Experimental determination of REE partition coefficients

between zircon, garnet and melt: a key to understanding high-temperature crustal processes

Journal: Journal of Metamorphic Geology

Manuscript ID: JMG-14-0026.R2

Manuscript Type: Original Article

Date Submitted by the Author: n/a

Complete List of Authors: Taylor, Richard; Curtin University, Applied Geology

Harley, Simon; University of Edinburgh, School of Geosciences Hinton, Richard; University of Edinburgh, School of Geosciences Elphick, Stephen; University of Edinburgh, School of Geosciences Clark, Chris; Curtin University of Technology, Applied Geology Kelly, Nigel; University of Colorado, Boulder, Department of Geological Sciences

Keywords: zircon, garnet, rare earth elements, strain-modelling, UHT metamorphism

1

Experimental determination of REE partition coefficients 1

between zircon, garnet and melt: a key to understanding 2

high-temperature crustal processes 3

R.J.M. TAYLOR,1* S.L. HARLEY,2 R.W. HINTON,2 S. ELPHICK2, C. CLARK1 AND N.M. 4

KELLY3 5

1 Department of Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia 6

2 School of GeoSciences, University of Edinburgh, Edinburgh EH9 3JW, UK 7

3 Department of Geological Sciences, University of Colorado, Boulder CO, USA 8

9

*Corresponding Author- email: richard.taylor@curtin.edu.au phone: +61(0) 8 9266 7625 10

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ABSTRACT 12

The partitioning of rare earth elements (REE) between zircon, garnet and silicate melt 13

was determined using synthetic compositions designed to represent partial melts 14

formed in the lower crust during anatexis. The experiments, performed using 15

internally heated gas pressure vessels at 7 kbar and 900–1000 °C represent 16

equilibrium partitioning of the middle to heavy REE between zircon and garnet 17

during high-grade metamorphism in the mid to lower crust. The DREE (zircon/garnet) 18

values show a clear partitioning signature close to unity from Gd to Lu. Because the 19

light REE have low concentrations in both minerals, values are calculated from strain 20

modelling of the middle to heavy REE experimental data; these results show that 21

zircon is favoured over garnet by up to two orders of magnitude. The resulting general 22

concave-up shape to the partitioning pattern across the REE reflects the preferential 23

incorporation of middle REE into garnet, with DGd (zircon/garnet) ranging from 0.7–24

1.1, DHo (zircon/garnet) from 0.4–0.7, and DLu (zircon/garnet) from 0.6–1.3. There is 25

no significant temperature dependence in the zircon-garnet REE partitioning at 7 kbar 26

and 900–1000 °C, suggesting that these values can be applied to the interpretation of 27

zircon-garnet equilibrium and timing relationships in the UHT metamorphism of low-28

Ca pelitic and aluminous granulites. 29

30

31

Key words: zircon; garnet; rare earth elements; strain modelling; UHT 32

metamorphism 33

34

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35

INTRODUCTION 36

One of the most challenging applications of zircon geochronology is to define the 37

ages of events in high-grade metamorphic terranes, most of which have experienced 38

complex and polyphase P–T histories. Linking zircon growth and modification zones 39

to specific metamorphic events is, in principle, achievable by identifying and using 40

reliable chemical signatures. For example, the Ti content and REE patterns of zircon 41

can be tied to temperature and equilibration with coexisting minerals and which are 42

independent of the U-Pb system (e.g. Rubatto, 2002; Kelly & Harley, 2005; Tomkins 43

et al., 2005; Harley & Kelly, 2007; Harley et al., 2007; Rubatto & Hermann, 2007b; 44

Baldwin & Brown, 2008; Clark et al., 2009; Hermann & Rubatto, 2009; Kotková & 45

Harley, 2010; Schoene et al., 2010; Marsh et al., 2012; Jiao et al., 2013; Korhonen et 46

al., 2013; Harley & Nandakumar, 2014; Taylor et al., 2014). 47

As a modally abundant and important sink for trace elements such as Y and 48

the heavy REE (e.g. Bea et al., 1994; Hermann, 2002; Rubatto & Hermann, 2003), 49

garnet also acts as a key competitor with zircon for the middle to heavy REE in high-50

grade metamorphism and anatexis, with many studies using textural relationships 51

between these minerals to infer equilibrium trace element signatures (Harley et al., 52

2001; Rubatto, 2002; Rubatto & Hermann, 2003; Whitehouse & Platt, 2003; Hokada 53

& Harley, 2004; Kelly & Harley, 2005; Buick et al., 2006). However, at granulite 54

facies conditions there remains some uncertainty as to the temperature dependence 55

and magnitude of the equilibrium distribution coefficient of the REE (DREE) between 56

zircon and garnet, with two possibilities being proposed based on combinations of 57

empirical and experimental observations as follows. 58

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(a) The distribution of MREE–HREE describes a flat pattern with values 59

remaining at unity, or slightly favouring garnet throughout the middle and heavy 60

REE, both for UHT (T>900 °C) conditions (Harley et al., 2001; Hokada & Harley, 61

2004; Kelly & Harley, 2005) and more typical granulite facies temperatures (750-62

900C: Whitehouse & Platt, 2003; Harley & Kelly, 2007). This interpretation of D is 63

based principally on zircon-garnet REE determinations from granulite facies mineral 64

assemblages in pelites and leucogranitic rock types. 65

(b) The distribution of MREE–HREE describes a steep pattern with 66

approximately an order of magnitude increase in D values towards the HREE, which 67

strongly favours zircon over the typical range of granulite facies temperatures (750–68

900 °C) (Rubatto, 2002; Hermann & Rubatto, 2003; Buick et al., 2006). This model, 69

proposed on the basis of zircon-garnet REE measurements from granulite migmatites 70

and eclogite, has found some support in the experimental results of Rubatto and 71

Hermann (2007a), the only experiments so far performed to address this problem. 72

Those experiments indicated that near-equipartitioning of the MREE–HREE between 73

zircon and garnet (DHREE near unity and DGd = DYb) under UHT conditions (1000 °C), 74

whereas steep HREE distribution patterns (DHREE >5 and increasing from Dy to Lu) 75

reflect equilibrium at more normal granulite temperatures of 800–850 °C. However, 76

although the Rubatto and Hermann (2007a) experiments are significant in 77

demonstrating the importance of understanding the distribution of REE between 78

zircon and garnet, they are not directly applicable to the granulite facies and UHT 79

metamorphism of pelites and pelitic migmatites in the middle crust (800–1000 °C and 80

6–12 kbar: (e.g. Harley, 1989; Brown, 2007) as they were conducted at 20 kbar and 81

involved garnets with appreciable Ca contents. 82

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In the present communication we report the results of experiments carried out 83

to fill this gap between the natural rock studies and laboratory-based determinations 84

of DREE (zircon/garnet). Experiments have been conducted in internally heated gas 85

pressure vessels using synthetic starting materials based on realistic mineral and melt 86

compositions to produce DREE (zircon/garnet) values from measured mineral REE 87

concentrations. The P-T conditions of these experiments, 900°C–1000 °C and 7 kbar, 88

complement the previous 20 kbar experiments of Rubatto and Hermann (2007a) and 89

are highly relevant to the interpretation of zircon–garnet mineral and zircon–garnet–90

melt relationships during mid- to deep-crustal metamorphism and partial melting of 91

pelites under UHT conditions. 92

Several tests are applied to the experimental and analytical methodology to 93

ensure the dataset is a robust and accurate determination of zircon–garnet partitioning. 94

The experimental data are evaluated using a strain modelling approach (Blundy & 95

Wood, 1994) which produces an excellent fit for the middle to heavy REE with 96

realistic values of the crystallographic site parameters. The strain model allows 97

estimation of the partition coefficients of LREE between the phases, which are not 98

determined from the experiments directly. 99

100

EXPERIMENTAL METHODS 101

Choice of bulk compositions and experimental conditions 102

The experiments have been designed to simulate, garnet–melt and zircon–garnet–melt 103

trace element relationships established during melting of fertile pelite protoliths in the 104

middle to deep crust during high-grade metamorphism and anatexis. Hence, the 105

experiments have been run at 900 °C, 950 °C and 1000 °C at 7 kbar in internally 106

heated gas apparatus and the melt composition has been chosen to be appropriate to 107

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these conditions. An initial pilot study at 5 kbar was run to determine the most 108

reliable method for determining trace element compositions of zircon (Appendix S1). 109

This pilot study also included two experiments carried out at different run durations 110

(45 hours and 468 hours) to evaluate the effect of experimental run time on the 111

zircon–melt partitioning results and hence allow an assessment of whether 112

equilibrium was approached in the main set of experiments. 113

The experimental protocol of Carrington & Harley (1995), who determined 114

biotite-bearing phase equilibria and related melt compositions in the KFMASH 115

system, were followed in the selection of the melt composition. A model 116

peraluminous granitic NKFMASH melt composition reflecting that formed from the 117

partial melting of pelitic rocks with intermediate Fe–Mg ratios during anatexis 118

(Harley & Carrington, 2001), has been used here as the basis for the starting gel 119

compositions (XMg = 0.31–0.52; ASI = 1.3–1.8). The major-element composition of 120

the starting materials is reported in Table 1. Water was added to the experimental 121

charges in order to produce H2O-undersaturated melts with a range of water contents 122

(3–4 wt% H2O) comparable to those produced during the anatexis of pelitic rocks. 123

Starting materials 124

All experiments used mixtures of two gels as starting materials (Gels G3 and G4, 125

Table 1). Gel starting materials were produced using the ‘co-precipitation gel’ method 126

(Hamilton & Henderson, 1968; Biggar & O'Hara, 1969) using reagent grade materials 127

for major and trace elements. Gels have several advantages over other starting 128

materials such as sintered oxides or rock powders such as their very fine grain size 129

(~10 times smaller than sintered oxides) and their flexibility in making customised 130

compositions involving many major and trace components. Gels also have the 131

advantage of creating a homogenous starting material without the need for multiple 132

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fusions and grinding (Hamilton & Henderson, 1968). Further details on the use of 133

starting materials can be found in Appendix S1. 134

Experimental setup 135

A total of 8 experimental runs were performed for this study, each containing 3–6 136

capsules, using the internally heated gas pressure vessels at The NERC Recognised 137

Experimental Geoscience Facility, University of Edinburgh. Experiments were 138

performed at 7 kbar and 900–1000 °C and run for 220–264 hours. Internally heated 139

gas pressure vessel procedures follow those described by Carrington & Harley (1995), 140

leading to precisions in temperature of ±10 °C and in pressure of ±0.1 kbar. Internally 141

heated gas pressure vessels allow ƒO2 to be maintained constant over the long 142

durations of this type of experiment (e.g. Burnham & Berry, 2012). 143

Runs used 20 mg of starting material, including 3–4 wt% H2O loaded into 144

Ag50Pd50 metal capsules with 2mm external diameters. This capsule material is highly 145

resistant to the absorption of Fe from the experimental charge. Up to six 2 mm 146

diameter capsules were placed within a Mo container in the furnace vessel with two 147

thermocouples placed 1/3 and 2/3 of the way along the length of the capsule to 148

monitor temperature gradients. Tantalum chips were placed in the Mo container to 149

help maintain an oxygen fugacity appropriate for melting under reducing conditions 150

during the runs, close to 1 log unit below QFM (Carrington, 1993; Carrington & 151

Harley, 1995). An initial test on promoting garnet growth demonstrated the best 152

results if the starting material had been further reduced prior to the experiments. 153

Therefore gel starting materials for garnet-bearing experiments were reduced to one 154

log unit above iron-wustite (IW+1) at 900 °C in a H2–CO2 1 atm gas-mixing furnace 155

prior to loading into experimental capsules. Care was taken to ensure the tantalum did 156

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not interfere with either the thermocouples or the capsules. Full details on the run 157

conditions for these experiments can be found in Table 2. 158

The 5 kbar pilot study included short duration runs, single-stage runs at 1050 159

°C as well as runs with simple, stepped and sawtooth temperature drops. These tests 160

demonstrated that a single down-temperature stepped approach was highly successful 161

in ensuring nucleation of abundant acicular zircon, but could not generate zircon 162

coarse enough for analysis by SIMS as single grains. Following these results, the 7 163

kbar experiments involved a two-stage heat-settling process in which all experiments 164

were heated to 1050 °C for at least 2 hours at the start of the experiment, then brought 165

down to the run temperature over a period of approximately 10 minutes. After this 166

short-term pre-equilibration stage the experiments were held at the required run 167

temperature for the remaining duration of the experiment (220–264 hours). This 168

initial temperature overstep/heat-settling approach was adopted in order to ensure that 169

all Zr is initially dissolved in melt (in the case of the low-Zr gel) and promote 170

dissolution of any early-nucleated zircon, and encourage zircon crystallisation 171

through oversaturation (Watson & Harrison, 1983) as temperature is decreased to the 172

final run temperature. 173

RUN PRODUCTS 174

Full details on the imaging and analytical techniques used for experimental and 175

natural samples in this study can be found in Appendix S1. Experimental run products 176

were imaged on the SEM prior to analysis using the Back Scattered Electron (BSE) 177

detector to examine textures and select areas for analysis. Glass formed from the 178

quenching of the granitic melt comprises ~80% of the run product, the remainder 179

generally being zircon and garnet grains crystallised during the experiment, and minor 180

orthopyroxene (<5 vol%) inferred from its textural relations to have formed in the 181

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glass as the experiments are quenched. Figure 1 shows BSE images of typical run 182

products, and high resolution EDS images for Fe and Mg of garnet and glass. 183

Zircon crystals, easily recognised by their high BSE response compared to the 184

glass (Fig. 1), form approximately 7–15% by volume of the experimental charge. 185

Experiments consistently produced zircon grains up to 15 µm in length and up to 3–5 186

µm in diameter, with some larger grains, variably distributed throughout the 187

experimental capsule. No zoning is visible in the grains using BSE imagery, though 188

the small grain size may make any zoning difficult to detect. Although the usual grain 189

size of the zircon produced in the experiments is too small for a single spot analysis 190

with the ion microprobe the variable spatial distribution, density and clustering of 191

crystals makes them ideal for the production of zircon–melt mixing lines. The REE 192

composition of the average zircon in each run can be extrapolated by linear regression 193

of these mixing lines for each trace element, following the approach adopted by 194

Rubatto and Hermann (2007a). Rubatto & Hermann (2007a) demonstrated that this 195

approach is a viable method for constraining experimental zircon REE concentrations, 196

even when extrapolating from considerably lower zircon contents (1–5 vol%) than 197

those achieved here (up to 25%, 7 kbar; up to 50%, 5 kbar pilot study). The linear 198

regression approach was developed during the 5 kbar pilot study and subsequently 199

adopted for zircon REE analysis in this study. 200

Up to 12% of the volume of the capsule is composed of newly formed garnet 201

(Alm47-60Pyp39-52; XMg = 0.40–0.47) with grain sizes up to 40 µm. The largest 202

variation in garnet chemistry is between the two 900 °C runs (run z26-1: XMg = 0.40; 203

run z26-4: XMg = 0.47; Table 4). Oxide values based on 12 oxygens and calculated 204

assuming full site occupancy suggest that there is some Fe2O3 component in garnet 205

from experiment z26-4, potentially elevating the XMg value in this experiment. If the 206

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average of the garnet values for these two runs is used (XMg = 0.44) then almost no 207

variation in garnet chemistry is evident in this study. There was a variation in the 208

modal proportion of garnet in the runs, with the higher temperature runs (1000 °C) 209

containing a larger number of garnets grains that were also the largest in size (Fig. 1). 210

This is considered to reflect more favourable growth kinetics in granitic melts at 211

higher temperature. 212

213

EXPERIMENTAL RESULTS 214

Melt (glass) compositions 215

In all experiments clean, zircon-free melt patches or areas were analysed for the REE, 216

Zr, Hf, Y, P and Li contents, referenced against Si. Clean melt analyses are important 217

in anchoring one end of the mixing line for zircon REE regression. These areas were 218

also analysed by electron microprobe for their major-element compositions so that the 219

SIMS REE and Zr measurements could be correctly converted into ppm. These trace-220

element contents are reported along with major-element compositions of the 221

experimental glasses in Table 3. 222

Melts (glass) from all experiments at 900 °C, 950 °C and 1000 °C have similar 223

major-element compositions, with SiO2 contents ~65–66%. These melts also have 224

very similar REE concentrations (Table 3) with Yb at 1.7–2.8 ppm (in zircon–garnet 225

runs), a strong negative Eu anomaly (as dictated by the Eu-depleted starting material 226

composition), and LREE increasing from 5.7–8.0 ppm at Sm to 34–42 ppm at La. The 227

compositions are close to those predicted from modelling of the initial starting 228

compositions and expected abundances of crystallised zircon. Errors on these analyses 229

are based on propagation of the analytical uncertainties (< 5–15% error on each 230

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element) along with the point-to-point variations in concentration (2–4 melt analyses 231

in each experiment). 232

The Zr contents of the zircon-saturated melts produced in these experiments is 233

compared with previous experimental and modelling studies (Watson & Harrison, 234

1983; Boehnke et al., 2013) that demonstrate a consistent and systematic variation of 235

saturation Zr content with temperature. This comparison is depicted in terms of 236

ln(DZr) plotted against inverse temperature (10000/T) in Fig. 2. The melt Zr data from 237

this study vary systematically with inverse temperature, as required by zircon–melt 238

equilibrium, but are slightly offset in Zr concentration to higher values than those 239

given by the predictive model of Watson & Harrison (1983) and its updated 240

calibration by Boehnke et al. (2013). This offset (to higher ln(DZr) at a given T) may 241

be an effect of the Ca-free chemistry of the model granitic melts in our runs. 242

Melt compositions in all experiments approximate those of granitic minimum 243

melts in near-haplogranitic systems (NKFMASH), and lie near the model melt 244

composition of Carrington & Harley (1995) for their intermediate XMg composition 245

[XMg = 0.31–0.52]. Melt REE contents are similar to HREE-depleted granite 246

compositions and differ from the starting gel granite composition in being depleted in 247

HREE following the growth of HREE-bearing phases, as expected and inherent in the 248

experimental design. 249

Zircon compositions and zircon/melt distribution coefficients (DREE) 250

Between 30 and 45 SIMS analyses of zircon-melt mix areas with differing 251

proportions of zircon were used to calculate the REE contents of the average zircon 252

produced in each experiment. Each zircon REE (e.g. Dy, Yb) was estimated 253

individually using linear regression of ppm (REE) against Zr wt% in the analyses, and 254

the zircon REE patterns then established collectively from all of the element-specific 255

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regressions (see Appendix S1 for further details and example regressions). Maximum 256

Zr contents for the zircon–melt mix regression lines used to reconstruct these zircon 257

MREE–HREE compositions were between 75,000–130,000 ppm (approx. 15–25% 258

zircon). Values for the zircon REE concentrations are presented in Table 4 and Fig. 259

3a. 260

DREE (zircon/melt) values are presented in Table 5 and Fig. 4a. DREE are 4–5 at 261

Sm, decrease to near unity at Eu (though this is difficult to measure with high 262

precision given the low concentrations in each phase) and then increase towards the 263

HREE (DHREE), with DGd 8–14, and DLu 52–68. These DREE are comparable to those 264

obtained in previous experimental studies (e.g. Dickinson et al., 1981; Okano et al., 265

1987; Rubatto & Hermann, 2007a) and are very similar to the natural rock study of 266

zircon and related melt inclusions by Thomas et al. (2002). 267

Garnet/melt distribution coefficients (DREE) 268

Garnet and melt major element and REE compositions are presented in Tables 3 and 269

4. DREE (garnet/melt) values are presented in Table 5 and Fig. 4b. The XMg values for 270

each phase are close to those expected from the initial bulk composition, and define 271

Kd (Fe–Mg) values (0.83–1.87) that conform to previous equilibrium experiments in 272

KFMASH and related systems (Ellis, 1986; Carrington & Harley, 1995). All garnet 273

REE analyses were on areas of grains large enough for single spots using SIMS 274

analysis with a field aperture in the secondary column. A test study of garnet–melt 275

partitioning was performed in a zircon-absent run (z26-1, 900 °C, 7 kbar), to enable 276

the determination of DREE (garnet/melt) in a simple, single-mineral experiment based 277

on multiple garnet and glass analyses. DREE (garnet/melt) values show the LREE 278

partition into the melt, while the middle to heavy REE partition strongly into the 279

garnet, with DGd ~10 and DHREE ~100. The glass was analysed using the electron 280

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microprobe to determine major element compositions along with Zr and Y. The 281

analyses showed that there was no significant or consistent zoning in the glass 282

surrounding the garnet in these runs, with variations within 2% being smaller than 283

analytical error. High resolution EDS images of Fe and Me (Fig. 1e–f; run z33, 1000 284

°C, 7 kbar) show flat profiles in both the garnet and the surrounding melt in terms of 285

major element composition. A close approach to major element equilibrium is 286

therefore suggested, supported by the measured Grt/melt Fe–Mg KD values that are 287

consistent with the results of Ellis (1986) and Carrington & Harley (1995). The 288

systematic variation of XMg in garnet relative to melt with temperature is consistent 289

with previous experimental data (e.g. Ellis, 1986) and also reported by Rubatto & 290

Hermann (2007a) as supporting garnet growth during the final run temperature (not 291

the cooling step). Care had to be taken in the garnet analyses, as zircon inclusions 292

were often present at or just beneath the surface (e.g. Fig. 1b, d). Careful selection of 293

target areas from BSE images combined with attention to Zr content in the analysis 294

was used to discriminate zircon-contaminated analyses. The very shallow depth of 295

analysis pits from the ion microprobe decreases the likelihood of intersecting zircon 296

inclusions beneath the surface. 297

Four runs, ranging from 900–1000 °C, produced clean garnet analyses, free 298

from contamination by zircon grains or glass. DREE (garnet/melt) values for the 299

zircon-bearing runs (DGd 5–19, DHREE ~ 40–80) closely match those from the zircon-300

absent run (z26-1), despite very different total REE concentrations, showing that the 301

presence of zircon has not prevented the garnet grains from achieving REE 302

equilibrium with the melt. Whilst D (garnet/melt) values for the HREE are very 303

similar across the temperature range studied, D (garnet/melt) values for the MREE 304

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show a tendency to decrease with increasing temperature, consistent with the trends 305

reported by Nicholls & Harris (1980). 306

Zircon/garnet distribution coefficients (DREE) at 7 kbar 307

The garnet REE concentrations from ion microprobe analysis, combined with zircon 308

REE concentrations produced from regression analysis in the same run can be used to 309

determine DREE (zircon/garnet) values for 900–1000 °C and 7 Kbar. This was done 310

for all four experiments that produced clean garnet data (900 °C, z26-4; 950°C, z31-2; 311

1000 °C, z33-2 & z33-4). Two further experiments at 950 °C (z31-1 & z31-3) 312

produced zircon data, but the garnet analyses were contaminated by small melt 313

inclusions, identified by an increased LREE content without any increase in measured 314

Zr. The DREE (zircon/melt) data for these two capsules was combined with the DREE 315

(garnet/melt) for run z31-2 to produce two additional DREE (zircon/garnet) datasets for 316

950 °C. This was deemed a valid approach as all three experimental capsules were 317

from the same run, using the same starting material and run conditions. Data for the 318

DREE (zircon/garnet) is presented in Table 5 and Fig. 4c. Errors on calculated DREE 319

(zircon/garnet) are based on the 95% confidence limits for the regressed zircon 320

composition and the analytical error on the glass and garnet compositions from ion 321

microprobe analysis, leading to typical errors in DHREE of ±0.1–0.3 at Lu and ± 0.1 at 322

Dy, and of ± 0.2–0.7 at Sm. 323

All DREE (zircon/garnet) results show the same general pattern, broadly a 324

‘concave up’ shape with Sm favouring zircon, the MREE favouring garnet and the 325

HREE approximately equipartitioned (i.e. DHREE (zircon/garnet) = 1). DSm shows 326

moderate variation, probably as a consequence in the errors in extrapolation of zircon 327

Sm values from the mixing lines when D (zircon/melt) is close to unity. Although D 328

values for Eu are also variable, due to the low concentrations present in the 329

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experiments, the average DEu is ~1. Eu therefore appears to mark the ‘pivot’ where 330

the REE change from favouring zircon, in the LREE (see later modeling) and Sm, to 331

favouring garnet for Gd and beyond through the HREE. For the majority of runs the 332

strongest partitioning into garnet occurs at Dy, with DDy (zircon/garnet) ranging from 333

0.5–0.7 ± 0.1–0.2. DHREE approach values around unity at Er and beyond, e.g. DLu 334

varies from 0.6–1.3 ± 0.1–0.3. One run at 1000 °C (z33-2) recorded DMREE-HREE 335

values favouring garnet (~0.4–0.6; Fig. 4c), whilst the other 1000 °C run, which had 336

lower REE in zircon, resulted in a similar DMREE–HREE (zircon/garnet) pattern but at 337

slightly higher D values (0.6–0.8). 338

The experimental design employed allows for these DMREE-HREE 339

(zircon/garnet) results to be tested by analysis of zircon inclusions in garnet. If there is 340

an order of magnitude variation in the DMREE–HREE values for these elements, 341

increasing towards the heavier HREE such as Yb (e.g. Rubatto, 2002; Hermann & 342

Rubatto, 2003; Buick et al., 2006), then garnet analyses with large numbers of zircon 343

inclusions, as monitored by image analysis and total Zr content, would show 344

significant increases in the HREE compared to inclusion-free garnet analyses. The 345

capsules that had large numbers of garnet grains were analysed with the deliberate 346

aim of sampling inclusion-bearing garnets for comparison with the ‘clean’ analyses 347

(examples of zircon inclusions can be seen in Fig. 1d). 348

Garnet analyses from capsule z33-2 (1000 °C) had Zr contents ranging from 349

~1000 ppm (inclusion poor) to over 20,000 ppm (many inclusions: estimated to be 350

equivalent to ~4 wt% zircon) covering over an order of magnitude variation in the 351

amount of zircon in the analysis. The HREE contents of the garnets only vary by 352

approximately 10% (<10% for the MREE), and show no correlation with Zr content, 353

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supporting the conclusion that the distribution of the HREE between zircon and 354

garnet is very close to 1:1 at 1000 °C. 355

356

DISCUSSION 357

Tests and limitations of experimental method 358

The methodology for determining DREE has been checked and assessed using the 359

alternative technique of estimating DREE based on modal analysis and mass balance. 360

The approach to equilibrium in these experiments has then been assessed as a group 361

using a comparison of the main DREE (zircon/melt) set with DREE determined from an 362

experiment with a different effective melt composition; a time study of zircon REE 363

compositional change using results from a short-duration run and one longer-duration 364

run; and a determination of the effects of texturally recognizable disequilibrium, in 365

the form of microlite crystallization in melt, on resultant DREE. The short duration run 366

and textural disequilibrium runs are described in the 5 kbar pilot study (Appendix S1). 367

Zircon REE estimation: modal mass balance test of the mixing line approach 368

The zircon–melt mixing lines produced by spatially targeted microanalysis of 369

domains with differing zircon modes and hence wt% Zr can be compared, in simple 370

zircon–melt experiments, with zircon REE concentrations estimated based on image 371

analysis of the analysed ion microprobe pits, which are typically 20–25 µm in 372

diameter and 2–3 µm deep. As these pits are no deeper than the typical thickness of 373

the zircon grains the only material being analysed is that which was visible at the 374

surface of the original sample. 375

Appendix S1, Fig. 2 is a ‘before and after’ image showing the shallow depth 376

of the pits and their size in relation to the experimental zircons. The experimental run 377

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products were re-imaged using the SEM after they had been analysed on the ion 378

microprobe in order to pinpoint the exact location of the pits. The high BSE response 379

of zircon compared to the host glass enabled straightforward digital image review of 380

each area analysed and from that calculation of the modal proportion of zircon in each 381

data point on the regression line. This modal proportion data, regressed to 100%, 382

plotted against each REE provides a separate method for extrapolating zircon REE 383

concentration that is independent of the measured Zr concentration of each analysis 384

spot. The resultant zircon REE patterns based on this modal proportion method are 385

very similar to those determined using the mixing line approach (Appendix S1, Fig. 386

3). 387

Initial tests of approach to MREE–HREE equilibrium 388

Initial attempts at producing garnet resulted in an experiment (z26-6, conducted at 389

900 °C with 9 wt% H2O) that crystallised zircon and orthopyroxene (XMg = 0.35–0.5; 390

Al2O3 = 11–13%; up to 1 mm x 100 µm in size) instead of zircon and garnet. The 391

crystallization of orthopyroxene resulted in a melt chemistry that differed from the 392

zircon–garnet experiments in having an upward-sloping normalized HREE pattern, 393

consistent with mass balance considerations. The zircon HREE pattern calculated 394

using the mixing-line approach for this garnet-absent run was also upward sloping to 395

the heaviest HREE (LuN/GdN ~4). As a consequence, the resultant zircon/melt DHREE 396

values are identical to those calculated for the garnet-bearing experiments, DGd = 3.6 397

and DLu = 33.9. The maintenance of the same DHREE values despite the difference in 398

zircon and melt HREE contents provides good evidence in support of the attainment 399

of zircon–melt MREE–HREE equilibrium in the experiments as a whole. 400

Page 17 of 47

18

Further assessment of equilibrium 401

The validity of experimental approach to determining mineral/melt distribution 402

coefficients relies on both the zircon and garnet grains being in trace-element 403

equilibrium with the melt in which they form. 404

Zircon 405

The consistency of the zircon MREE–HREE data produced, despite whether they are 406

processed using chemical Zr mixing line or physical modal abundance criteria, shows 407

that the methodology is reliable, but does not in itself demonstrate equilibrium. A first 408

chemical test for equilibrium is whether the melt coexisting with zircon has a Zr 409

content consistent with zircon saturation at the experiment run conditions (Watson & 410

Harrison, 1983). The Zr contents of melts in our experiments vary systematically with 411

inverse temperature, consistent with a good approach to equilibrium (Fig. 2). 412

Apparent run temperatures calculated using our melt compositions and the Zr 413

saturation thermometer of Watson & Harrison (1983) and its updated re-calibration 414

(Boehnke et al., 2013) lie close to those of the experiments in this study (Fig. 2). The 415

reason for the positive offset in the values from this study compared to the modelled 416

values is unclear. The calculated ‘M’ values for the melts (0.8–1.1) fall outside the 417

range for which the Boehnke et al. (2013) model is calibrated (1.1–2.0). It is possible 418

that the lack of Ca in the bulk compositions used in our experiments is responsible for 419

this deviation from the expected model values, as Ca has twice the effect of the other 420

network modifiers when calculating the parameter ‘M’ (Watson & Harrison, 1983; 421

Boehnke et al., 2013). Rubatto & Hermann (2007a) demonstrated a negative offset in 422

Zr saturation data compared to the models (with a similar magnitude of variation as 423

that seen in this study), attributed to a possible pressure effect on the calibration that 424

is unlikely to be the case in this study. As well as a deviation from the expected model 425

Page 18 of 47

19

values there is also some variation between samples at the same temperature in this 426

study. The electron probe technique analyses a sample volume greater than that which 427

can be seen at the sample surface, potentially incorporating some minor proportion of 428

sub-surface zircon. 429

One source of disequilibrium lies in the very slow intracrystalline diffusion of 430

cations in zircon (Cherniak et al., 1997; Cherniak & Watson, 2003). For example, 431

Luo & Ayers (2009) and Burnham & Berry (2012) reported obvious sector and 432

asymmetric zoning using CL imaging in run product zircons of up to 50 µm diameter. 433

However, zircon grains produced in this study have maximum thicknesses of 5 µm 434

(more typically 1–2 µm), and in any one zircon–melt microanalysis spot there are 435

numerous of these fine grains, which would average out any potential sub-micron 436

scale zoning. In addition to the issues of zoning within larger zircon grains, Luo & 437

Ayers (2009) also highlighted the possibility of zones of REE-depleted melt around 438

large zircon grains due to slow REE3+ diffusion in the melt. However, this problem 439

has been largely discounted in light of the study of Burnham & Berry (2012), who 440

noted that because Zr diffusion is an order of magnitude slower than the REE3+ in a 441

silicate melt (Koepke & Behrens, 2001) it will be the rate-limiting step in zircon 442

growth, and therefore REE3+ in zircon should be in equilibrium with the melt. 443

Diffusion of P is expected to occur at a very similar rate to the REE within the melt 444

(e.g. Rapp & Watson, 1986) and therefore P, an important charge balancing ion in 445

zircon REE substitution, is also not considered to be rate limiting in terms of REE 446

equilibrium. This is consistent with the results of our short-duration time study 447

experiment, in which anomalously high MREE in the zircon can be attributed to 448

retarded zircon growth relative to REE availability. Furthermore, the mixing line 449

regression technique used in this study and that of Rubatto & Hermann (2007a) is less 450

Page 19 of 47

20

susceptible to localized melt REE depletion. If the melt was REE-depleted due to 451

zircon growth then the analyses sampling a small zircon volume would measure a 452

different melt composition to those sampling a large proportion of zircon, resulting in 453

a curved or kinked mixing line. There is no such curvature in zircon–melt mixing 454

lines in this study, which extend up to 45 wt% zircon in some 5 kbar pilot study 455

experiments, suggesting no REE depletion in melt proximal to growing zircon has no 456

impact on the results. The similarity of the zircon MREE–HREE estimated from 457

modal mass balance methods (Appendix S1) with those determined using the mixing 458

line approach supports this conclusion. 459

The arguments made above for equilibrium REE contents in zircon apply only 460

to the MREE–HREE, which are clearly partitioned into zircon relative to melt and so 461

lead to statistically significant mixing lines. As the LREE do not partition strongly 462

into the zircon structure (e.g. Hoskin & Schaltegger, 2003) mixing line fits are not 463

statistically significant from these elements. These, and Sm, are instead estimated in 464

the next section using strain modelling anchored by our experimental zircon–melt 465

DREE determined from the REE from Sm to Lu. 466

Garnet 467

The attainment of garnet–melt equilibrium can be evaluated by considering the extent 468

of zoning within the product garnet crystals. Only minor zoning in XMg was observed 469

in the experimental garnets, with the most significant zoning observed (experiment 470

z33-2) amounting to variation in XMg of only 0.02, with no observable core to rim 471

variation. The EDS images of Fe and Mg seen in Fig. 1e–f highlight the absence of 472

zoning within the experimental garnets. The zoning in HREE was assessed using 473

electron microprobe analysis of Y. Under the EMPA analytical conditions Y could be 474

measured to ± 8% relative, for total concentrations of ~1400 ppm. Variation of Y 475

Page 20 of 47

21

within a grain was within or similar to analytical error, less than 10% relative, and no 476

pattern of variation from core to rim was observed. Combined with the absence of 477

zoning in the experimental glasses, this evidence suggests that the garnets grown in 478

this study closely approach chemical equilibrium with the melt. 479

In some experiments garnets contain fine inclusions of zircon. Whilst imaging 480

was used to avoid obvious inclusions there nevertheless is some potential for zircon 481

inclusions to occur below the polished surface in the garnet and so affect the resultant 482

DREE (zircon/garnet) values. The 1000 °C experiments (z33-2, z33-4) contained 483

enough garnet to determine statistically consistent Zr contents using the electron 484

probe. The resulting Zr content, ~930 ± 32 ppm, was lower than the lowest Zr content 485

recorded from SIMS analysis (~1300 ± 20 ppm). This suggests that at least some 486

zircon, equivalent to 0.1 wt% of the analysed spot, may have been included in even 487

the best SIMS analysis. As a ‘worst case scenario’ test the data was modelled with all 488

the Zr in the SIMS analyses assumed to be due to zircon inclusions. The REE content 489

representing this proportion of zircon was removed from the garnet data to produce a 490

model zircon-free, ‘pure’ garnet and the DREE (zircon/garnet) values recalculated for 491

this new, modelled garnet composition. One experiment (z31-2, 950 °C) produced 492

‘model’ DREE (zircon/garnet) values that were ~1% (relative) different from the 493

measured data, while in the other 3 experiments the differences were less than 1%. 494

These calculations demonstrate that any unrecognized zircon inclusions in apparently 495

clean garnet analyses only influence the DMREE–HREE (zircon/garnet) data well within 496

their uncertainties and are therefore ignored. 497

Strain modelling of mineral–melt DREE 498

The measured zircon/melt and garnet/melt MREE–HREE distribution coefficients 499

have been plotted on Onuma Diagrams (Onuma et al., 1968)(e.g. Fig. 5a–d), which 500

Page 21 of 47

22

show the dependence of DREE values on ionic radius of the partitioning cation. The 501

logarithm of the distribution coefficients defines a parabolic shape on these diagrams, 502

with the apex of the parabola being at the optimal cation size. 503

Zircon–melt 504

In the case of zircon the distribution coefficients for the HREE from Lu to the MREE 505

at Sm, with the exception of Eu, show a strong dependence on ionic radius that fits 506

this parabolic shape, with a systematic increase in DREE value towards the smaller 507

ionic radii HREE. This is consistent with the ionic radius of the Zr4+ site being 508

smaller than the REE3+ cations. Ionic radii are obtained from Shannon (1976). 509

The experimental data has been evaluated using the mineral–melt partitioning 510

model developed by Blundy & Wood (1994)(Equation 1) which links the parabolic 511

shape of the Onuma diagram to the elastic properties of the site into which the cation 512

is substituting. The key variables in this modelling are the idealised ‘strain free’ site 513

radius (r03+) and the elastic response parameter (E

3+), for which fitted D0 values are 514

defined from the height of the Onuma parabola at the radius r03+, 515

516

Di = D0(M )

n+ × exp−4πNAEM

n+ 1

2r0(M )

n+ ri − r0(M )

n+( )2

+1

3ri − r0(M )

n+( )3

RT

. (1) 517

518

In this study the elastic strain model has been fitted assuming a relationship 519

between r0 and E (e.g. Draper & van Westrenen, 2007; van Westrenen & Draper, 520

2007), which for zircon-melt systems is anchored by the study of (Hanchar et al., 521

2001), rather than constraining r0 to the radius of Zr (e.g. Luo & Ayers, 2009). This 522

method (referred to in Table 6 as Hanchar and van Westrenen method) is preferred 523

Page 22 of 47

23

over a fixed r0, as many recent studies (e.g. van Westrenen & Draper, 2007) have 524

shown that r0 in almost all minerals is rarely identical to the radius of the site into 525

which the ion is partitioning, and can also vary with pressure and temperature (van 526

Westrenen & Draper, 2007). It is essential to fit the strain model to the zircon–melt 527

experiments in this study using this method, because of the absence of Sc, which 528

would constrain the other (low ionic radius) side of the parabola. As with the 529

measured DREE values the modelled values show no statistical variation with 530

temperature when examined using this approach. 531

Deviations of DEu from this simple one site strain model are systematic, and 532

can be explained by the Eu being dominantly present in the divalent state in the 533

experimental zircons. The values used to fit the parameters of the strain modelling 534

equation can be found in Table 6, with D0 representing the partition coefficient for an 535

ideally sized cation, E represents the elastic response of the site, and r0 the ionic 536

radius of the site. 537

The strain model approach anchored by our MREE–HREE zircon–melt data 538

allow estimates to be made for the LREE in zircon. The resultant values (Table 7) are 539

lower than all experimental and some empirical studies (e.g. Hinton & Upton, 1991; 540

Thomas et al., 2002), but lie close to the values of Sano et al. (2002). The 541

combination of the statistically reliable DMREE–HREE values obtained in the 542

experiments with the DLREE values calculated from strain modelling as fitted to those 543

MREE–HREE data is considered to provide the most reliable and internally consistent 544

set of DREE that can be extracted from this study (Table 7). 545

Garnet–melt 546

The measured DREE (garnet/melt) values show a very good correlation with ionic 547

radius, with peak D values occurring in the middle to heavy REE (Fig. 4b). Attempts 548

Page 23 of 47

24

to fit these measured DREE (garnet/melt) values to the strain model using 549

thermodynamic constraints and predictive modelling (Draper & van Westrenen, 2007; 550

van Westrenen & Draper, 2007) did not provide a good match with the measured 551

values, as the ‘best fit’ r0 values are too small. The predictive model produced from 552

those studies is particularly suited for anhydrous melting experiments typically at 2–3 553

GPa with garnets comprising a majorite component, and the authors state that it 554

poorly represents H2O-bearing experiments, and studies in lower pressure systems 555

with pyropic garnet (Draper & van Westrenen, 2007). In contrast, unconstrained 556

modelling of the lattice strain parameters (Blundy & Wood, 1994) yields a very good 557

fit to the experimental DREE (garnet/melt) data, aided by the ideal cation size (r03+ = 558

0.93– 0.98) being within the range for the MREE. 559

DREE (zircon/garnet) values (Fig. 6 and Table 7) have been produced from the 560

strain modelling equations by dividing the DREE (zircon/melt) by the DREE 561

(garnet/melt) modelled data. The key variables/parameters fitted to the experimental 562

results are summarised in Table 6. The derived DMREE–HREE lie near unity or slightly 563

favour garnet (<1), as expected from the measured experimental zircon/garnet data. 564

The 1000 °C modelled data are lower through the HREE and display a flatter DHREE 565

(zircon/garnet) pattern favouring garnet compared with the lower-T modelled data, 566

features attributable to a slightly smaller modelled D0 for garnet–melt at this 567

temperature. Whilst, as expected, the strain modelled data are very similar to the 568

measured values for the MREE–HREE, the modelled fits provide the only realistic set 569

of DREE values for the LREE due to the difficulty in measuring these elements using 570

the methods in this study. Therefore the strain-modelled values extracted here provide 571

the best evaluation of DREE (zircon/garnet) values for the whole suite of REE at 7 kbar 572

and 900–1000 °C and for Fe–Mg garnets with intermediate XMg. 573

Page 24 of 47

25

Comparison to other experimental data 574

There is only one other experimental dataset constraining DREE (zircon/garnet) 575

partition coefficients. Rubatto & Hermann (2007a) performed a series of piston 576

cylinder experiments at 20 kbar and 800–1000 °C, and pioneered the use of the 577

mixing line method to determine the composition of zircon within the runs. Zircon 578

REE were obtained from mixing lines with maximum Zr contents equivalent to ~5% 579

zircon, compared with up to 45% in this study. Rubatto & Hermann (2007a) applied 580

several lines of argument to support equilibrium growth of the main mineral phases, 581

including Kd (Fe–Mg) between garnet and melt consistent with previous studies (e.g. 582

Ellis, 1986), and systematic variation of Zr with temperature in the melt (Watson & 583

Harrison, 1983). As demonstrated in previous sections, these lines of argument also 584

apply to this experimental study. 585

The results from Rubatto & Hermann (2007a) show the DREE (zircon/garnet) 586

values as being temperature-dependent (Fig. 7a). The DMREE–HREE (zircon/garnet) 587

pattern was close to unity for their high temperature runs (1000 °C), but at lower 588

temperatures the HREE increasingly favoured zircon over garnet, with a steep 589

partitioning pattern (Lu/Gd up to 10) being obtained at 850–800 °C. A direct 590

comparison between this study and the equivalent T range (900–1000 °C) in the 591

Rubatto & Hermann (2007a) experiments (Fig. 7a) shows that the latter has generally 592

higher DHREE (zircon/garnet) values, e.g. Yb always favours zircon (DYb = 1–3) The 593

results from Rubatto & Hermann (2007a) can be interpreted to explain the large range 594

of DREE (zircon/garnet) values from the literature (e.g. Harley et al., 2001; Rubatto, 595

2002; Rubatto & Hermann, 2003; Whitehouse & Platt, 2003; Hokada & Harley, 2004; 596

Kelly & Harley, 2005; Buick et al., 2006) as reflecting equilibrium at different 597

temperatures in different terranes, with the steeper DREE patterns being recorded from 598

Page 25 of 47

26

lower-T granulites and migmatites and the flat and near-equipartitioned DREE recorded 599

from UHT regions. However, this attractive explanation does not fit with known P–T 600

conditions of some of the studied areas. For example, the granulites described by 601

Whitehouse & Platt (2003) formed at 750–800 °C, but would require temperatures of 602

950–1000 °C in order for the measured DREE (zircon/garnet), if representing 603

equilibrium, to be consistent with the Rubatto & Hermann (2007a) experimental DREE 604

(zircon/garnet) values. The present study shows no significant correlation between 605

DMREE–HREE (zircon/garnet) and temperature for the T range 900–1000 °C. Over a 606

comparable T range Rubatto & Hermann (2007a) found a 3- to 4-fold increase in 607

DMREE–HREE for Dy, Er, Yb and Lu, and at 800–900 °C the HREE favoured zircon 608

very strongly over garnet (e.g. DYb(900 °C) = 4; DYb(800 °C) = 10). These differences 609

in DREE between the two experimental studies may reflect the different pressures 610

under which the experiments were conducted (7 kbar vs. 20 kbar). Alternatively or 611

additionally, the differences in DREE behavior may arise from variations in garnet 612

chemistry. The steep DREE (zircon/garnet) values obtained at 800 °C in the Rubatto & 613

Hermann (2007a) experiments involved garnets with >20% grossular component, 614

whereas their DREE (zircon/garnet) values that lie closer to unity (950–1000 °C) 615

involved garnets with 7–8% grossular. These lower-Ca garnet DREE data are more 616

comparable with the present study that involves Ca-free garnet. 617

CONCLUSIONS 618

The experiments show a clear signature for the equilibrium distribution of the REE 619

between coexisting zircon and Fe–Mg garnet, DREE (zircon/garnet), relevant to high-620

grade crustal metamorphism at UHT P–T conditions of 900–1000 °C. The 621

experimentally-determined DREE (zircon/garnet) for MREE–HREE provide a robust 622

basis for site-strain based modelling that enable extension of the results to include the 623

Page 26 of 47

27

LREE. DREE (zircon/garnet) are therefore defined for all REE over the P–T range 624

relevant to granulite/UHT crustal metamorphism and anatexis of low-Ca pelites. 625

The experimental DREE (zircon/garnet) values determined in this study 626

describe a flat or concave-up pattern with DMREE–HREE near unity or slightly favouring 627

garnet (i.e. DREE<1). This distribution pattern is similar to that observed in a number 628

of natural rock studies from both UHT (Harley et al., 2001; Whitehouse & Platt, 629

2003; Hokada & Harley, 2004; Kelly & Harley, 2005; Harley & Kelly, 2007) and 630

800–900 °C granulite areas (Whitehouse & Platt, 2001)(Fig. 7b). 631

These experiments, and their modelling, do not result in the steep and 632

fractionated DMREE-HREE (zircon/garnet) patterns similar to those reported from other 633

studies of granulites (Rubatto, 2002; Hermann & Rubatto, 2003; Buick et al., 2006), 634

or evidence for any statistically significant temperature dependence of DREE 635

(zircon/garnet) values over the 900–1000 °C temperature range, in contrast to the 636

higher-pressure and Ca-bearing experiments of Rubatto and Hermann (2007a). The 637

reasons for these differences are difficult to evaluate, but may include disequilibrium 638

or non-equilibrium between zircon and garnet in some of the studied natural rock 639

examples, the effects of garnet Ca–Fe–Mg compositional variations, and/or the 640

influence of pressure on DREE in the case of experimental and some natural rock 641

studies. 642

643

Acknowledgements 644

This study was supported by the Natural Environment Research Council 645

(NERC Grant NE/B504157/1, S.L. Harley). We thank the members of the NERC 646

recognised Experimental Geoscience Facility and the Ion Microprobe Facility 647

(EMMAC) at the University of Edinburgh. We would like to acknowledge Mike 648

Page 27 of 47

28

Brown for editorial support, and Daniella Rubatto and one anonymous reviewer for 649

their constructive comments on the manuscript. Thanks also go to Ian Butler, Nic 650

Odling and Stephan Klemme for help with experimental techniques and John Craven 651

for analytical support. We acknowledge the Centre for Materials Research at Curtin 652

University. 653

654

References 655

656 Baldwin, J. A. & Brown, M., 2008. Age and duration of ultrahigh-temperature 657

metamorphism in the Anápolis-Itauçu Complex, Southern Brasília Belt, 658

central Brazil - Constraints from U-Pb geochronology, mineral rare earth 659 element chemistry and trace-element thermometry. Journal of Metamorphic 660 Geology, 26(2), 213-233. 661

Bea, F., Pereira, M. D. & Stroh, A., 1994. Mineral/leucosome trace-element 662 partitioning in a peraluminous migmatite (a laser ablation-ICP-MS study). 663 Chemical Geology, 117(1-4), 291-312. 664

Biggar, G. M. & O'Hara, M. J., 1969. A comparison of gel and glass starting materials 665 for phase equilibrium studies. Mineralogical Magazine, 36, 198-205. 666

Blundy, J. & Wood, B., 1994. Prediction of crystal-melt partition coefficients from 667 elastic moduli. Nature, 372(6505), 452-454. 668

Boehnke, P., Watson, E. B., Trail, D., Harrison, T. M. & Schmitt, A. K., 2013. Zircon 669 saturation re-revisited. Chemical Geology, 351, 324-334. 670

Brown, M., 2007. Metamorphic conditions in orogenic belts: A record of secular 671 change. International Geology Review, 49(3), 193-234. 672

Buick, I. S., Hermann, J., Williams, I. S., Gibson, R. L. & Rubatto, D., 2006. A 673 SHRIMP U-Pb and LA-ICP-MS trace element study of the petrogenesis of 674 garnet-cordierite-orthoamphibole gneisses from the Central Zone of the 675 Limpopo Belt, South Africa. Lithos, 88(1-4), 150-172. 676

Burnham, A. D. & Berry, A. J., 2012. An experimental study of trace element 677 partitioning between zircon and melt as a function of oxygen fugacity. 678

Geochimica et Cosmochimica Acta, 95, 196-212. 679 Carrington, D. P., 1993. Partial melting and phase relations in metapelitic granulites. 680 Carrington, D. P. & Harley, S. L., 1995. Partial melting and phase relations in high-681

grade metapelites: an experimental petrogenetic grid in the KFMASH system. 682 Contributions to Mineralogy and Petrology, 120(3-4), 270-291. 683

Cherniak, D. J., Hanchar, J. M. & Watson, E. B., 1997. Rare-earth diffusion in zircon. 684 Chemical Geology, 134(4), 289-301. 685

Cherniak, D. J. & Watson, E. B., 2003. Diffusion in zircon. In: Hanchar JM, Hoskin, 686 PWO (eds) Zircon. Reviews in Mineralogy 53, Mineralogical Society of 687 America, 113-143. 688

Clark, C., Collins, A. S., Santosh, M., Taylor, R. & Wade, B. P., 2009. The P-T-t 689 architecture of a Gondwanan suture: REE, U-Pb and Ti-in-zircon 690

Page 28 of 47

29

thermometric constraints from the Palghat Cauvery shear system, South India. 691 Precambrian Research, 174(1-2), 129-144. 692

Dickinson, J. E., Hess, P. C. & Rutherford, M. J., 1981. Zircon saturation and REE 693 partition coefficients in lunar basalts and granites. Lunar Planet. Sci., 12, 229-694 231. 695

Draper, D. S. & van Westrenen, W., 2007. Quantifying garnet-melt trace element 696 partitioning using lattice-strain theory: Assessment of statistically significant 697 controls and a new predictive model. Contributions to Mineralogy and 698 Petrology, 154(6), 731-746. 699

Ellis, D. J., 1986. Garnet-liquid Fe2+-Mg equilibria and implications for the 700 beginning of melting in the crust and subduction zones. American Journal of 701 Science, 286(10), 765-791. 702

Hamilton, D. L. & Henderson, C. M. B., 1968. The preparation of silicate 703 compositions by a gelling method. Mineralogical Magazine, 36, 832-838. 704

Hanchar, J. M., Finch, R. J., Hoskin, P. W. O., Watson, E. B., Cherniak, D. J. & 705 Mariano, A. N., 2001. Rare earth elements in synthetic zircon: Part 1. 706 Synthesis, and rare earth element and phosphorus doping. American 707 Mineralogist, 86(5-6), 667-680. 708

Harley, S. L., 1989. The origins of granulites: a metamorphic perspective. Geological 709

Magazine, 126(3), 215-247. 710 Harley, S. L. & Carrington, D. P., 2001. The distribution of H2O between cordierite 711

and granitic melt: H2O incorporation in cordierite and its application to high-712 grade metamorphism and crustal anatexis. Journal of Petrology, 42(9), 1595-713 1620. 714

Harley, S. L. & Kelly, N. M., 2007. The impact of zircon-garnet REE distribution 715 data on the interpretation of zircon U-Pb ages in complex high-grade terrains: 716 An example from the Rauer Islands, East Antarctica. Chemical Geology, 717 241(1-2), 62-87. 718

Harley, S. L., Kelly, N. M. & Möller, A., 2007. Zircon behaviour and the thermal 719

histories of moutain chains. Elements, 3(1), 25-30. 720 Harley, S. L., Kinny, P., Snape, I. & Black, L. P., 2001. Zircon chemistry and the 721

definition of events in Archean granulite terrains. In: Fourth International 722 Archean Symposium, Extended Abstract Volume, AGSO Geoscience Australia 723 Record, 2001/37, 511-513. 724

Harley, S. L. & Nandakumar, V., 2014. Accessory Mineral Behaviour in Granulite 725 Migmatites: a Case Study from the Kerala Khondalite Belt, India. Journal of 726 Petrology, 55(10), 1965-2002. 727

Hermann, J., 2002. Allanite: Thorium and light rare earth element carrier in subducted 728 crust. Chemical Geology, 192(3-4), 289-306. 729

Hermann, J. & Rubatto, D., 2003. Relating zircon and monazite domains to garnet 730 growth zones: Age and duration of granulite facies metamorphism in the Val 731 Malenco lower crust. Journal of Metamorphic Geology, 21(9), 833-852. 732

Hermann, J. & Rubatto, D., 2009. Accessory phase control on the trace element 733 signature of sediment melts in subduction zones. Chemical Geology, 265(3-4), 734 512-526. 735

Hinton, R. W. & Upton, B. G. J., 1991. The chemistry of zircon: Variations within 736 and between large crystals from syenite and alkali basalt xenoliths. 737 Geochimica et Cosmochimica Acta, 55(11), 3287-3302. 738

Hokada, T. & Harley, S. L., 2004. Zircon growth in UHT leucosome: Constraints 739

from zircon-garnet rare earth elements (REE) relations in Napier Complex, 740

Page 29 of 47

30

East Antarctica. Journal of Mineralogical and Petrological Sciences, 99(4), 741 180-190. 742

Hoskin, P. W. O. & Schaltegger, U., 2003. The composition of zircon and igneous 743 and metamorphic petrogenesis. Reviews in Mineralogy and Geochemistry, 53, 744 27-62. 745

Jiao, S., Guo, J., Harley, S. L. & Peng, P., 2013. Geochronology and trace element 746 geochemistry of zircon, monazite and garnet from the garnetite and/or 747 associated other high-grade rocks: Implications for Palaeoproterozoic 748 tectonothermal evolution of the Khondalite Belt, North China Craton. 749

Precambrian Research, 237, 78-100. 750 Kelly, N. M. & Harley, S. L., 2005. An integrated microtextural and chemical 751

approach to zircon geochronology: Refining the Archaean history of the 752 Napier Complex, east Antarctica. Contributions to Mineralogy and Petrology, 753 149(1), 57-84. 754

Koepke, J. & Behrens, H., 2001. Trace element diffusion in andesitic melts: An 755 application of synchrotron X-ray fluoresence analysis. Geochimica et 756 Cosmochimica Acta, 65(9), 1481-1498. 757

Korhonen, F. J., Clark, C., Brown, M., Bhattacharya, S. & Taylor, R., 2013. How 758 long-lived is ultrahigh temperature (UHT) metamorphism? Constraints from 759

zircon and monazite geochronology in the Eastern Ghats orogenic belt, India. 760 Precambrian Research, 234, 322-350. 761

Kotková, J. & Harley, S. L., 2010. Anatexis during high-pressure crustal 762 metamorphism: Evidence from garnet-whole-rock REE relationships and 763 zircon-rutile Ti-Zr thermometry in leucogranulites from the Bohemian Massif. 764 Journal of Petrology, 51(10), 1967-2001. 765

Luo, Y. & Ayers, J. C., 2009. Experimental measurements of zircon/melt trace-766 element partition coefficients. Geochimica et Cosmochimica Acta, 73(12), 767 3656-3679. 768

Marsh, J. H., Gerbi, C. C., Culshaw, N. G., Johnson, S. E., Wooden, J. L. & Clark, C., 769

2012. Using zircon U-Pb ages and trace element chemistry to constrain the 770 timing of metamorphic events, pegmatite dike emplacement, and shearing in 771 the southern Parry Sound domain, Grenville Province, Canada. Precambrian 772 Research, 192-195(1), 142-165. 773

Nicholls, I. A. & Harris, K. L., 1980. Experimental rare earth element partition 774 coefficients for garnet, clinopyroxene and amphibole coexisting with andesitic 775 and basaltic liquids. Geochimica et Cosmochimica Acta, 44(2), 287-308. 776

Okano, O., Watson, E. B. & Tatsumoto, M., 1987. Partition coefficients for REE and 777 Hf between zircon and liquid: Inferences for lunar granite petrogenesis. Lunar 778 planet. Sci., 74, 740-741. 779

Onuma, N., Higuchi, H., Wakita, H. & Nagasawa, H., 1968. Trace element partition 780 between two pyroxenes and the host lava. Earth and Planetary Science 781 Letters, 5(C), 47-51. 782

Rapp, R. P. & Watson, E. B., 1986. Monazite solubility and dissolution kinetics: 783 implications for the thorium and light rare earth chemistry of felsic magmas. 784 Contributions to Mineralogy and Petrology, 94(3), 304-316. 785

Rubatto, D., 2002. Zircon trace element geochemistry: Partitioning with garnet and 786 the link between U-Pb ages and metamorphism. Chemical Geology, 184(1-2), 787 123-138. 788

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Rubatto, D. & Hermann, J., 2003. Zircon formation during fluid circulation in 789 eclogites (Monviso, Western Alps): Implications for Zr and Hf budget in 790 subduction zones. Geochimica et Cosmochimica Acta, 67(12), 2173-2187. 791

Rubatto, D. & Hermann, J., 2007a. Experimental zircon/melt and zircon/garnet trace 792 element partitioning and implications for the geochronology of crustal rocks. 793 Chemical Geology, 241(1-2), 38-61. 794

Rubatto, D. & Hermann, J., 2007b. Zircon behaviour in deeply subducted rocks. 795 Elements, 3(1), 31-35. 796

Sano, Y., Terada, K. & Fukuoka, T., 2002. High mass resolution ion micropobe 797

analysis of rare earth elements in silicate glass, apatite and zircon: Lack of 798 matrix dependency. Chemical Geology, 184(3-4), 217-230. 799

Schoene, B., Latkoczy, C., Schaltegger, U. & Günther, D., 2010. A new method 800 integrating high-precision U-Pb geochronology with zircon trace element 801 analysis (U-Pb TIMS-TEA). Geochimica et Cosmochimica Acta, 74(24), 802 7144-7159. 803

Shannon, R. D., 1976. Revised effective ionic radii and systematic studies of 804 interatomic distances in halides and chalcogenides. Acta Crystallogr., 32, 751-805 767. 806

Taylor, R. J. M., Clark, C., Fitzsimons, I. C. W., Santosh, M., Hand, M., Evans, N. & 807

McDonald, B., 2014. Post-peak, fluid-mediated modification of granulite 808 facies zircon and monazite in the Trivandrum Block, southern India. 809 Contributions to Mineralogy and Petrology, 168(2). 810

Thomas, J. B., Bodnar, R. J., Shimizu, N. & Sinha, A. K., 2002. Determination of 811 zircon/melt trace element partition coefficients from SIMS analysis of melt 812 inclusions in zircon. Geochimica et Cosmochimica Acta, 66(16), 2887-2901. 813

Tomkins, H. S., Williams, I. S. & Ellis, D. J., 2005. In situ U-Pb dating of zircon 814 formed from retrograde garnet breakdown during decompression in Rogaland, 815 SW Norway. Journal of Metamorphic Geology, 23(4), 201-215. 816

van Westrenen, W. & Draper, D. S., 2007. Quantifying garnet-melt trace element 817

partitioning using lattice-strain theory: New crystal-chemical and 818 thermodynamic constraints. Contributions to Mineralogy and Petrology, 819 154(6), 717-730. 820

Watson, E. B. & Harrison, T. M., 1983. Zircon saturation revisited: temperature and 821 composition effects in a variety of crustal magma types. Earth and Planetary 822 Science Letters, 64(2), 295-304. 823

Whitehouse, M. J. & Platt, J. P., 2003. Dating high-grade metamorphism - Constraints 824 from rare-earth elements in zircon and garnet. Contributions to Mineralogy 825 and Petrology, 145(1), 61-74. 826

827

Supporting Information 828

Additional supporting information can be found in the online version of this article: 829

Appendix S1. Analytical techniques used in this study, and 5 kbar pilot study 830

describing the method for producing zircon-melt mixing lines and the calculation of 831

zircon REE concentrations. 832

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32

Please note: Wiley are not responsible for the content or functionality of any 833

supporting materials supplied by the authors. Any queries (other than missing 834

material) should be directed to the corresponding author for this article. 835

Figure captions 836

Fig 1. Images of run products showing the major phases present in the 837

experimental capsules during 7 kbar zircon-garnet experiments. a–d) BSE 838

images showing major phases present in the runs. Zircon is distinguished by its 839

acicular habit and bright BSE response. Glass, formed from the quench 840

crystallization of granitic melt has the lowest BSE response. Garnet is present as 841

equant grains up to 40μm often containing small inclusions of both zircon and 842

glass. Minor orthopyroxene is present in the glass and interpreted to have 843

formed during quenching of the experiments. e–f) EDS images of Fe and Mg in 844

garnet and melt showing no zonation in either phase. 845

846

Fig 2. Zr saturation in melt for the experiments in this study (symbols larger 847

than errors). Values are compared to calculated values from Watson and 848

Harrison (1983) and the updated melt model (Boehnke et al., 2013). Model 849

values based on calculated melt parameter ‘M’ for melt compositions used in this 850

study. 851

852

Fig 3. Experimentally derived REE concentrations from Sm to Lu in major phases 853

in this study at 900–1000 °C and 7 kbar. a) REE in zircon. b) REE in garnet. c) 854

REE in glass. 855

856

Fig 4. Experimental DREE values from Sm to Lu calculated for all 7 kbar 857

experiments in this study. a) DREE (zircon/melt). b) DREE (garnet/melt. c) DREE 858

(zircon/garnet). 859

860

Fig 5. Example Onuma diagrams with strain modeling parameters and fits for 861

zircon–melt and garnet–melt partitioning data from this study based on REE 862

from Sm to Lu. Zircon strain model fits based on Hanchar and van Westrenen 863

method (see text), garnet modelling based on unconstrained fits. a) z26-4 zircon-864

melt. b) z31-2 zircon-melt. c) z26-4 garnet-melt. d) z31-2 garnet-melt. 865

866

Fig 6. DREE (zircon/garnet) produced by strain modelling using modelled D 867

values for zircon/melt and garnet/melt. DREE (zircon/melt) modelling is pinned 868

by the work of Hanchar et al. (2001). DREE (garnet/melt) values are produced 869

using unconstrained fits of good quality based on Blundy and Wood (1994). 870

871

Fig 7. a) Comparison of the strain modelled data produced from this study with 872

the DREE (zircon/garnet) experiments of Rubatto and Hermann (2007a). There is 873

a strong similarity between this study with the highest temperature results of 874

Rubatto and Hermann (2007a), which also contain garnets with the closest 875

composition to this study in terms of being low in Ca content. b) Comparison of 876

the strain modelled data produced from this study with empirically determined 877

Page 32 of 47

33

DREE (zircon/garnet) values interpreted to represent zircon-garnet equilibrium 878

(Harley et al., 2001; Rubatto, 2002; Hermann & Rubatto, 2003; Whitehouse & 879

Platt, 2003; Hokada & Harley, 2004; Kelly & Harley, 2005; Buick et al., 2006). 880

881 882

Page 33 of 47

140x177mm (300 x 300 DPI)

Page 34 of 47

136x133mm (300 x 300 DPI)

Page 35 of 47

214x421mm (300 x 300 DPI)

Page 36 of 47

214x435mm (300 x 300 DPI)

Page 37 of 47

150x188mm (300 x 300 DPI)

Page 38 of 47

154x115mm (300 x 300 DPI)

Page 39 of 47

175x256mm (300 x 300 DPI)

Page 40 of 47

Table 1. Starting material compositions

Gel 3 (G3) Granite + Garnet Gel 4 (G4) Granite + Garnet + Zircon

Wt% oxide ppm REE ppm (n) Wt% oxide ppm REE ppm (n)

SiO2 65.5 Si 305803 SiO2 60.0 Si 279438

Al2O3 16.2 Al 85632 Al2O3 14.0 Al 73821

FeO 6.7 Fe 52009 FeO 6.2 Fe 48010

MgO 3.1 Mg 18668 MgO 2.9 Mg 17421

Na2O 3.4 Na 25191 Na2O 2.8 Na 20695

K2O 5.1 K 42278 K2O 4.2 K 34732

ZrO2 0.0 Zr 200 ZrO2 9.9 Zr 73213

P 649.1 P 647.5

Y 89.9 Y 149.4

Hf 3.0 Hf 2393.9

Li 144.8 Li 119.5

La 41.4 La 176.3 La 41.3 La 175.8

Ce 91.9 Ce 152.3 Ce 91.6 Ce 151.9

Pr 9.3 Pr 104.1 Pr 9.3 Pr 103.8

Nd 34.4 Nd 76.0 Nd 34.3 Nd 75.8

Sm 8.6 Sm 58.7 Sm 8.6 Sm 58.6

Eu 0.6 Eu 11.1 Eu 0.6 Eu 11.1

Gd 16.0 Gd 81.2 Gd 19.2 Gd 97.8

Tb 2.8 Tb 76.5 Tb 3.6 Tb 98.2

Dy 17.9 Dy 73.6 Dy 23.8 Dy 98.1

Ho 3.7 Ho 66.3 Ho 5.3 Ho 94.6

Er 11.3 Er 70.8 Er 16.3 Er 102.8

Tm 1.7 Tm 70.0 Tm 2.6 Tm 108.1

Yb 11.2 Yb 68.7 Yb 18.2 Yb 111.9

Lu 1.6 Lu 66.7 Lu 2.5 Lu 101.0

Page 41 of 47

Table 2. Run Conditions and major phases

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4 z26-1 z26-6

T (°C) 1000 1000 950 950 950 900 900 900

P (kbar) 7 7 7 7 7 7 7 7

Length (hrs) 225 225 220 220 220 264 264 264

% H2O 3.1 3.4 3.0 3.3 3.1 3.1 3.1 9.4

Run product phases

Glass x x x x x x x x

Zircon x x x x x x x

Garnet x x x x x x x

Opx x x

Page 42 of 47

Table 3. Glass major element and REE compositions

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4 z26-1

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C 900°C

P (kbar) 7 7 7 7 7 7 7

1σ 1σ 1σ 1σ 1σ 1σ 1σ

Major Elements

SiO2 72.0 ± 1.6 70.8 ± 1.0 72.1 ± 1.5 70.7 ± 1.7 70.1 ± 0.4 68.6 ± 2.4 71.7 ± 0.4

Al2O3 14.6 ± 0.4 14.2 ± 0.9 14.0 ± 0.5 13.7 ± 0.2 14.2 ± 0.8 13.8 ± 0.4 14.6 ± 0.3

FeO 3.1 ± 0.6 4.1 ± 0.6 2.2 ± 1.3 3.3 ± 1.4 3.8 ± 1.5 5.0 ± 1.7 1.7 ± 0.2

MgO 0.8 ± 0.7 1.6 ± 0.1 0.6 ± 1.2 1.6 ± 1.1 1.7 ± 1.5 3.0 ± 1.5 0.4 ± 0.1

Na2O 2.8 ± 1.0 3.3 ± 0.1 3.9 ± 0.5 3.7 ± 0.2 3.7 ± 0.3 3.2 ± 0.3 4.1 ± 0.0

K2O 3.4 ± 0.4 3.4 ± 0.1 3.7 ± 0.3 3.5 ± 0.2 3.5 ± 0.2 3.4 ± 0.3 4.2 ± 0.0

Total 96.6 97.4 96.5 96.4 96.9 97.0 96.7

XMg 0.16 0.23 0.17 0.27 0.26 0.32 0.16

ASI 1.8 1.6 1.4 1.4 1.4 1.5 1.3

AFM 0.7 0.6 0.8 0.6 0.6 0.5 0.8

Zr (ppm) 310 447 270 214 287 175 129

REE ppm 1σ 1σ 1σ 1σ 1σ 1σ 1σ

La 34.4 ± 0.1 34.0 ± 0.6 39.9 ± 0.9 38.9 ± 0.5 40.2 ± 0.6 34.2 ± 0.3 41.7 ± 1.9

Ce 76.1 ± 2.0 76.6 ± 3.3 89.0 ± 2.5 87.9 ± 1.6 92.0 ± 1.5 77.2 ± 0.6 97.7 ± 2.2

Pr 6.9 ± 0.2 6.9 ± 0.4 8.4 ± 0.4 7.8 ± 0.2 8.5 ± 0.2 6.9 ± 0.1 9.0 ± 0.7

Nd 27.5 ± 0.1 27.9 ± 0.6 35.3 ± 0.4 31.1 ± 0.9 34.9 ± 1.0 27.4 ± 0.9 36.8 ± 0.6

Sm 5.7 ± 0.1 6.0 ± 0.5 6.4 ± 0.3 6.8 ± 0.4 6.4 ± 0.4 6.0 ± 0.4 8.0 ± 0.1

Eu 0.5 ± 0.03 0.5 ± 0.04 0.6 ± 0.03 0.6 ± 0.1 0.7 ± 0.1 0.5 ± 0.1 0.8 ± 0.03

Gd 9.6 ± 0.1 9.4 ± 0.1 9.1 ± 0.2 8.1 ± 0.3 8.7 ± 0.4 7.2 ± 0.3 17.1 ± 0.5

Tb 1.6 ± 0.2 1.4 ± 0.1 0.9 ± 0.1 1.2 ± 0.1 1.1 ± 0.1 1.1 ± 0.1 3.0 ± 0.1

Dy 7.7 ± 0.9 6.0 ± 0.8 5.1 ± 0.6 6.0 ± 0.6 5.2 ± 0.6 5.3 ± 0.3 16.6 ± 0.1

Ho 1.2 ± 0.03 0.9 ± 0.2 1.1 ± 0.1 1.0 ± 0.1 0.8 ± 0.1 0.9 ± 0.1 3.5 ± 0.2

Er 3.5 ± 0.02 2.8 ± 0.4 2.4 ± 0.4 2.1 ± 0.2 2.2 ± 0.2 1.9 ± 0.2 9.2 ± 0.8

Tm 0.5 ± 0.1 0.3 ± 0.04 0.3 ± 0.1 0.3 ± 0.1 0.3 ± 0.1 0.3 ± 0.03 1.5 ± 0.05

Yb 2.8 ± 0.4 2.1 ± 0.1 2.0 ± 0.8 2.3 ± 0.3 1.7 ± 0.3 2.0 ± 0.2 9.8 ± 2.0

Lu 0.3 ± 0.1 0.3 ± 0.04 0.3 ± 0.1 0.3 ± 0.1 0.2 ± 0.1 0.3 ± 0.03 1.2 ± 0.2

Y 27.8 ± 1.2 22.8 ± 0.1 16.6 ± 1.0 17.1 ± 0.6 17.5 ± 0.8 18.5 ± 0.7 11.7 ± 0.9

Hf 15.8 ± 5.6 15.2 ± 2.3 8.3 ± 3.8 6.7 ± 3.2 8.7 ± 4.6 13.0 ± 3.1 8.2 ± 3.8

P 1130.7 ± 27.3 1148.4 ± 2.5 1104.3 ± 7.7 1234.5 ± 11.0 1105.5 ± 17.9 1008.0 ± 9.9 636.9 ± 12.4

Trace element errors calculated from multiple SIMS analyses of glass

XMg = Mg/(Fe+Mg) (molar) ASI = Al/(Na+K) (molar) AFM = Al/(Al+Fe+Mg) (molar)

Page 43 of 47

Table 4. Zircon REE and garnet major element & REE composition

REE composition (ppm and ±1σ) of zircon

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

REE ppm

La 21.7 ± 10.5 28.4 ± 7.0 29.3 ± 10.2 39.2 ± 11.9 29.5 ± 17.0 32.6 ± 23.9

Ce 68.5 ± 22.2 83.3 ± 18.8 85.0 ± 21.8 105.9 ± 28.5 84.1 ± 37.6 98.3 ± 55.6

Pr 5.9 ± 2.5 9.5 ± 2.0 8.8 ± 1.8 11.9 ± 2.2 8.5 ± 3.2 8.9 ± 4.9

Nd 38.1 ± 11.3 43.1 ± 10.2 50.1 ± 9.6 65.1 ± 12.2 52.9 ± 16.7 58.8 ± 20.4

Sm 24.9 ± 4.5 20.0 ± 4.3 27.2 ± 4.4 32.7 ± 3.0 32.2 ± 3.8 32.9 ± 5.7

Eu 1.3 ± 0.6 1.8 ± 0.6 1.8 ± 0.6 2.0 ± 0.5 1.4 ± 0.5 1.5 ± 0.8

Gd 57.3 ± 11.4 62.2 ± 15.8 77.3 ± 16.4 94.6 ± 15.0 97.6 ± 14.6 99.6 ± 12.2

Tb 13.3 ± 2.9 15.3 ± 3.6 17.0 ± 3.8 19.9 ± 3.7 20.0 ± 3.5 21.0 ± 3.5

Dy 76.0 ± 19.3 95.9 ± 21.5 109.9 ± 22.3 113.1 ± 24.5 124.6 ± 26.6 112.4 ± 37.3

Ho 22.4 ± 5.6 27.0 ± 5.4 28.3 ± 5.4 29.0 ± 6.3 31.9 ± 7.1 31.8 ± 6.4

Er 73.6 ± 14.3 86.7 ± 14.6 97.2 ± 13.7 92.6 ± 19.5 98.8 ± 21.4 94.4 ± 21.1

Tm 14.4 ± 2.7 16.4 ± 2.4 17.8 ± 2.7 16.8 ± 3.2 17.9 ± 3.7 16.2 ± 3.6

Yb 99.2 ± 15.4 111.9 ± 17.6 116.7 ± 15.3 115.2 ± 20.3 124.5 ± 24.2 115.3 ± 24.6

Lu 14.4 ± 2.5 15.4 ± 2.4 15.1 ± 2.3 14.8 ± 2.8 17.2 ± 3.1 15.0 ± 3.1

Y 536 ± 97 625 ± 103 667 ± 106 710 ± 111 381 ± 131 716 ± 127

Hf 17241 ± 356 16964 ± 690 19015 ± 237 17770 ± 373 17530 ± 1171 19759 ± 305

P 2084 ± 227 1656 ± 123 2046 ± 143 2232 ± 747 1181 ± 614 1789 ± 476

Errors on zircon trace elements calculated as 95% confidence limits on linear regression

Major element (Wt% oxide and ±1σ) REE composition (ppm and ±1σ) of garnet

Run No. z33-2 z33-4 z31-1* z31-2 z31-3*§

z26-4 z26-1§

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C 900°C

P (kbar) 7 7 7 7 7 7 7

Major Elements 1σ 1σ 1σ 1σ 1σ 1σ 1σ

SiO2 39.5 ± 0.9 39.5 ± 0.3 39.4 ± 0.1 40.3 ± 0.8 40.2 ± 1.3 37.9 ± 0.6 39.6 ± 0.7

Al2O3 22.6 ± 0.5 22.5 ± 0.2 22.3 ± 0.1 21.7 ± 0.3 22.8 ± 0.4 21.6 ± 0.2 22.1 ± 0.3

FeO 26.0 ± 0.3 26.0 ± 0.3 26.2 ± 0.2 26.5 ± 0.4 25.4 ± 0.6 26.4 ± 0.4 27.3 ± 0.3

MgO 12.2 ± 0.5 11.4 ± 0.3 11.4 ± 0.1 11.2 ± 0.5 11.0 ± 0.8 13.2 ± 0.4 10.3 ± 0.3

Na2O 0.01 ± 0.01 ± 0.01 ± 0.04 ± 0.1 0.18 ± 0.2 0.03 ± 0.1 0.09 ± 0.1

K2O 0.02 ± 0 ± 0.02 ± 0.08 ± 0.1 0.14 ± 0.2 0.02 ± 0.1 0.05 ± 0.1

Total 100.3 99.27 99.34 99.85 99.67 99.17 99.36

XMg 0.46 0.44 0.44 0.43 0.44 0.47 0.40

S/FM 0.99 1.02 1.01 1.04 1.07 0.90 1.04

AFM 0.25 0.25 0.25 0.25 0.26 0.23 0.25

REE ppm 1σ 1σ 1σ 1σ 1σ

La 1.0 ± 0.9 0.3 ± 1.0 5.8 ± 1.5 2.8 ± 1.0 11.0 0.8 ± 0.5 0.7

Ce 2.5 ± 1.4 1.5 ± 1.6 13.8 ± 2.9 7.3 ± 1.6 24.3 3.4 ± 0.8 4.0

Pr 0.5 ± 0.3 0.6 ± 0.3 1.6 ± 0.3 1.2 ± 0.3 2.6 1.4 ± 0.2 1.2

Nd 5.4 ± 1.5 8.5 ± 1.7 12.0 ± 0.7 12.5 ± 1.7 13.5 18.4 ± 1.4 17.0Sm 9.1 ± 0.8 13.3 ± 0.9 16.7 ± 0.0 20.7 ± 0.9 13.4 33.4 ± 0.8 30.7

Eu 0.8 ± 0.1 1.1 ± 0.2 1.5 ± 0.2 2.0 ± 0.2 1.3 3.6 ± 0.1 2.4

Gd 47.8 ± 1.2 56.8 ± 1.3 70.2 ± 3.8 89.6 ± 1.3 53.5 135.6 ± 1.4 196.7

Tb 20.5 ± 0.3 22.5 ± 0.3 23.7 ± 1.5 29.5 ± 0.3 17.7 40.8 ± 0.3 80.7

Dy 176.8 ± 1.9 170.0 ± 1.9 166.7 ± 11.4 203.9 ± 1.9 125.2 259.3 ± 1.9 736.3

Ho 56.9 ± 0.4 46.3 ± 0.4 41.6 ± 2.2 48.7 ± 0.4 32.9 61.1 ± 0.4 237.3

Er 182.9 ± 1.4 147.4 ± 1.4 112.6 ± 5.1 128.7 ± 1.4 92.8 146.7 ± 1.4 808.8

Tm 33.1 ± 0.3 25.6 ± 0.3 18.4 ± 0.6 19.5 ± 0.3 15.5 23.5 ± 0.3 151.9

Yb 213.7 ± 1.4 162.9 ± 1.4 113.7 ± 2.7 121.5 ± 1.5 97.5 148.3 ± 1.4 1055.4

Lu 24.1 ± 0.2 19.0 ± 0.2 12.4 ± 0.8 13.3 ± 0.2 11.0 17.8 ± 0.2 130.9

*high LREE values are the result of unavoidable melt inclusions incorporated in garnet analysis

§ Garnet REE analyses based on single SIMS analysis for this sample, therefore have not propogated error for the composition

XMg = Mg/(Fe+Mg) (molar) S/FM = Si/(Fe+Mg) (molar) AFM = Al/(Al+Fe+Mg) (molar)

Page 44 of 47

Table 5. Experimentally determined partition coefficients

Experimentally determined DREE (zircon/melt)

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

La 0.6 ± 0.3 0.8 ± 0.2 0.9 ± 0.3 1.0 ± 0.3 0.9 ± 0.5 1.0 ± 0.7

Ce 0.9 ± 0.3 1.1 ± 0.3 1.1 ± 0.3 1.2 ± 0.3 1.1 ± 0.5 1.3 ± 0.7

Pr 0.9 ± 0.4 1.4 ± 0.3 1.3 ± 0.3 1.5 ± 0.3 1.2 ± 0.5 1.3 ± 0.7

Nd 1.4 ± 0.5 1.6 ± 0.4 1.8 ± 0.4 2.1 ± 0.4 1.9 ± 0.7 2.1 ± 0.8

Sm 4.2 ± 1.0 3.4 ± 0.9 4.6 ± 1.0 4.8 ± 0.7 5.4 ± 1.0 5.5 ± 1.3

Eu 2.6 ± 1.5 3.6 ± 1.5 3.6 ± 1.5 3.5 ± 1.2 2.7 ± 1.2 3.0 ± 1.8

Gd 8.0 ± 1.9 8.7 ± 2.5 10.8 ± 2.7 11.6 ± 2.2 13.6 ± 2.5 13.9 ± 2.2

Tb 12.4 ± 3.3 14.3 ± 4.1 15.9 ± 4.3 16.4 ± 3.7 18.7 ± 4.2 19.7 ± 4.2

Dy 14.4 ± 4.4 18.2 ± 5.0 20.9 ± 5.3 18.9 ± 4.9 23.7 ± 6.3 21.4 ± 6.4

Ho 26.0 ± 8.1 31.3 ± 8.2 32.8 ± 8.2 29.5 ± 8.0 36.9 ± 10.5 36.8 ± 9.7

Er 39.5 ± 11.3 46.5 ± 12.1 52.2 ± 12.6 43.7 ± 12.7 53.0 ± 16.3 50.7 ± 16.0

Tm 50.0 ± 14.9 56.8 ± 14.7 61.9 ± 16.3 51.4 ± 14.7 62.3 ± 19.7 56.3 ± 18.6

Yb 49.8 ± 11.6 56.2 ± 13.2 58.6 ± 12.2 50.8 ± 12.4 62.5 ± 17.0 57.9 ± 16.8

Lu 57.0 ± 16.1 60.8 ± 16.3 59.5 ± 15.5 51.6 ± 14.6 68.0 ± 19.6 59.2 ± 18.9

Y 19.3 ± 4.3 27.4 ± 4.6 40.1 ± 8.8 41.4 ± 8.0 21.8 ± 8.5 38.6 ± 8.4

Hf 1089.4 ± 465 1116.9 ± 217 2299.8 ± 1365 2652.8 ± 1729 2006.5 ± 1620 1521.3 ± 405

P 1.84 ± 0.25 1.44 ± 0.11 1.85 ± 0.14 1.81 ± 0.62 1.07 ± 0.57 1.77 ± 0.49

Experimentally determined DREE (garnet/melt)

Run No. z33-2 z33-4 z31-1* z31-2 z31-3* z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

La 0.03 ± 0.9 0.01 ± 1.8 0.1 ± 2.0 0.1 ± 1.3 0.1 ± 1.7 0.02 ± 1.3

Ce 0.03 ± 1.9 0.02 ± 2.4 0.1 ± 2.1 0.1 ± 1.1 0.1 ± 1.6 0.04 ± 1.1

Pr 0.07 ± 1.8 0.08 ± 2.2 0.1 ± 2.3 0.1 ± 1.5 0.1 ± 1.8 0.2 ± 1.5

Nd 0.2 ± 1.2 0.3 ± 1.6 0.4 ± 1.5 0.4 ± 1.8 0.4 ± 1.9 0.7 ± 1.8

Sm 1.6 ± 2.2 2.2 ± 2.8 3.3 ± 2.5 3.1 ± 2.6 3.3 ± 2.7 5.6 ± 2.6

Eu 1.6 ± 1.5 2.2 ± 1.6 3.4 ± 1.6 3.5 ± 1.7 3.1 ± 1.8 7.0 ± 1.7

Gd 5.0 ± 2.5 6.1 ± 2.5 9.9 ± 2.6 11.0 ± 2.6 10.3 ± 2.7 19.0 ± 2.6

Tb 13.2 ± 3.5 16.4 ± 3.2 31.8 ± 3.4 24.3 ± 3.1 26.1 ± 3.3 38.3 ± 3.1

Dy 23.0 ± 3.3 28.2 ± 3.3 40.0 ± 3.2 34.0 ± 2.9 39.1 ± 3.1 49.2 ± 2.9

Ho 46.1 ± 2.8 49.5 ± 3.2 44.1 ± 3.1 49.6 ± 2.8 58.2 ± 2.9 70.8 ± 2.8

Er 52.6 ± 3.0 53.4 ± 3.3 54.0 ± 3.4 60.7 ± 3.1 58.8 ± 3.2 78.8 ± 3.1

Tm 71.6 ± 3.8 73.5 ± 3.5 55.9 ± 3.6 59.6 ± 3.5 63.6 ± 3.7 81.8 ± 3.5

Yb 75.5 ± 3.3 77.4 ± 3.0 60.4 ± 3.7 53.6 ± 3.1 71.1 ± 3.3 74.5 ± 3.1

Lu 76.3 ± 3.6 64.2 ± 3.3 43.0 ± 3.7 46.2 ± 3.2 54.0 ± 3.4 70.2 ± 3.2

Experimentally determined DREE (zircon/garnet)

Run No. z33-2 z33-4 z31-1* z31-2 z31-3* z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7.00

La 21.6 ± 12.3 95.4 ± 55.4 10.4 ± 3.8 13.9 ± 4.6 10.5 ± 6.4 43.4 ± 37.2

Ce 27.6 ± 10.6 55.1 ± 17.9 11.6 ± 3.2 14.5 ± 4.2 11.5 ± 5.4 28.7 ± 17.5

Pr 11.9 ± 6.5 17.1 ± 5.3 7.6 ± 1.9 10.3 ± 2.3 7.3 ± 3.1 6.4 ± 3.7

Nd 7.1 ± 2.8 5.1 ± 1.5 4.0 ± 0.9 5.2 ± 1.2 4.2 ± 1.5 3.2 ± 1.2

Sm 2.7 ± 0.7 1.5 ± 0.4 1.3 ± 0.3 1.6 ± 0.2 1.6 ± 0.2 1.0 ± 0.2

Eu 1.6 ± 0.9 1.6 ± 0.6 0.9 ± 0.3 1.0 ± 0.3 0.7 ± 0.3 0.4 ± 0.2

Gd 1.2 ± 0.3 1.1 ± 0.3 0.9 ± 0.2 1.1 ± 0.2 1.1 ± 0.2 0.7 ± 0.1

Tb 0.6 ± 0.1 0.7 ± 0.2 0.6 ± 0.1 0.7 ± 0.1 0.7 ± 0.1 0.5 ± 0.1

Dy 0.4 ± 0.1 0.6 ± 0.1 0.5 ± 0.1 0.6 ± 0.1 0.6 ± 0.1 0.4 ± 0.1

Ho 0.4 ± 0.1 0.6 ± 0.1 0.6 ± 0.1 0.6 ± 0.1 0.7 ± 0.2 0.5 ± 0.1

Er 0.4 ± 0.1 0.6 ± 0.1 0.8 ± 0.1 0.7 ± 0.2 0.8 ± 0.2 0.6 ± 0.1

Tm 0.4 ± 0.1 0.6 ± 0.1 0.9 ± 0.2 0.9 ± 0.2 0.9 ± 0.2 0.7 ± 0.2

Yb 0.5 ± 0.1 0.7 ± 0.1 1.0 ± 0.1 0.9 ± 0.2 1.0 ± 0.2 0.8 ± 0.2

Lu 0.6 ± 0.1 0.8 ± 0.1 1.1 ± 0.2 1.1 ± 0.2 1.3 ± 0.3 0.8 ± 0.2

* garnet/melt and zircon/garnet D values produced using z31-2 garnet composition due to contamination of z31-1 & z31-3 garnet by melt inclusions

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Table 6. Strain modelling fit parameters

Strain modelled DREE (zircon/melt) - Hanchar and van Westrenen model

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

1σ 1σ 1σ 1σ 1σ 1σ

R0 0.96 ± 0.004 0.94 ± 0.009 0.96 ± 0.005 0.96 ± 0.005 0.95 ± 0.006 0.97 ± 0.003

E (GPa) 430 ± 45 410 ± 52 449 ± 15 441 ± 21 463 ± 18 440 ± 35

D0 42 ± 1 69 ± 5 68 ± 2 65 ± 2 91 ± 3 67 ± 1

Strain modelled DREE (garnet/melt) - unconstrained fits

Run No. z33-2 z33-4 z31-1* z31-2 z31-3* z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

R0 0.985 0.986 1.000 1.000 1.000 1.000

E (GPa) 1150 1040 1150 1150 1150 1050

D0 77 75 63 63 63 90

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Table 7. Strain modelled D values

Strain modelled DREE (zircon/melt)

Run No. z33-2 z33-4 z31-1 z31-2 z31-3 z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

La 0.03 ± 0.3 0.02 ± 0.2 0.02 ± 0.3 0.0 ± 0.3 0.03 ± 0.5 0.03 ± 0.7

Ce 0.1 ± 0.3 0.1 ± 0.3 0.1 ± 0.3 0.1 ± 0.3 0.1 ± 0.5 0.1 ± 0.7

Pr 0.3 ± 0.4 0.2 ± 0.3 0.3 ± 0.3 0.3 ± 0.3 0.4 ± 0.5 0.4 ± 0.7

Nd 0.9 ± 0.5 0.6 ± 0.4 0.9 ± 0.4 1.0 ± 0.4 1.1 ± 0.7 1.3 ± 0.8

Sm 3.7 ± 1.0 3.2 ± 0.9 4.6 ± 1.0 4.9 ± 0.7 5.6 ± 1.0 6.0 ± 1.3

Eu 6.3 ± 1.5 5.7 ± 1.5 8.3 ± 1.5 8.6 ± 1.2 10.0 ± 1.2 10.3 ± 1.8

Gd 9.8 ± 1.9 9.6 ± 2.5 13.6 ± 2.7 14.0 ± 2.2 16.6 ± 2.5 16.5 ± 2.2

Tb 14.3 ± 3.3 15.1 ± 4.1 20.9 ± 4.3 21.2 ± 3.7 25.4 ± 4.2 24.5 ± 4.2

Dy 19.6 ± 4.4 22.2 ± 5.0 30.0 ± 5.3 29.9 ± 4.9 36.4 ± 6.3 34.0 ± 6.4

Ho 24.9 ± 8.1 30.2 ± 8.2 39.4 ± 8.2 38.8 ± 8.0 47.9 ± 10.5 43.4 ± 9.7

Er 29.6 ± 11.3 38.2 ± 12.1 48.2 ± 12.6 46.9 ± 12.7 58.6 ± 16.3 51.6 ± 16.0

Tm 33.5 ± 14.9 45.7 ± 14.7 55.7 ± 16.3 53.6 ± 14.7 67.6 ± 19.7 58.1 ± 18.6

Yb 36.4 ± 11.6 52.2 ± 13.2 61.5 ± 12.2 58.6 ± 12.4 74.7 ± 17.0 62.6 ± 16.8

Lu 38.3 ± 16.1 57.5 ± 16.3 65.6 ± 15.5 62.0 ± 14.6 79.6 ± 19.6 65.4 ± 18.9

Strain modelled DREE (garnet/melt)

Run No. z33-2 z33-4 z31-1* z31-2 z31-3* z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

La 0.00003 ± 0.9 0.0001 ± 1.8 0.0002 ± 1.3 0.0004 ± 1.3

Ce 0.001 ± 1.9 0.002 ± 2.4 0.003 ± 1.1 0.006 ± 1.1

Pr 0.01 ± 1.8 0.02 ± 2.2 0.03 ± 1.5 0.06 ± 1.5

Nd 0.1 ± 1.2 0.1 ± 1.6 0.2 ± 1.8 0.4 ± 1.8

Sm 1.5 ± 2.2 2.3 ± 2.8 3.5 ± 2.6 5.7 ± 2.6

Eu 4.3 ± 1.5 5.9 ± 1.6 8.6 ± 1.7 13.5 ± 1.7

Gd 10.3 ± 2.5 12.8 ± 2.5 17.7 ± 2.6 26.9 ± 2.6

Tb 21.0 ± 3.5 24.2 ± 3.2 30.8 ± 3.1 45.6 ± 3.1

Dy 36.5 ± 3.3 39.4 ± 3.3 45.7 ± 2.9 66.3 ± 2.9

Ho 52.8 ± 2.8 54.6 ± 3.2 57.1 ± 2.8 82.0 ± 2.8

Er 66.3 ± 3.0 66.4 ± 3.3 62.6 ± 3.1 89.4 ± 3.1

Tm 74.5 ± 3.8 73.2 ± 3.5 62.0 ± 3.5 88.7 ± 3.5

Yb 77.0 ± 3.3 75.0 ± 3.0 57.3 ± 3.1 82.2 ± 3.1

Lu 75.0 ± 3.6 72.8 ± 3.3 50.5 ± 3.2 72.9 ± 3.2

Strain modelled DREE (zircon/garnet)

Run No. z33-2 z33-4 z31-1* z31-2 z31-3* z26-4

T (°C) 1000°C 1000°C 950°C 950°C 950°C 900°C

P (kbar) 7 7 7 7 7 7

La 192.7 ± 12.3 270.3 ± 55.4 116.6 ± 3.8 137.5 ± 4.6 141.6 ± 6.4 66.5 ± 37.2

Ce 49.9 ± 10.6 64.0 ± 17.9 30.6 ± 3.2 35.3 ± 4.2 37.2 ± 5.4 19.2 ± 17.5

Pr 15.5 ± 6.5 18.6 ± 5.3 9.9 ± 1.9 11.2 ± 2.3 12.1 ± 3.1 6.7 ± 3.7

Nd 5.7 ± 2.8 6.6 ± 1.5 4.0 ± 0.9 4.4 ± 1.2 4.8 ± 1.5 2.8 ± 1.2

Sm 1.5 ± 0.7 1.7 ± 0.4 1.2 ± 0.3 1.3 ± 0.2 1.5 ± 0.2 0.9 ± 0.2

Eu 1.0 ± 0.9 1.1 ± 0.6 0.9 ± 0.3 0.9 ± 0.3 1.1 ± 0.3 0.7 ± 0.2

Gd 0.7 ± 0.3 0.8 ± 0.3 0.7 ± 0.2 0.7 ± 0.2 0.9 ± 0.2 0.6 ± 0.1

Tb 0.5 ± 0.1 0.6 ± 0.2 0.6 ± 0.1 0.6 ± 0.1 0.8 ± 0.1 0.5 ± 0.1

Dy 0.5 ± 0.1 0.5 ± 0.1 0.6 ± 0.1 0.6 ± 0.1 0.7 ± 0.1 0.5 ± 0.1

Ho 0.4 ± 0.1 0.5 ± 0.1 0.6 ± 0.1 0.6 ± 0.1 0.8 ± 0.2 0.5 ± 0.1

Er 0.4 ± 0.1 0.5 ± 0.1 0.7 ± 0.1 0.7 ± 0.2 0.9 ± 0.2 0.5 ± 0.1

Tm 0.4 ± 0.1 0.6 ± 0.1 0.8 ± 0.2 0.8 ± 0.2 1.0 ± 0.2 0.6 ± 0.2

Yb 0.4 ± 0.1 0.6 ± 0.1 1.0 ± 0.1 0.9 ± 0.2 1.2 ± 0.2 0.7 ± 0.2

Lu 0.5 ± 0.1 0.7 ± 0.1 1.2 ± 0.2 1.1 ± 0.2 1.5 ± 0.3 0.8 ± 0.2

* garnet/melt and zircon/garnet D values produced using z31-2 garnet composition due to contamination of z31-1 & z31-3 garnet by melt inclusions

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