Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earth's mantle conditions

    Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earths mantle conditions Bastian Joachim, Alison Pawley, Ian C. Lyon, Katharina Marquardt (ne´e Hartmann), Torsten Henkel, Patricia L. Clay, Lorraine Ruzi´e, Ray Burgess, Christopher J. Ballentine PII: DOI: Reference:

S0009-2541(15)30010-3 doi: 10.1016/j.chemgeo.2015.08.012 CHEMGE 17670

To appear in:

Chemical Geology

Received date: Revised date: Accepted date:

18 November 2014 12 August 2015 13 August 2015

Please cite this article as: Joachim, Bastian, Pawley, Alison, Lyon, Ian C., Marquardt (ne´e Hartmann), Katharina, Henkel, Torsten, Clay, Patricia L., Ruzi´e, Lorraine, Burgess, Ray, Ballentine, Christopher J., Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earths mantle conditions, Chemical Geology (2015), doi: 10.1016/j.chemgeo.2015.08.012

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ACCEPTED MANUSCRIPT Experimental partitioning of F and Cl between olivine, orthopyroxene and silicate melt at Earth´s mantle conditions

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Katharina Marquardt (neé Hartmann), 2Torsten Henkel, 2Patricia L. Clay, 2Lorraine Ruzié,

Ray Burgess, 1Christopher J. Ballentine

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Authors: 1,2,*Bastian Joachim (corresponding author), 2Alison Pawley, 2Ian C. Lyon,

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Department of Earth Sciences, University of Oxford, South Parks Road, Oxford OX1 3AN, United Kingdom; email: [email protected], phone: +43 512 507 54619 2 School of Earth, Atmospheric and Environmental Sciences, Williamson Building, University of Manchester, Manchester M13 9PL, United Kingdom 3 German Research Centre for Geosciences (GFZ), Section 3.3 Chemistry and Physics of Earth Materials, Telegrafenberg, 14473 Potsdam, Germany 4 Bayerisches Geoinstitut, University of Bayreuth, 95447 Bayreuth, Germany

*Present address: University of Innsbruck, Innrain 52, 6020 Innsbruck, Austria

ACCEPTED MANUSCRIPT Abstract Halogens have, because of their volatile behavior and incompatibility, the potential to act as

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key tracers of volatile transport processes within the Earth´s mantle. A better understanding of halogen behavior during partial melting processes will improve our understanding of volatile input mechanisms into the Earth´s mantle and give insight into its evolution over the Earth´s

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history. This study introduces Time of Flight Secondary Ion Mass Spectroscopy (TOF-SIMS) as an analytical method for the determination of halogen partition coefficients and

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significantly extends the available dataset for fluorine and chlorine partitioning between

Basalt (OIB) source regions.

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mantle minerals and silicate melts to conditions of partial melting processes in Ocean Island

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Halogen partitioning between olivine, orthopyroxene and silicate melt has been determined in experiments at 1.0-2.3 GPa and 1350-1600°C. Combining our data with results of recent studies (O´Leary et al. 2010; Beyer et al. 2012; Dalou et al. 2012, 2014; Rosenthal et al. 2015) shows that fluorine and chlorine partitioning between olivine and melt increases by

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about 1.5 - 2 orders of magnitude between 1350°C and 1600°C (fluorine: 0.005(3)-0.31(16);

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chlorine: 0.005(45)- 0.17(9)) and does not show any pressure dependence between 1.0 and 2.3 GPa. Chlorine partitioning between orthopyroxene and melt increases by about 1 order of magnitude between 1450°C and 1600°C (0.015(8)-0.16(9)) at a constant pressure of 2.3 GPa. Fluorine partitioning between orthopyroxene and melt increases by 1.5 orders of magnitude between 1250°C and 1600°C (0.029(6)-0.20(14)) and does not show any pressure dependence. Transmission Electron Microscopy measurements show that halogens are not incorporated in the form of humite-type defects in olivine. The most reasonable incorporation mechanism for halogens is via point defects in the olivine and orthopyroxene lattice, where they are inferred to be charge-balanced via oxygen defects.

ACCEPTED MANUSCRIPT By combining our partition coefficients with natural halogen concentrations in oceanic basalts, we are able to give estimates for fluorine and chlorine abundances in Mid Ocean

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Ridge Basalt (MORB) ( F=3-14; Cl=0.6-14ppm) and OIB (F=34-76; Cl=21-71 ppm) mantle source regions. Comparing these with estimates of bulk silicate Earth (BSE) concentrations (F = 18±8 ppm, Lyubetskaya and Korenaga, 2007; F = 25±10 ppm, Palme and O´Neill, 2003; Cl

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= 30±12 ppm Palme and O´Neill, 2003) indicates that the upper mantle is degassed by 2288% in fluorine and 22-99% in chlorine relative to the primitive mantle. The OIB source

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mantle region has a chlorine concentration that is similar to primitive mantle estimates, but is

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enriched in fluorine by a factor of 1.4-4.2 relative to the primitive mantle. An explanation for the relative fluorine enrichment in the OIB source region is that compared to chlorine, fluorine may be incorporated to a greater extent into the crystal structure of minerals that are

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stable at high P-T conditions and may thus be recycled more efficiently into the deeper mantle through subduction of oceanic crust.

Keywords: halogen; fluorine; chlorine; partitioning; Earth´s mantle; recycling; TOF-SIMS;

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olivine; pyroxene; forsterite; Mid Ocean Ridge Basalt (MORB); Ocean Island Basalt (OIB)

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1 Introduction

Halogens are reactive due to their high electronegativity and show a range from moderate (F, Cl) to highly (I) volatile behavior. This and their moderate (F) to highly (I) incompatible behavior imply that their distribution is influenced by fluid mobility and processes such as partial melting, fractionation and degassing. This makes them very good tracers for transport processes of volatile elements in Earth and planetary systems (e.g. Schilling et al. 1978, 1980; Ito et al. 1983; Jambon et al. 1995). A better understanding of the distribution and behavior of volatile elements and processes in these systems may help us to identify the mechanisms by which the terrestrial planets in our solar system acquire, retain and redistribute volatiles, which remains a fundamental question within Earth and planetary sciences. The determination of halogen abundances and ratios in different reservoirs in the Earth´s mantle will give us the

ACCEPTED MANUSCRIPT ability to understand and quantify volatile input mechanisms into the mantle (e.g. during subduction) and provide insight into the evolution of the mantle over Earth´s history.

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Many studies have determined the halogen content of mid ocean ridge basalts (MORB) and ocean island basalts (OIB) to quantify the distribution and concentration of halogens in the major geochemical reservoirs in the Earth (Schilling et al. 1978, 1980; Ito et al. 1983;

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Michael and Schilling 1989; Déruelle et al. 1992; Wedepohl 1995; Jambon et al. 1995; Saal et al. 2002; Shaw et al. 2008; Wanless and Shaw 2012; Le Voyer et al. 2014). However, the

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very low halogen abundances in basaltic rocks, especially of the heavy halogens Cl, Br and I

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(ppm-ppb), have led to analytical challenges. This has meant that until recently little progress has been made in quantifying distributions and concentrations of halogens in Earth´s geochemical reservoirs (Pyle and Mather 2009). Smith et al. (1981) found that the most

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common mantle minerals only contain halogens at or below the detection limit, which was between 20 and 50 ppm in 1981. This estimate is in overall agreement with the range of values summarized by Newsom (1995) for fluorine, but much higher than recent estimates for

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heavy halogens (Aiuppa et al. 2009). Recent studies (e.g. Ruzié et al. 2012; Kendrick et al.

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2012a, 2012b, 2013) have determined halogen ratios and bulk concentrations in OIB and MORB by using the noble gas technique, which provides detection limits unmatched by any other study (Johnson et al. 2000). Combining these with experimentally determined halogen partition coefficients will allow us to estimate halogen abundances in MORB and OIB mantle source regions. The heavy halogens are expected to be highly incompatible during partial melting of peridotite due to their large ionic radii (Schilling et al. 1980; Déruelle et al. 1992). Fluorine is expected to be moderately incompatible because its smaller ionic radius compared to the heavy halogens allows it to substitute for OH- in hydrous or nominally anhydrous rock forming minerals (Luth, 2003). For a long time, the partitioning behavior of fluorine and chlorine during partial melting of the Earth´s mantle was roughly estimated from correlations

ACCEPTED MANUSCRIPT with other incompatible trace elements measured in natural samples. For example, Schilling et al. (1980) showed that F/P, F/Sr and F/Nd ratios are roughly invariant in basaltic rocks,

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suggesting that these four elements are equally incompatible during partial melting. Similar bulk partition coefficients of F and Nd in MORB-source regions during partial melting of peridotite were also suggested by Saal et al. (2002) and Workmann et al. (2006). Similar

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behavior has been proposed for Cl/K (e.g. Schilling et al. 1980; Saal et al. 2002; Salters and Stracke, 2004) and Cl/Nb in MORB (e.g. Roux et al. 2006). Recent estimates of fluorine and

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chlorine abundances in the primitive Earth´s mantle are given in Table 5, together with

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fluorine and chlorine estimates of the MORB and the OIB source mantle. These estimates are mainly based on element ratios such as F/Sr or Cl/K. Literature values of the fluorine OIBsource concentration range from 8 to 55 ppm, which covers a range from depleted up to

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enriched abundances relative to primitive mantle estimates. A completely independent approach that enables us to determine the halogen partitioning behavior between major mantle minerals and corresponding melt are laboratory experiments

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that simulate partial melting processes at Earth´s mantle conditions. The major upper mantle

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host minerals for halogens are olivine and pyroxene (Bell and Rossman, 1992) as well as possibly minerals from the humite group (Bromiley and Kohn, 2007). Other potential halogen bearing minerals, for example hydrous minerals such as apatite, are unlikely to be present in the mantle source regions of basaltic rocks (Aoki et al. 1981; Sigvaldason and Oskarsson, 1986). Recent studies have produced the first experimentally determined halogen partitioning data for partial melting processes at Earth´s upper mantle conditions (Hauri et al. 2006; O´Leary et al. 2010; Beyer et al. 2012; Dalou et al. 2012, 2014; Rosenthal et al. 2015). However none of these studies reached the high temperatures of OIB source regions. Here, we present experimentally derived fluorine and chlorine partitioning data between olivine, orthopyroxene and corresponding silicate melts in the CaO-MgO-Al2O3-SiO2+F+Cl+Br+I

ACCEPTED MANUSCRIPT (CMAS+F+Cl+Br+I) system at pressures ranging from 1.0 to 2.3 GPa and temperatures between 1350 and 1600°C. In so doing, we have extended the available dataset describing the

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F and Cl partitioning behavior between mantle minerals and melt to P-T conditions reflecting partial melting processes in the OIB source region. Combining these with results of recent studies that have determined halogen ratios and bulk concentrations in OIB and MORB

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source regions by using the noble gas technique (e.g. Ruzié et al. 2012; Kendrick et al. 2013) allows us to estimate MORB and OIB mantle source region halogen abundances by using an

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independent method that does not rely on any element ratios. This enables us to better

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understand the global halogen cycle, particularly how halogens are recycled back into the Earth´s mantle through subduction of oceanic crust.

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2 Materials and experimental methods 2.1 Rationale

Experiments were performed using the model primitive Earth mantle composition proposed by Jagoutz et al. (1979), simplified to the four components CaO-MgO-Al2O3-SiO2 according

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to the procedure of O´Hara (1968). Defined small amounts of halogens were added as CaF2,

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CaCl2, CaBr2 and CaI2. Experiments were run at 1.0 GPa and 2.3 GPa and between 1350°C and 1600°C. Backscattered electron (BSE) images were taken to examine the phase distribution in each sample. Two olivine crystals were also analyzed via transmission electron microscopy (TEM) to investigate whether halogens are incorporated in the form of humitetype defects in the crystal lattice. Fluorine and chlorine at crystal-melt interfaces were mapped to assess equilibrium and quantified via time of flight secondary ion mass spectrometry (TOF-SIMS). Concentrations of bromine and iodine in the crystals were largely below the detection limit of the TOF-SIMS analysis. Determination of their partitioning behavior will be part of a subsequent contribution. Three new MORB-glass standards with defined halogen concentrations and two halogen-free glass standards were synthesized to improve the calibration of the TOF-SIMS

ACCEPTED MANUSCRIPT analyses and allow the correct quantification of higher (up to 0.5 wt%) halogen concentrations.

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2.2 Starting materials Starting material compositions calculated from their initial weight are shown in Table 1. The first two represent the model primitive Earth mantle composition proposed by Jagoutz et al.

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(1979) with the addition of halogens, and the third is similar to one of the starting compositions used by Beyer et al. (2012). They were prepared from pure, analytical grade

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oxides, carbonate, fluoride, chloride, bromide and iodide. First, dried CaCO3, MgO, Al2O3

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and SiO2 powders were thoroughly mixed under acetone in an agate mortar and heated in a platinum crucible to 1500°C for 3 h to release CO2 and adsorbed water. Quenched glasses were then ground and dried CaF2, CaCl2, CaBr2 and CaI2 powders were added as halogen

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sources. Halogen concentrations in the experiments exceed natural halogen abundances in MORBs and OIBs by a factor of 100-400 (Table 1) to enable the quantification of halogens in olivine and pyroxene crystals via TOF-SIMS. Powders were thoroughly mixed again using an

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80°C.

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agate mortar to obtain homogeneous starting materials, and then dried for at least 30 min at

2.3 High pressure-temperature (partitioning) experiments Dried starting materials were placed into annealed platinum capsules of 3.0 mm outer diameter, 2.7 mm inner diameter and 8 mm length, which were sealed shut immediately using an arc welder. All experiments were performed in a Boyd and England type piston cylinder apparatus (Boyd and England, 1960). The pressure assembly consisted of a ½” talc-pyrex tube and a straight-walled graphite heater of 6 mm diameter. The capsule was positioned in a crushable alumina container near the hotspot of the graphite furnace. A pressure correction of 10% was applied for friction loss on the talc-pyrex sleeves and the crushable alumina spacers with an overall uncertainty in pressure determination of ±0.1 GPa. Temperatures were

ACCEPTED MANUSCRIPT determined using a W97Re3-W75Re25 (C-Type) thermocouple that was positioned directly above the sample. Measured temperatures have an uncertainty of ± 20°C.

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Experiments CMAS_1,2 and 4-6 were first heated to 1720°C, held for 30 min and then cooled slowly to the target temperature (1500°C-1600°C; Table 2). Two different cooling rates (1°C/min and 10°C/min) were applied, allowing us to test whether derived partition

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coefficients depend on growth rate. After reaching the target temperature, experimental conditions were kept constant for 5 h before samples were quenched by cutting off the power.

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Experiments CMAS_12 and CMAS_17 were heated to 1600°C, held for 30 min and then

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cooled at 1°C/min to their target temperature (1350°C and 1450°C, respectively). In these experiments, target temperatures were kept constant for 24 h before quenching. After the runs,

(2002). 2.4 Analytical methods

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samples were polished and mounted in pure indium following the method of Hauri et al.

2.4.1 Electron Microprobe Analysis, EMPA

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All samples were examined using a Cameca SX 100 electron microprobe. Grain sizes and

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textures of minerals in partitioning experiments were determined from BSE images (Fig. 2). Chemical compositions of crystals and melts were obtained by wavelength dispersive analysis. Analysis conditions were 15 kV and 20 nA with defocused beam. Counting times were 20 s on the peak and 10 s on the background. Well-determined synthetic and natural standards were used for EMPA standardization. In every sample 10-20 spots with varying distances to the crystal-melt contact were analyzed for each phase to determine the major element concentrations (CaO, MgO, Al2O3 and SiO2; Table 3). In a second session, 10 spots were analyzed for fluorine and chlorine from each phase of sample CMAS_4 and from the olivine in sample CMAS_5. 2.4.2 Time of Flight – Secondary Ion Mass Spectrometry, TOF-SIMS

ACCEPTED MANUSCRIPT Fluorine and chlorine contents at interfaces of large homogeneous melt pools and large, euhedral orthopyroxene and olivine crystals were mapped and quantified via TOF-SIMS.

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Halogen distributions and abundances were analyzed using the IDLE instrument described by Henkel et al. (2006) equipped with a multi-channel plate detector followed by a scintillator and photomultiplier. A ~1 nA (dc) beam of Au+ ions focused to a spot size of ~1 μm was used

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to sputter clean the surface over an area typically 100 μm across.

Pulsing the Au+ beam and using the technique of delayed extraction (Henkel et al. 2006)

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allowed the acquisition of high mass resolution (m/Δm > 4000) secondary ion mass spectra, which is adequate to resolve 19F from the observed isobars 18OH and 16OH3. Chlorine

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abundances were measured as the sum of 35Cl and 37Cl negative secondary ion abundances. A typical challenging interference for the analyses of Cl is the SH molecule, which is not

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relevant in this study because the system is sulfur free. No significant isobaric interferences were detected and the measured 35Cl/37Cl isotope ratio agreed within counting statistic error with the known ratios, which gives confidence that no significant undetected interferences

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affected the measured abundance of this element.

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Negative secondary ions were acquired with counting durations between 15-30 minutes and measured as mass resolved images that span areas of interest containing melt, orthopyroxene and/or olivine. Regions of interest (ROI) within each mineral or melt phase were defined (maximum distance to crystal-melt contact = 20 µm) and mass spectra for the ROI were computed. Two to four measurements of each phase in each sample were performed and averaged. Relative sensitivity factors for calibrating the efficiency of formation of each species of halogen secondary negative ions relative to O- were determined by analyzing well-determined (NIST610, ALV) and newly synthesized glass standards of known composition and halogen abundances (details are provided in the appendix). It is known that changing the chemical composition of an analyzed material and its structure (crystal/amorphous) can have small

ACCEPTED MANUSCRIPT effects on relative sensitivity factors (eg. Shimizu and Hart, 1982). For that reason newly developed standards with MORB-like composition were used and the analyzed areas on

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standards and samples were heavily pre-sputtered for about 5 minutes before the actual analyses to amorphize the surface of crystalline materials to a depth of several tenths of nm before the actual analysis. It has been demonstrated, using this technique, that relative

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sensitivity factors obtained from silicate glass standards can be applied to amorphous as well as crystalline silicate samples pre-sputtered with argon ions (Stephan, 2001; Stephan and

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Lyon, 2013). Botazzi et al. (1992) showed that glasses can be employed to calibrate REE ion

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signals in crystals for quantitative purposes. Their data compare, however, only the relative ionization efficiency between REE and Si in crystals and glasses. A detailed study, which compares the matrix effect of glass standards with defined halogen concentration used for the

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analyses of crystalline samples, is still missing. For that reason, we included an additional uncertainty of a factor of 2 in the determination of crystal halogen concentrations. This makes sure that a potential systematic error stemming from the matrix effect is definitely covered by

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the total uncertainty.

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During the TOF-SIMS measurements, background arises from F and Cl in the vacuum and on the surface. F and Cl signals were observed to increase with time after the surface was sputter cleaned due to redeposition of those species from the vacuum in the cumulative time in between the ion pulses hitting the surface and the next pulses arriving. This results in detection limits for the halogen analysis of 0.01 mg/g for fluorine and chlorine. Abundance measurements presented here are from the sputter-cleaned surface before the background levels of these species increased. 2.4.3 Structural analysis (Transmission Electron Microscopy (TEM) and Focused Ion Beam (FIB) sample preparation) TEM lamellae (15 x 8 x 0.1 µm³) of two olivine crystals (from samples CMAS_1 and CMAS_4) were prepared using the FIB technique (Lee et al. 2003; Overwijk et al. 1993;

ACCEPTED MANUSCRIPT Phaneuf 1999; Wirth 2004). This method allows a large sample orientation freedom so that lamellae with constant thickness and defined orientations can be produced. In this study, the

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orientation of the FIB lamellae was chosen to allow a zone axis to be reached where (001) diffraction spots of the forsterite are visible.

We used conventional TEM as well as analytical and high-resolution transmission electron

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microscopy (HRTEM) on a Tecnai F20 X-Twin TEM operated at 200 kV with a fieldemission gun (FEG) as electron source. The TEM is equipped with a post-column Gatan

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imaging filter (GIF Tridiem). For brightfield imaging and lattice fringe imaging we applied an

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energy filter of 10 eV on the zero loss peak, thus excluding the inelastically scattered electrons from image formation. The TEM has a high angle annular dark field detector (HAADF) and an EDAX EDS analyzer with an ultra-thin window. These were combined

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with EDX in scanning transmission mode to verify major element compositions. The Gatan DigitalMicrograph software was used to analyze the transmission electron micrographs.

3 Results

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Experimental run conditions and products are presented in Table 2, and major element and

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halogen concentrations are presented in Table 3. 3.1 Phase distribution and major element composition In experiments CMAS_1 to CMAS_6 we observed either olivine crystals only, or both pyroxene and olivine, in a MORB-like melt, thus representing the stable assemblage at the target experimental P-T conditions in the CMAS-pyrolite system (Fig. 1). In experiments CMAS_12 and CMAS_17 we observed either olivine (CMAS_12) or orthopyroxene (CMAS_17) crystals in a MORB-like melt. These two assemblages are the same as obtained by Beyer et al. (2012) at similar conditions, with the exception that spinel is absent in experiment CMAS_17. The reason for this is unclear. BSE images of 1 GPa experiments show a mixture of a few anhedral and many euhedral olivine grains, which are approximately rectangular with a side length varying from 50 to 300

ACCEPTED MANUSCRIPT µm (Fig. 2a). No zoning or growth patterns were observed within the crystals. The olivinemelt contact is distinct and sharp. EMPA shows that the olivine is forsterite and reveals a

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small amount of Ca substituting for Mg in the crystal lattice. The CaO-content is homogeneous within each sample and does not show any gradient between crystal rims and cores. This observation is independent of the individual cooling rate and experimental target

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temperature (Table 3). BSE images of experiments CMAS_4 and CMAS_5 reveal a mixture of euhedral olivine and pyroxene grains (Fig. 2b). The olivine major element composition,

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size, and nature of the crystal-melt contact, is within uncertainty identical to those analyzed in

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1 GPa experiments. Pyroxene crystals in experiment CMAS_4 are almost euhedral with roughly rectangular shape, have no visible voids or inclusions, and have a side length of up to 200 µm. In contrast to the olivine crystals, they show in some places slightly blurred, but still

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distinct crystal-melt contacts. EMPA identifies the pyroxene as almost pure enstatite (Table 3) with no sign of any melt or fluid inclusions within the crystals. Some orthopyroxene crystals show, however, compositional zoning of their CaO- and Al2O3-content, which has already

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been observed in high-pressure (0.7-3.5 GPa) melting experiments (Walter and Presnall,

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1994). In experiment CMAS_5 only one pyroxene crystal, with a side length of about 100 µm, was found. Following Beyer et al. (2012), we present only orthopyroxene EMP analyses located close to the orthopyroxene-melt interface (maximum distance to crystal-melt contact = 30 µm), assuming that orthopyroxene halogen concentrations in rims represent equilibrium conditions with the surrounding melt. At these positions, major element EMP analyses are within uncertainty homogeneous (Table 3). Melt pools in all partition experiments appear homogeneous and do not show any major element variations (Table 3). Variations in melt pool major element compositions between experiments are strongly dependent on the target experimental P-T conditions. Near-solidus conditions result in a higher crystal/melt ratio (Table 2), which affects the composition of the melt. Because of the comparably high MgO and low CaO, Al2O3 and halogen content of the

ACCEPTED MANUSCRIPT minerals (especially olivine), melt pool composition is shifted to higher CaO, Al2O3 and halogen, and lower MgO concentrations with increasing crystal/melt ratio. The MgO content

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of the melt pool in sample CMAS_6 is anomalously high (Table 3), suggesting contamination of the starting material of this particular sample. 3.2 Halogen distribution and concentration

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3.2.1 Halogen map of mineral-melt contacts

Halogen maps of areas surrounding crystal-melt contacts are important for determining

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whether experimentally determined partition coefficients represent equilibrium conditions.

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This question is addressed in detail in section 4.1. Figure 3 shows examples of the fluorine and chlorine distribution at olivine-melt contacts (CMAS_1; Fig. 3a-c,e-g and i-k), an olivineand an orthopyroxene-melt contact (CMAS_4; Fig. 3d-f) and orthopyroxene-melt contact

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(Fig. 3h). TOF-SIMS maps in Figures 3a-h show sharp and distinct crystal-melt interfaces of samples with varying target temperatures and cooling rates, with no visible concentration gradient in any phase. This is a strong argument that the determined partition coefficients are

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representative for equilibrium conditions in these particular experiments. In contrast, Fgure 3i

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shows a concentration maximum in the melt located close to the crystal-melt interface, and Figure 3h inhomogeneous 19F distribution in a slow-cooled sample targeted at 1500°C (CMAS_2). These are two examples of non-equilibrium features at crystal-melt interfaces. 3.2.2 Experimentally derived partition coefficients The composition of the newly synthesized glass standards and the amorphous structure of all standards is directly comparable to the composition and structure of the melt in our experiments, so that no further uncertainty needs to be considered for the determination of halogen concentrations in this phase. The potential structural effect (crystalline/amorphous) on relative sensitivity factors for the determination of halogen concentrations in olivine and orthopyroxene was considered with an additional uncertainty of a factor 2 as described in detail in Section 2.4.2. The total uncertainty also includes the uncertainty of the analytical

ACCEPTED MANUSCRIPT method, the uncertainty in determining the slope of the calibration line, and the reproducibility of two to four individual measurements of each phase in each sample. It

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should be noted that the resulting uncertainty in crystal-melt partition coefficients is small compared to the effect of temperature on the observed partitioning behavior. In samples CMAS_4 and CMAS_5, fluorine and chlorine concentrations were also

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determined via EMPA. The analysis of olivine in sample CMAS_5 yields cFol = 1.2(5) mg/g and cClol = 0.4(2) mg/g (uncertainties in parentheses are given as 1). Average concentrations

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of each phase in sample CMAS_4 are cFopx = 0.5(3) mg/g and cClopx = 0.6(4) mg/g; cFol =

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0.5(3) mg/g and cClol = 0.5(3) mg/g; cFmelt = 4.2(5) mg/g and cClmelt = 7.7(6) mg/g. Overall, concentrations determined via EMPA and TOF-SIMS (Table 3) are within uncertainty of each

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other, providing a first order independent calibration of the TOF-SIMS technique. Halogen concentrations determined using TOF-SIMS have, however, a big advantage in that halogen maps with a high spatial resolution of the analyzed areas can be generated. These are a powerful tool to evaluate whether determined concentrations are representative of chemical

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equilibrium (see section 4.1. for details). Furthermore, crystal halogen concentrations could

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not be determined by EMPA in experiments below 1600°C due to the halogen abundances being below the detection limit. For these reasons, only TOF-SIMS data were used for the determination of halogen partition coefficients (Table 3 and 4). We considered only monocrystalline, large and euhedral crystals and determined partition coefficients only at places where TOF-SIMS maps show clear and distinct crystal-melt interfaces with no visible halogen concentration gradient either in the crystal or the melt. Halogen partition coefficients (DXmineral/melt with “X” being fluorine or chlorine) were calculated from the halogen concentrations in the mineral (CXmineral) and corresponding quenched melt (CXmelt) (Table 3) according to DXmineral/melt = CXmineral / CXmelt (Table 4). The uncertainty in determining the relative sensitivity factor (RSF) affects the absolute halogen concentrations in both minerals and glasses (Table 3) in an identical manner, if we

ACCEPTED MANUSCRIPT exclude a potential (crystalline/amorphous) matrix effect. For that reason, only the potential matrix effect, the uncertainty of the analytical method and the reproducibility were considered

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for the determination of a total uncertainty for halogen partition coefficients. Excellent agreement within uncertainty between our partition coefficients (particularly experiment CMAS_12) and data at the same P-T conditions of Beyer et al. (2012), who used the ion

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implantation method as an independent approach to determine relative sensitivity factors for crystalline materials, is a strong indicator of the quality and accuracy of halogen partitioning

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data provided in this study (Fig. 4a).

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Partition coefficients of slow-cooled samples targeted at 1500°C do not represent equilibrium conditions and are therefore excluded from Table 4 and from temperature dependences shown in Figure 4 (see section 4.1. for details). Combining our data with results of recent studies

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(O´Leary et al. 2010; Beyer et al. 2012) shows that fluorine and chlorine partitioning between olivine and melt increases by about 2 orders of magnitude between 1350°C and 1600°C (fluorine: 0.005(3)-0.31(16); chlorine: 0.005(45)- 0.17(9)) and does not show any pressure

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dependence at least within the pressure range of this study (1.0-2.3 GPa). Figure 4d shows

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that chlorine partitioning between orthopyroxene and melt increases by about 1 order of magnitude between 1450°C and 1600°C (0.015(8)-0.16(9)) at a pressure of 2.3 GPa. Combining our newly determined fluorine partition coefficients between orthopyroxene and melt with the data from Beyer et al. (2012) and Rosenthal et al. (2015) suggests that fluorine partitioning into orthopyroxene exponentially increases by about 1.5 orders of magnitude between 1200 and 1600°C and does not show any pressure dependence at least between 1.02.3 GPa (Fig. 4b). 3.2.3 Structural observations TEM was used to investigate the local defect structure of olivine in experiments CMAS_1 and CMAS_4. We explicitly looked for (001)-planar defects such as clinohumite lamellae, because earlier studies showed that fluorine can be incorporated in clinohumite, which might

ACCEPTED MANUSCRIPT be present as lamellae in the olivine structure (Stalder and Ulmer, 2001). If present, clinohumite lamellae can be observed using TEM (examples are given in Drury, 1991;

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Hermann et al. 2007; Risold et al. 2001; Wirth et al. 2001). The lattice parameters of forsterite and clinohumite are comparable in the a (4.76 Å and 4.74 Å) and b (10.23 Å and 10.25 Å) dimensions, whereas c of clinohumite is more than twice that of forsterite (13.66 Å compared

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to 5.99 Å, lattice parameters from Brown, 1970). This makes clinohumite-type lamellae occur perpendicular to c and visible in orientations with (001) diffraction spots. Single oxygen

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defects cannot be detected using TEM, whereas they might become visible if sufficient

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oxygen vacancies condense or order. TEM analyses of crystals in CMAS_1 and CMAS_4 show perfectly homogeneous olivine. Neither melt nor fluid inclusions are detectable; no chemical variations or defects indicating the presence of lamellae were observed. A

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representative bright field image and the corresponding diffraction pattern are depicted in Figure 5.

4 Discussion

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4.1. Assessment of equilibrium

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It is often the case that only small concentrations of incompatible elements such as halogens are likely to be detected in crystalline phases of a crystallisation experiment. To determine halogen concentration in these crystals, analytical methods with a very low detection limit such as SIMS or Laser Ablation Mass spectrometry are required. Due to the spatial resolution, a prerequisite for the use of these analytical methods are crystal sizes of at least 20-50 µm in diameter to make sure that any melt is completely excluded from the analysed area. Crystals of this size cannot be obtained experimentally by keeping the P-T conditions constant. Nearliquidus conditions will result in few very small crystals. Near-solidus conditions will enhance nucleation and result in numerous small crystals. In recent studies, two methods have been used to obtain large crystals in laboratory experiments:

ACCEPTED MANUSCRIPT 1. Addition of small crystal seeds to the starting material that are used as starting nuclei (e.g. Dalou et al., 2012, 2014).

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2. Slow cooling to the experimental target temperature, which results in a low nucleation rate and a high crystal growth rate (Winter, 2001). During cooling of the samples to the target temperature the crystals do not grow at equilibrium conditions. However,

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the halogen concentrations at crystal-melt interfaces might represent equilibrium conditions at those conditions. This method was used by Beyer et al. (2012) and

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Rosenthal et al. (2015) and was also used in this study.

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In the first method, it is extremely unlikely that halogen concentrations in the starting seeds reflect exactly the partitioning behaviour between crystals and melt, so that these will differ between the crystal core and the growing rim. Halogen concentrations will depend on the size

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of the seeds, the halogen concentration gradient between the seed and the growing crystal, the halogen diffusivity at experimental P-T conditions, the crystal growth rate and the dimension and position of the analysed area. A similar problem can also affect crystals grown using the

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second method, if partitioning is temperature dependent, because crystal growth starts before

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the target temperature is reached. This may result in crystal cores which have a halogen concentration that differs from the concentration in the crystal rim. In both cases, concentration gradients in the crystal are a sign of disequilibrium between core and rim (Fig. 3k) due to slow diffusive equilibration. Incompatible elements will be slightly enriched in the melt located at the crystal/melt interface during crystal growth. This effect is negligible if crystal growth rates are relatively slow, since these elements will diffuse quickly away from the interface. If crystal growth is, however, too fast with respect to diffusivities in the melt close to the crystal/melt interface, these elements may not diffuse away fast enough, so that a concentration maximum develops. For that reason it is crucial to make sure in both methods that no concentration maximum of

ACCEPTED MANUSCRIPT incompatible elements develops in the melt located at the melt/crystal interface. An example of such a concentration maximum is shown in Figure 3i.

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We suggest the following criteria to assess whether experimentally determined partition coefficients are representative of natural equilibrium partial melting processes: 

Crystals should be euhedral, monocrystalline and should show no voids, melt or fluid

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inclusions. This is, however, not a sufficient argument, because potential concentration gradients in the crystal or melt cannot be detected. Different cooling rates strongly affect the crystal growth rate (Winter, 2001). If the

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

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determined partition coefficients are identical at different cooling rates (and varying crystal growth rates), this is a strong argument that partition coefficients represent

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equilibrium conditions. If fast-cooled experiments (higher crystal growth rate) show, however, higher partition coefficients this clearly indicates that at least the fast-cooled experiments do not represent equilibrium conditions. 

Homogeneity of major elements in crystals and melt (EMPA) may also give an



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growth.

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indication that experimental crystal growth is comparable to equilibrium crystal

High-resolution halogen distribution maps provided via TOF-SIMS are a powerful tool that allows us to assess whether halogen concentrations at crystal-melt interfaces represent equilibrium conditions. Equilibrium conditions are represented in Figures 3b,c and 3e-h, which show examples of sharp and distinct crystal-melt contacts with no concentration gradient in the crystal or melt. On the other hand, Figure 3i shows a typical example of a concentration maximum located in the melt close to the crystalmelt interface, indicating disequilibrium. Figure 3k is an example of a concentration gradient in a crystal, which can develop if parts of the crystal grow before the target temperature is reached or if seeds are used as starting nuclei (see comments above). In

ACCEPTED MANUSCRIPT both cases, observed concentration gradients indicate that equilibrium partition coefficients cannot be determined.

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Samples in this study were cooled at two different rates (1°C/min and 10°C/min) to the experimental target temperature to vary crystal growth rates (Table 2). At an experimental target temperature of 1600°C, the derived halogen partition coefficients between olivine

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and melt are within uncertainty unaffected by varying cooling rates, thus providing a strong argument that determined partition coefficients represent equilibrium conditions in

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all experiments at this target temperature. At 1500°C target temperature, however, partition coefficients of slow-cooled samples (CMAS_2: DFfo/melt=0.15(6) and

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DClfo/melt=0.12(7); cooling rate 1°C/min) are significantly larger than those of fast-cooled experiments (Table 4). The reason for this is that slow cooling rates allow large parts of

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the olivine crystals to grow before reaching the target temperature. As a consequence, the halogen content of the crystals does not reflect the halogen content at the target temperature, but is elevated as a result of enhanced halogen incorporation into olivine

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during the cooling process caused by the strong temperature dependence of the

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partitioning behavior (Fig. 4a and c). Consequently, partition coefficients of slow-cooled samples are overestimated and do not represent equilibrium conditions at 1500°C target temperature. Fast-cooled samples do, however, represent equilibrium conditions at this target temperature, because the cooling duration and consequently the crystal growth during cooling is negligibly small compared to overall crystal growth. With reference to the criteria for equilibrium listed earlier, this assumption is supported by the following: 

BSE images (Fig. 2) and TEM analyses (Fig. 5) reveal that the analyzed olivines are perfectly monocrystalline and do not show any hints of alteration, growth patterns, fluid or melt inclusions or other halogen-bearing crystalline phases (e.g. clinohumite).



The major element distribution of neither olivine crystals nor the corresponding melt shows any chemical gradient of any major component (CaO, MgO, SiO2, Al2O3).

ACCEPTED MANUSCRIPT 

Halogen maps (Fig. 3b,c) reveal a sharp and distinct olivine-melt interface without any halogen concentration gradients, either within the melt or within the crystal.

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As a consequence of the above arguments, at 1500°C target temperature, only partition coefficients derived from the fast-cooled experiment were used (Fig. 4a and c). Fluorine and chlorine concentrations in orthopyroxene are very high for slow-cooled samples

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at 1600°C (Table 3, experiment CMAS_5). It might be that the slow cooling process affects the halogen incorporation into the crystal. However, these concentrations are based on the

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analysis of the only orthopyroxene crystal that could be located in sample CMAS_5 and need

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to be verified in the future. We can safely assume that the determined concentrations and consequently the halogen partition coefficients between orthopyroxene and melt in sample CMAS_5 do not represent equilibrium conditions at the experimental target temperature.

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Experiments targeted at 1350°C and 1450°C (CMAS_12 and CMAS_17 respectively) were slow-cooled but equilibrated significantly longer (24h) at their respective target temperatures, so that, in contrast to slow-cooled experiments targeting at 1500°C (temperature kept constant

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for 5h), the cooling duration is short compared to the total experiment duration. Following the

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same criteria for equilibrium described above, partition coefficients were determined at locations where halogen maps show a sharp crystal-melt interface and no gradient in the melt or crystal (Fig.3g, h). We can safely assume that crystal and melt are in equilibrium at these locations, although further time dependent experiments would be desirable to further verify this assumption. 4.2 Halogen partitioning at Earth´s mantle conditions 4.2.1 Olivine Overall, our results show the following trend between halogen partition coefficients in olivine: DFol/melt > DClol/melt. This partitioning behavior is in agreement with the results of Dalou et al. (2012) and Fabrizzio et al. (2013), who showed that DFol/melt > DClol/melt , which

ACCEPTED MANUSCRIPT can be explained by the fact that it is usually more difficult to include a larger anion in a mineral structure.

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Combining our newly determined partition coefficients with data from Beyer et al. (2012) and O’Leary et al. (2010) shows that halogen partitioning into olivine is temperature dependent at representative P-T conditions for partial melting processes within the Earth´s upper mantle

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(Fig. 4a). Fluorine partitioning increases by about 2 orders of magnitude between 1350°C and 1600°C. Fluorine partitioning data provided by Hauri et al. (2006) and Dalou et al. (2014) are

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about 0.5-1.0 orders of magnitude higher than the trend shown in Fig. 4a. This might be

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explained by the fact that water was introduced into these samples (1.7-25 wt% H2O and 2.6 wt% H2O in the melt in experiments of Hauri et al. (2006) and Dalou et al. (2014), respectively), which potentially led to an increase of the fluorine partitioning into olivine

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(Crépisson et al. 2014) as a result of hydrolytic weakening (Brodholt and Refson, 2000). Our data suggest that chlorine partitioning into olivine also increases by about 1.5-2 orders of magnitude between 1350°C and 1600°C, which is comparable to the temperature effect on

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fluorine partitioning in this temperature range. The one datapoint provided by Dalou et al.

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(2014) might indicate that the presence of H2O (melt water content 2.6 wt%) increases chlorine partitioning into olivine. Further detailed studies are required to examine the effect of water on the halogen partitioning behavior. 4.2.2 Orthopyroxene Figure 4b illustrates the fluorine partitioning behavior between orthopyroxene and melt by combining data of this study with the studies of Beyer et al. (2012) and Rosenthal et al. (2015). We have not incorporated the data of Dalou et al. (2012) as they state that their experiment giving the most elevated partition coefficient (DFopx/melt = 0.1841(89)) does not represent equilibrium conditions, and their suggestion of increasing partition coefficients with increasing melt polymerization (described as “viscosity”) is inconsistent with the data of Beyer et al. (2012), which do not show any effects of variations in melt polymerization on the

ACCEPTED MANUSCRIPT partitioning behavior of fluorine. In their study, neither variation of the major element chemistry nor of the bulk fluorine concentration was observed to affect the fluorine

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partitioning behavior, providing a strong argument for conditions that are representative for chemical equilibrium between minerals and melt during their experiments. Taking this into account it is likely that the “viscosity” effect described by Dalou et al. (2012) is a result of

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disequilibrium during crystal growth. Partitioning data presented by Hauri et al. (2006) are about 0.5 orders of magnitude higher than our fluorine partitioning data between

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orthopyroxene and melt as also found for olivine. Again this might be explained by the fact

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that these experiments had a melt water content of 1.7-25 wt% H2O, which is significantly higher than the melt water content in the experiments presented by Rosenthal et al. (2015) (melt water content: 0.28-0.86 wt%).

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Our data for chlorine partitioning into orthopyroxene (Fig. 4d) show that chlorine partitioning into orthopyroxene increases by about 1-1.5 orders of magnitude for a temperature increase of 100°C between 1300°C and 1600°C at constant pressure. Combining results of this study with

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the dataset of Dalou et al. (2012) seems to suggest that chlorine partitioning into

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orthopyroxene decreases with increasing pressure at constant temperature (Fig. 4d). However, further studies are needed to verify this. 4.3 Possible incorporation mechanisms of halogens in orthopyroxene and olivine It is important to note that our experiments were not specifically designed to investigate the halogen incorporation mechanisms in olivine and orthopyroxene. To better understand the exact mechanisms, specifically designed experiments combined with structural refinement methods (e.g. nuclear magnetic resonance spectroscopy, single crystal X-ray diffraction) are needed. Bromiley and Kohn (1997) measured a fluorine content in forsterite of up to 0.45 wt%. This high concentration might be the result of an intergrowth of clinohumite lamellae and olivine, as the presence of fluorine in clinohumite may increase its stability to higher temperatures,

ACCEPTED MANUSCRIPT whereas fluorine-free clinohumite is not stable at Earth´s mantle conditions (Stalder and Ulmer, 2001). Our TEM investigations show that halogens are not incorporated in olivine as

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clinohumite lamellae (Fig. 5). Moreover, TEM and BSE images show no sign of voids or melt inclusions. This leaves halogen incorporation in olivine in association with oxygen vacancies as a plausible explanation, as previously suggested in several studies (Bromiley and Kohn,

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2007; Bernini, 2011; Beyer et al. 2012; Dalou et al. 2012; Bernini et al. 2013). Bernini et al. (2013) suggested that fluorine solubility in forsterite at 1100°C and 2.6 GPa

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may be controlled by the substitution [MgO2]2- = [F2]2-. This is the analog mechanism of

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OH incorporation in garnet, in which protonation of oxygen coupled with the formation of Mg vacancies in the octahedral site is the predominant mechanism (Smyth et al. 2006). Another potential mechanism involves the replacement of a [SiO4]4- tetrahedron by a

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[halide]4- quadruplet. This mechanism has been proposed for fluorine incorporation in calcic and magnesian garnets (Valley et al. 1983; Smyth et al. 1990; Visser 1993). Bernini (2011) showed, by using first-principle calculations, that a predominant [SiO4]4- = [halide]4-

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substitution mechanism increases the solubility of fluorine in forsterite exponentially with

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increasing temperatures whereas the effect of pressure is almost negligible. This is in agreement with the halogen partitioning behavior observed in this study and might be a sign that this mechanism is predominantly controlling the halogen incorporation in olivine. This assumption is supported by recent findings of Crépisson et al. (2014), who suggest a coupled incorporation mechanism of fluorine and hydrogen in forsterite. In the presence of water, clumped OH/F defects are formed which are coupled with the formation of a [Si4+] vacancy. Their results indicate that mixed OH/F defects display an even greater stability than solely hydrolytic weakening, so that the combined presence of water and fluorine might increase the partitioning of fluorine into olivine. The authors showed that at 1250°C and 2.0 GPa, fluorine contents up to 1715 ppm can be reached without formation of clinohumite lamellae. This

ACCEPTED MANUSCRIPT might indicate that fluorine and potentially also chlorine incorporation in olivine is associated with the formation of a [Si4+] vacancy in the olivine lattice.

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A possible incorporation mechanism of halogens in these orthopyroxenes could be the coupled substitution of Al3+ and [halide]- with Si4+ and O2- as suggested by Dalou et al. (2012) and Beyer et al. (2012). Mierdel et al. (2007) proposed that OH- is incorporated by the

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coupled substitution of Al3+ and H+ for Si4+ as well as Al3+ and H+ for 2Mg2+, which implies by analogy that the substitution mechanism Al3+ + [halide]- = [Mg2O]2+ is also a potential

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halogen incorporation mechanism in the orthopyroxene structure. These two potential

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mechanisms were recently also proposed as incorporation mechanisms for fluorine in clinopyroxene (Mosenfelder and Rossman, 2013a , 2013b). Other trivalent cations such as Cr3+ or Fe3+ may affect this substitution mechanism (Stalder et al. 2005). It is important to

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note that these halogen incorporation mechanisms require a strong correlation of halogen content with Al2O3 content but only very little effect of pressure and temperature, which is in disagreement with the findings of this study.

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On the basis of our experimental results, we can neither completely support nor disprove any

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specific halogen incorporation mechanisms in olivine and orthopyroxene. The discussed halogen incorporation mechanisms do not provide a simple explanation for the observed chlorine partitioning behavior between orthopyroxene and silicate melt at high temperatures (1600°C), which is only slightly lower than fluorine partitioning at these conditions (Table 4). Further investigations are necessary to fully understand the incorporation of halogens in olivine and pyroxene. 4.4 Halogen concentrations in MORB and OIB mantle source regions Recent estimates of fluorine and chlorine abundances in the primitive Earth´s mantle are given in Table 5, together with fluorine and chlorine estimates of the MORB and the OIB source mantle. These estimates are mainly based on element ratios such as F/Sr or Cl/K. Combining our new experimental partitioning data with recent measurements of MORB and

ACCEPTED MANUSCRIPT OIB bulk halogen concentrations allows us to present a simplified model, which estimates halogen concentrations in the respective mantle source regions by using an independent

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method that does not rely on any element ratios. We apply our partitioning data to natural bulk fluorine concentrations estimated in OIB and normal MORB (N-MORB) samples (see details below). Segregation of partial melts in

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MORB source mantle regions occurs at about 30 km depth (Winter, 2001), i.e. at about 1 GPa and 1250°C (Hirschmann, 2000). Partial melt separation from the solids in OIB source

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regions is assumed to occur at about 2.5 GPa and 1500°C (Davis et al. 2011). We assume that DFcpx/melt/DFopx/melt is about 1.5 based on the datasets of Hauri et al. (2006), Dalou et al. (2012,

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2014) and Rosenthal et al. (2015). DClcpx/melt/DClopx/melt is about 5 (Dalou et al. 2012). Furthermore, following Beyer et al. (2012), we assume that spinel and garnet contribute only

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negligible amounts of halogens to the source rock, due to their low modal abundances in the Earth´s mantle (McDonough, 1990). Bulk fluorine and chlorine partition coefficients are calculated by using an average peridotite composition of 62% olivine, 24% orthopyroxene,

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12% clinopyroxene and 2% minor phases (McDonough, 1990). For the calculation of bulk

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fluorine and chlorine concentrations in MORB- and OIB-source regions (Table 5), we used the model for accumulated fractional melting for the determination of minimum values and the model for equilibrium batch modal melting for the determination of maximum values (Shaw, 1970). We assumed a melt fraction of 6-10% in MORB source regions (Beyer et al. 2012) and 1-5% in OIB source regions (Dasgupta et al. 2007). It is important to note that several simplifications are used in our model compared to natural systems. We use the model primitive Earth mantle composition proposed by Jagoutz et al. (1979) and neglect the potential effect of mantle inhomogeneity on halogen partitioning. We assume modal melting only and do not consider the potential effects of incongruent melting. Our starting composition is nominally volatile and Fe-free. Crépisson et al. (2014) proposed that the incorporation mechanism of fluorine in olivine is not expected to be significantly

ACCEPTED MANUSCRIPT modified in the presence of iron. Their experimental results indicate, however, that a coupled incorporation mechanism of fluorine and hydrogen in olivine may increase fluorine

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partitioning into olivine significantly (see detailed comments above). This is in agreement with fluorine partitioning data provided by Hauri et al. (2006) and Dalou et al. (2014) that are about 0.5-1.0 orders of magnitude higher than the trend shown in Figure 4a.

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Further studies are urgently required to investigate the effect of variations in the chemical system and presence of iron or volatiles such as H2O on the halogen partitioning behavior.

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Our results are, however, in very good agreement with recent estimates (Table 5) of MORB

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and OIB source halogen concentrations, which gives confidence to our approach for estimating them that is independent of any element ratios.

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4.4.1 Fluorine

Bulk fluorine partition coefficients are estimated as D F (MORB source) ≈ 0.005 and D F (OIB source) ≈ 0.08. We apply our partitioning data to natural bulk fluorine concentrations estimated in olivine-hosted melt inclusions of OIB (400-600 ppm, Hauri et al., 2002), and to

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olivine-hosted melt inclusions and groundmass glasses from primitive MORBs of the

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Siqueiros transform fault (50-135 ppm, Saal et al. 2002). Using these, the MORB source region bulk fluorine concentration is calculated as 3-15 ppm, which is in good agreement with recent literature estimates (Table 5). Our data imply that the upper mantle is degassed in fluorine relative to the primitive mantle by 22-88% (Table 5). The bulk fluorine composition of OIB source regions is calculated to be 34-76 ppm. This is in good agreement with the results of Kovalenko et al. (2006), who estimated average fluorine concentrations of about 55 ppm for the OIB source mantle based on melt inclusions in olivine and quenched glasses. This and the fact that our calculation is based on an average peridotite composition (McDonough 1990) suggest that fluorine may be effectively stored in olivine, orthopyroxene and clinopyroxene. No other fluorine-bearing mineral such as humite is needed to store

ACCEPTED MANUSCRIPT fluorine effectively in the mantle, which supports the assumption that olivine (Bell and Rossmann, 1992), orthopyroxene and clinopyroxene (Beyer et al. 2012) are regarded as the

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major host minerals for fluorine in the Earth´s upper mantle. Comparing our calculated OIB source fluorine concentration with estimated fluorine abundances of the primitive mantle (Table 5) implies that the OIB source mantle is enriched

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in fluorine relative to the primitive mantle by a factor of 1.4-4.2. 4.4.2 Chlorine

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Bulk chlorine partition coefficients are calculated as DCl (MORB source) ≈ 0.001 and DCl

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(OIB source) ≈ 0.03. Ruzié et al. (2012) measured chlorine concentrations in N-MORB samples using the noble gas technique, which provides detection limits unmatched by any

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other study (Johnson et al. 2000). Their measured chlorine concentrations range from 10 to 140 ppm, which agrees with results presented by Kendrick et al. (2013), who used the same method, and with concentrations found in N-MORB samples from the East Pacific and MidAtlantic Ridge presented by Jambon et al. (1995), determined via EMPA.

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We calculate the MORB-source chlorine concentration to be 0.6-14 ppm, which agrees well

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with estimates of several studies (Table 5) such as Burgess et al. (2002), who calculated halogen abundances in the subcontinental mantle to be 3 ppm, based on the 40Ar/Cl ratio in African and Siberian coated diamonds. Comparing our data with estimates for primitive mantle chlorine concentrations (Table 5) shows that the upper mantle is degassed in chlorine relative to the primitive mantle by 22-99%. This is similar to our calculations for fluorine and indicates that the upper mantle is depleted in fluorine and chlorine. Representative OIB chlorine concentrations (EM1 and EM2) from the “Pitcairn” and “Society” localities range from 600 to 900 ppm (Kendrick et al. 2013) and were obtained using the noble gas technique. These concentrations agree well with the data of Stroncik and Haase (2004). We calculate OIB source chlorine concentrations to be 21-71 ppm, which is in good agreement with estimates from Kovalenko et al. (2006).

ACCEPTED MANUSCRIPT Our calculations, which are based on average peridotite (McDonough, 1990), imply that olivine, orthopyroxene and clinopyroxene may be regarded as the major hosts for chlorine in

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the Earth´s mantle. As with fluorine, no other halogen-bearing mineral such as amphibole is needed to store chlorine effectively. This supports the assumptions made by Bell and Rossmann (1992), Beyer et al. (2012) and Mosenfelder and Rossman (2013a, 2013b), who

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state that olivine and pyroxenes are the major Earth´s mantle host minerals for halogens. Our calculated OIB source chlorine concentration is in the range of estimates for chlorine

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abundances in the primitive mantle (Table 5). This implies that chlorine is, in contrast to

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fluorine, less efficiently recycled to OIB-source regions through subduction of oceanic crust. 4.5. Recycling of halogens into the Earth´s mantle through subduction of oceanic crust If we assume that no incompatible elements, such as chlorine, are recycled to the OIB source

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region through subduction of oceanic crust, we would expect the OIB source mantle to be slightly depleted in incompatible elements. The similarity or slight enrichment of the chlorine abundance relative to primitive mantle estimates (Table 5) might be a sign that at least small

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amounts of chlorine are subducted into the OIB source mantle region. Several studies provide

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strong indications for this assumption: Scambelluri et al. (1997) showed that hydrous ultramafic systems can act as large-scale carriers of seawater and associated elements, such as halogens, into the deep mantle. John et al. (2004) suggest that the most likely source of the eclogitization-triggering fluid is the dehydration of serpentinites followed by a long distance transport of the released (halogen-bearing) fluid through the slab. Sumino et al. (2010) showed that a marine pore fluid signature can be found in subducted materials stemming from a depth of at least 100 km. Literature values of the fluorine OIB-source concentration range from 8 to 55 ppm, which covers a range from depleted up to enriched abundances relative to primitive mantle estimates (Table 5). Our estimates for fluorine abundances imply that the OIB source mantle is significantly enriched in fluorine relative to the primitive mantle by at least a factor of 1.4-4.2

ACCEPTED MANUSCRIPT (Table 5). The dichotomy between fluorine and chlorine concentrations in the OIB source is fully consistent with an efficient fractionation of chlorine from fluorine during the subduction

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process. While chlorine is more efficiently returned to the surface via magmatism, only comparably small amounts of fluorine leave the subducting slab before reaching deeper mantle regions (Straub and Layne, 2003; Kovalenko et al. 2006). An explanation for this

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partial separation during subduction might be that chlorine is more likely to escape from the subducting slab in hydrous slab fluids at an early subduction stage due to its incompatibility,

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especially at low P-T conditions, as a result of its large ionic radius (1.81 Å) and high fluid

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mobility (Straub and Layne, 2003). A greater amount of fluorine is, however, likely to remain in the slab during subduction as its incorporation into the crystal lattice increases the thermal stability of hydrous amphibole and mica, and it can be incorporated into the anhydrous high-

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pressure phases of eclogite such as garnet (Zhu and Sverjensky, 1991). Another potential mineral that might promote the partial separation of fluorine and chlorine during subduction is apatite. Fluorine heavily partitions into apatite in metamorphic rocks whereas chlorine tends

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5 Conclusions

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to be found only in minor amounts (Spear and Pyle, 2002; Harlov, 2015).

Our data extend the temperature range of previous fluorine and chlorine partitioning experiments, and show a strong temperature dependence for their partitioning in both olivine and orthopyroxene. Where there are reliable data over a range of pressures (fluorine and chlorine in olivine, and fluorine in orthopyroxene), no pressure dependence is observed. A potential effect of pressure on the chlorine partitioning in orthopyroxene needs to be verified in future studies. Back-scattered electron images do not show any fluid or melt inclusions either in olivine or in pyroxene. Furthermore, TEM measurements show that halogens are not incorporated in the form of humite lamellae in olivine. This leaves as a plausible explanation that the halogens

ACCEPTED MANUSCRIPT are incorporated in the form of point defects in the olivine and orthopyroxene lattice, where they are inferred to be charge-balanced via oxygen defects.

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Estimates of fluorine and chlorine abundances in MORB (F=3-14; Cl=0.6-14 ppm) and OIB (F=34-76; Cl=21-71 ppm) source regions were calculated by combining our experimentally determined partition coefficients with natural halogen concentrations in oceanic basalts. Our

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calculations, which are based on Fe- and volatile-free average peridotite modal olivine (62%), orthopyroxene (24%) and clinopyroxene (12%) proportions (McDonough, 1990), are in good

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agreement with literature estimates of halogen concentrations in MORB and OIB source

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regions (Table 5). This implies that these minerals are the major hosts for halogens in the Earth´s mantle. No other halogen-bearing minerals such as humite or amphibole are needed to store fluorine or chlorine effectively in the mantle.

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A comparison of the MORB and OIB source region halogen abundances with estimates of halogen concentrations in the primitive mantle indicates that the Earth´s upper mantle is significantly depleted in fluorine (22-88%) and chlorine (22-99%) relative to the primitive

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mantle. In contrast, the estimated OIB source mantle chlorine concentration is similar or even

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slightly enriched compared to primitive mantle estimates. If we expect an OIB source mantle slightly depleted in incompatible elements, this suggests that at least small amounts of chlorine are recycled into the OIB source mantle region. The OIB source region is, however, significantly enriched in fluorine relative to the primitive mantle by a factor of at least 1.4-4.2 through recycling of subducted oceanic crust. An explanation for the partial separation of chlorine and fluorine during subduction might be that chlorine is more likely to escape from the subducting slab in hydrous slab fluids at an early subduction stage whereas significant amounts of fluorine are likely to remain in the slab during subduction, possibly incorporated in the lattice of apatite, hydrous amphibole or mica, or in anhydrous high-pressure phases of eclogite.

Acknowledgements

ACCEPTED MANUSCRIPT We thank B. Gale and C. Dixon for extensive technical support and J. Charnock for support during microprobe measurements. K. Marquardt thanks the Helmholtz foundation for

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financial support through the Helmholtz-postdoc grant. This work was performed in the framework of the “NOBLE” (The Origin, Accretion and Differentiation of Extreme Volatiles in Terrestrial Planets) research project and funded by the European Research Council (ERC)

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grant No: 267692 under the European Commission Seventh Framework Programme (FP7). We also thank several anonymous reviewers for their comments, which substantially

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improved this paper.

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References

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Aoki, K., Ishiwaka, K., Kanisawa, S., 1981. Fluorine geochemistry of basaltic rocks from continental and oceanic regions and petrogenetic application. Contrib. Mineral. Petrol. 76, 5359. Bell, D.R., Rossmann, G.R., 1992. Water in the Earth´s mantle-the role of nominally anhydrous minerals. Science 255, 1391-1397.

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ACCEPTED MANUSCRIPT Figures Figure 1: Phase relations in the anhydrous CMAS-Pyrolite system. Figure is modified from

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Litasov and Ohtani (2002) and illustrates P-T paths for experiments CMAS_1, 2 and 4-6. Experiments were first heated to 1720°C (grey crosses) and after holding for 30 min were cooled at constant rates of 1°C/min and 10°C/min (grey arrows) to the experimental target

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temperatures (black crosses).

Figure 2: Back-scattered electron images of piston cylinder runs showing a MORB-like melt

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(CMAS_4). See text for further explanation.

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containing (a) olivine grains (CMAS_1) and (b) olivine and orthopyroxene grains

Figure 3: Secondary ion images (a, d), and high resolution 19F (b, e, g, i, k) and 35Cl (c, f, h)

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TOF-SIMS maps of crystal-melt contacts. Color-coding represents number of counts per pixel. Images (a-h) show a sharp crystal-melt contact and homogeneous halogen concentrations in both the minerals and the melt, indicating conditions representing chemical equilibrium during crystal growth. Inhomogeneous halogen distribution in the melt (i) and

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crystal (h) indicate conditions that do not represent equilibrium during crystal growth. (a-c)

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olivine-melt contact in sample CMAS_1 (d-f) olivine-melt and orthopyroxene-melt contact in sample CMAS_4. Similar color-coding indicates similar halogen partition coefficients between these two crystals and melt in this particular experiment. (g) olivine-melt interface in sample CMAS_12 (h) orthopyroxene-melt interface in sample CMAS_17. (i) olivine-melt interface in sample CMAS_12. A concentration maximum in the melt located at the crystalmelt interface clearly indicates disequilibrium. (k) olivine-melt interface in sample CMAS_2, which shows an inhomogeneous 19F distribution in the crystalline phase as a result of temperature-dependent partitioning and substantial crystal growth during the cooling process. Figure 4: Plots of temperature vs. halogen partition coefficients between olivine and melt (a, c), and orthopyroxene and melt (b, d). Red open squares = fast cooling rate (10°C/min), red open diamonds = slow cooling rate (1°C/min). Data from O´Leary et al. (2010) are shown in

ACCEPTED MANUSCRIPT orange, data from Beyer et al. (2012) in brown and data from Rosenthal et al. (2015) in grey. Data from Dalou et al. (2014) (2.6 wt% melt water content) and data from Hauri et al. (2006)

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(1.7-25 wt% melt water content) are shown in blue. Nominally dry experiments from Dalou et al. (2012) are shown in green. (a,c) Dashed lines represent the best fit through the data excluding the slow-cooled 1500 °C samples and experiments with melt water content. (b)

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Dashed lines represent the best fit through the data excluding samples with a high melt water content (1.7-25 wt%) and the dataset of Dalou et al. (2012) (see section 4.2.2. for further

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explanation) (d) Numbers represent pressure conditions (GPa) of each experiment. Dashed

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lines represent the estimated effect of temperature at constant pressures of 1.2 GPa (left, based on dataset of Dalou et al. 2012) and 2.3 GPa (right, based on results of this study). Figure 5: Bright field image of forsterite from experiment CMAS_4 oriented close to [100].

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Upper right image shows the corresponding diffraction pattern viewed down the [100] zone axis. Neither image gives any hint of the existence of any other phase such as clinohumite in the olivine lattice.

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Figure A1: (a) Fluorine/oxygen and (b) Chlorine/oxygen actual ratios vs. measured ratios

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used for TOF-SIMS calibration. Actual ratios (x-axis) are either well known (black circle: 610, Holskin 1999; Jochum et al. 2011; black rectangle: ALV, Koleszar et al. 2009; Clay et al. 2013) or were determined via EMPA (black triangles: “new”).

ACCEPTED MANUSCRIPT Tables Table 1: Composition of starting materials for partitioning experiments CMAS_1, CMAS2;4-

halogen concentrations in mg/g. MgO

Al2O3

SiO2

F (mg/g)

CMAS_1

7.8

42.11

5.03

43.76

1.9

3.9

3.8

3.3

CMAS_2; 4-6

8.21

41.35

4.94

42.97

3.6

7.5

7.5

6.7

CMAS_12;17

9.64

28.65

11.99

47.42

4.2

5.2

6.2

7.3

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CaO

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Sample

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6 and CMAS_12;17 calculated from their initial weight. Major elements are given in wt%,

Cl (mg/g) Br (mg/g)

I (mg/g)

ACCEPTED MANUSCRIPT Table 2: Experimental run conditions, observed product phases (ol= olivine, opx=orthopyroxene, gl=glass) and crystal/melt ratio (calculated from BSE images).

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Phases ol, gl ol, gl ol, opx, gl ol, opx, gl ol, gl ol, gl opx, gl

Crystal/melt ratio 0.43 (16) 0.63 (24) 0.65 (26) 0.68 (26) 0.23 (8) 0.17 (10) 0.74 (18)

1.0

gl

-

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pressure (GPa) 1.0 1.0 2.3 2.3 1.0 1.0 2.3

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-

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1600

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Sample CMAS_1 CMAS_2 CMAS_4 CMAS_5 CMAS_6 CMAS_12 CMAS_17 Halogen standards

Final temperature cooling rate (°C) (°C/min) 1500 10 1500 1 1600 10 1600 1 1600 1 1350 1 1450 1

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Uncertainties are given as 2 values.

ACCEPTED MANUSCRIPT Table 3: Major element compositions (wt%, analyzed via EMPA) and halogen concentration (mg/g, analyzed via TOF-SIMS) of experimental products. Note that in the case of

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orthopyroxene, only EMP analyses located close to the crystal-melt interface (distance to crystal-melt contact max. 30 µm) were taken into account. Uncertainties of major element concentrations are given as 2 values. Determination of uncertainties of TOF-SIMS

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measurements (F and Cl) is described in detail in section 3.2.2. The glass composition in

contaminated. CaO

MgO

Al2O3

SiO2

F mg/g

Cl mg/g

Total

CMAS_1

0.25 (2)

56.21 (24)

0.11 (6)

CMAS_2

0.15 (2)

56.28 (76)

0.09 (2)

43.17 (56)

0.19 (11)

0.23 (13)

99.78 (68)

43.03 (52)

0.91 (39)

1.05 (68)

99.76 (78)

CMAS_4

0.17 (20)

56.43 (88)

0.11 (4)

42.43 (92)

1.30 (69)

1.45 (78)

99.41 (83)

CMAS_5

0.39 (39)

56.63 (44)

0.63 (48)

42.39 (120) 2.56 (135)

0.50 (26)

100.14 (76)

CMAS_6

0.20 (2)

57.09 (42)

CMAS_12

0.21 (2)

56.35 (56)

0.22 (14)

42.74 (108)

1.15 (60)

0.33 (18)

100.40 (98)

0.14 (10)

42.66 (184)

0.02 (1)

0.017 (10)

99.36 (124)

CMAS_4

1.14 (53)

34.40 (88)

7.25 (50)

55.40 (70)

1.12 (65)

1.42 (74)

98.37 (114)

CMAS_5

1.89 (82)

34.12 (130)

CMAS_17

2.31 (92)

35.17 (110)

7.65 (38)

53.32 (22)

3.95 (196)

2.44 (138)

98.74 (145)

7.49 (74)

55.13 (156)

0.74 (39)

0.21 (11)

100.20 (154)

10.49 (60)

23.60 (112)

14.62 (180)

51.29 (168)

4.79 (23)

12.42 (52)

101.72 (125)

CMAS_2 CMAS_4

11.61 (70)

19.60 (134)

14.53 (106)

51.95 (120)

5.95 (23)

8.55 (31)

99.15 (148)

12.26 (128)

15.86 (10)

15.50 (196)

52.28 (30)

5.51 (24)

8.48 (36)

97.31 (156)

CMAS_5

13.34 (108) 15.52 (102)

15.41 (34)

52.21(182)

8.17 (35)

3.59 (11)

97.67 (101)

48.56 (66)

4.98 (23)

1.99 (9)

CMAS_1

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Glass

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Orthopyroxene

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Sample olivine

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sample CMAS_6 suggests that the starting material of this particular sample might have been

CMAS_6

7.64 (48)

35.13 (112)

7.88 (26)

99.92 (106)

CMAS_12

12.57 (12)

19.36 (30)

15.19 (34)

51.64 (60)

4.34 (13)

3.12 (10)

99.82 (98)

CMAS_17

16.56 (84)

17.42 (60)

17.38 (58)

47.12 (164)

11.90 (39)

13.80 (79)

101.42 (135)

ACCEPTED MANUSCRIPT Table 4: Fluorine and chlorine partition coefficients between olivine, orthopyroxene and silicate melt of experiments that allow determination of partition coefficients at conditions

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that represent chemical equilibrium (detailed explanations are provided in sections 4.1.1. and 4.1.2.). Partition coefficients were calculated as Dmineral/melt = Cmineral/Cmelt. Determination of uncertainties is described in detail in section 3.2.2.

0.019 (11) 0.17 (9) 0.14 (8) 0.17 (9) 0.0050 (45)

1500 1600 1600 1600 1350

0.16 (9) 0.015 (8)

1600 1450

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DClmineral/melt

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Experiment DFmineral/melt Olivine 0.040 (24) CMAS_1 0.24 (13) CMAS_4 0.31 (16) CMAS_5 CMAS_6 0.23 (12) CMAS_12 0.005 (3) Orthopyroxene 0.20 (14) CMAS_4 0.06 (4) CMAS_17

Final cooling temperature rate pressure (°C) (°C/min) (GPa) 10 10 1 1 1

10 1

1.0 2.3 2.3 1.0 1.0 2.3 2.3

ACCEPTED MANUSCRIPT Table 5: Estimates of fluorine and chlorine abundances in the primitive mantle, the MORB source mantle region and the OIB source mantle region from the literature, and estimates of

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MORB source and OIB source halogen abundances from this study. All values are given in ppm. See section 4.4 for further discussion. Fluorine 181-252 6-153,4 84-5513 3-14 34-76

Lyubetskaya and Korenaga (2007), 2Palme and O´Neill (2003), 3Salters and Stracke (2004), 4Beyer et al. (2012), Schilling (1978), 6Michael and Cornell (1998), 7Burgess et al. (2002), 8Mc Donough (2003), 9Bonifacie et al. (2008), 10Dreibus and Wänke (1987), 11Sharp and Draper (2013), 12Straub and Layne (2003), 13Kovalenko et al. (2006)

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Chlorine 30±122 0.5-212;5-11 21(+23/-11)13 0.6-14 21-71

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primitive mantle MORB source (literature) OIB source (literature) MORB source (this study) OIB source (this study)

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ACCEPTED MANUSCRIPT Table A1: Major element composition and halogen concentration of new synthesized halogen standards. Major element compositions (wt%) and fluorine and chlorine concentrations of

Al2O3

SiO2

F mg/g

Cl mg/g

Total

13.58 (11)

22.32 (24)

15.51 (18)

48.70 (36)

b.d.

b.d.

100.11 (54)

14.54 (20)

20.70 (44)

15.47 (28)

49.71 (47)

b.d.

b.d.

100.42 (63)

12.97 (6)

21.29 (25)

15.32 (18)

48.48 (24)

1.2(1)

3.4(1)

99.05 (50)

13.48 (63)

21.43 (25)

15.95 (15)

48.90 (41)

2.6(2)

1.4(1)

99.96 (63)

13.31 (6)

20.81 (14)

15.94 (14)

48.45 (56)

3.0(1)

7.4(1)

99.36 (77)

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MgO

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Halogen standard 0 Halogen standard 0b Halogen standard 1 Halogen standard 2 Halogen standard 3

CaO

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standards (mg/g) were analyzed via EMPA. Uncertainties are given as 1 values.

ACCEPTED MANUSCRIPT Appendix TOF-SIMS calibration (and synthesis of new halogen standards)

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To the best of our knowledge, no well-characterized standards with halogen concentrations greater than 0.1 wt% exist. However, the expected concentration of the individual halogens within the melt is up to 0.5 wt%. For that reason, two new halogen-doped glass standards and

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two halogen-free “blanks” were produced to enable reliable quantitative analyses of the melt halogen concentrations. We chose a CMAS-MORB-like composition, which is representative

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of the composition of melt pools in the partitioning experiments (Table 3). Various amounts

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of fluorine, chlorine, bromine and iodine were added to give a similar range of concentrations to those expected in the melt. In this study, we will focus on fluorine and chlorine contents

well-known standards.

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only. TOF-SIMS calibrations were performed using a combination of new synthesized and

Synthesis of halogen standards and “blanks” All starting materials were prepared from pure, analytical grade oxides, carbonates, fluorides,

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chlorides, bromides and iodides. Dried CaCO3, MgO, Al2O3 and SiO2 powders were

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thoroughly mixed under acetone in an agate mortar and subsequently heated to 1500°C for at least 2 h. After quenching, these glasses are volatile free, which was confirmed via EMPA, so that blanks were not treated further (Halogen standard 0 and 0b, Table A1). Quenched glasses used for halogen bearing standards were reground immediately. For halogen-bearing standards, dried CaF2, CaCl2, CaBr2 and CaI2 powders were added as halogen sources and thoroughly mixed with the glass using an agate mortar. Prepared mixtures were dried for at least 30 min at 80°C before loading into a platinum capsule of 3.0 mm inner diameter, 3.2 mm outer diameter and 8 mm length, which was sealed shut immediately using an arc welder. Standards were heated up to 1600°C at 1 GPa using the pressure assembly and piston cylinder apparatus as described in section 2.3 and quenched after 5 h. Afterwards, the standards were

ACCEPTED MANUSCRIPT polished, mounted in pure indium following the method of Hauri et al. (2002) and examined via EMPA.

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Electron Microprobe Analysis The major element composition as well as the fluorine and chlorine concentrations of all new synthesized standards was examined using a Cameca SX 100 electron microprobe as

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described in section 2.4.1. In each standard, 30 spots were analyzed and averaged. BSE images of all halogen standard materials show a monophasic glass. EMP analysis shows

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a composition (Table A1) similar to the melt pool compositions analyzed in partitioning

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experiments (Table 3). Absence of voids, bubbles, major element or halogen gradients reveals an almost perfectly homogeneous composition of the glass standards. F/O and Cl/O ratios were calculated using oxygen totals assuming that all major elements are oxidized, and the

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fluorine and chlorine concentrations derived via EMP measurement. Fluorine and chlorine concentrations of the two “halogen-free” glasses were below the detection limit of the method.

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TOF-SIMS calibration

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Two external glass standards (NIST610 (fluorine: 295 (16) ppm, Holskin 1999; chlorine: 330 (40) ppm, Jochum et al. 2011) and ALV (Koleszar et al. 2009, Clay et al. 2010)), the newly synthesized glass standards (halogen standards 1,2 and 3; Table A1) and two blanks (Table A1) were analyzed via TOF-SIMS. Two measurements in each sample were performed and averaged. A good agreement of the individually measured halogen/oxygen ratios confirms that F and Cl are homogeneously distributed within each standard. Figure A1 combines the TOF-SIMS halogen/oxygen ratios vs. ratios of the two well-known glass standards NIST610 and ALV and of the ratios of the newly synthesized standards. The uncertainties for the new halogen standards shown in Figure A1 include the uncertainty of the TOF-SIMS analysis, the reproducibility of the TOF-SIMS analysis, and the uncertainty of the microprobe analysis. F/O-ratios (Fig. A.1a) and Cl/O-ratios (Fig. A.1b) show very well fitted

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sensitivity factor (RSF). The calibration does not show a 1:1 line because halogens form negative ions more easily than oxygen. Derived calibration lines were used for the determination of halogen concentrations (Table 3) and accordingly partition coefficients

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(Table 4).

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ACCEPTED MANUSCRIPT Highlights New experimental data describe the F and Cl partitioning between mantle minerals and melt in MORB and OIB source regions.

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Introduction of a TOF-SIMS technique for measuring halogens.

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Halogens are incorporated as point defects in the olivine and orthopyroxene lattice.

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Cl and F are both depleted relative to BSE in the MORB source but equal (Cl) or are greater (F) than BSE in the OIB source.

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Excess F in the OIB source suggests that F and Cl are partially separated during subduction.

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