SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION DOI: /NGEO1959 Helium in Earth s Early Core M.A. Bouhifd 1,2, A.P. Jephcoat 1, V.S. Heber 3,4 and S.P. Kelley 3 1 University of Oxford, Department of Earth Sciences, South Parks Road, Oxford, OX1 3AN, UK 2 Laboratoire Magmas et Volcans, Université Blaise-Pascal, CNRS UMR 6524, OPGC IRD, 5 Rue Kessler, Clermont-Ferrand Cedex, France 3 Department of Environment, Earth and Ecosystems, Open University, Milton Keynes, MK7 6AA, UK 4 Department of Earth and Space Sciences, University of California, Los Angeles, USA. Starting materials Pure Fe (High purity %) and FeNiCo (54 wt% Fe, 29wt% Ni, 17 wt% Co) metal foils were obtained from Goodfellow Cambrdige Ltd. The CI-model composition (SiO wt%, MgO 35.1 wt%, FeO 8.5 wt%, Al 2 O wt%, CaO 3.3 wt%) is the same starting material used in our previous work (Bouhifd & Jephcoat, 2006). The proportions of SiO 2, Al 2 O 3, MgO and CaO were chosen to be that of model CI-chondrite. The mix of CI-chondrite with an FeNi-alloy consisted of 66 wt% of CI-maj, 24 wt% Fe metal and 10 wt% Ni metal (Bouhifd & Jephcoat, 2003). High-pressure set-up In the LHDAC He solubility and partitioning experiments, we used diamond anvils with 500 micron culets, and stainless-steel gaskets pre-indented to a thickness of 50 microns and drilled to a diameter of 150 microns. Samples were mounted in the pressure chamber and helium loaded with a high-pressure gas-loading technique at 200 MPa (Jephcoat et al., 1987). Nominal pressures were measured by the ruby fluorescence method and the hydrostatic pressure scale. In our experiments, the fluorescence emission of one or two ruby grains at the edge of the gasket hole were systematically recorded at room temperature before and after heating for each run. No differences were observed on both pressures. Such pressures may differ from the real pressure of the sample during heating as thermal pressure may be generated in the laser spot. In our configuration it is impossible to quantify effects of the thermal pressure, but an increase of a few GPa on top of the nominal pressures reported was deduced from the LHDAC studies of olivines and periclase (Andrault et al., 1998, Fiquet et al., 1996). NATURE GEOSCIENCE 1

2 The temperatures were determined spectro-radiometrically with a fit to a grey-body Planck function. During melting of samples (silicate or metal) under pressure, the surrounding solid helium melts by conductive heating from the heated sample, allowing dissolution of He into the melts. To reduce the temperature gradient across the samples, we used a relatively broad, defocused beam (hot-spot size around 20 microns, with central temperature gradients of less than 20 K/μm). With this temperature gradient, the maximum difference between the temperatures at the centre and the edge of the laser spot is about 200 degrees which was taken as a conservative uncertainty on the temperatures. During a heating, when the melt is detected optically, the laser power was adjusted to give temperature higher by about 100 degrees in comparison with the estimated melting temperatures. In the Figure S1 we reported the measured temperatures for two runs at 10 and 16.7 GPa. Figure S1. Measured temperatures for two runs at 10 and 16.7 GPa, respectively. The average temperature (dashed and plain lines) is taken as the mean of the 6-7 temperature measures during the heating stage Average = 2892 K Standard deviation = 110 K Metal/silicate partitioning experiment at 16.7 GPa Average = 2515 K Standard deviation = 78 K Metal/silicate partitioning experiment at 10 GPa Number of measures

3 Attainment of equilibrium during High-Pressure and High-Temperature experiments in the LHDAC For HP-HT experiments in the LHDAC, a key factor in assuring the quality of data produced by the experiments is attention to the attainment of equilibrium. The good agreement between the Ar contents of the silicate liquids studied using LHDAC and those determined from multianvil or piston-cylinder experiments provides the strongest evidence for the attainment of the equilibrium during the LHDAC experiments (White et al., 1989; Carroll and Stolper, 1991; Chamorro-Perez et al., 1998; Bouhifd and Jephcoat, 2006). Another way to demonstrate equilibrium during the LHDAC experiments is that in the range of the pressures investigated 2 20 GPa, the diffusion coefficients for most elements in silicate liquids range from to 10-9 m 2 /s in the K temperature interval (Reid et al., 2001) for Ar and He, for example, the diffusion coefficient is about and m 2 /s in a tholeiite basalt at 1350 ºC and at ambient pressure (Lux, 1987) -- Assuming an average of the diffusion coefficient of m 2 /s at HP-HT, the characteristic diffusion length ( 2 Dt ; where D is the diffusion coefficient and t is time) in a minutes experiment is between 600 and 850 microns which is significantly larger than the typical diameter of our sample chamber. Another factor which can influence our noble gases solubility is their loss from the heated samples during the quench. The much higher thermal diffusivity in a LHDAC (about 10-6 to 10-7 m 2 /s) in comparison to the noble gases diffusion in silicate melts at HP rules out a potential ex-solution of noble gases from the heated samples during quenching. Analytical technique All He analyses with the UV laser ablation microprobe were carried out at the Open University, UK. The laser system consists of a NewWave UP213 combined laser/microscope system uses a 10 Hz quadrupled Nd-YAG laser producing 10 ns pulses at 213 nm. The laser beam has a flat top profile which can be focussed through a range of apertures, and the laser is thus capable of producing flat bottom pits with sizes ranging from 10 to 250 microns. Individual laser pits for the present experiments were produced by ablating the sample for periods of 1 to 5 seconds. The extracted gas was cleaned using three SAES AP-10 getters to remove active gases before analysing helium with a MAP noble gas mass spectrometer. Because the

4 analysed 4 He concentrations were low, a multiplier collector was used. Analyses were corrected for 4 He measured blank values. It was generally necessary to ablate relatively large areas (20 to 40 microns diameter for a single spot size) to extract measurable quantities of helium from the metals. The Helium contents data for CI chondrite melt and Fe-rich alloys are reported in Tables S1 S3. The reported values represent the bulk (sub-surface) average of 5 to 20 analyses. Error bars correspond to 1σ (ranging from 7 to 25 %) of the average of the chemical analyses. Errors quoted for He concentrations are reported as 1σ statistical error of the average analyses. In this work the uncertainties were between 7 and 25% for silicate melt analyses, but they can be as high as 100% for the very low concentrations measured in metals. We have previously studied the release of He from laser ablation pits, and ensured the complete extraction from within the pit with relatively little released from laboratory grown crystals in glass. Further details regarding the UV laser technique can be found in Kelley et al. (1994) and Heber et al. (2007). In common with the work of Heber et al. gas quantities in this study were calibrated using known amounts of gas from a pipette system. To convert measured He abundances into concentrations, the volumes of all laser pits were precisely determined using a vertical scanning white light interferometer (Zygo Instrument at Imperial College, London), with nanometre-scale vertical precision. One example of the output from the interferometer is shown in figure S2-a, which shows a contour map of the sample surface within the gasket (note the three laser pits). Figure S2-b shows a software generated cross section of one of the pits showing depth measurements. A similar technique was used to quantify the laser penetration for a given laser power for different materials, and the calibration used to estimate volumes ablated during the experiments. Spatial resolution A possible artifact in our experiments/analyses is silicate contamination of the metal phase, giving apparent high partition coefficients. Additional experiments were performed in which pure alloy starting material was used (either FeNiCo or pure Fe metal foils), to test the influence of the existing silicate composition in the same charge. Comparison of the He concentrations in the metallic phases (metal near the silicate, during partitioning experiments, and in the pure metal, during solubility experiments) showed similar values, hence ruling out any major contamination of the metal with silicate during the LHDAC experiments and UVLAMP analyses (Tables S1-S3).

5 Figure S2-a. Example of the output of typical LHDAC sample after UVLAMP analyses from the interferometer showing a contour mapp of the sample surfacee with the three laser pits. Figure S2-b. A measurement. software generated cross section of one of the pits showing depth

6 Table S1. Helium concentrations (ppm) in molten CI-chondrite. P (GPa) T (K) He contents (ppm) Bulk CI-chondrite (± 365) (± 89) (± 145) (± 100) (± 22) (± 85) (± 71) Table S2. Helium concentrations (ppm) in molten CI-chondrite and Fe 80 Ni 20 alloy liquid analysed in the same run products. P (GPa) T (K) He contents (ppm) He contents (ppm) D He Bulk CI-chondrite Bulk Fe 80 Ni 20 alloy (± 290) 4.0 (± 1.5) 1.7 (± 0.8) (± 330) 2.0 (± 1.0) 6.7 (± 4.1) (± 190) 2.0 (± 1.0) 8.0 (± 4.6) (± 380) 6.7 (± 2.0) 3.3 (± 1.6) (± 35) 5.9 (± 2.0) 1.7 (± 0.7) 10-2

7 Table S3. Helium concentrations in pure metals. Pure Iron P (GPa) T (K) He contents (ppm) He contents (ppm) D He Bulk molten Fe CI-chondrite melt* (± 0.5) 2500 (± 190) 6.4 (± 2.5) (± 1.0) 2050 (± 400) 1.7 (± 0.8) (± 2.0) 600 (± 100) 8.8 (± 4.8) (± 1.4) 600 (± 150) 1.0 (± 0.5) (± 1.2) 350 (± 50) 9.4 (± 4.8) 10-3 Fe 54 Ni 29 Co 17 alloy P (GPa) T (K) He contents (ppm) He contents (ppm) D He Bulk molten FeNiCo CI-chondrite melt* (± 1.0) 2970 (± 330) (± 1.5) 2500 (± 100) 2.3 (± 0.7) (± 0.8) 600 (± 100) 5.3 (± 2.2) (± 1.2) 360 (± 50) 1.3 (± 0.5) (± 1.0) 250 (± 50) 6.0 (± 5.2) (± 1.0) 300 (± 50) 4.7 (± 4.1) 10-3 *He contents (ppm) interpolated from the experimental results taken from Table S1. - All analyses were corrected for 4 He measured blank values taken before and after each ablation. - The reported He concentrations above in both bulk silicate and bulk Fe-rich alloys are the average of 5 to 20 analyses (as those reported in Figures 2a-2b in the main text). - From the reported He analyses above, we show that the He concentrations in the metallic phases (metal near the silicate, during partitioning experiments, and in the pure metal, during solubility experiments) are similar, ruling out any contamination of the metal with silicate during the LHDAC experiments and UVLAMP analyses.

8 Uncertainties on D He The errors on D He were calculated using the following relation:, where D He is the partition coefficient of He and X metal and X silicate are respectively the concentration of He (ppm) into the metal and silicate phases, and ΔX metal and ΔX silicate are the uncertainties on X metal and X silicate, respectively. ΔX metal and ΔX silicate were determined as 1 standard deviation from the He analyses either in silicate or metal phases. These uncertainties (ΔX metal and ΔX silicate ) include the uncertainties derived from the volume determination. In all cases there is roughness or variable depth for an individual pit and the reported uncertainties of He are an indication of this variation. Effects of silicate melt and metal compositions on the D He To estimate an effect of the silicate melt composition on the solubility of He we used a recent model of calculation published by Iacono-Marziano et al. (2010). Although the database for He solubility in silicate melts used in this model is limited to ~200 MPa and 1500 C, we noticed that He solubility in CI-chondrite melt at 1000 MPa and 2000 C is in the same range as the He solubilities determined at the highest pressures in our study (12-16 GPa). We then used the model of Iacono-Marziano et al. at 1 GPa and 2000 C to determine the He solubility in various silicate melts (including all known chondrites that can constitute the building block of our planet: CI, EH, L-LL, CV, CO and CM model chondrite compositions as well as the PUM primitive upper mantle taken from Javoy et al. (2010)). The D He obtained in this way (assuming that He solubility in Fe-rich metal is 2 ± 1 ppm) ranges from to , well within the experimental values we reported in our study. Although more experiments are needed to test the calculations made above, we are confident that changing the Mg/Si and Al/Si to match the present mantle, or the building blocks of our planet, will not significantly affect the partition coefficient of He between molten metal and silicate liquid. About metal, we performed solubility experiments on pure metals as well as metal alloy silicate liquid partitioning studies. The He solubility experiments are listed in Tables S1-S3 and show that the chemical composition of the metal (pure Fe, Fe 80 Ni 20, Fe 58 Ni 29 Co 17 ) has only a small effect on the solubility of He into molten Fe-rich alloys. Indeed, within the experimental uncertainty, all the measured values of solubility appear equivalent. So we believe that an alloy with a proportion of 95:5 of Fe and Ni will have the same range of He

9 contents. In addition, a recent study by Zhang and Yin (2012) obtained a D He for a metallic composition containing (96.4 wt% Fe, 2.4 wt% Si, 1.1 wt% O) similar D He of ~(9±3) Given the presence of other light elements in the core, we do not expect the alloy composition by itself to have a strong effect on D He between metal and silicate.

10 References Andrault, D. et al. Thermal pressure in the laser-heated diamond-anvil cell: An X-ray diffraction study. Eur. J. Mineral. 10, (1998). Bouhifd, M.A. & Jephcoat, A.P. The effect of pressure on partitioning of Ni and Co between silicate and iron-rich metal liquids: a diamond-anvil cell study. Earth Planet. Sci. Lett. 209, (2003). Bouhifd, M.A. & Jephcoat, A.P. Aluminium control of argon solubility in silicate melts under pressure. Nature 439, (2006). Carroll, M.R. & Stolper, E.M. Noble gas solubilities in silicate melts and glasses: New experimental results for argon and the relationship between solubility and ionic porosity, Geochim. Cosmochim. Acta 57, (1993). Chamorro-Perez, E., Gillet, P., Jambon, A., Badro, J. & McMillan, P. Low argon solubility in silicate melts at high pressure. Nature 393, (1998). Fiquet, G., Andrault, D., Itie, J.P., Gillet, P. & Richet, P. X-ray diffraction of periclase in a laserheated diamond-anvil cell. Phys. Earth Planet. Int. 95, 1-17 (1996). Heber V.S., Brooker R.A., Kelley S.P. & Wood B.J. Crystal-melt partitioning of noble gases (helium, neon, argon, krypton and xenon) for olivine and clinopyroxene. Geochim. Cosmochim. Acta 71, (2007). Iacono-Marziano G., Paonita A., Rizzo A., Scaillet B. & Gaillard F. Noble gas solubilities in silicate melts: New experimental results and a comprehensive model of the effects of liquid composition, temperature and pressure. Chem. Geol. 279, (2010). Javoy M. et al. The chemical composition of the Earth: Enstatite chondrite models. Earth Planet. Sci. Lett. 293, (2010). Jephcoat, A.P., Mao, H.K., & Bell, P.M. in Hydrothermal Experimental Techniques (Wiley-Interscience, New York, 1987). Kelley, S.P., Arnaud, N.O. & Turner, S.P. High spatial resolution 40 Ar/ 39 Ar investigations using an ultra-violet laser probe extraction technique. Geochim. Cosmochim. Acta 58, (1994). Lux G. The behavior of noble gases in silicate liquids: Solution, diffusion, bubbles and surface effects, with applications to natural samples. Geochim. Cosmochim. Acta 51, (1987). Reid, J.E., Poe, B.T., Rubie, D.C., Zotov, N. & Wiedenbeck, M. The self-diffusion of silicon and oxygen in diopside (CaMgSi 2 O 6 ) liquid up to 15 GPa. Chem. Geol. 174, (2001). White, B.S., Brearly, M. & Montana, A. Solubility of argon in silicate liquids at high pressures. Am. Mineral. 74, (1989). Zhang, Y. & Yin, Q.Z. Carbon and other light element contents in the Earth s core based on first-principles molecular dynamics. PNAS 109, (2012).