Nitrogen speciation in upper mantle fluids and the origin of Earth s nitrogen-rich atmosphere

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1 Supporting Online Material for Nitrogen speciation in upper mantle fluids and the origin of Earth s nitrogen-rich atmosphere Sami Mikhail & Dimitri Sverjensky S1. Supplementary discussion S1.1 The selection of N 2, 20 Ne, 36 Ar, 84 Kr, and 130 Xe In comparing the chemistry of telluric planetary atmospheres, several factors must be considered to enable accurate comparisons between different planets. The relative abundances of atmospheric N 2 and primordial noble gases can be used to investigate the role of mantle petrology and geochemistry on volcanic degassing, and to place constraints on the similarities and differences during the evolution of the terrestrial planets (Table S1). Here we use the noble gases and nitrogen because Earth s atmosphere has been terraformed by life 1, resulting in a CO 2- poor and O 2- rich atmosphere, whereas the atmosphere of the lifeless terrestrial planets (Venus and Mars) contain >95% CO 2 and only trace quantities of O 2 2. To enable comparative investigations of these planetary atmospheres the data must be selective to avoid effects of biological processes and geology/weathering (i.e. the decay of 40 K to 40 Ar). Most Ar in the atmospheres of Earth and Mars is not primordial, but instead produced through the decay of radiogenic 40 K (with 40 Ar/ 36 Ar ratios of 298 and 1900 ± 300 respectively) 1-2, whereas the ratio of primordial to radiogenic Ar in the Venusian atmosphere is anomalously low, with a 40 Ar/ 36 Ar ratio of 1.03 ± Therefore, the abundance of total Ar (combined 40 Ar, 38 Ar and 36 Ar) in planetary atmospheres appears to be more a function of the abundance of exposed K to the atmosphere, and not the amount of degassed Ar released through volcanism. NATURE GEOSCIENCE 1

2 Our approach is to use total atmospheric N 2 relative to the abundance of primordial noble gases ( 20 Ne, 36Ar, 84 Kr, and 130 Xe). This approach is based on the following factors. Firstly, the ratio of surficial nitrogen (biosphere, sediments and oceans) to atmospheric nitrogen is only ~ , therefore Earth s atmospheric nitrogen abundance is not controlled by the biosphere (contrary to CO 2, CH 4, O 2 and H 2O). Secondly, the primordial noble gas abundances in the atmosphere are not fractionated in the biosphere. S1.2 The reason for ruling out the role of bridgmanite to explain the data in Figure 1 Historically, geochemists have assumed that they should group nitrogen with the noble gases (Ne, Ar, Kr, Xe). This is because molecular nitrogen (N 2) and noble gases are inert gases within planetary atmospheres and are highly incompatible elements in common mantle minerals 5-6. However, recent experimental data show this assumption to be incorrect regarding mantle differentiation. Molecular nitrogen is indeed incompatible in silicate minerals, but ammonic nitrogen can be a moderately compatible element in K- Ca- Na bearing silicates phases, such as phlogopite and clinopyroxene 5-6. Likewise, Ne, Ar and Kr have been show to be soluble in bridgmanite (a recently named mineral, formerly known as MgSiO 3- Perovskite), whereas Xe remains incompatible 7. This means nitrogen can be fractionated from the noble gases, depending upon speciation, and theoretically, the noble gases can be fractionated from nitrogen during bridgmanite crystallization. If bridgmanite was involved in the origin of Earth s atmospheric N 2/noble gas enrichment one would expect the Martian and Venusian atmospheres to show very different results for ratios such as Ne/Xe or Ar/Xe because of the large differences in bridgmanite modal abundances within the mantles of these planets. However, this is certainly not the case (Figs.S1 and 1 respectively). S1.3 The reason for ruling out the role of Earths magnetic field to explain the data in Figure 1 It is theoretically possible that Earths magnetic field has enabled more atmospheric nitrogen retention relative to the Martian and Venusian atmospheres. This shielding effect has certainly occurred, but cannot explain the data in Fig. 1 for three reasons. Firstly, the Venusian atmosphere contains more nitrogen by 2 NATURE GEOSCIENCE

3 SUPPLEMENTARY INFORMATION mass relative to Earth (factor of 3) 3. Secondly, one would expect mass- dependent fractionation reflected from N to Xe abundances, and also between light and heavy primordial noble gases during loss to space, which would be recorded within the given planetary atmosphere and is not seen here. Instead, the 20Ne/ 84 Kr ratio of Earth s atmosphere falls alongside the Venusian and Martian values (Fig.S1). Finally, the high 40 Ar/ 36 Ar ratio of the Martian atmosphere (1900±300) relative to Earth s (298) 1-2 has been proposed to be a function of early loss of more primordial 36 Ar from a weak Martian atmosphere relative to Earth, which was followed by volcanic degassing of 40 Ar produced by 40 K decay on both planets. Interestingly, the 40Ar/ 36 Ar ratio of the Venusian atmosphere is ~1 3, and shows comparable 36 Ar/primordial noble gas and 20Ne/primordial noble gas abundances to Earth and Mars (Fig.S1 and 1 respectively). Therefore the shielding effect of Earth s magnetic field cannot explain the differences between Earth s N 2- enrichment and the relative, and comparable, N 2- depletions for Mars and Venus. S1.4 The reason for ruling out N- rich cores for Mars and Venus relative to Earth to explain the data in Figure 1 Another hypothetical possibility is that Nitrogen should partition into a metallic phase during metal- silicate differentiation under equilibrium conditions 8. However, because of their similarity in size (within 5%), Earth and Venus would likely have had similar P- T- fo 2 conditions of core formation, provided that the giant moon- forming impact and core formation occurred under similar conditions. Ergo, Mars should be an outlier to the Earth- Venusian system, which is not the case (assuming comparable chemical composition for the starting materials during accretion). Overall then, the partitioning of N during core formation is not adequate to explain the differences for the atmospheric N 2/primordial noble gas ratios for the atmospheres of Earth and Venus S1.5 The reason for ruling out preferential primordial noble gas loss from Earth during the Moon- forming impact to explain the data in Figure 1 It could be hypothesized that loss of Earth s early atmospheric N 2 and the primordial noble gases during NATURE GEOSCIENCE 3

4 a large impact (i.e. the proposed Moon- forming impact) would be manifested as a distinct geochemical signature in Earths atmosphere and not in the atmospheres of Venus and Mars. In other words, we would predict highly fractionated primordial noble gas ratios relative to the Martian and Venusian atmospheres 9. In fact, Earth s atmosphere does show lower total concentrations of noble gases and molecular nitrogen relative to Venus, but there is no indication of fractionation of the heavy/light primordial noble gases (i.e. 20Ne/ 36 Ar and 20 Ne/ 83 Kr ratios; Fig.S1). This provides evidence that the relative noble gas patterns are preserved through secondary loss processes (i.e. the Moon- forming Giant Impact). Therefore, major loss of atmosphere appears to be possible without significantly fractionating the relative proportions of the primordial noble gases and molecular nitrogen 9. There is also the question of how much of the telluric planetary volatiles were delivered by the late veneer, which is required to explain the moderate to volatile elements (such as H, C, S, and Se) 10, and the HSE The late veneer was widespread throughout the solar system and certainly post- dates the formation of the moon However, it is unlikely that Earth received its N 2/primordial noble gas enrichment from the late veneer, because the Venusian and Martian atmospheres show comparable N 2/primordial noble gas abundances. The late veneer cannot explain these data in Figs 1a+b and S1. This is because the late veneer should/would have affected all planets in the inner solar system as a function of size (surface area). Ergo, the atmospheres of Earth and Venus should be comparable, and not different. In addition, the late veneer would have affected the atmospheres of Mars and Venus differently, however, they display comparable N2/primordial noble gas ratios. In fact, as shown in Fig.1a, and S1, the three planets exhibit comparable primordial noble gas/ primordial noble gas ratios (e.g. 20 N/ 36 Ar), something that would not be predicted if the late veneer was the explanation for the Earth s N2/primordial noble gas enrichment. S1.6 The redox state of the interiors of the terrestrial planets The Earth s upper mantle redox state (expressed as fo 2 in log units) can be determined by studying basaltic glasses and mantle xenoliths. These data show the LOGfO 2 of Earth s volcanic sampling field to be 4 NATURE GEOSCIENCE

5 SUPPLEMENTARY INFORMATION around the QFM redox buffer (average LOGfO 2 = ΔQFM 0 ± 2) 13. However, if the data for these mantle xenoliths are subdivided into two groups, a clearer picture emerges. Mantle xenoliths from arc settings above the mantle wedge are shown to be more oxidizing (LOGfO 2 = ΔQFM 0 to +2) relative to primitive basalts and kimberlitic/oceanic xenoliths (LOGfO 2 = ΔQFM 0 to - 3) 13-15, which thermodynamic data show should become even more reducing with depth (i.e. below ca. 250 km the mantle is buffered around IW, not QFM) 13. The LOGfO 2 of the Martian mantle has been determined from using the most primitive SNC meteorites to be around ΔQFM - 1 to Due to the scarcity of data, any comments on the redox state of the Venusian mantle are highly speculative. Data from landers Veneras 13 and 14 show FeO contents of basaltic rocks on the Venusian surface to be between ca wt.% 17, these compositions are similar to mid- ocean ridge basalts on Earth. 17. Because Venus and Earth are of a comparable size and bulk composition, their respective mantles should be dominated by bridgmanite, which during core- mantle differentiation forces the disproportionation of ferrous iron into ferric iron plus metal 18. This process has previously been described as an oxygen pump, which would have injected ferric iron into the Earth s upper mantle during mantle differentiation 18 thus raising the ambient redox state from near IW towards QFM. This process would not have occurred within the Martian interior because of the limited stability of bridgmanite in the smaller planet 18. This explains the upper mantle redox discrepancy between Earth and Mars, and would imply the Venusian mantle should have a redox state akin to the Earth s ambient mantle. Therefore, we assume the fo 2 of the Venusian mantle is similar to that of Earth s primitive mantle (LOGfO 2 = ΔQFM 0 to - 3). NATURE GEOSCIENCE 5

6 S1.7 References cited S1. Porcelli, D. & Pepin, R. O. The Origin of Noble Gases and Major Volatiles in the Terrestrial Planets, Treatise on Geochemistry, , (2003) S2. Mahaffy et al., Abundance and Isotopic Composition of Gases in the Martian Atmosphere from the Curiosity Rover, Science, 341, (2013) S3. Hoffmann, J. H., Oyama, V. I. & Zahn, U. V. Measurements of the lower atmospheric composition: A comparison of Results. J. Geophys. Res. 85, (1980) S4. Canfield, D. E., Glazer, A. N, & Farlkowski, P. G. The evolution and future of Earth s nitrogen cycle. Science, 330, (2010) S5. Watenphul et al., Ammonium- bearing clinopyroxene: A potential nitrogen reservoir in the Earth's mantle. Chem. Geol. 270, (2010) S6. Li et al. Nitrogen solubility in upper mantle minerals. Earth Planet. Sci. Lett. 377, (2013) S7. Shcheka, S.S., & Keppler, H. The origin of the terrestrial noble- gas signature, Nature, 490, (2012) S8. Roskosz et al. Nitrogen solubility in molten metal and silicate at high pressure and temperature. Geochim. Cosmochim. Acta, 121, (2013) S9. Halliday, A. N. The origins of volatiles in the terrestrial planets, Geochim. Cosmochim. Acta, 105, (2013) 6 NATURE GEOSCIENCE

7 SUPPLEMENTARY INFORMATION S10. Wang, Z, & Becker, H. Ratios of S, Se and Te in the silicate Earth require a volatile- rich late veneer, Nature, (2013) S11. Dale et al., Late Accretion on the Earliest Planetesimals Revealed by the Highly Siderophile Elements, Science, 336, (2012) S12. Day, J. M. D, et al. Late accretion as a natural consequence of planetary growth. Nat. Geo., 5, (2012) S13. Frost, D. J, & McCammon, C. M. The Redox State of Earth's Mantle. Annu. Rev. Earth Planet. Sci. 36, (2008) S14. Wood, B. J., Bryndzia, L. T. & Johnson, K. E. Mantle oxidation state and its relationship to tectonic environment and fluid speciation. Science 248, (1990) S15. Parkinson, I. J. & Arculus, R. J. The redox state of subduction zones: insights from arc- peridotites. Chem. Geol. 160, (1999) S16. Herd, C. D. K., et al. Oxygen fugacity and geochemical variations in the Martian basalts: implications for Martian basalt petrogenesis and the oxidation state of the upper mantle of Mars. Geochim. Cosmochim. Acta, 66, (2002) S17. Hunten et al., Venus, University of Arizona press, 1983 S18. Wade, J. & Wood, B. J. Core formation and the oxidation state of the Earth. Earth Planet. Sci. Lett. 236, (2005) NATURE GEOSCIENCE 7

8 S2. Supplementary data Table S1: Data used in this study for the atmospheres of Earth 1, Mars 1-2 and Venus 1,3, NATURE GEOSCIENCE

SUPPLEMENTARY INFORMATION Figure S1: The abundances of atmospheric 20 Ne relative to the abundance of molecular nitrogen and the primordial noble gases of Earth 1, Mars 1-2

9 SUPPLEMENTARY INFORMATION Figure S1: The abundances of atmospheric 20 Ne relative to the abundance of molecular nitrogen and the primordial noble gases of Earth 1, Mars 1-2 and Venus 1,3. Molecular nitrogen and the primordial noble gases are listed in order of their relative abundances ref.1. NATURE GEOSCIENCE 9

Volatiles in the terrestrial planets. Sujoy Mukhopadhyay University of California, Davis CIDER, 2014

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