High pressure redistribution of nitrogen and sulfur during planetary stratification
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Abstract
Figures and Tables
Figure 1 Concentrations of volatile elements (H, C, S, and N) in Earth’s observable reservoirs normalised to CI chondrites. Nitrogen appears depleted in Earth compared to other volatiles. Data are from Hirschmann (2016) and Wasson and Kallemeyn (1988). | Figure 2 Controls on nitrogen partitioning. (a) Oxidising conditions make nitrogen more siderophile. Literature data are recalculated to 1973 K and 1 GPa. (b) Nickel and (c) Sulfur make nitrogen less siderophile. (d) Hotter temperatures make nitrogen less siderophile. Extrapolation of temperature effect at 1 GPa underpredicts values compared to values measured for higher pressure LH-DAC experiments. (e) A 1∶1 comparison of observations versus predictions from Equation 1. The R2 for the literature fit is 0.76. (f) Application of Equation 1 to mantle liquidus geotherm at ΔIW−2. Nitrogen becomes more siderophile with depth. | Figure 3 Models for redistribution of N and S in response to planetary differentiation. (a) An example of a model iteration of S/NOE as function of core formation pressure and atmospheric fO2. Solid lines are model solutions for a given atmospheric fO2 (log unit spacing). Horizontal dotted lines delineate the estimated range of S/NOE for Earth (Hirschmann, 2016). Observable Earth is the sum of the atmosphere and magma ocean. (b) Fractional success of the model between iterations for producing a S/NOE ratio that matches Earth. (c) An example of a model iteration of S/NOE.MO as function of core formation pressure and atmospheric fO2. Horizontal dotted lines delineate the estimated range of S/NOE for Earth. Observable Earth is only the magma ocean. (d) Fractional success of the model between iterations for producing a S/NOE ratio that matches Earth. (e) Predicted bulk planet S as a function of core formation pressure. Dashed lines are 1 sigma that propagate uncertainties from the observable Earth S and parameterised values. The dotted line is the estimate of observable Earth S from the volatility trend (5600 ppm; Dreibus and Palme, 1996). | Table 1 Experimental run conditions and chemical analyses used in parameterisation. |
Figure 1 | Figure 2 | Figure 3 | Table 1 |
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Introduction
Nitrogen is the most depleted element in the observable Earth when compared to CI chondrites, materials that most faithfully record the bulk composition of the solar system (observable Earth is the sum of the atmosphere, oceans, crust, and mantle) (e.g., Marty, 2012
Marty, B. (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on earth. Earth and Planetary Science Letters 313–314, 56–66.
; Halliday, 2013Halliday, A.N. (2013) The origins of volatiles in the terrestrial planets. Geochimica et Cosmochimica Acta 105, 146–171.
). The depletion of N is a fundamental expression of the integrated processes that determine the volatile budget of planets (Fig. 1).There are many early acting processes that can deplete nitrogen and other volatile elements, either in a bulk planet or its observable reservoirs. Chondrites have various volatile element patterns that may reflect formation in regions of the Solar System with fractionated volatiles or the operation of incomplete condensation/evaporation reactions (Bergin et al., 2015
Bergin, E.A., Blake, G.A., Ciesla, F., Hirschmann, M.M., Li, J. (2015) Tracing the ingredients for a habitable earth from interstellar space through planet formation. Proceedings of the National Academy of Sciences 112, 8965–8970.
). With accretion and differentiation, terrestrial bodies form atmospheres, and these atmospheres can be lost (Tucker and Mukhopadhyay, 2014Tucker, J.M., Mukhopadhyay, S. (2014) Evidence for multiple magma ocean outgassing and atmospheric loss episodes from mantle noble gases. Earth and Planetary Science Letters 393, 254–265.
; Schlichting et al., 2015Schlichting, H.E., Sari, R., Yalinewich, A. (2015) Atmospheric mass loss during planet formation: The importance of planetesimal impacts. Icarus 247, 81–94.
). Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013Roskosz, M., Bouhifd, M.A., Jephcoat, A.P., Marty, B., Mysen, B.O. (2013) Nitrogen solubility in molten metal and silicate at high pressure and temperature. Geochimica et Cosmochimica Acta 121, 15–28.
; Kadik et al., 2015Kadik, A., Koltashev, V., Kryukova, E., Plotnichenko, V., Tsekhonya, T., Kononkova, N. (2015) Solubility of nitrogen, carbon, and hydrogen in FeO–Na2O–Al2O3–SiO2 melt and liquid iron alloy: Influence of oxygen fugacity. Geochemistry International 53, 849–868.
; Li et al., 2016Li, Y., Marty, B., Shcheka, S., Zimmermann, L., Keppler, H. (2016) Nitrogen isotope fractionation during terrestrial core-mantle separation. Geochemical Perspectives Letters 2, 138–147.
; Dalou et al., 2017Dalou, C., Hirschmann, M.M., von der Handt, A., Mosenfelder, J., Armstrong, L.S. (2017) Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth. Earth and Planetary Science Letters 458, 141–151.
; Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
,bGrewal, D.S., Dasgupta, R., Sun, C., Tsuno, K., Costin, G. (2019b) Delivery of carbon, nitrogen, and sulfur to the silicate earth by a giant impact. Science Advances 5, eaau3669.
; Speelmanns et al., 2019Speelmanns, I.M., Schmidt, M.W., Liebske, C. (2019) The almost lithophile character of nitrogen during core formation. Earth and Planetary Science Letters 510, 186–197.
).The role of pressure and temperature on core formation chemistry become larger for larger planetary bodies, as indicated by the refractory siderophile element concentrations observed for Earth, Mars, and Vesta (Righter and Drake, 1996
Righter, K., Drake, M.J. (1996) Core formation in Earth’s moon, Mars, and Vesta. Icarus 124, 513–529.
). Studies agree that increasing temperature makes nitrogen less siderophile (Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
; Speelmanns et al., 2019Speelmanns, I.M., Schmidt, M.W., Liebske, C. (2019) The almost lithophile character of nitrogen during core formation. Earth and Planetary Science Letters 510, 186–197.
). Extrapolating the temperature effect on metal-silicate partition coefficients for nitrogen (, atomic) to the average core formation temperature for Earth implies lithophile behaviour (e.g., Speelmanns et al., 2019Speelmanns, I.M., Schmidt, M.W., Liebske, C. (2019) The almost lithophile character of nitrogen during core formation. Earth and Planetary Science Letters 510, 186–197.
). In the absence of a large effect of pressure, core formation would apparently have little ability to modify the observable budget of nitrogen during the accretion of larger rocky worlds, such as Earth.Previously published high pressure experiments suggest that pressure favours N incorporation into cores (Roskosz et al., 2013
Roskosz, M., Bouhifd, M.A., Jephcoat, A.P., Marty, B., Mysen, B.O. (2013) Nitrogen solubility in molten metal and silicate at high pressure and temperature. Geochimica et Cosmochimica Acta 121, 15–28.
, Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
); however, the pressure effect cannot be confidently resolved to be different from zero (Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
). Towards this end, we report values from experiments conducted up to 26 GPa and 3437 K, using a laser heated diamond-anvil cell (LH-DAC). We supplement our LH-DAC data, with a systematic series of piston cylinder experiments to enable a robust parameterisation.top
Methods
We conducted lower P-T partitioning experiments (1773–2413 K, 0.95–2.38 GPa) using a piston cylinder (PC) and higher P-T experiments (3046–3437 K, 23.2–25.6 GPa) using a LH-DAC (Tables 1, S-1). Experimental fO2 conditions ranged from ΔIW−6.6 to ΔIW−1.0 (log unit deviations from the iron-wüstite buffer). We quantified the composition of reacted metal-silicate pairs using field emission electron microprobe analysis (Table 1). An example backscatter image is provided in Figure S-1, along with a typical time-temperature path for LH-DAC experiments. We also compare major element partitioning data of LH-DAC experiments to literature data in Figure S-2 as an evaluation of data quality. Table S-2 reports starting materials compositions. Further details on methods are provided in the Supplementary Information.
Table 1 Experimental run conditions and chemical analyses used in parameterisation.
Experiment | Note | Series | T, K | T, ± | P, GPa | P, ± | log(DN), atomic | log(γFe) | Ni, atomic | S, atomic | C, atomic | N, atomic | ΔIW |
PC_N_EXP1 | P, T, S, Ni, N, fO2, IP | 1973 | 10 | 0.95 | 0.05 | 1.40 | −0.08 | 0.000 | 0.000 | 0.159 | 0.024 | −1.59 | |
PC_N_EXP2 | low S metal | S | 1973 | 10 | 0.95 | 0.05 | 1.44 | −0.06 | 0.000 | 0.014 | 0.133 | 0.038 | −1.62 |
PC_N_EXP2 | high S metal | S | 1973 | 10 | 0.95 | 0.05 | 1.17 | 0.18 | 0.000 | 0.372 | 0.025 | 0.020 | −1.80 |
PC_N_EXP3 | fO2 | 1973 | 10 | 0.95 | 0.05 | 0.01 | −0.01 | 0.000 | 0.000 | 0.053 | 0.002 | −6.56 | |
PC_N_EXP4 | fO2 | 1973 | 10 | 0.95 | 0.05 | −0.68 | −0.04 | 0.000 | 0.000 | 0.109 | 0.003 | −5.28 | |
PC_N_EXP5 | fO2 | 1973 | 10 | 0.95 | 0.05 | −0.59 | −0.06 | 0.000 | 0.000 | 0.136 | 0.007 | −4.65 | |
PC_N_EXP6 | P | 1973 | 10 | 2.38 | 0.05 | 1.36 | −0.05 | 0.000 | 0.000 | 0.119 | 0.106 | −1.61 | |
PC_N_EXP7 | high S metal | S | 1973 | 10 | 2.38 | 0.05 | 1.43 | −0.04 | 0.000 | 0.109 | 0.091 | 0.086 | −1.57 |
PC_N_EXP8 | fO2 | 1973 | 10 | 0.95 | 0.05 | 0.97 | −0.07 | 0.000 | 0.000 | 0.142 | 0.031 | −2.50 | |
PC_N_EXP9 | T | 2373 | 10 | 1.05 | 0.05 | 0.77 | −0.06 | 0.000 | 0.000 | 0.151 | 0.019 | −1.64 | |
PC_N_EXP10 | T | 2403 | 10 | 1.05 | 0.05 | 0.46 | −0.07 | 0.000 | 0.000 | 0.160 | 0.013 | −1.62 | |
PC_N_EXP11 | low S metal | S | 2413 | 10 | 1.05 | 0.05 | 1.05 | −0.06 | 0.000 | 0.012 | 0.143 | 0.033 | −1.69 |
PC_N_EXP12 | low S metal | S | 1973 | 10 | 1.66 | 0.05 | 1.37 | −0.05 | 0.000 | 0.030 | 0.111 | 0.074 | −1.69 |
PC_N_EXP12 | high S metal | S | 1973 | 10 | 1.66 | 0.05 | 1.20 | 0.11 | 0.000 | 0.301 | 0.026 | 0.050 | −1.78 |
PC_N_EXP13 | Ni | 1973 | 10 | 1.00 | 0.05 | 0.80 | −0.05 | 0.225 | 0.000 | 0.117 | 0.019 | −1.57 | |
PC_N_EXP14 | T, N | 1773 | 10 | 0.95 | 0.05 | 1.56 | −0.07 | 0.000 | 0.000 | 0.138 | 0.039 | −1.70 | |
PC_N_EXP15 | Ni | 1973 | 10 | 0.95 | 0.05 | 0.84 | −0.07 | 0.012 | 0.000 | 0.143 | 0.032 | −2.48 | |
PC_N_EXP17 | T, N | 2173 | 10 | 1.05 | 0.05 | 0.90 | −0.06 | 0.000 | 0.000 | 0.147 | 0.029 | −1.64 | |
PC_N_EXP18 | T, N | 1873 | 10 | 1.05 | 0.05 | 1.47 | −0.07 | 0.000 | 0.000 | 0.146 | 0.033 | −1.60 | |
PC_N_EXP19 | IP | 1973 | 10 | 0.95 | 0.05 | 0.92 | −0.08 | 0.000 | 0.000 | 0.157 | 0.030 | −1.75 | |
PC_N_EXP22 | IP | 1973 | 10 | 0.95 | 0.05 | 0.83 | −0.06 | 0.000 | 0.000 | 0.137 | 0.042 | −1.71 | |
PC_N_EXP23 | IP | 1973 | 10 | 0.95 | 0.05 | 0.82 | −0.07 | 0.000 | 0.000 | 0.149 | 0.036 | −1.61 | |
PC_N_EXP24 | Ni | 1973 | 10 | 0.95 | 0.05 | 1.11 | −0.07 | 0.104 | 0.000 | 0.148 | 0.009 | −1.85 | |
DAC_N_EXP1 spot 2 | P | 3142 | 46 | 23.80 | 2.35 | 1.55 | −0.06 | 0.256 | 0.000 | 0.160 | 0.135 | −1.18 | |
DAC_N_EXP1 spot 3 | P | 3437 | 255 | 25.60 | 3.90 | 1.78 | −0.10 | 0.215 | 0.000 | 0.219 | 0.054 | −0.96 | |
DAC_N_EXP1 spot 4 | P | 3046 | 13 | 23.20 | 2.10 | 1.59 | −0.06 | 0.303 | 0.000 | 0.154 | 0.118 | −1.15 |
Series: P, pressure; T, temperature; S, sulfur; Ni, nickel; N-C, nitrogen-carbon; fO2, oxygen fugacity; IP, ionic porosity; Ni, S, C, N, atomic fraction of metal phase.
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Results
We completed independent series of experiments to quantify the P-T-X controls on values (Fig. 2, Table 1). Individual series correlations indicate values depend strongly on fO2 (R2 = 0.99, p value = 0.005, n = 4), temperature (R2 = 0.97, p value = 0.001, n = 6), and pressure (R2 = 0.98, p value < 0.001, n = 9) over other conditions relevant to magma oceans (Fig. 2a,d), whereas , , , and the ionic porosity of silicate liquid have significant (p values < 0.05) but minor effects (Figs. 2b,c, S-4, S-5). Our fO2 and temperature findings accord with previous work (Dalou et al., 2017
Dalou, C., Hirschmann, M.M., von der Handt, A., Mosenfelder, J., Armstrong, L.S. (2017) Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth. Earth and Planetary Science Letters 458, 141–151.
; Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
; Speelmanns et al., 2019Speelmanns, I.M., Schmidt, M.W., Liebske, C. (2019) The almost lithophile character of nitrogen during core formation. Earth and Planetary Science Letters 510, 186–197.
). Our lower pressure experiments indicate that N-C interactions in Fe alloy make both elements less siderophile and yield an epsilon value () of 7.6 ± 1.4 (R2 = 0.90, p value = 0.004, n = 6; Fig. S-4), qualitatively consistent with the Steelmaking Data Sourcebook (1988)Steelmaking Data Sourcebook (1988) The Japan Society for the Promotion of Science: The 19th Committee on Steelmaking. Gordon and Breach Science Publishers, New York.
and values collected under C-free conditions (Grewal et al., 2021Grewal, D.S., Dasgupta, R., Hough, T., Farnell, A. (2021) Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets. Nature Geoscience 14, 369–376.
). Our value calculations follow the approach of Ma (2001)Ma, Z. (2001) Thermodynamic description for concentrated metallic solutions using interaction parameters. Metallurgical and Materials Transactions B 32, 87–103.
.Our LH-DAC experiments consistently demonstrated siderophile behaviour (Fig. 2d) despite their high temperature, and this requires a large, positive pressure effect. These high P-T experiments contain C and Ni, and were relatively oxidising. Quantifying the pressure effect therefore requires simultaneous consideration of these other parameters, which is enabled by our PC experiments (Equation 1, see below). Our determination of a positive effect of pressure is consistent with previous work, but the larger range of P we investigated results in more precise determination of the P-T coefficients in Equations 1 and 2 (c.f., Roskosz et al., 2013
Roskosz, M., Bouhifd, M.A., Jephcoat, A.P., Marty, B., Mysen, B.O. (2013) Nitrogen solubility in molten metal and silicate at high pressure and temperature. Geochimica et Cosmochimica Acta 121, 15–28.
, Grewal et al., 2019aGrewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
).To parameterise our data, we first recalculate experiments to a carbon-free baseline using our newly derived value. We then conduct an equal weight, least squares regression on parameters identified as significant in their individual series (1/T, P/T, , and ΔIW). Our approach yields the following expression (R2 = 0.95, p value < 0.001, n = 22; Fig. 2e):
or if the effect of carbon is included in the parameterisation
Uncertainties are reported as 1σ. Note that , , and refer to the expanded concentration expression associated with the notation of Ma (2001)
Ma, Z. (2001) Thermodynamic description for concentrated metallic solutions using interaction parameters. Metallurgical and Materials Transactions B 32, 87–103.
, and that positive coefficients indicate a reduction in the value. Application of Equation 1 to a mantle liquidus geotherm at ΔIW−2 indicates a monotonic increase in with depth (Fig. 2f).We parameterise our data alone because we completed systematic series of experiments to isolate specific effects on partitioning. Inclusion of all published data is accompanied by a large number of free parameters, and prevents resolution of N, S and C effects that are clearly observable in our results (Figs. 2, S-4). Predictions of Equation 1 are compared to literature data in Figures 2e, S-6. Additional details regarding this regression are provided in the Supplementary Information. The covariance matrix for Equation 1 is reported in Table S-3. We also report our fitting of literature data with the effects identified as significant here as Equation S-7.
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Modelling N Distribution throughout Earth
Having established that pressure strongly modulates values, we now calculate the equilibrium distribution of N and S between cores, magma oceans, and atmospheres for a range of plausible stratification conditions. We include S because multiple high pressure studies show agreement and now constrain its metal-silicate partitioning up to extreme P-T conditions. Our parameterisation of values is applied here and is detailed in the Supplementary Information (Fig. S-8).
Mass balance is used to solve for the equilibrium distribution of N and S as required by partitioning and magma solubility constraints (Equation 1; Libourel et al., 2003
Libourel, G., Marty, B., Humbert, F. (2003) Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity. Geochimica et Cosmochimica Acta 67, 4123–4135.
). The mantle is assumed to be completely molten, and the mass fraction of the core is 0.325. Uncertainties are evaluated by varying and according to their covariance matrices and iterating the mass balance model. No S is assumed to be present in the atmosphere given its relatively high solubility in magma. All scenarios assume a bulk planet S/N ratio set by enstatite chondrite (85) and that the magma ocean contains 225 ± 25 ppm S (Hirschmann, 2016Hirschmann, M.M. (2016) Constraints on the early delivery and fractionation of Earth’s major volatiles from C/H, C/N, and C/S ratios. American Mineralogist 101, 540–553.
). We focus on enstatite chondrites because their average S/N ratio is intermediate between gas-rich (29; CI and CM) and ordinary chondrites (404; Wasson and Kallemeyn, 1988Wasson, J.T., Kallemeyn, G.W. (1988) Compositions of chondrites. Philosophical Transactions of the Royal Society of London Series A, Mathematical and Physical Sciences 325, 535–544.
). Models that consider S/N ratios for other groups of chondrites are presented in the Supplementary Information (Figs. S-9, S-10).Our first goal is to quantify how the S/N ratio of observable Earth (S/NOE, 82 ± 29, dotted lines in Fig. 3a,c) varies in response to differentiation conditions. We consider models with core formation fO2 of ΔIW−2 while varying core formation pressure and atmospheric fO2 (Fig. 3). We focus on ΔIW−2 because the FeO content of Earth implies an average fO2 for core formation near this value. Pressures of metal-silicate equilibration range up to 60 GPa, an upper limit in the single stage framework for Earth (e.g., Fischer et al., 2015
Fischer, R.A., Nakajima, Y., Campbell, A.J., Frost, D.J., Harries, D., Langenhorst, F., Miyajima, N., Pollok, K., Rubie, D.C. (2015) High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O. Geochimica et Cosmochimica Acta 167, 177–194.
). Our model takes a single stage approach, and while accretion is a multi-step process, single stage calculations capture average P-T-X conditions that can be readily compared between elements (Siebert et al., 2011Siebert, J., Corgne, A., Ryerson, F.J. (2011) Systematics of metal–silicate partitioning for many siderophile elements applied to earth’s core formation. Geochimica et Cosmochimica Acta 75, 1451–1489.
). Atmospheric fO2 varies in our model between ΔIW−3 and ΔIW+3. Atmospheric fO2 conditions are a free parameter in light of evidence for the depth dependence of the Fe+3/Fetot ratio of magma in equilibrium with iron (Zhang et al., 2017Zhang, H., Hirschmann, M., Cottrell, E., Withers, A. (2017) Effect of pressure on Fe3+/Fe ratio in a mafic magma and consequences for magma ocean redox gradients. Geochimica et Cosmochimica Acta 204, 83–103.
; Armstrong et al., 2019Armstrong, K., Frost, D.J., McCammon, C.A., Rubie, D.C., Ballaran, T.B. (2019) Deep magma ocean formation set the oxidation state of earth’s mantle. Science 365, 903–906.
; Deng etal., 2020Deng, J., Du, Z., Karki, B., Ghosh D., Lee, K. (2020) A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres. Nature Communications 11, 2007.
) and the temperature dependence of Fe+3/Fetot for any fO2 (Sossi et al., 2020Sossi, P.A., Burnham, A.D., Badro, J., Lanzirotti, A., Newville, M., O’Neill, H.S.C. (2020) Redox state of earth’s magma ocean and its venus-like early atmosphere. Science Advances 6, eabd1387.
).Our models demonstrate the strong sensitivity of S/NOE ratios to core formation pressure; at low pressure (<10 GPa), S/NOR ratios are low, but with increasing pressure, S/NOE ratios rise as S becomes less siderophile and N becomes more siderophile. The rise is such that core formation near 30 GPa results in S/NOE ratios that match Earth for more reducing atmospheres. More oxidising atmospheres require higher pressure core formation to satisfy Earth’s S/NOE ratio, as more N remains in the atmosphere, unable to partition into the core (Fig. 3a,b).
It is possible that the atmosphere is partially lost during accretion, leading to preferential depletion of N relative to S. We model this possibility as the end member of the magma ocean being the only contributor of N and S to later-forming observable reservoirs (Fig. 3c,d). In these cases S/NOE,MO (S/N ratio of the magma ocean) also matches Earth near 30 GPa, and model results are relatively insensitive to atmospheric fO2. Cases of intermediate atmospheric loss plot between the two end members for atmospheric contribution considered here (c.f., Fig. 3a–d).
Taken together, our models demonstrate that higher pressure core formation scenarios can satisfy the S/NOE ratio of Earth, assuming a bulk planet S/N ratio similar to enstatite chondrite. Bulk planet S/N ratios closer to gas-rich chondrites (low S/N) require even higher core formation pressures, while bulk planet S/N ratios closer to ordinary chondrites (high S/N) require lower pressure to satisfy the S/NOE ratio constraint (Figs. S-9, S-10). This all serves to highlight the importance of core formation pressure for modulating planetary volatile budgets.
It is significant that higher pressure core formation (>30 GPa) and a core-mantle fO2 of ΔIW−2 can produce S/NOE ratios equal to Earth because these are the same differentiation conditions implied by moderately siderophile element and FeO concentrations in the observable Earth in a single stage framework (e.g., Siebert et al., 2011
Siebert, J., Corgne, A., Ryerson, F.J. (2011) Systematics of metal–silicate partitioning for many siderophile elements applied to earth’s core formation. Geochimica et Cosmochimica Acta 75, 1451–1489.
; Fischer et al., 2015Fischer, R.A., Nakajima, Y., Campbell, A.J., Frost, D.J., Harries, D., Langenhorst, F., Miyajima, N., Pollok, K., Rubie, D.C. (2015) High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O. Geochimica et Cosmochimica Acta 167, 177–194.
). Importantly, higher pressure core formation also yields a bulk planet with 6650–2050 ppm S (30–60 GPa) (Fig. 3e), and these values compare favourably with estimates of 5600 ppm S for bulk Earth based on the volatility trend (Dreibus and Palme, 1996Dreibus, G., Palme, H. (1996) Cosmochemical constraints on the sulfur content in the Earth’s core. Geochimica et Cosmochimica Acta 60, 1125–1130.
). The multiple successes of higher pressure models for explaining Earth’s volatile budget are important because they suggest that volatiles are modulated by the same core formation events that modulate refractory elements. This suggestion contrasts with previous hypotheses that smaller, volatile-rich bodies preferentially contributed to Earth’s volatile budget (e.g., Grewal et al., 2019bGrewal, D.S., Dasgupta, R., Sun, C., Tsuno, K., Costin, G. (2019b) Delivery of carbon, nitrogen, and sulfur to the silicate earth by a giant impact. Science Advances 5, eaau3669.
), decoupling the accretion of life-enabling elements from refractory elements.It is well established within the Solar System that planetary body size correlates with average core formation pressures (Righter and Drake, 1996
Righter, K., Drake, M.J. (1996) Core formation in Earth’s moon, Mars, and Vesta. Icarus 124, 513–529.
), and our work therefore predicts a direct relationship between planet size and its distribution of volatiles. This link should enable more precise evaluations of exoplanet habitability, worlds for which size remains a central constraint on their geologic evolution.top
Acknowledgements
We acknowledge discussions with Ian Ocampo during a summer internship at NMNH. We additionally acknowledge the support provided by C. Prescher, E. Greenberg, and V. Prakapenka during the experiments conducted at GSECARS and in data reduction. We thank Rob Wardell and Tim Rose for their support of the microscopy work associated with this manuscript. We thank Tim McCoy for discussions regarding the model results.
CRMJ acknowledges support by NSF-EAR grant 1725315, NASA grant 80NSSC21K0377, Smithsonian GVP fellowship, Carnegie Institution for Science (Geophysical Lab) Fellowship, and start up from Tulane University. Experiments were partially supported by NASA grant to YF. ZD thanks the support from Carnegie Fellowship, grants from Chinese Academy of Sciences and State Key Laboratory of Isotope Geochemistry (No. 29Y93301701, 51Y8340107), as well as Strategic Priority Research Program (B) (XDB18030604). Portions of this work were performed at GeoSoilEnviroCARS (The University of Chicago, Sector 13), the Advanced Photon Source, Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation–Earth Sciences (EAR 1128799) and the Department of Energy–GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357.
Editor: Cin-Ty Lee
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References
Armstrong, K., Frost, D.J., McCammon, C.A., Rubie, D.C., Ballaran, T.B. (2019) Deep magma ocean formation set the oxidation state of earth’s mantle. Science 365, 903–906.
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Atmospheric fO2 conditions are a free parameter in light of evidence for the depth dependence of the Fe+3/Fetot ratio of magma in equilibrium with iron (Zhang et al., 2017; Armstrong et al., 2019; Deng etal., 2020) and the temperature dependence of Fe+3/Fetot for any fO2 (Sossi et al., 2020).
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Bergin, E.A., Blake, G.A., Ciesla, F., Hirschmann, M.M., Li, J. (2015) Tracing the ingredients for a habitable earth from interstellar space through planet formation. Proceedings of the National Academy of Sciences 112, 8965–8970.
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Chondrites have various volatile element patterns that may reflect formation in regions of the Solar System with fractionated volatiles or the operation of incomplete condensation/evaporation reactions (Bergin et al., 2015).
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Dalou, C., Hirschmann, M.M., von der Handt, A., Mosenfelder, J., Armstrong, L.S. (2017) Nitrogen and carbon fractionation during core–mantle differentiation at shallow depth. Earth and Planetary Science Letters 458, 141–151.
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Our fO2 and temperature findings accord with previous work (Dalou et al., 2017; Grewal et al., 2019a; Speelmanns et al., 2019).
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Deng, J., Du, Z., Karki, B., Ghosh D., Lee, K. (2020) A magma ocean origin to divergent redox evolutions of rocky planetary bodies and early atmospheres. Nature Communications 11, 2007.
Dreibus, G., Palme, H. (1996) Cosmochemical constraints on the sulfur content in the Earth’s core. Geochimica et Cosmochimica Acta 60, 1125–1130.
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The dotted line is the estimate of observable Earth S from the volatility trend (5600 ppm; Dreibus and Palme, 1996).
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Importantly, higher pressure core formation also yields a bulk planet with 6650–2050 ppm S (30–60 GPa) (Fig. 3e), and these values compare favourably with estimates of 5600 ppm S for bulk Earth based on the volatility trend (Dreibus and Palme, 1996).
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Fischer, R.A., Nakajima, Y., Campbell, A.J., Frost, D.J., Harries, D., Langenhorst, F., Miyajima, N., Pollok, K., Rubie, D.C. (2015) High pressure metal–silicate partitioning of Ni, Co, V, Cr, Si, and O. Geochimica et Cosmochimica Acta 167, 177–194.
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Pressures of metal-silicate equilibration range up to 60 GPa, an upper limit in the single stage framework for Earth (e.g., Fischer et al., 2015).
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It is significant that higher pressure core formation (>30 GPa) and a core-mantle fO2 of ΔIW−2 can produce S/NOE ratios equal to Earth because these are the same differentiation conditions implied by moderately siderophile element and FeO concentrations in the observable Earth in a single stage framework (e.g., Siebert et al., 2011; Fischer et al., 2015).
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Grewal, D.S., Dasgupta, R., Holmes, A.K., Costin, G., Li, Y., Tsuno, K. (2019a) The fate of nitrogen during core-mantle separation on earth. Geochimica et Cosmochimica Acta 251, 87–115.
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Studies agree that increasing temperature makes nitrogen less siderophile (Grewal et al., 2019a; Speelmanns et al., 2019).
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Previously published high pressure experiments suggest that pressure favours N incorporation into cores (Roskosz et al., 2013, Grewal et al., 2019a); however, the pressure effect cannot be confidently resolved to be different from zero (Grewal et al., 2019a).
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Previously published high pressure experiments suggest that pressure favours N incorporation into cores (Roskosz et al., 2013, Grewal et al., 2019a); however, the pressure effect cannot be confidently resolved to be different from zero (Grewal et al., 2019a).
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Our fO2 and temperature findings accord with previous work (Dalou et al., 2017; Grewal et al., 2019a; Speelmanns et al., 2019).
View in article
Our determination of a positive effect of pressure is consistent with previous work, but the larger range of P we investigated results in more precise determination of the P-T coefficients in Equations 1 and 2 (c.f., Roskosz et al., 2013, Grewal et al., 2019a).
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Grewal, D.S., Dasgupta, R., Sun, C., Tsuno, K., Costin, G. (2019b) Delivery of carbon, nitrogen, and sulfur to the silicate earth by a giant impact. Science Advances 5, eaau3669.
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This suggestion contrasts with previous hypotheses that smaller, volatile-rich bodies preferentially contributed to Earth’s volatile budget (e.g., Grewal et al., 2019b), decoupling the accretion of life-enabling elements from refractory elements.
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Grewal, D.S., Dasgupta, R., Hough, T., Farnell, A. (2021) Rates of protoplanetary accretion and differentiation set nitrogen budget of rocky planets. Nature Geoscience 14, 369–376.
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Our lower pressure experiments indicate that N-C interactions in Fe alloy make both elements less siderophile and yield an epsilon value () of 7.6 ± 1.4 (R2 = 0.90, p value = 0.004, n = 6; Fig. S-4), qualitatively consistent with the Steelmaking Data Sourcebook (1988) and values collected under C-free conditions (Grewal et al., 2021).
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Halliday, A.N. (2013) The origins of volatiles in the terrestrial planets. Geochimica et Cosmochimica Acta 105, 146–171.
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Nitrogen is the most depleted element in the observable Earth when compared to CI chondrites, materials that most faithfully record the bulk composition of the solar system (observable Earth is the sum of the atmosphere, oceans, crust, and mantle) (e.g., Marty, 2012; Halliday, 2013).
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Hirschmann, M.M. (2016) Constraints on the early delivery and fractionation of Earth’s major volatiles from C/H, C/N, and C/S ratios. American Mineralogist 101, 540–553.
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All scenarios assume a bulk planet S/N ratio set by enstatite chondrite (85) and that the magma ocean contains 225 ± 25 ppm S (Hirschmann, 2016).
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Horizontal dotted lines delineate the estimated range of S/NOE for Earth (Hirschmann, 2016).
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Data are from Hirschmann (2016) and Wasson and Kallemeyn (1988).
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Kadik, A., Koltashev, V., Kryukova, E., Plotnichenko, V., Tsekhonya, T., Kononkova, N. (2015) Solubility of nitrogen, carbon, and hydrogen in FeO–Na2O–Al2O3–SiO2 melt and liquid iron alloy: Influence of oxygen fugacity. Geochemistry International 53, 849–868.
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Li, Y., Marty, B., Shcheka, S., Zimmermann, L., Keppler, H. (2016) Nitrogen isotope fractionation during terrestrial core-mantle separation. Geochemical Perspectives Letters 2, 138–147.
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Libourel, G., Marty, B., Humbert, F. (2003) Nitrogen solubility in basaltic melt. Part I. Effect of oxygen fugacity. Geochimica et Cosmochimica Acta 67, 4123–4135.
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Mass balance is used to solve for the equilibrium distribution of N and S as required by partitioning and magma solubility constraints (Eq. 1; Libourel et al., 2003).
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Ma, Z. (2001) Thermodynamic description for concentrated metallic solutions using interaction parameters. Metallurgical and Materials Transactions B 32, 87–103.
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Our value calculations follow the approach of Ma (2001).
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Note that , , and refer to the expanded concentration expression associated with the notation of Ma (2001), and that positive coefficients indicate a reduction in the value.
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Marty, B. (2012) The origins and concentrations of water, carbon, nitrogen and noble gases on earth. Earth and Planetary Science Letters 313–314, 56–66.
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Nitrogen is the most depleted element in the observable Earth when compared to CI chondrites, materials that most faithfully record the bulk composition of the solar system (observable Earth is the sum of the atmosphere, oceans, crust, and mantle) (e.g., Marty, 2012; Halliday, 2013).
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Righter, K., Drake, M.J. (1996) Core formation in Earth’s moon, Mars, and Vesta. Icarus 124, 513–529.
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The role of pressure and temperature on core formation chemistry become larger for larger planetary bodies, as indicated by the refractory siderophile element concentrations observed for Earth, Mars, and Vesta (Righter and Drake, 1996).
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It is well established within the Solar System that planetary body size correlates with average core formation pressures (Righter and Drake, 1996), and our work therefore predicts a direct relationship between planet size and its distribution of volatiles.
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Roskosz, M., Bouhifd, M.A., Jephcoat, A.P., Marty, B., Mysen, B.O. (2013) Nitrogen solubility in molten metal and silicate at high pressure and temperature. Geochimica et Cosmochimica Acta 121, 15–28.
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Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
View in article
Previously published high pressure experiments suggest that pressure favours N incorporation into cores (Roskosz et al., 2013, Grewal et al., 2019a); however, the pressure effect cannot be confidently resolved to be different from zero (Grewal et al., 2019a).
View in article
Our determination of a positive effect of pressure is consistent with previous work, but the larger range of P we investigated results in more precise determination of the P-T coefficients in Equations 1 and 2 (c.f., Roskosz et al., 2013, Grewal et al., 2019a).
View in article
Schlichting, H.E., Sari, R., Yalinewich, A. (2015) Atmospheric mass loss during planet formation: The importance of planetesimal impacts. Icarus 247, 81–94.
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With accretion and differentiation, terrestrial bodies form atmospheres, and these atmospheres can be lost (Tucker and Mukhopadhyay, 2014; Schlichting et al., 2015).
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Speelmanns, I.M., Schmidt, M.W., Liebske, C. (2019) The almost lithophile character of nitrogen during core formation. Earth and Planetary Science Letters 510, 186–197.
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Studies agree that increasing temperature makes nitrogen less siderophile (Grewal et al., 2019a; Speelmanns et al., 2019).
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Extrapolating the temperature effect on metal-silicate partition coefficients for nitrogen (, atomic) to the average core formation temperature for Earth implies lithophile behaviour (e.g., Speelmanns et al., 2019).
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Our fO2 and temperature findings accord with previous work (Dalou et al., 2017; Grewal et al., 2019a; Speelmanns et al., 2019).
View in article
Core formation occurs in parallel to planetary accretion, and because nitrogen can display both siderophile and lithophile behaviour, the effect of core formation on the apparent nitrogen depletion in Earth remains uncertain (Roskosz et al., 2013; Kadik et al., 2015; Li et al., 2016; Dalou et al., 2017; Grewal et al., 2019a,b; Speelmanns et al., 2019).
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Siebert, J., Corgne, A., Ryerson, F.J. (2011) Systematics of metal–silicate partitioning for many siderophile elements applied to earth’s core formation. Geochimica et Cosmochimica Acta 75, 1451–1489.
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Our model takes a single stage approach, and while accretion is a multi-step process, single stage calculations capture average P-T-X conditions that can be readily compared between elements (Siebert et al., 2011).
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It is significant that higher pressure core formation (>30 GPa) and a core-mantle fO2 of ΔIW−2 can produce S/NOE ratios equal to Earth because these are the same differentiation conditions implied by moderately siderophile element and FeO concentrations in the observable Earth in a single stage framework (e.g., Siebert et al., 2011; Fischer et al., 2015).
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Sossi, P.A., Burnham, A.D., Badro, J., Lanzirotti, A., Newville, M., O’Neill, H.S.C. (2020) Redox state of earth’s magma ocean and its venus-like early atmosphere. Science Advances 6, eabd1387.
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Atmospheric fO2 conditions are a free parameter in light of evidence for the depth dependence of the Fe+3/Fetot ratio of magma in equilibrium with iron (Zhang et al., 2017; Armstrong et al., 2019; Deng etal., 2020) and the temperature dependence of Fe+3/Fetot for any fO2 (Sossi et al., 2020).
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Steelmaking Data Sourcebook (1988) The Japan Society for the Promotion of Science: The 19th Committee on Steelmaking. Gordon and Breach Science Publishers, New York.
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Our lower pressure experiments indicate that N-C interactions in Fe alloy make both elements less siderophile and yield an epsilon value () of 7.6 ± 1.4 (R2 = 0.90, p value = 0.004, n = 6; Fig. S-4), qualitatively consistent with the Steelmaking Data Sourcebook (1988) and values collected under C-free conditions (Grewal et al., 2021).
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Tucker, J.M., Mukhopadhyay, S. (2014) Evidence for multiple magma ocean outgassing and atmospheric loss episodes from mantle noble gases. Earth and Planetary Science Letters 393, 254–265.
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With accretion and differentiation, terrestrial bodies form atmospheres, and these atmospheres can be lost (Tucker and Mukhopadhyay, 2014; Schlichting et al., 2015).
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Wasson, J.T., Kallemeyn, G.W. (1988) Compositions of chondrites. Philosophical Transactions of the Royal Society of London Series A, Mathematical and Physical Sciences 325, 535–544.
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Data are from Hirschmann (2016) and Wasson and Kallemeyn (1988).
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Zhang, H., Hirschmann, M., Cottrell, E., Withers, A. (2017) Effect of pressure on Fe3+/Fe ratio in a mafic magma and consequences for magma ocean redox gradients. Geochimica et Cosmochimica Acta 204, 83–103.
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Atmospheric fO2 conditions are a free parameter in light of evidence for the depth dependence of the Fe+3/Fetot ratio of magma in equilibrium with iron (Zhang et al., 2017; Armstrong et al., 2019; Deng etal., 2020) and the temperature dependence of Fe+3/Fetot for any fO2 (Sossi et al., 2020).
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Supplementary Information
The Supplementary Information includes:
- Experimental Methods
- Demonstrations of Equilibrium
- Validation of Predictive Power of Equations 1 and 2
- Further Details for Nitrogen Distribution Model
- Model Results for Other Chondrite Groups
- Tables S-1 to S-3
- Figures S-1 to S-10
- Supplementary Information References
Download Tables S-1 to S-3 (Excel).
Download the Supplementary Information (PDF).