origin and 87rb 87sr age of porewaters in low permeability … · 2020. 10. 12. · for sr...

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Origin and 87Rb–87Sr age of porewaters in low permeabilityOrdovician sediments on the eastern flank of the Michigan Basin,Tiverton, Ontario, Canada1Laurianne Bouchard, Ján Veizer, Laura Kennell-Morrison, Mark Jensen, Ken G. Raven, and Ian D. Clark

Abstract: Porewater extractions and acid leachates of rock core from a 250 m thick sequence of low-permeability Ordovician-ageshales and limestones, on the eastern flank of the Michigan Basin, were analysed for strontium isotope ratios in an attempt toinfer porewater ages from observed 87Sr/86Sr enrichments. The porewaters originated as Ordovician seawater, which subse-quently mixed with evaporated Silurian seawater infiltrating from above, and, to some extent, with a deep brine—with anenriched 87Sr/86Sr signature—from the underlying crystalline shield or deep basin. The porewater 87Sr/86Sr ratios are moreradiogenic than contemporaneous seawater but show no obvious correlation to those leached from the solid rock phases.Accepting that the initial 87Sr/86Sr signatures in porewaters were dominated by Late Silurian brine, potentially with an additionaldeep brine component, the excess of radiogenic 87Sr appears to represent ingrowth from 87Rb decay over a time span of some420 million years, approaching the depositional age of the rocks. Similarly, Rb/Sr errochron ages of acid leachates of solid phases,and the calculated initial 87Sr/86Sr isotopic ratios, are consistent with a proposition that the calcites inherited their Sr fromOrdovician seawater and were dolomitized shortly afterwards by infiltrating Mg-enriched evaporative brine, indicating long-term conservative behaviour for the enclosing carbonate rocks. The errochron for leachates from (alumino)silicates yields a highinitial 87Sr/86Sr, but with an errochron age of about 340 ± 48 Ma, likely owing to variable admixtures of diagenetic illite in theshales. Overall, the data provide evidence for a stable hydrologic regime since Paleozoic time.

Résumé : Des extractions d’eau interstitielle et de lixiviats acides de carottes de roches d’une séquence de 250 m d’épaisseur deshales et calcaires ordoviciens de faible perméabilité sur le flanc est du bassin du Michigan ont été analysées pour en déterminerles rapports d’isotopes du strontium, dans le but d’inférer les âges des eaux interstitielles sur la base de l’enrichissement observéen 87Sr/86Sr. Les eaux interstitielles étaient à l’origine de l’eau de mer ordovicienne qui s’est ensuite mélangée avec de l’eau demer silurienne évaporée s’infiltrant par le haut et, dans une certaine mesure, avec une saumure profonde—caractérisée unesignature en 87Sr/86Sr enrichie—provenant du bouclier cristallin ou bassin profond sous-jacent. Les rapports 87Sr/86Sr des eauxinterstitielles sont plus radiogéniques que ceux de l’eau de mer contemporaine, mais ne présentent aucune corrélation évidenteavec les eaux lixiviées des phases solides des roches. En présumant que les signatures initiales de 87Sr/86Sr dans les eauxinterstitielles étaient dominées par des saumures d’âge silurien tardif et, éventuellement, par une autre composante de saumureprofonde, l’excédent de 87Sr radiogénique semble représenter le résultat de la décomposition du 87Rb sur une période de quelque420 millions d’années, s’approchant de l’âge du dépôt des roches. De même, les âges erreurchrones de lixiviats acides issus dephases solides et les rapports 87Sr/86Sr initiaux calculés concordent avec la proposition voulant que les calcites aient hérité leurSr d’eau de mer ordovicienne et ont été dolomitisées peu après par une saumure issue de l’évaporation enrichie en Mg, ce quitémoigne d’un comportement conservateur à long terme des roches carbonatées encaissantes. L’errochrone pour les lixiviatsd’(alumino)silicates donne un 87Sr/86Sr initial élevé, mais avec un âge errochrone d’environ 340 ± 48 Ma, découlant probablementde degrés variables de mélange d’illite diagénétique dans les shales. Globalement, les données offrent des preuves d’un régimehydrologique stable depuis le Paléozoïque. [Traduit par la Rédaction]

IntroductionA 400 m thick Upper Ordovician sedimentary sequence of shale

(200 m) and underlying carbonate (200 m) situated on the easternflank of the Michigan Basin comprise an aquiclude proposed for aDeep Geological Repository for low- and intermediate-level nu-clear waste (Raven et al. 2011; Clark et al. 2013, 2015; Al et al. 2015).Here, porewaters and host minerals have been studied to under-

stand groundwater system evolution and solute mobility as partof site investigations. The approximate age of porewaters inferredfor the low permeability (≤10−18 m2) argillaceous and carbonatesediments has been assessed using strontium (Sr) isotopes fromextracted porewaters and sequential leachates from the hostrocks. A significant enrichment in the 87Sr/86Sr ratio over that ofcontemporaneous seawater has been observed in the porewaters.

Received 7 March 2018. Accepted 12 August 2018.

Paper handled by Guest Editor Ihsan Al-Aasm.

L. Bouchard, J. Veizer, and I.D. Clark. Department of Earth and Environmental Sciences, University of Ottawa, 25 Templeton St., Ottawa, ON K1N 6N5,Canada.L. Kennell-Morrison and M. Jensen. Nuclear Waste Management Organization, 22 St. Clair Ave. E, Toronto, ON M4T 2S3, Canada.K.G. Raven. Geofirma Engineering Ltd., 1 Raymond St., Ottawa, ON K1R 1A2, Canada.Corresponding author: Ian D. Clark (email: [email protected]).1This paper is part of a Special Issue entitled “Advances in low-temperature geochemistry, diagenesis, and seawater and climate evolution through theEarth’s history: a tribute to Jan Veizer”.

Copyright remains with the author(s) or their institution(s). Permission for reuse (free in most cases) can be obtained from RightsLink.

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Can. J. Earth Sci. 00: 1–8 (0000) dx.doi.org/10.1139/cjes-2018-0061 Published at www.nrcresearchpress.com/cjes on 28 November 2018.

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mailto:[email protected]://www.nrcresearchpress.com/page/authors/services/reprintshttp://dx.doi.org/10.1139/cjes-2018-0061

The evidence supports the existence of an isolated hydrologicsystem, in which fluid and solute migration are associated withresidence times on geologic time scales, and hence a novel appli-cation of Sr isotopes has been employed in an effort to obtainestimates of porewater age.

The aims of this paper are to (i) estimate the approximate ageof porewaters and authigenic solid phases in the Ordoviciansedimentary sequence, and (ii) qualify inferences regarding thepotential influence of water–rock interaction on porewater 87Srenrichment.

Geological settingThe Paleozoic stratigraphy of the 840 m thick sedimentary se-

quence beneath the investigated site, situated near Tiverton, On-tario, on the eastern flank of the intracratonic Michigan Basin, isshown in Fig. 1 (Raven et al. 2011). During the Paleozoic, present-day eastern North America was located in the tropics, covered byinland seas, and the related marine sediments represent a com-posite sequence from Cambrian to the earliest Devonian (Armstrongand Carter 2010). The Ordovician sediments, of specific interest tothis study, were deposited during a marine transgression thatresulted in deposition of a sequence of clastics and carbonates,from lagoonal to shoal to deep shelf facies (Kobluk and Brookfield

1982). The Ordovician-age samples studied were obtained fromthe formations described below, at depths ranging between 458and 685 m below ground surface (refer to Fig. 1).

The Cobourg Formation consists of blue–grey to grey–brownnodular, fine- to coarse-grained, fossiliferous limestones and ar-gillaceous limestones (Armstrong and Carter 2010). The Colling-wood Member consists of dark grey and black calcareous shales,impure limestones, and lime marlstones (Macauley et al. 1990).The Blue Mountain, Georgian Bay, and Queenston formations arefine-grained clastics deposited during inundation that followedthe collapse of the carbonate platform at the onset of Taconicorogeny. The Blue Mountain Formation consists of blue–grey togrey–brown shales with intermittent siltstone, sandstone, andlimestone interbeds. The Georgian Bay Formation is similar, buthas more greenish- to bluish-grey color and contains more fossils.In contrast, the 80 m thick Queenston Formation is a red to ma-roon illitic shale, with interbeds of green shales, siltstones, sand-stones, and limestones (Donaldson 1989). The top of this formationrepresents a geologic unconformity caused by sea level drop. Thetop of the Queenston Formation to the base of the Cobourg For-mation represent what is defined below as the Upper Ordovicianaquiclude. Key petrophysical attributes of the formations are out-lined in Table 1 (Raven et al. 2011).

Fig. 1. Composite stratigraphic section at the Bruce nuclear site.

Table 1. Key petrophysical parameters.

Formation

Wet bulkdensity (g/cm3)

Effective diffusion,De (m2/s)

Totalporosity (%)

Brinesaturation (%)

Avg. SD Range Avg. SD Avg. SD

Silurian formations 2.63 0.12 — 8.9 6.8 91 13Ordovician shales 2.65 0.04 1.7–3.8 E-11 7.4 1.7 93 10Ordovician limestones 2.69 0.04 2.0–7.8 E-12 1.9 1.3 86 13


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Groundwater flow at this site is restricted to permeable zoneswithin Silurian to Devonian formations and the underlying Cam-brian sandstone. Within the Ordovician section studied, fluids arerestricted to the porewater network, representing from 8% to lessthan 1% volumetric water content. Porewaters were extracted inprevious studies to establish their geochemical and isotopic char-acteristics and for insights on their origin (Raven et al. 2011; Clarket al. 2013). In this Upper Ordovician aquiclude, porewaters con-stitute a 6 M Na–Ca–Cl post-diagenetic brine originating as highlyevaporated Silurian seawater. Both Cl− and �18O follow a slightdown-section depletion trend into the underlying limestonesaccompanied by a downward enrichment trend for deuteriumexcess, which have been suggested to result from mixing in theunderlying Ordovician carbonates with basal brine (Clark et al.2013).

A detailed mineralogical (microprobe, XRD, SEM) profile of thePaleozoic stratigraphic section (Raven et al. 2011) is reproduced inFig. 2, showing calcite, dolomite, quartz, and clays to be the dom-inant phases in respective lithologies. The present study will con-centrate on separation of these major components to elucidatetheir origin and potential influence on porewater composition.

Methodology for extraction of porewatersPorewaters were extracted from the rocks using the crush-and-

leach method (Raven et al. 2011) and diluted with 1% HNO3 foranalysis of K+, Rb+, Ca2+, and Sr2+ by ICP-OES (Varian Vista 116 Pro®)with precision better than 1% relative standard deviation. Solutemasses from these leaches have been normalized to the porewatermass for each sample to yield molal porewater concentrations inparts per million (ppm; Table 2). Concentrations of 87Rb werecalculated from total Rb concentration using the natural abun-dance ratio of 2.59. For Sr isotopes, 1.5 mL of each sample wasdried at 90 °C on a hotplate in a clean Teflon beaker. Once dried,residues were spotted with a few drops of 7 N HNO3 and driedagain, following which 1.5 mL of 2.5 N HCl was added to eachbeaker to perform Sr separation with a Dowex 50-X8 cation ex-change resin. Sr-enriched eluents were collected in pre-labelledclean Teflon containers and then dried at 90 °C on a hotplate.

Residues were re-dissolved in 4.2 �L H3PO4, and 3 �L of eachsample were deposited on single tantalum filaments and dried.All samples, along with a standard (NIST SRM–987; 87Sr/86Sr =0.71024 ± 0.00002) (Carleton University 2012), were analyzed bythermal ionization mass spectrometry (TIMS) the following day.Additional technical specifications are available in Bouchard(2015).

Fig. 2. Pattern of major mineralogical composition across the stratigraphic column. Modified from Raven et al. (2011).

Table 2. Normalized concentrations of cations in pore-waters and groundwaters.

Depth DGR-1/2(mBGS)

Ca2+

(ppm)Sr2+

(ppm)K+

(ppm)Rb+

(ppm)

Porewaters458.01 52 200 1100 12 600 16.9473.82 51 300 1040 14 100 18.8484.59 54 900 1210 14 800 19.3515.12 52 600 1030 13 000 17.0522.20 61 500 1530 15 200 20.5554.92 35 900 1000 13 500 21.5581.28 39 000 1120 12 400 17.3582.42 53 900 1550 15 200 18.9589.11 52 500 1470 13 500 16.9589.41 62 200 1710 13 300 17.2589.98 50 300 1410 13 500 16.4605.17 46 200 1250 11 400 14.0607.67 48 400 1290 12 200 15.4619.36 54 700 1710 14 300 17.8625.33 44 300 1220 10 100 12.2636.57 63 300 1710 14 200 16.4649.71 52 700 1490 13 000 14.5666.57 40 000 1710 11 100 5.05676.15 41 200 1760 1800 4.64682.45 25 900 977 6150 4.70685.58 37 900 1540 12 700 8.85

Groundwaters377.42 37 900 543 4380 2.67847.53 41 470 799 947 1.55


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Methodology for separation of major componentsof the rocks

As documented by Raven et al. (2011), and summarized in Fig. 2,the major mineral phases hosting these formation porewaters arecalcite, dolomite, and sheet silicates (clays). Whole-rock sequen-tial dissolution was chosen to separate each of these phases forsubsequent Sr isotope measurements. Following porewater ex-tractions (method described in Clark et al. 2010), rocks were driedand crushed to fine powders. Two hundred milligrams of eachsample were put in clean Teflon containers. To remove labile andexchangeable ions that could contaminate the isotope data, 3 mLof nanopure water was added to each sample and containers werecapped (Gosselin et al. 2004). Samples were left in an ultrasonicbath for 20 min before being taken out to rest for 5 min. Sampleswere then centrifuged, and the supernatants pipetted, transferredto pre-labeled Teflon containers, and stored for further analysis.The subsequent rapid leaching by 5% acetic acid (Tessier et al.1979; Lerouge et al. 2010) that dissolved mostly calcite was fol-

lowed by dissolution of residual carbonates, principally dolomite,by 6 N HCl (Sharp et al. 2002). Note that treatment by HCl candissolve cations from the surface of clay particles with minordissolution of silicate phases (Clauer et al. 1993) and can alsopartially attack hydrated oxides, sulfides, and phosphates (Gosselinet al. 2004). The final step includes dissolution of the remainingsilicates, mostly clays, by a mixtures of 50% HF – 12 N HNO3(Jacobson et al. 2002) with the sample left capped on a hotplate at140 °C until complete dissolution. Containers were then uncappedand a few drops of 7 N HNO3 added to the residues, which werethen evaporated, followed by the addition of 3 mL 6 N HCl. Fol-lowing this, the samples were left on the hotplate at 90 °C over-night. On the following morning, all leachates were evaporated,spotted with 7 N HNO3, and the residues re-dissolved in 3 mL of2.5 N HCl. Each sample was then divided into two subsamples of1.5 mL, dried, dissolved in 1.5 mL HNO3, and stored in clean plastictubes for further ICP analyses (as detailed in the porewater sec-tion). The mass of the major ions in the leachates were normalized

Table 3. Normalized concentrations of cations in acid leachates.

K+ (mg/kg) Rb+ (mg/kg) Ca2+ (mg/kg) Sr2+ (mg/kg)

Depth DGR1/2(mBGS)

H2Oleach

AcOHleach

HClleach

HF/HNO3dissolution

H2Oleach

AcOHleach

HClleach

HF/HNO3dissolution

H2Oleach

AcOHleach

HClleach

HF/HNO3dissolution

H2Oleach

AcOHleach

HClleach

HF/HNO3dissolution

458.01 854 232 1389 24 000 3.36 1.06 9.37 104 385 94 500 4460 303 4.34 99.6 9.7 26.3484.59 540 550 1250 19 500 1.49 0.89 6.92 133 765 94 400 42 900 2640 5.16 134 29.4 22.7522.20 236 379 570 19 400 0.65 0.87 3.20 99.4 722 79 600 62 600 5060 4.56 91.6 28.8 21.1554.92 355 467 713 13 100 1.28 0.88 5.52 30.3 327 14 800 28 300 335 3.32 65.6 15.0 7.46581.28 453 426 640 13 300 0.99 0.79 5.19 42.1 465 22 500 13 000 338 5.48 32.9 13.9 8.12589.41 103 297 435 15 000 0.68 0.52 2.48 80.3 245 129 500 64 100 4650 1.66 213 65.0 24.7666.57 109 189 372 7350 0.33 0.29 1.39 31.7 860 122 000 174 000 17 800 2.06 197 282 22.9671.01 85.4 129 227 5050 0.27 0.24 1.13 24.7 1190 130 000 175 000 14 800 2.62 181 253 17.1682.45 134 250 313 9010 0.40 0.47 1.65 39.9 559 187 000 136 000 8540 2.44 260 197 17.0685.58 93 129 193 5160 0.28 0.26 1.07 20.1 1741 176 000 160 000 10 900 2.84 223 219 13.3

Fig. 3. Monovalent and divalent Ca, Sr, K, and Rb concentrations in porewaters (ppm porewater) and leachates (ppm of rock). The concentrationsfor the Silurian and Cambrian groundwaters are from Heagle and Pinder (2010).




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to the dried rock mass and reported in units of ppm (Table 3). Theremaining solution underwent Sr extraction using Dowex 50-X8Cation resin. Sr enriched eluents were collected in clean contain-ers, left on a hotplate at 90 °C to dry overnight, and spotted with7 N HNO3 on the following morning to remove all traces of organicmolecules. Samples were then dissolved in 4.2 �L H3PO4, and 3 �Lof each sample were deposited on single tantalum filaments anddried for Sr isotope measurements by TIMS at Carleton University.

Porewater and mineral phase geochemistryVertical profiles for the cation concentrations (Ca, Sr, K, and Rb)

in the rock (ppm per kgrock) are shown for each leach solution inFig. 3. While the sequential leaches unlikely sample discrete min-eral phases in these rocks, some general trends can be noted.Monovalent cations (K+, Rb+) show greatest concentrations in themineral phases with the HF leach solution, reflecting a partition-ing into the aluminosilicate (clay) phases over the carbonatephases. In contrast, the divalent cations (Ca2+, Sr2+) show a prefer-ential partitioning into the carbonate phases over the clays. Ver-tical trends reflect the upward transition in lithology from thepredominantly limestone Cobourg Formation to clays in the Or-dovician shales (Fig. 2).

The geochemistry of the porewaters (black dots in Fig. 3), ex-pressed in ppm per kgwater, is from earlier work (Clark et al. 2013)showing relatively uniform concentrations through the section,which resemble concentrations in groundwaters from the overly-ing Silurian (Guelph) and the underlying Cambrian formations(Heagle and Pinder 2010; Al et al. 2015). The vertical trends appearto show little similarity with those for the mineral leachates,particularly for Rb and Sr. Sr concentrations in the porewatersgreatly exceed those in any of the leachates, suggesting an addi-tional source term for Sr in the porewater system, leading to thehypothesis that in-situ 87Rb decay to 87Sr in the porewaters is aviable source of excess Sr, under conditions of limited solute mo-bility over hundreds of millions of years—which has been docu-mented for this sedimentary rock mass with support from othergeochemical studies (e.g., Al et al. 2015; Clark et al. 2013, 2015).

Given the similar geochemical affinities of Rb and K, their dis-tribution patterns in porewaters should be comparable if bothoriginated from the influx of Silurian brine. With the exception ofan excursion to lower Rb concentrations in the Cobourg Forma-tion, these two alkalis have very similar porewater profilesthrough the section. The porewater profile for Rb does not showthe same overall trend as the profiles for the leach solutions,however, suggesting a lack of mineral control on the Rb porewa-ter concentrations. These observations, paired with the Sr isotopedata presented in the following section, lend support to the pro-posal above that much of the enrichment in the 87Sr values ap-pears to be derived from 87Rb decay since Silurian time, ratherthan from interaction with detrital aluminosilicate minerals de-rived from the craton.

Estimates of age of porewaters and mineral phasesin the leachates

The observed 87Sr/86Sr ratios of leachates (Table 4) and porewa-ters (Table 5) all plot at or above the values for coeval seawater(Fig. 4). For the leachates, the 87Sr/86Sr ratio increases up-section,from carbonates to shales, and with the strength of the leachingagents. In contrast, the porewater 87Sr/86Sr profile is exceedinglyuniform. The porewater 87Sr/86Sr also is more radiogenic than theleast aggressive leach (acetic acid), yet considerably less radio-genic than either of the more aggressive leachates (HCl and HF).This absence of coherency between the porewater profile and theleachate profiles, and the observation that the porewaters aremore radiogenic than the most soluble (and arguably most ex-changeable) mineral phase, suggests that the porewaters have notinherited their radiogenic signal entirely from the enclosingrocks.

As noted above, evidence suggests that the Ordovician pore-waters represent a mixture of evaporated Silurian seawater, with acomposition assumed to resemble the overlying Silurian Guelph

Table 4. 87Sr/86Sr ratios in rock leachates.

Depth DGR-1/2(mBGS)

AcOH HCl HF

87Sr/86Sr ±2� 87Sr/86Sr ±2� 87Sr/86Sr ±2�

458.01 0.70919 0.000007 0.7263 0.0001 0.79236 0.00001484.59 0.70886 0.00003 0.71150 0.00002 0.79360 0.00001522.20 0.70904 0.00006 0.70975 0.00002 0.78228 0.00007554.92 0.71029 0.00002 0.71410 0.00002 0.78879 0.00001581.28 0.71053 0.00002 0.71358 0.00001 0.78066 0.00002589.41 0.70846 0.00006 0.70917 0.00002 0.76230 0.00002666.57 0.70817 0.00001 0.70805 0.00001 0.73280 0.00002671.01 0.70817 0.00002 0.70801 0.00002 0.73424 0.00002682.45 0.70813 0.00002 0.70812 0.00002 0.75409 0.00002685.58 0.70818 0.00002 0.70804 0.00002 0.74092 0.00002

Table 5. 87Sr/86Sr ratios in porewatersand groundwaters.

Depth DGR-1/2(mBGS) 87Sr/86Sr ±2�

Porewaters458.01 0.70965 0.00005473.82 0.70964 0.00003484.59 0.70970 0.00002515.12 0.70971 0.00003522.20 0.70983 0.00002554.92 0.70988 0.00002581.28 0.70990 0.00002582.42 0.70992 0.00002589.11 0.70991 0.00002589.41 0.70987 0.00003589.98 0.70991 0.00003605.17 0.70993 0.00002607.67 0.70992 0.00003619.36 0.70989 0.00002625.33 0.70992 0.00002636.57 0.70994 0.00003649.71 0.70993 0.00003666.57 0.70988 0.00001676.15 0.70996 0.00002682.45 0.70989 0.00002685.58 0.70997 0.00003

Groundwaters393.36 0.709127 0.000012841.96 0.710228 0.000011


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Fig. 4. Strontium isotope profiles in porewaters and rock leachates. The trend for Ordovician seawater is from Veizer et al. (1999).

Fig. 5. Measured 87Sr/86Sr ratios for porewaters and calculated initial ratios based on an ingrowth period of 420 million years. Shown also isthe mixing line for the proposed groundwater brine endmembers during the Silurian. [Colour online.]

350

450

550

650

750

850

0.7091 0.7093 0.7095 0.7097 0.7099 0.7101 0.7103

Dept

h (m

BGS

DGR-

1/2)

87Sr/86Sr

Porewaters Groundwater mixing line Calculated original ra�o




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Formation groundwater, and a component of radiogenic shield-like brine similar to that in the underlying Cambrian groundwater.If fluid mixing is assumed to be the cause of the present-dayenriched signatures, then the measured 87Sr/86Sr ratios wouldplot in the vicinity of the mixing line between these two endmem-bers, which clearly is not observed (see groundwater mixing lineon Fig. 5). The 87Sr/86Sr of the Guelph endmember (0.70911; de-rived from Raven et al. 2011; Hobbs et al. 2011) is close to that ofLate Silurian seawater (0.7087; Veizer et al. 1999), and the valueobserved in the Cambrian sandstone (0.71024; derived from Ravenet al. 2011; Hobbs et al. 2011) is similar to the enriched valuesmeasured in crystalline brine in Sudbury, Ontario (>0.7100; seeFrape et al. 1984) and in Cambrian sedimentary formation watersat the site and in the neighbouring Appalachian Basin (Hobbset al. 2011). Further, this line brackets the range for 87Sr/86Srinitialdetermined by Harper et al. (1995) for Ordovician fluids at the baseof the Paleozoic section in southern Ontario. As a result, the mix-ing line represents both present-day and the assumed groundwa-ter system conditions during the Late Silurian. Assuming that theobserved excess of 87Sr in porewaters is due to in situ decay of 87Rbsince an approximately Late Silurian timeframe—a requirementof the hypothesis because the influx of highly evaporated Silurianseawater has been conceptualized to have occurred as large-scalefluid flux along discrete higher-permeability conduits in the sed-imentary package—the back calculation of the proposed initial87Sr/86Sro for the porewaters should yield values that plot alongthe mixing line for these two endmembers, which appears tobe the case (green symbols on Fig. 5). It must be noted for claritythat the linear mixing scenario is very simplified for the back-calculation of the 87Sr/86Sr ratios in the system following somemeasure of interaction with brine fluids from both above andbelow.

The leaching agents are designed to dissolve sequentially theincreasingly less soluble phases of the rocks. The acetic acid lea-chates, mostly from dissolution of calcites, yield an errochron(Fig. 6) with an initial 87Sr/86Sr isotope ratio of about 0.7080 that isconsistent with their precipitation from Ordovician seawater(Veizer et al. 1999). However, the slope of this errochron intimatesan unrealistic Archean “age”, suggesting that the leaching proce-dure dissolved phases with high present-day 87Sr, such as loosely-bound Sr on clays and (or) dissolution of gypsum and (or) celestite(Reardon and Armstrong 1987), which have been observed in theQueenston and Blue Mountain formations as infilling minerals. Incontrast, the HCl leaches, designed to dissolve mostly dolomites,yield more consistent results with a near-marine initial ratio of0.7080 (Fig. 6) and an Ordovician errochron “age” of 454 ± 16 Ma.This suggests that dolomitization of precursor limestones occurredearly in the sedimentary history of the basin (i.e., Ordovician–Silurian), potentially by infiltrating seawater (Raven et al. 2011).

The wide scatter of the HF/HNO3 leaches (Fig. 6) broadly impliesan “age” of about 340 ± 48 Ma for the clays, within the range foundby Ziegler and Longstaffe (2000a, 2000b) for diagenetic clay phasesin the Ordovician formations on the southeastern side of the Al-gonquin Arch formed from basin fluids evolved from seawater(Ziegler and Longstaffe 2000b). This is some 100 Ma younger thanthe actual depositional age of the sequence, and may suggest thatillitization of the Ordovician mixed layer phases followed at sometime after the infiltration of K+-rich brines from the evaporatedSilurian seas, possibly associated with changes in temperature,pressure, and fluid dynamics during rapid subsidence in the De-vonian and achievement of peak burial in the Carboniferous. Themeasured high 87Sr/86Sr ratios in the HF/HNO3 leachates (Table 4)and the projected initial ratio of 0.7198 (Fig. 6) suggest that theclays are mostly of detrital nature. Closer inspection of the data inTable 4 shows that the leaches from shale sequences cluster at thehigh end of the plot (87Sr/86Sr > 0.78), while those from the car-bonates trend toward the low range of values. The overall erro-chron may, therefore, be an artifact of lumping together leachates

from the two lithologies, where the shale sequences contain al-most exclusively detrital clays with radiogenic Sr inherited fromthe shield, while clays in the carbonates may be at least partiallyof authigenic origin.

ConclusionsThis study explores the dynamics and interaction between

rocks and water in the context of understanding the origin ofporewaters within an Ordovician-age sedimentary sequence onthe eastern flank of the Michigan Basin, comprised of near-horizontally layered low-permeability carbonates and shales.Based on earlier studies (Raven et al. 2011; Clark et al. 2013), com-plemented by our results, the porewaters in the Ordovician aqui-clude are considered to have originated as Ordovician seawaterthat was chemically overprinted by evaporated seawater that in-

Fig. 6. Errochron plots of measured 87Sr/86Sr and 87Rb/86Sr valuesfor all leachates. [Colour online.]


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filtrated the section during Late Silurian time and mixed with apre-existing brine from deeper in the hydrostratigraphic section.

This work highlights a novel application of Sr isotopes. Mea-surements show that the porewaters contain 87Sr/86Sr ratios thatare greater than that of Ordovician seawater, with no obviouscorrelation to leachates from the different mineral constituentsof the host rocks. The excess radiogenic 87Sr in porewaters istherefore not likely to have been inherited from coeval seawateror from interaction with the enclosing rocks. Assuming that pore-waters inherited their initial Sr from mixtures of groundwaterbrines in the host Silurian and underlying formations, the excessradiogenic 87Sr is suggested to result from in-growth caused by87Rb decay in the porewaters. This provides evidence for the exis-tence of a stable groundwater regime in which mass transport hasbeen limited for hundreds of millions of years—a conclusion sup-ported by earlier studies of D, 18O, He, CH4, and CO2 isotope trac-ers (Clark et al. 2013, 2015), all of which provide evidence for adeep groundwater system in which porewater residence timesapproach the age of the enclosing rocks.

While the apparent ages and initial 87Sr/86Sr ratios obtainedfrom Rb/Sr errochrons of rock leachates support a scenario inwhich the limestones deposited from Ordovician seawater haveremained more or less stable during their subsequent geologicalhistory, the scenario for the shale sequence is more complex.Their high initial 87Sr/86Sr ratios reflect the fact that they aremostly of detrital origin (i.e., eroded from the continental shieldduring the Taconic Orogeny), with a portion of authigenic illitegenerated during a later, deep burial interval in the Late Paleozoic.

We conclude from the uniform enrichment in 87Sr in the pore-waters over that of Ordovician and Late Silurian seawater, andconvergence of Rb–Sr isochrons on an approximate age of 420 Ma,that both the porewaters and their enclosing rocks have actedas a stable system, with limited solute mobility, since the mid-Paleozoic.

AcknowledgementsWe would like to thank Pingqing Zhang and Nimal De Silva

from the University of Ottawa for the ICP-OES and ICP-MS analy-ses, as well as Shuangquan Zhang from the Isotope Geochemistryand Geochronology Research Centre at Carleton University forhelp with strontium isotope analyses. We would also like to thankJohn Avis from Geofirma Engineering Ltd. for his help with fig-ures, and the Nuclear Waste Management Organization for finan-cial support.

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http://dx.doi.org/10.1016/j.chemgeo.2015.03.005http://dx.doi.org/10.1016/j.chemgeo.2015.03.005http://iggrc.carleton.ca/http://dx.doi.org/10.1130/G34372.1http://dx.doi.org/10.1016/j.orggeochem.2015.03.006http://dx.doi.org/10.1016/0009-2541(93)90287-Shttp://dx.doi.org/10.1016/0016-7037(84)90331-4http://dx.doi.org/10.1139/e95-116http://dx.doi.org/10.1016/S0016-7037(01)00755-4http://dx.doi.org/10.1016/S0016-7037(01)00755-4http://dx.doi.org/10.1016/j.gca.2010.02.013http://dx.doi.org/10.1016/j.gca.2010.02.013https://www.opg.com/generating-power/nuclear/nuclear-waste-management/Deep-Geologic-Repository/Documents/Submission/18.DGSM.pdfhttps://www.opg.com/generating-power/nuclear/nuclear-waste-management/Deep-Geologic-Repository/Documents/Submission/18.DGSM.pdfhttp://dx.doi.org/10.1016/0016-7037(87)90007-Xhttp://dx.doi.org/10.1016/0016-7037(87)90007-Xhttp://dx.doi.org/10.1016/S0016-7037(01)00798-0http://dx.doi.org/10.1021/ac50043a017http://dx.doi.org/10.1016/S0009-2541(99)00081-9http://dx.doi.org/10.1180/000985500546620http://dx.doi.org/10.1346/CCMN.2000.0480407

ArticleIntroductionGeological settingMethodology for extraction of porewatersMethodology for separation of major components of the rocksPorewater and mineral phase geochemistryEstimates of age of porewaters and mineral phases in the leachatesConclusions

AcknowledgementsReferences

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