Reactive ammonia in the solar protoplanetary disk … · Reactive ammonia in the solar protoplanetary disk . and the origin ... Stepped trenches on either side of the Pt ... a Cameca

Reactive ammonia in the solar protoplanetary disk and the origin of Earth’s nitrogen

Dennis Harries, Peter Hoppe, Falko Langenhorst

SUPPLEMENTARY INFORMATIONDOI: 10.1038/NGEO2339

NATURE GEOSCIENCE | www.nature.com/naturegeoscience 1

© 2015 Macmillan Publishers Limited. All rights reserved

http://dx.doi.org/10.1038/ngeo2339

Supplementary Methods Materials. Yamato 791198 is an unbrecciated CM2 chondrite with a limited degree of hydrothermal alteration31,32 induced by aqueous fluids on the CM parent body. The absence of brecciation and the occurrence of ubiquitous fine-grained rims around chondrules and other coarse-grained objects indicate that it is a primary rock which experienced little secondary modification in the parent body33. The state of organic macromolecules in Yamato 791198 indicates that the meteorite experienced no significant thermal metamorphism at temperatures above those of the hydrothermal alteration34. It is free of discernible shock-metamorphic features.

Yamato 793321 is a brecciated CM2 chondrite also showing a rather limited degree of hydrothermal alteration32. The partial dehydration of phyllosilicates and alteration of macromolecular organic matter indicate that Yamato 793321 experienced a period of thermal metamorphism34,35. Metamorphic temperatures reached up to 770 K, likely during residence in the surface regolith of its parent body where it was implanted with noble gases of the solar wind35.

Focused ion beam (FIB) preparation. FIB preparation of electron-transparent lamellae for TEM study commenced in the following way:

1. Grains of interest were selected based on SEM imaging and EDXS.

2. Platinum straps of 10 to 20 µm length, ~1.5 µm width, and ~1.5 µm height were deposited on the intended extraction locations of the lamellae via ion-beam induced deposition using the GIS.

3. Stepped trenches on either side of the Pt straps were formed by sputtering material (‘milling’) with the Ga+ ion beam operated at 30 keV energy and 5 to 30 nA beam current.

4. The remaining lamellae were then thinned to approximately 0.5 to 1 µm thickness using sequentially lower beam currents at 30 keV energy, starting at 5 nA and ending at 0.5 or 0.3 nA.

5. The still thick lamellae were partially released from the trenches by undercutting and removal of material at the walls of the trenches. This involved a beam current of 1 or 3 nA at 30 keV energy.

6. The thick lamellae were then approached by the tungsten needle of the micromanipulator, attached to it by Pt deposition, and finally fully released from the trenches by sputtering away the last contact point.

7. After lift-out from the sample, the thick lamellae were transferred to a post-type copper TEM grid and attached there by Pt deposition.

8. Final thinning to electron transparency (~100 to 120 nm) on both sides of the lamellae was carried out using sequentially lower beam currents of 0.5 to 0.1 nA at 30 keV energy. At this final step of the preparation, the lamellae were subjected to gracing incidence of the ion beam only. This avoided strong implantation of Ga and beam damage. Control of the thinning process was carried out by SEM, which imaged the lamellae at 52°. The last


polishing step invoked an ion beam energy of 5 keV and a beam current < 0.1 nA in order to further minimize damage layers.

The mounted and thinned lamellae were transferred to the TEM without further treatment. A list of the samples studied is given in Supplementary Table 3.

Transmission electron microscopy (TEM). At Bayreuth we used a Philips CM20 field-emission TEM equipped with a Thermo-Noran high-purity germanium energy-dispersive X-ray (EDX) spectrometer and a Gatan parallel electron energy loss spectrometer (PEELS). At Jena we used a LEO 922 energy-filtered TEM equipped with a Li-doped silicon EDX detector. Both instruments were operated at accelerating voltages of 200 kV. Images were recorded using sheet films (Kodak SO-163) and scanned at 2000 dpi resolution. Only linear contrast and brightness changes were applied. EDX spectra were recorded in scanning TEM mode and semiquantitative evaluation of the spectra was accomplished by the Cliff-Lorimer method using pre-calibrated k-factors integrated into the Thermo-Noran software.

Secondary ion mass spectrometry (NanoSIMS). Isotopic in situ analyses were obtained using a Cameca NanoSIMS 50 ion probe at the Max Planck Institute for Chemistry at Mainz. The instrument was operated with a Cs+ primary ion beam of ~1 pA which was focused into a spot size of ~100 nm. Secondary ions measured were 12C−, 12C14N−, 12C15N−, and 32S−. Ion intensities were recorded in multi-collection using four electron multipliers on movable trolleys. All measured species were free of isobaric interferences. This is particularly important for 12C15N− which was fully separated from 13C14N−. As reference and for the control of instrumental mass fractionation we used a synthetic mixture (C231) of chromium nitride (Alfa Aesar 12149, Lot 31012B) and natural pentlandite (Sudbury, Canada), which were mixed as fine powders and hot-pressed in a piston cylinder apparatus at 1 GPa pressure and 800 °C (this resulted not in a nanocrystalline material and measurements were conducted on larger, µm-sized chromium nitride grains). Because this reference material and its intermediate products were produced at high temperatures, it can be assumed that it preserved the isotopic composition of the atmospheric nitrogen precursor. The pentlandite-CrN reference was mounted along with the samples of Y-791198 into the same aluminium disk. The instrumental mass fractionation (IMF) of 12C15N/12C14N was determined as −13 ± 12 ‰ (95% confidence interval) and the PCS data were corrected accordingly. All measurements of PCS grains were conducted by scanning the primary ion beam over square areas of 3 × 3 to 10 × 10 µm2. Except for grain A-GR03, scanned at the smallest field size of 3 × 3 µm2, all scans were recorded in imaging mode. All count rates were corrected for detector dead time (44 ns). QSA corrections36 can be neglected because 12C14N count rates were relatively low (several 10000 cps for a 1 pA Cs+ primary beam) and of comparable magnitude in the synthetic standard and the PCS grains. The NanoSIMS results are tabulated in Supplementary Table 2 and secondary ion distribution images are shown in Supplementary Figure 4. Gas mixing calculations. The calculations were performed using the Cantera software suite37. Elemental compositions of gas mixtures were used as input and atomic and molecular gas species and their partial pressures were calculated at given temperatures and total pressures using the thermodynamic equilibrium solver of the suite. Thermodynamic species data were obtained from a database of NASA Glenn Coefficients38 provided with Cantera. The species considered were: H, He, C, N, O, S, H2, O2, H2O, CO, CO2, CH4, CH3OH, C2H2, HCHO, N2, NH3, NH2, NH, NO, NO2, N2O, HCN, HNCO, S2, S8, SH, H2S, CS, CS2, SO, SO2, SO3, SN, COS. The input elemental compositions for H, He, C, N, O, and S of the mixing components ‘ice’ and ‘solar gas’


(endmember compositions) were calculated from the production rates of comet Hale-Bopp19 and the composition of the solar photosphere39, respectively (Supplementary Table 4). For the ice endmember, the abundance of He was set to zero. The C/O ratio of the ice endmember was adjusted by recalculating the C abundance based on the desired C/O ratio and the O abundance. The suppression of the species N2 was accomplished by removing N2 from the list of species to be considered by the equilibrium solver.

Non-equilibrium, mechanical mixtures were calculated by using the Hale-Bopp production rates as direct estimates of the speciation of an ice-derived gas. For the solar gas a mixture of H2 and He without the elements C, H, O, N, and S was considered for simplicity. These calculations did not involve thermodynamic calculations and were based on the equation:

Xi,mix = Xi,ice × fice + Xi,solar × (1 – fice)

(Xi,gas = abundance of the species i in the mixed gas, in the ice gas, and in the solar gas, fice = molar fraction of the ice gas). The endmember compositions are given in Supplementary Table 5.

Kinetic calculations of homogeneous gas reactions were also performed in Cantera using an isothermal, zero-dimensional (perfectly stirred) reactor model. Reactions and kinetic parameters were taken from the GRI-Mech 3.0 reaction mechanism model40 and were supplemented by reactions specific to ammonia decomposition41. The calculations included the species H2, H, O, O2, OH, H2O, HO2, H2O2, C, CH, CH2, CH2(singlet), CH3, CH4, CO, CO2, HCO, CH2O, CH2OH, CH3O, CH3OH, C2H, C2H2, C2H3, C2H4, C2H5, C2H6, HCCO, CH2CO, HCCOH, N, NH, NH2, NH3, NNH, N2H2, N2H3, N2H4, NO, NO2, N2O, HNO, CN, HCN, H2CN, HCNN, HCNO, HOCN, HNCO, NCO, N2, C3H7, He, Ar, C3H8, CH2CHO, CH3CHO. As input species compositions for ice-derived and solar gas we used those of Supplementary Table 5, wherein the ‘Rest’ was substitute by non-reacting He. Because no sulfur-bearing species were available in the models (and reaction data are generally scarce), H2S was also substituted by He. The homogeneous reaction kinetics are expected to be reasonably accurate in the absence of solids. Solids, like metallic iron, will likely enhance reaction rates while being modified by the reactions with the gas.

Supplementary Note Technical nitriding: Technical nitriding using ammonia-hydrogen mixtures is conducted at temperatures at which ammonia is not a thermodynamically favourable molecule with respect to N2. However, its dissociation is kinetically slow and, therefore, ammonia remains as a metastable molecule in the gas phase. The metal surface acts as a catalyser of ammonia dissociation and therefore establishes local thermodynamic equilibrium, which is described by the nitriding potential. The bulk of the gas remains in a non-equilibrium state.

As a result of ammonia nitriding, chromium nitride precipitates occur evenly distributed within in the host alloy, and, for symmetry reasons, form in three mutually perpendicular orientation variants, resulting in preferred orientations. In body-centred cubic (bcc) iron, the morphology of CrN precipitates is governed by the Baker-Nutting orientation relationship, (001)bcc||(001)CrN and [110]bcc||[100]CrN., which arises from a matching pair of Cr-Cr distances in CrN (approx. 0.42 nm) and Fe-Fe distances in bcc iron (approx. 0.41 nm). In this setting, the interface energy of CrN is highly anisotropic and only one set of parallel (001)CrN cube planes is allowed to develop coherent, low energy interfaces, resulting in platelet equilibrium shapes42.


The platelet-shaped precipitates within the metallic matrix are often referred to as ‘continuous’ precipitates to distinguish them from massive, lamellar and ‘discontinuous’ CrN precipitates, which occur at the metal-gas interface when additional nitriding agents such as nitrous oxide (N2O) are used12. We did not observe such massive CrN precipitates probably due to comparably lower nitrogen activities, which, however, were sufficient enough to produce the continuous CrN precipitates. Our gas mixing calculations indicate that N2O should be less abundant than nitric oxide (NO), which is expected to be poorly reactive in terms of providing atomic nitrogen at the metal surface.

In technical nitriding size gradients of CrN precipitates are often observed (e.g., ref. 13) on scales of several hundred µm. Such observable gradients are likely absent in our samples due to fragmentation or initially small metal grains.


Supplementary Figures

Supplementary Figure 1 | Bright-field TEM micrographs and selected area electron diffraction (SAED) patterns of PCS grains from Y-791198. a, Grain 98F03 consisting of nanocrystalline pentlandite (n-Pn) and carlsbergite (CrN). b, SAED pattern of the interior of grain 98F03. c, Dark-field scanning TEM image of the PCS portion of grain aggregate 98F05. The inset shows a convergent beam electron diffraction pattern of a single carlsbergite platelet. d, SAED pattern of the PCS portion of grain aggregate 98F05. e, Grain 98F11 (FIB section after after NanoSIMS analysis). Relatively large carlsbergite platelets and grains of brezinaite (Cr3S4) occur within nanocrystalline pentlandite. f, SAED pattern of the dark (strongly diffracting) carlsbergite-brezinaite aggregate in the lower central part of e. Carlsbergite and brezinaite show a crystallographic orientation relationship.


Supplementary Figure 2 | Bright-field TEM micrographs and selected area electron diffraction (SAED) patterns of PCS grains from Y-793321. a, Grain 21F04 consisting of carlsbergite and nanocrystalline pentlandite. b, SAED pattern of the interior of grain 21F04. The continuous rings of nanocrystalline pentlandite are better defined compared to Y-791198 (e.g., 311 and 222 are resolved). Ring segments correspond to carlsbergite platelets, which show preferred orientations. c, Grain 21F07 showing square-shaped carlsbergite platelets in nanocrystalline pentlandite. d, SAED pattern of the interior of grain 21F07. e, Grain 21F08 showing scarce carlsbergite and large domains of troilite (FeS) intergrown with nanocrystalline pentlandite. f, SAED pattern of grain 21F08.


Supplementary Figure 3 | Electron energy loss (EEL) spectra recorded from carlsbergite platelets and nanocrystalline pentlandite matrix from PCS of Y-791198 and Y-793321. a, Carlsbergite platelet from grain 98F03 (Y-791198) showing clear absorption edges of N and Cr. b, Carlsbergite platelet from grain 21F04 (Y-793321) showing clear N and Cr edges. Visible O and Fe edges may be due to adjacent oxide or sulfide material. c, Nanocrystalline pentlandite matrix of grain 98F03showing Fe and Ni edges and minor Cr and O. Cr is visible probably due to small carlsbergite particles. The O edge likely results from interstitial oxide or phosphate in the nanocrystalline material. d, Nanocrystalline pentlandite matrix of grain 21F04 showing Fe and Ni edges and minor O. Also here, the O edge likely results from interstitial oxide or phosphate in the nanocrystalline material.


Supplementary Figure 4 | NanoSIMS isotopic analysis of PCS grains in Y-791198. a, Backscattered electron SEM image of grain C-GR28. The red, dashed square indicates the area (10 × 10 µm2) scanned by NanoSIMS for isotopic analysis. The green dashed line indicates the approximate location of a FIB section removed after NanoSIMS analysis (sample 98F11). b, 12C14N ion distribution. The colour scale indicates the number of accumulated detector counts. c, 12C15N ion distribution image. d, 12C ion distribution. Areas of probable organic carbon are marked by arrows. e, 32S ion distribution. f, Backscattered electron SEM image of grain C-GR29. g, 12C14N ion distribution. h, 12C15N ion distribution. i, 12C ion distribution. j, 32S ion distribution. k, Backscattered electron SEM image of grain A-GR15. l, 12C14N ion distribution. m, 12C15N ion distribution. n, 12C ion distribution. o, 32S ion distribution.


Supplementary Figure 5 | Zoned PCS-bearing sulfide aggregates in Y-791198. a, Backscattered electron SEM images of a concentric aggregate in Y-791198. PCS is exposed, but no Fe,Ni metal is evident. b, Backscattered electron SEM images of Aggregate 98F05 sampled by FIB for TEM study. PCS is not visible at the polished surface and was found during FIB sectioning. c, Bright-field scanning TEM image of the FIB section marked in b. Chr = chromite, CrN = carlsbergite, Dau = daubréelite, Kam = kamacite, Mag = magnetite, n-Pn = nanocrystalline pentlandite, Pho = Ca-rich phosphate (probably merrillite), Pn = pentlandite (normal), Po = 4C-pyrrhotite, Sch = schreibersite. The white dashed line indicates the border to the phyllosilicates-rich matrix. d, Composite X-ray distribution map obtained by EDX spectrometry. Red = sulfur Kα, green = nickel Kα, blue = iron Kα. The dashed white lines separate the inner reduced zone from the outer oxidized shell. e, Composite X-ray distribution map. Red = sulfur Kα, green = chromium Kα, blue = oxygen Kα. Carlsbergite stands out as bright green regions. f, Composite X-ray distribution map. Red = calcium Kα, green = phosphorus Kα, blue = oxygen Kα. Schreibersite stands out as green regions, Ca-rich phosphate as pink-coloured regions.


Supplementary Figure 6 | TEM analysis of minerals in the outer shell of the grain aggregate 98F05. a, Bright-field TEM image of 4C-pyrrhotite. b, Bright-field TEM image of magnetite. c, SAED pattern of 4C-pyrrhotite obtained from the region labelled in a. d, Detail of the SAED pattern marked in c. The three reflections in the central row indicate a twinned 4C-pyrrhotite with the twin domains a (reflections 111 and 110) and b (reflection 200). e, SAED pattern of magnetite obtained from the grain labelled in b.


Supplementary Figure 7 | Thermochemical relationships in the Fe-S system. The field of pyrrhotite (Fe1-xS) and troilite (FeS, x = 0) is bounded by the reaction forming pyrite (FeS2; solid line: ref. 43; dashed line: ref. 44), the reaction forming metallic iron (solid line: ref. 43; dashed line: ref. 44), the melting curve (from ref. 45), and the condensation of elemental sulfur (JANAF data, ref. 46). The composition of pyrrhotite is variable within the field of Fe1-xS and depends on the sulfur fugacity and temperature. Isopleths for specific pyrrhotite compositions (x = 0.125 for 4C-pyrrhotite, Fe7S8; x = 0.100 for Fe9S10; x = 0.083 for Fe11S12) are shown as red lines [solid: ref. 44; dashed: ref. 45). The line for x = 0.125 intersects the pyrite-pyrrhotite equilibrium at about 750 K. Below this temperature 4C-pyrrhotite is not stable with respect to the formation of pyrite.


Supplementary Figure 8 | Gas mixing calculations for temperatures of 750, 850 and 950 K. In addition to Fig. 4, the magenta mixing trajectory shows the mechanical, non-equilibrium mixture between the ice species and hydrogen. The variation of the C/O ratio of the ice (red dashed line) does not alter the direction of the mixing trajectories in the compositional space, but has an influence on the extent of the mixing results. a+c+e, Equilibria involving NH3. The vertical dashed line in a indicates a nitriding potential for successful nitridation under laboratory conditions22. The effects of temperature variations are small. The non-equilibium mixing line (magenta) is mostly congruent with the N2-suppressed trajectory at the same total pressure, showing that the suppression of N2 models a metastable retention of NH3 for most of the mixing ratios. Shown are phase boundaries for Cr sulphides and nitrides (black) and roaldite (Fe4N) and


pentlandite (green). The mixing trajectories trend along the line of coexistence of brezinaite (Cr3S4) and carlsbergite, consistent with the topotactic intergrowth of both minerals shown in Supplementary Fig. 1e,f. The stability field of roaldite lies rather far from the mixing trajectories. Its absence in our samples is likely due the actually achieved nitriding potentials having been too low or due to subsequent sulfidation (or oxidation) of this mineral. b+d+f, Phase equilibria involving H2S and H2O (ref. 43-47). Note that below 750 K 4C-pyrrhotite (Fe0.875S) is not stable with respect to pyrite formation. Note also that in a purely solar gas (diamond symbol), troilite (FeS, x = 1) is not stable at 750 K and starts to form at about 700 K when the composition falls on the metal/pyrrhotite boundary. However, if just a small fraction of ice contribution is present in the gas phase, troilite/pyrrhotite can form at higher temperatures. The congruency of the non-equilibrium mixing line (magenta) with the equilibrium trajectories shows that H2S and H2O are stable molecules with respect to dissociation.


Supplementary Figure 9 | Kinetic modelling of gas evolution. The kinetic model shows the time evolution of homogeneous (gas-gas) reactions in gas mixtures between Hale-Bopp-like and solar composition (‘Ice’ and ‘Solar’ compositions of Supplementary Table 5; H2S and ‘Rest’ have been replaced by non-reacting He). fice is the molar fraction of the ice-derived gas. NH3 is metastable at the temperatures shown but decays only very slowly to N2 and H2. Dashed lines represent the equilibrium states at given temperature and pressure (note colour and corresponding symbol on the right of each line).


Supplementary Tables Supplementary Table 1 | Semi-quantitative TEM-EDX analyses of bulk PCS (weight %). The analysed areas were selected to contain no other phases than carlsbergite and nanocrystalline, P-bearing pentlandite. The relative uncertainty (based on a 95 % interval) is about 10 %, but may range up to 70 % for Cr and N when the carlsbergite crystals were few and the areas suitable for analysis were small (especially the case for samples 98F05, 98F11, 21F07, and 21F08). Minor amounts of oxygen may be present due to interstitial oxide phases (e.g., phosphates). The content of nitrogen has been estimated by assuming a molar ratio of Cr/N = 1.

Sample P S Cr Fe Co Ni N

98F03 4.6 24.0 3.8 30.6 1.8 33.5 1.7

98F05 6.9 25.7 3.0 27.8 1.7 33.6 1.3

98F11 7.0 23.6 2.0 27.6 2.1 36.8 0.9

21F04 6.3 25.4 2.7 32.9 1.7 29.8 1.2

21F07 6.2 26.4 4.8 34.9 1.3 24.3 2.2

21F08 7.3 24.9 2.1 34.2 1.9 28.6 1.0

Supplementary Table 2 | Results from the NanoSIMS analysis of PCS grains from Y-791198 and a synthetic pentlandite-CrN reference material (R1 C231) used as isotope standard. For individual measurements of 15N/14N ratios 95% confidence intervals (CI) were calculated from counting statistics. Instrumental mass fractionation (IMF) corrections were applied based on the measured standard. The Y-791198 IMF corrected data, include the error of the IMF correction. Given averages are error-weighted averages. δ15N values are relative to air composition [(15N/14N)air = 0.003676]: δ15N = [(15N/14N)sample/(15N/14N)air −1] × 1000.

Sample Area size (µm2)

15N/14N measured

95% CI

15N/14N IMF corr.

95% CI

δ15N (‰)

95% CI

R1 C231 Spot #1 3 × 3 3.671 × 10−3 5.3 × 10−5 Spot #2 3 × 3 3.640 × 10−3 4.2 × 10−5 Spot #3 3 × 3 3.590 × 10−3 3.9 × 10−5 Average 3.627 × 10−3 4.5 × 10−5 3.676 × 10−3 4.5 × 10−5 0 12 Y-791198 A-GR03 3 × 3 3.859 × 10−3 3.0 × 10−5 3.911 × 10−3 5.7 × 10−5 64 15

A-GR15 5 × 5 3.809 × 10−3 2.2 × 10−5 3.860 × 10−3 5.2 × 10−5 50 14

C-GR28 10 × 10 3.791 × 10−3 2.3 × 10−5 3.842 × 10−3 5.3 × 10−5 45 14

C-GR29 5 × 5 3.785 × 10−3 2.1 × 10−5 3.836 × 10−3 5.2 × 10−5 44 14

Average 3.805 × 10−3 2.8 × 10−5 3.856 × 10−3 5.5 × 10−5 49 15


Supplementary Table 3 | Samples studied by FIB-TEM and NanoSIMS. Abbreviations: 4C-Po = 4C-pyrrhotite, Bre = brezinaite, Chr = chromite, CrN = carlsbergite, Dau = daubréelite, Esk = eskolaite, Kam = kamacite, Mag = magnetite, Mer = Merrillite, n-Pn = nanocrystalline pent-landite, Pn = pentlandite, PCS = phosphorus- and chromium-bearing sulfide, Sch = schreibersite.

Sample Meteorite Description Mineral assemblage TEM SIMS

98F03 Y-791198 grain with frayed border, isolated in matrix

CrN, n-Pn, Sch

98F04 Y-791198 irregular grain, isolated in matrix

n-Pn, Sch; no CrN

98F05 Y-791198 grain aggregate with FGR, isolated in matrix

CrN, n-Pn, Sch, Kam, Dau, Pn, 4C-Po, Mag, Chr, Mer

98F06 Y-791198 grain aggregate with FGR, isolated in matrix

Pn, 4C-Po, Mag, Ca phosphate; no PCS

98F07 Y-791198 irregular metal grain in matrix

Fe,Ni metal; no CrN


Fe,Ni metal; no CrN


Fe,Ni metal; no CrN


Fe,Ni metal; no CrN

98F11 C-GR28 Y-791198 irregular aggregate at chondrule-FGR border

CrN, n-Pn, Bre, Sch, Esk? (peripheral)

C-GR29 Y-791198 irregular aggregate at chondrule-FGR border

Sch (not sampled for TEM)

A-GR15 Y-791198 grain with irregular border, in chondrule FGR

(not sampled for TEM)

A-GR03 Y-791198 grain with well-defined border, in chondrule FGR

(not sampled for TEM)

21F04 Y-793321 grain with frayed border, isolated in matrix

CrN, n-Pn

21F07 Y-793321 irregular grain, isolated in matrix

CrN, n-Pn

21F08 Y-793321 grain at chondrule border CrN, n-Pn, Tro

21F09 Y-793321 metal grain in chondrule Fe,Ni metal; no CrN


Supplementary Table 4 | Input elemental abundances for thermodynamic equilibrium calculations (molar fractions). Endmember compositions are for ‘ice’ based on Hale-Bopp productions rates (data from ref. 19) and for ‘solar gas’ based on spectrometry of the solar photosphere (data from ref. 39).

Ice Solar gas

H 5.000 × 10−1 9.209 × 10−1

He 0 7.838 × 10−2

C 1.104 × 10−1 2.261 × 10−4

N 2.642 × 10−3 5.549 × 10−5

O 3.808 × 10−1 4.209 × 10−4

S 5.985 × 10−3 1.271 × 10−5

Supplementary Table 5 | Input species abundances for non-equilibrium (mechanical) mixing calculations (molar fractions). Endmember compositions are for ‘ice’ based on Hale-Bopp productions rates (data from ref. 19) and for ‘solar gas’ represented by a mixture of H2 and He with a solar ratio (data from ref. 39).

Ice Solar gas

H2 0 8.545 × 10−1

He 0 1.455 × 10−1

H2O 6.611 × 10−1 0

CO 1.521 × 10−1 0

CO2 1.322 × 10−1 0

CH4 3.967 × 10−3 0

CH3OH 1.587 × 10−2 0

H2CO 7.272 × 10−3 0

NH3 4.628 × 10−3 0

HCN 1.653 × 10−3 0

H2S 9.917 × 10−3 0

Rest 1.128 × 10−2 0


Supplementary Discussion Solubility of nitrogen in liquid iron-rich alloys: Based on our TEM-EDXS measurements, bulk PCS contains about 1–2 wt% nitrogen (Supplementary Table 1). In order to test the idea that an equivalent amount of nitrogen could have been dissolved in the precursor metal, we used the model of Kunze et al.48 to calculate the weight fraction of dissolved nitrogen in liquid Fe-rich alloys as a function of temperature and N2 partial pressure. The reason for assuming a liquid state is that nitrogen solubility is largest in liquid iron alloys49. Therefore, the calculations yield estimates of the lower bounds of N2 partial pressures required to dissolve certain amounts of nitrogen in the metal. We conducted the calculations for two types of metal compositions: CM metal (Fe92.3Ni6.3Co0.4Cr0.7P0.3)50 and a Ni- and minor element-enriched metal (Fe47Ni38Co2Cr5P8) corresponding approximately to sulfur-free PCS.

The results (Supplementary Fig. 9) show that in equilibrium with a solar gas of 10 Pa total pressure (where the partial pressure of N2 is approximately 5 × 10−4 Pa, ref. 18) no more than about 0.03 parts per million (ppm) by weight of nitrogen is dissolved in the liquid metal. In order to dissolve nitrogen in the weight per cent range, partial pressures of nitrogen on the order of 108 Pa are required. There are currently no known and reasonable scenarios of protoplanetary processes that could provide such high partial pressures of N2.

Supplementary Figure 9 | Solubility of nitrogen in liquid Fe-rich alloys as a function of temperature and partial pressure of N2. The partial pressure of nitrogen of 5 × 10−4 Pa approximately corresponds to a solar gas at a total pressure of 10 Pa (calculation based on ref. 48).


Kinetics of nitridation: In order to estimate the time scales required for nitridation of Cr-bearing Fe-rich alloys we used the approach of Schacherl et al.51 to calculate nitriding depths as a function of time. Diffusion coefficients of N in solid Fe as a function of temperature were calculated based on the parameters of Fast & Verrijp52,53. The solubility of N in solid Fe as a function of temperature and nitriding potential was calculated using the parameters of Hosmani et al.54. The results are shown in Supplementary Figure 10.

Supplementary Figure 10 | Nitriding depths for precipitation of CrN in a Fe99Cr1 alloy as a function of time, temperature, and nitriding potential. Note that at temperatures of 350 K and 300 K ammonia is a stable molecule under protoplanetary conditions. Therefore, the actual activity of atomic nitrogen at the metal surfaces will be orders of magnitude smaller than the hypothetical activities that would correspond to the nitriding potentials shown in this plot for 350 K and 300 K. Accordingly, nitriding times would be much longer at these temperatures (calculations based on ref. 51).

At 750 K the nitridation to depths of 50 nm to 10 µm occurs within seconds to hours. Because 50 nm is the approximate size and spacing of carlsbergite crystals within the PCS and 10 µm is approximately the maximum size of PCS grains as a whole, the times needed to nitride the metal to these depths bracket the timescales needed to form the PCS grains. The sulfidation apparently followed in short succession on the nitridation of the outer metal layer, and, hence, the time for nitridation of a 10 µm metal grain can be considered a conservative maximum, because the porous sulfide shell developing on the metal grain increases the permeability of reactive gases toward the nitridation front (compared to a purely diffusive transport within a non-sulfidated metal grain).


The effect of total gas pressure is accounted for in the nitriding potential, but the rate of atomic nitrogen uptake by the Fe metal surfaces in principal depends on the partial pressure of H2 (ref. 50), and, hence, on the total pressure. For an increase of the partial pressure of H2 from 1 Pa to 100 Pa during passage of a shock wave22, the uptake rate of N increases accordingly by a factor of 100. However, because the uptake rate approaches zero when the metal surface approaches nitrogen saturation53, the diffusion of nitrogen within the metal will be the rate determining process once a steady-state concentration at the surface (close to saturation) has been achieved. Because the diffusion and growth within the metal are arguably slower than the transport of gas to the metal surface, the appropriate surface concentration for sustained carlsbergite growth is not expected to be strongly affected by the total pressure, as long as the impingement rate of NH3 is considerably higher than the maximum nitrogen uptake rate. For example, at relevant total pressures between 10 to 1000 Pa, a temperature of 750 K, H2 partial pressures determined by gas mixtures containing 90 % ice, and a fixed nitriding potential of 3 × 10−4 Pa−1/2 (and corresponding NH3 partial pressures), the maximum nitrogen uptakes rates (based on ref. 53) range between 1.5 × 10−11 mol s−1 m−2 (at 10 Pa) and 1.7 × 10−9 mol s−1 m−2 (at 1000 Pa). In comparison, the impingement rates of NH3 calculated from the kinetic theory of ideal gases range between 7.3 × 10−6 mol s−1 m−2 (at 10 Pa) and 9.0 × 10−3 mol s−1 m−2 (at 1000 Pa) and are several orders of magnitude larger.

Based on the considerations above, the timescales for the formation of PCS and the enclosed carlsbergite on the order of hours to days appears reasonably feasible at temperatures around 750 K. These times are consistent with the duration of shock-wave induced gas conditions and mineral reactions22.

At lower temperatures between 300 and 350 K, where aqueous alteration on chondritic parent bodies took place and resulted in the alteration of metals55, the diffusion rates of nitrogen in the metal are much slower (Supplementary Fig. 10). Although the aqueous activity in the CM parent body at these temperatures apparently lasted for several million years56, individual CM chondrites probably experiences much shorter episodes of alteration. In particular, Y-791198 is one of the least altered CM chondrites and certainly was subjected to aqueous fluids for a much shorter time interval (or at much a smaller temperature or water-to-rock ratio) than other CM chondrites33. This preserved fine-grained (and easily altered) anhydrous silicates in the fine-grained rims, where also PCS is found. Although we do not know exact parameters of the alteration of Y-791198, it seems highly unlikely that time or temperatures were sufficient for the nitridation of metal.

Besides the kinetic difficulties, low temperature nitriding faces thermodynamic problems. Under the temperature and redox conditions of the aqueous alteration in the CM parent body ammonia is thermodynamically stable. Therefore, the activity of atomic nitrogen at the reaction interface will be much lower than in the case of metastable ammonia, and the dissociation reaction will not yield sufficient amounts of nitrogen to drive the precipitation of carlsbergite. Because, from a thermodynamical point of view, ammonia is also expected to be the dominant nitrogen-bearing species in the fluids of the CM parent body, there is a lack of any known alternative nitriding agent. This is emphasized by the fact that there is no known technical nitriding agent that would allow nitriding temperatures below ~500 K, although such an agent would be technically highly desirable57.


Supplementary References

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33. Chizmadia, L. J. & Brearley, A. J. Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002. Geochim. Cosmochim. Acta 72, 602-625 (2008).

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55. Palmer, E. E. & Lauretta, D. S. Aqueous alteration of kamacite in CM chondrites. Meteorit. Planet. Sci. 46, 1587-1607 (2011).

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