Constraints on the timing of Marinoan “Snowball Earth” glaciation by 187Re–187Os dating of a Neoproterozoic, post-glacial black shale in Western Canada

Earth and Planetary Science Letters 222 (2004) 729 – 740 www.elsevier.com/locate/epsl

Constraints on the timing of Marinoan ‘‘Snowball Earth’’ glaciation by 187Re– 187Os dating of a Neoproterozoic, post-glacial black shale in Western Canada Brian S. Kendall a,*, Robert A. Creaser a, Gerald M. Ross b, David Selby a a

Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 ESB, Edmonton, Alberta, Canada T6G 2E3 b P.O. Box 458, Kula HI 96790, USA Received 14 October 2003; received in revised form 1 April 2004; accepted 2 April 2004

Abstract New Re – Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re – Os isochron date of 634 F 57 Ma (MSWD = 65, n = 5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO3 – H2SO4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8 F 4.7 Ma (MSWD = 1.2). Both dates are in agreement with existing U – Pb age constraints that bracket the Old Fort Point Formation between f 685 and f 570 Ma. The distinctive Re – Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO3 – H2SO4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8 F 4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan ( f 620 – 600 Ma) ice age and older than the f 580-Ma Gaskiers glaciation of northeastern North America. The new Re – Os age determined from the CrO3 – H2SO4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re – Os date obtained with the aqua regia protocol. These results also indicate the potential for Re – Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC < 1%) of the Old Fort Point Formation precluded the determination of a precise Re – Os depositional age using the CrO3 – H2SO4 analytical protocol. D 2004 Elsevier B.V. All rights reserved. Keywords: Re – Os absolute-age dating; black shales; Old Fort Point Formation; Neoproterozoic; Marinoan

1. Introduction

* Corresponding author. Tel.: +1-780-492-2942; fax: +1-780492-2030. E-mail address: [email protected] (B.S. Kendall). 0012-821X/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2004.04.004

The worldwide distribution of Neoproterozoic glaciogenic deposits may represent the geologic record of multiple, global synchronous ice ages [1 – 4]. Paleomagnetic measurements have conclusively dem-

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onstrated that several of the glacial deposits were the products of widespread, low-latitude glaciation (see [5] for a review). An obvious test for a ‘‘Snowball Earth’’ event is precise radiometric age determinations on apparently equivalent glacial intervals from several different continents. However, the extent, timing, number, and duration of the Neoproterozoic ice ages are poorly constrained because only a few glacial deposits have reliable age constraints—usually provided by U –Pb dates from zircon [5]. This problem results from a scarcity of suitable igneous rocks amenable for radiometric dating that are either directly incorporated within glacial units or are temporally and spatially related to the diamictites. New radiometric dating methods are thus needed to provide more comprehensive age information for the glacial horizons. The 187Re (rhenium) – 187Os (osmium) radioisotope system represents one long-lived geochronometer capable of providing a precise depositional age for some clastic sedimentary rocks like black shales [6 – 10]. Both Re and Os become concentrated within anoxic sediments by redox reactions at or below the sediment –water interface, and are known to be incorporated in the organic matter of black shales [6,9 – 17]. In addition, Re and Os in these rocks are primarily hydrogenous in origin (i.e., derived from seawater; [6,7]). Following deposition, Re and Os can behave as a closed system [7– 10] in which radioactive 187Re undergoes beta decay to radiogenic 187Os over time (k187Re = 1.666  10 11 year 1 [18]). This forms the basis of a viable deposition-age geochronometer, assuming the non-hydrogenous component of Re and Os is negligible [6– 12]. Application of the Re –Os radioisotope system to black shales in close stratigraphic proximity to Neoproterozoic diamictites is a potential dating tool for correlation of these glacial deposits worldwide. Here, we demonstrate the utility of the Re –Os geochronometer by precisely dating the upper Old Fort Point Formation, a post-glacial, black shale marker horizon from the middle Windermere Supergroup in western Canada. By dating this unit, we also demonstrate that precise Re – Os depositional ages may be obtained from black shales metamorphosed to chlorite-grade (i.e., slates) and on shales containing low total organic carbon contents (TOC < 1%). To achieve a precise Re –Os age from such rocks requires use of a CrO3 –

H2SO4 digestion technique, which minimizes analysis of detrital Re and Os [10]. By comparison with conventional inverse aqua regia digestion, we show that this dissolution method has widespread utility for Re – Os dating of low-TOC shales.

2. The Old Fort Point Formation, Windermere Supergroup, Western Canada The Old Fort Point Formation (hereafter OFP) is a widespread, fine-grained triad (siltstone, rhythmic marble-pelite, and sulfidic, carbonaceous pelite) marker unit exposed over an area of 35,000 km2 throughout southeastern British Columbia and southwestern Alberta, Canada (Fig. 1) [19]. This succession is sandwiched between thick units of feldspathic sandstones, granule conglomerates, and pelites (collectively termed the ‘‘grit’’ division [20]) of the Neoproterozoic Windermere Supergroup in the southeastern Canadian Cordillera. Along with most of the Windermere Supergroup in this region, the grits were deposited in a northwest-trending longitudinal, basinal turbidite system along either a west-facing passive margin [21,22] or within a 2500-km-long intracontinental rift system [23]. The fine-grained nature of the OFP represents the development of anoxic conditions and the termination of clastic sediment deposition as a result of a eustatic sea-level rise throughout the Windermere basin [19]. The uppermost carbonaceous interval of the OFP records a highstand systems tract produced by maximum flooding conditions [22]. The OFP is capped by coarse-grained turbiditic grits that locally contain carbonate clasts with pseudomorphic calcite crystal rosette and teepee structures [22] similar to those present within the Teepee Dolomite (cap carbonate of the glacial Ice Brook Formation) in the Mackenzie Mountains [25]. Such structures are also found within the carbonate clasts from the carbonaceous pelites (basal Framstead Formation) overlying the glaciogenic Mount Vreeland cap carbonate in northeastern British Columbia [22,24]. In the latter case, the carbonate clasts were interpreted as olistoliths derived from cap carbonates deposited to the east on the Windermere continental shelf [24]. On the basis of these lithological similarities, the OFP is interpreted as the deep-water facies

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Fig. 1. Distribution of the Windermere Supergroup within the regional geological setting of the southeastern Canadian Cordillera. The known localities of the OFP (A – G) and the sampling location (52j52.346VN; 118j17.722VW; denoted by a star) are also shown. SRMT = Southern Rocky Mountain Trench. Modified after Ross et al. [22].

equivalent of the cap carbonates of the second Windermere glaciation (Fig. 2) [22]. Within the southeastern Canadian Cordillera, there are no diamictites that are correlative with this glaciation because of the distal offshore paleogeographic position within the Windermere basin. The eustatic sea-level rise recorded by the OFP thus indirectly represents the only record of the second Windermere ice age in this region. However, field studies have demonstrated an abrupt southwest lateral facies change of the Mount Vreeland diamictites into turbiditic grits and pelites of the Windermere ‘‘grit’’ division [24,26]. This is also reflected by the position of the coarsest grits directly beneath the OFP.

There are currently no age constraints for the OFP and its correlative units in western Canada. The maximum age of the Windermere Supergroup is f 762– 728 Ma based on U – Pb zircon and Sm – Nd ages from granitic and volcanic rocks that unconformably underlie the Windermere or intrude its basal succession [22,27]. U – Pb SHRIMP zircon dates of 685 F 7 and 684 F 4 Ma were obtained from felsic volcanic rocks of the Edwardsburg Formation (Idaho) that are interbedded with glaciogenic rocks of the first Windermere glaciation [28]. The Toby Formation and Rapitan Group represent possible correlatives of these diamictites in the southern and northern Canadian Cordillera. A U – Pb zircon date of 569.6 F 5.3 Ma

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Fig. 2. Stratigraphic columns for the Windermere Supergroup in the Canadian Cordillera showing the inferred correlations between the diamictites (Ice Brook and Mount Vreeland Formations) and cap carbonates (Teepee Dolomite and basal Framstead Formation) of the second Windermere glaciation and their relationship to the OFP. Known age constraints for the Windermere Supergroup, including the Re – Os age reported in this study, are shown on the right (* denotes present study). Modified after Ross et al. [22].

was determined for volcanic rocks of the Hamill Group that unconformably overlie the Windermere Supergroup [23]. The stratigraphic position of the second Windermere glaciation well above f 685 Ma glaciogenic rocks suggests it may be correlative with either ‘‘Marinoan’’ (ca. 620 –600 Ma [5]) glacial deposits found worldwide [22] or the Gaskiers glaciation ( f 580 Ma [29]). Re – Os dating of the OFP thus represents an excellent opportunity not only to provide a minimum age constraint for the second Windermere glaciation, but also establish radiometric age control on the very thick (up to 6 km), otherwise undated, post-Toby/Rapitan succession of the Windermere Supergroup in the Canadian Cordillera.

3. Sampling and analytical methods Samples of sulfidic slates (containing f 0.5% TOC [Ross, unpublished data]) from the upper OFP carbonaceous interval were collected along a single Highway 16 roadcut outcrop approximately 15 km west of Jasper, Alberta (52j52.346VN, 118j17.722VW). Stratigraphic sampling was limited to a vertical interval of 50 cm. Rocks of the Windermere Supergroup in the Rocky Mountain Fold and Thrust Belt were deformed and metamorphosed to the lower greenschist facies as a result of Mesozoic orogenic events along the western continental margin of North America [30,31]. Because the robustness of the Re – Os radioisotope system in

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black shales during metamorphism is unknown, it was necessary to choose a sampling locality where the metamorphic grade was lowest. For this reason, the OFP at the above locality was chosen for sampling rather than higher-grade units such as the basal Framstead Formation, even though the latter has a more intimate stratigraphic association with the Mount Vreeland diamictites. Strata exposed in the sampled roadcut are folded and display well-developed cleavage, and in this area, metamorphism is of chlorite-grade [30] and XRD analysis indicates a chlorite-grade mineral assemblage for the upper OFP samples used here. The biotite isograd is located far to the west near Tete Juane Cache, British Columbia ( f 85 km from the sampled roadcut) [30]. Processing of samples to produce fine powder ( f 30 Am) follows the procedure outlined in [9]. Only the unweathered portions of each whole-rock sample were used for Re – Os analysis because a significant fraction of the Re, Os, and organic carbon budgets in black shales are lost during weathering processes [15 –17]. In addition, to avoid any potential possible small-scale disturbance of the Re –Os system by metamorphism, large amounts of rock (85 –120 g, f 30 –45 cm3) were processed for each sample. Rhenium and Os isotope analyses were carried out at the Radiogenic Isotope Facility of the Department of Earth and Atmospheric Sciences, University of Alberta. To test the effects of detrital Os on the Re – Os isotope systematics of the OFP, two separate digestion methods were carried out in Carius tubes [32] for each sample. The inverse aqua regia method is known to dissolve both the non-hydrogenous (detrital and meteoritic particulate) and hydrogenous Re and Os fractions from black shales [10]. This technique involves dissolution of a sample and an isotopically enriched 185Re – 190Os spike in a 1:2 acid mixture containing 3 ml of 12 N HCl and 6 ml of 16 N HNO3 [9]. In contrast, the CrO3 – H2SO4 dissolution technique minimizes detrital Os and Re digestion by selectively dissolving organic matter that is dominated by hydrogenous Os [10]. However, any extraterrestrial component from cosmic dust is likely digested as well. Following sample digestion, chemical separation and purification of Re and Os were carried out using previously published procedures [9,10]. The prepared Re and Os fractions were loaded onto Ni and Pt

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filaments, respectively, and the concentrations and isotopic compositions were determined by isotope dilution—negative thermal ionization mass spectrometry (ID-NTIMS) [33,34] on a Micromass Sector 54 mass spectrometer. Rhenium and Os analyses were carried out using Faraday collectors in static mode and an electron multiplier in single collector peak-hopping mode, respectively. Aqua regia Re and Os measurements were also made using Faraday collectors prior to development of the CrO3 – H2SO4 dissolution technique and these are included here as replicate analyses. Corrections were made to the Re and Os isotopic ratios for isobaric oxygen interferences, instrumental mass fractionation (using 185Re/187Re = 0.59738 [35] and 187Os/188Os = 3.08261 [36]), and blank and spike contributions. Procedural blanks are 4.5 pg Re and 0.24 pg Os (CrO3 – H2SO4), and 1.7 –4.5 pg Re and 1.2– 3.1 pg Os (aqua regia). The blank 187Os/188Os isotopic composition was 0.27 and 0.26 for CrO3 – H2SO4 and aqua regia digestion, respectively. Inhouse standard solutions of Re (AB-1) and Os (AB2) were repeatedly analyzed to monitor long-term mass spectrometer reproducibility. The AB-1 standard solution yielded a 185Re/187Re ratio of 0.59813 F 0.00047 (1 S.D., n = 51) whereas the AB-2 solution gave 187Os/188Os ratios of 0.1068 F 0.0002 (1 S.D., n = 31) and 0.10683 F 0.00009 (1 S.D., n = 39) using electron multiplier and Faraday collectors, respectively. Regressions were performed with the program Isoplot [37] using a 187Re decay constant of 1.666 10 11 year 1 [18]. Uncertainties for 187 Re/ 188 Os and 187 Os/188Os isotope ratios were calculated by numerical error propagation and include uncertainties in the spike calibration, spike and sample weighing (for Re, Os abundances only), mass spectrometric analysis, Re standard bias, oxygen, spike and blank isotopic composition and blank abundance. The error correlation (rho) between 187Re/188Os and 187Os/188Os is significant for these samples with 187Re/188Os of f 250 – 600, and ranges from f 0.4 to 0.6.

4. Results The Re and Os concentrations and isotope compositions obtained through inverse aqua regia and CrO3 – H2SO4 digestion of five samples from the

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OFP are presented in Table 1. Elemental concentrations are broadly similar for the two methods and are significantly enriched relative to average Re and Os crustal values of f 2 ppb [38] and 0.03 –0.05 ppb [39 – 41], respectively. Notably, the Re (6– 16 ppb), total Os (125 – 249 ppt), and 192Os (30 – 59 ppt) concentrations and TOC contents ( f 0.5%) are significantly lower than those previously reported for other black shales dated with the Re – Os radioisotope system [6,7,9,42]. Duplicate aqua regia analyses show generally good agreement for Re concentrations (1 – 3% variation), but significant variation in Os concentrations (up to 10% variation), which primarily controls coupled variation in 187Re/188Os and 187Os/188Os well beyond 2r analytical uncertainties (see Table 1). Similar observations have been previously recorded, and may be ascribed partially not only to sample powder inhomogeneties [9], but also to the presence of multiple and/or separate host phases for Re and Os [9,10]. The isotope ratios for 187Re/188Os and 187Os/188Os range between f 250 and 600, and f 3.2 and 6.9, respectively. Regression of the Re –Os aqua regia data set using Isoplot yields an imprecise date of 634 F 57 Ma, with considerable scatter about the regression line (MSWD = 65; 2r, Model 3, Fig. 3). In contrast, a well-fitted isochron yielding a precise date of

607.8 F 4.7 Ma (MSWD = 1.2; 2r, Model 1, Fig. 3) is obtained for the same sample powders using the CrO3 –H2SO4 digestion method. For all samples except BK-01-015A, the 187Re/188Os and 187Os/188Os isotope ratios obtained from the aqua regia digestion are higher than those determined with CrO3 – H2SO4 digestion, using the same sample powder.

5. Discussion 5.1. Effect of detrital Os on the Re – Os isotope systematics of the OFP Determining accurate and precise depositional ages for black shales with the 187Re – 187Os radioisotope system requires the fulfillment of several criteria. All samples must be the same age and have the same initial 187Os/188Os ratio that reflects the composition of seawater at the time of sediment deposition. In addition, Re and Os must remain a closed system within the sediments following deposition (i.e., negligible disturbance of these elements through postdepositional processes such as weathering and metamorphism). To determine a precise age, a sufficient range in 187Re/188Os ratios must be obtained in order to generate a corresponding range in present-day

Table 1 Re and Os concentrations and isotope data for the OFP slates Samplea BK-01-014B BK-01-014B BK-01-015A BK-01-015A BK-01-015B BK-01-015B BK-01-015C BK-01-015C BK-01-015D BK-01-015D a

ar rpt cr ar rpt cr ar rpt cr ar rpt cr ar rpt cr

Re (ppb)

Os (ppt)

192 Os (ppt)

187

Re/188Osb

15.57 15.27 15.43 6.340 6.249 6.357 12.40 12.12 12.53 8.463 8.346 8.543 7.299 7.116 7.279

238.7 232.8 249.4 170.6 153.3 164.7 216.1 206.4 218.9 185.4 185.0 192.0 129.6 125.1 144.8

52.6 51.0 58.5 50.1 43.3 48.1 52.2 49.2 53.8 49.9 50.2 53.3 31.0 29.7 38.2

588.40 595.49 524.67 251.86 287.36 262.93 472.53 490.26 463.71 337.27 330.70 318.70 468.01 476.21 379.58

(2.57) (2.76) (2.25) (1.10) (1.37) (1.17) (2.04) (2.33) (2.01) (1.53) (1.54) (1.39) (2.33) (2.59) (1.78)

187

Os/188Osb

6.8138 6.9017 5.9542 3.2403 3.6773 3.2991 5.5639 5.7443 5.3523 4.2138 4.1250 3.8588 5.6804 5.7760 4.4750

(0.0192) (0.0213) (0.0175) (0.0091) (0.0123) (0.0105) (0.0148) (0.0201) (0.0158) (0.0137) (0.0133) (0.0115) (0.0204) (0.0242) (0.0157)

Rho

IOs(608

0.457 0.458 0.404 0.436 0.460 0.426 0.458 0.454 0.427 0.468 0.430 0.427 0.615 0.625 0.514

0.82 0.84 0.61 0.68 0.75 0.62 0.75 0.75 0.63 0.78 0.76 0.61 0.92 0.93 0.61

‘‘ar’’ denotes inverse aqua regia digestion, ‘‘cr’’ denotes CrO3 – H2SO4 digestion, and ‘‘rpt’’ denotes a replicate analysis. Numbers in parentheses denote measured 2r uncertainty in the isotope ratio calculated by numerical error propagation. c IOs(608 Ma) = initial 187Os/188Os isotope ratio calculated at 608 Ma. b

c Ma)

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Fig. 3. Re – Os isochron diagram for the OFP black shales. Inverse aqua regia digestion analyses are represented by open ellipses; regression of these points yields the dashed isochron. CrO3 – H2SO4 digestion analyses are represented by filled ellipses and their regression by the solid isochron. To make a direct comparison of the regressed Re – Os data set for the aqua regia and CrO3 – H2SO4 analytical protocols, the replicate analyses were not included in the aqua regia regression. The inset diagram shows the deviation of each point from the CrO3 – H2SO4 digestion analyses best-fit regression line.

187

Os/188Os ratios. Finally, the Re – Os radiometric dating method assumes that Re and Os in black shales are entirely hydrogenous in origin. Detrital and extraterrestrial materials are sources of non-hydrogenous Os in black shales that may cause scatter in isochron regressions. Previous Re – Os studies on these rocks suggested the meteoritic flux is a negligible source of non-hydrogenous Os (i.e., f 0.01 – 0.1% of the total Os budget) assuming this flux is similar to that of the Cenozoic [7,9]. While quantitative estimates of the Neoproterozoic meteoritic flux are unknown, if they are also broadly similar to the average Cenozoic flux (e.g., a meteorite flux of f 3.7  104 T/year, a chondritic meteorite Os concentration of 486 ppb, and a sediment

accumulation rate of 50 m/Ma yields a meteoritic Os contribution of f 0.28  10 12 g Os/g mudrock [7]), then extraterrestrial Os contributions to the OFP account for f 0.1– 0.2% of the total Os budget in these rocks and can thus also be considered negligible. The effects of detrital Os may not be negligible. Ravizza et al. [11] showed that mixing of a detrital Os component with hydrogenous Os could lead to Re– Os dates that are younger or older than the true depositional age. Also, detrital Os effects were interpreted to have affected the Re – Os systematics in black shales of the Exshaw Formation (Alberta, Canada) [9]. Small differences in accuracy and precision of the depositional age determinations were

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observed between the inverse aqua regia (358 F 10 Ma [9]) and CrO 3 – H 2 SO 4 (366.1 F 9.6 and 363.4 F 5.6 Ma [10]) digestion protocols, although these ages were still equivalent at the 2r level. Thus, a detrital Os component represents a potential source of geological error that could result in imprecise and/ or inaccurate depositional age determinations. Under conditions of a constant detrital Os flux, this effect would be more pronounced for black shales with small amounts of hydrogenous Re and Os because the detrital Os budget represents a larger fraction of the total Os. This situation might be expected in lowTOC rocks such as the OFP given the low Re and Os contents relative to other black shales (see above). Both Re –Os dates obtained for the OFP using aqua regia and CrO3 –H2SO4 digestion are consistent with the known age constraints for the unit, and its stratigraphic position between the f 685 Ma Edwardsburg Formation and f 570 Ma Hamill Group volcanic rocks. However, the high degree of scatter (MSWD = 65) and imprecise date (9% uncertainty) derived from the aqua regia digestions contrasts strongly with the low degree of scatter (MSWD = 1.2) and very precise date (607.8 F 4.7 Ma) obtained by CrO3 – H2SO4 digestion of the same sample powders. These observations are unlikely to relate to any geological process such as weathering or metamorphism, as the same sample powders were used for the two digestion methods. Rather, these results are consistent with the findings and conclusions of [10], that the inverse aqua regia analytical

protocol dissolved a significant detrital component of Os, but the CrO3 – H2SO4 digestion did not. Notably, all inverse aqua regia analyses plot above the CrO3 – H2SO4 digestion regression on the Re – Os isochron diagram (Fig. 3), consistent with the inclusion of a radiogenic detrital component of Os in the former analyses. Using the f 608-Ma isochron as a reference line, a wide range in IOs(608 Ma) values is apparent from aqua regia analyses (0.68 – 0.92; Table 1), indicating that the detrital Os component was apparently not homogenous over the 50-cm stratigraphic interval of sampling. This may reflect changes in the Os isotope composition and/or magnitude of the detrital Os flux to the OFP during deposition. These results demonstrate the CrO3 – H2SO4 analytical protocol as a superior technique to aqua regia for accurately determining the timing of deposition for black shales [10]. Given the very low degree of scatter associated with the CrO3 –H2SO4 isochron, we therefore interpret 607.8 F 4.7 Ma as the depositional age for the OFP. The initial 187Os/188Os isotopic composition of 0.62 F 0.03 derived from this regression likely represents the composition of the local seawater at the time of sediment deposition. 5.2. Timing of the Marinoan and Gaskiers glaciations The Re –Os depositional age of f 608 Ma determined for the OFP contributes to a growing body of evidence for a global Marinoan ice age at f 620– 600 Ma [5] (Table 2). As previously noted, lithostrati-

Table 2 Summary of absolute age constraints on the Marinoan and Gaskiers ice ages Glacial interval Minimum age constraints

Maximum age constraints

Marinoan (620 – 600 Ma) 1

Nantuo (China)

Mount Vreeland and Ice Brook (Canada)2 Congo-Sao Francisco and Western Africa3 None

Gaskiers (580 Ma) 584 F 26 Ma (Lu – Hf) 599.3 F 4.2 Ma (Pb – Pb) 607.8 F 4.7 Ma (Re – Os)

Gaskiers (Canada)4

580 Ma (U – Pb)5

Squantum (Northeast US)5

570 Ma (U – Pb)

620 – 600 Ma (U – Pb) Squantum (Northeast US)5 Loch na Cille (Scotland)6 Gaskiers (Canada)4

596 F 2 Ma (U – Pb) 601 F 4 Ma (U – Pb) 580 Ma (U – Pb)

Sources of data: (1) Barfod et al. [43]; (2) Present study; (3) Evans [5]; (4) Bowring et al. [29]; (5) Thompson and Bowring [45]; (6) Dempster et al. [46]. Rb – Sr dates from shales are excluded because detrital and authigenic considerations generally hamper the determination of depositional ages.

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graphic correlations indicate that the OFP is stratigraphically equivalent to cap carbonates that overlie diamictites of the second Windermere glaciation. Thus, the Re – Os depositional age of f 608 Ma provides a minimum age constraint for the timing of this ice age in western Canada, which correlates it with the Marinoan glacial event. Other age information for this glacial epoch includes several U –Pb age constraints for diamictites from the Congo, Sao Francisco, and West African cratons that suggest a glacial episode at f 620 – 600 Ma (although reliable age constraints are generally weak for many of the diamictites; see Evans [5] for discussion). In addition, the Nantuo glacial interval in southern China has minimum age constraints of 584 F 26 Ma (Lu– Hf) and 599.3 F 4.2 Ma (Pb – Pb) that were obtained from phosphorites within the disconformably overlying Doushantuo Formation [43]. In contrast, Neoproterozoic glacial deposits from northeastern North America may only represent a regional glaciation rather than a global ice age. The age of the glaciogenic Gaskiers Formation on the Avalon Peninsula of Newfoundland is constrained to f 580 Ma through U – Pb dating of ash beds which lie below, within, and above the glacial deposits (Table 2) [29]. In addition, Bowring et al. [29] suggest that the < 1-Ma time period of deposition for the Gaskiers Formation either does not represent a ‘‘Snowball Earth’’ event or that such events are much shorter in duration than the 4– 30-Ma predicted by Hoffman et al. [3] on the basis that at least 0.12 bars of atmospheric CO2 are needed to initiate widespread melting of ice sheets [44]. The Squantum tillite in the Boston Basin, Massachusetts, has maximum and minimum U –Pb ages of 596 F 2 Ma (welded tuff clast within the tillite) and f 570 Ma (youngest dates from zircon in an ash bed from the overlying Cambridge Argillite) [45]. A U – Pb zircon age of 601 F 4 Ma determined for the Tayvallich Volcanic Formation (Dalradian Supergroup, Scottish Highlands provides a maximum age constraint on the timing of the Loch na Cille glaciation [46]. The narrow overlap between the f 608-Ma minimum age constraint on the second Windermere ice age determined here, and the f 601-Ma maximum age constraint for the Loch na Cille glaciation suggests the latter could be correlative with either the Marinoan or Gaskiers ice ages.

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Recently, a Re – Os date of 592 F 14 Ma was reported for black shales of the Neoproterozoic Aralka Formation (Amadeus Basin, Australia) that underlies the glaciogenic Olympic Formation, interpreted to represent the Marinoan glaciation, and overlies the glaciogenic Areyonga Formation, interpreted as the Sturtian glaciation ([47] and references therein). The ca. 592-Ma date is thus considerably younger than that expected from global correlation studies [47], but does derive from a subset of three analyses from a total of nine analyses that yield a date of 623 F 18 Ma. Because the Aralka black shales are relatively organic-poor ( < 1% TOC) and were digested with aqua regia, it is possible that the ca. 592-Ma date reported for this unit may reflect mixing of hydrogenous and detrital Re and Os (note this date is not included in Table 2). The accuracy of the aqua regia date obtained for the Aralka Formation should thus be verified through CrO3 –H2SO4 digestion of multiple samples from a short depositional interval within this unit. Available absolute-age information also suggests there were two ‘‘Sturtian’’ glacial episodes at f 740 – 720 Ma [5] and f 685 Ma [28] and thus the current geochronologic database now allows for four distinct Neoproterozoic ice ages. Although this database is small compared to the number of known Neoproterozoic localities containing diamictites of glacial origin (at least 78 distinct deposits [5]), it does allow the possibility of global glaciations in that available age constraints for the Marinoan and older Sturtian events come from several, widely spaced localities worldwide. Because black shales are often deposited during the post-glacial, eustatic sea-level rise following the meltback of Neoproterozoic ice sheets, they represent attractive targets for providing stronger constraints on the timing, number, magnitude, and duration of the Neoproterozoic ice ages. 5.3. Implications for future Re – Os dating of black shales The results reported here indicate that the potential of the Re – Os radioisotope system for providing depositional ages on black shales is greater than previously realized. Earlier studies utilized only organic-rich material containing greater than 2% TOC for determining Re – Os depositional ages [6,7,9,42].

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This strategy maximized the chances of analyzing material with sufficiently high Re and Os concentrations in order to obtain a precise depositional age. However, the results of the present study indicate that precise and accurate depositional ages can still be obtained on black shales containing much lower ( < 1%) TOC contents. The precision of the 607.8 F 4.7-Ma Re – Os age reported here ( F 0.8%, 2r) exceeds the precision of earlier Re –Os dating studies on higher-TOC rocks analyzed through inverse aqua regia [7,9] or nickel-sulfide fire assay [6,42] (minimum of 2 – 3%). The improvement in precision is ascribed to the selective nature of the CrO3 –H2SO4 digestion method, which serves to minimize variation in the initial 187Os/188Os isotopic composition of individual samples through minimization of non-hydrogenous (detrital) Os. Recently, Creaser et al. [9] demonstrated that the Re –Os systematics of black shales are not disturbed during hydrocarbon maturation processes (temperatures up to 150 jC) as regression of immature, mature, and overmature samples from the Exshaw Formation yielded ages within error of established radiometric age constraints. The present study employed material that had been subjected to chlorite-grade metamorphism (i.e., temperatures of f 300– 400 jC). The excellent CrO3 – H2SO4 isochron systematics preserved by the OFP suggest that open-system behavior of Re and Os during Mesozoic metamorphism either did not occur or was not detectable at the scale of sampling undertaken. Further systematic study is needed to determine the relative degree of mobility and reaction histories of Re and Os during prograde metamorphism and the general metamorphic conditions at which the Re – Os system can no longer provide primary depositional age information.

parison of the Re –Os data set obtained with the two analytical protocols indicates that a variably, radiogenic detrital component of Os is present in the Old Fort Point Formation. These results verify the CrO3 – H2SO4 technique as a superior method to aqua regia for determining precise and accurate Re – Os depositional ages on mudrocks. The best estimate of the depositional age for the OFP is thus 607.8 F 4.7 Ma while the initial 187Os/188Os isotopic ratio of 0.62 F 0.03 from this regression likely represents the composition of the seawater at the time of sediment deposition. The Old Fort Point Formation is stratigraphically comparable to carbonates that cap Neoproterozoic glacial deposits of the second Windermere ice age in western Canada. From the Re –Os age of f 608 Ma, this glaciation must be Marinoan (620 – 600 Ma) in age, and older than the f 580-Ma Gaskiers ice age of eastern North America. This new Re – Os age provides further support to a growing body of evidence for a global Marinoan glacial episode. The Re – Os radioisotope system is applicable to rock types commonly associated with Neoproterozoic glaciogenic deposits, and will potentially allow the concept of global glaciations (i.e., ‘‘Snowball Earth’’) to be fully tested. Re – Os dating of the Old Fort Point Formation represents the first demonstration that Re –Os depositional ages can be obtained from chlorite-grade black shales, and that black shales with < 1% total organic carbon can be dated using suitable analytical protocols. The application of the Re –Os radioisotope system as a tool for providing depositional ages on black shales, and their low-grade metamorphosed equivalents, thus has much greater potential than previously realized.

6. Conclusions

Acknowledgements

Two 187Re – 187Os dates of 634 F 57 Ma (MSWD = 65) and 607.8 F 4.7 Ma (MSWD = 1.2) were obtained for black shales of the Old Fort Point Formation, Windermere Supergroup, using inverse aqua regia and CrO3 – H2SO4 digestion protocols, respectively. Both dates are in excellent agreement with existing age constraints that bracket the Old Fort Point Formation between f 685 and f 570 Ma. However, com-

The comments of Martin Kennedy and an anonymous reviewer improved the manuscript. Stacey Hagen and Kelly Nixon are thanked for their technical assistance. Thomas Chacko provided valuable insights and comments. This study was supported by a Natural Sciences and Engineering Research Council (NSERC) of Canada Discovery Grant to RAC and an NSERC PGS-A graduate scholarship to BSK. The

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