bock tac on i an sediment ology 2005

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Scale and timing of Rare Earth Element redistributionin the Taconian foreland of New England

BA RBA RA BO CK1, J . A. HUROWITZ, S. M. MCLENNAN and G. N. HANSON

Department of Geosciences, ESS Building, State University of New York at Stony Brook, Stony Brook,NY, 11794-2100, USA

ABS TR ACT

Clastic sedimentary rocks, deposited on eastern North America in response tothe Taconian Orogeny, commonly have Sm/Nd isotope relationshipsindicating substantial isotope disturbance near or subsequent to the time ofsedimentation that may be associated with severe depletion in light rare earthelements (LREE). Affected units [Normanskill Formation (Austin Glen andPawlet Members), Frankfort Formation and Perry Mountain Formation] arewidely separated both geographically (western New York to western Maine)

and stratigraphically (Middle Ordovician to Silurian). A model is proposed forthe most likely explanation of the observed REE and Sm/Nd isotoperelationships involving a two-stage process. In the first stage, REE areredistributed on a mineralogical scale (dissolution/precipitation on a samplescale) often with the involvement of REE-enriched trace phases such as apatiteand monazite. This stage typically takes place during diagenesis but may alsotake place later during metamorphism and/or recent weathering, and results inisotope re-equilibration on a sample scale. The second stage occurs when oneor more of these phases is redissolved and REE are transported on largeadvective scales. Where LREE-enriched phases are involved, this gives rise toLREE depletion in whole rocks. The timing of this second stage cannot beconstrained from Sm/Nd isotope data and may take place at any time

subsequent to the isotope re-equilibration. Such complex histories of REEredistribution may result in serious errors in estimating Nd model ages but notin estimating the Nd isotope composition at the age of sedimentation. Thus,Sm/Nd ratios even of unmetamorphosed sedimentary rocks have to becarefully evaluated before the calculation of depleted mantle model ages forthe provenance.

Keywords Nd isotope resetting, provenance, Rare Earth Element redistribu-tion, Taconian Orogeny, USA.

INTRODUCTION

Rare earth elements (REEs) are generally thoughtto be immobile during weathering, transport andsedimentation (e.g. McLennan, 1989). Althoughredistribution of REEs on a mineralogical scale isto be expected where recrystallization has takenplace during diagenesis or metamorphism (Ohr

et al., 1991), some studies have also demonstra-ted REE transport on a much larger scale. Frac-

tionation of the REE has been attributed todiagenesis and/or early metamorphism (McLen-nan & Taylor, 1979; Milodowski & Zalasiewicz,1991; Ohr et al., 1991; Bock et al., 1994) as well asto weathering (Nesbitt, 1979; Banfield & Eggleton,1989; McDaniel et al., 1994; Hannigan & Sholk-ovitz, 2001). In some cases, this fractionationclearly leads to a substantial disturbance of theSm-Nd isotope system (Zhao et al., 1992; Bocket al., 1994, 1996; McDaniel et al., 1994; Levet al., 1998, 1999).

1Present address: IFM-Geomar, Leibniz-Institut furMeereswissenschaften, Dienstgebaude Ostufer, Wis-chhofstr. 13, 24148 Kiel, Germany (E-mail:

[email protected])

Sedimentology (2004) 51, 885897 doi: 10.1111/j.1365-3091.2004.00656.x

2004 International Association of Sedimentologists 885


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The Middle Ordovician Taconian Orogenyaffected much of the New England region ofNorth America. During the Middle Ordovician toSilurian, clastic sedimentary rocks were derivedfrom the Taconian Orogen and deposited in theforeland on eastern North America. In this paper,the results of several regional REE and Nd

isotope studies are reviewed to show that theredistribution of REE appears to be a widespreadprocess (geographically and stratigraphically)within the Taconian foreland. The data that arereviewed and discussed in this paper werepublished by Bock et al. (1994, 1998; AustinGlen Member), Andersen & Samson (1995;Frankfort Formation), Cullers et al. (1997; Range-ley and Perry Mountain Formation) and Huro-witz (2001; Pawlet Member). In addition, newdata are presented from one hand specimen of alate Middle Ordovician greywacke from the

Normanskill Formation that shows an extremeexample of small-scale REE redistribution. It isthe purpose of this paper to demonstrate that: (1)rare earth redistribution and disturbance of theSm/Nd isotope system was widespread, bothgeographically (from Maine to New York State)and stratigraphically (from the Ordovician to theSilurian, about 40 million years), within thesedimentary successions of the Taconian fore-land; (2) - substantial Sm/Nd isotope re-equili-bration at a mineralogical scale is likely to haveoccurred typically near the time of sedimenta-tion; and (3) open-system transport of REE,

occurring on greater than whole rock scale,affected these rocks. Such multiple and complexREE redistribution histories may be difficult tounravel in detail, but a schematic model showshow these processes may occur and the conse-quences for the calculation of mantle model ages.The calculation of eNd at the time of sedimenta-tion will allow the evaluation of the presence orabsence of Nd isotope re-equilibration events.Systematic variations in the Sm/Nd ratios withdepleted model ages are an indicator of distur-bances on large scales (larger than sample size).

If such events go unrecognized, the undifferen-tiated use of Nd isotopes will lead to aberrantmantle model ages and possibly incorrect prov-enance inferences.

GEOLOGICAL BACKGROUND

The Taconian Orogeny (Middle to Late Ordovi-cian) caused increased subsidence and drowningof the Cambrian to Early Ordovician stable

carbonate platform along the east coast of Laur-entia. As a result of thrust loading and bending ofthe continental margin, the carbonate platform ofthe passive margin (Trenton Group; Fig. 1) wasoverstepped from the east first by deeper watershales, which are themselves overstepped by aflysch sequence. This overstepping occurred dia-

chronously in space and time. Therefore, theflysch deposited to the east (Austin Glen Memberof the Normanskill Formation) is older than theflysch deposited further west (Schenectady andFrankfort Formation) in the foreland basin (Row-ley & Kidd, 1981; Stanley & Ratcliffe, 1985;Bradley, 1989).

In the studies by Bock et al. (1994, 1998) andHurowitz (2001) of the Austin Glen and thePawlet Members, the uppermost units of theNormanskill Formation were analysed (Fig. 1).Both the Austin Glen and the Pawlet Members are

thought to be derived from sources to the east(Rowley & Kidd, 1981). As shown in Fig. 2,samples of the Austin Glen and Pawlet Memberswere collected from southern and northern NewYork State, respectively, so it is difficult toestablish the relative stratigraphic positions ofthe samples. The Austin Glen and Pawlet Mem-bers are sedimentologically indistinguishablefrom one another and, for the purpose of thisstudy, both members will be considered to beabout 465 Ma (graptolite zone Nematograptusgracilis according to Riva, 1974).

The Late Ordovician (Ashgill, c. 440 Ma) Frank-

fort Formation, analysed in a study by Andersen& Samson (1995), was also derived from easterlysources and deposited in western New York State(Fig. 2). The Frankfort Formation is autochtho-nous but sedimentologically indistinguishablefrom the allochthonous flysch (Rowley & Kidd,1981).

The Rangeley and Perry Mountain Formationsare part of the Silurian clastic wedge, which wasdeposited on top of a thick Late Ordovicianclastic sequence in the Merrimack Synclinorium(Osberg et al., 1968; Moench, 1971). The REE and

Nd isotope data of these formations were pub-lished by Cullers et al. (1997). The Early SilurianRangeley Formation is considered to be a flysch-type deposit. The Perry Mountain Formation ismore uniform than the Rangeley Formation and issupposedly more reworked than the former. Bothformations are interpreted as being derived fromthe Taconian highlands in the west, so that theymay have a provenance similar to that of the olderrocks deposited in the Taconian foreland basin(Osberg et al., 1989).

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METHODS

Most of the REE abundances and Nd isotope datadiscussed in this paper were collected in theisotope laboratory at SUNY Stony Brook and aretherefore of comparable quality. Samples fromBock et al. (1994, 1998) and the previouslyunpublished data from the hand specimen ofthe Normanskill Formation were analysed byTIMS for REE abundances and Nd isotopes. Fordetails on sample preparation, see the originalpublications. The total analytical uncertainties onthe REE abundances are < 1%, and the uncer-tainty on eNd is better than 03 at the 2 r level.

Sm and Nd concentrations and Nd isotope meas-urements on the Rangeley and Perry MountainFormation published by Cullers et al. (1997) werealso performed in the isotope laboratory at SUNYStony Brook. Therefore, the methods and errorsare the same as the ones stated above. The errorquoted by Andersen & Samson (1995) on their Ndisotope measurements is 03 e at the 2 r level.

All 143Nd/144Nd ratios were normalized to146Nd/144Nd 07219, and those analysed in theisotope laboratory of SUNY Stony Brook (all data

except those from Andersen & Samson, 1995)were corrected to correspond to a 143Nd/144Nd

ratio of 0511865 for the La Jolla Nd-Standard. eNd

(the deviation of the Nd isotopes relative to thechondritic uniform reservoir) is calculated as(143Nd/144Ndsample/

143Nd/144NdChur)1)*10 000with 143Nd/144NdChur 0512638; TDM (thedepleted mantle model age) is calculated as1/k* ln[(143Nd /144Ndsample ) 051315)/(

147Sm /144Ndsample)0217) + 1].

REE REDISTRIBUTION AND ITS EFFECTSON Nd ISOTOPES

REE patterns and Sm/Nd isotope data are widelyused in clastic sedimentary rocks as a proven-ance indicator because REE are generally regar-ded as the least mobile elements duringsedimentary processes. Although this is gener-ally the case, recent studies have demonstratedthat, under certain conditions of weathering,diagenesis and perhaps low-grade metamor-phism, REEs may be mobilized on mineralogicaland/or advective scales from REE-rich trace

Fig. 1. Stratigraphic relations of the Normanskill, Frankfort, Rangeley and Perry Mountain Formations (modifiedfrom Rowley & Kidd, 1981).

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phases, organic matter, clay minerals and labilerock fragments (McLennan & Taylor, 1979; Eld-erfield et al., 1981; Elderfield & Sholkovitz, 1987;Banfield & Eggleton, 1989; German & Elderfield,1989; Milodowski & Hurst, 1989; Leventhal,1990; Milodowski & Zalasiewicz, 1991; Murrayet al., 1991, 1992; Ohr et al., 1991, 1994; Zhaoet al., 1992; McLennan et al., 1993; Bock et al.,1994; McDaniel et al., 1994; Rasmussen &Glover, 1994; Bouch et al., 1995; Sturesson,

1995; Cullers et al., 1997; Lev et al., 1998,1999; Kidder et al., 2003). A common featureamong most studies is that REE redistributionsare strongly controlled by dissolution and/orreprecipitation of phosphatic trace minerals(apatite, monazite, florencite and rhabdophane)and petrogenetically related diagenetic minerals(e.g. calcite).

The most detailed studies of mineralogicalscale redistribution of REE and Sm/Nd isotopesare those by Ohr et al. (1991) and Lev et al. (1998,

1999). In these studies, there is strong evidence ofREE redistribution on a mineralogical scale as aresult of reactions among organic matter, clayminerals, apatite, calcite and REE-enriched phos-phatic minerals (monazite, rhabdophane, floren-cite). In the areas studied by Ohr et al. (1991,1994), the geochemical system was thought to beessentially closed on a whole rock scale becauseof reduction of pore space and fluid volumes withincreasing depth. However, a number of other

studies (Bock et al., 1994, 1996; McDaniel et al.,1994; Cullers et al., 1997; Lev et al., 1998, 1999)have demonstrated nearly contemporaneous (orsubsequent) transport of REE, leading to partialresetting of the Sm/Nd isotope system on greaterthan whole rock scale. In each of these cases,mass balance of REE strongly suggested theredistribution of trace phosphatic phases as thecause of the isotopic disturbance. In the case ofLev et al. (1998), the inferences from massbalance were confirmed by petrographic

Fig. 2. Modified map (after Bradley,1989) showing the approximatesample locations. (1) Rangeley andPerry Mountain Formations fromCullers et al. (1997). (2) FrankfortFormation from Andersen & Samson(1995). (3) Pawlet Member fromHurowitz (2001). (4) Austin GlenMember from Bock et al. (1994,1998). (5) Hand specimen from theNormanskill Formation.

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identification of diagenetic reactions involvingapatite and monazite.

Austin Glen and Pawlet Members

Combined REE and Sm/Nd isotope data for theAustin Glen Member have been published by

Bock et al. (1994, 1998). Many of the samplesshow substantial LREE depletion with147Sm/144Nd ratios commonly being > 013, whichcompares with ratios of 0105012 for mostclastic sedimentary rocks (McLennan & Hem-ming, 1992) and the remainder of the AustinGlen. Sm/Nd isotope data calculated at the timeof sedimentation (about 465 Ma) provide uniformeNd values of )80 06 (n 23), indicating anold age of mantle extraction, 18 Ga (Bock et al.,1994). However, when data are plotted on aSm-Nd evolution diagram (Fig. 3), they scatter

about a 465 Ma reference line, and the trend isdistinct from a 18 Ga or any significantly olderreference line. Samples taken from a single 70 mstratigraphic section are especially coherent inFig. 3, and a regression of these data (solid line inFig. 3) provides an age of 547 76 Ma, which isonly slightly older than the sedimentation age

(about 465 Ma) and indicates a substantial degreeof disturbance of the Sm/Nd isotope system.

The Pawlet Member samples display a range of147Sm/144Nd from 00985 to 01390 (Hurowitz,2001). Values for eNd calculated at 465 Ma rangefrom )82 to )97 (n 8). The model age calcu-lated for one sample with a normal 147Sm/144Nd

ratio (01089) is 1

8 Ga, consistent with results for

undisturbed samples from the Austin Glen Mem-ber (Bock et al., 1994). The remaining PawletMember samples define a range in TDM from 17 to24 Ga. A regression through the data from thePawlet Member (Fig. 4) yields an age of493 240 Ma (with the exception of one anom-alous sample), indicating Sm-Nd isotopic distur-bance at around the time of sedimentation. Theanomalous Pawlet Member sample (Fig. 4) hasmost probably undergone a later, or more pro-tracted, diagenetic disturbance, such that the age

of Nd isotopic resetting is considerably youngerthan the sedimentation age. Such a diagenetichistory would also explain the low eNd of thissample when calculated back to the time ofsedimentation ()97 at 465 Ma).

Bock et al. (1994) demonstrated with massbalance calculations that the LREE-depleted char-acter of Austin Glen Member samples was incon-sistent with removal of a detrital phase or mixingof an unusual provenance component. Instead, atwo-stage process best explains the Sm/Nd iso-tope systematics shown in Figs 3 (Austin Glen

Fig. 3. Sm-Nd isochron diagram for sediments fromthe Austin Glen Member (data from Bock et al., 1994,1998). The solid line is a regression of samples from asingle stratigraphic section that gives an age of547 76 Ma. Dashed lines are reference isochrons of1800 and 465 Ma, designed to intersect the isochron at147Sm/144Nd of about 0115, a 147Sm/144Nd ratio typicalof average shale. Note that the regional samples, whichare more poorly stratigraphically constrained, havegreater scatter with most scatter from a single locationnear Wappinger Falls.

Fig. 4. Sm-Nd isochron diagram for sediments fromthe Pawlet Member (Hurowitz, 2001). The solid line is aregression of samples that gives an age of 493 240 Ma. The dashed line is a reference isochronof 1800 Ma, designed to intersect the regression line at147Sm/144Nd of about 0115.




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Member) and 4 (Pawlet Member). In the firststage, the Sm/Nd isotope system is essentially re-equilibrated on an unknown scale. No observablemineralogical relationships are preserved in thesesamples that indicate the exact character of thisre-equilibration event. However, other studies ofREE redistribution in clastic sedimentary rocks

(e.g. Milodowski & Zalasiewicz, 1991; Ohr et al.,1994; Evans & Zalasiewicz, 1996; Lev et al., 1998,1999) have pointed to the breakdown of variousrelatively labile provenance components, such asvolcanic glass, organic matter, Fe-Mn oxyhydrox-ides and so forth, to form clay minerals andvarious trace phosphatic phases (such as apatite,monazite, rhabdophane, florencite) during dia-genesis and/or very early metamorphism.

The second stage of this process involves loss ofan LREE-enriched component that leads to thedramatic increase in whole rock Sm/Nd ratios.

These samples show no evidence of substantialchange in geochemical composition apart fromREE patterns (Bock et al., 1994, 1998). This ledBock et al. (1994) to suggest that loss of an LREE-enriched trace phase was probably the majorcause of changing Sm/Nd ratios, and this isconsistent with other studies of similar rocks(Milodowski & Zalasiewicz, 1991; Evans & Zala-siewicz, 1996; Lev et al., 1998, 1999). This secondstage may occur essentially contemporaneouslywith the first stage or may post-date it by sometime. The fact that the data align so closely to theage of sedimentation does not necessarily imply

that the loss of LREE must have occurred veryclose to the time of sedimentation, but only thatthe isotopic re-equilibration took place at thattime. This is discussed further below.

Frankfort Formation

Sm/Nd isotope systematics of the Frankfort For-mation in western New York State were investi-gated by Andersen & Samson (1995) as part of aregional provenance study of Taconian sedimen-tary rocks. The Frankfort Formation lies far to the

west of the Austin Glen Member and is in theorder of 20 Ma younger based on graptolite zona-tions (early Ashgill or about 445440 Ma). Wholerock Sm/Nd ratios display a similar range to thoseseen in the Austin Glen Member, and Sm/Ndisotope data align along a slope that approximatesthe age of sedimentation, indicating that the Sm/Nd isotope system has been strongly disturbed(Fig. 5). REE mobility was not an issue in thisstudy and, to avoid any complications related toREE disturbance, the authors reported only eNd

values at the time of sedimentation eNd (T) and noTDM values, which may be severely affected.

Andersen & Samson (1995) conducted a leach-ing experiment on one sample, and isotoperesults are shown in Fig. 5. The Sm/Nd system-atics of the leach/whole rock/residue set arealmost identical to those found by Ohr et al.

(1994) for Ordovician shales from Wales, wherethe leachates reflect the presence of early dia-genetic apatite. Although complete REE patternsare not available for these rocks, Fig. 6 shows thatthe process giving rise to variable Sm/Nd ratios isdifferent for the Frankfort Formation than for theAustin Glen/Pawlet Member and the Perry MtnFormation. In the Austin Glen/Pawlet/Perry Mtnsamples, high Sm/Nd ratios correspond to severedepletion of LREE in whole rocks, leading to anegative correlation between Nd concentrationand Sm/Nd ratio, which led Bock et al. (1994) to

suggest loss of a highly LREE-enriched phase,such as monazite, to be the cause of the change inSm/Nd ratio for the Austin Glen Member (Fig. 6).In contrast, Sm/Nd ratios in the Frankfort Forma-tion whole rocks vary essentially independentlyof Nd concentration (Fig. 6). Thus, for the Frank-fort Formation, there is no necessity for a stronglyLREE-enriched phase to be involved, and the datacan be explained by redistribution among variousdiagenetic phases with generally similar REEconcentrations but variable Sm/Nd ratios.

Fig. 5. Sm-Nd isochron diagram for sedimentary rocksand leachates from the Frankfort Formation (data fromAndersen & Samson, 1995). The solid line is a regres-sion of whole rock/leachate/residue for sample OC-4,which gives an age of 450 40 Ma. Note that a regres-sion of the whole rock data would give rise to ayounger, but statistically indistinguishable, age.

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The slope of the leachate/whole rock/residue inFig. 5 is consistent with a mineralogical scaleredistribution of REE at about the time of sedi-mentation (c. 450 Ma). Whole rock data appear todefine a slightly shallower trend than the leac-

hate/residue, but the slopes are statistically indis-tinguishable. A shallower slope might indicatethat the whole rock disturbance took place overan extended period of time, consistent with a latediagenetic resetting of the isotope system.

Perry Mountain Formation

The Silurian Rangeley and Perry Mountain For-mations are exposed in western Maine (Fig. 2)and are significantly younger than the AustinGlen/Pawlet Members or the Frankfort Formation

(Fig. 1). They also differ substantially from theAustin Glen/Pawlet Members and the FrankfortFormation in that they were later metamorphosedto garnetstaurolite grade during the Acadianand/or Alleghenian Orogenies.

REE and Sm/Nd isotope characteristics werestudied by Cullers et al. (1997). The REE datashow substantial depletion of the LREE in sam-ples from the Perry Mountain Formation, gener-ally similar to samples of the Austin Glen/PawletMembers, but far more complex in detail (e.g.

variable Ce and Eu anomalies). Sm/Nd isotopedata for the LREE-depleted samples of the PerryMountain Formation are highly scattered (Fig. 7)but, for the most part, clearly distinct from anyreasonable 18 Ga reference line and indicative of

substantial isotopic disturbance close to and/orsubsequent to the age of sedimentation(c. 420 Ma). Cullers et al. (1997) suggested thatthese rocks were most probably affected by morethan one episode of Sm/Nd disturbance. Themost likely times when this may have occurredare during diagenesis (c. 420 Ma), metamorphism(Acadian and perhaps Alleghenian) and perhapseven recent weathering.

Recent REE redistribution in the NormanskillFormation

A sample from the Normanskill Formation dis-plays incipient exfoliationcorestone relation-ships characteristic of weathering. Thisweathering appears to be relatively recent, prob-ably occurring since the last glaciation (Pleisto-cene). The hand specimen has a grey, non-porous,apparently unweathered core of about 10 cmdiameter surrounded by an 2 cm wide weath-ered porous rim (porosity resulting from the loss of2030% carbonate). The grain size of the sample is

Fig. 6. Plot of Nd vs. 147Sm/144Nd. Samples from theAustin Glen and Pawlet Member and Perry Mountain

Formation form a negative correlation consistent withthe change in Sm/Nd ratio being dominated by loss ofLREE. In contrast, the Frankfort Formation shows nocorrelation between Nd abundances and 147Sm/144Nd,suggesting that resetting of the Sm/Nd system is con-trolled by redistribution of REE without substantial lossof LREE.

Fig. 7. Sm-Nd isochron diagram for metasedimentaryrocks from the Silurian Rangeley and Perry MountainFormations (data from Cullers et al., 1997). Shown forreference are 1

8 Ga, 420 Ma and 250 Ma reference is-

ochrons designed to intersect at the average Sm/Ndratio of the three Rangeley Formation samples withlowest Sm/Nd. The Perry Mountain Formation samplesare highly scattered with very high Nd model ages thatcorrelate with Sm/Nd ratios, suggesting a complexhistory of Sm/Nd isotopic redistribution.




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medium to fine sand (125250 lm) graded acrossthe sample. The hand specimen was cut into about1 cm thick slices for thin section chips and also tofacilitate separating the rim from the core. Thecore was crushed and then aliquoted for analysis,whereas the rim was sampled according to grainsize. A composite as well as pieces of coarse- and

fine-grained weathered rim material were ana-lysed for REE abundances. A soft (cold 1 N HCl)leachate of powdered core and its residue wereanalysed for REE abundances (Table 1).

The brownish colour of the weathered rimresults from iron coatings that surround quartzgrains. The Fe may be derived from ankerite thatwas observed in the core. The major differencebetween rim and core is the increased porosity ofthe rim due to the loss of the abundant carbonatecomponent that is present in the core (2030%) aslarge twinned single crystals and as smaller

grains replacing feldspar. Notable features ob-served in the core are small rhombs that showhigh luminescence. These rhombs are surroundedby thin iron-rich, non-luminescent rims and havea very brightly luminescent core that does nothave a crystal shape. This relationship mayindicate that an earlier dolomite or ankeriticrhomb-shaped crystal was replaced by the car-bonate. Accordingly, it is clear that the core is not

entirely pristine but also underwent significantpost-depositional changes (i.e. carbonate replace-ment). Minor phases are chlorite, biotite, apatite,zircon and rutile. A few monazite grains 3010 lm in size were observed in both core and rim.

Rare earth element abundances

Rare earth element and Sm/Nd isotope data forthe core and rim of the Normanskill Formationsample are given in Tables 1 and 2. Chondrite-normalized REE patterns for three core samplesand the rim composite and various grain sizes areshown in Fig. 8a. The main differences betweenthe patterns are the 7% higher Sm-Nd ratio of theweathered rim and the negative Ce anomaly of thecore. The rim samples (Table 1) differ in absoluteabundances, but the shapes of the patterns aresimilar and none shows negative Ce anomalies. InFig. 8b, the rim composite and the results from

the 1

0 N HCl leaching experiment are shown.Samples plotted in Fig. 8b are normalized to theaverage core composition, so the positive Ceanomalies are artifacts of the negative Ce anomalypresent in the core (Fig. 8a), but the core leachatehas a chondrite-normalized negative Ce anomaly(Ce/Ce* 075). The difference in REE patternsbetween core and rim results largely from the lossof a soluble middle REE-enriched phase(s) with

Table 1. REE abundances (p.p.m.) of a hand specimen from the Normanskill Formation.

Core Core Core Core av. Rimcomp. Rimcoarse Rimfine Rim vs.fine Rim av. Coreleach Coreresidue

La 2351 2234 1984 2190 1928 1798 2892 2698 2329 2434 1431Ce 3767 3596 3754 3706 4451 4053 5700 6138 5086 4841 3304Nd 2567 2518 2695 2593 2420 2030 3433 3540 2856 4966 1401Sm 563 553 603 573 580 476 883 906 711 1388 231Eu 127 127 134 129 127 104 211 213 164 320 045Gd 640 670 663 658 580 471 960 951 741 1492 195Dy 508 526 599 544 527 445 830 848 663 1173 225Er 348 317 336 334 302 257 480 468 377 685 160Yb 330 267 292 296 292 249 431 445 354 463 168Sum 112 108 111 110 112 99 158 162 133 178 72Sm/Nd 0219 0220 0224 0221 0240 0235 0257 0256 0249 0280 0165Ce/Ce* 074 074 082 076 102 103 089 099 098 075 111

Eu/Eu* 065 0

64 0

64 0

64 0

67 0

67 0

70 0

70 0

69 0

68 0

64

Table 2. Nd isotopes of a hand specimen from the Normanskill Formation.

Sample Sm (p.p.m.) Nd (p.p.m.) 147Sm/144Nd 143Nd/144Nd eNd (0)

Core 553 2518 01330 0512012 )122Core 603 2695 01347 0512011 )122Rim composite 580 2420 01443 0511997 )125

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chondrite-normalized negative Ce anomalies.Mass balance calculations show that mixing30% of leachate and 70% of residue results inan REE pattern similar to the core. These propor-tions agree with the estimated carbonate compo-

nent based on petrography.The loss of the carbonate component cannot be

responsible for the flattening of the REE pattern ofthe rim because the Sm/Nd ratio of the leachedcore (residue) is 0165, which is lower than theSm/Nd ratio of the unleached core (0221) andmuch lower than the Sm/Nd ratio of the rim(0249) (Table 1). Thus, the leaching experimentdid not result in a residue that is representative ofthe rim composition. This could be interpreted intwo ways: (1) the leaching removed not only REEs

held in the carbonate component, but alsoremoved REEs from other sites (perhaps REEsabsorbed on clays); or (2) the leaching removedonly the carbonate component, but the differencebetween residue and rim results from morecomplex processes such as preferential retentionand/or loss of the REEs by other minerals present

in the rock. The leaching experiment has shownthat the REE component that is most easilyremoved from the core has a high Sm/Nd ratio.

Timing and character of Sm/Nd isotopere-equilibrationThe Sm/Nd isotope data are given in Table 2 andplotted in Fig. 9 where they are compared withsamples from the single stratigraphic section inthe Austin Glen Member. Rim and core havediffering Sm/Nd ratios but essentially identical143Nd/144Nd ratios consistent with recent isotopic

re-equilibration and REE redistribution. It isworth noting that the intersection of the 0 Mareference isochron and any reasonable 465 Mareference isochron is at relatively high147Sm/144Nd (> 012) and 143Nd/144Nd ratios con-sistent with an earlier disturbance of these sam-ples. The cause of the re-equilibration event is notobvious but probably related to the recent weath-ering that affected these samples.

On the other hand, the presence of Ce anomaliesis not necessarily the direct result of recentweathering. Where fractionation of the REEs

Fig. 8. (a) Chondrite-normalized REE pattern for a rimcomposite, various grain sizes of the rim and threeanalyses of the core. (b) Average core-normalized REEpattern of a soft (1 N HCl) leach, its residue and, forcomparison, the rim composite.

Fig. 9. 143Nd/144Nd vs. 147Sm/144Nd for the core andrim. Also shown are 18 Ga, 465 Ma and 0 Ma referenceisochrons as well as the data points of the singlestratigraphic section of the Austin Glen Member (opencircles).




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occurs during weathering processes in an oxid-izing environment, Ce anomalies may be found(Banfield & Eggleton, 1989; Braun et al., 1990;Marsh, 1991) and can be attributed to the oxida-tion of Ce3+ to the less mobile Ce4+, which is co-precipitated with oxyhydroxides or precipitatedas the extremely insoluble CeO2 (cerianite; Braun

et al., 1990). Therefore, weathering residues oftenexhibit positive Ce anomalies (Ce/Ce* < 1),whereas weathering products are commonly de-pleted in Ce (Ce/Ce* < 1). However, carbonateminerals may also carry a negative Ce anomaly,which is generally interpreted as being inheritedfrom sea water (Elderfield et al., 1981). As the coreis interpreted as relatively unweathered, thepresence of a Ce anomaly is unexpected. Onepossibility is that the core is only apparentlyunweathered; for example, Price et al. (1991)noted that incipient alteration often causes the

most erratic REE patterns (see also Banfield &Eggleton, 1989), whereas more severely alteredsamples more closely reflect the unaltered materi-al. The presence of carbonate with a pronouncednegative Ce anomaly in the core (indicated by theleach; see Fig. 8) is the alternative explanation,and the REE pattern of the rim supports thisconclusion as the rim is carbonate-free and has nochondrite-normalized negative Ce anomaly.

DISCUSSION

The data reviewed and discussed in this paperclearly establish that isotopic re-equilibration ofthe Sm/Nd system and redistribution of REE onlarger than hand sample scales are not necessarilyisolated effects. In the Taconian foreland of NewEngland, tectonically and sedimentologicallyrelated sedimentary rocks that are separated intime (sedimentation ages range from c. 465 toc. 420 Ma) and space (western New York towestern Maine) have been affected. That is notto say, however, that all units within this tectono-stratigraphic sequence have been so affected as

there are a number of published data fromstratigraphic units that show no evidence forREE redistribution (e.g. Andersen & Samson,1995; Cullers et al., 1997). Nevertheless, it isremarkable that evidence for REE redistributionhas been found in so many tectonically relatedstratigraphic units.

The exact timing of re-equilibration appears tobe variable. In the Austin Glen/Pawlet Membersand Frankfort Formation, re-equilibration is prob-ably diagenetic, and this appears to be the most

common situation (Stille & Clauer, 1986; Broset al., 1992). However, in the Perry MountainFormation, isotopic disturbance may have oc-curred during diagenesis and again later duringAcadian and/or Alleghenian metamorphism. Inthe extreme case of the corerim sample, thedisturbance appears to be essentially recent and

may result from weathering. Thus, in sedimentarysuccessions that are susceptible to Sm/Nd iso-topic disturbance, the disturbance can occur atwhatever time(s) at which the appropriate condi-tions come into existence, whether that be duringdiagenesis, metamorphism or weathering.

In the study of Early Silurian turbidites inWales, Ohr et al. (1991) recognized Sm/Nd re-equilibration but saw no evidence that sampleswere affected on anything greater than a minera-logical scale. This is probably because conditionsthat give rise to mineral dissolution and changes

to whole rock Sm/Nd ratios did not occur, as isthe case for the Taconian samples. In cases wherechanges in Sm/Nd ratios (thus permitting therecognition of isotopic disturbance on a wholerock scale) result simply from dissolution of oneor more phases (e.g. monazite, apatite), the timingof the REE mobility may post-date the isotopic re-equilibration by any amount of time. The exacttiming of this second stage cannot be evaluated byREE or Sm/Nd isotope data alone and must beinferred from petrographic, geochemical andother independent data. This can be seen in theschematic isochron diagram presented in

Fig. 10a. In a simplified case, a sample has hadthe Sm/Nd system fully re-equilibrated among thevarious minerals at time T0. If, at some interme-diate time (T1), one of the phases is lost, the effecton the whole rock is to change both Sm/Nd ratioand Nd isotope composition such that the wholerock remains on the isochron. Of note, however,is that this stage of REE loss gives rise to variableSm/Nd ratios that permit the entire disturbance(both isotopic and REE loss) to be identified on awhole rock scale.

The complications that these processes may

impose on any interpretation of provenance areillustrated in Fig. 10b. Here, it can be seen thatthe change in bulk rock Sm/Nd results in anestimate of mantle extraction age (generallyequated to mean age of provenance) that isaberrant. On the other hand, it is valid tocalculate the Nd isotope composition of thewhole rock back to the time of re-equilibration(T0). Where this is essentially equivalent to theage of sedimentation (i.e. diagenetic resetting),the estimate of eNd at the time of sedimentation

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will be robust and can be compared with poten-tial provenance reservoirs.

As pointed out by Ohr et al. (1991), regardless

of whether or not Nd isotopic re-equilibration hasoccurred, as long as there is no open systemtransport of REE, the whole rock Sm/Nd isotopesystematics remain intact. In cases such as thosedescribed in this paper, that is obviously not thecase. In Fig. 11, TDM is plotted against Sm/Nd,and it can be seen that there is a strong relation-ship, providing an important test for open systemtransport of REE. In effect, where a correlationexists between model age and Sm/Nd ratio,disturbance of the Sm/Nd isotope system should

be suspected and evaluated with more thoroughpetrographic and geochemical studies.

CONCLUSIONS

In this paper, it has been demonstrated that rareearth redistribution and disturbance of the Sm/Nd isotope system were widespread both geo-graphically (from Maine to New York State) andstratigraphically (from the Ordovician to theSilurian, about 40 million years) within thesedimentary successions of the Taconian fore-land.

Fig. 10. (a) and (b). Schematic Sm/Nd isochron diagram illustrating the effects of mineral scale Sm/Nd isotope re-equilibration in a single sample and subsequent dissolution of an REE-bearing mineral (mineral A). Where a certainmineral(s) contain(s) a substantial fraction of the rocks REE budget and an Sm/Nd ratio significantly different fromthe whole rock, loss of the mineral will result in a changed Sm/Nd ratio of the whole rock. The earlier resetting eventwill still be recorded in the remaining minerals, and the eNd at the time of the resetting will also remain unchanged.(b) However, the Nd model age will be disturbed to a degree controlled by the REE characteristics (Sm/Nd, Ndcontent) of the REE-bearing mineral and the time of its dissolution.




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Furthermore, it has been demonstrated thatsubstantial Sm/Nd isotope re-equilibration at amineralogical scale is likely to have occurredtypically near the time of sedimentation becausethe eNd values converge to homogeneous valueswhen back-calculated to the time of sedimentation.

And, finally, it was observed that open-systemtransport of REE, occurring on greater than wholerock scale, affected these rocks. This can be recog-nized by the REE patterns that deviate from averageupper crustal REE patterns and by the positiverelationship of Sm/Nd ratios and TDM values.

ACKNOWLEDGEMENTS

This study was funded by an NSF grant (EAR-8957784) to S. M. McLennan. Thanks toS. Samson, M. Schoonen and D. Davis for readingan earlier version. Thanks to R. Hannigan andR. Cullers for their thorough reviews, whichhelped to improve the manuscript.

REFERENCES

Andersen, C.B. and Samson, S.D. (1995) Temporal changes inNd isotopic composition of sedimentary rocks in the Sevierand Taconic foreland basin: Increasing influence of juvenilesources. Geology, 23, 983986.

Banfield, J.F. and Eggleton, R.A. (1989) Apatite replacementand rare earth mobilization, fractionation, and fixationduring weathering. Clays Clay Minerals, 37, 113127.

Bock, B., McLennan, S.M. and Hanson, G.N. (1994) Rare earthelement redistribution and its effects on the neodymiumisotope system in the Austin Glen Member of the Norman-skill Formation, New York, USA Geochim. Cosmochim.Acta, 58, 52455253.

Bock, B., McLennan, S.M. and Hanson, G.N. (1996) TheTaconian orogeny in southern New England: Nd-isotopeevidence against addition of juvenile components. Can. J.Earth Sci., 33, 16121627.

Bock, B., McLennan, S.M. and Hanson, G.N. (1998) Geo-chemistry and provenance of the Middle Ordovician AustinGlen Member (Normanskill Formation) and the TaconianOrogeny in New England. Sedimentology, 45, 635655.

Bouch, G.C., Hole, M.J., Trewin, N.H. and Morton, A.C. (1995)Low-temperature aqueous mobility of the rare earth ele-ments during sandstone diagenesis. Geol. Soc. London, 152,985998.

Bradley, D.C. (1989) Taconic plate kinematics as revealed byfore deep stratigraphy, Appalachian Orogen. Tectonics, 8,10371049.

Braun, J.-J., Pagel, M., Muller, J.-P., Bilong, Michard, A. andGuillet, B. (1990) Cerium anomalies in lateritic profiles.Geochim. Cosmochim. Acta, 54, 781795.

Bros, R., Stille, P., Gauthier-Lafaye, F., Weber, F. and Clauer,N. (1992) Sm-Nd isotopic dating of Proterozoic clay mater-ial: An example from the Francevillian sedimentary series,Gabon. Earth Planet. Sci. Lett., 113, 207218.

Cullers, R.L., Bock, B. and Guidotti, C. (1997) Elemental dis-tributions and neodymium isotopic compositions of Silur-ian metasediments, western Maine, USA: redistribution ofthe rare earth elements. Geochim. Cosmochim. Acta, 61,18471861.

Elderfield, H. and Sholkovitz, E.R. (1987) Rare earth elementsin the pore waters of reducing nearshore sediments. EarthPlanet. Sci. Lett., 82, 280288.

Elderfield, H., Hawkesworth, C.J. and Greaves, M.J. (1981)Rare earth element geochemistry of oceanic ferromanganesenodules and associated sediments. Geochim. Cosmochim.Acta, 5, 513528.

Evans, J. and Zalasiewicz, J. (1996) U-Pb, Pb-Pb and Sm-Nddating of authigenic monazite: implications for the dia-genetic evolution of the Welsh Basin. Earth Planet. Sci.Lett., 144, 421433.

German, C.R. and Elderfield, H. (1989) Rare earth elements inSaanich Inlet, British Columbia, a seasonally anoxic basin.Geochim. Cosmochim. Acta, 53, 25612571.

Hannigan, R.E. and Sholkovitz, E. (2001) The development ofmiddle rare earth element enrichments in freshwaters:weathering of phosphate minerals. Chem. Geol., 175, 495508.

Hurowitz, J.A. (2001) The Geochemical and Nd-IsotopicEvolution of the Eastern Margin of North America in NewEngland: Late Proterozoic to Middle Ordovician. MSThesis, Stony Brook, State University of New York, 137pp.

Kidder, D.L., Krishnaswamy, R. and Mapes, R.H. (2003) Ele-mental mobility in phosphatic shales during concretiongrowth and implications for provenance analysis. Chem.Geol., 198, 335353.

Lev, S.M., McLennan, S.M., Meyers, W.J. and Hanson, G.N.(1998) A petrographic approach for evaluating trace-elementmobility in a black shale. J. Sed. Res., 68, 970980.

Fig. 11. Plot of TDM vs.147Sm/144Nd for samples from

the Taconian foreland. Hatched area encloses

approximate range of TDM and 147Sm/144Nd for samplesunaffected by REE redistribution. Note that changes inSm/Nd result in geologically unreasonable model ages,and such a pattern is a good indicator of disturbance onthe Sm/Nd isotope system in clastic sedimentary rocks.

896 B. Bock et al.



13/13

Lev, S.M., McLennan, S.M. and Hanson, G.N. (1999) Minera-logic controls on REE mobility during black-shale diagen-esis. J. Sed. Res., 69, 10711082.

Leventhal, J.S. (1990) Comparative geochemistry of metals andrare earth elements form the Cambrian alum shale and kolmof Sweden. In: Sediment-Hosted Mineral Deposits (Ed. J.Parnell), Int. Assoc. Sedimentol. Spec. Publ., 11, 203215.

McDaniel, D.K., Hemming, S.R., McLennan, S.M. and Han-

son, G.N. (1994) Resetting of neodymium isotopes andredistribution of REEs during sedimentary processes: TheEarly Proterozoic Chelmsford Formation, Sudbury Basin,Ontario, Canada. Geochim. Cosmochim. Acta, 58, 931941.

McLennan, S.M. (1989) Rare Earth Elements in sedimentaryrocks: influence of provenance and sedimentary processes.Rev. Mineral., 21, 169200.

McLennan, S.M. and Hemming, S. (1992) Samarium/neo-dymium elemental and isotopic systematics in sedimentaryrocks. Geochim. Cosmochim. Acta, 56, 887898.

McLennan, S.M. and Taylor, S.R. (1979) Rare earth elementmobility associated with uranium mineralization. Nature,282, 247250.

McLennan, S.M., Hemming, S., McDaniel, D.K. and Hanson,G.N. (1993) Geochemical approaches to sedimentation,

provenance and tectonics. In: Processes Controlling theComposition of Clastic Sediments (Eds M.J. Johnsson and A.Basu), GSA Spec. Paper, 284, 2140.

Marsh, J.S. (1991) REE fractionation and Ce anomalies inweathered Karoo dolerite. Chem. Geol., 90, 189194.

Milodowski, A.E. and Hurst, A. (1989) The authigenesis ofphosphate minerals in some Norwegian hydrocarbon res-ervoirs: evidence for the mobility and redistribution of rareearth elements (REE) and Th during sandstone diagenesis.In: Proceedings of International Symposium on Water Rock Interaction (Ed. D.L. Miles), pp. 491494. Balkema,Rotterdam.

Milodowski, A.E. and Zalasiewicz, J.A. (1991) Redistributionof rare earth elements during diagenesis of turbidite/hemi-pelagite mudrock sequences of Llandovery age from central

Wales. In: Developments in Sedimentary Provenance Stud-ies (Eds A.C. Morton et al.), GSA Spec. Paper, 57, 101124.

Moench, R.H. (1971) Geologic Map of the Rangeley and Phil-lips Quadrangles, Franklin and Oxford Counties, Maine.USGS, Reston, VA, USA.

Murray, R.W., Buchholtz ten Brink, M.R., Gerlach, D.C.,Russ, D.P. and Jones, D.L. (1991) Rare earth, major and traceelements in cherts from the Franciscan Complex and Mon-terey Group, California: assessing REE sources to finegrained marine sediments. Geochim. Cosmochim. Acta, 55,18751895.

Murray, R.W., Buchholtz ten Brink, M.R., Gerlach, D.C.,Russ, D.P. and Jones, D.L. (1992) Interoceanic variation inthe rare earth, major, and trace element depositional

chemistry of chert: perspectives gained from the DSDP andODP record. Geochim. Cosmochim. Acta, 56, 18971913.

Nesbitt, H.W. (1979) Mobility and fractionation of rare earthelements during weathering of a granodiorite. Nature, 279,206210.

Ohr, M., Halliday, A.N. and Peacor, D.R. (1991) Sr and Ndisotopic evidence for punctuated clay diagenesis, TexasGulf Coast. Earth Planet. Sci. Lett., 105, 110126.

Ohr, M., Halliday, A.N. and Peacor, D.R. (1994) Mobility andfractionation of rare earth elements in argillaceous sedi-ments: Implications for dating diagenesis and low-grademetamorphism. Geochim. Cosmochim. Acta, 58, 289312.

Osberg, P.H., Moench, R.H. and Warner, J. (1968) Stratigraphyof the Merrimack Synclinorium in west-central Maine. In:Studies of Appalachian Geology: Northern and Maritime(Eds E.A. Zen et al.), pp. 241253. Interscience, New York.

Osberg, P.H., Tull, J.F., Robinson, P., Hon, R. and Butler, J.R.(1989) The Acadian orogen. In: The Appalachian-OuachitaOrogen in the US, Vol. F-2 (Eds R.D. Hatcher, W.A. Thomasand G.W. Viele), Geol. Soc. Am., Boulder, CO, USA.

Price, R.C., Gray, C.M., Wilson, R.E., Frey, F.A. and Taylor,S.R. (1991) The effects of weathering on rare-earth element,Y and Ba abundances in Tertiary basalts from southeastern

Australia. Chem. Geol., 93, 245265.Rasmussen, B. and Glover, J.E. (1994) Diagenesis of low-mobility elements (Ti, REE, Th) and solid bitumen enve-lopes in Permian Kennedy Group Sandstone, WesternAustralia. J. Sed. Res., A64, 572583.

Riva, J. (1974) A revision of some Ordovician graptolites ofeastern North America. Paleontology, 17, 140.

Rowley, D.B. and Kidd, W.S.F. (1981) Stratigraphic relation-ships and detrital composition of the Medial Ordovicianflysch of western New England: implications for the tec-tonic evolution of the Taconic Orogen. J. Geol., 89, 199218.

Stanley, R.S. and Ratcliffe, N.M. (1985) Tectonic synthesis ofthe Taconian orogeny in western New England. Geol. Soc.Am. Bull., 96, 12271250.

Stille, P. and Clauer, N. (1986) Sm-Nd isochron age and

provenance of the argillites of the Gunflint Iron FormationOntario, Canada. Geochim. Cosmochim. Acta, 50, 11411146.

Sturesson, U. (1995) Llanvirnian (Ord.) iron ooids in Balto-scandia; element mobility, REE distribution patterns, andorigin of REE. Chem. Geol., 125, 4560.

Zhao, J.X., McCulloch, M.T. and Bennett, V.C. (1992) Sm-Ndand U-Pb zircon isotopic constraints on the provenance ofsediments from the Amadeus Basin, central Australia: evi-dence for REE fractionation. Geochim. Cosmochim. Acta,56, 921940.

Manuscript received 28 July 2003; revision accepted 23February 2004.



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