Cerium oxidation state in silicate melts: Combined fO2, temperature and compositional effects

Accepted Manuscript Cerium Oxidation State in Silicate Melts: Combined fO2, Temperature and Compositional Effects Duane J. Smythe, James M. Brenan PII: DOI: Reference:

S0016-7037(15)00449-4 http://dx.doi.org/10.1016/j.gca.2015.07.016 GCA 9370

To appear in:

Geochimica et Cosmochimica Acta

Received Date: Accepted Date:

24 September 2013 7 July 2015

Please cite this article as: Smythe, D.J., Brenan, J.M., Cerium Oxidation State in Silicate Melts: Combined fO2, Temperature and Compositional Effects, Geochimica et Cosmochimica Acta (2015), doi: http://dx.doi.org/10.1016/ j.gca.2015.07.016

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Cerium Oxidation State in Silicate Melts: Combined fO2, Temperature and Compositional Effects

Duane J. Smythe1,2*, James M. Brenan1

1

Department of Earth Sciences, University of Toronto, 22 Russell Street, Toronto, ON, M5S

3B1, Canada 2

Current address: Department of Earth Sciences, Oxford University, South Parks Road, Oxford,

OX1 3AN, UK

*Corresponding author. Phone: +441865272026, email: [email protected]

Abstract To quantify the relative proportions of Ce3+ and Ce4+ in natural magmas, we have synthesized a series of Ce doped glasses ranging in composition from basalt to rhyolite (± H2O) at 0.001 and 1 GPa, under fO2 conditions varying from FMQ –4.0 to FMQ +8.4, and temperatures from 1200 to 1500 °C. The Ce4+/Ce3+ ratio in the experimental run products was determined both potentiometrically and in situ, using Ce M4,5-edge x-ray absorption near-edge structure (XANES) spectroscopy. For a given melt composition, the change in Ce4+/Ce3+ ratio with fO2 follows the trend predicted from the reaction stoichiometry assuming simple oxides as melt species. In addition to fO2, melt composition and water content have been found to be secondary controls on Ce4+/Ce3+, with more depolymerized melts and hydrous compositions favoring the stabilization of Ce3+. The Ce4+/Ce3+ ratio can be expressed through the equation,

log

CeO 2 1 5705(±257) NBO = log fO 2 + − 0.8694(±0.005) CeO3/2 4 T T −3.856(±0.049) ⋅ xH 2 O − 3.889(±0.102)

where T is in Kelvin, NBO/T is the proportion of non-bridging oxygen to tetrahedrally coordinated cations, and xH2O is the mole fraction (calculated using molecular oxides, e.g. Al2O3, Na2O) of water dissolved in the melt. A recent study conducted by Burnham and Berry (2014, Chemical Geology) investigating Ce oxidation state in silicate melts using Ce L3-edge XANES, equilibrated under a subset of the conditions investigated here, showed a similar dependence of Ce4+/Ce3+ on T and fO2, however, melt composition was found to have the opposite effect, with decreasing melt polymerization resulting in an increased abundance of Ce4+. This likely arising from the presence of alkalies and H2O in the compositions presented in this study, which were absent in the study of Burnham and Berry (2014). Our results indicate that even at relatively low oxygen fugacity, trace amounts of Ce4+ will be present in most terrestrial igneous systems, suggesting that Ce partitioning could be a sensitive indicator of fO2.

1. INTRODUCTION AND THEORETICAL BACKGROUND Cerium is unique among the rare-earth elements (REEs) as it can exist as 4+ in addition to the 3+ valence state common to this element group. Although Ce4+ has been typically associated with Earth’s surface environment, thermodynamic data for the component oxides (Zinkevich et al., 2006) and measurements of Ce redox state in silicate glasses (Paul and Douglas, 1965; Darab et al., 1998) suggest that both Ce3+ and Ce4+ should also be present in igneous systems under conditions experienced by terrestrial magmas. This prediction is supported by the observation of positive Ce anomalies relative to other REE in igneous phases which have a higher affinity for tetravalent cations, such as zircon and cassiterite (Jiang et al.,

2004; Hanchar and van Westrenen, 2007). Laboratory partitioning studies (Luo and Ayers, 2009; Trail et al., 2011; Burnham and Berry, 2012) have also observed enhanced partitioning of Ce into zircon under highly oxidizing conditions, presumably reflecting the stabilization of Ce4+ at high fO2. As described by Ballard et al (2002), detailed knowledge of Ce4+/Ce3+ in silicate melts, combined with mineral/melt partitioning models, offers a potentially powerful means to assess magma redox state. A fundamental limitation to this pursuit, however, is information on the Ce4+/Ce3+ variation with fO2 and composition in melts. The redox state of Ce in molten silicate can be expressed via the homogeneous reaction: 1 melt + O melt  CeO 2melt CeO3/2 2 4

(1)

The corresponding equilibrium constant, K, for this reaction is:

K=

aCmeeOlt2

(2)

melt aCeO ⋅ fO 21/4 3/2

melt melt where aCeO and aCeO are the activities of the CeO2 and CeO3/2 components in the melt. Taking 2 3/2

the logarithm of both sides of this equation yields the expression:

log K = log

melt aCeO 2 melt CeO3/2

a

1 − log fO2 4

(3)

which, by separating the activity terms into their constituents yields:

log K = log

melt xCeO 2 melt xCeO 3/2

melt γ CeO 1 + log melt − log fO2 γ CeO 4 2

(4)

3/2

where x and γ are the mole fractions and activity coefficients, respectively, of CeO2 and CeO3/2 in the melt. Rearrangement of Eq. (4) results in the expression:

log

melt xCeO 2 melt xCeO 3/2

γ CeO 1 = log fO2 − log melt 2 + log K γ CeO3/2 4 melt

(5)

melt melt For low concentrations of Ce it may be assumed that γ CeO and γ CeO are constants for a given 3/2 2 melt melt melt composition (Henry’s Law). Since both log K and log γ CeO are constants under set γ CeO 2 3/2

conditions, plotting the logarithm of the mole ratio verses log fO2 should then yield a straight melt melt line, with a slope of 1/4, and an intercept corresponding to the sum of – log γ CeO and log γ CeO 2 3/2

K. Both K and the ratio of the activity coefficients are implicit functions of temperature and melt composition. The temperature dependence of K can be explicitly expressed with the form, Δ r G° = −2.30259 RT log K = Δr H ° − T Δr S °

(6)

where ΔrG°, ΔrH°, and ΔrS° are the standard state Gibbs energy, enthalpy, and entropy of the reaction, R is the gas constant, and T is temperature in Kelvin. Based on heat capacity (CP) data for the component Ce oxides, the change in log K due to ΔrCP over the temperature range investigated can be considered to be negligible relative to the ΔrH° (Barin and Knacke, 1973). Therefore, log K =

−Δ r H ° Δr S ° + 2.30259 RT 2.30259 R

(7)

Assuming both ΔrH° and ΔrS° are constant with T, this can then be expressed as: log K =

a +b T

(8)

where a and b are constants for a particular melt composition. Previous work has shown K may also be a function of melt composition (Sack et al., 1980) in addition to fO2 and T, fully described by the empirical expression:

log

melt xCeO 2 melt CeO3/2

x

1 a = log fO2 + + ∑ bi xi + c 4 T i

(9)

where bi and c are constants, and xi is the mole fraction of component i in the melt. In this study we employ two different methods to measure Ce valence state. The first involves digestion of the sample in a HF – H2SO4 solution then titrating against a standard reducing agent, in this case ammonium Fe(II) sulfate. In systems where other redox sensitive elements are present, analyses through wet chemical techniques become problematic. Apart from the potential for charge transfer on quench, re-equilibration of the redox pairs in the aqueous solution during digestion is an unavoidable consequence of the analytical method. To obviate these complexities, experiments were done with Fe-free analogues of natural silicate melts. Furthermore, the detection limits for Ce4+/Ce3+ determined by this technique were found to be ~0.01 at best. Evaluation of Ce oxidation state at low fO2 was therefore not possible through wet chemical analysis. To complement the wet chemical method, determinations of Ce oxidation state have also been done using synchrotron based x-ray absorption. Measurement of Ce4+/Ce3+ using synchrotron radiation typically exploits the Ce L3-edge X-ray absorption near-edge structure (XANES) spectra, which involves a transition of a 2p electron to an unoccupied 5d state. Depending on oxidation state the L3-edge shifts from ~5723 eV for Ce3+ to ~5740 for Ce4+, resulting from differences in the coulomb interaction between the 2p core hole and the valence band. A preliminary attempt to apply this technique to the silicate glass samples of this study resulted in reduction of Ce4+ to Ce3+ during exposure to the X-ray beam. This is not necessarily a surprising result as a similar phenomenon has been observed during Eu and Sm L3-edge XANES of borate glasses (Shimizugawa et al., 2001) as well as S K-edge XANES of silicate glasses (Wilke et al., 2008). Therefore, in the present study determination of Ce4+/Ce3+ was done using Ce M4,5-edge XANES. The Ce M4,5 absorption edge results from a 3d → 4f electron transition

and is characterized by two distinct line groups located at 902.4 (M4) and 883.8 eV (M5) corresponding to 3d3/24f5/2 and 3d5/24f7/2, respectively. This transition has the advantage of being at a lower energy, minimizing the potential for modification of the Ce redox state under beam exposure. Past experimental work by Schreiber et al. (1980) suggested that Ce4+ was unstable in magmatic systems and would be converted to Ce3+ by oxidation of Fe2+. This result, however, is clearly at odds with the inferred presence of Ce4+ in natural igneous rocks. It now seems probable that Ce4+ was eliminated in the glasses produced in the Schreiber et al. (1980) study as a result of electron exchange with Fe2+ during quench, as shown for Cr redox state in Fe-bearing melts (Berry et al., 2003). In this study we use both wet chemical methods and XANES analysis to determine the oxidation sate of Ce over an fO2 interval which spans 12 orders of magnitude. Melt compositions were both hydrous and anhydrous and vary from rhyolite to basalt, but are free of other multivalent cations (with the exception of H in the 1 GPa runs) to prevent charge transfer during quench. The resulting dataset allows for the accurate prediction of Ce valence over a range of T, fO2 and composition relevant to magmas generated on Earth and other solar system bodies.

2. MATERIALS AND METHODS Experiment starting materials were prepared from reagent grade oxides and carbonates (>99.99% purity). Each starting composition was doped with approximately 1.1 wt% CeO2. At this concentration Ce is within the Henry’s law region (Beattie, 1994; Prowatke and Klemme, 2006), yet at high enough concentration to produce robust x-ray absorption spectra. Starting materials were first ground using an agate mortar and pestle under ethanol until dry before being

calcined at 1100 °C for 8 to 12 hours. The mixtures were then fused twice at 1500 °C in a platinum crucible for approximately 30 min and quenched in water. After each fusion the resulting glass was ground twice under ethanol until dry, so as to ensure homogenous starting materials. Bulk compositions of the starting materials are provided in Table 1, which consist of synthetic basalt (BH09), andesite (AA08), and rhyolite (RH08). Experiments done at atmospheric pressure used a vertical-tube gas mixing furnace, with oxygen fugacity controlled using either CO-CO2 gas mixtures or pure O2. Owing to space limitations within the furnace, the internal furnace temperature and fO2 were not monitored during each experiment. Instead, temperature within the furnace was checked before and after each experiment using a Pt-Pt10%Rh thermocouple. The fO2 of the furnace atmosphere was also measured before and after each experiment using a SIRO2 C700+ solid zirconia electrolyte oxygen sensor purchased from Ceramic Oxide Fabricators©. A typical sample contained approximately 100 mg of glass starting material mixed with polyvinyl alcohol and applied to a wire loop made from either Pt or Pt10%Rh, then dried. The loop + glass were initially suspended within the cool end of the sealed furnace from a fused silica glass rod, which was lowered into the hot spot after the furnace atmosphere had been allowed to equilibrate. Experiments were terminated by quenching the melt bead in cold H2O. In some cases up to four samples were run simultaneously using a “chandelier” arrangement made from fused silica rod. Loss of alkali elements through volatilization is a common problem in one atmosphere high temperature experiments done at fO2 below FMQ, as the dominant alkali species in the gas phase are monatomic Na and K (O'Neill, 2005). To prevent the loss of alkalis from our experiments under these conditions we have employed the use of a silicate alkali reservoir melt.

This technique has been shown to fix the activity of alkali metal oxides in experiments done at one atmosphere (O'Neill, 2005; Borisov et al., 2006). The configuration for these experiments is shown in Fig. 1a, and consists of a Pt crucible containing ~1 g of reservoir melt suspended under the sample. The sample was positioned within the walls of the crucible and approximately 3 cm above the reservoir melt. Initial experiments using the alkali-disilicate melt employed by Borisov et al. (2006) resulted in an overabundance of alkali elements in experimental run products. In subsequent experiments, we found that reservoir melts of approximately the same composition as the sample, but with a relative enrichment in Na and K of 10%, resulted in run products with alkali contents varying by less than 0.1 wt% from their initial content. To determine the run durations necessary for the Ce oxidation state to reach equilibrium values, we performed a time series on the RH08 composition, as this is the most polymerized composition and likely the slowest to react. Samples were run at 1200°C under pure oxygen for durations of 2, 12, and 48 hours. Run products were analyzed spectroscopically (see below) and Ce4+/Ce3+ was found to be consistent for all durations investigated. In light of these results, all subsequent experiments were equilibrated for ~12 h. We also performed a series of hydrous experiments at 1 GPa using a piston cylinder apparatus. Starting materials were weighed to 100 mg and thoroughly mixed with a noble metal oxide buffer (Ru-RuO2, Ir-IrO2; O'Neill and Nell, 1997) to promote rapid equilibration at the imposed fO2. To prevent exhaustion of the buffer due to H2 diffusion into the experimental charge, additional buffer was added to the top and bottom of the glass starting material. Samples were sealed in Pt capsules containing 2-6 μL of deionized H2O. Capsules were placed vertically into holes drilled into a graphite slug, which was contained in a fired pyrophyllite cup to prevent contact with the graphite furnace. The piston-cylinder apparatus employed a 1.905 cm bore

pressure vessel with pressure cells consisting of MgO filler pieces and a graphite furnace fit into concentric sleeves of Pyrex (inner) and NaCl (outer, Fig. 1b). After quenching, samples were reweighed to check for H2O loss. Run products were optically transparent and varied from clear to pale yellow in colour. The yellow colour was most intense in glasses with the highest Ti content, as would be expected given that Ce-Ti interaction is a known colouring agent in glass (Paul, 1976). All run products were inspected optically to ensure that the samples were crystal-free, thus indicating that the equilibration temperature was above the liquidus for the given composition. A summary of experimental run conditions is provided in Table 2. 3. ANALYTICAL 3.1. Electron Probe Micro-Analysis All experimental run products were analyzed for major and minor elements using a Cameca SX50 electron probe micro-analyzer housed in the Department of Earth Sciences, University of Toronto. Samples were prepared for analysis by mounting in epoxy, then polishing with diamond grit down to 1 μm, followed by 0.3 μm alumina. Beam conditions employed for analyses were 15 kV acceleration voltage, 20 μm defocused beam and 5 nA beam current. Standards used for calibration were natural basaltic glass (Mg, Ca), obsidian (Si, Al), albite glass (Na), sanidine (K), TiO2 (Ti), and CePO4 (P, Ce). To prevent their underestimation caused by migration under the electron beam, alkali elements were analyzed at the beginning of the acquisition cycle. The ZAF correction routine was used to convert raw count rates to element concentrations. Typically, 8 to 10 spots were measured on each sample. Table 3 provides the averages of these analyses.

Water contents off hydrous glasses were determined by difference from the EPMA totals. This method was validated by gravimetric analysis of sample DS08-C1-09. The sample was powdered and heated to 1300°C over 5 hours under an atmosphere of pure O2 to prevent volatilization of alkalies. This yielded a H2O content of 6.1 wt.%, identical to the value determined by EPMA. 3.2. Potentiometric Titrations Potentiometric determinations of the Ce4+ content of samples were done using the method described in detail in Smythe et al. (2013) which is briefly summarized here. Approximately 20 mg of sample was crushed in an agate mortar then dissolved in 1.0 mL of a 12% HF – 7% H2SO4 mixture which was cooled in an ice bath. Following the digestion, 250 mg of boric acid was added to the sample solution to complex with the excess HF, preventing the gradual reduction of Ce4+. The resulting solution was then diluted with 35 mL of deionized H2O and titrated against a 0.0001 N ammonium Fe (II) sulfate solution. Voltage measurements were made using a Pt-pin platinum indicator electrode and a Ag-AgCl reference electrode which were connected to an Orion model 525a+ pH meter. Calibration of this technique was done by mixing ~20 mg of Ce-free blank glass with measured amounts of Ce standard compounds, which included Ce(SO4)2, (NH4)4Ce(SO4)4·2H2O, and (NH4)3Ce(NO3)6. A calibration curve was then constructed by plotting the volume of titrant added at the equivalence point versus the mass of Ce4+ added. Titrations of standard material were run both before and after the unknowns to ensure oxidation of the ammonium Fe (II) sulfate solution had not occurred over the course of the analyses. 3.3. Ce M4,5-edge XANES

Cerium M4,5-edge XANES spectra were collected on the Spherical Grating Monochromator (SGM) undulating beamline at the Canadian Light Source, University of Saskatchewan. Samples consisted of freshly cleaved coarse glass chips which were mounted on stainless steel disks with carbon tape. The disks were placed in the absorption chamber under a vacuum of <10-7 Torr. The samples were faced approximately 45° toward the 1000 μm x 100 μm incident beam. Spectra were collected at room temperature in the region between 870 and 920 eV with a 0.2 eV sampling step size over the edge region in both fluorescence yield (FLY) and total electron yield (TEY) modes. The TEY spectra were found to have significantly higher noise than the FLY spectra resulting from charge buildup due to the insulating nature of the glasses. As a result, only the FLY spectra were used for redox measurements. X-ray energies were calibrated using CeO2 as a standard. Three spectra were recorded per sample, which were then normalized and averaged. Some averages consisted of only two spectra if one was found to be too noisy. Although this was not a significant problem, approximately 10% had signal to noise ratios on the order of 10:1 or lower, in which case the spectra were not used for redox determinations. The Ce3+ M-edge consists of two edges centered at ~882.5 (M5) and ~899.8 eV (M4) with resolvable satellite peaks located at approximately 879.6, 881.5, 896.6 and 898.4 eV. The Ce4+ M-edge is shifted to higher energies by approximately 2 eV with main peaks at ~884.1 (M5) and ~902.0 eV (M4) and smaller features at ~889.0 and ~907.0 eV (Fig. 2). Details of the curve fitting procedure are described in detail in Smythe et al. (2013), where we found that the Ce4+/Ce3+ obtained from M4,5-edge area measurements somewhat overestimates the true Ce4+/Ce3+, as determined potentiometrically. The M4,5-edge measurements are corrected for this effect using the empirical relation: Ce 4+ / ΣCe = 578.7[Ce 4+ / ΣCe ± 5%]XANES A( ±0.004)

(0 < Ce

4+

/ ΣCe < 0.4

)

(10)

where A is a function of the NBO/T value of the glass expressed as: log A( ±0.004) = 0.754 ( log NBO/T ) − 1.119 ( log NBO/T ) 4

3

+0.577 ( log NBO/T ) − 0.119 ( log NBO/T ) + 0.709

(11)

2

[Ce4+ / ΣCe]XANES is the Ce4+contribution to the total area of the M4,5-edge, and Ce 4+ /ΣCe is the actual fraction of Ce4+ in the glass. For experiments containing H2O the hydrous NBO/T values were used to calculate A.

4. RESULTS 4.1. Effect of Temperature To investigate the dependence of Ce oxidation state on the equilibration temperature a series of experiments were done from 1300 to 1500 °C and constant fO2. For most of these experiments, pure O2 was chosen for the furnace atmosphere to yield the highest concentration of Ce4+, ensuring that the Ce4+/Ce3+ ratio would be well above the detection limit of the wet chemical technique. Two experiments were also done at more reducing conditions (log fO2 = -2.4). The temperature dependence for a given melt composition can then be evaluated through Eq. (9), which can be re-written as:

log

melt xCeO 2 melt CeO3/2

x

=

a +B T

(12)

In which B is a constant for a given melt composition at fixed fO2. Over the temperature range investigated, log Ce4+/Ce3+ varies linearly with the reciprocal temperature for all compositions (Fig. 3). The slope of the resulting regression (5705 ± 257, R2 = 0.982) yields the enthalpy of the reaction, determined using the Van’t Hoff relation (Eq. 7). Values are negative, as is expected for an exothermic oxidation reaction. There appears to be no

discernible compositional or fO2 dependence on the enthalpy of the redox reaction, with values for all samples being within error of -109.2 (± 4.8) kJ/mol. This similarity of reaction enthalpy differs from results obtained in binary alkali-oxide silica systems, in which enthalpy correlates strongly with melt basicity (Paul and Douglas, 1965). The average enthalpy determined in this study is substantially higher than values obtained for melts of Na2O·2SiO2 (-33.5 kJ/mol; Johnston, 1965), 3Li2O·7SiO2 (-76 kJ/mol) and alkali-borate melt compositions (-10 to -76 kJ/mol; Paul and Douglas, 1965). This is not surprising, however, given the markedly different compositions in these investigations. Of the compositions investigated by Paul and Douglas (1965), those most similar in composition (3Na2O·7SiO2 and 3K2O·7SiO2) to the range investigated in this study agree fairly well (-100 and -130 kJ/mol, respectively) with the enthalpy determined here. The close agreement between these alkali-silicate melts and the more complex alkali-poor compositions of this study suggests the enthalpy of the reaction in Eq. (1) should remain essentially constant within the range of terrestrial silicate melts. The enthalpy of the Ce redox reaction of -109.2 kJ/mol determined in this study is also much lower than that observed in previous studies by Schreiber et al. (1980) and Burnham and Berry (2014) on CMAS compositions. The results of Schreiber et al (1980) show a very small enthalpy for the redox reaction of -1.2 kJ/mol. This is, however, likely results from the small temperature range investigated which only spanned 50 °C. The study by Burnham and Berry (2014) found an enthalpy -36.7 kJ/mol for anorthite-diopside eutectic glass, similar to that of Johnson (1965). 4.2. Effect of Oxygen Fugacity melt melt As discussed previously, Eq. (5) predicts a linear relation between log xCeO and / xCeO 2 2/3

log fO2 with an expected slope of 1/4 for data obtained at constant temperature and a given melt

composition. For the compositions analyzed in this study, the regressed slopes are 0.225 (± 0.025, R2 = 0.986), 0.231 (± 0.031, R2 = 0.973) and 0.274 (± 0.028, R2 = 0.982), for RH08, AA08 and BH09, respectively. The model, therefore, holds true as a slope of 1/4 can be fit through the data within error (Fig. 4). Such results are in agreement with the observations from earlier work involving simple silicate-, and borate-based glass compositions (Johnston, 1965; Paul and Douglas, 1965; Schreiber et al., 1980; Burnham and Berry, 2014). 4.3. Melt Composition In addition to redox conditions and temperature, the redox state of Ce was also found to change systematically with the composition of the host melt. This is seen in Fig. 3 and Fig. 4 where Ce4+/Ce3+ in the basaltic composition BH09 is lower relative to the more silicic compositions AA08 and RH08 under the same T – fO2 conditions. Given the number of components present in the three compositions investigated here, regression analysis to determine values of bi in Eq. (9) is not possible. To apply a model of the form of Eq. (9) would require investigation of at least as many compositions as components. Instead, we sought a single compositional parameter which accounts for the bulk solution properties of the melt. A number of melt parameterizations have been put forth to describe the solution properties of silicate melts, including optical basicity (Duffy and Ingram, 1971), M, the cation ratio (Na + K + 2Ca)/(Al · Si), (Watson and Harrison, 1983), metal to oxygen ratio (M/O) (Henderson et al., 1985) as well as the number of non-bridging oxygens (NBO) to tetrahedrally-coordinated cations (T), NBO/T (Virgo et al., 1980). Here we have chosen to use NBO/T as this yields the best fit to our data out of the parameters mentioned (Fig. 5). By expressing the melt composition through NBO/T, Eq. (9) becomes:

log

melt xCeO 2 melt CeO3/2

x

1 NBO a = log fO2 + + b +c 4 T T

(13)

in which values of b and c are constants determined through regression analysis. Fitting all of the anhydrous data to Eq. (13) yields b = -0.8694 (±0.005, R2 = 0.928) and c = -3.889 (±0.102). 4.4. Water Content A series of experiments investigating the effect of H2O on Ce4+/Ce3+ were done at 1 GPa in sealed Pt capsules. These experiments contained water contents ranging from 2.8 – 9.1 wt% H2O. This encompasses much of the range of H2O observed in natural silicate melts, though lower than of some pegmatites forming melts which have been shown to have contained in excess of 20 wt% H2O (Thomas and Davidson, 2012). Determination of the Ce4+/Ce3+ in hydrous glasses using potentiometric methods is complicated by possible reaction with the metal + oxide added to buffer fO2 during synthesis. Although the noble metal oxide buffers have very low solubilities in silicate melts (discussed in Section 5.1), these were finely dispersed through the glass to facilitated equilibration. Consequently, we use the Ce M4,5-edge XANES to evaluate the effect of H2O on Ce redox equilibria. Based on the XANES analysis, the presence of H2O was found to result in a decrease in the proportion of Ce4+ in the melt (Fig. 5). This result is expected given the known depolymerizing effect of OH- on silicate melt structure (Mysen and Virgo, 1986). However, the resulting increase in NBO/T caused by the addition of H2O is insufficient to explain the decrease in Ce4+ concentration using Eq. (13). We therefore calculate NBO/T on an anhydrous basis, and treat water separately from the other melt components, through the introduction of an additional H2O term to Eq. (13):

log

melt xCeO 2 melt CeO3/2

x

1 NBO a = log fO2 + + b + c + d ⋅ xH2O 4 T T

(14)

Where d is a constant, determined by regression, to equal -3.856 (±0.049, R2 = 0.963), and xH2O is the mole fraction of total H2O dissolved in the melt. Although the hydrous

experiments were done at 1 GPa, as opposed to the an hydrous experiments done at 0.001 GPa, the observed change in Ce4+/Ce3+ is unlikely an effect of pressure. Burnham and Berry (2014) were unable to resolve any effect of pressure on the Ce4+/Ce3+ ratio of their experiments and concluded that any pressure effect must be small. This is in contrast to the 1 GPa hydrous experiments presented here where a significant shift in Ce4+/Ce3+ is observe which has been found to correlate with H2O concentration. Substituting the regressed constants into Eq. 14 yields a final expression of : log

melt xCeO 2 melt CeO3/2

x

=

1 5705( ±257) NBO log fO 2 + − 0.8694( ±0.005) 4 T T

(15)

−3.856(±0.049) ⋅ xH 2 O − 3.889( ±0.102)

5. DISCUSSION 5.1. Quench Modification of Ce Oxidation State A possible complication when determining the redox state of Ce in hydrous glasses is the modification of the Ce4+/Ce3+ by reaction between dissolved Ce and either the buffer material or dissolved H during quench. For the case of the buffer, two types of reactions seem likely. The first is between Ce and the dispersed solid oxide and metal, constituting a heterogeneous reaction involving either decomposition of the metal oxide (Ce3+ oxidation) or oxidation of the metal (Ce4+ reduction). Both reactions require bulk transport of oxygen through the melt. The diffusive lengthscale for this process, as estimated from data on oxygen diffusion in hydrous rhyolitic melt (Zhang and Ni, 2010), is ~3.0 μm over the ~10 seconds required to cool the experiment below the glass transition (~700 °C). Given this short length scale, any modification of the Ce redox state in the melt will be negligible. The second type of reaction involving the

buffer is the homogeneous equilibrium involving dissolved components in the melt. Under the oxidizing conditions investigated in the study Ru3+ (Borisov and Nachtweyh, 1998) and Ir2+ (O'Neill et al., 1995), or possibly Ir3+ (Borisov and Palme, 1995), are likely melt species. Oxidation of Ce3+ may then occur through the electron exchange reaction:

nCe3+ + Men+ → nCe4+ + Memetal

(16)

where Me is Ru or Ir. The estimated solubilities of Ru and Ir in the melt at the fO2 of our experiments are ~85 ppm Ru (Borisov and Nachtweyh, 1998), and ~1 ppm Ir (Brenan and McDonough, 2009), which are 103 to 104 times less than the amount of dissolved Ce, indicating that the Ce4+/Ce3+ will not be shifted significantly by this effect. Charge transfer between Ce and H is more of a potential problem and would take place through the reaction, 1 1 CeO 2 (melt ) + H 2 (melt )  H 2 O(melt ) + CeO3/ 2 ( melt ) 2 2

(17)

The redox pair with the lower enthalpy for the oxidation reaction will be reduced during the quenching process. The enthalpy of the Ce redox reaction has been determined in this study, whereas the equivalent data for hydrogen in silicate melts is not well constrained. The enthalpy of formation of H2O under the P, T conditions of our experiments ranges from -256 to -261 kJ/mol (Dow Chemical Co., 1971). Assuming this is an approximation for the equivalent homogeneous reaction involving hydrogen in silicate melts, the oxidation of H2 would then be more exothermic than that for Ce. This would suggest that if charge transfer were occurring on quench, it would be through the reduction of Ce4+, requiring the consumption of H2. The amount of H2 in our experiments will be governed by the dissociation of H2O through the reaction:

H2O  1/ 2O2 + H2

(18)

The extent of H2O dissociation can be calculated through the equation, log K d =

−Δ r G ° 2.30259 RT

(19)

where ΔrG° is the free energy of the reaction in the standard state, and Kd is the equilibrium constant for the dissociation reaction,

fO21/2 ⋅ fH 2 Kd = fH 2O

(20)

Using values for Kd from the JANAF thermochemical tables (Dow Chemical Co., 1971) and assuming an H2O activity of unity, the fH2 at known fO2 can be determined. All of the hydrous samples reported in this study were equilibrated under highly oxidizing conditions, in which the maximum fH2 would be 0.0723. The H2O fugacity at saturation under these conditions would be ~2.1 GPa , giving a molar hydrogen to water ratio of 10-5.46. This translates to a concentration of ~0.5 ppm hydrogen in our experiments, which should be taken as a maximum value as it assumes H2O saturation (all of the hydrous experiments in this study were water undersaturated). Given the low availability of H2, any reduction of Ce4+ would, therefore, not shift the measured Ce4+/Ce3+ by any resolvable amount. If the application of the standard state enthalpy of formation of H2O to the oxidation reaction is not valid for silicate melt systems and it is in fact lower than the enthalpy of the Ce oxidation, charge transfer would then be expected to result in the oxidation of Ce3+. Increased water content would then result in an intensification of this process. Contrary to this expectation, we have found that samples with higher water contents trend to lower Ce4+ (Fig. 5) suggesting that the Ce4+/Ce3+ ratio established at temperature was not shifted during quench. 5.2. Comparison to Previous Work

A dependence of Ce oxidation state on melt composition similar to that documented in this study has also been noted in previous work (Paul and Douglas, 1965; Schreiber et al., 1980; Patra et al., 2000). The compositional range studied in earlier work differs greatly from that of the present study, however, making it difficult to replicate past results within the context of the model presented. Some general trends are in agreement, however. For example, our results show the stabilization of Ce4+ with increasing silica content, which is consistent with the observation that Ce4+ is nearly the sole Ce species in Ce-doped SiO2 glass under an atmosphere of pure oxygen (Patra et al., 2000). In detail our model would predict that the SiO2 glass synthesized by Patra et al. (2000) should have Ce4+/ΣCe of 0.80 ± 0.05, which is in remarkably good accord, given that pure SiO2 is well outside of the composition range investigated. Furthermore, a subset of the results from Johnston (1965), Paul and Douglas (1965), Darab et al. (1998) and Burnham and Berry (2014), and one of the compositions of Schreiber et al. (1980) are within error of the Ce4+/Ce3+ calculated by the model presented here (Fig. 6). In an experimental study investigating zircon/melt trace element partitioning, Burnham and Berry (2012) estimated the Ce4+/Ce3+ of the melt based on the variation in partitioning with fO2. These experiments were done at ~1300 °C and 1 atm using an alkali-earth aluminosilicate composition containing ~56 wt% SiO2 (approximating a synthetic andesite, but with ~2.9 wt% ZrO2), over an fO2 range of 16 log units. Using the model presented here we have calculated the Ce4+/Ce3+ ratio under the run conditions in the composition investigated by Burnham and Berry (2012). Our results are in close agreement with those determined through partitioning, especially considering difference in melt composition compared to the range investigated here, and are within 0.5 log units in all cases (Fig. 7). Furthermore, since the results of Burnham and Berry (2012) are based on the partitioning of Ce between zircon and melt, the calculated Ce4+/Ce3+ are

not subject to quench effects. This validates the inherent assumption that the Ce4+/Ce3+ ratios measured in our glasses are representative of those at temperature. There are some apparent discrepancies between our results and previously published data which warrant discussion. The compositional dependence on Ce redox state observed by Paul and Douglas (1965), and Schreiber et al. (1980) has the opposite correlation with NBO/T to that observed for our samples. In the glasses investigated by Paul and Douglas (1965), increased alkali content appears to favor the stabilization of Ce4+, and compositions with the highest concentration of alkalis show the greatest deviation from the model presented in this study (Fig. 6), with much higher Ce4+/Ce3+ than predicted. It is difficult to evaluate the role alkalis play in our experiments, primarily due to the relatively restricted range investigated, and the co-variation of alkalis with other melt components. However, similar to the observations of Paul and Douglas (1965) we also find that Ce4+/Ce3+ increases with alkali content. It should also be noted that the compositions investigated by Paul and Douglas (1965) extend to alkali contents in excess of 40 wt% oxide, far outside those investigated here, as well as the natural range. The source of the inconsistency between our results and those from Schreiber et al. (1980) is uncertain, but may be linked in part to differences in Al content, as more Al in the melt correlates with a decrease in the proportion of Ce4+ (Patra et al., 2000). For the range of compositions evaluated in this study, we observe a weak negative correlation between Ce4+/Ce3+ and Al/Si, which can also be seen in the compositions analyzed by Schreiber et al. (1980). One of the compositions in that study which only contained 2.3 wt% Al2O3, well below the natural range of aluminum contents in silicate melt systems, showed a relatively high Ce4+ concentration compared to our results. Since both Si and Al are network formers (within the compositions investigated here) this seems to suggest that the NBO/T parameter used here to describe the

compositional dependence of Ce valence may in fact be an over simplification, and the oxidation state of Ce in silicate melts may therefore be controlled by more complicated factors. Application of this model to compositions with low Al2O3 contents, such as komatiitic and picritic melts, should therefore be approached with caution. Values of Ce4+/Ce3+ measured in the glasses produced by Darab et al. (1998) are generally higher than predicted by our model. However, the glass compositions they synthesized contain high concentrations of boron (7 to 12 wt% B2O3) and alkalis (15.5 to 19.5 wt% A2O) both of which have been shown to stabilized Ce4+ in glasses at high concentrations (Paul and Douglas, 1965). Lastly, the work of Burnham and Berry (2014) shows a small but systematic shift to higher Ce4+/Ce3+ than our model would predict. The results of Burnham and Berry (2014) are broadly similar to those of Schreiber et al. (1980) where Ce4+ is favored in more depolymerized melt compositions. As both studies investigated melt compositions in CMAS system, it seems likely that the source of this inconsistency is the presence of alkali elements in the compositions studied here, which were absent in these previous investigations. 5.3. Geological Implications Anomalous chondrite normalized concentrations of Ce relative to its neighboring REEs have been documented in whole rock analyses of igneous lithologies from a variety of tectonic settings (Frey and Green, 1974; Fodor et al., 1992; Shimizu et al., 1992). The presence of Ce4+ in natural systems would not be expected to affect rare earth element patterns in whole rock analyses, however, as our results indicate that the proportion of Ce in a 4+ oxidation state would typically be 1% or less. Therefore, Ce anomalies observed in these samples are likely imparted through alteration in the surface environment (Marsh, 1991; Cotten et al., 1995) due to the

greater solubility of Ce3+ in aqueous fluids or through recycling of sediments carrying a positive or negative Ce anomaly (Neal and Taylor, 1989). This also implies that element ratios commonly used as indicators of source region characteristics, such as Ce/Pb, would not be greatly affected by subtle variations in fO2, and hence Ce oxidation state. Our results predict ~0.03-0.29% of Ce will be present as Ce4+ in granitic compositions over the range of terrestrial fO2 (taken to be FMQ ±2). Based on lattice strain predictions (Blundy and Wood, 1994), mineral phases such as zircon, baddeleyite, and cassiterite, which have a relatively large 4+ cation as a major component, should efficiently discriminate between the two oxidation states of Ce. As the estimated difference in the mineral/melt partitioning of Ce4+ and Ce3+ in these phases is as large as 6 orders of magnitude (Ballard et al., 2002), Ce anomalies would be expected in the above phases even if they crystalized at conditions well below FMQ. This explains the ubiquitous presence of positive Ce anomalies observed in zircon (Ballard et al., 2002). Contemporaneous positive Ce anomalies (resulting from the presence of Ce4+) and negative Eu anomalies (caused by the stabilization of Eu2+) are a common observation in terrestrial zircons. This has posed somewhat of a paradox, as Ce4+, interpreted to represent oxidizing conditions, and Eu2+, which is stable under reducing conditions, were thought to not coexist (Hoskin and Schaltegger, 2003; Schreiber et al., 1980). This leads to the suggestion that other mechanisms, such as the fractionation of plagioclase, prior to or during zircon crystallization, are required to form simultaneous Ce and Eu anomalies (Hoskin et al., 2000). Using the model presented here for Ce redox state, and that of Drake (1975) for Eu, it is possible to estimate the coexisting Ce and Eu anomalies in zircon at equilibrium (Fig. 8a). This shows that the high affinity of zircon for Ce4+, coupled with the stabilization of trace amounts of Ce4+ at

relatively reducing conditions (where a significant portion of Eu will exist in a 2+ oxidation state) can also account for this observation. This is not to say that the observed Eu anomalies in zircon are strictly the result of the prevailing redox conditions, or that one would expect fO2 calculated from coexisting Ce and Eu anomalies to be concordant, as the crystallization of plagioclase would indeed have large effect on the budget of Eu in a melt (shown in Fig. 8b). Nevertheless, this does demonstrate that the simultaneous Ce and Eu anomalies can form under equilibrium conditions. A similar conclusion was reached by Burnham and Berry (2012) and Trail et al. (2012) based on direct measurements of zircon-melt partitioning over a range of fO2. Analysis of synthetic glasses containing both Fe and Ce done by Schreiber et al. (1980) showed evidence for interaction between the Ce3+ – Ce4+ and Fe2+ – Fe3+ redox pairs. The authors suggested that in the presence of Fe2+, Ce4+ will be reduced through the reaction,

Fe2+ (melt ) + Ce4+ (melt ) → Fe3+ (melt ) + Ce3+ (melt )

(21)

The authors argued that, since Fe2+ will always be more abundant than Ce4+ in natural magmas, Ce4+ will not be a stable melt species. If true, this would require that the observed Ce anomalies in phases like zircon are the result of a process other than the differences in partitioning of the two species. Numerous studies have shown that zircons with positive Ce anomalies form from magmas that lack such anomalies. It is difficult to envision a process other than the preferential uptake of Ce4+ to account for these observations. On examination of the data from Schreiber et al. (1980) it appears that the experiments containing Fe and Ce from which this conclusion was based may have been subject to charge transfer on quench through the reaction in Eq. (21). A similar conclusion was also reached by Burnham and Berry (2014). Although the enthalpy of the Fe oxidation reaction in most natural silicate melts is lower than that of Ce, values can vary significantly with melt composition (Sack et al., 1980; Mysen et al., 1985; Kress and

Carmichael, 1991). Schreiber et al. (1980) did not measure the enthalpy of the Fe2+ → Fe3+ reaction in their experiments, but their compositions are close to those investigated by Mysen et al. (1985). These authors studied Fe3+/Fe2+ equilibria by Mössbauer spectroscopy in the CaOMgO-Al2O3-SiO2-FeO system and found reaction enthalpies as high as 147.3 kJ/mol. This is considerably more exothermic than that of the Ce3+ → Ce4+ reaction determined in this study, and indicates that charge transfer during quenching was a possible mechanism to reduce Ce4+ in the experiments of Schreiber et al. (1980). It is of note that Schreiber et al. (1980) infer the presence of Ce-O-Fe “complexes” in their run-product glasses, based on electron paramagnetic resonance (EPR) measurements and optical absorption spectroscopy. Thus, the close proximity of these cations would serve to facilitate the charge transfer process.

6. CONCLUSIONS The oxidation state of cerium has been determined in a series of geologically relevant melt compositions. The measured redox state of Ce was found to have a dependence on fO2 predicted from the reaction stoichiometry. The variation in Ce4+/Ce3+ with temperature indicates the reaction is exothermic with an enthalpy of -109.2 (± 4.8) kJ/mol, which is constant between all investigated melt compositions. The average redox state of Ce was observed to increase with polymerization of the melt and decrease with the addition of water. These results show that Ce will exist primarily in a 3+ oxidation state under terrestrial magmatic conditions, however, Ce4+ will be present in low abundances over a significant range of fO2. Though the low proportions of Ce4+ in silicate melts would not be expected to affect the whole rock REE patterns, as most rock forming minerals will not effectively discriminate

between Ce4+ and Ce3+, these Ce4+ concentrations will result in elevated Ce content in accessory phases, such as zircon, which have a much greater affinity for 4+ cations.

ACKNOWLEDGMENTS We are grateful to John Hanchar, James Mungall, Mike Hamilton, and Grant Henderson for their comments on a previous version of this manuscript, and to Tom Regier for technical support on the SGM beamline. Comments from the anonymous GCA reviewers, as well as AE Wim Van Westrenen are appreciated. Funding for this project was provided by a NSERC Discovery grant to JMB and a GSA student research grant to DJS. The Canadian Light Source is funded by NSERC and the Canadian Foundation for Innovation.

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Fig. 1. Experimental assemblies for a) low fO2 experiments at 1 atm employing an alkalireservoir melt, and b) piston cylinder experiments done at 1 GPa.

Fig. 2. Ce M4,5-edge XANES spectra of samples DS09-C3-17, DS09-C3-28, and DS11-C64 as well as CeO2 and CeF3. The samples are all of BH composition, synthesized at 1400°C and equilibrated at different fO2s, expressed relative to FMQ .

Fig. 3. Arrhenius plot of the variation in the Ce redox equilibrium versus inverse temperature (K). Shaded symbols were done at log fO2 = 0 (pure O2) and open symbols at log fO2 ~-2.4. Squares, diamonds and triangles correspond to compositions RH08, AA08 and BH09, respectively. All samples measured potentiometrically. Lines are fit to the data by linear regression.

Fig. 4. Plot of Ce4+/Ce3+ as function of fO2, expressed in log units relative to the fayalitemagnetite-quartz oxygen buffer (ΔFMQ). Lines through the data have slope of 1/4 in log-log space (as per Eq. 3) with the intercepts determined by linear regression. Ce4+/Ce3+ determined potentiometrically (open symbols) and by XANES (shaded symbols). Squares, diamonds and triangles correspond to compositions RH08, AA08 and BH09, respectively.

Fig. 5. Values for the compositional term ‘B’ ( = ∑ bi xi from Eq. 9) versus the dry NBO/T values i

for the different compositions investigated in this study.

Squares, diamonds and triangles

correspond to compositions RH08, AA08 and BH09, respectively. Shaded symbols = anhydrous compositions, open = hydrous experiments. Dashed lines show the displacement in Ce redox equilibria with the mole percent of H2O calculated for the experiment (numbers beneath lines).

Fig. 6. Measured versus calculated log (Ce4+/Ce3+) comparing data determined in this study with previous measurements on silicate glasses. Symbols are as follows: X’s, this study; grey +’s are the results of Burnham and Berry (2014); light and dark grey diamonds are FAS and FAD compositions, respectively (Schreiber et al., 1980); light, medium and dark circles are Li, Na, and K silicate glasses, respectively (Paul and Douglas, 1965); light and dark triangles are Pu10SI and Pu10S-II compositions, respectively (Darab et al., 1998); and squares are Na-silicate glasses (Johnston, 1965). Dashed arrows in inset are in the direction of increasing alkali content for compositions from Paul and Douglas (1965). Error bars omitted for clarity. Errors on calculated log (Ce4+/Ce3+) are approximately 0.5 log unit.

Fig. 7. log (Ce4+/Ce3+) versus log fO2 determined from the zircon-melt partitioning experiments of Burnham and Berry (2012, grey triangles). Grey line shows best fit to the data, solid black line shows the values for log (Ce4+/Ce3+) calculated from the model presented in this study, for the melt composition and run conditions of the partitioning experiments. Our model predicts the experimental results of Burnham and Berry within 0.5 log units in all cases, with 6 of their 13 experiments being within 1 σerror of the Ce4+/Ce3+ ratio predicted by this study.

Fig. 8. Chondrite normalized REE diagrams showing the predicted magnitude of Ce and Eu anomalies in zircon crystalizing from a hydrous granitic melt with a) no plagioclase crystallization, and b) 20% plagioclase fractionation. Zircon/melt partition coefficients for 3+ REEs from Sano et al. (2002) and Ce4+ from Ballard et al. (2002). Plagioclase/melt partition coefficients from Aigner-Torres (2007). Partitioning of Eu2+ into zircon was considered to be negligible relative to Eu3+.

Table 1 Bulk Composition of starting materials (wt.%) BH09

AA08

RH08

SiO2

51.7

61.5

72.1

TiO2

1.1

0.6

0.3

Al2O3

16.8

18.7

14.8

MgO

11.9

4.2

0.7

CaO

15.9

11.5

1.7

Na2O

2.6

2.8

6.4

0.7

3.9

K2O P2O5

0.1

All compositions contained 1.0-1.2 wt% CeO2

Table 2 Experiment run conditions. Sample

Composition

Temperature (K)

log fO2a

Pressure (GPa)

wt% H2Ob

DS11-C38c

RH08

1476

-0.02

0.001

-

DS11-C40c

RH08

1474

-0.02

0.001

-

DS11-C41c

RH08

1478

-0.02

0.001

-

DS11-C44c

RH08

1473

-0.02

0.001

-

DS09-C2-13

RH08

1572

0.00

0.001

-

DS09-C3-10

RH08

1571

-2.73

0.001

-

DS09-C3-05

RH08

1571

-6.02

0.001

-

DS09-C3-19

RH08

1672

-0.02

0.001

-

DS09-C3-24

RH08

1672

-4.30

0.001

-

DS09-C3-32

RH08

1672

-6.31

0.001

-

DS11-C66

RH08

1672

-9.31

0.001

-

DS11-C36

RH08

1773

0.00

0.001

-

DS11-C24

RH08

1772

-2.40

0.001

-

DS11-C28

RH08

1772

-6.77

0.001

-

DS08-C1-02

RH08

1373

-2.72 (Ru)

1

9.1

DS08-C1-20

RH08

1373

-0.06 (Ir)

1

6.1

DS09-C3-37

RH08

1573

-1.30 (Ru)

1

7.3

DS09-C2-09

AA08

1572

0.00

0.001

-

DS11-C50

AA08

1574

-0.01

0.001

-

DS09-C3-08

AA08

1571

-2.73

0.001

-

DS09-C3-06

AA08

1574

-6.01

0.001

-

DS09-C3-15

AA08

1574

-7.28

0.001

-

DS09-C3-18

AA08

1672

-0.02

0.001

-

DS09-C3-23

AA08

1674

-4.29

0.001

-

DS09-C3-29

AA08

1674

-6.30

0.001

-

DS11-C63

AA08

1672

-9.31

0.001

-

DS11-C34

AA08

1773

0.00

0.001

-

DS11-C22

AA08

1772

-2.40

0.001

-

DS08-C1-09

AA08

1573

1.08 (Ir)

1

6.1

DS09-C2-06

AA08

1573

1.08 (Ir)

1

7.2

DS09-C3-22

AA08

1673

1.54 (Ir)

1

6.0

DS09-C2-12

BH09

1572

0.00

0.001

-

DS11-C49

BH09

1574

-0.01

0.001

-

DS09-C3-07

BH09

1572

-2.73

0.001

-

DS09-C3-03

BH09

1571

-5.32

0.001

-

DS09-C3-13

BH09

1574

-7.28

0.001

-

DS09-C3-14

BH09

1575

-7.28

0.001

-

DS09-C3-17

BH09

1672

-0.02

0.001

-

DS09-C3-20

BH09

1672

-4.30

0.001

-

DS09-C3-28

BH09

1674

-6.30

0.001

-

DS11-C64

BH09

1673

-9.31

0.001

-

DS11-C33

BH09

1773

0.00

0.001

-

DS11-C21

BH09

1772

-2.40

0.001

-

DS09-C3-09

BH09

1573

-1.30 (Ru)

1

2.8

DS09-C3-04

BH09

1573

1.08 (Ir)

1

a

Ru and Ir indicate the noble metal oxide buffer used in piston cylinder experiments.

b

Determined by difference from EMP analysis

c

Run as part of a time series. DS11-C38 = 2 h, DS11-C41 = 6 h, DS11-C44 = 12 h, DS11-C40 = 48 h.

3.6

Table 3 Electron probe micro-analyses of glass run products ( in wt%). Errors given in brackets. n = number of analyses Sample DS11DS11DS11DS11DS09DS09DS09DS09C38 C40 C41 C44 C2-13 C3-10 C3-05 C3-19 n 10 10 9 10 8 9 10 10 SiO2 71.05 71.23 71.15 71.60 72.34 71.44 71.42 72.01 (1.06) (0.27) (1.09) (0.95) (0.42) (0.32) (0.27) (0.42) TiO2 0.32 0.25 0.18 0.25 0.37 0.25 (0.1) 0.22 0.30 (0.1) (0.13) (0.15) (0.13) (0.12) (0.11) (0.14) Al2O3 14.62 14.37 14.56 14.30 14.35 14.36 14.16 14.29 (0.67) (0.16) (0.72) (0.67) (0.19) (0.13) (0.14) (0.15) MgO 0.66 0.64 0.64 0.63 0.66 0.7 (0.04) 0.68 0.69 (0.04) (0.04) (0.03) (0.06) (0.05) (0.05) (0.04) CaO 1.79 1.83 1.81 1.74 1.83 1.84 1.90 1.89 (0.08) (0.06) (0.08) (0.08) (0.07) (0.04) (0.06) (0.07) Na2O 6.57 6.49 6.36 6.45 6.06 6.39 (0.2) 6.36 6.04 (0.28) (0.15) (0.35) (0.39) (0.11) (0.21) (0.22) K2O 3.79 3.76 3.71 3.73 3.55 (0.1) 3.65 3.76 3.51 (0.09) (0.09) (0.13) (0.08) (0.14) (0.09) (0.09) P2O5 0.14 0.09 0.11 0.14 0.09 0.09 0.12 0.08 (0.14) (0.12) (0.07) (0.13) (0.05) (0.04) (0.06) (0.05) Ce2O3 0.92 0.94 0.97 0.94 (0.1) 1.17 1.10 1.17 1.18 (0.08) (0.12) (0.07) (0.10) (0.09) (0.13) (0.08) Total NBO/T1 log Ce4+/Ce3+

Sample n SiO2 TiO2 Al2O3 MgO CaO Na2O K2O P2O5 Ce2O3

Total

DS09C3-24 10 72.09 (0.37) 0.30 (0.1) 14.38 (0.17) 0.71 (0.03) 1.91 (0.07) 6.02 (0.2) 3.64 (0.12) 0.11 (0.14) 1.18 (0.1)

99.90 (0.73) 0.082 (0.007) -0.15 (0.13)

99.63 (0.51) 0.083 (0.002) -0.05 (0.13)

99.55 (0.38) 0.078 (0.007) -0.19 (0.13)

99.82 (0.62) 0.081 (0.007) -0.15 (0.13)

100.48 (0.74) 0.074 (0.002) -0.41 (0.01)

99.87 (0.44) 0.084 (0.002) -0.99 (0.03)

99.84 (0.27) 0.090 (0.002) -1.63 (0.13)

100.04 (0.56) 0.077 (0.002) -0.60 (0.03)

100.38 (0.39) 0.078 (0.003) -1.58 (0.13)

DS09C3-32 10 71.83 (0.27) 0.28 (0.14) 14.27 (0.12) 0.72 (0.04) 1.92 (0.08) 6.43 (0.26) 3.53 (0.08) 0.06 (0.07) 1.17 (0.09)

DS11C66 10 71.88 (0.71) 0.3 (0.1)

DS11C24 9 71.23 (0.39) 0.33 (0.11) 14.59 (0.16) 0.65 (0.02) 1.79 (0.06) 5.93 (0.27) 3.64 (0.1)

DS11C28 10 72.16 (0.44) 0.29 (0.11) 14.55 (0.15) 0.66 (0.03) 1.75 (0.11) 5.37 (0.2)

DS08C1-20 10 67.42 (0.22) 0.26 (0.14) 13.38 (0.13) 0.63 (0.04) 1.72 (0.04) 5.82 (0.23) 3.43 (0.1)

DS09C2-09 10 59.70 (0.44) 0.55 (0.2)

13.18 (0.3) 0.58 (0.1)

17.76 (0.23) 3.88 (0.1)

3.53 (0.13) 0.07 (0.09) 0.98 (0.11)

DS08C1-02 10 65.44 (0.5) 0.31 (0.14) 13.19 (0.2) 0.64 (0.04) 1.62 (0.03) 5.11 (0.16) 3.40 (0.09) 0.11 (0.05) 1.03 (0.11)

DS09C3-37 9 66.63 (0.97) 0.34 (0.1)

3.56 (0.08) 0.07 (0.08) 1.03 (0.15)

DS11C36 10 72.11 (0.56) 0.34 (0.12) 14.52 (0.16) 0.65 (0.05) 1.82 (0.06) 6.16 (0.27) 3.59 (0.17) 0.02 (0.04) 0.98 (0.09)

1.61 (0.26) 5.57 (0.22) 3.56 (0.11) -

11.27 (0.13) 2.72 (0.12) 0.67 (0.05) -

1.01 (0.17)

1.08 (0.12)

100.26 (0.57)

99.97 (0.45)

100.20 (0.8)

99.37 (0.31)

99.41 (0.47)

90.89 (0.81)

93.85 (0.47)

92.62 (0.53)

97.64 (0.83)

14.76 (0.3) 0.73 (0.07) 1.99 (0.26) 5.62 (0.2)

0.13 (0.11) 1.02 (0.1)

0.10 (0.12) 1.05 (0.15)

NBO/T1 log Ce4+/Ce3+

Sample n SiO2 TiO2 Al2O3 MgO CaO Na2O K2O

0.087 (0.002) -2.09 (0.13)

0.063 (0.003) -2.57 (0.13)

0.070 (0.003) -0.82 (0.03)

0.067 (0.002) -1.58 (0.13)

0.052 (0.002) -1.83 (0.13)

0.065 (0.003) -1.47 (0.11)

0.080 (0.002) -0.56 (0.11)

0.075 (0.005) -1.62 (0.11)

0.273 (0.012) -0.44 (0.11)

DS11C50 10 61.53 (0.28) 0.58 (0.14) 18.42 (0.17) 3.96 (0.1)

DS09C3-08 10 61.01 (0.29) 0.69 (0.15) 17.96 (0.16) 4.02 (0.07) 11.74 (0.15) 2.80 (0.09) 0.66 (0.03) 1.15 (0.1)

DS09C3-06 10 60.84 (0.37) 0.58 (0.19) 17.90 (0.14) 4.09 (0.1)

DS09C3-18 10 60.99 (0.34) 0.60 (0.13) 18.08 (0.14) 4.11 (0.06) 11.74 (0.17) 2.88 (0.16) 0.65 (0.04) 1.21 (0.11)

DS09C3-23 10 61.21 (0.36) 0.65 (0.14) 18.17 (0.17) 4.08 (0.09) 11.72 (0.13) 2.79 (0.06) 0.67 (0.06) 1.21 (0.13)

DS09C3-29 9 60.91 (0.25) 0.60 (0.11) 17.89 (0.12) 4.10 (0.1)

11.75 (0.19) 2.83 (0.19) 0.67 (0.06) 1.16 (0.1)

DS09C3-15 10 60.64 (0.27) 0.60 (0.09) 18.32 (0.15) 4.13 (0.05) 11.55 (0.13) 2.86 (0.14) 0.65 (0.04) 1.20 (0.11)

11.69 (0.13) 2.74 (0.17) 0.64 (0.03) 1.13 (0.11)

DS11C63 10 61.36 (0.33) 0.56 (0.15) 18.09 (0.11) 3.94 (0.11) 11.7 (0.11) 2.42 (0.15) 0.72 (0.05) 0.96 (0.12)

DS11C34 10 60.96 (0.34) 0.68 (0.13) 18.01 (0.2) 3.93 (0.09) 11.77 (0.22) 2.83 (0.13) 0.72 (0.05) 0.96 (0.1)

11.59 (0.12) 2.66 (0.15) 0.7 (0.05)

P2O5 Ce2O3

0.95 (0.09)

Total

100.43 (0.5) 0.264 (0.008) -0.57 (0.02)

100.05 (0.48) 0.284 (0.009) -1.26 (0.11)

99.87 (0.68) 0.290 (0.011) -2.01 (0.11)

99.96 (0.38) 0.280 (0.007) -1.93 (0.11)

100.29 (0.49) 0.288 (0.009) -0.87 (0.03)

100.54 (0.6) 0.282 (0.009) -1.72 (0.11)

99.74 (0.4) 0.286 (0.009) -2.33 (0.11)

99.79 (0.49) 0.267 (0.008) -3.04 (0.11)

99.89 (0.46) 0.280 (0.01) -1.03 (0.09)

DS11C22 10 51.58 (0.24) 1.20 (0.2)

DS08C1-09 8 57.36 (0.38) 0.54 (0.16) 17.01 (0.13) 3.72 (0.09) 10.87 (0.11) 2.65 (0.18) 0.66 (0.01) 1.17 (0.09)

DS09C2-06 10 56.11 (0.19) 0.64 (0.18) 16.57 (0.14) 3.56 (0.1) 10.68 (0.12) 3.34 (0.18) 0.65 (0.05) 1.08 (0.07)

DS09C3-22 10 57.3 (0.31) 0.60 (0.08) 16.87 (0.17) 3.73 (0.08) 10.95 (0.26) 2.91 (0.22) 0.60 (0.03) 1.07 (0.13)

DS09C2-12 10 51.88 (0.35) 1.01 (0.16) 16.39 (0.16) 11.46 (0.12) 16.29 (0.12) 2.11 (0.13) -

DS11C49 10 51.86 (0.48) 1.17 (0.15) 16.38 (0.18) 11.28 (0.14) 16.30 (0.15) 2.14 (0.07) -

DS09C3-07 10 51.65 (0.27) 1.05 (0.15) 16.28 (0.14) 11.53 (0.18) 16.28 (0.12) 2.24 (0.15) -

DS09C3-03 9 51.52 (0.3) 0.95 (0.17) 16.26 (0.17) 11.40 (0.15) 16.24 (0.15) 2.28 (0.14) -

DS09C3-13 10 51.57 (0.37) 1.09 (0.19) 16.60 (0.15) 11.61 (0.18) 16.14 (0.14) 2.17 (0.09) -

1.13 (0.12)

1.12 (0.19)

1.11 (0.1)

1.12 (0.1)

1.10 (0.1)

93.95 (0.53) 0.276 (0.009)

92.67 (0.54) 0.295 (0.009)

94.07 (0.54) 0.287 (0.01)

100.31 (0.58) 0.766 (0.002)

100.29 (0.47) 0.758 (0.035)

100.19 (0.61) 0.777 (0.026)

99.8 (0.62) 0.774 (0.029)

100.30 (0.39) 0.765 (0.031)

NBO/T1 log Ce4+/Ce3+

Sample n SiO2 TiO2 Al2O3

K2O

16.27 (0.12) 11.35 (0.12) 16.22 (0.22) 2.14 (0.15) -

P2O5 Ce2O3

1.09 (0.1)

Total

99.88 (0.41) 0.764 (0.026)

MgO CaO Na2O

NBO/T1

log Ce4+/Ce3+

-1.68 (0.11)

-0.94 (0.11)

-1.09 (0.11)

-1.02 (0.11)

-1.09 (0.07)

-0.93 (0.02)

-1.46 (0.09)

-2.09 (0.11)

-2.83 (0.11)

Sample

DS09C3-14 10 51.32 (0.34) 1.00 (0.11) 16.48 (0.23) 11.46 (0.13) 16.15 (0.17) 2.23 (0.1)

DS09C3-17 10 51.35 (0.33) 1.06 (0.13) 16.52 (0.25) 11.54 (0.08) 16.09 (0.13) 2.30 (0.08) 1.12 (0.1)

DS09C3-20 10 52.23 (0.37) 1.07 (0.16) 16.53 (0.15) 11.52 (0.09) 16.35 (0.1) 1.94 (0.12) 1.14 (0.15)

DS09C3-28 10 51.62 (0.26) 1.08 (0.16) 16.25 (0.16) 11.60 (0.13) 16.32 (0.22) 2.11 (0.15) 1.14 (0.05)

DS11C64 10 51.99 (0.19) 0.92 (0.19) 16.4 (0.13) 11.61 (0.16) 16.32 (0.16) 1.6 (0.15)

DS11C21 10 51.57 (0.26) 1.02 (0.11) 16.28 (0.24) 11.40 (0.14) 16.23 (0.14) 2.12 (0.08) 1.15 (0.11)

DS09C3-09 10 50.16 (0.42) 0.99 (0.2)

1.14 (0.09)

DS11C33 9 51.91 (0.45) 1.04 (0.22) 16.50 (0.13) 11.32 (0.19) 16.18 (0.16) 2.15 (0.16) 1.09 (0.12)

15.86 (0.16) 11.10 (0.2) 15.81 (0.22) 2.18 (0.13) 1.05 (0.05)

DS09C3-04 10 48.94 (0.35) 1.06 (0.23) 16.18 (0.23) 10.95 (0.14) 15.32 (0.1) 2.84 (0.12) 1.13 (0.04)

100.04 (0.59) 0.769 (0.031) -1.04 (0.11)

100.82 (0.55) 0.757 (0.029) -2.47 (0.11)

100.17 (0.34) 0.779 (0.027) -2.80 (0.11)

100.01 (0.34) 0.758 (0.023) -3.57 (0.11)

100.23 (0.77) 0.753 (0.035) -1.28 (0.08)

99.80 (0.61) 0.768 (0.028) -1.96 (0.32)

97.21 (0.59) 0.771 (0.037) -1.60 (0.11)

96.44 (0.6) 0.772 (0.036) -1.12 (0.10)

n SiO2 TiO2 Al2O3 MgO CaO Na2O K2O P2O5 Ce2O3

1.12 (0.09)

Total

99.81 (0.59) 0.768 (0.031) -2.98 (0.11)

NBO/T1 log Ce4+/Ce3+ 1

Non-bridging oxygen (NBO) vs. tetrahedrally coordinated cations (T). T = Si + Ti + Al* + P in atomic percent, NBO = 2O - 4T, where and O is atomic percent oxygen. *All Al assigned to T since Al < Na + K + 2Ca + 2Mg in all compositions. Calculated as anhydrous for all experiments.

a.Figure 1 Pt wire Pt Crucible Sample Na/K reservior melt

b.

W-Re thermocouple (Al2O3 sheath) NaCl Pyrex ® MgO Pyrophyllite Graphite Sample 10 mm

Absorption (arb. units)

Figure 2

6.3 0 -3.0

CeO2 CeF3

870

880

890 900 Energy (eV)

910

920

Figure 3

1

RH08

log (Ce4+/Ce3+)

0

8 0 A A BH09

-1

9 BH0

-2

-3 5.0

5.5

6.0 10000/T

6.5

7.0

Figure 4

R A 09 A08 H08

1

log (Ce4+/Ce3+)

BH

0

-1

-2

-3

-4 -4

0

4 ∆FMQ

8

12

Figure 5

0.0 -0.2 -0.4

B

-0.6 -0.8 -1.0

0

-1.2

5

0.0

0.2

0.4

10

15

20

25

0.6 NBO/T

0.8

1.0

Figure 6

1

log (Ce4+/Ce3+)Meas.

0

-1

-2

0

-3

-1.5

-4

1 : 1

-1.0 -1.5

-4

-3

-2

-1

log(Ce4+/Ce3+)Calc.

-1.0 0

1

Figure 7

FMQ

IW

-1

log (Ce4+/Ce3+)

-2

-3

-4

-5 -15

-10

-5 log fO2

0

Figure 8

104 a. 10

FMQ +2

102

FMQ

Zircon/Chondrite

3

101 100 10-1

FMQ -2

10-2 b. Zircon/Chondrite

10

3

FMQ +2

102 101 100

FMQ

10-1

FMQ -2

10-2 La

Ce

Pr Nd

Sm Eu Gd Tb Dy Ho

Er Tm Yb

Lu