these datessuggest the Postmasburg Group was deposited between ca. 2430and 2380 Ma (Moore et al., 2012), thus providing a minimum agefor the Koegas Subgroup.

3. Analytical methods

All Re–Os isotope analyses were carried out at the RadiogenicIsotope Facility of the Department of Earth and Atmospheric Sci-ences, University of Alberta. Each sample comprises 35–55 g ofdark gray shale that was ground to remove drilling and saw marks,broken into small chips without metal contact, and powderedin an automated agate mill. To assess the impact of detrital Oson the Re–Os isotope systematics of each sample, one samplepowder aliquant (∼1.0–1.3 g) was digested in a CrVI-H2SO4 solu-tion whereas a second powder aliquant (∼0.6–0.7 g) was reactedwith inverse aqua regia. A smaller amount of powder was usedfor inverse aqua regia digestions to ensure complete acid-sample



Table 1Re–Os data for the Klipputs Member, drillhole GEC01.

Sample (depth) Analysis type Re (ppb) 2� Os (ppt) 2� 192Os (ppt) 2� 187Re/188Os 2� 187Os/188Os 2� rho (“�”) IOs (2479 Ma)

GEC174.50-174.98-2 CrVI-H2SO4 0.737 0.004 136.8 0.6 48.3 0.2 30.33 0.20 1.419 0.008 0.40 0.14(174.53–174.57 m) Inverse AR 0.632 0.008 119.7 0.5 41.8 0.1 30.06 0.41 1.522 0.006 0.19 0.25









Note: Analysis type refers to whole-rock dissolution with CrVI-H2SO4 or inverse aqua regia. IOs (2479 Ma) represents the initial sample 187Os/188Os ratio calculated at 2479 Ma.

reaction and sample-spike equilibration (Kendall et al., 2009a). Thepowders were weighed, and loaded into Carius tubes, which werethen chilled in a dry ice – methanol slurry, followed by addition of aknown amount of a 185Re + 190Os spike solution, and either 8 mL ofCrVI-H2SO4 solution or 9 mL of inverse aqua regia. The Carius tubeswere then sealed and heated at 240 ◦C for 48 h. Chemical proceduresfor sample digestion and purification of Re and Os are describedelsewhere (inverse aqua regia: Selby and Creaser, 2001; Creaseret al., 2002; CrVI-H2SO4: Selby and Creaser, 2003; Azmy et al., 2008).Average total procedural blanks for Re and Os using CrVI-H2SO4solutions prepared from Fluka Chemika CrO3 (Lot #1177884; alsoused by Kendall et al., 2009c) were 10.8 ± 1.5 pg (1�, n = 9) and0.12 ± 0.05 pg (1�, n = 8), and the average blank 187Os/188Os was0.24 ± 0.08 (1�, n = 8). Procedural blanks for Re and Os using inverseaqua regia were 0.67 ± 0.18 pg (1�, n = 8) and 0.37 ± 0.06 pg (1�,n = 5), respectively, with 187Os/188Os of 0.28 ± 0.09 (1�, n = 5).

We obtained Re and Os abundances and isotope compositions byisotope dilution – negative thermal ionization mass spectrometry(ID-NTIMS) on a Micromass Sector 54 instrument (for CrVI-H2SO4digestions) and a Thermo TRITON instrument (for inverse aquaregia digestions). Rhenium measurements were carried out usingFaraday collectors in static mode whereas Os measurementswere made with a pulse-counting electron multiplier in single-collector peak-hopping mode. During a seven month interval thatincluded the CrVI-H2SO4 analyses of the Klipputs Member on theMicromass Sector 54 instrument, our in-house standard solu-tions yielded average 185Re/187Re and 187Os/188Os isotope ratiosof 0.59889 ± 0.00044 (1�, n = 43) and 0.1069 ± 0.0001 (1�, n = 46),respectively. During the three month interval in which the inverseaqua regia analyses of the Klipputs Member were performed onthe Thermo TRITON, the in-house standard solutions yielded aver-age 185Re/187Re of 0.59814 ± 0.00076 (1�, n = 5) and 187Os/188Os of0.10688 ± 0.00012 (1�, n = 19). Standard compositions for Re andOs on both instruments are identical within error.

Data reduction followed the methods established in previousstudies (Kendall et al., 2004). Regressions were performed withIsoplot V.4.15 (Ludwig, 2011) using � 187Re = 1.666 × 10−11 yr−1

(Smoliar et al., 1996), 2� uncertainties for 187Re/188Os and187Os/188Os determined by numerical error propagation, and theerror correlation (�) between 187Re/188Os and 187Os/188Os (Kendallet al., 2004). When comparing our new Re–Os age from the KlipputsMember with other Paleoproterozoic U–Pb zircon ages, we include

the ±0.31% uncertainty for the 187Re decay constant (Smoliar et al.,1996). Model 1 dates are calculated by Isoplot V.4.15 by assumingall scatter about a regression line arises from the input 2� uncer-tainties for 187Re/188Os and 187Os/188Os and the value of �. Whenadditional scatter is present, then Isoplot V.4.15 calculates a Model3 date that assumes the excess scatter arises from an unknown butnormally distributed variation in 187Os/188Os (Ludwig, 2011).

4. Results

The Klipputs Member contains Re abundances of 0.3–0.8 ppbthat are similar to the average upper crustal abundance of∼0.2–2 ppb (Esser and Turekian, 1993; Peucker-Ehrenbrink andJahn, 2001; Hattori et al., 2003; Sun et al., 2003) (Table 1). TotalOs and 192Os (non-radiogenic or “common Os” isotope) abun-dances in the Klipputs Member are 34–177 ppt and 10–66 ppt,respectively, which overlap average upper crustal abundances of30–50 ppt and 12–21 ppt, respectively (same references as forRe). The Re and Os abundances derived from CrVI-H2SO4 andinverse aqua regia digestion have an average, non-systematic, off-set of ∼20% between the two methods. It is well known thatincomplete powder homogenization during sample grinding canproduce a heterogenous distribution of Re and Os in powders,thus resulting in non-reproducibility of Re and Os abundancesand isotope ratios (e.g., Creaser et al., 2002; Selby and Creaser,2003; Kendall et al., 2004; Turgeon et al., 2007; Rooney et al.,2011). Hence, comparisons of Re and Os abundances derived fromthe CrVI-H2SO4 and inverse aqua regia protocols are difficult. The187Re/188Os and 187Os/188Os isotope ratios of the Klipputs Memberare 15–84 and 0.75–3.66, respectively. Whereas the five samplesat 176.06–176.35 m depth encompass the full range in isotoperatios, the four samples at 174.53–174.83 m exhibit a narrowrange in 187Re/188Os (19–30) and 187Os/188Os (0.97–1.52) isotoperatios.

Regression of the Re–Os isotope data derived from CrVI-H2SO4digestions of the five samples at 176.06–176.35 m yields a preciseRe–Os date of 2479 ± 20 Ma (±22 Ma if the 187Re decay constantuncertainty is included, 2�, Model 1, Mean Square of WeightedDeviates [MSWD] = 0.01, initial 187Os/188Os [IOs] = 0.12 ± 0.01). Bycontrast, inverse aqua regia digestions of the same sample pow-ders yield an imprecise Re–Os date of 2862 ± 190 Ma (2�, Model



Fig. 4. Re–Os isochron diagram for Klipputs Member black shales (calculated with Isoplot V.4.15; Ludwig, 2011). (a) Interval 176.06–176.35 m (5 samples from throughout thisinterval) and (b) interval 174.50–174.98 m (4 samples from 174.53 to 174.83 m only). Open ellipses: CrVI-H2SO4 digestions and filled ellipses: inverse aqua regia digestions.The inset diagrams show the deviation of individual sample analyses from the CrVI-H2SO4 digestion best fit line. Open squares: CrVI-H2SO4 digestions and filled squares:inverse aqua regia digestions.

3, MSWD = 148, IOs = 0.01 ± 0.15) (Fig. 4a). Because of the limitedrange in isotope ratios, imprecise Re–Os dates were obtainedfor the four samples at 174.53–174.83 m. The CrVI-H2SO4 andinverse aqua regia digestions of these samples yield Re–Os datesof 2479 ± 990 Ma (2�, Model 3, MSWD = 12, IOs = 0.14 ± 0.46) and2810 ± 740 Ma (2�, Model 3, MSWD = 80, IOs = 0.06 ± 0.32), respec-tively (Fig. 4b).

5. Discussion

5.1. Re–Os isotope systematics of the Klipputs Member

To evaluate the influence of detrital material on the Re–Osisotope systematics of the Klipputs Member, we carried out a com-parison of Re–Os isotope data using the CrVI-H2SO4 and inverse

aqua regia protocols (Selby and Creaser, 2003; Kendall et al., 2004,2009a; Rooney et al., 2011). Because the comparison is based ondata derived from aliquants of the same sample powders, it is notlikely that the differences in Re–Os isotope data between the twomethods arise from post-depositional processes.

For the five samples from the 176.06 to 176.35 m interval,the CrVI-H2SO4 protocol yields a precise Model 1 Re–Os date(2479 ± 20 Ma) with low MSWD (0.01), but the inverse aqua regiamethod yields an imprecise Model 3 Re–Os date (2862 ± 190 Ma)with high MSWD (148). Significantly, all inverse aqua regia analysesplot above the regression line defined by the CrVI-H2SO4 analy-ses. This is illustrated most clearly by the inset diagram of Fig. 4,which shows the percent deviation of individual samples from theCrVI-H2SO4 regression line. From this observation, we infer that theinverse aqua regia method dissolved a component of radiogenic



detrital Os from the Klipputs Member ORS whereas the CrVI-H2SO4medium did not. Using the regression age of 2479 Ma, we can cal-culate individual sample IOs values. Whereas the five CrVI-H2SO4analyses yield identical IOs (2479 Ma) values of 0.12, the inverse aquaregia data reveal a range of more radiogenic IOs (2479 Ma) values(0.22–0.50), implying that the detrital component was not homoge-nous during deposition of the 176.06–176.35 m interval. We inferthat changes in the Os isotopic composition and/or magnitude ofthe detrital Os flux contributed to the high MSWD of the inverseaqua regia regression. An identical conclusion was reached forthe Old Fort Point Formation (Windermere Supergroup, WesternCanada; Kendall et al., 2004), which had higher Re (6–16 ppb) butgenerally similar 192Os (30–59 ppt) abundances compared to theKlipputs Member.

Despite the limited spread in 187Re/188Os isotope ratios and thepoor precision of the Re-Os dates from the 174.53 to 174.83 m inter-val, the CrVI-H2SO4 protocol yields a Re–Os date whose MSWD(12) is also lower compared to the inverse aqua regia regression(80). Once again, a radiogenic detrital Os component is indicated byinverse aqua regia analyses plotting above the CrVI-H2SO4 regres-sion line as well as higher individual sample IOs (2479 Ma) for theinverse aqua regia (0.18–0.25) compared to CrVI-H2SO4 (0.13–0.16)analyses. However, an MSWD � 1 and slightly more radiogenicIOs (2479 Ma) compared to the 176.06–176.35 m interval raises thepossibility that the CrVI-H2SO4 medium liberated some detrital Osfrom 174.53 to 174.83 m samples during digestion. We do not favorthis scenario because the average Os abundance of the shales from174.53 to 174.83 m (total Os: 151 ppt; 192Os: 54 ppt) is significantlyhigher than the shales from 176.06 to 176.35 m (total Os: 79 ppt;192Os: 27 ppt). Alternatively, the excess scatter in the CrVI-H2SO4regression of the 174.53–174.83 m interval is caused by small-scalepost-depositional mobility of Re and Os, or a small degree of initial187Os/188Os isotope heterogeneity arising from variations in sea-water 187Os/188Os. Because of these considerations, and the limitedspread in 187Re/188Os isotope ratios, we did not proceed with anyfurther analyses from the 174.53 to 174.83 m interval.

Given the excellent linear fit of the CrVI-H2SO4 regression from176.06 to 176.35 m, we interpret the Re–Os date of 2479 ± 22 Ma(including the 187Re decay constant uncertainty) as the first directestimate for the depositional age of the Koegas Subgroup. TheRe–Os age is statistically indistinguishable from U–Pb SHRIMP zir-con ages (e.g., 2460 ± 5 Ma) from the older Kuruman Iron Formationthat are inferred to record syndepositional volcanism (Pickard,2003). Given the uncertainties in the two ages, it is possible forthe Klipputs Member to be as much as 8 Myr younger than theKuruman Iron Formation. The presence of hundreds of meters ofiron formation between the Kuruman Iron Formation and the Klip-puts Member is compatible with this time window considering thatcompacted sedimentation rates for iron formation can be relativelyrapid. For example, Trendall et al. (2004) suggested an averageof 180 m per Myr for Hamersley Group iron formation based onU–Pb geochronological constraints for the Joffre Member of theBrockman Iron Formation. The Re–Os age is also compatible withthe inferred depositional age of ca. 2430–2380 Ma for the youngerPostmasburg Group (Bau et al., 1999; Moore et al., 2012; Faireyet al., 2013). Hence, our interpretation is consistent with publishedgeochronological age constraints for the timing of Koegas Subgroupdeposition.

If the Re–Os date derived from the CrVI-H2SO4 analyses waserroneously old, then it is necessary to invoke a mixing line (withpositive slope on the isochron diagram at the time of deposi-tion, t = 0) between a detrital and hydrogenous component. Sucha mixing line could be represented by an unradiogenic Os detritalend-member with a low Re/Os ratio and a radiogenic seawater Osend-member with a high Re/Os ratio (cf. Ravizza et al., 1991). How-ever, comparison of the data obtained using the CrVI-H2SO4 and

inverse aqua regia methods revealed that radiogenic detrital Oswas preferentially liberated by the inverse aqua regia medium. Thealternative scenario is a radiogenic Os detrital end-member with ahigh Re/Os ratio and an unradiogenic seawater end-member witha low Re/Os ratio. However, obviously supracrustal 187Re/188Osratios (>150) are commonly observed in late Archean ORS (Anbaret al., 2007; Yang et al., 2009), so early Paleoproterozoic seawaterprobably had a higher Re/Os ratio than detrital material. Further-more, liberation of the detrital Os component by inverse aqua regiatypically produces a Model 3 regression with significant scatter(this study; Selby and Creaser, 2003; Kendall et al., 2004; Rooneyet al., 2011). Hence, we suggest the Klipputs Member Re–Os age isnot erroneously old.

Instead, a Re–Os depositional age of 2479 ± 22 Ma for the Klip-puts Member suggests the unpublished Pb–Pb date of 2415 ± 6 Mafrom the Rooinekke Formation (cited by Kirschvink et al., 2000;Polteau et al., 2006; Schröder et al., 2011; Moore et al., 2012) is erro-neously young. The Re–Os age also places a minimum constraint onthe amount of time separating deposition of the Klipputs Memberand Makganyene Formation. Given that the Makganyene Formationmust have been deposited after 2436 ± 7 Ma (youngest U–Pb detri-tal zircon age; Moore et al., 2012) and using the youngest possibleage for the Klipputs Member permitted by the Re–Os age uncer-tainty (2457 Ma), then more than 20 Myr of time likely separates theKlipputs Member from the Makganyene Formation. Our data is con-sistent with a significant time gap at the contact between the Ghaapand Postmasburg Groups (Schröder et al., 2011). However, if thecontact between the two groups is truly conformable in the deep-water sections (Polteau et al., 2006), then the long time intervalbetween Klipputs Member and Makganyene Formation depositionwould require the presence of condensed section(s) near the top ofthe Koegas Subgroup.

5.2. Implications for low-level Re–Os geochronology of ORS

Our work contributes to a growing body of evidence that sug-gests the CrVI-H2SO4 digestion protocol can yield precise Re–Osdepositional ages from ORS containing small amounts of hydroge-nous Re and Os, and relatively limited in situ growth of radiogenic187Os. Rooney et al. (2011) obtained a Model 1 Re–Os date of659.6 ± 9.6 Ma (MSWD = 0.01) and a Model 3 date of 655 ± 49 Ma(MSWD = 16) using the CrVI-H2SO4 and inverse aqua regia methods,respectively, for the Neoproterozoic Ballachulish Slate Formation(Dalradian Supergroup, Scotland). Compared to the Klipputs Mem-ber, the Ballachulish Slate Formation had similar or higher Reabundances (two samples with 0.3–0.4 ppb; three samples with1–2 ppb) but lower total Os (26–54 ppt) and 192Os (8–17 ppt)abundances. Based on the differences in Re–Os isotope data fromthe CrVI-H2SO4 and inverse aqua regia methods, Rooney et al.(2011) concluded that inverse aqua regia liberated a componentof unradiogenic detrital Os but the CrVI-H2SO4 medium did not.However, that study suffered from the drawback that independentgeochronological age constraints that could verify the accuracyof the Re–Os date were located several km above and below thedated horizon. Our study provides additional confidence that theBallachulish Re–Os date records a depositional age.

In both the Klipputs Shale and Ballachulish Slate examples, theRe and Os abundances derived from the CrVI-H2SO4 and inverseaqua regia methods are generally similar and do not show system-atic differences. Similar observations were made for the ExshawFormation (western Canada), Old Fort Point Formation (westernCanada), Aralka Formation (central Australia) and Tindelpina ShaleMember (basal Tapley Hill Formation, southern Australia) (Selbyand Creaser, 2003; Kendall et al., 2004, 2009a; Kendall, 2008). Inthe case of the Australian examples, Os abundances of only ∼2and 12 ppt were obtained by inverse aqua regia measurements



of solid residues left behind after CrVI-H2SO4 digestion of ORScontaining ∼82 ppt and 113 ppt Os, respectively (Kendall, 2008;Kendall et al., 2009a). Collectively, these observations could implythat the Os abundance of the detrital fraction of ORS is generallylower than the upper crustal average of ∼30–50 ppt. However, arecent comparison of major and trace element abundances in ORSderived from inverse aqua regia and quantitative whole-rock dis-solution (using concentrated HF + HClO4 + HNO3 + HCl) shows thatinverse aqua regia dissolves only a small proportion (<10–30%) ofthe detrital mineral fraction (Xu et al., 2012). Hence, a majorityof the Re and Os liberated during inverse aqua regia digestion islikely hydrogenous in origin (plus 187Os produced by in situ 187Redecay), consistent with the similar abundances obtained from theCrVI-H2SO4 and inverse aqua regia methods. The clear contrasts inthe Re–Os isotope systematics of the two methods for several ORS(Model 1 CrVI-H2SO4 versus Model 3 inverse aqua regia regres-sions; this study, Selby and Creaser, 2003; Kendall et al., 2004;Rooney et al., 2011) further show that CrVI-H2SO4 digestions resultin negligible release of detrital Os from ORS.

Other examples of Model 1 Re–Os ages (using the CrVI-H2SO4 protocol) are known from ORS with slightly higherhydrogenous Re but similar total Os and 192Os abundances com-pared to the Klipputs Member. Basal Albian gray clay of theGault Formation (Germany; Re = 0.9–2.4 ppb, total Os = 93–117 ppt,192Os = 35–44 ppt) yielded a Model 1 Re–Os age of 108.9 ± 6.2 Ma(MSWD = 0.12) that is statistically indistinguishable from the cur-rently accepted Aptian–Albian boundary age of 112.0 ± 1.0 Ma(Selby et al., 2009). Kendall et al. (2006) obtained a Model 1Re–Os age of 645.1 ± 4.8 Ma (MSWD = 1.2) from the basal Tap-ley Hill Formation (Adelaide Rift Complex, southern Australia;Re = 1.6–4.7 ppb, total Os = 100–145 ppt, 192Os = 33–42 ppt). A sta-tistically identical Model 1 Re–Os age of 647 ± 10 Ma (MSWD = 0.79)was derived from a stratigraphically correlative interval contain-ing higher abundances of Re (5.6–10.0 ppb), similar abundancesof Os (93–135 ppt), and lower abundances of 192Os (20–28 ppt).Creaser and Stasiuk (2007) also reported a Model 1 Re–Osage of 1541 ± 13 Ma (MSWD = 1.5) from the Douglas Formation(Athabasca Group, Canada; Re = 1.6–4.4 ppb, total Os = 95–125 ppt,192Os = 12–35 ppt).

These examples suggest that low-level Re–Os geochronologyof ORS will find widespread application for placing time pins insedimentary basins. Non-mass-dependent S isotope signaturesare not observed in the Mooidraai Formation of the upper Post-masburg Group and its possible stratigraphic equivalents (theupper Duitschland/Rooihoogte formations) in the Transvaal basin(Bekker et al., 2004; Guo et al., 2009; Moore et al., 2012). Bycontrast, non-mass-dependent S isotope signatures have beenreported from the Koegas Subgroup (Ono et al., 2008). Hence, theKlipputs Member was likely deposited before the Great OxidationEvent. Previous Re–Os geochronology of Archean ORS used mate-rial containing >5 ppb Re (Anbar et al., 2007; Yang et al., 2009). Ournew Re–Os age from the Klipputs Member suggests that ArcheanORS containing low hydrogenous Re and hydrogenous + radiogenicOs enrichments – reflecting limited oxidative weathering of uppercrust and low seawater Re and Os concentrations – are amenableto geochronology.

5.3. Seawater 187Os/188Os across the Archean–Paleoproterozoictransition

The IOs from a Re–Os ORS isochron regression provides an esti-mate for the local seawater 187Os/188Os at the time of sedimentdeposition (Ravizza and Turekian, 1992; Cohen et al., 1999; Ravizzaand Paquay, 2008). As summarized by Peucker-Ehrenbrink andRavizza (2000), seawater 187Os/188Os reflects a balance betweenthe riverine input of radiogenic Os from oxidative weathering and

erosion of the upper continental crust (modern 187Os/188Os ∼ 1.5;Levasseur et al., 1999) and unradiogenic Os from dissolution ofcosmic dust and the hydrothermal alteration of oceanic crust andperidotites (modern 187Os/188Os ∼ 0.13; Walker et al., 2002a,b;Sharma et al., 2007). Hence, the IOs of Precambrian ORS is relevantto reconstructions of temporal trends in Earth surface oxygen-ation because oxidative weathering is required for mobilizationand transport of radiogenic Os to the oceans (Hannah et al., 2004;Kendall et al., 2009a).

Two Archean ORS, the 2695 ± 14 Ma Joy Lake Sequence (south-western Superior Province, Minnesota, U.S.A.; Yang et al., 2009)and the 2495 ± 18 and 2464 ± 41 Ma Mt. McRae Shale (Hamers-ley basin, Western Australia; Anbar et al., 2007), yield IOs values(0.15 ± 0.16, 0.06 ± 0.09, and 0.86 ± 0.86, respectively) that cannotbe statistically resolved from the late Archean chondritic/mantlebaseline of ∼0.11. The 2479 ± 22 Ma Klipputs Member also yieldsmantle IOs of 0.12 ± 0.01. Thus far, ORS deposited before the GreatOxidation Event do not reveal a clear Os isotope signature for oxida-tive weathering. Pronounced Re and Mo enrichments together withMo, S, and N isotope data from the upper Mt. McRae Shale pointto an episode of oxidative weathering and surface ocean oxy-genation at ∼2.50 Ga (Anbar et al., 2007; Kaufman et al., 2007;Garvin et al., 2009; Reinhard et al., 2009; Duan et al., 2010).The ORS with the highest Re and Mo abundances yielded an ageof 2464 ± 41 Ma and an IOs value of 0.86 ± 0.86. The poor preci-sion arises from all samples having high present-day 187Re/188Osand 187Os/188Os. However, regression of all Re-Os isotope datafor the Mt. McRae Shale (covering 20 m in the same drill core)yields a Model 1 age of 2501 ± 8 Ma and an mantle IOs valueof 0.04 ± 0.06. Hence, there is no clear evidence for a contribu-tion of radiogenic crustal Os to seawater during this “whiff” ofO2.

5.4. Implications for correlations between the Kaapvaal andPilbara cratons

Our new Re–Os age from the Klipputs Member may requirea revision of stratigraphic correlations between the Ghaap andHamersley groups. Previous correlation schemes suggested thelower Koegas Subgroup is temporally equivalent to the upper Bool-geeda Formation and younger in age than the Woongarra and WeeliWolli formations of the upper Hamersley Group (e.g., Fig. 6 ofBeukes and Gutzmer, 2008). In this scenario, U–Pb SHRIMP zirconages of ca. 2446–2440 Ma from the upper Kuruman Iron Forma-tion are interpreted to record the age of syndepositional volcanism,suggesting correlation with the Weeli Wolli Formation. In con-trast, Pickard (2003) interpreted these dates as influenced by Pbloss or post-depositional resetting, preferring instead to use datesof 2461 ± 6, 2460 ± 5, 2460 ± 8, and 2464 ± 6 Ma as the best esti-mates for the age of upper Kuruman deposition (Fig. 5). Our newRe–Os age of 2479 ± 22 Ma from the Klipputs Member is statisti-cally older than the U–Pb zircon ages from the Weeli Wolli andWoongarra formations, thereby supporting the interpretation ofPickard (2003). It is unclear if decay constant uncertainties areincluded in the U–Pb zircon ages, but our interpretation does notchange if this uncertainty is added. Furthermore, the Re–Os agefrom the Klipputs Member is statistically indistinguishable fromthe U–Pb SHRIMP zircon dates of tuffaceous beds within the Brock-man Iron Formation of the middle Hamersley Group (Pickard,2002; Trendall et al., 2004). As shown in Fig. 5, we suggest theBrockman Iron Formation is broadly equivalent in age to the Kuru-man and Griquatown iron formations and the Koegas Subgroup.It then follows that the upper Hamersley Group (Weeli Wolli,Woongarra, and Boolgeeda formations) is younger than the GhaapGroup.



Fig. 5. Proposed stratigraphic correlations between the Ghaap and Hamersley groups based on U–Pb zircon ages (normal type) and Re–Os ORS ages (bold). The 187Re decayconstant uncertainty was included when calculating the uncertainty on Re–Os ORS ages because the Re-Os ages are compared with U-Pb zircon ages. Sources of data: (1)Arndt et al. (1991), (2) Trendall et al. (2004), (3) Rasmussen et al. (2005), (4) Trendall et al. (1998), (5) Anbar et al. (2007), (6) Pickard (2002), (7) Barley et al. (1997), (8)Walraven and Martini (1995), (9) Martin et al. (1998), (10) Altermann and Nelson (1998), (11) Barton et al. (1994), (12) Sumner and Bowring (1996), (13) Pickard (2003), and(14) this study.

5.5. Implications for global correlations of Paleoproterozoicglacial deposits

Recently, Hoffman (in press) synthesized the debate on thecorrelation of early Paleoproterozoic glacial deposits, which havelong been hampered by the limited number of identified glacialdeposits and reliable geochronological ages. Hoffman (in press)proposes a simple correlation scheme that takes into account thestratigraphic horizon where mass-independent fractionation ofsulfur isotopes disappears from the rock record (Fig. 6). The firsttwo Paleoproterozoic glaciations are suggested to precede the dis-appearance of non-mass dependent sulfur isotope signatures (asimilar scheme is shown in Fig. 1 of Bekker and Holland, 2012).Hence, the Makganyene glaciation was assigned to the second ofthree Paleoproterozoic glaciations, consistent with the absenceof non-mass-dependent sulfur isotope signatures in the youngerMooidraai Formation (Guo et al., 2009; Hoffman, in press) and thepresence of such signatures in the older Koegas Subgroup (Onoet al., 2008).

The Re–Os age from the Klipputs Member does not improve ageconstraints on the low-latitude Makganyene glaciation beyond thatof Moore et al. (2012). However, it does require that the diamictitelens in the Doradale Formation be older than 2479 ± 22 Ma. Hence,the Doradale diamictite may not be correlative with the <2444+8/−4 Ma diamictites in the Ramsay Lake Formation (HuronianSupergroup; age from Krogh et al., 1984) or with the <2445 ± 4 Madiamictites in the Meteorite Bore Member (Kungarra Formation,lower Turee Creek Group, which overlies the Hamersley Group inWestern Australia; age from Trendall et al., 2004). The MeteoriteBore Member could readily be assigned to the middle Paleoprotero-zoic glaciation, consistent with both geochronological and sulfurisotope data (Williford et al., 2011; Hoffman, in press). How-ever, a nullification of the Doradale – Ramsey Lake correlation

Fig. 6. Revised global correlations of Paleoproterozoic glacial deposits. The correla-tion scheme is modified from Hoffman (in press) to show that diamictite from theDoradale Formation may be older than other G1 diamictites based on the Re–Os agefrom the Klipputs Member. The box pattern represents cap carbonates above theVagner and Bruce diamictites. Dark shading and light shading indicate anoxic andoxic atmospheric conditions, respectively, based on the presence/absence of non-mass-dependent sulfur isotope signatures, redox-sensitive detrital minerals, andpaleosols. White shading indicates an absence of paleoredox data (see Hoffman, inpress, for further explanation). Depositional age constraints include: (1) U–Pb zir-con age from rhyolite near the base of the Huronian Supergroup (Krogh et al., 1984),(2) youngest U–Pb detrital zircon age from the Makganyene Formation (Mooreet al., 2012), (3) Re–Os age from the Klipputs Member gray shale (this study), (4)youngest U–Pb detrital zircon age from volcaniclastic beds in the Hekpoort Forma-tion (Dorland, 2004), (5) Re–Os age from synsedimentary to early diagenetic pyritein black shale of the Rooihoogte and Timeball Hill formations (Hannah et al., 2004),and (6) U–Pb zircon age from tuff in the Woongarra rhyolite (Trendall et al., 2004).

may mean that global correlations are more complicated thanthat recently proposed by Hoffman (in press). Alternatively, theHoffman (in press) correlation scheme can be preserved if theDoradale diamictite does not have a glacial origin and the evidencefor the oldest Paleoproterozoic glaciation in the Griqualand West



basin has been removed by erosion (or otherwise has yet to be iden-tified), or if the geochronological constraint on the Ramsey LakeFormation is incorrect. Additional geochronological, sulfur isotope,and other paleoredox data are needed to fully resolve the rela-tionship between Paleoproterozoic glaciation and Earth’s surfaceoxygenation.

6. Conclusions

We report a new Re–Os depositional age of 2479 ± 20 Ma(±22 Ma if the 187Re decay constant uncertainty is included) forthe Klipputs Member (Nelani Formation, Ghaap Group, TransvaalSupergroup, South Africa) using the now-widely-adopted CrVI-H2SO4 digestion protocol for ORS. Through a comparison withRe–Os isotope data derived from inverse aqua regia digestion,we are able to show that, despite low abundances of hydroge-nous Re (0.3–0.8 ppb) and hydrogenous + radiogenic Os (total Osand 192Os of 34–177 and 10–66 ppt, respectively), detrital Os didnot adversely affect the quality of the CrVI-H2SO4 Re–Os isochronregression. Our Re–Os age from the Klipputs Member statisticallyagrees with published U–Pb zircon ages from underlying and over-lying stratigraphic units. Our results further highlight the potentialfor widespread application of low-level Re–Os ORS geochronology,including rocks deposited before the Great Oxidation Event. Theinitial 187Os/188Os from the isochron regression reveals that con-temporaneous local seawater had a chondritic 187Os/188Os isotopecomposition, suggesting minimal oxidative weathering and trans-port of radiogenic crustal Os to the oceans at this time. Integrationof the Klipputs Member Re–Os age with existing U–Pb zircon agesfor the Ghaap and Hamersley groups suggest that the upper Hamer-sley Group (above the Brockman Iron Formation) is younger thanthe Ghaap Group. The depositional age of diamictite in the DoradaleFormation, which stratigraphically underlies the Klipputs Memberin the Koegas Subgroup, is older than 2479 ± 22 Ma. Comparisonwith other geochronological constraints suggests that the Doradalediamictites are not correlative with glacial deposits assigned to theoldest Paleoproterozoic glaciation.

Acknowledgements

This research was supported by a Natural Sciences and Engi-neering Research Council (NSERC) Discovery Grant to R.A. Creaser.We thank Joseph Kirschvink for facilitating access to the drill core atthe California Institute of Technology. The Radiogenic Isotope Facil-ity at the University of Alberta was supported, in part, by an NSERCMajor Facilities Access grant for this study. Constructive commentsby Hari Tsikos, an anonymous reviewer, and editor Randall Parrishimproved the manuscript.

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Depositional age of the early Paleoproterozoic Klipputs Member, Nelani Formation (Ghaap Group,...

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