Evaluating chemical equilibrium in metamorphic rocks using ... · Evaluating chemical equilibrium...

Chemical Geology 401 (2015) 151–168

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Chemical Geology

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Evaluating chemical equilibrium in metamorphic rocks using majorelement and Sm–Nd isotopic age zoning in garnet, Townshend Dam,Vermont, USA

Matthew P. Gatewood a,⁎, Besim Dragovic b,1, Harold H. Stowell a, Ethan F. Baxter b,David M. Hirsch c, Rose Bloom c

a Department of Geological Sciences, University of Alabama, Box 870338, Tuscaloosa, AL 35487-0338, USAb Department of Earth and Environment, Boston University, 685 Commonwealth Ave., Boston, MA 02215, USAc Geology Department, Western Washington University, 516 High St., Bellingham, WA 98225-9080, USA

⁎ Corresponding author. Tel.: +1 251 709 8634.E-mail addresses: [email protected] (M.P

(B. Dragovic), [email protected] (H.H. Stowell), efb@[email protected] (D.M. Hirsch), rose.vail.bloom@gm

1 Present address: Department of Geological Sciences, VState University, 1405 Perry St., Blacksburg, VA 24061, US

http://dx.doi.org/10.1016/j.chemgeo.2015.02.0170009-2541/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 15 March 2014Received in revised form 21 January 2015Accepted 18 February 2015Available online 26 February 2015

Editor: L. Reisberg

Keywords:MetamorphismChemical equilibriumGarnet Sm–Nd geochronologyPorphyroblast growth ratesTownshend Dam, VT

A quantitative assessment of metamorphic chemical equilibrium derived from correlation of spessartine contentand garnet Sm–Nd ages suggests that major element matrix equilibrium was maintained (to a first order)throughout a ca. 40 cm-wide rock sample during garnet growth; however cm-scale Sm–Nd isotopic heterogene-ity limits the Sm–Nd age precision required to evaluate more subtle age differences within individual garnetcrystals. Central wafers from 1–3 cm diameter garnet grains within a 1.21 × 104 cm3 block of pelitic schistwere used to document concentric growth zoning of major elements, with decreasingMn and Ca and increasingFe and Mg from cores to rims. Garnets also preserve growth zoning patterns for HREE and MREE and showevidence for resorption and partial recrystallization of the outermost rims. Similar garnet core compositionsand identical garnet rim compositions for large like sized porphyroblasts throughout the sample suggest thatgarnet growth occurred at near equilibrium P–T–X conditions for major elements over the sample volume.Comparison of 28 rock Sm–Nd isotope values from the sample indicates substantial cm-scale heterogeneity,which precludes meaningful use of local garnet rock isotope pairs for isochron age calculation. Therefore,Sm–Nd isotopic compositions of thirty-eight concentric core to rim garnet segments from ten large (1–3 cm)garnets and two small (1–4 mm) bulk garnets, with narrow ranges of Mn content, are paired with sixteenmatrix/whole-rock Sm–Nd isotopic compositions collected over the rock volume to define a range of isochronages from 383.1 ± 6.8 Ma to 324.5 ± 3.3 Ma. Four of the garnets have anomalously young rims that likely resultfrom post-growth alteration. Chlorite, quartz, and xenotime haloes around garnet suggest that anomalouslyyoung garnet rim ages reflect post-growth resorption/recrystallization effects. Excluding these young rims yieldsa range of ages from 383.1± 6.8 (oldest core) to 374.9± 1.8Ma (youngest rim). Sm–Nd age precisions N1.5m.y.(and high MSWD) result primarily from isotopic heterogeneity in the finely layered metasedimentary rockmatrix. However, garnet cores with high Mn (n = 7), mantles with intermediate Mn (n = 14), and rims withlow Mn (n = 8; including the 2 smaller bulk garnet analyses), define three distinct multi-grain isochrons of380.3 ± 2.0 Ma (n = 23, MSWD = 14), 377.3 ± 1.4 Ma (n = 30, MSWD = 18), and 376.5 ± 1.0 Ma (n = 24,MSWD= 18), respectively, yielding an average garnet growth duration of 3.8 ± 2.2 m.y. These three compositeMn-age zones define aMn vs. age relationship that reflects depletion ofMn in the rockmatrix as it is sequesteredby growing garnet. Correlation of garnetmajor element compositions throughout the sample suggests thatmajorelement matrix equilibrium was generally maintained (to a first order) throughout the ca. 4 m.y. duration ofgarnet growth.

© 2015 Elsevier B.V. All rights reserved.

. Gatewood), [email protected] (E.F. Baxter),ail.com (R. Bloom).irginia Polytechnic Institute andA.

1. Introduction

Metamorphic rocks are generally considered as equilibriumpressure–temperature–composition-mineralogical (P–T–X-M) systems. Althoughthis equilibrium view of metamorphism is widely used for interpretingP–T histories of rocks, it is disequilibrium that drivesmetamorphic reac-tions and produces mineral compositional zoning and metamorphicrock textures (e.g. Carlson, 2002). Therefore, to fully understand the

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152 M.P. Gatewood et al. / Chemical Geology 401 (2015) 151–168

metamorphic process, the specific chemical elements and the spatial,temporal, and compositional scales over which rocks achieve (or donot achieve) chemical equilibrium need to be rigorously evaluated.

During metamorphic mineral growth, certain elements are prefer-entially incorporated into a growing crystal. When the interior portionsof crystals no longer equilibrate with the matrix, the ‘effective’ bulkcomposition of the rock matrix outside the growing crystal will changeif the rock is closed to the elements incorporated into the new crystal(e.g. Hollister, 1966; Carlson, 2002). Porphyroblasts, such as garnet, pref-erentially incorporate elements, such asMn, which become progressivelydepleted in the matrix as more crystals nucleate and grow. The familiardisequilibrium texture of a chemically zoned porphyroblast is producedvia this process. Although the chemical cores of zoned porphyroblastsare out of equilibrium with the matrix, growing porphyroblast rims maycontinually maintain chemical equilibrium with the rock matrix duringchanging P–T–X-M conditions. Therefore, major-element, trace-element,and isotopic zoning in garnet provide a record of the spatial and temporalscales over which these changes occur during porphyroblast growth andprogressive metamorphism. This study evaluates concentric Mn growthzoning in garnet in order to determine the scale of chemical equilibrationbetween garnet and the rockmatrix aswell as the applicability ofMn asa proxy for age for calculating the rates of garnet growth. Our worktests the hypothesis that garnet Mn concentrations will correlate withSm–Nd ages within individual garnets throughout a hand specimen.

Garnets from the spillway of Townshend Dam, Vermont, USA, wereselected because pelitic schists from this location are well exposed,appear to reflect onemetamorphic event, haveMn-rich bulk-rock com-positions, and contain large syn-kinematic “snowball” garnet crystals.Several workers have investigated the stratigraphic (Doll et al., 1961),structural (Rosenfeld, 1970), petrologic (Kohnand Spear, 1990) and iso-topic (Chamberlain and Conrad, 1993; Kohn andValley, 1994) characterof rocks from this locality. Christensen et al. (1989) used Rb–Sr isotopesto calculate a 10.5 ± 4.5 m.y. interval for syn-kinematic garnet growthin these rocks.

2. A quantitative test for chemical equilibrium

The use of equilibrium thermodynamics in metamorphic petrologydates back to the first use of the phase rule to relate observedmetamor-phic mineral assemblages to bulk rock composition by Goldschmidt(1912). Over the next century, studies of metamorphic rocks have usedequilibrium thermodynamics to construct P–T paths for orogenic beltsand models of metamorphic processes (e.g. Powell and Holland, 2010).However, documentation of chemical equilibrium during metamor-phism is often limited to observations of textural relationships betweenphases, and many studies have documented chemical disequilibriumduring metamorphism via measurements of stable and radiogenic iso-topes (e.g., Barnett and Bowman, 1995; Baxter and DePaolo, 2002b;Eppel and Abart, 1997; Baxter and DePaolo, 2004, etc.). Failure to fullyequilibrate locally between solid phases and grain boundary fluidssuggests sluggish local reaction rates. Failure to equilibrate at the rock-scale across heterogeneous domains (either reflecting protolith hetero-geneities, or metamorphically induced heterogeneities due to melting,metasomatism, Rayleigh fractionation depletion) suggests sluggishrock-wide chemical transport rates. Comparison of local reaction ratesand rock-wide transport rates permits quantification of an “equilibriumlengthscale” for a given element in the system (e.g. DePaolo and Getty,1996; Baxter and DePaolo, 2002a). When the equilibrium lengthscaleis very large, all minerals crystallizing within that volume will sharethe same equilibrium surface compositions. When the equilibriumlengthscale is very small, minerals crystallizing outside of that volumemay have different compositions. In the study presented here, we willtest for the scale of rockwidemajor element (Mn) chemical equilibriumas well as 143Nd/144Nd and 147Sm/144Nd equilibrium by comparing Mnand Sm–Nd isotopic zonation in garnet crystals.

Kretz (1973, 1974) first used chemical zoning patterns in garnetsas a proxy for porphyroblast growth rates. This technique, andmodifica-tions of it, have been used in several other studies to address garnetgrowth rates (e.g. Cygan and Lasaga, 1982; Finlay and Kerr, 1987;Carlson, 1989). As noted by Carlson (1989), all garnets that grow fromthe same medium under the same equilibrium conditions at the sametime should preserve identical compositions for any particular growthincrement. Concentric zoning of Mn in garnet is particularly useful forevaluating garnet growth processes because Mn is strongly partitionedinto garnet and thereby provides a recordof changingphysical and chem-ical conditions during metamorphism (e.g. Harte and Henley, 1966;Hollister, 1966; Tracy, 1982; Spear and Daniel, 2001; Hirsch et al.,2003). Based on similarities in modeled and observed concentric Mnzoning in garnet, Hollister (1966) proposed that Mn is fractionatedinto growing garnet by a Rayleigh distillation process. Following thispioneering work, concentric Mn zoningwas noted in garnets from sim-ilar rocks and peak Mn garnet compositions of concentrically-zonedgarnets have been widely used to infer initial garnet growth composi-tions (e.g. Chernoff and Carlson, 1997; Spear and Daniel, 1998). Yangand Rivers (2002) argue against using Mn as a proxy for garnet growthage, based on the occurrence of high-Mn annuli in their samples. How-ever, monotonic simple concentric Mn zoning is common in garnetfrom many locations and all garnets observed from Townsend Damhave concentric zoning profiles with decreasing Mn and lack high-Mnannuli.

This study tests the idea that rock-wide equilibrium should result incorrelation betweenMn concentration in garnet and garnet growth age.To test for the scale of rock-widemajor element equilibrium during gar-net growth,we present garnetmajor element compositions and isotoperatios to evaluate the correlation of garnet compositions and growthages. If major element equilibrium, particularly of Mn, was maintainedthroughout garnet growth, (i.e. identical garnet Mn compositions pre-cipitated at the same time), then Mn compositions and growth agesshould correlate across the equilibrium volume. The Sm–Nd method isused for age determination, but this too requires the assumption ofinitial equilibrium (for 143Nd/144Nd) between the growing garnet andan identifiable matrix reservoir from which the garnet grew. Thus, ourstudy will also by necessity investigate the lengthscale of heterogeneityand/or equilibrium for Sm and Nd within the rock system. Severalworkers have utilized the Sm–Nd isotopic system to investigate garnetgrowth and metamorphic rates (e.g. Vance and O'Nions, 1990; Burtonand O'Nions, 1991; Hensen and Zhou, 1995; Vance and Harris, 1999;Stowell et al., 2001; Baxter et al., 2002; Ducea et al., 2003; Stowell andTinkham, 2003; Pollington and Baxter, 2010). Recently developed tech-niques for Sm–Nd analysis of small volumes of garnet (Harvey andBaxter, 2009; Pollington and Baxter, 2011) allow for microsampling ofcompositionally-specific segments of individual garnet crystals andcorrelation of garnet Mn compositions with growth age across a givenvolume of rock.

3. Geologic setting

The geology of southeastern Vermont is structurally complex,consisting of a deformed andmetamorphosed passivemargin sequencestructurally overlying Mid-Proterozoic Grenville basement (Dollet al., 1961). The passive margin sequence was thrust onto the NorthAmericanmargin during the Taconic orogeny (470–455Ma) and subse-quently metamorphosed during the Acadian orogeny (390–360 Ma).The spillway at Townshend Dam exposes part of this metamorphosedand deformed passive margin sequence.

The Acadian orogeny is thought to be the result of a protracted ca.30 m.y. collision of Laurentia and the composite Avalon terrane(Tucker et al., 2001). Acadianmetamorphism in NewEngland is spatial-ly and temporally discontinuous. Karabinos (1984) showed that garnetsfrom southeastern Vermont preserve textural and chemical discontinu-ities, thereby recording two deformational and metamorphic events.

153M.P. Gatewood et al. / Chemical Geology 401 (2015) 151–168

However, the rocks at Townshend Dam only preserve evidence for asingle staurolite zone/amphibolite facies metamorphic event (Rosenfeld,1968; Ratcliffe et al., 1992). P–T estimates for peak metamorphic condi-tions range from 550 to 625 °C and 8 to 11 kbar (Kohn and Spear, 1990;Ratcliffe et al., 1992; Kohn and Valley, 1994; Dragovic, 2013).

Christensen et al. (1989) used Rb and Sr isotopes to determinegrowth durations for garnets from Townshend Dam. By analyzing coreand rim segments of garnets from the Pinney Hollow Formation, theyshowed that synkinematic garnet porphyroblasts grew over an averagetime interval of 10.5 ± 4.5 m.y., between ca. 380 Ma and 370 Ma. Fur-thermore, they used the curvature of helicitic inclusions in garnet tocalculate strain rates of 2.4 × 10−14 s−1. This landmark paper was thefirst use of isotopically zoned garnet porphyroblasts to evaluate therates of tectonic processes. Their interpretation of the Rb–Sr data relieson the assumption that the 87Sr/86Sr isotopic evolution of the rock ma-trix is solely due to the decay of 87Rb, the 87Sr/86Sr in garnet resultsfrom incorporation of Sr from the adjacent matrix, and the matrix hasnot been affected by interaction with Sr-rich fluids. Metasomatic fluidscould substantially change the Sr budget; however, Kohn and Valley(1994) showed that metamorphic fluid transport during garnet growthwas restricted to layer-parallel flow with potentially little to no fluidflow at all, allowing the possibility of closed system Rb–Sr behavior.The Rb–Sr system is also particularly susceptible to local scale matrixheterogeneities, even at the grain scale, due to the wide range of Rb/Srratios in common rock-forming minerals (e.g. Sousa et al., 2013). Inthis study the Sm–Nd isotopic system is utilized because Sm and Nd ingarnet are less susceptible to fluid-flux alteration due to their relativeimmobility in most crustal fluids (Baxter and DePaolo, 2002b) and lesssusceptible to local-scale heterogeneities because the range in Sm/Ndamongst rock forming minerals is small (e.g. Sousa et al., 2013). Fur-thermore, Sm and Nd concentrations in garnet are generally very lowcompared to the rock matrix (e.g. Baxter and Scherer, 2013; Table 1),meaning that rock matrix Sm/Nd ratios are not significantly changedduring garnet growth.

4. Sample selection

Although samples were collected from all formations exposed inthe spillway, this detailed geochemical and isotopic study focuseson a single sample from the Pinney Hollow formation of Doll et al.(1961). Biotite-rich lithologies were purposely avoided to eliminatepotential problems associated with the garnet polycrystals that occurin Townshend Dam schists (Whitney et al., 2008). The sample (TD09-14) was collected ca. 130 m southeast of the spillway wall. The contactwith the southeasternmost large amphibolite layer in the spillway is ca.3 m to the NW of this sample (see outcrop sketch in Chamberlainand Conrad, 1993). A large sample of garnet-bearing schist was col-lected and trimmed to ca. 1.21 × 104 cm3 (sample dimensions are ca.37 cm × 21 cm × 16 cm; Fig. 1). Garnets extracted from cuttings fromthe main volume (TD09-14a) were used for preliminary work (TD09-14b-Grt3 and TD09-14a2-Grt7). Sample TD09-14 is not significantly al-tered by weathering and contains abundant garnets (4.75% by volume)that are mostly large (1–3 cm), euhedral, and relatively inclusion-poor(Fig. 1). It is a coarse-grained pelitic schist that contains themetamorphicassemblage quartz+muscovite+paragonite+ garnet+ plagioclase+biotite with accessory ilmenite, rutile, apatite, epidote/clinozoisite,xenotime, and zircon (Fig. 1b). Garnets contain inclusions of quartz,white mica, biotite, plagioclase, apatite, epidote/clinozoisite, ilmenite,rutile, zircon, and xenotime. Xenotime occurs as inclusions only inthe outermost garnet rims and just outside the garnet-matrix grainboundaries. Retrograde chlorite cuts across the rock fabric and occursalong the margins of garnet (Fig. 1b). Based on the mineralogy andbulk rock composition, garnet likely grew at least in part via the reactionmuscovite+chlorite+quartz⇒Mg-chlorite+garnet+biotite+H2O.The schistosity in the rock is defined by alignment of micas and quartz. Aprominent S–C fabric in the sample suggests a component of non-coaxial

shear affected the rock, which may have also produced the heliciticinclusion patterns preserved in the garnets (Rosenfeld, 1970).

5. Analytical methods

This research is part of a collaborative multi-purpose projectdesigned to evaluate garnet growth duringmetamorphism. This holisticapproach to evaluating porphyroblast growth in a single sample drawsfrom three key datasets: (1) High-Resolution X-ray Computed Tomog-raphy (HRXCT); (2) rock and mineral major and trace-element com-positions; and (3) rock and mineral isotopic compositions and ages.HRXCT data were collected at the University of Texas and identificationof candidate crystals for this study was done at Western WashingtonUniversity (WWU). Selection, extraction, and wafering of garnet crys-tals for analysis and major and trace element characterization wasconducted at University of Alabama Central Analytical Facility. Allmicrosampling, garnet purification procedures, REE elution chemistry,and isotopic analyses were performed at the Boston University TIMSFacility.

5.1. Extraction of garnet

The candidate sample was trimmed on a large saw to obtainsubvolumes for preliminary study. Two garnets, TD09-14b-Grt3 andTD09-14a2-Grt7, were extracted from across the fabric for preliminaryEPMA and isotopic work. The main volume of the candidate sample(1.21 × 104 cm3) was sent to the University of Texas at Austin forHRXCT scanning. HRXCT data was collected following the methodsdescribed in Ketcham et al. (2005). The HRXCT scan produced a totalof 518 image slices with a spacing of 0.75 mm between slices and0.2766 mm between pixels. Here, the HRXCT image slices were onlyutilized to select target garnets that were suitable for study (large,euhedral, and inclusion-poor) and to document the spatial distributionof these target grains.

The main sample volume (TD09-14a) was initially cut into fiveblocks. The surface of each block was then visually matched to anHRXCT image slice, effectively registering the sample to the images(Figs. 1 and 2a). Then, an electric drill press with a diamond core bit(100 mm long, 35 mm inside diameter) was used to overcore targetgarnets (Fig. 2a). Drill cores were then serial sectioned perpendicularto their long axis into 2-mm thick sections with a wafering saw to ex-pose central sections of the target garnets and the attached rock matrix(Fig. 2b). Garnet central sections and adjacent rock matrices were thenmounted to glass slides with Crystalbond, polished, and imaged prior tochemical analyses.

5.2. Compositional mapping of garnet and xenotime

Mineral compositions were obtained using the JEOL 8600 electronprobe microanalyzer (EPMA) at the University of Alabama. Major-element compositional maps of garnet central sections were construct-ed by contouring a grid of EPMA point analyses obtained using fivewave length dispersive spectrometers (WDS). Grid spacings variedbetween 300 and 500 μm depending on garnet diameter, producingover 1200 point analyses per garnet wafer. To minimize analyticaltime, a “garnet quick map” routine was developed that simultaneouslycounts Kα X-rays of Fe, Mg, Mn, Ca, and Si for 5–7 s and uses a 15 keVaccelerating voltage with a 200 nA, 1 μm-diameter beam. Al concentra-tions were calculated stoichiometrically, assuming 2 Al cations for each3 Si. The garnet chemical mapping routine was calibrated with mineralstandards using Probe for Windows v.8.21 software (Donovan et al.,2007). When possible, point grid boundaries were delineated to avoidlarge inclusions and/or embayments. Point analyses of smaller inclu-sions and non-stoichiometric analyses were manually filtered fromthe data after collection. The analyses were then contoured by their

Table 1Sm and Nd isotopic compositions and ages of Townshend Dam garnet wafers and rocks with garnet Mn compositions⁎.

Grt Mn Compositions(M.F.)

Sample/Zone Mass⁎⁎

(mg)Sm(ppm)

Nd(ppm)

ng Ndloaded

147Sm/144Nd 143Nd/144Nd ±2 S.E.(abs)

Age(Ma)⁎⁎⁎

±2s MSWD Min Max Avg Mn

TD09-14b-Grt3 Zone 1 (core) 30.88 0.251 0.099 2.4 1.537 0.515496 0.000014 380.8 3.2 16 0.190 0.230 0.211TD09-14b-Grt3 Zone 2 (mantle) 72.50 0.341 0.113 6.3 1.832 0.516220 0.000010 379.7 2.5 16 0.070 0.170 0.115TD09-14b-Grt3 Zone 3 (rim) 51.53 0.622 0.124 4.9 3.028 0.5191240 0.0000088 376.1 1.4 16 0.001 0.040 0.012TD09-14a2-Grt7 Zone 1 (core) 10.30 0.150 0.056 0.44 1.619 0.515721 0.000036 382.8 4.7 16 0.160 0.210 0.186TD09-14a2-Grt7 Zone 2 (mantle) 30.10 0.283 0.073 1.7 2.342 0.517479 0.000021 379.1 2.4 16 0.050 0.150 0.112TD09-14a2-Grt7 Zone 3 (rim) 90.27 0.682 0.144 10 2.872 0.5187501 0.0000073 376.6 1.5 16 0.001 0.040 0.019TD09-14a-Grt3 Zone 1 (core) 81.04 0.040 0.042 2.6 0.5841 0.513156 0.000050 no age reported† 0.195 0.240 0.206TD09-14a-Grt3 Zone 2 (mantle) 201.10 0.114 0.101 16 0.6862 0.513238 0.000020 no age reported† 0.070 0.170 0.123TD09-14a-Grt3 Zone 3 (rim) 183.77 0.216 0.083 12 1.573 0.515437 0.000013 365.2 3.0 15 0.001 0.030 0.017TD09-14a-Grt4a Zone 1 (core) 89.09 0.039 0.026 1.9 0.9182 0.513955 0.000051 381.0 11 16 0.160 0.220 0.186TD09-14a-Grt4a Zone 2 (mantle) 187.60 0.343 0.151 24 1.372 0.515022 0.000011 373.1 3.4 16 0.070 0.140 0.104TD09-14a-Grt4a Zone 3 (rim) 260.79 0.646 0.158 35 2.467 0.5174453 0.0000078 356.8 1.7 15 0.001 0.040 0.018TD09-14a-Grt4b Zone 1 (core) 39.42 0.229 0.182 5.4 0.7635 0.513577 0.000012 383.1 6.8 16 0.200 0.232 0.212TD09-14a-Grt4b Zone 2 (mantle) 36.73 0.223 0.214 5.9 0.6316 0.513241 0.000019 no age reported† 0.150 0.180 0.165TD09-14a-Grt4b Zone 3 (mantle) 6.88 0.339 0.275 1.4 0.7465 0.513559 0.000017 no age reported† 0.110 0.130 0.12TD09-14a-Grt4b Zone 4 (mantle) 58.51 0.384 0.196 8.6 1.183 0.514594 0.000012 378.1 4.1 16 0.060 0.090 0.074TD09-14a-Grt4b Zone 5 (mantle) 33.10 0.621 0.337 8.4 1.114 0.5144275 0.0000088 378.7 4.2 16 0.030 0.040 0.035TD09-14a-Grt4b Zone 6 (rim) 64.25 0.751 0.222 11 2.048 0.516703 0.000012 375.5 2.3 16 0.001 0.010 0.005TD09-14a-Grt8 Zone 1 (core) 35.74 0.237 0.108 3.2 1.332 0.514988 0.000025 381.1 4.7 16 0.210 0.242 0.228TD09-14a-Grt8 Zone 2 (mantle) 37.38 0.220 0.122 3.8 1.087 0.514331 0.000025 374.2 5.8 16 0.120 0.170 0.185TD09-14a-Grt8 Zone 3 (mantle) 32.26 0.272 0.115 3.1 1.430 0.515169 0.000038 373.9 5.6 16 0.110 0.150 0.135TD09-14a-Grt8 Zone 4 (mantle) 23.35 0.410 0.151 2.7 1.645 0.515686 0.000033 372.9 4.3 16 0.070 0.090 0.08TD09-14a-Grt8 Zone 5 (mantle) 47.63 0.643 0.143 5.3 2.712 0.518358 0.000019 376.9 1.9 16 0.020 0.040 0.029TD09-14a-Grt8 Zone 6 (rim) 55.70 0.719 0.137 5.9 3.179 0.519470 0.000022 374.9 1.8 16 0.001 0.010 0.005TD09-14a-Grt9 Zone 1 (core) 170.05 0.210 0.214 27 0.5961 0.5131142 0.0000075 no age reported† 0.160 0.235 0.194TD09-14a-Grt9 Zone 2 (mantle) 134.75 0.263 0.162 16 0.9826 0.5141235 0.0000065 382.6 4.8 16 0.050 0.130 0.088TD09-14a-Grt9 Zone 3 (rim) 39.40 0.601 0.157 4.6 2.324 0.5174504 0.0000086 380.2 1.9 16 0.001 0.030 0.014TD09-14a-Grt10 Zone 1 (core) 169.19 0.209 0.097 14 1.303 0.5144731 0.0000065 324.5 3.3 14 0.190 0.252 0.218TD09-14a-Grt10 Zone 2 (mantle) 110.37 0.253 0.113 11 1.354 0.5149120 0.0000072 365.0 3.3 15 0.110 0.170 0.141TD09-14a-Grt10 Zone 3 (mantle) 70.48 0.445 0.138 8.2 1.948 0.5164853 0.0000067 377.9 2.2 16 0.040 0.070 0.054TD09-14a-Grt10 Zone 4 (rim) 108.40 0.747 0.149 14 3.041 0.5186539 0.0000068 350.0 1.4 15 0.001 0.020 0.009TD09-14a-Grt13 Zone 1 (core) 88.04 0.224 0.120 8.9 1.130 0.5144464 0.0000075 375.5 4.1 16 0.190 0.243 0.216TD09-14a-Grt13 Zone 2 (mantle) 81.27 0.291 0.148 10 1.191 0.5146123 0.0000082 377.9 3.9 16 0.110 0.160 0.135TD09-14a-Grt13 Zone 3 (mantle) 60.20 0.506 0.178 9.0 1.721 0.5159261 0.0000070 378.1 2.6 16 0.040 0.070 0.054TD09-14a-Grt13 Zone 4 (rim) 57.36 0.598 0.159 7.7 2.272 0.5172668 0.0000092 376.4 2.0 16 0.001 0.020 0.010TD09-14a-Grt16 Zone 1 (core) 47.26 0.159 0.086 3.1 1.113 0.5144338 0.0000081 380.2 4.2 16 0.190 0.220 0.204TD09-14a-Grt16 Zone 2 (mantle) 268.13 0.229 0.105 22 1.321 0.5148898 0.0000085 372.2 3.5 16 0.070 0.170 0.121TD09-14a-Grt16 Zone 3 (rim) 208.58 0.710 0.145 23 2.966 0.5188451 0.0000080 369.4 1.4 16 0.001 0.040 0.018TD09-14a 3-4 mm Grt 30.04 0.553 0.180 2.2 1.861 0.516264 0.000017 377.4 3.2 19 0.005 0.051 0.028TD09-14a 1 mm Grt 17.60 0.651 0.143 3.0 2.762 0.518467 0.000019 376.0 1.9 16 0.004 0.041 0.0225

Near-garnet matrix samplesTD09-14b-Grt3 Mtx 2.06 6.05 30.4 44 0.1204 0.5119498 0.0000050TD09-14a2-Grt7 Mtx 2.20 6.67 33.8 52 0.1192 0.5119627 0.0000049TD09-14a-Grt3 Mtx 2.15 6.66 36.1 54 0.1115 0.5119649 0.0000063TD09-14a-Grt4a Mtx# 2.04 8.51 44.5 70 0.1157 0.5119859 0.0000076TD09-14a-Grt4b Mtx 1.18 6.92 35.9 36 0.1166 0.5119605 0.0000061TD09-14a-Grt8 Mtx 3.02 4.13 21.1 54 0.1181 0.5119409 0.0000070TD09-14a-Grt9 Mtx 1.66 9.25 49.4 62 0.1133 0.5119501 0.0000045TD09-14a-Grt10 Mtx 3.17 3.62 19.4 52 0.1131 0.5119530 0.0000051TD09-14a-Grt13 Mtx 3.14 7.27 37.0 98 0.1188 0.5119445 0.0000050TD09-14a-Grt16 Mtx 2.03 3.30 16.8 24 0.1188 0.5119504 0.0000050TD09-14a 3-4 mm Grt Mtx 1.60 6.93 34.6 39 0.1211 0.5119599 0.0000080TD09-14a 1 mm Grt Mtx 1.60 5.97 31.4 35 0.1151 0.5119690 0.0000095

Whole rock samplesWhole rock — red 1 1.48 6.45 34.9 26 0.1120 0.5119535 0.0000091Whole rock — red 2 1.44 7.36 38.7 28 0.1150 0.5119509 0.0000087Whole rock — blue 1.51 9.16 48.6 37 0.1140 0.5119460 0.0000068Whole rock — violet 2.06 6.09 30.8 32 0.1200 0.5119450 0.0000051Whole rock — green 1.53 6.45 33.4 22 0.1170 0.5119552 0.000011

Matrix traverse samplesMatrix traverse 1# 2.09 2.32 11.6 12 0.1209 0.511971 0.000011Matrix traverse 2# 2.12 8.94 44.9 48 0.1204 0.511934 0.000014Matrix traverse 3# 2.09 2.08 10.6 11 0.1189 0.5119621 0.0000081Matrix traverse 4# 2.15 3.36 16.8 18 0.1207 0.511957 0.000011Matrix traverse 5# 2.13 6.53 32.6 58 0.1210 0.5119400 0.0000070Matrix traverse 6# 1.55 4.98 24.8 29 0.1214 0.5119644 0.0000052Matrix traverse 7# 1.94 7.47 38.2 37 0.1184 0.5119554 0.0000081Matrix traverse 8# 1.99 5.97 29.2 29 0.1236 0.5119562 0.0000076


Table 1 (continued)

Grt Mn Compositions(M.F.)

Sample/Zone Mass⁎⁎

(mg)Sm(ppm)

Nd(ppm)

ng Ndloaded

147Sm/144Nd 143Nd/144Nd ±2 S.E.(abs)

Age(Ma)⁎⁎⁎

±2s MSWD Min Max Avg Mn

Matrix traverse 9# 2.10 1.59 7.19 13 0.1335 0.511973 0.000010Matrix traverse 10# 1.52 2.60 11.8 15 0.1337 0.511931 0.000018Matrix traverse 11# 1.47 7.14 37.4 46 0.1155 0.5119528 0.0000034Unleached garnet powders (b63μm)†

TD09-14b Grt3 Zone 1 unleached powder 9.60 8.70 43.2 190 0.1217 0.5119492 0.0000034TD09-14b Grt3 Zone 2 unleached powder 9.73 9.24 45.3 230 0.1234 0.5119689 0.0000029TD09-14b Grt3 Zone 3 unleached powder 11.6 6.28 28.9 26 0.1313 0.5120085 0.0000029TD09-14a2 Grt7 Zone 1 unleached powder 9.93 6.09 30.6 24 0.1205 0.5119568 0.0000030TD09-14a2 Grt7 Zone 2 unleached powder 9.77 1.25 35.4 27 0.0214 0.5119639 0.0000026TD09-14a2 Grt7 Zone 3 unleached powder 9.56 7.91 36.9 27 0.1295 0.5119783 0.0000038TD09-14a Grt3 Zone 1 unleached powder 2.17 2.40 11.5 17 0.1265 0.5120043 0.0000070TD09-14a Grt3 Zone 2 unleached powder 2.15 3.20 15.5 24 0.1249 0.5119944 0.0000046TD09-14a Grt3 Zone 3 unleached powder 2.07 3.32 7.00 10 0.2867 0.5120962 0.0000077TD09-14a Grt4a Zone 3 unleached powder 2.13 2.25 8.56 14 0.1588 0.5120678 0.0000060TD09-14a Grt4b Zone 1 unleached powder 5.32 2.27 10.8 43 0.1272 0.5119873 0.0000055TD09-14a Grt4b Zone 2 unleached powder 5.39 2.07 9.89 40 0.1269 0.5120035 0.0000056TD09-14a Grt4b Zone 3 unleached powder 5.26 2.66 12.8 51 0.1251 0.5119742 0.0000067TD09-14a Grt4b Zone 4 unleached powder 5.24 2.68 12.5 49 0.1299 0.5119901 0.0000077TD09-14a Grt4b Zone 5 unleached powder 5.11 2.22 8.55 33 0.1569 0.5120608 0.0000057TD09-14a Grt4b Zone 6 unleached powder 5.13 1.74 5.90 23 0.1784 0.5121089 0.0000095TD09-14a Grt8 Zone 1 unleached powder 2.01 1.93 9.06 15 0.1286 0.511980 0.000011TD09-14a Grt9 Zone 1 unleached powder 4.15 3.99 20.2 63 0.1198 0.5119700 0.0000045TD09-14a Grt9 Zone 2 unleached powder 4.16 4.61 23.2 72 0.1205 0.5119700 0.0000059TD09-14a Grt9 Zone 3 unleached powder 4.05 2.54 9.77 30 0.1575 0.5120576 0.0000060TD09-14a Grt10 Zone 1 unleached powder 2.14 2.82 13.9 21 0.1231 0.5119842 0.0000085TD09-14a Grt10 Zone 2 unleached powder 1.01 2.71 13.2 9.3 0.1239 0.511969 0.000010TD09-14a Grt10 Zone 3 unleached powder 1.00 3.97 18.8 13 0.1278 0.5119902 0.0000077TD09-14a Grt10 Zone 4 unleached powder 1.00 2.60 10.3 7.2 0.1528 0.5120517 0.0000078TD09-14a Grt13 Zone 1 unleached powder 2.13 2.68 13.0 19 0.1250 0.511993 0.000010TD09-14a Grt16 Zone 1 unleached powder 1.98 7.37 36.3 50 0.1230 0.5119656 0.0000036TD09-14a Grt16 Zone 2 unleached powder 3.07 10.0 50.0 110 0.1213 0.5119735 0.0000038TD09-14a Grt16 Zone 3 unleached powder 3.07 10.6 51.5 110 0.1251 0.5119790 0.0000036

⁎Data table modified from Dragovic (2013).⁎⁎Post-partial dissolution mass reported for leached garnet samples.⁎⁎⁎Isochron ages for individual garnet crystal segments calculated by pairing with 16 matrix/whole-rock analyses.#Isotopic composition excluded from all age calculations (outlier data point).†unreliable age, rejected based on Sm/Nd b 0.75.147Sm/144Nd external precision is 0.023% based on repeat analyses of a mixed normal solution.


Mn content (in mole fraction) and digitally overlain on optical and/orBSE images of the grain to create a Mn chemical contour map.

Uncertainties of the EPMA analyses of garnet were evaluated bycomparison of a central linescan (rim to rim) extracted from the maproutine with an identical one collected using a full-quantitative routineand by obtaining the uncertainties for each elemental analysis fromProbe for Windows. Near-identical values for major elements collectedvia full-quantitative and quick map routines suggest that the quick maproutine adequately quantifies the garnet compositions (Supplementaldataset A). Because themagnitudes of EPMA analytical errors are strong-ly correlatedwith the concentration of the analyte element in theminer-al, a curvewas calibrated that associates the analytical errorwithmineralcomposition (Supplemental dataset B). This calibrationwas then used toassign a unique analytical uncertainty to eachMn compositional contour.

High-resolution X-ray intensity maps were obtained to identify ac-cessory phases in and adjacent to Townshend Dam garnets that couldbe REE sinks (e.g. apatite, xenotime). The Ca, Mg, Ce, Y, and Zr X-rayintensity maps were collected using the five-spectrometer CamecaSX50 electron microprobe at the University of Massachusetts, Amherst.X-ray intensities were collected for 25ms at each analysis point over anentire thin section using a point spacing of 30–50 μm. Run conditionswere 300 nA beam current and a 15 keV accelerating voltage. For acces-sory phase geochronology, the U and Pb content of small xenotime crys-tals was determined on the Cameca SX Ultrachron also at the Universityof Massachusetts, Amherst. Operating conditions were 15 keV, 200 nAbeam current, and a beam diameter of b1 μm. Pb Mβ and U Mβ X-rayswere collected by LPET. U values were corrected for interference by

Th, K, and Gd, and Pb values were corrected for interference by Ce.Compositional maps of Pb, U, Y, and Yb in selected xenotime crystalswere obtained as WDS grid maps on the University of Alabama EPMA.The 4 micron spacing grid was obtained at 15 kV with a 1 micron beamsize and 20 nA beam current.

Trace-element and REE compositions of Townshend Dam garnetswere measured with the CETAC LSX-500 laser ablation system attachedto a Perkin-Elmer ELAN6000 quadrupole mass spectrometer (LA-ICPMS) at the U.S. Geological Survey, Lakewood, CO. Spot analyses wereperformedwith the following operating conditions: 266 nmwavelength;9 mJ energy; 150 μm spot size; and a 10 Hz laser pulse rate. The compo-sition of the carrier gas used was 1.05 l/min Ar. The USGS standard GSE-1 g was used as the calibration standard and Ca, as determined viaEPMA, was used as an internal standard. Measured raw intensitieswere converted to element concentrations using the GeoPro offlinedata processing software. Chemical maps were constructed by firstmanually removing data from laser cycles displaying elemental concen-trations indicating inclusions and contouring a grid of ca. 150 μm pitswith a 320 μm spacing.

5.3. Extraction of compositionally-specific garnet segments

Mn concentration maps were used to guide microsampling ofcompositionally-specific garnet crystal segments using a New WaveMicroMill at the Boston University TIMS facility. Similar techniqueshave been developed to microsample garnet for other purposes (e.g.Ducea et al., 2003; Pollington and Baxter, 2011; Dragovic et al., 2012).

Grt3(14b)

Grt7(14a2)

Grt16

Violet Plane

Green Plane

Blue Plane

Red Plane

Grt13

Grt8Grt9

Grt3

Grt4

Grt10

Foliation Trace

Foliatio

n Trace

1 cm

A B

C D

Fig. 1. Images of sample TD09-14 from TownshendDam, VT. A. Sample photo prior to cutting and garnet extraction. B. Thin section showing helicitic structures in garnet, their discordancewith matrix foliation, and pressure shadows around garnet porphyroblasts. C. Perspective view of HRXCT data with partially transparent matrix showing the orientation of the dominantfabric in the sample. X, Y, and Z axis units aremillimeters. Bright spots are garnet near the periphery of the sample. D. Sketch of the image in (C)with the same scale, showing the locationsand sizes of garnet used for major element and isotopic analyses.


The ca. 2 mm-thick garnet wafers were glued onto a graphite block.Using the Mn maps as a guide, microdrill trenches were delineated toclosely follow Mn compositional contours (isopleths) and cut concen-tric rings of the garnet wafers (Fig. 2c). Garnet was drilled in waterwith a tapered diamond-encrusted drill bit producing trenches rangingfrom ca. 800 μm (top) to ca. 500 μm (bottom) wide (e.g. Pollington andBaxter, 2011). The number of crystal segments extracted from eachgarnet wafer depends on the garnet size and the overall Mn content ofthe segment. The outermost drill trench approximately correspondedto the grain boundary. The rock matrix adjacent to the microsampledgarnet wafer was collected and processed for analysis along with theassociated garnet. Sample weights of individual garnet and matrix frac-tions processed for Sm–Nd analyses are listed in Table 1. Because eachgarnet segment contains a range ofMn compositions, a linearweightingbetween isopleths was used to calculate an averageMn composition foreach segment. The uncertainties of the Mn isopleths within each zonewere combined in a similar manner to calculate the overall uncertaintyof the Mn composition of the segment.

5.4. Garnet purification and Sm–Nd geochronology

All garnet purification and sample preparation for Sm–Nd geochro-nology was conducted at the Boston University TIMS facility (Dragovic,2013). After microsampling, garnet annuli were removed from thegraphite block and ultrasonicated in acetone to remove any remainingCrystalbond. Each garnet segment was then crushed in a tungsten car-bide percussion mortar and sieved to a size of 63–106 μm to exposesmall inclusion phases. The fine powders that passed through the sieve(b63 μm) were also collected, dissolved (see below), and analyzed,

without further purification techniques. Crushed, 63–106 μm, garnetwas processed through a Frantz magnetic separator, cleaned via partialdissolution in 7NHNO3 for 3 h at 120 °C, and handpicked under amicro-scope to remove any remaining optically-impure garnet.

Optically pure garnet (N63 μm)was then cleaned by a partial disso-lution procedure modified from Anczkiewicz and Thirlwall (2003) andPollington and Baxter (2011). The purpose of this partial dissolution isto remove micro-mineral inclusions that could otherwise contaminatethe garnet and lead to imprecise and inaccurate ages. This issue hasbeen discussed and addressed by numerous papers (e.g. Zhou andHensen, 1995; Amato et al., 1999; Prince et al., 2000; Scherer et al.,2000; Luais et al., 2001; Thöni, 2003; Martin et al., 2010; Baxter andScherer, 2013; and references therein). Several partial dissolution ex-periments were initially performed on bulk garnet separates to deter-mine the best procedure (choice of grain size, acids, temperatures, anddurations) for purifying these garnets (the results of these experimentsare presented in Supplemental dataset C). The partial dissolutionprocedure that optimally purifies TownshendDamgarnet (as evidencedby high 147Sm/144Nd and low Nd concentrations; e.g. Baxter andScherer, 2013) uses a combination of HF, HClO4, and HNO3 acids to re-move silicate and oxide inclusions and H2SO4 to remove phosphates.The 63–106 μm garnet fractions were partially dissolved in sealed Tef-lon vials in 1:2 concentrated HF:1.5 N HCl at 120 °C for 60 min, thenin 2:1 concentrated HClO4:1.5 N HCl at 150 °C overnight, then in 7 NHNO3 at 120 °C for 180 min, and lastly in concentrated H2SO4 at120 °C for 180 min. Each acid step involved a series of ultrasonic treat-ments. Samples were washed in 1.5 N HCl and nanopure H2O andultrasonicated between acid treatments to remove any remainingleachate residue.

2 cm

10 cmA

B

C

Fig. 2. Sample TD09-14 garnet sampling. A. Post-sample cutting and overcoring photoof the sawed green surface from Fig. 1c and d, with drill cores still in place. Numberscorrespond to targeted garnet grains. B. Photo of central wafer of garnet 3. C. Photo ofmicro-drilled garnet wafer, cut into concentric rings to followMn isopleths. (For interpreta-tion of the references to color in thisfigure legend, the reader is referred to thewebversion ofthis article.)


Full dissolution of purified garnet, unpurified garnet powders,whole-rock, and matrix samples was accomplished with high-purityconcentrated HF, concentrated HNO3, and 1.5 N HCl. Samples wereplaced in sealed Teflon vials containing concentrated HF and HNO3 andkept at ca. 150 °C until no visible solids remained. The solution was sub-sequently dried down and redissolved several times using high-purityHNO3 and 1.5 N HCl to prevent formation of secondary fluorides. Thedissolved samples were equilibrated with a mixed 147Sm−150Nd spikeprior to chromatographic separation of Sm and Nd for isotopic analysis.To extract high-purity Sm and Nd splits from the dissolved samples, athree-part column procedure was used (Harvey and Baxter, 2009).First, the dissolved samples were passed through a cation exchangeresin to remove Fe,which has been shown to overwhelm the subsequentTRU-spec column. The REEs were then separated using a TRU-spec col-umn. Finally, Sm and Nd were isolated using a 2-methyl lactic acid col-umn. Procedural blanks, processed in parallel with the samples, yielded0.8 to 2.6 pg of Sm and 4.3 to 22.9 pg of Nd.

Sm and Nd isotopic analyses were performed using the Thermo-Finnigan TRITON thermal ionization mass spectrometer at the BostonUniversity TIMS facility, following the methodology of Harvey andBaxter (2009). Neodymium was loaded in nitric acid onto single rhe-nium filaments with 2 μl of H3PO4 and Ta2O5 activator slurry and wascollected as NdO+ in static modewith amplifier rotation. Samariumwas

loaded in nitric acid onto rheniumdoublefilaments. Over theduration ofdata collection for this study, repeat analyses of 4 ng loads of an in-houseNd standard solution (Ames metal) produced a mean 143Nd/144Nd of0.5121311 ± 0.0000077 (15.0 ppm, 2 RSD, n = 67). External precision(2σ) on 147Sm/144Nd is 0.023%, based on repeat solution analyses(n = 11; including column chemistry and TIMS) of an in houseSm–Nd gravimetric mixed standard. Analytical uncertainties propa-gated through age calculations use the poorer (higher) of the internal2 S.E. (reported in Table 1) or the external 2 S.D. of the standard (i.e.0.000077). No blank corrections to 143Nd/144Nd or 147Sm/144Nd areattempted as correlated blank effects on calculated isochron ages arenegligible as compared to the propagated analytical uncertainties. AllSm–Nd isochron ages were calculated with the linear regression algo-rithm of York (1969) using the program ISOPLOT v. 3 (Ludwig, 2008).

6. Results

6.1. Garnet major element compositions

Ten garnets analyzed from the candidate sample volume areremarkably similar in their major-element compositions and zoning pro-files (Fig. 3). Garnet are dominantly almandine (XFe = 0.52–0.78), withsubordinate amounts of pyrope (XMg = 0.03–0.12), spessartine(XMn=0.00–0.25), and grossular (XCa=0.12–0.21).Major-element zon-ing profiles of garnet show radially increasing Fe and Mg and decreasingMn, from core to rim. Ca is low in the garnet cores, then increases slightlybefore decreasing at the rims. All garnets display bell-shaped Mn zoningprofiles and remarkably similar overall major-element zoning profiles(Fig. 3). Increases in Fe and Mg compensate for decreases in Mn and Cain all garnets (Fig. 3). Furthermore, the magnitude of the zoning for allmajor elements is similar in all of the large garnets because they havesimilar core and rim compositions, although the smaller garnets havelower core Mn and higher core Fe and Mg than do the larger garnets(Table 1). Slight differences in the garnet core compositions may reflectslices which missed the compositional center of some grains. Some gar-nets have narrow (b80 μm) outer rims in which the Mn content slightlyincreases, the Fe content increases, andMg slightly decreases (Fig. 4). Thesignificance of the garnet outer rim compositions is discussed below.

Concentric zoning of Mn around the cores of the garnet is bestillustrated by the quantitative compositionalmaps contoured forMn con-tent shown in Fig. 5. The smooth, concentric Mn zoning is only disturbedaround inclusion phases, which were mostly filtered out of the dataset.Mn isopleths are generally parallel to garnet crystal faces except for anal-yses collected adjacent to inclusions. The maps show that the composi-tional cores may not correspond precisely to the geometric centers ofthe wafers. Broadly spaced Mn isopleths around the chemical cores giveway to closely-spaced isopleths near the rims. Mn isopleths deviateslightly and truncate against embayments into some of the grains. Thequantitative Mn compositional maps were used to delineate the Mn-specific zones that were microsampled for Sm–Nd dating (Fig. 6).

6.2. Garnet trace element compositions

The central wafer of TD09-14b-Grt3 was analyzed for 43 major,minor, and trace elements, including 14 REEs (Supplemental dataset D).Quantitative compositionalmaps of selected trace elements in this garnetare shown in Fig. 7. Trace elements and REE are zoned in TownshendDam garnet and HREE are enriched relative to LREE (Fig. 8). However,the garnet rims are enriched in the MREE relative to the HREE, asshown by the change in slope for the garnet rims in Fig. 8. All traceelement and REE concentrations increase abruptly at the outermostrim, within 100 μm of the grain boundary (Fig. 7).

Nd is weakly-zoned in pure garnet, slightly increasing from core torim and bulls-eye patterns on the maps are interpreted to reflect thestrong influence of incompletely filtered LREE-rich inclusion phases(Fig. 7). Nd concentrations show an irregular pattern with local highs

0.10

0.15

0.20

0.25

15000100005000050001000015000

Mo

le F

ract

ion

Gro

ssu

lar

Distance across Garnet Wafer (μm)

Ca

0.05

0.10

0.15

0.20

0.25

0.30

Mo

le F

ract

ion

Sp

essa

rtin

e

Mn

0.05

0.10

0.15

Mo

le F

ract

ion

Pyr

op

e

Mg

0.55

0.60

0.65

0.70

0.75

0.80

14a-Grt314a-Grt4a14a-Grt4b14a-Grt814a-Grt914a-Grt1014a-Grt1314a-Grt1614b-Grt314a2-Grt7

Mo

le F

ract

ion

Alm

and

ine

Fe

Fig. 3. Major element zoning profiles for Townshend Dam garnet, TD09-14. Distancesacross garnet wafers are centered on peak Mn concentrations. Note the similarities inthe character and magnitude of the zoning in all ten grains. Locations of linescans areshown in Fig. 5.

0.00

0.01

0.06

0.08

0.10

0.12

0.14

0.75

0.77

0.79

0.81

1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

Distance from Garnet Rim (mm)

Sps

Decreasing Sps

Growth Zoning

PerturbedRim ZoningOuter Rim

Prp

Grs

Alm

Mo

le F

ract

ion

Alm

, Prp

, Sp

s, G

rs

Location ofLinescan

5 mm

Chl

A

B

Fig. 4. Garnet compositions from the outermost rim of Grt4b showing a narrow zone(b80 μm) of reversed zoning in spessartine and pyrope compared to zoning shownin Fig. 3. A. Back scattered electron image showing Grt and location of analyses. B. Molefractions of Almandine (Alm), Grossular (Grs), Pyrope (Prp), and Spessartine (Sps) alongthe line shown in A. This reversal only occurs at the outermost rims typically where thegarnet is in contact with chlorite (Chl).


(bulls-eyes) N1 ppm and a background low value b0.2 ppm that ex-tends from core to rim. Local high-Nd spot analyses do not necessarilycorrelate with fluctuations in the Sm concentrations (Fig. 7). Thus, Ndanalyses N0.3 ppm are interpreted to be inclusion-contaminated analy-ses, with purest garnet having Nd concentrations b0.2 ppm. Sm and Euconcentrations range from ca. 0.2–0.4 ppm in the cores to ca. 0.5 ppmat the rim (Fig. 7). Gd and Dy patterns form low-MREE concentrationannuli around the garnet cores and gently increase toward the rims(Fig. 7). Lu and Y show distinct concentric zoning, with the concen-trations of these elements decreasing from core to rim (Figs. 7 and 8).Sparse bull's-eye patterns suggest that a few Y-rich inclusions arepresent (Fig. 7).

6.2.1. Sm–Nd isotope dataThirty-eight Mn-specific concentric zones from ten large individual

garnet crystalswere analyzed for their SmandNd isotopic compositions(Fig. 6; Table 1). Two small (1–4 mm) garnets, processed as “bulk”garnet splits (i.e. not microsampled), were also analyzed for their Smand Nd isotopic compositions (Table 1). These garnet crystal segmentshave Nd concentrations ranging from 0.026 to 0.337 ppm (average corevalue 0.097 ppm; average rim value 0.158 ppm), Sm concentrationsranging from0.039 to 0.751ppm(average core value 0.184ppm; averagerim value 0.647 ppm), and 147Sm/144Nd from 0.584 to 3.179 (averagecore value 1.202; average rim value 2.543). These are in good agreementwith observed Nd and Sm concentrations from LA-ICPMS analyses(Fig. 7), confirming that our cleansing procedures have generallysucceeded in providing pure garnet for isotopic analysis. Sm and Nd con-centration as well as 147Sm/144Nd generally increase from core to rimin cleansed garnets analyzed via TIMS, also consistentwith LA-ICPMS ob-servations. In general, high Sm/Nd (N1.0) and low Nd concentration

14a-Grt3 14a-Grt914b-Grt3

14a2-Grt714a-Grt10

14a-Grt16

14a-Grt13

14a-Grt4a

14a-Grt4b

14a-Grt81 cm

.24

.20

.18

.16

.14

.12

.10

.08

.06

.04

.02

.00

.22

Mo

le F

ract

ion

Mn

Fig. 5. Shaded contour map of Mn contents in garnets from sample TD09-14. Contours are 0.01 mole fraction Sps. Note the similarity in the magnitude of Sps zoning in all garnets. Whitelines show the locations of linescans in Fig. 3.


(b0.5 ppm) are good indications of sufficiently pure inclusion free garnetto eliminate the significant effects of inclusion contamination on age (e.g.Baxter and Scherer, 2013). Here, isotopic analyses of garnet crystal seg-ments yielding higher Nd (Nd concentrations N0.2 ppm) and/or lower147Sm/144Nd (b0.75) likely reflect incomplete removal/cleansing ofinclusions. Samples with the lowest 147Sm/144Nd are most likely tohave their apparent ages affected by inclusions, so the five garnet crystalsegmentswith 147Sm/144Nd b0.75were not used for age calculations.Wereport thirty-five garnet Sm–Nd ages (33 garnet crystal segment ages; 2small bulk garnet ages) for our candidate sample in Table 1.

Low Nd concentrations (b0.2 ppm) and high 147Sm/144Nd ratios(≥1.0) obtained for most garnet crystal segments suggest the uti-lized partial dissolution procedure was adequate for removing anysignificant effects of inclusions from most garnet segments. The ef-fect of the partial dissolution procedure for removing REE-bearinginclusions is shown by comparison of the non-purified garnet pow-ders for several of the garnets to the cleaned garnet splits (Supple-mental dataset E). Compared to the garnet splits that were cleanedby our partial dissolution procedure, the uncleaned garnet powdershave much higher Nd concentrations (5.903 to 51.490 ppm) andlower 147Sm/144Nd (0.021 to 0.287), suggesting that the partial dis-solution procedure made a significant difference and succeeded inremoving significant age effects of problematic REE-bearing inclu-sions (Table 1; Supplemental dataset E).

6.2.2. Age effects of local matrix heterogeneity in Sm–NdInitially, age calculations were made for individual garnet crystal

segments paired with the isotopic composition of the rock matrixdirectly adjacent to each of the 10 garnet crystals. This requires the as-sumption that the adjacentmatrix composition is an accurate represen-tation of the matrix composition (at present day) from which thatgarnet grew. If the matrix is homogeneous over lengthscales largerthan the size of a garnet crystal, this is probably a good assumption.The lengthscale of Sm–Nd heterogeneity within the rock matrix wasevaluated using a series of matrix samples across the foliation at 1 cmspacing (n= 11), and several large whole-rock slab samples averagingover the entire rock width (n = 5; Table 1). These analyses in additionto the near-garnet matrix samples show that there is appreciablevariation in the matrix and whole-rock 143Nd/144Nd and (especially)147Sm/144Nd values, which range from 0.511931 ± 0.000018 to0.5119859 ± 0.0000076 and 0.111505 ± 0.000026 to 0.133713 ±0.000031, respectively. Significant matrix heterogeneity at the cm- (orsmaller) scale (i.e. smaller than the size of individual garnet crystals)makes it impossible to identify the exact matrix composition that eachgarnet segment grew in equilibrium with.

Isotopic heterogeneity of the matrix and whole-rock across thesample precludes identification of equilibrium rock compositions withwhich the garnet crystal segments grew. Whereas it is possible to pairisotope ratios of individual garnet segments with individual matrix

.24

.20

.18

.16

.14

.12

.10

.08

.06

.04

.02

.00

.22

Mo

le F

ract

ion

Mn

14a-Grt3 14a-Grt914b-Grt3

14a2-Grt714a-Grt10

14a-Grt16

14a-Grt13

14a-Grt4a

14a-Grt4b

14a-Grt81 cm

Fig. 6.Compositional-specificmicrosampled volumes of garnet used for Sm–Nd isotopic analyses. Contourmaps depictmole fraction Sps and are identical to those shown in Fig. 5. Red lineoverlays showareas of discarded garnet from800-micronwidemicromill trenches. AverageMn compositions for each zonewere calculated based on the surface area occupied by eachMncontour within each zone (see text for discussion). Unshaded white areas show inclusion rich areas or embayments not used for Sm–Nd analyses. (For interpretation of the references tocolor in this figure legend, the reader is referred to the web version of this article.)


and/or whole-rock isotopic compositions to construct two-point iso-chrons, cm-scale heterogeneity in the matrix and whole-rock isotopiccompositions suggest that garnet segments may have equilibrated withlocally distinct matrix isotopic compositions. Furthermore, lengthscalesfor garnet-rock isotopic equilibrium likely changed as individual garnetsgrew and incorporated atoms from larger rock volumes. Thus, assign-ment of a single matrix or whole-rock analysis (for example, to createa precise two-point isochron age) is unjustified in this case, and wouldresult in inaccuracies of N1 m.y. (Fig. 9). Small-scale matrix heterogene-ity encounteredwithin Townshend Dampelitemay be common in rockswith layered clastic sedimentary protoliths. In contrast, meta-igneous orother protoliths which are more homogeneous at the sample scale likelywill not have heterogeneities of this magnitude (e.g., Pollington andBaxter, 2010; Dragovic et al., 2012). Isotopic compositions of the matrixtraverse samples were excluded from age calculations because theyhave uniformly higher 147Sm/144Nd than the near-garnet matrix andwhole-rocks and are devoid of garnet (Table 1). The isotopic compositionof each garnet crystal segment is paired with 11 near-garnet matrixand 5 whole-rocks to calculate seventeen-point isochrons for each gar-net zone sampled. These ages have lower precision (compared to twopoint isochrons) and largerMSWD's (14–19; Table 1), which accuratelyreflects the real uncertainty in the equilibrated matrix composition

at the time of garnet growth. Though less precise, these are the mostaccurate individual garnet segment age interpretations we can reportgiven the degree of heterogeneity of the rock matrix.

Seventeen-point isochrons calculated for 35 individual garnet crys-tal segments (8 cores; 15 mantle zones; 10 rims; 2 small bulk garnets)range from 383.1 ± 6.8 Ma (MSWD = 16; TD09-14a-Grt4b-Zone 1)to 324.5 ± 3.3 Ma (MSWD = 14; TD09-14a-Grt10-Zone 1). Althoughthe core and mantle ages of 324.5 ± 3.3 Ma (TD09-14a-Grt10-Zone 1)and 365.0 ± 3.3 Ma ages (TD09-14a-Grt10-Zone 2) cannot be rejectedon the basis of their Nd concentration or 147Sm/144Nd, they are obviousoutlier ages (when compared to other core and mantle zones) and are,in fact, younger than the outer zones collected from this crystal (i.e.reverse-age zoning). Excluding all rejected a-priori ages and outliercore and mantle ages for TD09-14a-Grt10, isochron ages calculatedfor individual garnet crystal segments range from 383.1 ± 6.8 Ma to350.0 ± 1.4 Ma (Table 1). Two-sigma uncertainties of calculated iso-chron ages range from ±11 to ±1.4 m.y., and are related to the sizeof the Nd load (after microsampling and cleansing), the magnitude ofthe 147Sm/144Nd separation between garnet andmatrix, and thematrixheterogeneity as discussed above (Table 1).

Excluding five a-priori rejected data (2 cores and 3 mantles) and afew outlier ages (1 core, 1 mantle, 4 rims), garnet crystal segments

1 cm 1 cm

1 cm 1 cm

1 cm 1 cm

1 cm 1 cm

Fig. 7. Shaded color contourmaps of trace element concentrations in TD09-14b-Grt3.Mapswere constructed byfirstmanuallyfiltering out obvious inclusion spots and then contouring thegrid of 504 ca. 150 μm-diameter pits with a 320 μm spacing. Note the bullseye patterns on the unfiltered Ndmap indicative of inclusion-contaminated analyses. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version of this article.)


with similar Mn contents yielded similar Sm–Nd ages. However, the un-certainties of the Sm–Nd ages are insufficient to distinguish betweensubtle age differences in many adjacent zones from within individualcrystals (Table 1). Grouping like zones from different garnet crystals(i.e. cores, mantles, rims) into multi-garnet isochrons may yield morerobust estimates for the timing of garnet core, mantle, and rim growth

for large (1–3 cm) garnets in the rock volume (Fig. 10; Table 1). Excludinga single outlier (TD09-14a-Gt10-zone 1), seven garnet cores containingXMn ranging from 0.252 (maximum inner core isopleth) to 0.160(minimum outer core isopleth) define an isochron of 380.3 ± 2.0 Ma(n=23,MSWD=14; Fig. 10a). This is interpreted as the best constraintof rock-wide garnet core growth for the larger garnets in the rock

0.0

0.1

1.0

10.0

100.0

1000.0

10000.0

Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

Core1

Core2

Mid1

Mid2

Mid3

Mid4

Rim1

Rim2

Sam

ple

/ C

ho

nd

rite

TD09-14b-Grt3

Fig. 8. Chondrite-normalized REE concentrations of Townshend Dam garnet, TD09-14b-Grt3. Each line represents a spot analysis of inclusion-free garnet from point analysesalong a linescan beginning in the core (Grt Core 1) and progressing to the outer rim (GrtRim 2). Note the depletion of HREE at the garnet rims relative to the inner zones, likelyreflecting post-growth resorption/recrystallization accompanied by growth of xenotimeor another HREE-sequestering phase. Chondrite REE concentrations from Anders andGrevesse (1989).


volume. Similar grouping of fourteen interior garnet zones (mantles)yields a 377.3 ± 1.4 Ma isochron (n = 30, MSWD = 18; 3 rejected; 1outlier excluded; Fig. 10b). These garnet mantle zones together encom-pass the largest range of Mn compositions (highest inner isoplethXMn = 0.170; lowest outer isopleth XMn = 0.020). Therefore, groupingthesemantle zones provides a constraint on the timing of garnetmantlegrowth for the larger (1–3 cm) garnet in the rock volume.

Garnet rims are combined with the two small-garnet analyses (allsharing lowMn) in a third multi-garnet isochron. Microsampled garnetrim zones range from 0.051 (highest inner isopleth) to 0.001 (lowestouter rim isopleth) in XMn and from 380.2 ± 1.9 Ma to 350.0 ±1.4 Ma in age (Table 1). In contrast to the core and mantle data, severalgarnet rims give significantly younger ages, none of which can berejected a priori on the basis of their Nd concentrations or 147Sm/144Ndratios. Excluding the isotopic ratios of the apparently younger rims forgarnets 3, 4, 10, and 16, yields a 24-point (6 garnet rims; 2 small garnet;16 whole-rock/matrix) isochron age of 376.5 ± 1.0 Ma (MSWD = 18;

0.511

0.512

0.513

0.514

0.515

0.516

0.00 0.50 1.00 1.50

14b-Grt3-Zone2Sm/Nd=1.83

Inset (Fig. 9b)

WR/MtxG3b-Z2 with All WR & Mtx

G3b-Z2 with Grt3mtx

G3b-Z2 with violet WR

143 N

d/1

44N

d

147Sm/144Nd

A

Fig. 9. The effects of Sm and Nd isotope heterogeneity on isochron ages. A. Example of threesegment were chosen because it has similar 147Sm/144Nd to the average of all the garnet se(A) for TD0914b-Grt3-Zone2, illustrating the range of possible ages for 2-point isochrons from 3three outliers; n = 17) and 14b-Grt3-Z2 is the age (379.7 ± 2.5 Ma) reported in Table 1. Agesvariation than ages calculated for garnet segments with lower 147Sm/144Nd.

Fig. 10c). The younger rims of garnets 3, 4, 10, and 16 do not fall on asingle isochron and their significance is discussed below.

6.3. Correlation of garnet Mn content and age

Comparison of the XMn ranges and isochron ages calculated forindividual garnet crystal segments show an overall trend of decreasingMn content with decreasing age, but scatter in the dataset precludes arobust quantification of a Mn-age relationship (Fig. 11a). In fact, someof these data show that certain ranges of Mn compositions (particularlythose sampled by the interior crystal segments) yield statistically dis-tinct Sm–Nd isotopic ages, compatible with a lack of perfect rock-wideequilibrium between Mn, Sm, and Nd (Fig. 11a). However, groupedisotopic ages for cores, mantles, and rims, do suggest a first order corre-lation between Mn content and garnet age rock wide (Fig. 11b).

6.4. Xenotime characterization and growth ages

Several xenotime crystals adjacent to Grt 8 were identified by map-ping X-ray intensities of Ca, Mg, Ce, Y, and Zr and dated using U and Pbcompositional data from the Cameca Ultrachron in order to betterunderstand the wide range of garnet Sm–Nd rim ages. Small (~50 μm),anhedral, complexly-zoned xenotime crystals occur as inclusions inthe outer rim of Grt 8 and are concentrated along the garnet-matrixgrain boundary (Fig. 12a). Preliminary dates for 11 xenotime grainsrange from ca. 425 to 320 Ma (Fig. 12) with distinct cluster of ages at370–360 and 340–310 Ma (Supplemental dataset F).

7. Discussion

7.1. Young age of garnet rims 3, 4, 10, and 16

Six of the ten garnet rim and both of the small-garnet bulk ages areequivalent within their uncertainties at 376.5 ± 1.0 Ma (Fig. 10c). Theother four garnet rim dates (Gt 3, 4, 10, 16) range down to ca. 350 Masuggesting another process is at play in these four crystals. One possibil-ity is that we failed to cleanse these four garnet rims of their inclusions;thus their skewed ages reflect contamination by older inherited lowSm/Nd inclusions such as monazite which would lead to an apparentdecrease in age (e.g. Prince et al., 2000; Baxter and Scherer, 2013). How-ever, all of these anomalously young garnet rims have similarly high

0.51193

0.51194

0.51195

0.51196

0.51197

0.51198

0.51199

0.110 0.115 0.120 0.125 0.130 0.135

To G

rt3b-Z2

377.6±1.1Ma

To G

rt3b-Z

2

379.7±2.5Ma

To G

rt3b-Z

3

381.2±1.1Ma

Grt3b-Z2 with all WR/Mtx (N=17)

Grt3b-Z2 with Grt3mtx (N=2)Grt3b-Z2 with violet WR (N=2)

Mtx Near GrtWhole Rocks

147Sm/144Nd

B

isochrons calculated for TD09-14b-Grt3-Zone 2 (mantle). Isotopic ratios for this crystalgments analyzed (~1.81). B. Expanded view of WR/Mtx portion of isochrons shown in77.6± 1.1Ma to 381.2± 1.1Ma. The isochron fit to allWR/Mtx isotopic ratios (excludingfor garnet segments with higher 147Sm/144Nd are less sensitive to matrix and whole-rock

0.51100

0.51200

0.51300

0.51400

0.51500

0.51600

0.51700

0.51800

0.51900

0.00 0.50 1.00 1.50 2.00 2.50 3.00

147Sm/144Nd

143 N

d/14

4 Nd Grouped Garnet Rims

Isochron Age = 376.5±1.0 MaMSWD=18; N=24 points(5 WR; 11 Mtx; 8 Grt Rims)

Mtx-WR (N=16)Grt Rims (N=8)Rejected Grt Rims (Sm/Nd<0.75; N=0)Outlier Grt Rims (N=4)

0.51100

0.51200

0.51300

0.51400

0.51500

0.51600

0.51700

0.51800

0.51900

0.00 0.50 1.00 1.50 2.00 2.50

Mtx-WR (N=16)Grt Mantles (N=14)Rejected Grt Mantles (Sm/Nd<0.75; N=3)Outlier Grt Mantles (N=1)

147Sm/144Nd

143 N

d/14

4 Nd Grouped Garnet Mantle Zones

Isochron Age = 377.3±1.4 MaMSWD=18; N= 30 points(5 WR; 11 Mtx; 14 Grt Mantle Zones)

0.51100

0.51200

0.51300

0.51400

0.51500

0.51600

0.00 0.20 0.40 0.60 0.80 1.00 1.20 1.40 1.60 1.80

Mtx-WR (N=16)Grt Cores (N=7)Rejected Grt Cores (Sm/Nd<0.75; N=2)Outlier Grt Cores (N=1)

147Sm/144Nd

143 N

d/14

4 Nd Grouped Cores

Isochron Age = 380.3±2.0 MaMSWD=14; N=23 points(5 WR; 11 Mtx; 7 Grt Cores)

A

B

C

Fig. 10. Garnet-rock Sm–Nd isochrons of Townshend Dam grouped garnet cores (A),grouped mantles (B), and grouped rims (C).

0.00

0.05

0.10

0.15

0.20

0.25

0.000 0.500 1.000 1.500 2.000 2.500 3.000 3.500 4.000147Sm/144Nd

Zo

ne

Mn

Co

nte

nt

(Mo

le F

ract

ion

) Garnet CoresGarnet Mantles

Garnet Rims

y = -0.075x + 0.2276R2 = 0.55

376 375377378379380381382383

Sample TD09-14 Grouped Garnet Sm-Nd AgesCores (n = 7)Mantles Zones (n = 14)Rims (n = 6 and 2 small bulk grt)Error bars are 2σ uncertainties of the isochron age of the grouped garnet zones with 5 whole-rock and 11 matrix

0.00

0.05

0.10

0.15

0.20

0.25

Sm/Nd Age (Ma)

Zo

ne

Mn

Co

nte

nt

(Mo

le F

ract

ion

)

Garnet Cores380.3 ± 2.0 Ma n=23; MSWD = 14)

Garnet Rims376.5±1.0Man = 24MSWD = 18

Garnet Mantles377.3 ± 1.4 Man = 30; MSWD = 18

0.00

0.05

0.10

0.15

0.20

0.25

370 365375380385390395

14b-Grt314a2-Grt714a-Grt4a14a-Grt4b14a-Grt814a-Grt914a-Grt1014a-Grt1314a-Grt16Small Grts

Sm/Nd Age (Ma)

Zo

ne

Mn

Co

nte

nt

(Mo

le F

ract

ion

)

A

B

C

Fig. 11.Mn vs. age relationship for Townshend Dam garnet growth. A. Mn vs. Sm–Nd agefor individual garnet crystal segments. X-axis error bars correspond to the age uncer-tainties listed in Table 1 for each crystal segment. Y-axis error bars represent the rangeof Mn compositions sampled for each segment. B. Mn-Age plot constructed for groupedages (cores, mantles, rims). X-axis error bars show the 2-sigma isochron age uncertaintyfor grouped cores (n = 7), mantles (n = 14), and rims (n = 8). Y-axis error bars showthe range of Mn contents sampled for each group. C. Variation of 147Sm/144Nd with Mncontent for garnet cores, mantles, and rims.


147Sm/144Nd (1.573 to 3.041) and low Nd concentrations (0.083 to0.158 ppm) as do the other garnet rims, suggesting that they are allsimilarly clean (Table 1; Fig. 11c).

Post-growth garnet resorption accompanied by recrystallizationand/or back diffusion is another possible explanation for the observedyoung garnet rims of crystals 3, 4, 10, and 16. Chlorite and quartzrims, on most garnet provide textural evidence for some resorption ofthe garnet rims (Fig. 4a). Most garnets have irregular grain boundariesadjacent to pressure shadows that are embayed with chlorite, quartz,and accessory phases, compatible with late-stage resorption. Closely-spaced (ca. 5 μm) EPMA data for the garnet rims show perturbationsin major-element zoning adjacent to irregular grain boundaries (e.g.Spear, 1993; Fig. 4). In particular, decreasing Mg and increasing Mn

compositions within ca. 80 μm of the rim contrast with the overallgrowth zoning patterns for these elements, suggesting that some late-stage processes affected the remaining rim compositions (Figs. 3 and 4).Decreasing Mg concentration near the garnet rim is consistent withpost-growth garnet resorption at the expense of chlorite, which wouldsequester the released Mg. The slight uptick in Mn at the outermostgarnet rim is consistent with either back-diffusion of Mn into the

Dis

tan

ce (

mic

ron

s)

Distance (microns)

X360

50

40

30

20

10

00 10 20 30 40 50

Ag

e (M

a)50

045

040

035

030

025

0

Xenotime high Ce

X3

A) Garnet Ca K Map B) Xenotime Ages C) Xenotime Yb M Map

Fig. 12.Garnet and accessory phases in TD09-14a-Grt8. A. Ca Kα X-raymapwith associated xenotime grains and high Ce spots detected by EPMAmapping. Xenotime grains aremarked inblue and those analyzed on the Cameca Ultrachron are labeled as X1-X12 B. Xenotime ages. C. Yb Mα X-ray intensity map for xenotime X3 in the upper right of the garnet grain shown in(A). Orange corresponds to high (ca. 1.9 elemental weight percent) and blue to lowYb (ca. 0.3 elementalweight percent). (For interpretation of the references to color in thisfigure legend,the reader is referred to the web version of this article.)


remaining garnet during resorption or recrystallization of a resorbedgarnet rim.

The garnet rim zones have higher Sm and 147Sm/144Nd than the coreand mantle zones (Fig. 11c; Table 1). This is corroborated by LA-ICPMStrace element maps that show relatively flat zoning profiles for theMREE (Sm, Eu, Gd, and Dy) that slightly increase near the garnet rims(Fig. 7). Increasing 147Sm/144Nd from core to rim may seem counter-intuitive, given that garnet slightly fractionates Sm over Nd duringgrowth. Models of garnet growth predict flat or slightly decreasingzoning profiles for 147Sm/144Nd (Lapen et al., 2003; Kohn, 2009).However, increasing 147Sm/144Nd from core to rim has been document-ed in other garnets that preserve growth zoning (e.g. Vance andO'Nions,1990; Stowell et al., 2001; Dragovic et al., 2012; Dragovic et al., 2015)and likely results from concurrent growth of garnet with other Nd-sequestering phases. Thus, initial garnet grows unzoned with respectto 147Sm/144Nd; then, growth of clinozoisite (or perhaps some otherNd sequestering phase) reduces the amount of Nd available to the gar-net mantles and rims and the observed zoning profile is developed.Garnet-compatible HREE and Y decrease concentrically away from thecore, then increase at the rim (Fig. 7). The observed concentric decreasein HREE and Y demonstrates the affinity of these elements in garnet andare typical of low-temperature garnet that preserves growth zoning (e.g.Kohn, 2009; Anczkiewicz et al., 2012). The increase of Y and HREE at theoutermost ~100 μm in the garnet rim (e.g. the tiny red portions in Fig. 7)is interpreted to reflect back-diffusion of these elements into the garnetduring resorption of the rim (e.g. Kelly et al., 2011). Overall,MREE, HREE,and Y zoning patterns in Townshend Dam garnet are interpreted torepresent primary growth fractionation of these elements followed bylate-stage resorption of the rim and re-incorporation of garnet-compatible elements back into the outermost remaining garnet.This re-incorporation could have occurred via wholesale garnet recrys-tallization following resorption, and/or by back diffusion. Diffusionlengthscales for REE back into garnet at peak temperatures will be lessthan 0.1 mm even with a 10 m.y. duration at peak ~600 °C temperature(e.g. Pollington and Baxter, 2011 using the diffusion data of Tirone et al.,2005; diffusion lengthscales could be even smaller based on the slowerREE diffusion data of Carlson, 2012), so this mechanism can account foronly the outermost REE increases we observe (Fig. 7). Any modificationof garnet compositions penetrating several hundred microns is morelikely related to wholesale recrystallization following resorption.

Small xenotime crystals are concentrated along the outermost partof the garnet rims,with a fewoccurring as inclusion phases. The concen-tration of xenotime crystals around the garnet indicates that xenotime

grew as a result of garnet consumption and is compatible with releaseof Y during breakdown of the garnet rim after most garnet growth(e.g. Pyle and Spear, 1999; Fig. 12a). The 370–360 Ma xenotime agegroup is indistinguishable (within uncertainties) from the majorityof our anomolously young garnet rim Sm–Nd results (Fig. 12; Table 1;Supplementary dataset F). Locally irregular garnet rim shapes mayreflect new growth of garnet subsequent to the resorption (i.e. recrys-tallization). This interpretation is consistent with the high Mn, Y, andREE concentrations observed in the garnet rims (Figs. 4b and 7). Theseelements are inferred to have been released from garnet during resorp-tion, then sequestered by xenotime plus secondary garnet, thereby pro-ducing the textures and zoning patterns observed near the garnet rims.

The younger xenotime age cluster (340–310Ma) indicates a seconddistinct event, possibly associated with later heating and fluid influxthat produced new xenotime growth which is indicated by the youngages and complex zoning of individual xenotime grains (Fig. 12c). Weinfer that this event also led to localized recrystallization of garnetwhich thus yield anomalously young Sm–Nd dates. Thus, garnet “rim”

dates that are young outliers (i.e. the black diamond data points onFig. 10c) reflect mixed ages between primary rim growth at 376.5 ±1.0 Ma and later resorption and recrystallization (with new xenotime)between 340 and 310 Ma.

7.2. Reverse age zoning and anomalously young age of garnet core 10

One of the garnets used in this study (TD09-14a-Gt10) has innerzones (higher Mn) that are apparently younger than the outer zones(lower Mn). In fact, the date for this garnet core (324.5 ± 3.3) is theyoungest in the entire dataset. The reverse age zoning observed in thisgarnet persists, regardless of which matrix and/or whole-rock isotopicratios the zones are paired with for isochron ages. These “ages” cannotreflect primary growth ages given simple geometric constraints. Possi-ble geologic causes for this include: recrystallization of core driven bycracking and fluid infiltration; crystal clustering (polycrystals); and/orisotopic disequilibrium during initial garnet growth.

Fluid driven recrystallization of the garnet core is unlikely becausemajor-element zoning in the core is not visibly disrupted and no atolltextures have been observed. Garnet crystal clusters are common inthe Townsend Dam rocks and these clusters often have simple concen-tric major element zoning that crosses distinct lattice orientations, asdocumented by Whitney et al. (2008). If this is the cause for reverseage zoning in some Townshend Dam garnets, then one or more outer(young) zones of garnet could have been inadvertently sampled with


or without older core material. Another younger garnet zone could haveimpinged on one or both of these grains and the results are mixed agesbetween the two grains. Alternatively, this grain with anomalouslyyoung inner zones may have nucleated in areas of the sample thatretained locally anomalous isotopic signatures and never isotopicallyequilibratedwith thematrix.Whatever the cause, this young core outlierdate is not included in our subsequent interpretations.

7.3. Prograde garnet growth duration at Townshend Dam, Vermont

The difference between the garnet core (380.3 ± 2.0 m.y.) and rim(376.5 ± 1.0 m.y.) age is 3.8 ± 2.2 m.y. (2σ) which gives the durationof garnet growth rock wide. While our garnet ages are generally con-sistent with those reported by Christensen et al. (1989), our durationis somewhat shorter than theirs (10.5 ± 4.3 m.y.). This is probablybecause we were able to recognize and filter out the anomalouslyyoung garnet rim ages from our data, whereas these late resorptionevents likely affected the rim age data of Christensen et al. (1989) andincreased their apparent prograde growth duration.

7.4. Chemical and isotopic equilibrium during garnet growth

ComparableMn compositions and concentric zoning patterns of gar-nets throughout the sample supports, to a first order, the hypothesisthat all garnets growing at the same time equilibrated with the sameuniform matrix reservoir that was being progressively depleted in Mn(Figs. 3 and 5); that is, that the equilibrium lengthscale for Mn in therock matrix was greater than the sample dimensions (~40 cm). All ofthe divalent cations in garnet throughout the sample correlate welland showuniformmajor element zoning from core to rim (Fig. 13). Gar-net Fe and Mn compositions show a strong negative linear correlation,reflecting decreasing Mn and increasing Fe from garnet cores to rims(Fig. 13a). Mn also correlates negatively with Mg and positively with Ca(although non-linearly) throughout the sample (Fig. 13b, c). Mg and Caare anti-correlated (Fig. 13f). Correlations of these cations for separategarnet grains of different size throughout the sample volume supportthe interpretation that every increment of garnet had nearly the samemajor element composition during growth and imply that these elementswere homogeneously distributed (to a first order) in the intergranularmatrix. Localized second order (i.e. shorter wavelength; e.g. Baxter andDePaolo, 2002a) major element heterogeneities may have persisted dur-ing metamorphism but these are not be resolvable with our analysis.

Sm and Nd isotopic compositions of thewhole-rock andmatrix varyconsiderably throughout the sample (Fig. 9b); that is, the equilibriumlengthscale for Sm and Nd in the rock matrix is ca. 1 cm or smaller.This variation likely reflects the heterogeneous distribution of detritalor metamorphic accessory phases (xenotime, zircon, apatite, epidote,etc.) throughout the sample and/or post-garnet growth alteration ofma-trix compositions by fluids. Thus, it is difficult to document the exactwhole-rock or matrix compositions with which each zone of garnetisotopically equilibrated because some or all of the accessory phasesmay not have isotopically equilibrated with the garnets or the whole-rock during garnet growth, and because the garnet crystals themselvesare larger than the lengthscale of heterogeneity. Furthermore, thelengthscale of heterogeneity sampled by garnet certainly changedduringmetamorphism as the garnets grew. Because chemical equilibrium re-quires that reactant-liberating metamorphic reactions and intergranulardiffusion keep pace with crystal growth, limited availability and longerequilibrium lengthscales formajor elementsmay hinder slowly diffusingREE from equilibrating rock-wide (e.g. Skora et al., 2006). Equilibriumlengthscales for the REE and their isotopes tend to be very small (dueto low solubility in most crustal fluids and consequently low bulk diffu-sivity) such that even small scale protolith heterogeneities are generallypreserved (e.g. Baxter and DePaolo, 2002b). The contrast between con-sistent first order garnet major element compositions and inconsistentSm and Nd isotopic compositions, rockwide, suggests that the garnet

and rock matrix may have continuously maintained major-elementchemical equilibrium but not Sm and Nd isotopic equilibrium. This effectwould be accentuated by the presence of refractory REE-rich phases(zircon, epidote, apatite, xenotime, etc.) in the matrix that may not havereacted at the metamorphic conditions experienced by TownshendDam rocks. Alternatively, matrix isotopic compositions could have initial-ly equilibrated rockwide and subsequently been changed during fluidinflux (eg. Chamberlain and Conrad, 1993). However, an oxygen isotopestudy of Townshend Dam rocks by Kohn and Valley (1994) suggestsvery localized fluid flow, if any at all, occurred. The relatively robustSm–Nd isochrons for grouped isotope data from across our sample sug-gest that any post-garnet growth chemical alteration (presumably dueto fluid flow) was very localized or not significant. Thus, the variationin matrix isotopic compositions likely results from the heterogeneousdistribution of accessory phases throughout the sample.

Consistentmajor-element compositions of zoned garnet throughoutthe sample volume suggest that major-element chemical equilibriumwas generally maintained (to a first order) over the duration of garnetgrowth. However, the newdataset documents a narrower garnet growthduration (3.8±2.2m.y.) thanwas previously proposed (10.5±4.5m.y.;Christensen et al., 1989), and isotopic heterogeneity of the matrix pre-cludes garnet-rock Sm–Nd isochron age precisions needed to distinguishbetween ages of adjacent garnet crystal segments. Additionally, evenwiththe improved sampling resolution afforded by theMicroMill andmodernTIMS Sm–Nd analysis, the range of Mn compositions sampled for eachcrystal segment varied based on garnet wafer size (sample volume),thereby producing a large range of Mn contents for each grouped zone.However, the consistency of the garnet and rockmajor element composi-tions throughout the sample does suggest that major element chemicalequilibrium was maintained to a first order throughout garnet growth.Further testing of chemical equilibrium via evaluation of chemicalchanges through time in zoned minerals, such as garnet, would requirea sample with a more homogeneous distribution of isotopic composi-tions throughout the rock matrix. Additionally, avoiding samplesthat may contain secondary garnet growth textures (resorptionand/or recrystallization), reverse age zonation, polycrystals, orfluid-flux alteration will aid in interpreting compositional and agedata for the evaluation of chemical equilibrium during metamorphicmineral growth.

8. Conclusions

A new technique is described for evaluating chemical equilibrium inmetamorphic rocks by testing the idea that Mn contents and Sm–Ndages of concentric garnet crystal segments should correlate over a vol-ume of rock that has maintained matrix equilibrium during growth ofzoned garnets. One to three cm diameter garnet in a 1.21 × 104 cm3

block of pelitic schist from Townshend Dam, VT, are concentricallyzoned in major elements, especially Mn. Similar garnet compositionsand element zoning patterns throughout the sample qualitatively sug-gest that major element equilibrium at the garnet-matrix interfacewas maintained throughout garnet growth. However, Sm and Nd ele-mental and isotopic composition in the rock matrix is heterogeneousat the b1 cm scale – smaller than the size of individual garnet crystals –suggesting that the equilibrium lengthscale for Sm and Nd was smalland rockwide equilibrium of Sm andNdwas not achieved duringmeta-morphism of the layered sedimentary protolith. As a consequence, theprecision of Sm-Nd ages is compromised for large zoned garnet crystalswhere the exact composition of the matrix equilibrating with garnetcannot be known. In part due to this uncertainty, Sm–Nd ages of garnetcrystal segments range from383.1±6.8Ma to 324.5±3.3Ma. However,the documented isotopic heterogeneity of thematrix cannot explain all ofthis variation. Exclusion of four significantly younger garnet rim ages thatcontain recrystallization and/or resorption textures at their outermostrims further constrains the ages from 383.1 ± 6.8 to 374.9 ± 1.8 Ma.Grouped Sm–Nd isotopic compositions for 7 high-Mn garnet cores, 14

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Fig. 13.Correlation of divalent cations (reported asmole fraction) in ten TownshendDamgarnet from sample TD09-14. A. Alm versus Sps. B. Prp versus Sps. C. Grs versus Sps. D. AlmversusPrp. E. Alm versus Grs. F. Prp versus Grs. 10,370 data points are plotted for 10 garnets (after filtering). Correlation of all garnetmajor element compositions throughout the sample volumeis compatible with the interpretation that major element matrix equilibrium was generally maintained throughout garnet growth. Note that TD09-14a-Grt4b (purple crosses) was not acentral wafer explaining the slight differences in composition. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)


medium-Mn mantles, and 8 low-Mn rims, define three distinct Sm–Ndisochron ages of 380.3 ± 2.0 Ma, 377.3 ± 1.4 Ma, and 376.5 ± 1.0 Ma,respectively, suggesting an average rock-wide garnet growth durationof 3.8 ± 2.2 m.y. Grouped cores, mantles, and rims define a simple firstorder Mn-age relationship that reflects a depletion of Mn in the rockmatrix as garnet grew.

Acknowledgments

Stéphanie Duchêne and an anonymous reviewer are thanked forthorough technical reviews of themanuscript. The journal editor, Laurie

Reisberg, is thanked for helpful suggestions and efficient handlingthroughout the review and publication process. This workwas support-ed by National Science Foundation grants EAR-0911491 EAR-0911582,and EAR-0911402 to Stowell, Baxter, and Hirsch, respectively. JeremyInglis provided invaluable help with isotopic analyses at Boston Univer-sity. All Sm–Ndgeochronologywere conducted in the Boston UniversityTIMS Facility which was funded by NSF grants EAR-0521266 and EAR-0949390. Rob Holler and the University of Alabama Central AnalyticalFacility provided assistance obtaining garnet major element composi-tional maps on the JEOL 8600 electron probe microanalyzer. NationalScience Foundation grant EAR-0744467 provided an upgrade to the


JEOL 8600 which was essential for producing the garnet compositionalmaps. Alan Koenig is thanked for providing trace element compositionsfor garnet obtained with the LA-ICP-MS in the U.S. Geological Surveylaboratory at the Lakewood Federal Center, CO. Richard Ketcham andthe UT-Austin High Resolution X-Ray CT Facility are thanked for assis-tance in obtaining the high resolution images of the sample. MichaelWilliams and Michael Jercinovic and the University of MassachusettsAmherst Geosciences Electron Microprobe/SEM Facility are thankedfor providing high resolution compositional maps and U–Pb xenotimeages. The U.S. Army Corps of Engineers and Donna Vondle are thankedfor providing access to the Townshend Dam spillway for sampling.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2015.02.017.

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